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W9RAN
04-07-2010, 01:00 PM
A buddy is looking for an inexpensive way to make round slugs out of 3/8" thick Ultra high molecular weight polyethylene (UHMW). He needs two different sizes, 1.465" and 1.100" diameter (no holes, just a blank slug) - and the quantities would be in the range of 2,500 to 5,000 a year.

He's thinking that a punch die in a hydraulic press would be the way to go. Seems to me he'd still need custom made hardened steel tooling for these non-standard size blanks. He says a couple of toolmakers he's approached have blown him off, so I offered to post the question here. He's trying to get a home-based business going so cost is a major concern.

I've wondered if he could get round stock turned to the right diameter, then slice off 3/8" pieces with a chop saw. But the kerf waste would be a factor.

I've suggested laser cutting in the past, but don't know how good this material is for lasering, would appreciate any suggestion on tooling sources or other ideas welcome!

Thanks, Bob

ADGO_Racing
04-07-2010, 01:57 PM
I think it may tend to "extrude", rather than "shear". It would be an ideal candidate for water jet cutting. They have little waste, just buy the UHMW by the sheet and have it done. Water jet will leave very little burr to deal with. As most conventional cutting processes will leave a rather furry burr, that will need to be dealt with.

Machinist-Guide
04-07-2010, 02:04 PM
W9RAN I am a tool & die maker and may be able to help. I am on my way to work at this time so look for my post tonight when I get home from work
Donnie

form_change
04-07-2010, 05:40 PM
I second ADGO's view. At slow press speeds PE will extrude more than shear and a hydraulic press is slow. The other thing with punching is going to be the edge - soft materials in particular distort during the process and so the edge is not going to be square or straight (or probably clean either) - that is, the diameter in the middle of the punched part is likely to be smaller than at the top and bottom surfaces. It depends on what the discs are being used for but tolerances may come into play.
Michael

psomero
04-07-2010, 06:18 PM
can uhmw be cast or injection molded? it's a thermoplastic, so i'd assume so?

that way, you'd be able to minimize your waste

Lew Hartswick
04-07-2010, 07:16 PM
I've wondered if he could get round stock turned to the right diameter, then slice off 3/8" pieces with a chop saw. But the kerf waste would be a factor.



If you lay out a bunch of circles on a flat sheet the waste is going to be
almost the same as the kerf between two peices cut from a rod.
( It seems to me ) :-)
...lew...

chrsbrbnk
04-07-2010, 09:19 PM
I have made a similar item using acetal. I used a power miter box and a 80 carbide tooth blade and a stop. Use slow feed rate Use a thin kerf blade to avoid as much waste as possible.
The edge quality is the big determinate in die cutting. if a rolled edge is un acceptable your going to have to saw it. A 4140 die plate at about 40 rockwell C and an A2 punch 62 rockwell peen and shear the die to punch size and position so it will be a zero clearance die stroke press as fast as possible

snowman
04-07-2010, 09:34 PM
For that annual quantity, I'd do it on a pin router. The amount of scrap is negligible compared to the tooling cost...meaning, you aren't going to recover the cost of scrap by using a more efficient (and expensive) method like injection moulding or punching. You can make a pin router out of about $15 of wood from home depot, any basic router and some fasteners.

J Tiers
04-07-2010, 09:53 PM
Maybe,....

Pin router would be a decent idea for a regular not-too fancy piece...... particularly since the tooling is expensive, and to get a decent edge and sized part you'd probably need to rough blank and then trim the part.... two operations, two costs for tooling (might share a punch and use different dies, I suppose) and twice the labor time.

With the pin router you could likely do two or even 3 at a time by stacking the sheets.

Problem with pin router is with sizing.... not really accurate, except by woodworking standards.

You showed 3 place decimals, implying low thous accuracy.... maybe to 5 thou... Ain't gonna happen with the pin router, you need that much clearance for the pin in the template, easy.

Does the 3 place sizing you showed mean what it says?

He could turn the rods, and then gang saw them off in a horizontal mill, for instance.... cutting as many as a dozen at a time even

snowman
04-07-2010, 11:27 PM
Good point. In that case, I'd do it with a circular saw and a homemade rotary arbor. The part is clamped between two "pads" that rotate. The circular saw can be moved perpendicular to a radii the distance required for the diameter required. Saw is slowly plunged down as part is turning at slow rate.

Or hell, just pin route the blanks and clamp between live center pad and faceplate pad and pop the blanks out by the 100.

Machinist-Guide
04-08-2010, 01:41 AM
Sorry I took so long to respond.
I am not here to tell you how to do this job but I would like to help you understand what you may encounter by piercing so you can make your desecion as how you would like to do it.
First thing as mentioned in an above post you will not be able to get a squire edge on your slug (blank) if you pierce. Your edge will be concave.
Next you will need to develop your punch and button size. When piercing metal the size of your slug will be the same as the button. Not so with rubber. Rubber will compress before it shears. After it shears it will expand back making it larger then the button. If you decide to pierce I would sugest you use a cheaper CRS or HRS to experiment with different button and punch sizes until you get it right then make them from tool steel.
You will also need to experiment with the clearance between your punch and button. This clearance will efect curvature of the concave edge. the finish and size of the burr.
Last but defiantly not least you will need some type of die set to mount the punch and button because the ram on a hyd. press it not precision enough to guide the punch into the button.
I am extrapolating here but I think you would need less then .010 clearance on your punch which is a lot less then you would need on metal .375 thick
Sorry for getting long winded here just wanted to let ya know that not all toolmakers will blow you off. Most of us are very willing to help when we can.

doctor demo
04-08-2010, 01:55 AM
Machinist Guide, if He was to make the punch with a slightly concave end and then some serrations to give it shear would it produce a better slug in Your opinion?
Does uhmv punch like rubber?

Steve

Machinist-Guide
04-08-2010, 03:09 AM
Machinist Guide, if He was to make the punch with a slightly concave end and then some serrations to give it shear would it produce a better slug in Your opinion?
Does uhmv punch like rubber?

Steve

I have never tried a concave punch but I can see where you may be coming from
If the punch point is concave it may help prevent the material from compressing so much

Does uhmv punch like rubber?
I think its much harder to achieve the desired dimension with uhmv then it is rubber. Because of the elasticity I have had to in the past use a .600 punch to get a .500 hole. As the punch applies pressure to the material the material flows away from the punch when the punch is stripped from the material the material flows back toward the hole causing a under size hole

914Wilhelm
04-08-2010, 03:33 AM
How about turning a custom sized hole saw on the lathe and then using this hole saw to cut the pieces out of sheet goods? This would avoid punching and the associated distortion but admittedly might leave a burred edge.

chrsbrbnk
04-08-2010, 09:08 AM
the scrap rate for taking the part from flat stock will be huge punching makes it even worse the closer it get to the last part the more stripper pressure you'll need. the hole saw is the best idea for coming from the flat for the volume

concave punch hasn't helped me on the ultra high density but zero clearance high press speed and making it a compound die might help keep things flat

J Tiers
04-08-2010, 09:18 AM
If he needs the size, and wants to punch, seems that a trimming die, or possibly even two, would be needed, because teh plastic won't finish well on the blanking operation.

I'd think it was a lost cause to try for anything but a "roughing" operation out of flat sheet, with the blank well oversized.

Then trimming down in probably two steps... added die sets......

Turning to size and gang sawing looks pretty good to me for any sort of reliable sizing.

But I think we do not have any sensible response on the actual precision and finish needed.

snowman
04-08-2010, 09:46 AM
But I think we do not have any sensible response on the actual precision and finish needed.

Actually, my post about rough sawing a blank (or pin routing) then putting it on the lathe held by pressure between pads works quite well. I've done it in the past and easily held +/-0.003 with sharp tools.

I'm not a big fan of sawing from diameter, just because the rods are so much more expensive than the sheet. I used to make a line of small parts out of HDPE and found that the easiest way was to cut them out on my CNC router, but that's besides the point, as that has high overhead (and it still won't hold the tolerance)

JoeCB
04-08-2010, 11:23 AM
I kinda' skimmed thru the thread.. did anyone suggest using a plain old hole punch like you use to punch leather? I think that the knife edge of a hole punch aginst a soft backup block ( end grain wood) would not cause the distortion that a punch and die would. A little soap lub and an arbor press should knock these out pretty quickly.

Joe B

RKW
04-08-2010, 12:32 PM
Sounds like that would work for thinner material but 3/8" might be a bit thick. I'm not sure any standard punch would accommodate a slug that thick anyway. Punches usually deform the waste slug a bit too, at least from what I have seen.


I kinda' skimmed thru the thread.. did anyone suggest using a plain old hole punch like you use to punch leather? I think that the knife edge of a hole punch aginst a soft backup block ( end grain wood) would not cause the distortion that a punch and die would. A little soap lub and an arbor press should knock these out pretty quickly.

Joe B

W9RAN
04-08-2010, 12:34 PM
Thanks for your great ideas, guys. Turns out my buddy may be rethinking his choice of materials, so this ought to give him some options to consider for now. I hadn't thought of the pin router idea, which would definitely be worth pursuing if it were me. Thanks again!

ckelloug
04-08-2010, 01:07 PM
The last manufacturer I checked UHMW won't injection mold: The long molecular chains are damaged at those temperatures and degrade into a lower molecular weight polyethlyene which may not perform in the way desired.

I chucked up a really rough square piece of UHMW in my lathe and cut it with HSS tools a couple months ago when I needed to replace a cracked bushing in a window regulator. I can't say what tolerance I held because it was a non critical part but it certainly did turn reasonably well.

--Cameron

ADGO_Racing
04-08-2010, 01:26 PM
All of this requires a good bit of trial and error, and a substantial investment in time and materials. Water jet works fine. Yes it is out sourced, but it can be done in a few days, if the parts are properly nested the waste will be minimal. There will be little if any burr to worry about. If the guy is trying to get a business started, I would think it a major waste of time to fool around with all of this hard tooling, as a business owner you have far too much to worry about besides playing around with a rather simple problem. If he was just fooling around in the home shop out of his curiosity that would be different. Plus, using water jet all of the pieces will be the same, round and definitely well within the +/- .005 range

Go out to www.mfg.com post a drawing, you can request a quote for multiple quantities. (500, 1000, 1500, etc Whatever quantity) have the supplier supply the material. You will have a hundred competitive quotes within a week, and you can see the suppliers past track record before you buy.