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Mechanics homemade CNC machineAuthor - Sergey Pavlov ( Graf)

Why do I need a machine?

We are defining the technical characteristics of

appointment

cutting efforts and a stepper motor

Accuracy

Speed

Milling work area size

plain bearings and guides

materials

Non-metallic bearings

Resourcedecide

What to do and how?

Running screws and nuts

Helical gears with sliding friction

feed-screw

sliding nut

How to proceed?

Ball screws

machine design

Chassis

Transmission

Attach "commodity" form

application

Why do I need a machine?

"Would the machine, and that it do there," "Let us see what happens, then, and see," "Just wondering," "Not ajigsaw or a file to saw do not know how, here, let the machine and saws", " An interesting problem itself and theprocess of solutions "," I want the machine to to saw it many KIT-s and earn a lot of money ", etc. etc. Similarmotivations to start a construction of such a complex and expensive devices such as CNC machines, are notserious, though common.

My motive reason not coincide with any of the above. I knew what I would do on the machine - cut balsa parts fortheir aircraft. And why in the NC? And because he was tired and his hands too long. For example, here is a photoof the upper wing panels and a copy of the stabilizer of the aircraft I-5, designed for CNC lathe and completelysawn on it.

This is my first model, which is designed exclusively for the CNC. Ribs - balsa 1.5 mm, all tongued, 80% of theparts - unique. Hand is doing - worn out, but probably not to do. And imagine, bang a model in the first flight? Orthe second? Posedeesh! And then, he took and drank a new wing, or there ... stabilizer.

Oh well. A machine-why? Where not spit - office with laser-sharp! I gave files received the items, and notexpensive. Yes, it is, if the stamp KIT-s, but not in the process of development. OFFICE need volumes, they are

not interested cut 2-3 parts, 10 parts, they even cut will not, Give them 10 standard sheets. Yes, and not run up tothem.

To design inside and out, and then a sheet of laser cut, perfectly matched to all, can only be a simple model, butnot a copy. Maybe someone and it fails, but not me. Constructed node, sliced, glued, twisted in his hands, fixeddislike, moved on - that's my approach. And for this machine must be home.

Reading the forum dedicated to CNC machine tools on our website, I came to the conclusion that those wishing to

n ew s sh ow ca se Articles R eview s refe re nce s Th e sh op s C on ta cts To find
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build a machine a dime a dozen. But if with the electronics and software people, in general, friends, at least havean understanding of what and how to do something with the mechanical part of the machine - pipe. Article task - tointroduce those interested in the course of business on an example of the design of a particular machine. It wouldbe desirable to questions on forums are more meaningful and based on real facts and not on speculation. I haveno problem to teach and to specify exactly how you build your own machine. You can take my recommendationsinto consideration, and can be ignored, it is your right.

In this article, not a word is said about the electronics and software. And not only by the fact that this is a topic foranother article, which may have someone write. I do not want to offend anyone, but, in my opinion, today theelectronics - not a problem. In contrast to the mechanics of it can be quite easy to buy in full - connected andworking, and its cost is not more than a quarter of the total cost of the machine. But the mechanics of acceptablequality at a reasonable price - a problem. I wish that people other than the desire - I want the CNC machine, it wasalso an understanding that behind this.

We are defining the technical characteristics of

appointment

1. As already mentioned, the machine is needed primarily for cutting balsa plates - cut from them details of aircraft models. In thismaterial, the machine must have a maximum performance. Also to be milled balsa construction and aviation plywood, wood,plastic, glass fiber and carbon fiber. machine accuracy of these materials should not be worse than 0.1 mm at the maximumlength.

2. Also nonmetal machine should be well cut aluminum alloy cutters with a diameter of 3 mm with the filing of 150 ... 250 mm /min, with burial to 2 mm. Precision milling of aluminum alloys must be in the region 0.05 mm to 150x150 mm square.

3. Milling of steel is not provided, except for isolated cases, the speed and accuracy is not regulated.

4. It should be possible 3D milling of models and matrices of non-metallic materials and molding vykleyki wings, hoods, lamps,etc.

Optimally, compact desktop machine under the listed tasks must have frame construction.

cutting efforts and a stepper motor

There is a misconception that the milling cutter is necessary to press on to make it better cut. It is not right. Thinkof sawing a jigsaw, a little pressed - Sawing broke. sawing speed depends on how fast you perform reciprocating

jigsaw, and the severity of the saw blade. When milling cutters thin observed the same picture, set the wrongcutting data - mill broke. Therefore, we expect a sharp quality tool and optimum cutting conditions. Under theseconditions, a load on the spindle bearings and small expected response, within a few kilograms.

Expect those kilograms by the formulas is not necessary. You can easily and visually estimate the maximumpossible effort directly with your bare hands. To do this, take a thin end mill with a diameter of 1 mm, and try tobreak it in his hands. You will be surprised how easily you succeed. Cutter diameter 3 mm in the hands of the moredifficult to break, but still, these efforts are not extraordinary. The destruction of the mills in excess of the allowableloads and will be the fuse that will protect our machine from the critical stress and failure. The rigidity of themachine should be designed for the load, it is desirable to reserve a double.

Power of the stepping motor is not essentially needed for cutting, and to overcome friction forces in the pair and

the screw guides, and these forces depend on the quality of manufacture, gaps warps, lubrication. Can calculatethese forces, techniques exist, but the smaller gear, the less reliable results are obtained. So that the enginechoice for the machine power is the same shamanism, as the choice of engine for the aircraft model with internalcombustion engine: pull - will not pull, with a stock - to the limit, ie, from experience or based on the analysis of theprototype.

Stepper motors on the market in bulk. Select suitable to this abundance is not easy. Therefore we orient thoseengines, which are often used in this technique - Soviet inductor stepper motors DSHI-200-3 or 200-2-DSHI. Theydiffer in power. There are DSHI-200-1, but it is frankly weak. DSHI-200 engines are good, if you're lucky, you canfind these engines index OS (special series, military acceptance), the quality of their production better, but alsoquite ordinary level.

Here are the technical characteristics of the engine DSHI-200-3 (in brackets values for DSHI-200-2)

The maximum static moment, nt - 0.84 (0.46).

A single step, hail - 1.8 (1.8).

The error processing step,% - 3 (3).

The maximum frequency of pickup, Hz - 1000 (1000).

The current in the power phase A - 1.5 (1.5).

Power supply, V - 30 (30)

Power consumption - 16.7 (11.8).

Weight, kg - 0.91 (0.54).

Accuracy

Often confused resolution positioning and cutting accuracy. Resolution depends on the choice of the steppingmotor and the type of transmission. For example, a stepper motor DSHI-200-3 when working optimallypolushagovom mode makes 400 steps per revolution. Therefore, if we apply the screw gear increments screw 2mm, then one step actuator moves by 2/400 = 0.005 mm, ie 5 microns. In step 3 mm - 3/400 = 0.0075 mm, iewithin 2.5 microns, but will speed and a third higher.

If you use a transmission with a toothed belt, the picture is this. The minimum possible (because of designconsiderations), the average diameter of the drive gear - 14 mm. So, one way to get the turnover of 3.14 * 14 =43.96 mm, ie moving 1 step will be 43.96 / 400 = 0.11 mm. For balsa acceptable, with difficulty, of course, but itwould be possible to reconcile, if that was all. But this is, unfortunately, not all.

For milling accuracy, the value should be added authorization process backlash in the guide and transmission, aswell as the values of displacement due to elastic deformation due to the overall machine stiffness. Backlash can be


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calculated, but with the total hardness harder. Calculate it impossible.

When mass production initially designed and built a prototype (as a rule, on the basis of prior art, ie, the othermachine). Then, the machine experience, make careful measurements, and look, it meets the accuracyrequirements of TOR or not. If you do not respond, the structure is analyzed, define problem areas, where it isnecessary to increase the rigidity, conduct changes in the design documentation and launching a series ofinstallation. The process is repeated several copies already. This procedure is called a fine-tuning of the machine.

Amateur design also a kind of prototype, but unfortunately, it also turns out to be final. This forces lay in the designof the power circuit of the machine obviously excessive rigidity. Do not be afraid. Here it is better to be safe. Thedesire to create an elegant and original design can play a cruel joke with the designer. The machine can not gethard, but the second attempt can not be - too expensive.

Falsely understood "refinement" of the machine - the correction of errors in the power circuit screwing additionalcorners, gussets and ribs - the result does not. It's the same thing as tablets to treat teeth - temporary relief, andthen it gets worse. Learn to do robust, rigid construction impossible. The design must feel, it comes withexperience in the same way as the driver with the experience begins to feel the car.

If you want to build a reliable and durable machine for everyday use, and not for demonstrating fundamentalfeatures, but design experience you have is not enough, do not tempt fate, take a basis for a proven prototype, itwill save nerves, time and money.

If you do decide to develop the design of the machine itself, stick to a few simple rules:

Do not skimp on the stiffness. In doubtful cases reinsured. Adhere to the principle of equal strength andravnozhestkosti.

In the frame structure of the machine wherever possible, apply the deaf and press fit pins or because simplebolting stiffness does not.

Remember that on average, is proportional to the torsional rigidity of the square section dimensions andflexural - fourth degree, i.e. with increasing detail sectional dimensions twice, its rigidity is increased sixteentimes.

Do not get carried away with fins. Solid aluminum part is tougher than her equal in strength and weight of thesteel, but fins.

But we digress. The accuracy of the machine is declared in the specifications on the design basis of the tasks tobe performed on the machine. Here we have declared accuracy within 0.05 mm at a working mill area bounded bythe size of 150x150 mm. We will try to provide it. When the machine is ready, let's see what really happened, andwhile some estimates.

First. Toothed belt is not suitable for resolution. So screw. In terms of resolution, step screw 2 or 3 mm is notcrucial, both approaches. By the way, is another common misconception - the smaller the propeller pitch, thehigher the accuracy of the machine. Positioning resolution becomes higher, but the accuracy of the milling.

Second. Obviously, the most loaded in the machine for guiding the X-axis carriage Weight X is expected within 5kg, expected cutting force of 2 ... 3 kg. When loads such two shafts 16 mm, length 700 mm, made of steel glued40X deflection will be about 2-3 microns. Even 5 microns, it's still quite acceptable.

Third. We assume that we will be able to ensure the rigidity of the carriage body parts X one in which there will benoticeable deformation of the cutting forces. Then all the errors (of the order of 0.04 mm) will remain on thebacklash, mainly on the clearances in screw pairs and to a manufacturing error spindles.

Very stringent requirements, in fact, this is the maximum that can be obtained from the self-made machine. As forall the milling area, if we are to keep within 0.1 mm over a length of 700 mm, it will be just super.

The drive with toothed belt accumulated screws there is no error, but the belt only conditionally runs, it actuallystretches so cutting accuracy is low with it and is rarely better than 0.25 ... 0.3 mm over a length of 700 mm.

Speed

In the machine has two speeds - the speed of movement of the spindle during milling (supply) and the idle speed

(positioning). The first is defined by the cutting conditions and can vary over a wide range, and the second shouldbe possible. Obviously, if the maximum speed is lower than the optimum feed material for milling to which themachine is designed, machine performance will be insufficient.

For balsa optimal routing modes are:

Sheet thickness 1 to 2 mm - 0.6 mm cutter diameter (0.8 mm) feed 600 mm / min rate of 40,000 ... 50,000rev / min.

The sheet thickness of 2 to 6 mm - 0.8 mm cutter feed 500 mm / min at the same speed

For other supply less material. The speed depends on the spindle. Even if today I do not have a spindle 50,000rev / min, maybe he will be tomorrow, so the machine should be done on the supply of 500 ... 600 mm / min.

DSHI-200-3 has a frequency of 1000 Hz pickup in polushagovom mode is 150 rev / min, then the maximum feedscrew pitch of 3 mm is 450 mm / min. Little does not reach the optimum mode. With screw 2mm supply will beeven less, only 300 mm / min, which is clearly not enough. When the engine is in the normal mode, the maximum

speed is obtained 900 mm / min, but positioning accuracy drops to 0.015 mm. For balsa amiss for no aluminum.

Milling work area size

As they say, size does matter, and not only in terms of the optimal placement of the square billet (100h1000 forbalsa, 300h500 for balsa plywood). The size of the working plane milling machine is very cost depends on,especially if you apply a spiral gear. Here a compromise is needed. For myself, I found this compromise -


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700h300h70 mm. You have these dimensions may be others.

plain bearings and guides

For a relatively small-sized precision machines such as the fact that we design, alternative round steel guides withplain bearings are hard to find. At least in that price category, which we look forward.

In recent years, a large number of ball linear bearings of different types. Frankly, I do not understand the reasonsfor their growing popularity. In addition to a single-esteem - exceptional ease of movement (and therefore thepossibility to use less powerful engines), they have a solid disadvantages. The most important of which - the lowaccuracy and increased demands on the environment in which they operate. All sorts of tricks to protect the designof the bearings from dust, dirt and chips save bad. Furthermore, any additional detail in the bearing assembly,whether the cuff, a scraper or brush, besides increasing the cost of making a knot of unreliable elements.

For the same reasons will remove from consideration various design schemes using rails and wheels in the formof ball bearings as frivolous machine for a given accuracy, and closely examine the sliding support.

Enclosure with plain bearing.

Bearings have a small radial size and weight, making them requires no special equipment, they can carry heavyloads at high speeds. But in our case it is important to not that, what is important is their great advantage - theyare quiet and have high damping capacity under the influence of cyclical and shock loads.

materials

When selecting a material for sliding bearings will be guided by the available material having good frictioncharacteristics for our conditions. And these conditions are as follows:

Load range of 0.1 ... 10 MPa.

The sliding speed of 0.2 ... 5 m / s.

Friction type - medium-dry - and bearing surface of the guide in whole or in contact on long sections.Separating the oil layer is absent. Oil is only on the surface of the adsorption film.

Periodic lubrication.

For a guide of high accuracy, as in our case, special attention should be paid to ride, which depends primarily onthe difference of the coefficients of static friction and sliding friction (as no lubrication or lubrication when weak).This characteristic is particularly important for us, because we use a stepper motor, and the carriage on the guidewill move though scanty, but in spurts.

After simple searches turned here is a list of available and affordable ride (with a weak lubr ication) with coe fficientsof friction material on a steel shaft:

Grey cast iron - 0.15 ... 0.2.

Antifriction cast iron - 0.12 ... 0.15.

Bronze - 0.1 ... 0.15.

Textolite - 0.15 ... 0.25.

Polyamides, nylon - 0.15 ... 0.2.Nylon - 0.1 ... 0.2.

Teflon without lubrication - 0.04 ... 0.06.

Rubber under water lubrication - 0.02 ... 0.06.

In principle, the bearings can be used for any of the above materials other than rubber, which is shown forcomparison, and iron, which we discard as material for domestic machine exotic. Frankly, the choice is not great.By and large, it boils down to this - metal (bronze) or nonmetal (any of the above, except for the rubber).

For myself, I have long chosen bronze - a proven solution, we can say the standard, widely used, and does notrequire a detailed justification. But for the order, consider other options.

Non-metallic bearings

Nothing against the non-metallic bearings have not. If for any reason I would be unavailable bronze (though todayit is difficult to imagine such reasons), I would have chosen for bearings PCB . Textolite bearings are made frommulti-layered chiffon fabric impregnated with Bakelite and a molded under a pressure of about 1000 kg / cm2, at150 ... 180 degrees. They work best if the layers are arranged perpendicular to the friction surface. Textolite canbe processed with carbide tipped tools at low feed rates and high cutting speeds with relatively tight tolerances.

Kapron and nylon work well when there is insufficient lubrication or no lubrication at all. But, like all thepolyamides are difficult machining. Nylon and nylon bearings manufactured press-molding in metal molds withdimensional accuracy to within a few hundredths of a millimeter. In the production problems may arise with the
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required tolerances on universal processing equipment - does not take one.

Teflon (PTFE) great stuff, but, unfortunately, not very good for the manufacture of bearings due to the softness ofa large linear expansion coefficient, cold creep (the appearance of residual deformations under prolongedexposure to relatively low stress), and the total non-wetting oil.

All non-metallic bear ings are used in conjunction with guides high hardness (> HRC 50) . Under this condition, theyexhibit high durability. increased hardness requirement guide is not a lack of non-metallic bearings, it is a reality.By the way, and for bronze bushings too bad podkalit guide.

Resource

With regard to the bearing life, there must be taken into account the following considerations. If we adopted the

principle of equal strength and ravnozhestkosti as a fundamental concept in the design, there is nothing stoppingto take the same principle in relation to the basic resource nodes. What I mean? The main components of ourmachines are running screws with nuts and guides. It is logical to make them so that resource screw pair wascommensurate with the resource of plain bearings. Those. installing bearings once they have to work out all thetime while operating the screws and nuts. Upon leaving the screw pair of order, the machine will require majorrepairs, at this moment, and you can replace the bearings. Making replacement before impractical to set thebearings, which survive not only the screw pair, but you and me - too.

It is known that the conventional screw steam steel spindle and bronze nut is a very long time. With proper choiceof parameters and quality manufacturing such units operate for years every day in three shifts. I do not think thatmy machine will be loaded in this way. However, it is impossible to accurately calculate the resource. It can bepredicted based on the experience and knowledge of the subject. I think that in this case will serve as a pair ofspiral for about 8 years, even taking into account the fact that I saw on the machine KIT-s. During this time a lot ofwater will flow away, and the machine is morally obsolete, and new technologies will be, and the cost of productionfalls possible. It may make sense and will not be repaired.

The pair steel screw - nut bronze works in much more severe conditions than steel guide - bronze bearing, whichmeans that the bearing will theoretically have certainly more resources. But if the gap, resulting from production ofthe thread in the nut is adjusted, the gap in the bronze sleeve bearings - no. Therefore, we will (not from theceiling, and on the basis of an analysis of prototypes and very likely) that the screw and the bronze bearings willhave approximately the same resource.

Do as much live as a non-metallic bearing? Not sure. He can live, but maybe not. The principle is not fatal, it ispossible to provide for interchangeable inserts, but this increases the cost of the bearing assembly, and in addition,by investing a lot of money in the production of the machine, do not want to initially lay hemorrhoids withreplacement bearings.

decide

With this in mind, the design of the guide, you can take the following technical solution for the implementation ofthe bearing assembly:

hole in the hull under the drill sleeve with the minimum requirements for tolerances of form and position ofsurfaces (ie, quite rudely)

tightly pressed into the case details bronze plain bushes with an allowance on the inner diameter

bore sleeve for guiding a part of buildings with design tolerances.

Already we can say that such a solution seems appropriate, but still look at other options.

The first thing that comes to mind - why do bronze bushings, and then embed them and bore, when the market isfull of ready-made sleeves plain bearings, with much better properties than pure bronze, for example,metalfluoraplastic bearings? Is not it easier to buy them and just drive in?

Metalloftoroplastovogo sleeve.

We will understand. Metalloftoroplastovogo bearing is a steel liner with vacuum impregnated Teflon-leadcomposition, dispersion liquid to a porous anti-friction layer of spechnyh bronze alloys. By itself, the combination ofbronze and PTFE tempting and promises significant benefits in properties. The way it is. Metalloftoroplastovogobearing at low speeds and dry (!) Permits a very large friction load (up to 350 MPa) and remains operational in thetemperature range from -20 to +280 degrees. However, at loads within 0.1 ... 10 MPa, and sliding speeds of 0.2 ...5 m / s (as ours) the friction coefficient may vary from 0.1 to 0.2, i.e. be within the usual bearing materials in

boundary lubrication. It turns out the same thing that put the alloy wheels on wheels eared Zaporozhets - can, ofcourse, just makes no sense.

Then maybe we will win exactly simplify the machining and thus save? Also no. In the first case we do chiseledbronze bush, in the second, just have a seat bore under the sleeve in the housing, ie, expensive surgery on agood boring machine, we do not rule. Moreover, in the calculation of dimensional chains are included tolerance ofmisalignment, runout, roundness and others. The most purchase cartridges that need to be taken into account,


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provided that these tolerances are known and reliable, that is, it is good expensive bearings instead of unknownorigin liner - 3 rubles per bag. As a result, all our precision machine did not add, but rather the opposite.

bronze bushings The cost, which is simply a piece of pipe - 50 rubles, and good metalloftoroplastovogo bearing -about $ 10. Bearing these need 12 pieces. Calculate for yourself how much we overpay, almost without purchasinganything. The same can be said about other possible options purchased plain bearings - overpay, and the benefitsare not obvious.

Well, if there is no bronze? And this is, I'm sorry, complete garbage. If you have access to a decent stock ofmachines and expensive project started, do not find a piece of bronze bushings for smaller twelve and four spindlenut, just funny!

What to do and how?

Until now we have always said, "steel", "bronze" .... And what kind of bronze and steel, specifically?

If our requirements for durability (every day in three shifts will not work) and low requirements for the stability ofthe friction forces, the choice of grades of steel and bronze, as well as heat treatment of steel rails are essentialnot. Therefore, if you call me from the factory and asked: "This bronze (steel) that you recorded in the drawing, wedo not. Can we make the change for ...? ". I immediately and without hesitation reply: "It is possible! If only this wasindeed the bronze and steel was medium carbon. For example, steel 30, 40 or 45 '.

But the figure still need something to write, and write should be the best option. Worsened always plenty of time.For plain bearing bushings are well suited tin phosphorous (BrOF10-1) and zinc (BrOTsS5-7-12, BrOTsS6-6-3)bronze. Tinless bronze (BrAZh9-4, BrS30) work best with smoothly machined hardened rails, so in any case it isnecessary to guide temper to a hardness of 40 ... 50 HRC and polished with a roughness Ra 0,63.Malo whetherfrom what eventually narezhut sleeve. The inner surface of the sleeves do not need polishing, but the roughnessof her should not be worse Ra1,25.

Let us not forget that we have in addition to bearing bushings there are bronze running nuts. There the material ismore rigid requirements, but not much for our case. It makes sense to unify the material for spindle nut and slidingsleeves.

With regard to geometry and clearance, then there better not to allow liberties. To ensure the efficiency of ourproduct at a given accuracy the maximum guaranteed clearance between the sleeve and the guide (16 mmdiameter) should be about 0.034 mm, which corresponds to the chassis landing on 7th Qualitet (H8 / f7).

In practice, with unit shipments (not serial) manufacture do so. First bore pressed into the hub shell, with therequired tolerances of form and position of surfaces, and then accurately measure the resulting hole and then justgrind rails to size, providing the necessary clearance. Then the whole thing is marked to not further confuse anybody slide for some guides.

Also impor tant parameter of the gap of the bearing sleeve - its length. Rather length not, as such, and the ratio oflength to diameter (l / d). It is known that the supporting bearing capacity is proportional to the square of l / d.Taking into account the positive and negative influence of l / d on the load-bearing capacity, often adhere to themean values of l / d = 0,8 ... 1,2. With a diameter of 16 mm rail length range bushes - 12.8 ... 19.2 mm. However,in our design of the bearing load carrying capacity are concerned a little, we load small. More concerned sleevesensitivity to distortions. Obviously, the smaller the ratio l / d, the smaller the sensitivity. Therefore, the sleevelength is better to choose closer to 13 mm than to 20.

One last note. What should I do if I can not fulfill all the above recommendations in this chapter? Drop the caseand not to bathe? Well, why, just have to be prepared for the fact that in the end quality of the product (machine)will suffer. That's all. And suddenly gets hurt? Suffer suffer, the question is how? And this is with a precision no onewill say. Questions like: "What would happen if you replace bronze brass, or even make a couple of sliding - steelon steel?" - Does not make sense. Try do, then tell us. One thing is certain - it will be worse. By the way, guidesirresponsible low accuracy pair of sliding steel-steel is allowed, and the details of the pair should have a differenthardness, for example, the guide hardened and the sleeve, conversely, released.

Running screws and nuts

In practice, there may be only two options - a classic steel lead screw with brass nut fitted with backlashcompensation device, or ballscrew (ball screws).

Helical gears with sliding friction

Almost all of the general considerations expressed in the previous chapter regarding the choice of materials for theguides and plain bearings are also valid for helical gear with sliding friction, it makes no sense to repeat. Consideranother important property of a screw pair, which may be important in relation to our case - namely, the dampingability of helical gear sliding friction.

Stepper motors peculiar undesirable effect called resonance. The effect is manifested in the form of a sudden thefall at some speeds. This may lead to missing steps and loss of synchronism. The effect is manifested when thestep frequency coincides with the natural resonant frequency of the rotor. The fight against this effect can becarried out in two directions. Electronic methods such as the transition to micro-stepper motor operation (or at thelevel of the driver of the algorithm), and the organization of the mechanical damping.

It's a shame, making or buying controller and built a machine to run into the phenomenon of resonance.Therefore, we must take care that the motor during acceleration and braking smoothly pass the resonancefrequency. Daylight microstepping mode is not always acceptable because of the sharp loss of speed and torque

on the shaft. Yes, and if acceptable, will never have a bad mechanical damping means.

The resonant frequency is calculated by the formula F 0= (N * T H/ (J R+ J L))0.5/ 4 * pi,

Where:

F0 - the resonant frequency,


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N - the number of full steps per revolution,

TH - point deduction for the method used, the control and the phase current,

JR - the moment of inertia of the rotor,

JL - inertia of the load.

The formula shows that the resonance to a large extent depends on the load connected to the engine. Obviously,when rigidly attached to the spindle motor shaft moment of inertia of the system will increase significantly, whichshifts the resonance at lower frequencies, which appear good damping properties of viscous friction in threadsrunning thread. By choosing the number of turns in the thread and adjusting clearance (preload) resonance caneliminate symptoms.

Here much depends on the material of the nut. For a good adsorption of the oil to the material. For example,PTFE nut can not be used as a damper for the complete non-wetting oil. Capron, in this sense, is better, but nottoo well. Of Nonmetallic best PCB, which is friendly with oil. Bronze is good on all sides.

When used in connection with the load of the elastic coupling of elastic materials, it is necessary to make sure thatthe elastic material is facilitated absorption of energy in the resonant system and leads to damping of spuriousvibrations, and not vice versa. These couplings are not cheap, but in this case, the damping in the threads loses itsmeaning, which is why when using ball screws, in which the viscous friction is clearly not expressed, a goodcoupling is required.

Next, focus on the specifics of steel lead screws and bronze nuts.

feed-screw

The lead screw.

Running screws rely on strength durability and stability. The strength and efficiency, we are interested enough.Durability interested in terms of determining the average pressure on the working surfaces of the threads and thechoice of coil height. But, based on the calculation of the stability, we need to determine the diameter of the screwat a predetermined length and the selected pattern fixing screws in the supports. This scheme must also beselected.

I will then lighten up, to look wise and bore you with the calculations on tricky formulas. Especially since I myself,though I am able to do this for a long time already these things do not count. Our machine is not hard with the jackscrew cargo on a given multi-ton load, and precise mechanical device. Selection of geometric parameters of thescrew can and need to produce prototypes based on the analysis. If you analyze (analyze the need industrialequipment, not homemade), a large number of similar machines and devices similar scheme, you will find thefollowing:

screw supports: one end is rigidly fixed and the other is based directly on the stepper motor.

The minimum diameter of the screw: 12 mm in length and 700 mm in length 16 mm to 1200 mm.

Thread profile: trapezoidal or tape (with a rectangular profile).

In step 3 mm height profile of 1.5 mm thread.

You can perform calculations specifically for our machine and verify this, but time is a pity. When designing themain attention should be paid to materials and technology in this case is much more important. Next will bepresented to the technical requirements of the screws. For their implementation should be sought, but it's notalways possible and quite expensive. Here it is necessary to seek compromises. Things to sacrifice, and what cannot be a difficult question and is decided by each designer in different ways, according to their preferences. Do notinsist on your opinion, are the basic requirements, as it should be really.

For thermally unprocessed spindles normal and high accuracy of the best material is a hot-rolled steel A40G.Applied also steel 45 and 40X bette r. In this case, the material guide can be unified with screws material.

In the case of finishing cutter screws used U10A steel that is annealed to the hardness of 197 HB.

For hardened and grinded on the profile of the screw thread steel grades used 40HG and 65G, which has highwear resistance. This embodiment of the machine for the home is too steep, but the ball screw, by the way, only todo so.

Tolerances screws:

1. Maximum permissible cumulative pitch error, m:

in a single step - 3 ... 6

a length of 25 mm - 5 ... 9

a length of 100 mm - 6 ... 12a length of 300 mm - 9 ... 18

per 300 mm length is added - 3 ... 5

over the entire length of the screw, not more than - 20 ... 40.

2. Tolerances of the outer, middle and inner diameter of the thread is set not more than the corresponding tolerances fortrapezoidal thread according to GOST 9484-81, with the field of tolerance 7H according to GOST 9562-81.


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3. To ensure the accuracy of the screws on the step and to prevent the thread from the rapid loss of accuracy due to local wear onthe roundness deviation of the average diameter of the thread at a pitch of 3 mm should be 5 ... 7 mm.

4. The beating of the outer diameter of the screw when checking in centers with a length of up to 1 m - 40 ... 80 microns.

5. If the outer diameter of the screw serves as a technological basis for tapping (and almost always the case), the tolerance on theouter diameter appoint h5.

It is not difficult to guess that on variations according to claim 1 directly depends on the accuracy of the machine. Ifwe moved the carriage manually vernier, then so be it was, but in this case it is easier to live, because the machineCNC accumulated error can be compensated by software.

If we started a trapezoidal thread, you have to set out the requirements would have to add a bunch of importantbut slozhnovypolnimyh requirements corners thread. But the cost of the lead screw and so high as to produce a

special tool for cutting trapezoidal thread (and it is made for each case). When manufacturing unit shipmentswithout preparation special equipment will fit perfectly tape thread with a rectangular profile.

Still, better than trapezoidal thread as compared with the tape? Only one - the best wear resistance, because coilworking surface in trapezoidal thread is greater and the pressure on this surface, respectively, is less. The choicebetween the trapezoidal thread and tape - the question of a compromise between durability and cost. If you arewilling to pay good money (comparable to the value of ball screws) for durability, choose a trapezoidal thread. I'mpersonally not ready.

I foresee the issue of the series: "What would happen if ...?". What happens if you take a good rod and cut it metricthreads with triangular profile? The answer is - it will be worse. At a diameter of 12 mm standard cut metric threadwith a pitch of 1.75. Height 1,137 mm at it, it is not enough for durability. The next thread, suitable height profile(1,624) has a pitch of 2.5 and cut into 18 mm diameter. It turns out a decent club. But most importantly, to thescrew on the requirements of paragraphs 1-5 are the same. The gain in manufacturing costs if you will, somethingsmall.

Incidentally, the manufacturing costs of the screw increases exponentially from its length. This is due to the threadcutting technology and the use of special equipment. For example, for the manufacture of a screw length of 500mm requires a backrest, and 700 mm for a screw for two. Steady rest for a specific screw need to be furtherdeveloped, the cost of completion and other necessary equipment, as you know, part of the cost of the screw. Ifwe were doing 50 screws, or associated with the production, where these screws are doing commercially, it wouldbe cheaper, and so .... That is why from the beginning I laid in the machine working area in the X - 700 mm,instead of 1000. Expensive and not all do.

sliding nut

Usually the nuts are made of bronze and brands BrO10F1 BrO6Ts6S3. If you find a bronze, it will be very good,but not fatally, if you use any other. In general, all that we have said about the materials for the sliding sleeves,valid for the spindle nut.

Permissible deviations of nuts:

1. Point 2 applies for screws to nuts.

2. For a split nut outer diameter of the thread is assigned a condition for abutment to the screw nut on the profile, so it is set at a0.5 mm than in accordance with GOST 9484-81. The inner diameter of the prescribed conditions necessary clearance, so it isset at a 0.5 mm than in the same GOST-y.

3. In cases where the inner diameter of the nut serves as a technological basis for the final processing of the body nuts (youknow, the way it happens), the inner diameter of the nut is performed on the N6.

4. Tolerances profile and pitch are not regulated, and limited to the value of tolerance for the average diameter.

The presence of the gaps between the threads of a screw pair is cause backlash. Its elimination is achieved bydesign measures - due to tightening slotted nut screw, spring clamp or collet. The easiest way to make a split nutto tie it with the screw /

Floating nut.

How to proceed?

Remember what we said about the guides and bearings: "In practice, they do so. First bore sleeve, and thenpolished to a size guide that provides the necessary clearance. " So, with the lead screws and nuts, everythinghappens exactly the opposite - first do the screws, and then grind them nuts.

This fact promises great benefits. Screws virtually no wear (this is in the production of capitally repaired machines -

make the new to the old screws nuts), so you can bring to the plant the appropriate spindle, and it will make you anut. Suitable screws can be bought, can be removed from the old machines and devices found in a landfill, finally.This greatly reduce the cost of production of your machine, because cost spindles is more than half the total costof manufacturing of mechanics.

As always happens, this solution has not only advantages. Purchased (found) the screws already have to cut theends that dictate a very definite structure supports, you might not profitable, as well as the use of the bearings,which are suitable for the screw, rather than those who would like to put to you. Often there is need for additional


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supports for the manufacture of parts that add cost and are not required to, be design your screws and nuts. Thisis a real minus.

Recently, a lot of firms (including foreign), who sell ready-screw pair. In principle, the cost of purchase andproduction do not differ much, but there is a problem with the ends. Often these companies are ready to make foryou the correct length screws and bunching of all, you do draw, but the price at the same time grow in 1,5 ... 2times. In any case, to make its spindles or buy ready-made, you decide.

If you are not sure that you will be able to produce high-quality screw pair, and you have decided in your machineto use store bought or even "left" screws will be right at first to buy them or find, and only then taken forconstruction of the machine. More specifically for the constructive design, because the design is nothing special init.

Ball screws

The ball screw sliding friction is replaced by rolling friction. This can significantly increase the efficiency of themechanism of up to 95 ... 98%, as well as an order of magnitude larger resource. This explains the widespreaduse of ball screws in mechanical engineering.

Precision ball screw below the accuracy of helical gears with sliding friction. The reason is simple. In aconventional helical gear in contact are just two parts and the technological gap (backlash) is adjustable, and theball screws, except the same two parts (screw and nut) in the work includes a third item - a ball, or rather a bunchof balls, and adjust the backlash problematic. But this does not mean that the ball screw is not accurate. It isaccurate, only the accuracy of this technology is not given easily. Put it this way, if we compare the ball screw andscrew drive with friction slip exactly the same, the ball screw is obtained substantially more expensive.

I'm not a bad attitude to ball screws and do not stand up exclusively with classical screw nut. On the contrary, I likethe ball screw itself to make them dream machine. But. In addition, it is safe, beautiful, expensive and generallycool, it's a great responsibility. It is strange to see the ball screw next to the guide of the tubes for curtains and

kapron bearings, drilled with a drill. Conversely, a good guide with fashionable look metalloftoroplastovogobearings near the threaded rod, bought on the market, and hex nut for 3 rubles at least strange.

If we use ball screws, together with the good guides, good quality plain bearing sleeves, sturdy transition couplingsfor connection to the motor ball screw, and the remaining parts of the machine should be at. Otherwise it makesno sense. And that's another price category.

machine design

Tedious part of the story is finished. Next will be more fun with pictures and photos. The first thing we denote somekey points that will be based in the design.

1. Come up with a complex mechanism with a bunch of parts is not difficult. There are many do not mind. It's hard to come up witha mechanism of simple and technological, but which fulfills the same function as the complex. Why is it difficult to come up withan original bike? Because it is all invented, long time! The question arises whether it is necessary to engage in invention anddesign balancing act? The machine is needed for business, rather than to demonstrate the inflamed imagination of thedesigner. So, without further ado, rummage on the Internet and choose the machine ready constructive scheme that meets ourrequirements.

2. Details of the machine must have a simple geometric shape with a minimum of milling operations. Additionally, these partsmust be small. We already spend a lot of money on guides and lead screws and nuts, to splurge even on filigree, lace bodyparts.

3. No welding. This extra money, and besides, then still have to anneal the welded assembly in an oven to remove residualstresses, and put on the machine for machining.

4. Material of body parts - alloy D16T. Stiffness will recruit large monolithic sections, as to impart the required stiffness thick onepiece less than three thin bonded together.

5. As little as possible fasteners. Tapping also costs money.

6. It would be nice to lay in the design of the possibility of upgrading. For example, to change the operation of the machine fieldwith minimal modifications, if necessary.

Search the Internet yielded results. I liked the Austrian-German machine-the Four the Step ( http://cooltool.ru/) .

General view of the prototype.

Very suitable prototype. If it cost less, at least two times, one would never bother with its own production, butsimply to take and buy. By the way the site has a detailed price list. It is clearly apparent sharp increase in the priceof the machine, depending on the increase in stroke for X. This is exactly what we talked about in the previous
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chapter.

Here on this prototype and we will be guided. Let us proceed.

Chassis

Let's start with the carriage Z. This is just a bar with a built-in slide bearings and bracket mounting holes for thespindle.

The carriage Z.

Y carriage is already two bars with holes for bearings and guides Z. guides are inserted into holes in the dense(transition) landing and fixed mounting screws. Fixing screws need more to soothe the soul, than for a realattachment. Runners have to sit in the holes in his tracks. In the lower bar has a hole for the spindle bearingassembly and the top - a seat for the stepping motor.

The carriage Y.

X carriage - two walls having the same structural elements as the carriage bars Y. The wall thickness of 15 mm.Less can not be otherwise guides will stay bad. At the bottom of the housing walls are screwed slide bearings formoving the carriage along the guides located in the frame.

Carriage H.

In this form, the X slider has sufficient rigidity. The effort required to make the carriage visible elastic deformationsfar outweigh the effort required to break the cutter. (Remember, broke at the beginning of the article?) Pleasenote, the design allows, in some small range to compensate for the dismal admission to the holes coordinates forguides.

Rama, in the sense we are accustomed to, not at all. Instead, there will be two beams with the housing fordirecting collected on aluminum 30h60h5 corner.

Beam.

Put X in the carriage frame.
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Chassis assembly.

You're ready to tie the chassis of the machine a strong and rigid base for the corners of the beams. The base maybe, for example a piece of laminated board used for the manufacture of kitchen furniture tabletops, or just a desk.Beams frame themselves in the required position. they do not interfere with the main thing.

Note that changing the length of the guide can be easily done with any machine (within reasonable limits) sizemilling working plane without changing the housing parts.

Transmission

You can begin to install the screws.

Screw bearings and motor.

As we have said, o ne end of the screw ha ngs dir ectly on the stepping motor, an d the other is based on the bearingassembly, which consists of two angular contact bearings, which do not allow to move along the axis of the screw.One bearing provides emphasis to one side, the other - the other. Preload in bearings is created through the capnut sleeve, standing between the bearings. Bearing assembly, and hence the entire screw is fixed in the housingmounting screw through the hole in the outer ring.

Bearings can be any. I applied with dimensions 6h15h5. The idea is that there should be double angular contactball bearings (series 176 GOST 8995-75), but find it difficult. Even simple angular contact bearings on the marketdoes not lie in heaps, and even double and did not. You can put ordinary radial bearings. Axial forces and speedwe are not big, and if after a while zahrustyat, replace them easily, do not even have anything to disassemble.

On the motor shaft screw impaled through the sleeve with screw clamps.

torque transmission from the drive screw position X on the non-drive by a special plastic timing belt.

Toothed belt.

Sam toothed belt and gear purchased. Strap on a length hardly stretches and it is necessary to ensure a goodtightness. Is it reliable? Reliably. And you can put on the X-axis shagovika two, one for each of the screw? I do notknow, have not tried. I think it will be a synchronization problem. A belt - cheap and cheerful.

Finishing touch. Put the bracket spindle.
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General view of the machine.

That's all. You can cling to electronics, to put the spindle and run the machine. Everything should work. And itworks, I must say! In principle, nothing more is needed. Oh, and the limit switches must be put, but you can notput. This option, the machine works perfectly and without limit switches.

Limit switch.

We believe housing (except rails and spindles) the details that need to be ordered at the factory - 14 pieces! Plus 2corner, plus two parts to the spindle bracket. Total: 18 parts. A nomenclature, and even less, only 8 very goodresult!

Attach "commodity" form

Looking at a photo of the prototype from the site, we see that there stanochek solid, and we have some skeletistyand dead!

Right now, we do it!

Establish the bottom of the frame channels - base (thickness 5 mm) and close the spindles channel bar - housing(2 mm thick).

Establish traverse, also of U-sections. Thus, from one end of the belt drive will close, and the other on the beamcan install connectors from shagovikov.

On the carriage X Replace the cover that protects the lead screw the Y, and for him to screw trough, which will laythe cable from the carriage Z. The same trough screwed to the frame from the drive.

Do all of these give our covers the machine rigidity? Of course, give, but not very much. Strengthening the designand give it the overall rigidity in a similar way is impossible. The power circuit of the machine should work on its
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Discuss on the forum

own and without these props. But now the machine is easy to carry from place to place, and not to keep it bolted tothe desk.

We pose the cap, drank (sample) on the new machine boxes to hide them in the transition block for wires fromshagovikov. And the finishing touch, set the tracks for cables.

All! Now is differen t! We look at the prototype - a twin br other!

There was sheer nonsense - to draw drawings, competently assign tolerances on key dimensions and can begiven to the plant.

Some people say - licked, and rejoice! Please, I do not care these criticisms, I need a good machine, not a patentfor the invention. I'll tell you in another way - creatively reworked because it does not dull rubbings, and knew whatI was doing, why, and why it is so.

I am not a great expert in the field of metalworking and machine tool design specifically, so maybe somewhere waswrong or inaccurate, comrades know me better. In addition, for many years of real design in instrument makingand mechanical engineering I have developed certain stereotypes in the approaches to the design of machineparts (choice of design bases, especially the appointment of tolerances and fits, the adaptation of the structureunder the concrete factory equipment and so forth.), Perhaps you these approaches not satisfied, so they are hereand do not bring. But the design of this machine, I relied precisely on those general considerations that areoutlined in the article. And this machine works! As conceived! Whether it will last 8 years - I do not know, time willtell, but with the design documentation, I can make not only parts, but a couple of the same machine. If you need.

application

Bibliography:

1. V.I.Anurev. Manual Machinist-designer. In 3 volumes. Moscow. "Engineering". 2001.

2. I.Ya.Levin. Reference Design precision instruments. Moscow. Oborongiz. 1962.

3. F.L.Litvin. Design of mechanisms and details of devices. Leningrad. "Engineering". 1973.

4. P.I.Orlov. Fundamentals of design. In 3 volumes. Moscow. "Engineering". 1977.

5. Directory. Instrument ball bearings. Moscow. "Engineering". 1981.6. metalworker Directory. In 5 volumes. Ed. B.L.Boguslavskogo. Moscow. "Engineering". 1978.

Copying material about radio-controlled models
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