Wt4603 unit4 week5

WT4603 Wood Processing Safety & Practice

Lecture Unit 4 (Week 5)

PLANING MACHINES & SETTING PLANER KNIVES Lecturer: Mr. Joseph Lyster [email protected] Notes prepared by: Mr. Donal Canty, Mr. Des Kelly and Mr. Joseph Lyster Notes available on www.slideshare.net/WT4603

mailto:[email protected]

http://www.slideshare.net/WT4603

Surface PlanerPlaner Knives Important factors when selecting a planer knife • Suitability for cutter block

• Material being processed

• Finish required

• Volume being machined

• Clamping and setting mechanisms

Department of Manufacturing & Operations Engineering

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Surface Planer

•Planers can have 2,3,4,6……. Cutter knives.

•Most smaller machines such as those found in schools will have 2,3 or 4 knives.


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Surface Planer• Knives can be made from Chrome Vanadium steel

alloy. • This is suitable for machining softwoods and non

abrasive hardwoods. • However with advances in machining technology

better materials have been developed to machine wood and wood composites.

• Chrome Vanadium knives dull quickly on harder more dense material.

• This requires more sharpening, setting up and leads to a lot of time wastage.


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Surface Planer

•High Speed Steel (HSS) is a cobalt steel alloy with a small percentage of Tungsten added.

• It is more suitable for machining all types of wood than the chrome steel compounds.


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Surface Planer• Abrasive stock should be machined using solid or

tipped cutters.• Tungsten Carbide (TC) is the best tool compound

for machining manufactured boards.• For general work HSS cutters are preferred to TC • Cutters can be re-sharpened easily.• A keener edge can be achieved on HSS giving a better finish


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Surface Planer

•The reason for this is that steel compounds are smelted and shaped by rolling and forging while the metal is close to melting point.

•The molecules of the compound flow and align themselves in response to this pressure giving the material maximum strength and edge holding capabilities


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Surface Planer•Tungsten carbide is a sintered compound.

The fine grain powder from which the cutter will eventually will be made is compressed into a mould ( the ‘blank’ un-edged cutter required) under extremely high temperature (1500C) and pressure to form a solid block.

•Tungsten does not flow – it retains a granular structure and will chip rather than deform if abused.


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Surface Planer•Because if its brittle nature TC cutters

require a more obtuse sharpness angle than the HSS cutters (more support for the cutting edge).

•This makes it less satisfactory for cutting softwoods than HSS knives which can be ground to a more acute cutting angle.


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Surface Planer

CutterKnife

TC

KnifeCutter

HSS

Large grinding angle

to support cutting edge

Smaller grinding angle

produces keen edge


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Knife Cutter geometry

• Rake or Cutting angle

• Angle created between the face of the cutting knife and the centre of the cutter block

• Can have a wide range

• Softwoods 27° to 35 °• Hardwoods 15° to 25 °

CuttingAngle


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Knife Cutter Geometry

Bevel or Lip Angle

• Angle formed to give the cutting edge

• Minimum of usually 35°

• Greater for tipped cutters

BevelAngle


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Clearance Angle

• Angle formed between a line tangential to cutting circle and the bevel angle of the knife

• Must be present• Has a bearing on the life

of the cutting edge• Usually 10° to 15°

ClearanceAngle

Cutter CircleDiameter


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Peripheral Cutting Speed

• A constant speed in the range of 35-45 m/s will give best results

• Increase in speed may cause loss of dynamic balance due to vibrations

• Poor finish• Increased noise levels

Cutter Circ leDiameter

CutterRotation

Work Movement Direction


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Pitch distance

• Combination of a rotary cut and a linear feed will leave the surface of the material with a series of arcs on it called Curtate Trochoids

• The pitch and depth of these arcs will determine how smooth the surface finish will be

Work PieceSlower Feed Rate

Work Piece

Pitch

Pitch

tt

t = Cutter arc depth on machined surface

Fast Feed Rate


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Department of Materials Science & TechnologyUniversity of Limerick

Pitch Distance

• 2mm to 3mm for non obvious joinery and painted external work.

• 1mm to 1.5 mm for internal painted work.

• 0.5mm to 1mm for hardwood joinery and furniture.


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Pitch DistanceThe SI unit of time is the second , but the minute is acceptable. Feed rate on wood working machines is expressed in metres per minute. (m/min) The formula for the pitch of the cutter marks is given by:

fp = -------

nR where p = pitch of cutter mark

f = feed rate

n = number of effective cutters

R = revolutions per minute of block


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Pitch DistanceThe unit for “p” will be metres (m) f m/min m minp = ---- = --------- = ----- x ------ = m nR 1/min min 1


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Pitch DistanceProblem 1

Calculate the cutter pitch of a 4 cutter block revolving at 4200 rev/min with a feed speed of 24m/min.

F 24 24p = ------- = ------------ = --------- = 0.0014m = 1.4mm

nR 4 x 4200 16800(Internal painted work)


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Pitch Distance

If a graded surface is specified and the machine has a multi-speed feed gearbox, the same formula is used but “f” is expressed in terms of n ,p, and R.

f p = ------- f = nRp

nR


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Pitch DistanceProblem 2

From a cutter block which rotates at 4200 rev/min and has two cutting knives, a surface finish of a 4mm pitch is required. At what speed should the feed gearbox be set.

f = nRp = 2 x 0.004 x 4200 = 33.6 m/min


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Chip formation


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Riving


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Riving

Cracking occurs below the cut depth


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On the surface planer the in-feed table (A) acts as chip breaker and the downward pressure exerted by the operator also makes it act as the pressure bar.

A


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Chip formation


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Chip formation

• Chip breaking aid and pressure bar prevent riving and splintering


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Square cutter blocks


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• Clamping bolts are in tension

• Tension force increases with rotational speed

• Will be ejected at failure


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Clamping bolts are in tension


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Circular cutter block

Clamping bolts are in tension


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Circular cutter block

• Reduced noise levels

• Better balance

• Safer clamping mechanism

• Can run head at higher speeds (RPM)

• Can produce better finish

• Easier and quicker maintenance


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Knife clamping mechanism


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Cutter projection

Round form tool with limited cutter projection


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Cutter projection

Use of a limiter to achieve limited cutter projection


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Cutter design•Cutters should preferably be designed to be used in

dimensionally similar pairs, formed to the same profile.

•Pairs should be mounted directly opposite one another. The manufacturer should ensure that instructions on balancing the pairs of cutters after grinding are given to the user.


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Cutter projection•The design of cutter blocks should, as far as is

practicable, be such as to prevent excessive cutter projection.

•Where the mounting arrangement permits projection which could subject the cutter to unsafe stresses, the maximum permissible projection for given cutter types should be specified in the user’s instructions.


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Hand-fed machines• For machines designed for hand-fed operations,

where cutters are necessarily exposed, the use of chip limiting cutters should be recommended

• For other hand and semi-mechanical feed operations, cutter blocks should have as little cutter projection as is practicable.


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Surface PlanerCutter and Machine Maintenance Involves: 1. Grinding and setting of knives 2. Roller and pressure bar setting 3. Prevention of resin build up on table and rollers. 4. Attention to: bearing wear

feed complex adjustmentsrise and fall table


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Surface Planer•The grinding angle of a cutter can

vary between 30 to 35

•This is increased to 40 for hardwoods (cutting edge lasts longer)


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Surface PlanerOverheating •May produce micro cracks in the cutting

edge which can run into gaps when the cutter is used.

•May cause the cutter to bow due to expansion.


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Surface PlanerOverheating can be avoided• By taking light cuts.

• By ensuring that the grind wheel is ‘dressed’ when required to ensure that the face is open and not glazed when grinding the knives.

• By using a ‘soft’ grinding wheel on HSS cutters – the soft structure of the wheel allows its grains to break away as soon as they are blunt revealing sharper ones.

• By wet grinding – this is the grinding of cutters while partially submerged in a mixture of water and soluble oil. The water is a coolant to prevent frictional heat developing and to disperse it should it occur. The oil prevents rust in the cutters and it provides a degree of cutting lubrication.


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Surface PlanerSetting Cutters in Block • Before setting the following points should be checked. • The out feed table and cutter block must be clean and free

from dust resin. • Method of adjusting cutters.

• Area where setting device is used from should be free from resin and damage.

• Straightness of cutters.

• Cutters correctly balanced both in weight and end for end. Department of Manufacturing & Operations Engineering

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Surface PlanerSetting devicesThere are a number of cutter setting devices.This device and procedure will often be supplied with the machine.They can be loosely placed into the following four categories: 1. Bridge device 2. Precision cutter setter device 3. Pin locater device 4. Wooden straight edge device Cutters require accurate setting in the block because if the knives are not revolving

in the same cutting circle a poor finish will be produced.


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Surface Planing


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Setting Planer Knives•When planing wooden material a number of factors

combine to generate the flat surface.•Number of cutting knives in the block•Speed of the revolving block•Feed speed of the material•Knife cutter design•Chip breaking aids•Nature and species of the material


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Knife cutter design & Chip breaking aids

•These factors combine to produce an acceptable surface finish

•Tip of the cutter splits away the chip•The chip is forced away from the stock and up along

the face of the cutter which is exerting a tearing effect on the grain fibre

•As the knife exits the stock the chip is either cut or will “rive” or tear deeply along the grain line and lift as a long heavy splinter


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•The cutter projection and the shape of the block face cause the severed chip to bend back causing a crack across its width

•This makes long grain riving less likely(Chip breaker not shown)


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• Sharp tooling (a) will aid in the chip bending back and cracking across its width

• Blunt tooling (b) will aid riving


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Chip Formation

Before knife makes its cut


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Chip Formation

Chip to be formed


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Chip Formation

Chip breaker will help prevent riving (a)

a


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A. Large cutting angle which gives the knife a lifting action which will cause riving

B. Grinding a face bevel reduces the risk of riving as it changes the cutting angle


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Face bevel on cutter knife (B)


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Setting Cutter Knives

• Setting of knives will greatly depend on the type of cutter block

• Knife cutter projection• Chip breaker• Knife parallel to table• All knives in the same peripheral cutting circle(Refer to machine manual for setting)


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Process Sheet Example:Double Mortise and Tenon


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Marking Out


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Marking out:


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Cutting 1


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Cutting 2


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Completed Joint


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Wt4603 unit4 week5

Education

Transcript of Wt4603 unit4 week5