Milling Polyurethane 80 Shore D
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  1. #1
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    Default Milling Polyurethane 80 Shore D

    I have the opportunity to quote a small job which consists of (5) rectangular blocks of Black Polyurethane, 80 Shore D. The blocks are approx 2 x 2.5 x 3" with flat bottom pockets that have .50" radius. The pockets are 1.12 to 1.50" deep depending on the detail drawing. The tolerances are all plus/minus .010". Each block will also have two 3/8-16 tapped blind holes 3/4" deep. I have never machined polyurethane before. I would be using a manual Bridgeport mill with max spindle speed of 4200 rpm. Customer is supplying the material, so if I scrap any, it will be coming out of my pocket. And the customer doesn't usually supply any extra material for "practice". Any tips for cutting speeds, feeds, tooling be that high speed steel or carbide? Is liquid coolant necessary? I usually use a compressed air cooling gun for cooling. Am I going to be able to clamp this material in my milling vise? Since the radii in the pocket corners is 0.50", I would plan on using a 1" diameter end mill. I usually mill mild steel, brass, aluminum, and hard plastics. Appreciate any timely advice, as the quote is due in three days.

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    Nevermind, I was thinking Shore A. No experience machining harder than 85A.

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    How would 85A polyurethane compare with 80D?

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    Quote Originally Posted by Nevetslleksah View Post
    How would 85A polyurethane compare with 80D?
    Shore 85A is roughly equivalent to Shore 40D, so very much softer than Shore 80D. However, both are "hard". A hard rubber caster tire might be 85A, while a hard hat or other rigid plastic item might be 80D.

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    I think you have your work cut out for you! I, too, have only worked with 70-90 A material (on a lathe) and it is not much fun. I rough with carbide and finish with files and grinding. The stuff makes a mess and I'd never try to hit .010 tolerance. Maybe the 80D will be easier, but I think I'd rather make a mould and cast a part like that.

    I suspect you'll find that it pushes away from the cutter, creates long thin 'burrs' and tears the surface. I sincerely hope I'm wrong, and probably am since I've no experience with that particular material.

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    Default The harder the better!

    Never machined any that hard, but it's bound to be better than UHMW.

    durometer_with_logo_small_580.jpg

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    Quote Originally Posted by MrWhoopee View Post
    Never machined any that hard, but it's bound to be better than UHMW.

    durometer_with_logo_small_580.jpg
    UHMW is easy with the right tooling. It's internal stress relief that's given me issues on occasion - cuts fine, not quite the final shape I wanted...

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    Run! If you don't have practice cutting this stuff, quote it very high.You will not get the job. Quote it normal, you will never make money on it. Either way, you lose.

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    Quote Originally Posted by MrWhoopee View Post
    Never machined any that hard, but it's bound to be better than UHMW.

    durometer_with_logo_small_580.jpg
    Cool chart, thanks and

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    That is a great chart. And looking it over I take back what I said in post #5. It might not be bad at all. Sharp cutter and coolant may be quite workable.

    Nevetslleksah, if you do find something that works well, I'd appreciate it if you'd let us know. Thanks

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    I have turned a poly wheel from a pallet jack on a lathe. Long strings of the crap, but it cut ok with sharp hss tooling, high rake.

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    I would not squeeze it in a vise. I would capture it in a close fitting plywood box including a windowed screw-on lid.
    additionally, you might freeze it and work it as cold as possible.
    additionally, I would plunge / nibble it to depth. just set the quill end stop and plunge, move over, plunge, etc...

    /// just ideas. not much experience.

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    Quote Originally Posted by dsergison View Post
    I would not squeeze it in a vise. I would capture it in a close fitting plywood box including a windowed screw-on lid.
    additionally, you might freeze it and work it as cold as possible.
    additionally, I would plunge / nibble it to depth. just set the quill end stop and plunge, move over, plunge, etc...

    /// just ideas. not much experience.
    Freezing works better on Shore A materials, not shore D. I'd be worried about dropping below the glass transition temp on the harder Urethane if frozen. Then you'd have a brittle material on your hands, which is not recommendable for machining. Not sure what your stuff is, but based on what I know a lot of shore A polyurethanes have tg -30°C to -40°C or so, shore D stuff is only -10°C (teens°F) so a standard freezer can definitely get below that.

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    80D is extremely hard for a plastic, I don't think I have ever machined any plastics that hard. Acetals and nylons are in the 65-70D range if memory serves, polyethylenes are softer. 70 to 80D is a big jump in hardness.

    Can you get a sample to play with to see what you think before quoting? I would think sharp tools with high rake would work fine, ones designed for plastic and that are brand new. The biggest issue may be tapping it.

    I have never found freezing to work, the part heat back up in a minute once you start to clamp and cut it, and this is hard enough that it wouldn't make a difference, IMO.

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    We surface and centerless grind 85d polyurethane often. Holds dimension very well. Machining.....never had any luck. Better off making a mold and pouring it....which is what we do.

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    I get the sense that this might be a good material to use "dubbed" (small neutral rake flat) cutting edges on HSS tooling for satisfactory machining.

    Without trying a sample myself this is just a guess, but it would probably be my first approach at it. Be sure to get as sharp a perpendicular edge as possible, you want to scrape with as little force as you can, while minimizing heat buildup. Just a small flat is needed, I'd try ~.010-.015" to start.

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    A newbie at the material tasked with .3mm tolerances on a 30mm deep pocket in hard PU using a manual Bridgeport mill??!!

    RUN!!!

    General advice only, I'm a bloody noob too: as you probably know getting tolerances in that range out of plastic is a nightmare. Leave the material overnight, measure it again, it might and probably will have changed. Not even getting extra material is completely unreasonable.

    Twist drills, sharp HSS end mills with two flutes. To get a good thread you may need to run it either by pecking or run it twice or even three times. As much coolant as you can get to combat heat expansion. If you want the pockets to actually keep the shape it may be worth thinking about creating metal forms to plug into them after machining since the material is elastic, the same goes with holes in general... but "a small job"... sigh.

    Echoing the above this is a money losing job and should be molded.

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    I did order a chunk of 75D polyurethane from McMaster to practice on. They don’t sell 80D. Hopefully it will arrive in time for me to practice milling prior to quote due date. Otherwise I will just “no quote”. But according to the chart above, 80D may be as hard as a hard hat and not that bad to mill at all.

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    Default Results of mt trial milling on the McMaster 75 D Polyurethane

    Quote Originally Posted by Nevetslleksah View Post
    I did order a chunk of 75D polyurethane from McMaster to practice on. They don’t sell 80D. Hopefully it will arrive in time for me to practice milling prior to quote due date. Otherwise I will just “no quote”. But according to the chart above, 80D may be as hard as a hard hat and not that bad to mill at all.
    So for those of you with an interest. My piece of 75D polyurethane 3" x 3" x 6" arrived from McMaster today, McMaster.com part number 8631K85. Per the Shore Hardness Scales chart supplied above by one of the responders, it is very hard like the plastic a hard hat is made out of. It seems slightly harder than the UHMW and HDPE plastic that I have machined many times with no issues in the past. I was able to grip it in my Kurt milling vise with no issue. I milled it with a 2 inch diameter 3 insert carbide face mill at a cutting speed of 340 fpm and approx 6 ipm feed and got a fine looking surface finish. It deburrs well using a Shaviv deburring tool. I hogged out a pocket using a HSS Forstner bit at 80 fpm cuting speed, drills fine. Then used a brand new two flute HSS end mill to mill out the pocket. No problem holding the required +/- .010" tolerance on the print dimensions. It also files nicely using a smooth mill file. I also tapped two 3/8-16 holes with no issues. So I will go ahead and quote these details for my customer as I anticipate no issues making the parts should I get awarded the Purchase Order.

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    Quote Originally Posted by Nevetslleksah View Post
    So for those of you with an interest. My piece of 75D polyurethane 3" x 3" x 6" arrived from McMaster today, McMaster.com part number 8631K85. Per the Shore Hardness Scales chart supplied above by one of the responders, it is very hard like the plastic a hard hat is made out of. It seems slightly harder than the UHMW and HDPE plastic that I have machined many times with no issues in the past. I was able to grip it in my Kurt milling vise with no issue. I milled it with a 2 inch diameter 3 insert carbide face mill at a cutting speed of 340 fpm and approx 6 ipm feed and got a fine looking surface finish. It deburrs well using a Shaviv deburring tool. I hogged out a pocket using a HSS Forstner bit at 80 fpm cuting speed, drills fine. Then used a brand new two flute HSS end mill to mill out the pocket. No problem holding the required +/- .010" tolerance on the print dimensions. It also files nicely using a smooth mill file. I also tapped two 3/8-16 holes with no issues. So I will go ahead and quote these details for my customer as I anticipate no issues making the parts should I get awarded the Purchase Order.
    Here are some photos of my trial machining on the chunk of Shore 75D Polyurethane
    Attached Thumbnails Attached Thumbnails face_milling.jpg   forstner_bit_drilling.jpg   pocket_milled.jpg   pocket_chamfered.jpg   tapped_holes.jpg  



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