How is finish polishing on punch dies done?


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    Default How is finish polishing on punch dies done?

    Looking at making some dies for punching aluminum sheet (about 0.020 - 0.040. I understand the milling but these are usually more polished, smooth finish. is the final step done with other tools on a CNC mill or some other way? Form is not a smooth circle which eliminates some simple methods I can think of.

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    Community Moderator Jim Dawson's Avatar
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    Default Re: How is finish polishing on punch dies done?

    Typically tools are finished by grinding after heat treat. Depending on the tool shape, this may be done on a surface grinder. The other option is the old school method of a smooth file and final stoning. And yes, I have done a lot of that

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: How is finish polishing on punch dies done?

    How much is typically left for heat treat distortion and grinding? (I realize this may vary a lot due to shape/size). Doesn't a surface grinder only do flat parts (and maybe convex surfaces)?



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    Default Re: How is finish polishing on punch dies done?

    Quote Originally Posted by Jim27 View Post
    How much is typically left for heat treat distortion and grinding? (I realize this may vary a lot due to shape/size). Doesn't a surface grinder only do flat parts (and maybe convex surfaces)?
    It depends on what steel you use for the tool, D2 tends to swell a bit, A2 it pretty stable and is my favorite due to its ease of machining in the annealed state. I normally leave about 0.002 to 0.005 for grinding.

    It is possible to grind some very complex shapes on a surface grinder, it really depends on how well tooled your surface grinder is and how creative you are in the setup. You can grind anything from cylinders to flat parts. If you have a radius dresser you can dress the wheel for concave, convex, angles, or any combination to create form profiles. A tool & cutter grinder is also a useful tool for some operations, and can be used as a light duty surface grinder.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: How is finish polishing on punch dies done?

    How would you put together a punch die for an 8" square shape? Only the edge needs to me hardened obviously so is it practical to make the edges out of hardened steel in sections and dowel them to a non-hardened plate? That's a pretty big part if done as one piece of A2.

    On the grinding to finished size - I looked up some videos on surface grinders but don't see how I could finish the inside of a punch die (female side). Maybe the male side but still seems very difficult. Is it possible to do the grinding on a cnc mill? I understand the need to cover the ways to keep out the grit. Would be sort of like using a tool post grinder on a lathe I think. Are appropriate grinding tools readily available that I could put in the spindle collet? Speed of the spindle is also an issue - would need a high speed spindle or it would be very slow.



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    Community Moderator Jim Dawson's Avatar
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    Default Re: How is finish polishing on punch dies done?

    It is possible to fasten hardened punch sections to a non-hardened base. I have done this a number of times. Don't rely on dowel pins to carry the load, support the base of the tool. I normally use 4140 or 4150 for the tool holder. 1018 would work for light duty use.

    It's not possible to use a surface grinder for all shapes, cavities would require a different method. It is possible to do some light grinding/polishing in a milling machine. Yes, it would require a high speed spindle, but that's easy to do. Small router motors and air grinders make great high speed spindles for a mill. I have pencil grinder from Harbor Freight that I use for engraving and (very) light milling. Never done any grinding with it, but no reason it wouldn't work. I'm able to put the pencil grinder in a 5/8 tool holder, but you can also build a bracket that clamps on to the main spindle and hang a small spindle off to the side of the main spindle. A number of people had done this with router motors and air grinders.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: How is finish polishing on punch dies done?

    Yes, I have seen routers mounted beside the main spindle. What sort of bits should be used for grinding? What about the wear of the bit? Do I need to compensate for this as it is being used or is the wear little enough it doesn't change very quickly.

    As for the dowel pins I wasn't planning to have them take the sheer load but for location. I was thinking of placing a wide flat bar (say 1/4" x 1") flat on top of a thicker piece of 1018 and dowelled and bolted down. Then have the other die piece come down to sheer the sheet metal.

    Do you think it is feasible to use multiple pieces of A2 around the perimeter of the die? How tightly do I need to fit the joints? Make them slightly oversize and grind them to a tight fit after heat treat?



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    Community Moderator Jim Dawson's Avatar
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    Default Re: How is finish polishing on punch dies done?

    There will be wear and you need to compensate for it. But the wheels wear slower than you might think. The biggest problem is going to be dressing the wheel, a radius dresser is almost mandatory for what you want to do, they do both angles and radii.

    https://www.ebay.com/itm/SPI-Radius-...oAAOSwUMxZ~6b3


    For mounted wheels, take your choice. You'll have to experiment to find the right grit and hardness. Normally use softer wheels for harder materials and vice-versa, but that is just a general guideline. I have successfully used hard wheels on hardened material. And use some kind of coolant, normally water based, helps keep the wheel from loading up. Just be sure to buy wheels that are rated for the spindle speed, it sucks when they explode at 25,000 RPM

    https://www.google.com/search?biw=16....0.3-ZfctQnHa4

    I would grind to get the best fit possible. Any gap would show up in the finished product, but that may be acceptable.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: How is finish polishing on punch dies done?

    That's very helpful. Didn't know where to start on wheels. I read a few places that carbide could be used on hardened steel but that didn't seem right and you confirmed that.

    On dressing it seems like I could just get the point for the dresser and use the CNC to move the wheel rather than the dressing point to get the job done. I am guessing I need to do radius dressing because the stones don't come in small diameters needed to do the corners (haven't figured out the details yet but thinking the minimum radius will be about 1/4")?



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    Default Re: How is finish polishing on punch dies done?

    CNC wheel dressing ! I love it. Hadn't thought of that, and yes it would work just fine.

    You can get wheels in pretty much any size and shape, but the odds of the surface being concentric to the shaft is small. For hand work they are fine, but you need to dress the wheel to do precision machining.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: How is finish polishing on punch dies done?

    Maybe I can get by without the hardened steel and grinding? That would make things a lot easier, especially for a first go at this. I am punching up to about 0.030 soft aluminum sheet and only expect to do several hundred. I could mill the entire thing out of some other steel and if I wear it out I could machine out space for hardened edges. Or I could make the frame and use better steel for the cutting edges but not hardened steel. What would you suggest for moderately hard material this is not too difficult to machine and reasonably available?



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    Default Re: How is finish polishing on punch dies done?

    You might get away with using 1050 steel, but it's still pretty soft in the annealed state. I have used 4140 for temporary dies, for bending and forming it would work OK for awhile, but any shearing is going to require hardened edges.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: How is finish polishing on punch dies done?

    You realize you can build the die in sections too right?

    ".030" soft aluminum"

    Not good enough ... what grade and T#?

    You may get away with 4140PH ... or you may have to use A2 @ 58/60 Rc.



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How is finish polishing on punch dies done?

How is finish polishing on punch dies done?