October 14, 20196 yr 34 minutes ago, ForgeDad said: Frosty, did you add that many zeros on accident or did you truly mean .0001 clearance? Ten thou is too close? I admit my perspective is probably off. Dad considered a ten thou slop but we're not talking aero-space applications here. My good friend, Lee Marshal" who was teaching die blanking passed away a few years ago and I can not access any of his posts to check #s. I do remember him saying the closer the better, that it's more in the relief you grind than leaving a gap. Gap is bad. What tolerance do you recommend for blanking dies? Frosty The Lucky.
October 14, 20196 yr Clearance depends on the material thickness and alloy being cut. It is variable. Thicker materials use more clearance in general. If you are blanking .001" sheet copper for PC boards you would be tighter than .001", but if you are punching 1/2" steel .001" would be too tight. Tolerances also affect tonnage required to punch, as well as allowable deformation on the front and backsides of the part. Punch face angles also affect tonnage, and blank shape when ejected. For holes, having a cupped punch face can reduce tonnage by shearing towards the center, but if the part being ejected is wanted it would be curved and not flat. Full shape blanking takes the most tonnage as you are doing the whole perimeter at the same time. This is where speed comes into play.
October 14, 20196 yr You have a better handle than I do Bigguns. Lee and I spent time talking about the difference between a punch press and die blanking. While the same in principle there are real differences in use. What I recall clearly from time if Dad's shop was how hard the 12 ton punch press hit when punching just a center hole in 14 ga. Punching more than a couple holes required the 30 ton punch press and talk about bang the whole shop. We punched thicker stock after regular hours. I don't know if he ever worked 12 ga. in his shop if so I probably would've been the one DRILLING center hole, probably subbing it out for more than a couple few parts. Frosty The Lucky.
October 14, 20196 yr Biggun gave a nice description. Generally speaking on a blanking cut the space between the punch and die would be 10% of material thickness. In the case of this thread 12ga or .105” it would be .010” per side. As an example if we were to make a .500 square hole the die would be a.520 square As a second point to put a visual on Frosty’s number of .0001. The average piece of notebook paper is.004 thick. We would need to slice the paper into 40 thinner pieces to get .0001 Even as a professional tool maker I almost never work in that kind of tolerance.
October 14, 20196 yr I have to agree with ForgeDad on this one. We would use 8~12% material thickness per side depending on material type for trim steels and punches/die buttons with tighter clearance around sharp corners. It’s all a trade off on die roll, shear percentage, burr size and tool life. Always exception to the rules... Of course “fine blanking” is another ball game all together. 100% shear requirements, zero clearance between punch and die (which never enter...) everything made to micron tolerances. But I doubt any of us are going to be stamping out precision gears in our shops.
October 14, 20196 yr back to the OP, if you are making hardware, check out Donald Streeter's book, "Professional Smithing". I believe he shows a hand punch or a shear that he used to blank out colonial hardware. The tool works for many things. Also, as an alternative, you might look into a Beverly shear. A B-3 will shear 3/16" mild steel, and is designed to cut curves. Sweet Bev is a fine tool!
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