Wt4603 unit5 week6

WT4603 Wood Processing Safety & Practice

Lecture Unit 5 (Week 6) Postponed until Week 7

Circular Saws 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

Circular Sawing MachineNOTE Circular sawing machines are high risk woodworking machinery

Pupils should not be permitted to use this machine.BS 4163:2000

The machine should be included in a planned maintenance program that should include electrical safety tests.Read Circular Sawing Machines (Week 6 Notes)

Department of Manufacturing & Operations Engineering

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HazardsOperators should be aware of the following hazards.

a) The work piece can become jammed in the circular sawing machine or can kick back.

b) Hands or fingers can come into contact with the blade.c) Wood dust can be inhaled.d) Noise can cause permanent hearing damage.e) Inadvertent starting of the machine can present a

hazard.


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Risk control measuresIt is essential that the machine be provided with:

•A means of electrical isolation using a fused switch-disconnector on or adjacent to the machine, and that it is controlled by a starter incorporating overload protection and no-volt release

•A conveniently positioned, mushroom-headed stop button or suitable other control device that can quickly stop the machine in an emergency


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Risk control measures• Fixed guards (removable only with the use of a tool), or

alternatively interlocked guards that encloses the drive mechanisms.

• It should be possible to lock the machine to off when not in use, if a locking device is not incorporated in the machine.

• There should be sufficient space around the saw bench so that the timber can be handled safely. The floor of the work area should be level. The floor surface should be kept free from loose material and should be non-slippery.


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Saw Blade Terminology

•Hook Angle•Edge Clearance•Pitch•Gullet•Plate Tension•Riving Knife•Table Slot•Guards•Fence


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Hook Angle

Hook Angle


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Hook Angle• On most saw blades, the tooth faces are tipped either toward or

away from the direction of rotation of the blade, rather than being perfectly in line with the centre of the blade.

• Hook angle is the angle formed between the tooth face and a line drawn from the centre of the blade across the tip of the tooth.

• On a blade with a positive hook angle, the teeth are tipped toward the direction of the blade's rotation.

• A negative hook angle means that teeth tip away from the direction of rotation, and a zero degree hook angle means that the teeth are in line with the centre of the blade.


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Hook Angle

Positive Hook AngleDepartment of Manufacturing & Operations Engineering

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Hook Angle

Zero Hook AngleDepartment of Manufacturing & Operations Engineering

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Hook AngleIn both Rip and Cross-cutting saws the Hook angle determines

The feel of the cut The quality of the finish The power consumed


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Hook Angle

• The approach angle of the saw varies according to the relative position of the tooth in the downward cutting arc.

• This angle alters from the top plane of the timber where the tooth top makes first contact to compress the timber before the tooth point engages, to a plane where the tooth angle and the timber face are parallel.


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Hook Angle• The amount of Hook determines the degree to which the tooth

will drive into the timber during the cut.• The effect is of the timber being drawn forward.• The greater the hook angle the greater this tendency.

Too great of a hook angle will result in▫ Harsh cut▫ Tearing▫ Poor finish▫ Less rigid tooth▫ Vibration.


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Hook Angle• A blade with high positive hook angle (+20°) will have a very

aggressive cut and a fast feed rate. • A low or negative hook angle will slow the feed rate and will also

inhibit the blade's tendency to "climb" the material being cut. • A blade for ripping wood on a table saw will generally have a

high hook angle, where an aggressive, fast cut is usually what you want.

• Radial arms saws and sliding compound mitre saws, on the other hand, require a blade with a very low or negative hook angle, to inhibit overly fast feed rate, binding, and the blade's tendency to try to "climb" the material


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Tooth Configuration

• The shape of the saw blade tooth and the way the teeth are grouped also affect the way the blade cuts. The configuration of the teeth on a saw blade has a lot to do with whether the blade will work best for ripping, crosscutting, or laminates.

• Of course, no matter which tooth design you're looking at, more teeth will give you a smoother cut than fewer teeth.


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Tooth Configuration

A ripping blade will have a Flat Top Grind (FTG) for fast cutting with the grain.


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Tooth Configuration

A cross cut blade will do the best job with an Alternating Top Bevel (ATB), cutting across the grain like a knife and producing a very smooth cut.


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Tooth Configuration

A blade with Triple Chip Grind (TCG) is good for all-purpose cutting and also gives you a very clean cut. TCG blades are also good for cutting non-ferrous metals and plastics.


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Tooth Configuration


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Tooth Configuration• In general, blades with more teeth yield a smoother cut, and

blades with fewer teeth move material faster.

• A 250mm blade designed for ripping wood can have as few as 24 teeth, and is designed to quickly move material along the length of the grain.

• A rip blade isn't designed to yield a mirror-smooth cut, but a good rip blade will move through wood with little effort and leave a clean cut with a minimum of scoring.


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Tooth Configuration• A crosscut blade is designed to give a smooth cut across the

grain of the wood, without any splintering or tearing of the material.

• A crosscut blade will usually have from 60 to 80 teeth. More teeth mean that each tooth has to cut less material.

• The result is a cleaner cut on edges and a smoother cut surface. With a top-quality crosscut blade, the cut surface will appear polished.


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Gullet• The gullet is the space cut away from the blade plate in front

of each tooth to allow for chip removal. • In a ripping operation, the feed rate is faster than in

crosscutting and the chip size is bigger, so the gullet needs to be deep enough to make room for the large amount of material it has to handle.

• In a crosscutting blade the chips are smaller and fewer per tooth, so the gullet is much smaller. The gullets on some crosscutting blades are purposely sized small to inhibit a too-fast feed rate, which can be a problem, especially on radial arm and sliding mitre saws.


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Gullet

•The gullets of a combination blade are designed to handle both ripping and crosscutting. The large gullets between the groups of teeth help clear out the larger amounts of material generated in ripping. The smaller gullets between the grouped teeth inhibit a too-fast feed rate in crosscutting


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Riving Knife

Note :

The thickness of the riving knife is to be approx. 10% thicker than the plate of the saw blade but must be less than the saw kerf “B”.

The riving knife must be tapered at its leading edge


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Riving Knife

Riving knife should not be set higher than the highest point of the saw blade


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Riving Knife

Max and min distances between saw blade and riving knife.Distances are measured in a radial direction


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Riving Knife

Max and min distances between saw blade and riving knife.Distances are measured in a radial direction


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Riving Knife

Shaded area can be included in the riving knife profile

Example 2 reduces the Ø of blade that can be fitted to the machine and has a weak point


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Riving Knife

When viewed in the direction of the blade edge the riving knife should not be visible to the operator.


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Table Slot

Table slot width max. and min. measurements for saw blade ranges.Min. and max. measurements for distance from blade face to edge of table slot.


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Table Slot

Max. distance between saw blade and front edge of table slot.(Blade raised to highest level)


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Table Slot

Max. distance between saw blade and front edge of table slot on machine with a scoring blade.(Blade raised to highest level)


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Clearance

The supporting body of the cutting tip is ‘relieved’ or ground back (X) so that only the cutting edge itself makes work contact.

For any machine cutting tool to work efficiently, only the cutting edge of the tool should be making contact with the work.


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ClearanceWork clearance must be provided.

• The saw tooth provides this clearance.• The ‘Kerf’ produced by the teeth must be wider than the

supporting saw plate.• Steel saws had the kerf formed by bending or ‘setting’

alternate teeth laterally.•With tipped saws the tips are wider than the saw plate and

thus create the clearance.• Clearance or relief bevels are ground on the sides and the

top of each tooth.Department of Manufacturing & Operations Engineering

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Plate Tension

• A flat disc will remain flat and true if turned at a slow speed.• When variable stresses are created on this disc due to:

Braking effect of sawing Heating effect of friction Outward pull of centrifugal force

• the outer rim area of the disc will expand.

• If the whole area of the disc can expand at the same rate the disc will remain flat and true.

• This does not happen with a saw blade.• The central region of the blade is clamped between the collars and does

not expand.Department of Manufacturing & Operations Engineering

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Plate Tension• Only the teeth of the saw blade should make contact with the work and

a portion of the energy expended in cutting will unavoidably be converted to heat.

• The periphery of the blade will therefore tend to warm up more quickly than the main plate body.

• This will cause the periphery of the blade to expand.• If this is not taken into account the blade will distort.• To prevent this saw blades are ‘tensioned’ during the manufacturing

stage.• Rim speed will determine the amount of tension required in a particular

saw blade.• Thinner saw blades require greater tension.• Faster saws require more tension.


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Plate Tension• Traditionally this was done by highly skilled labour but modern saw

manufacturers use machine operated rollers to achieve a faster more uniform result.

• This within limits allows the plate to expand uniformly in uneven temperature gradients.

• A blade which has lost its tension will be seen to be throwing from side to side.

• This is most noticeable as the blade slows down after the machine is switched off.

• If this is the case the blade should be removed and sent for servicing.• This can be reduced by cooling the blade tip while in operation by

packing.


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Plate Tension•TC tipped blades have an extremely long life and to

assist the tension factor, slots are incorporated around the edge of the blade.

•These allow a degree of individual expansion between segments on the plate edge.

•They also break up harmonic frequencies, which build up during the sawing process.


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Top Guard•Covers the top edge of the saw blade.

•Deflects waste.

•Prevents accidental contact with the uppermost teeth of the blade.

• It can also limit the effects of material rejection.


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Top Guard


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Blade Guard

Blade guard that completely encloses the blade


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Blade Guard

Guard not connected to riving knifeGuard has lead in at the frontDust extraction integrated in the guard


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Fence


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Fence


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Fence


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Circular Sawing Machines

• Scoring blade rotates in the opposite direction to the saw blade

• Scoring blade cuts on the up stroke of the cut to ensure a clean cut is produced on the underside of the panel


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Work being fed into the bladeBlade rotates clockwise(Guarding not shown)


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Top surface – fibres being compressedBottom surface – fibres being ripped outWill damage the under side of the material


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Top cutting “into the wood”Bottom “cutting out of the wood”


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Scoring blade introducedWill make an initial shallow cut before the larger saw blade will cut the material


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In this diagram the scoring blade rotates in the same direction as the saw bladeIs this blade still cutting “out of the wood” on the underside ?


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Here the scoring blade revolves in an anti clockwise direction. As the material passes this blade the cut produced is “into the wood”


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• This can create a hazard as the blade is unguarded and tends to pull the material towards the saw blade

• Should only be used when crosscutting or ripping sheet material where a clean finished cut is required on both surfaces of the panel


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Calculations• Saw Calculations

• Rim Speed (P) = x x Spindle Speed (R)

• Where P = Rim SpeedR = Spindle Speed = Diameter in metres

• Units = m x 1/min = m/min

• This speed is expressed in metres per minute m/min• It is left in this format because spindle speed is expressed in

Rev/minDepartment of Manufacturing & Operations Engineering

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CalculationsProblem 1

A saw blade of 750mm is required to run at a rim speed of 3048m/minCalculate the number of revs per min that will achieve this speed.

P = x x R P 3048

R = ---------- = ----------------- = 1294.3 RPM x 3.142 x .750


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Calculations

• Problem 2

• A circular saw has a motor that revolves at 1500 Rev/min• The of the blade is 720mm.

• Calculate the rim speed for the blade.


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Saw Parts•Guard•Fence•Blade•Riving Knife• In feed table•Out feed table•Start / Stop•Foot stop


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Blade Guard


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Blade Guard


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Blade Guard


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Blade Guard


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Fence•Must be parallel to the

saw blade•Clamping mechanism

cam screw lock

•Sliding fence Horizontal position Vertical position


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Fence (Horizontal Position)


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Fence Position for cross-cutting


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Fence (Vertical Position)


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Fence position for cross-cutting


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Fence position for cross-cutting


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Incorrect setting


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Incorrect setting

• Material once cut is positioned between the rotating blade and the fixed fence

• When the piece is moved by operator or vibration it will contact the moving blade and get ejected

• This will result in possible injury to the operator, damage to the blade and damage to the material


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Riving Knife


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In feed table


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In feed table


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Out feed table


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Start / Stop


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Foot Stop


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RECAP


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List of Drawings (Drawer not included)

• Orthographic plan, elevation and end view (no hidden detail)• Orthographic of each joint• Double Haunch Mortise and Tenon• Finger Joint• Dovetail Joint

• Isometric of side/back rail and leg (show hidden)• Also show splay

• Isometric of top/mid rail and leg (show hidden)• Plan view of table top

Education

Wt4603 unit5 week6