Punch and die clearance

Customer is punching .021" thick 316 stainless 2 layer mesh (sintered wire screen filter). Its made of stainless wire that has been fused together to form a very fine filter. Starting with a .569 diameter punch and a .571 die button (6 at a time) , we are getting an unacceptable burr. The round slug is being used for testing and needs to be as bur free as possible. Kind of an odd material as it isn't a true .021 sheet. I'm at a loss trying to decide if I need to go tighter or looser (.001" per side) with the clearance. Anyone ever punched filter material ? Customer is using an air cylinder press which seems to have plenty of power. Typical die construction, die shoe, punch holder, punches, stripper, die plate with buttons.

large burr usually indicates excessive clearance, but I have only worked with sheet material, not screens

another cause might be dull cutting edge, cutting wires may require much more frequent sharpening compared to cutting only sheet stock, sharp edge is very important to initiate the fracture

RJT said:

Customer is punching .021" thick 316 stainless 2 layer mesh (sintered wire screen filter). Its made of stainless wire that has been fused together to form a very fine filter. Starting with a .569 diameter punch and a .571 die button (6 at a time) , we are getting an unacceptable burr. The round slug is being used for testing and needs to be as bur free as possible. Kind of an odd material as it isn't a true .021 sheet. I'm at a loss trying to decide if I need to go tighter or looser (.001" per side) with the clearance. Anyone ever punched filter material ? Customer is using an air cylinder press which seems to have plenty of power. Typical die construction, die shoe, punch holder, punches, stripper, die plate with buttons.

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Don't know specific to that material, but in the die makers handbook I got when doing my apprenticeship, it said Dayton had done a study and found 12% clearance was more optimal than 10% for *most* materials. Their case studies included burr formation, life, etc. What they found was on lower die clearance, the punch would drag the material into the die and bulge at the top, then spring back onto the punch creating a slight drag (more or less (: ). W

detailed photo (magnified) of the burr on the part and the left over might help figuring out the issue

Measure the hole in your die, and measure your punch. Are they what they say they are?

It sounds as if you want a correctly sized perfectly formed slug rather than a perfect hole. You might look at literature from your punch manufacturer to find out if they recommend a different tooling topology for that case.

The standard solution for unwanted burrs is to decrease the size difference between punch and die.

As mentioned above, you might have a dull punch. It might be prudent to order a couple more punches just to see if that solves your problem. Yes, you have to pay for them, but you wouldn't be pulling your hair out.

metalmagpie

I have had a good amount of success with making 7075 aluminum punches a couple thousandths oversize and making them "0" clearance on their first trip through the die for thin material and believe it or not aluminum punches will make thousands of punches through harder materials especially when there is no clearance.

Hi RJT:
If this is mesh made by two layers of wire crisscrossed over each other, then all of the wires on the bottom layer will not be supported by the punch and all of the wires on the top will not be supported by the die insert when contact is first made between the mesh and the halves of the die.

So none of the wires can shear immediately...they will all need to bend first until they touch both punch and die insert at which point they can be sheared and then torn in half.
This reality argues for smaller die clearance than usual so the wires can't extrude in between the die insert and the punch.

Remember the wires are only 0.0105" thick, they are not 0.021" thick, so your die clearance needs to be sized for the material the die will actually need to cut.
However the wires are shearing at an angle, so they will be effectively thicker than 0.0105" depending on how coarse the mesh is and therefore how much they need to bend before contact on both sides and that will be different for every wire depending on how close the shear point is to a cross wire that is acting as a pivot.

Of course, the clearance will be too tight for every point at which a pair of crossed wires intersects the profile of the punch, but the majority of the wires you have to cut will be un-crossed.
You may fight this forever; a good bit of what you're interpreting as excessive burr may well be bent wires.
Also when you shear at an angle, you may well tend to drag the break part of the cut up into a point just as you'd see with excessive die clearance on a punched sheet, making the burr even worse.

Do you have a microscope or a macro lens on a camera to get a really close look at the edges of the cut?

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

Its not a matter of dull punches, new or freshly sharpened punches and die buttons still leave a burr. I'm going to decrease the clearance and see if that helps. Yes the slug is what I am interested in eliminating the burr on. I don't care if a burr is on the sheet skeleton.

Fairly tight clearance on mesh, good strippers help a lot, most of the punch is punching air so shearing of the disk tougher, concave tools seem to help too
Mark

kustomizer said:

I have had a good amount of success with making 7075 aluminum punches a couple thousandths oversize and making them "0" clearance on their first trip through the die for thin material and believe it or not aluminum punches will make thousands of punches through harder materials especially when there is no clearance.

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what was the idea behind using 7075?

Look at the edge of the parts for a double break. There should be about 1/3 cut and 2/3 break. If there is a double break the material is being dragged and can cause a burr. Increase the clearance slightly.
Is the punch in the center of the die opening. Is the perf plate thick enough to properly support the punch and keep the punch where it belongs.
Roger

rogertoolmaker said:

Look at the edge of the parts for a double break. There should be about 1/3 cut and 2/3 break. If there is a double break the material is being dragged and can cause a burr. Increase the clearance slightly.
Is the punch in the center of the die opening. Is the perf plate thick enough to properly support the punch and keep the punch where it belongs. Check the die set.

Click to expand...

Roger

Laser cut it