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Precision Sheet Metal Workshop The Squaring Shear

So, you’re here to learn how to operate a squaring shear?...

Okay, lets start with a short

film.

Parts, pieces and drives

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Shears must be of an appropriate size and of sufficient strength for the materials to be cut.

The load required to shear a piece of material is dependant on three

basic factors:

the rake of the knives.

material thickness.

shear load.

Rake is measured in terms of inches of

rise per foot.

4” – 4”

8” – 2”

(Guillotine)

(Swing beam)

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“The rake and the knifes interact like a pair of scissors, only a small portion of the material being

cut at any one time.”

“With the scissor action, a full sheet of material can be cut

while the shear force remains constant.”

Scissor action

“A fixed rake shear will produce a better quality in a short cut length.”

“A variable rake machine produces better quality in long cuts.”

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“FYI: Some guillotines have

adjustable rakes.”

“A lower rake equals a better quality and a larger shear load.”

Variable rake machines are common with thicker materials; one-half inch and up.

Changing the rake allows for better cuts on a wide range of thicknesses and materials.

“Cut quality is described through: burr, bow, twist and camber.”

How is cut quality described?

“These are seen on the drop (behind shear) and are most pronounced in short pieces (under 6-inches).”

Bow is caused by the blade pushing the material down during the cutting process,

which creates a "bending force" at the shear point.

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Twist is caused by the same forces that create bow.

Both of which are amplified by cutting with the grain.

The slower hydraulic shear has a tendency to increase the amount of bow

and twist.

Increased bow

“Camber is a function of internal stress

within the material.”

“Only good quality, stress relieved or low-stress material can reduce camber in a cut.”

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Ram Incline

The purpose of the incline is to provide clearance as the blades overlap. This allows for better "shedding" of

narrow strips.

Note: The blade incline should be approximately 1/2 of a degree

Run out occurs at the end of the cut as the knife blades "breaks" through the material, just before the

cut piece drops..

The bevel is caused by the bending force from the

weight of the part.

“Run out can be reduced or alleviated by some kind of supporting structure or by increasing the rake angle.”

Reducing run out

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Shear Burnish

Shear "burnish" size (the cut before the fracture) is expressed

as a percentage of material type e.g.:

Mild Cold Rolled Steel 30%

5052 H32 Aluminum 60%

304 Stainless 15%

Brass or Copper 55%

20% burnish (cut)

Fracture

Burr

“Just look edge on!”

How far the shear blades penetrate into the material before fracturing is determined by the hardness and shear strength of

the material.

Penetration factor

“For example: a material with a penetration factor of 35% would allow the blades to penetrate 35% of the material

before the fracture occurs.”

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Gap: Clearance between the blades

set the burnish.

Clearances/Gap

Increasing clearances can reduce tonnage, to a point.

“As a general rule of thumb., the clearance should be set at 7 percent of the material thickness being

sheared.”

Excessive clearance can increase the tonnage requirements.

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Knives used beyond the point where they should have been sharpened will require an excessive amount of material removal to renew the condition.

“It is foolish to delay the sharpening of dull knives.”

and heavy burrs on the stock being cut.

damage to the shear from overloading.

Broken, chipped or dull knives will result in

“The knife can bow under load”

Deflection: Movement within the blades and beam

caused by the force of shearing is creating the bowing action.

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The upper knife should be bowed .002 inch nearer the lower knife than at the ends. This adjustment is necessary to overcome deflection of the

upper ram.

When shearing near capacity

“Checking the burr… can be give a good indication of clearance gap and/or dull

knifes”

General notes:

1) The thicker the material becomes, the greater

the amount of clearance required.

2) The harder a material type the greater the amount of

required clearance.

3) Dull knives will have the same effect on shearing as having too tight a clearance! Poor cut quality! Higher shear loads!

4) The mechanical shear is 60 strokes per minute 20 to 25 strokes

on a hydraulic machine.

5) The hydraulic shear is slower so there is more time for

bow and twist to develop.

6) The slower action of the hydraulic machine requires the gap the machine have a nominal blade gap setting.

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Never allow the gap in the ram gibbs to get looser than .001 inch.”

Following these simple rules will make your shearing jobs better!

Regularly square the side gauge with the

knives; do the same for the back gauge.”

Remember too much clearance can will reduce the blade life

and effect the everyday accuracy of the shear

“Soft aluminums have a greater shear load because the penetration factor is greater.”

“Harder aluminums require less shear load because the blade doesn't need to penetrate as far for fracture to happen.”

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Mt = material thickness S = Shear strength

P = Penetration factor R = Rake of the top blade in inches per foot

Shear load

Shear force = (S x P x Mt2) / (R / 12)

Remember when you go from a material thickness of .250 to .375 that it is a physical increase of 50%...

Watch your shear load!

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Cutting - squaring and strips

•Dull knives increase twist and camber.

•Knives must be kept sharp

•Do not expect a good cut on cuts smaller than eight times the material thickness!

Regardless of backgauge movement method…

…lash must be considered!

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Front stops

Pop up / push down

Scale

Scribing cut lines

Height gauges are best for layout

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Lining up scribe marks

Set backgauge by bringing the gauge forward to position on knife edge!

You may need to look between the guard and the ram.

Look close…

Use backgauge to set location.

Once the gauging angles are scribed.

Line up side gauge for angle.

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Half-shears…

…Can be used for gauging!

Locate half-shears with turret press.

Drop part over knife then pull back against lower knife.

part

drop

Shears and the shop clown

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A mirror can be mounted near the machine to allow an operator to see behind it.

Some states and countries require the use of rear guards and light curtains.

Always check with mfg when load and capacity are issues.

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Heavy or sharp parts dropping can cause serious injury to someone "spudding" or just carelessly standing around!

Watch your feet!

Conclusion

“Shears are not that difficult to operate but, they can still can be dangerous!....

….It only takes a little training to care for and operate them correctly!”