Mechanical or hydraulic guillotine | MIG Welding Forum

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Appie,

I'm not Forrest, but I hope that I can help you.

You don't say what kind of material that your customer is wanting to shear. If it is everything from stainless steel to plastic, just shave your head now so you don't have to pull out all of your hair.

For mild steels, 7.5% is usually considered optimum. If you look at the edge of a sheared piece of metal you will see about 1/3 of it cut (shiny) and about 2/3 fracture (dull). This tells the toolmaker that he is cutting (or punching) the material with the least amount of tonnage required to produce a good part.

If you increase the clearance above 7.5%, tonnage will initially go down. The problem is plastic deformation. The material will actually flow into the die opening until it's thickness is 13.33 times (inverse of 7.5%)the die opening and then fractures. The tonnage goes up at this point because the die is "seeing" thicker material. An easy way to spot large clearance is a rounded top edge and a large burr on the bottom.

Low clearance dies (fine blanking) were the fashion 10 years ago. In most cases, companies found that the added equipment costs were not worth the improvement in part quality. For every 0.5% in clearance that is removed, the require tonnage goes up 25% (this is aprox.). For example: a part with 7.5% takes 10 ton, the same part with 7.0% takes 12.5 ton to cut, and the very same part with 5.0% clearance will take 30.5 tons to cut. You can see where this is going.

I have built dies with 2.5% per side clearance and they take a lot of work to keep going. The sudden release of force when the material finally fractures will knock the dowel pins right out of the die. Grade 8 bolts break, and even M-2 tool steel will not hold an edge.

If the shear that you are working on has an adequate frame, in theory, it can be done. In reality, get another shear to do the small work.

JR

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