what kind of surface finish can one expect punching holes (say 1.5" long by .150 wide) in
1/8" thick 7075 Al ? The application is punching of reed holes in accordion reed plates.
I have seen some that have near-mirror surface finish and am puzzled as to how can it be achieved ?
I have made some dies/punches, with .005 clearance all around. With my hand-operated flywheel press I can get holes in a single punch, but surface finish if rather rough, so I have to file the edges to get the smooth finish I require.
What factors could be affecting it ? In a factory condition, much higher forces and speeds can be obtained, but would it affect the surface finish ? With higher forces, one can reduce die/punch clearance - what kind of effect would that have on surface finish ?
It sounds like you probably don't want a fine-blanking die.
They have (comparitively) high tonnage requirements because of the virtually zero punch-to-die clearance (in the low tenths). Tonnage requirement increases as clearance decreases.
They are fairly complicated and require excellent workmanship/accuracy as there is virtually zero punch-to-die clearance. Slight wear/ deformation over the life of the tool with cause crashes and the punches/dies will be difficult/expensive to replace.
They require a triple-action press (not really common/cheap).
Are you going to build the die yourself? How many parts are you making?
There are other options.
You can run a second punch through as a single-tooth broach (taking off perhpas .001-.002" per side), and you will likely get a machined finish without the workmanship/accuracy requirements of a fine- blanking die. Mind you, now you have to deal with the silly little chips that will be generated by the cutting action. These are a pain in the butt because you'll likely need a lubrication to prevent cold- welding so they'll stick to everything. You can take off more of a bite to yield a thicker slug, but your finish will suffer.
Depending on other requriements, you can burnish the inside of the hole with a punch that has a taper, and then flairs out to your final dimension. This is a forming operation so there is no cutting involved, but you'll likely get a certain amount of bulging around the edges of the hole, and the finish you'll get will likely be dependant on the finish from the original punching operation. This means lubrication, and you'll have to keep the cutting and forming punches well polished to prevent cold-working.
If you're not making many parts, finishing the holes with some type of abrasive may be most cost effective as you can get rolling immidiately.
Before I made my own set of punches/dies (I actually made a few for different reed styles - italian and russian), I read a bit about die making. I started with .002 clearance which I had to expand to .005 to reduce the effort required when punching.
The punches and dies are A2, milled and heat treated by truly yours. To file the edges, I made a set of thin, wide and tapered files - 2" wide tapering to about 1" (and few smaller ones), 1/8" thick (and thinner). Milled the single cut pattern on a mill with rotating head. These sure eat Al for lunch !
I think I got as close to the best recipe for what I need to do, in my conditions, as I possibly can .
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