Shaped sheet metal punch


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    Default Shaped sheet metal punch

    Here is a punch I made on a CNC mill for punching 0.032 aluminum. It is a test case for a bigger punch and has some of the shapes/curves of the full size one. Did the modeling/CAM in Fusion 360. Plates are 3.5" x 4" x 0.5 thick steel - no hardening since the number of hits is low. Clearance is 0.004". Dowel pins for alignment and also for a 1/8" hole. Edges are very smooth with no burr - better than I expected.
    Shaped sheet metal punch-20180101_180443-jpg
    Shaped sheet metal punch-20180101_180458-jpg

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    Community Moderator Jim Dawson's Avatar
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    Default Re: Shaped sheet metal punch

    Good job! Looks like it came out great.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Shaped sheet metal punch

    That does look nice great work ,I remember the punching plates when I worked in Pharmaceuticals for the tablet and capsule strips the guy that did those was very good at his job .



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    Default Re: Shaped sheet metal punch

    It's been much longer than I realized since much progress on this and still planning to do some larger punches and forming dies - about 8" x 8" overall. Have been reading up and just taken some machining classes at a local tech school. Also scored some 1/4" D2 plate. They are about 6" wide so not big enough for the final die in one piece but maybe in sections? A lot of wasted material for an 8" hole otherwise although the cutouts may be useful for other projects. I could get some more material - I am trying to do this on the cheap but the prices wouldn't be a show stopper.

    Thinking the D2 without heat treat should be plenty hard for the small trial run doing about a 1/3 scale version of the tool. I expect to learn some things along the way and don't want to redo the full size one if I can avoid it. Then maybe heat treat the final tools but for about 2000 parts it seems like it won't really be needed. Or I suppose I could mill the D2 to within about 0.005 of finished dimensions, have it heat treated (maybe have it tempered to slightly less than normal hardness), and hard mill the final dimensions. Thoughts?

    I have some 1" thick steel plate of unknown alloy to mount the D2 on. Is it reasonable to do the punch edge in sections? Say about a 2" wide strip. Should the joints be in the corners (mitered 45 degree joint between segments) or slightly away from the corners in a flat section (with joint at right angle to part outline)?

    A drop through punch would be easier to remove parts from - they would just drop out the bottom. But that would require the small holes to be another operation. I am also wondering about distortion of the holes when I draw the part into a cup as the holes will be in the drawn flange forms the sides of the cup. And I will be at near the limit of my press to do the outline so thinking I will do the holes as a separate operation after drawing.

    I will try to post some on my progress as I go along.



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    Community Moderator Jim Dawson's Avatar
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    Default Re: Shaped sheet metal punch

    It has been a long time. For only 2000 parts I would have those laser cut today. Laser technology has come a long way in the last few years, and the pricing is very competitive with stamped parts on short runs. By short runs, I mean < 50K parts. On the other hand I'm lazy and would prefer to open a package of finished parts rather than building the die and running the parts myself. The really nice thing about laser cutting is you can make design changes on the fly. Just provide a new .dxf and instant design change, no die modifications required.

    We have a lot of parts laser cut, it's actually less expensive than trying to do them in-house. We use these guys for some of our production and prototyping https://sendcutsend.com/ and also have a couple of local laser shops do some of our work.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Shaped sheet metal punch

    Hi Jim! Was just reviewing all the good input you provided in the past on this.

    The sheet aluminum has an applied paint coating that I need to retain. It's common colored "coil stock" used in the building trades for flashing, gutters, etc. It is a pretty tough coating that stays adhered during bending and forming. Laser will discolor the edge I think and is not acceptable. I can send them a quote request and see what they come back with. The second operation is the drawing to raise the flange around the edge. This is for my wife's small business - gotta keep the SAF* high. And I suppose my time is considered free. It is an interesting project and I get to buy some tools and tooling so that's a plus...

    *Spouse Approval Factor



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    Community Moderator Jim Dawson's Avatar
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    Default Re: Shaped sheet metal punch

    The SAF may be the most important factor here.

    D2 is pretty soft in the annealed state, but it will work harden if you look at it wrong. It might work without heat treating for your application. You might be able to edge harden it with a torch, but I've never tried that before.

    I would bolt and dowel pin the D2 to the base material, then do all the machining. If I recall correctly you have a CNC mill. Square the mating edges as best you can to minimize any gap. I don't think mitered joints would be helpful and are a bit of a PITA to get good alignment. If you have a surface grinder, that would be the best tool to get a good fit up. If you don't have one, maybe it's time for a new tool.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Shaped sheet metal punch

    Is 4140HT a good plan? If the D2 is hard to work with and needs to be hardened to have decent life the 4140HT looks to be pretty available. Rockwell C about 30 so thinking it would be ok without additional heat treat.

    How to cut the D2 or 4140HT? From what I can find the annealed D2 is about 26 HRC so only a little softer than the 4140HT. If I get 2" wide bars I could just mill it to needed lengths. Could do that on the larger plates too but wondering what the other options are.



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    Community Moderator Jim Dawson's Avatar
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    Default Re: Shaped sheet metal punch

    The 4140HT might be a better choice for this application. I would get a plate(s) that are large enough to make the whole die out of in one piece. Bolt it down to the base and carve out the die. I have never worked with 414HT but at RC30 is should cut fine with carbide tools.

    For order of operation I would stack up all of the pieces and work in layers all in one setup. Drill the alignment holes, bolt holes, and dowel pin holes through everything. Then work your way down through the layers.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Shaped sheet metal punch

    For alignment of the cutting edge layer to the base I was planning to use .25" dowel pins. Perhaps something a bit bigger. To ream the 4140 HT to size I will need a carbide reamer rather than HSS? Have heard they won't take any misalignment but I will be drilling and reaming in the same setup so should have no issue with alignment. For some of the plates I want to be able to flip them over to do the mirror image shape (for punch I can just flip the material but for the drawing I need to reverse the die). So I thinking of reaming the dowel hole about 0.001 oversize for a close slip fit - Since I need to flip the base and working plate I will need to make it oversize in both pieces. I suppose I could ream to exact size for a light press fit. Think you had said countersunk screws could be used for alignment and holding things together so that's an option although with needing to flip things over that complicates the countersink option. I feel dowels would give more precise location but I could be wrong.

    Thinking I may try to bolt it down from the back of the base plates. The base is much thicker so the head depth is much less of an issue.



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    Default Re: Shaped sheet metal punch

    I have been talking to someone locally that does tool and die work and they are telling me I should have fully hardened edges at Rockwell 60 or thereabouts. Seems like more than needed for 2000 pcs of aluminum material and he is doing high volume production dies so obviously for his work they want as long of life as possible. Heat treat and then grinding to get good flatness add quite a bit of processing that I would need to send out and would rather avoid unless really needed. Would be interested to hear others views on this. I located some off cuts of 4140 HT that are big enough to do the parts in one piece at a good price so want to move ahead on that.



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    Default Re: Shaped sheet metal punch

    I like carbide tipped reamers, a little more forgiving than solid carbide reamers. In most cases, I drill and ream in one setup.

    Dowel pins will provide much better alignment than flat head screws. But I do use flat head screws in some applications that are less demanding or for temporary fixturing. Either way has it's use, just depends on the application.

    Normally by the time I start to make chips on a project, I have already ''machined'' and ''assembled'' the pieces several times in my head so I don't have any surprises when I get to that point. A couple of days ago I made a part that was longer than my machine travel, and I had to machine all six sides. I used a single dowel pin in the part center as my zero for the entire project. The dowel pin had no other purpose.

    I agree with the local tool maker, 60RC+ is the proper way to heat treat a die for production. And for a production die I would make the die out of D2 or A2 tool steel. But in production I expect the die to last between 50,000 and 200,000 hits between sharpenings, depending on the part and material. For a low use manual operation, I think you'll be fine with 4140HT

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Shaped sheet metal punch

    Any suggestions for sources of carbide tipped reamers? I am finding limited sizes and sources. I got the HSS ones from MSC but their site doesn't seem to be working today.



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    Default Re: Shaped sheet metal punch

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Shaped sheet metal punch

    Found some 4140HT drops that are big enough to do the parts in single pieces but haven't ordered them yet. Have read a few places that it can be drilled and cut with HSS - is that true? If not wondering how I can cut the blanks out. Say I get a 12 x 24 piece and need to cut for 2 pcs of 12 x 12. I could mill it with a carbide EM but is there an easier way?

    They have some 4140/4142HR HT and 4140/4142HT decarb. So the HR HT will have scale that I need to mill off? Probably better to get the decarb material if that's the case. If not need to get thicker to allow for what gets removed.

    Plan to make the corners 0.100 higher and taper to the middle of the sides to reduce punch force. If I start with 3/8 or 1/2 material thinking I should have plenty of thickness for the taper - 0.275 min when starting with 3/8 - maybe just a bit less if I need to lightly surface the sides to clean them up. Will put the taper on the outside edge to minimize distortion of the inner piece.



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    Default Re: Shaped sheet metal punch

    Do you have a vertical band saw? That's how I would cut a 12x24 down to a 12x12. I have one of those 14'' Chinese band saws that I slowed down to cut metal. Picked it up at an auction for $25. But I guess you could slice it in half with an endmill.

    The mill scale is like carbide, really hard on tools. I normally remove it with a angle grinder before trying to machine the parts. If you can take a cut with an end mill deep enough to get below the scale then you're OK also. I would get the decarb free if it's available.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Shaped sheet metal punch

    Don't have a band saw but do have access to a horizontal band saw. Have found that in thick and/or harder material the horizontal is much better way to go. Wasn't sure the blade would handle RC30 material at least without coolant but from what you say it will. Will go that that. Years ago when involved with homebuilt airplanes with my dad I recall seeing some notes on slowing down band saws to cut metal.



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    Community Moderator Jim Dawson's Avatar
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    Default Re: Shaped sheet metal punch

    I use M42 cobalt bi-metal blades on both of my saws. They'll cut pretty much anything. I buy the blades from Lennox or Starrett, the blades cost more than my saws did. I have several TPI depending on the thickness of the material that I'm cutting, you want to keep at least 3 teeth in contact with the material. My standard go to blade is a 10-14 TPI variable pitch, but I use fewer TPI on thicker material.

    When cutting harder and hardenable material, slow the saw down to the minimum speed, and feed as much as the saw will take. You don't want to let the material work harden by rubbing the tool, this applies to milling also. Around 40 FPM and a good chip load.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Shaped sheet metal punch

    I have found a local company that has a list of drops at what seems like really good prices. Although the list may not be up to date and they don't seem to have any decarb free HT on the list, only DCF annealed. If I was to get HRHT material how much do I need to take off to get through the scale/decarb layer? I can grind the surface with an angle grinder on both sides and them put in the mill and lightly face both sides to get it flat and parallel. Trying to estimate how much time and trouble that is going to be but the cost difference seems significant. I am not clear on the similarity/difference between the decarb layer and the scale. As I understand it the decarb layer is where the material defuses and loses carbon near the surface. Seems like a quite different process than the scale formation which I am thinking is mostly oxidation, again during the heating cycles. Just quoted me 4140 DCF at $130 for 3/8 x 12 x 12" and I can get several times that size in 1/2" in HRHT for less on the drops list (all 4140). I need to see what heat treat would cost and have a recommendation on a local place for that. If I go that route I could mill leaving a bit of excess, then harden, and then final mill. It looks like max hardness for 4140 isn't much more than RC30 or 40. Tool steel is going to run a lot more and would require the heat treat so haven't seriously considered that (yet).

    What they have listed is HRHT. They have some flat bar and some plate. I am guessing the bar may have some rounded edges and not be quite as uniform thickness?



  20. #20
    Community Moderator Jim Dawson's Avatar
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    Default Re: Shaped sheet metal punch

    I'm not exactly sure what the scale chemistry is, but it is like carbide. I normally take at least a 0.020'' depth of cut to clean it up, using a carbide insert face mill. Whatever it takes to get below the scale and into the base material.

    I would probably go for the less expensive material, it's a bit more work to prepare, but it's just a bit more time. I have heat treated 4140 to RC50, but the heat treat shop will make it like you request.

    In most cases the thickness is close to uniform, but it's not always flat across the width.

    Jim Dawson
    Sandy, Oregon, USA


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