Building a Small Index Fixture for Your CNC Mill

T he following describes an odyssey that started with the need for a 15-tooth gear

with an obsolete tooth profile, and ended with an index fixture for my small ProLight CNC milling ma-chine. There are a limited number of dividing heads available that are small enough to fit the physical restrictions of small milling ma-chines. Another factor to consider is the cost of a commercial unit compared to the expected usage, making them quite expensive for a hobbyist’s budget. A fourth axis is

one way to meet indexing needs, but many small CNC machines do not have this capability, and it re-quires additional software as well. There is no computer interface with this device, allowing it to be used on either CNC or manual mill-ing machines, or even a drill press.

There are no dimensioned drawings included, as the large range of the readers’ machine capabilities and overall size differences makes this a moot point. However, there are some design ideas and construction THE INDEX PLATES

The plates are the heart of the project and were built on my small CNC mill. Fortunately, I have Rhinoceros V5 as my CAD system, paired with RhinoCAM 2012, to produce the G- and M-code necessary to machine the parts. We will deal with drawing and making the plates first.

The first step is to prepare a drawing template of the master plate. This includes the mounting holes, center bore, the outside profile, and the master pin hole (Figure 1). The use of the template saves drawing time, but it also ensures the plates are the same size, with the same details and, thus, are interchangeable.

There are two plates in the system. The first is the “fixed”

by Pete SorensonPhotos and Figures by Author

plate that fastens to the body of the indexer. There is a second “moving” plate that is fastened to the rotating shaft. The fixed plate is developed from the template drawing by doing a polar array of the pin hole with the desired number of holes. If you only

require the number of increments provided by

the fixed plate, there would be only the one pin hole in the moving plate. In the case of a fixed plate having 16 holes (Figure 2), with

16 being divisible by two, four, and eight, you

would have these available to you as well. Similar choices are available to

you with any non-prime numbers (composite numbers).

If you want to index any num-ber that is a direct multiple of 16, such as 32, 48, or 64, this can be provided by adding a hole(s) to the moving plate. To develop the

location of the pin holes in the moving plate for indexing 32 places with a 16-hole fixed plate, you need to polar array the master pin hole by 32 steps in your CAD. Then, array radial lines in 16 places to represent the 16 holes of the fixed plate. From the resulting array, pick one 32-hole that is not coincidental to a line in the 16-hole

techniques included to help one build their own tool. Overall accuracy will depend on the abilities of the builder, as well as the capabilities of the machines being used. The final result should be as accurate as the machine it was built with.

THE INDEX TOOLThe included photos are of the

first unit I built and used as a test bed, and it is obvious that some of the material is from the scrap bin or from repurposed parts (Photos 1 and 2). Originally, the intention was for a device to machine a single part, but “Project Creep” reared its ugly head and the rest is history. The original unit worked well for the intended purpose, until I needed a 30-tooth gear. The next step was designing the 30-step ability with as few changes to the base device as possible, this leading to the replaceable plates used for all of the other number combinations.

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pattern and delete all other holes, leaving the selected hole and

master hole (Figure 3).

To do 48- and 64-hole plates, you would follow

the same process. Notice in the drawing that the two 48-pin holes used are adja-cent to a 16-pin hole

(Figure 4). Choose the two 48-pin holes and

delete the holes not being used. Also note that you can

use any hole in the array, as long as there is one relative to the right of a 16-hole and one is left of a 16-hole. The 64-hole sequence is

done the same way, except it is the holes on each side of the 32 hole array.

You now have the CAD drawings for both the fixed and moving plates for a series of indexes relative to 16. A similar approach will work with just about any integer; just make sure that none of the pin holes are overlapping or are too close. If the number of holes gets large, make a sec-ond moving plate to avoid the confusion of having too many choices for the holes to use.

THE NUMBER STAMPING DEVICE

A good point to make at this time is to remember to perma-nently mark the holes. Failing to do so will result in some rather odd gear tooth configurations. I used number stamps because the permanent marker wiped or wore off with use. A stamped plate and the stamping fixture are shown in Photo 3.

USING THE INDEXERTo use the indexer, install the

two plates needed and proceed to do the cutting operation, using the pin in the master hole of the moving plate and, in succession, each of the holes in the fixed plate. For example, if using the

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16-hole plate set to produce 48 holes, do all the 16-holes first and follow by doing all the holes in the fixed plate with the pin in one of the 48-holes in the moving plate. Then, finish by doing all of the 16-holes with the pin in the remaining 48-hole.

LARGE PRIME NUMBERSWhen you get to larger prime

numbers, there are layout changes required. There needs to be a hole for each index on the fixed plate, creating the possibility of over-lapping pin holes. Using the template drawing as a start, draw concentric circles relative to the master pin circle. Create a line between the center of the plate and the center of the master pin hole. Polar array the line for the number of indexes you wish. At this time, place a pin hole at the intersection of a concentric circle and an arrayed line.

The restriction for the hole pattern is that the holes must not overlap. This will create the hole pattern for the fixed plate. For the moving plate you need a slot the width of the pin hole that will allow you to use the locking pin in all of the holes. It is important that this slot is centered on a radial line from the center of the plate to the master pin hole (Figure 5 and Photo 4).

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BUILDING THE INDEX PLATESThe method I used to make the

plates contributed to the accuracy of the system. The plates were cut out in one machine setup, from cutter paths derived from the template drawing. Because they were cut from plate there was no need to face either side, as the plate thickness was consistent.

The pin holes and mounting holes were drilled first, followed by the center bore, as this is where the accuracy is necessary. The fasteners on my indexer are 4-40, so the first drill program drills both the mounting holes and the pin holes. The second step was to use a 1/8", two flute end mill to drill the pin holes to size. This yielded an accurate pin hole, both in diameter and location, as well

as a hole that is closer to being perfectly round

than a drilled hole. The shaft bore was cut next, using a pocket cutter path, followed by a cleanup profile cutter path.

The last step was to cut the outside of the

plate. The plates are cut as a profile, with bridges, or tabs, to hold everything in place until finished (Photo 5). The tabs were removed with a jeweler’s saw and the mounting holes were finished with a countersink tool (Photo 6).

A LITTLE DISCUSSION ABOUT ACTUALLY CUTTING A GEAR:

Mounting the Gear Cutter The easiest way to cut an actual

gear is with the correct involute gear cutter. The decisions that need to be made when using an involute cutter are, choosing the correct cutter from the set, selecting a method of mounting the cutter, and determining the depth to cut the teeth.

To mount my cutter, I built a three piece arbor to fit a 1/2" tool holder

(Photo 7). There are a few impor-tant tasks involved in building this holder: A piece of bar stock was faced on one end, drilled and ream-ed for the 1/2" shaft, and then the shaft was fastened in the reamed hole with Loctite. The reason I used a 1/2" shaft, rather than making the holder from one piece, is there is less material to machine away, and the 1/2" shaft is a nice “no measure” fit in the collet. When the Loctite cured, the holder was drilled for a roll pin and pinned on; the Loctite is just there to keep the shaft from moving when drilling the roll pin hole.

The hole for the clamping screw was drilled and tapped while the tool holder was held in a 1/2" collet on the lathe. The final step was to turn the outside diameter, as well as the diameter of the spud that fits the cutter bore. This should result in a tool holder that is as accurate as your lathe is capable of producing. You will notice there is no key for the cutter, as I would rather have the cutter spin than slip the belts on the milling machine.

A Fly CutterA second choice for cutting a

gear would be to use a fly cutter. The bit seen in Photo 8 was cut with a wire EDM machine using a .dxf file of the tooth profile. The tooth profile, in this case, was derived from a photo of the gear on an optical comparator (Photo 9).

A 3D PocketA third choice for cutting the

gear profile would be to use a 3D tool path developed in modeling software, as I did with an old gear that needed replacing. Again, the tooth profile, actually the space between two adjacent teeth, was obtained from a photo of the origi-nal gear on an optical comparator. Because this is an obsolete tooth form, there are no commercial cutters available. In this instance, as well as with the fly cutter bit, the Rhinoceros software let me trace a background bitmap.

The bitmap used was the optical comparator screen photo. After tracing the best tooth, I arrayed the tracing by the tooth count and measured the resulting diameter. After measuring the diameter of the existing gear and the CAD model, I was able to scale the CAD model, starting with an oversized, full screen bitmap and then scaling down to the final size. Any slight errors in the drawing were

also reduced by the same percentage, resulting in an accurate gear.

The next step was to create a 3D pocket that was the size and cross section shape of the space between adjacent teeth. With RhinoCam, I devel-oped the cutter paths to machine the pocket.The last step was to machine the pocket and then index to

the next space. This particular gear is now earning its keep in an early 1900s street clock (Photo 10).

A further demonstration of the CAM pocket method was done with a 16-tooth, 6DP gear. GearGen is a Rhino script plug-in that draws the correct gear shape for involute gears. Using the correct pitch circle of 2.666", a 16-tooth gear was generated and modified by creating

the curves needed to build the sur-faces of a pocket. The pocket was used to build three separate pocketing tool paths in RhinoCam. You should make sure the pocket extends beyond the gear blank by at least one diameter of the end mill you plan on using (Figure 6).

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Figure 6

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indexed shape; the limiting factor being that there should be no under-cuts in the CAM generated pocket portion of the machining opera-tions. This method is much slower than using a dedicated cutter, but it allows you to produce profiles when there is no cutter available.

USING AND TESTING THE INDEXER

As with all indexers, it is import-ant to revolve in the same direction for each index. If a step is missed, it is best to rotate all the way around in the same direction until arriving

at the correct pin hole, rather than backing up to the missed hole.

Once a system of plates was developed, a test was in order to make sure all my ideas were ap- plicable and usable. This entailed a test block and a set of gears. The gears in the block seen in Photo 12 are 43-tooth and 32-tooth. Using the values generated from Machinery’s Handbook, I produced the gear blanks on the lathe to within .0005" of the correct size. I then proceed-ed to cut the teeth with the index

This gear was cut first with a 1/8" diameter ball end mill, and the second and third tool paths were done with a 1/16" ball end mill. The surface finish of the teeth should be evident in the close-up photo of the gear (Photo 11). In this photo, all of the teeth have been cut with the 1/8" end mill; the ones next to the smoothest teeth are with first the 1/8" end mill, followed by a 1/16" end mill. The final cut, also done with a 1/16" end mill, was done with a fine set over, as well as a very small depth of cut.

While a gear was used as an example, there is no reason it could not be a spline or any other

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54 DigitalMach in i s t

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fixture, using three depth cuts in a climb milling direction. The three passes lightened the forces on the holder and the climb milling resulted in a good finish. Using the three-wire method, the gears were measured and found to be in tolerance. The plate was drilled and reamed using the DRO on my Bridgeport mill, with values calcu- lated from Machinery’s Handbook. The shafts were then pressed into the holes and the gears evaluated, with satisfactory results.

DON QUIXOTE SAID, “SO MANY WINDMILLS, SO LITTLE TIME.”

Well, maybe not windmills, but surely many things to index. As the gear set worked well, it was my impression that the journey was over. Then, a discussion of bevel gears raised its ugly head, present-ing a challenge to build an index base with angle capabilities. I think by using Rhinoceros and GearGen with the CAD pocket method I should be able to produce a bevel gear. Please play with some of the ideas presented here and explore some of the options, but beware of the dreaded “Project Creep.”

RESOURCES: RhinocerosV5: www.rhino3d.com

RhinoCam 2012: www.mecsoft.com

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