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  1. #1
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    Default Need advice on machining delrin

    Hi Guys,
    I am used to machining fairly large steel parts but I have never machined plastic (delrin) this big. I need your opinion on how much the part could move after roughing ?. How much stock to leave for finish operations?. And any advice on tooling will be appreciated this is because I need to hold 32 finish.

    The part I have is 30 1/2 long 9 1/4 wide and 9 1/16 tall (screen shot and Isometric view of solid model attached).We couldn't find a rectangular stock, so we got a cylinder that is 12" in diameter and 32 1/2 long. All the tolerances are +-.015.

    I am planning on having the boring bar rough out the rectangular island leaving 1/4 per side. This will allow me to clamp the part in two 8" vises. I would then rough out the 8.66 X 30.51 (top view) along with the step leaving 1/16 all over. I will then flip the part and rough out the other side, leaving 1/32 on all sides. Then go back and finish top side and later finish the bottom side.

    I am a little unsure about the stock to leave for finish. So, Please let me know if I am leaving too much or too little for finish.

    The machine I am doing this job is a Kitamura Bridge 6 with a max RPM of 12K. I plan on finish scalloping the angles and some radii using GibbsCAM. But I am not sure if I can accomplish 32 finish when 1/2 bull nose that sticks out 6.5 inches (your advice will be appreciated).

    Thanks.
    Attached Thumbnails Attached Thumbnails solid-model-delrin-part.jpg   delrin-part.jpg  

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    That's a fun part, you'll enjoy the cleanup afterwards! Send the chips to Ox, he likes snow (inside joke). If you have access to a dust collector trying to collect the chips as they are made is best, just be sure to watch for static buildup, and don't let the hose get caught between the cutters and the part.

    If that's real Delrin, you might find a central "rough zone", which is characteristic of how the material is made. If it's an acetal copolymer, it should be more homogeneous. If your customer is looking for good cosmetics all over, that may matter to them.

    For the machining, I'd leave at least 1/4" for roughing on the top and bottom, the sides (being symmetric ) should be more stable but it won't hurt to leave the 1/4". You can semi-finish to 1/32 before taking a finish cut on the next setups. Use sharp HSS flycutters with a rounded nose for planar surfaces, this is where that type of tool shines. Endmills will side cut well, but make sure they're sharp and have not been used for metal cutting before. I have some massive 1" diameter carbide endmills if they could help you (PM or email me).

    If you really need that sharp transition of the angle to the lower flat surface you might as well fixture the part to cut the angle as a flat (tipping the part twice), but if they OK a transition radius at the intersection then a 1/2" ball EM should work with a fine enough pitch in the coding. If the bridge mill can cut at angles (right head attachments) so much the better. Make sure the spindle is thermally stable before starting the finish cuts if ball EMing.

    There are tricks to getting a nice finish on Delrin after milling, using a single edge razor as a scraper (round the ends a bit with a stone first) to remove a few tenths can make a rougher surface nice.

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    leave 1/4 for semi rough, then do you finish pass.
    delrin cuts extremely well and doesnt move all that much.
    best endmill I have found was garr alumistar 3 flute endmills and the sandvic alum cutting inserts for the face mill. a 32 finish wont me hard at all.
    if you have thin wall stuff, you can take a .005-.010 finish cut with no problem. I typically take no more than .020 finish pass.
    I generally dont go over 6500 rpms, and I dont push the feed as to that tends to warp parts.
    black delrin for some reason which could be just an optical illusion finishes much nicer.
    I use tons of coolant also for 3 reasons keeps part cool when cutting a bunch of stock, better finish and it cleans the oil out of the coolant


    if it were me cutting that part I would rough everything leaving 1/2" - 1/4" stock. take part out of machine lay it on a flat surface over night then finish in the am. this will make sure everything settles. dont tighten your vices to tight as it ill tweak the part.

    one other thing 2 8" vise are going to hold the part rigid enough for your part. I would at a min run Cut to size alum jaws 12" long on each vise, I dont personally think that will be good enough. you may have to make a fixture to hold it as your tol is pretty tight on that big of a part.

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    Fortunately delrin doesn't move much, I tend to leave .020 for finish usually.

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    I have ran some large rings out of Delrin a few times, and the first time I had them pretty much dead nuts when they came off the lathe, but the next morning they were .020 or possibly .040 (?) different in size. I don't recall if they were bigger or smaller.

    These were 20-24" D x 4" thick rings taken from solid.

    So now I round them up the day before and finish the next day or later.

    I added $50 each for cleanup costs!


    -------------------

    Think Snow Eh!
    Ox

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    Gravy train with biscuit wheels. Finish all sides with an aluminum facemill for the perimeter. Use brand new alum spec endmills for finishing the details. If you step down that angle with a bullnose endmill, it'll take small stepdowns for a smooth finish.
    I'd rough everything down to +.100" and finish with reclamping and dialing in the datum. You will Not need coolant. Try and run a debur path in the machine

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    I found the Kolroy "Ripper" mills to work good for plastic. I always used chrome plated inserts for aluminum in all my facemills and indexables to have more "teeth" rather than the 1 when using hss. Roughing one day and then finishing the next is highly recommended, even a couple days won't hurt but delrin isn't as bad as some plastics. Coolant will help reduce the movement amount but I never made as thick a part as this to say for sure and it makes cleanup harder so often best to run dry and letting it sit to "relax". In critical parts ,temperature matters as delron does change some, we had to switch to Turcite for a lot of parts because delrin and celcon moved with warmer temps so bad, cold not as much a problem.

    Delrin makes those nice little flakes,like snow,that is a pain to clean up, I liked to start with a oil and sticky-free(miserable dried coolant) machine so it doesn't stick to everything.

    .

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    Since no one mentioned it... and you usually run large steel parts. Make very sure you clean up the left over Delrin chips when you get done. If you happen to machine steel next, and throw some hot steel chips in a pile of Delrin, you will have a fire that will emit toxic gasses and kill you.

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    Quote Originally Posted by G00 Proto View Post
    Since no one mentioned it... and you usually run large steel parts. Make very sure you clean up the left over Delrin chips when you get done. If you happen to machine steel next, and throw some hot steel chips in a pile of Delrin, you will have a fire that will emit toxic gasses and kill you.
    Ok. Thanks.

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    Quote Originally Posted by Rstewart View Post
    Gravy train with biscuit wheels. Finish all sides with an aluminum facemill for the perimeter. Use brand new alum spec endmills for finishing the details. If you step down that angle with a bullnose endmill, it'll take small stepdowns for a smooth finish.
    I'd rough everything down to +.100" and finish with reclamping and dialing in the datum. You will Not need coolant. Try and run a debur path in the machine
    Do you think a 1/2 bullnose sticking out 6.5 inches out of the holder will leave a good finish?

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    Quote Originally Posted by asraraleem View Post
    Do you think a 1/2 bullnose sticking out 6.5 inches out of the holder will leave a good finish?

    I am not qualified to reply with much/any experience in such apps, but I would think that it would depend on how much is still in the holder and TIR amount.
    Not like tool pressure is much of an issue at finishing values.
    And normally Delrin responds [very] well to machining.


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    Think Snow Eh!
    Ox

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    If cutting dry, route a shop vac near the cutting tool...suck up the chips before they go everywhere. 2.5" Loc-line works great.

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    Quote Originally Posted by Ox View Post
    I am not qualified to reply with much/any experience in such apps, but I would think that it would depend on how much is still in the holder and TIR amount.
    Not like tool pressure is much of an issue at finishing values.
    And normally Delrin responds [very] well to machining.


    -----------------

    Think Snow Eh!
    Ox
    Haven't ordered any tools yet but Ill be sure to order a long enough tool that I can properly hold on to. Goal to have a .0005 TIR or better.Thank you all for your input.Ill keep you guys posted,once the chips start flying .

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    Quote Originally Posted by asraraleem View Post
    Do you think a 1/2 bullnose sticking out 6.5 inches out of the holder will leave a good finish?
    if you have the endmill already try it, you have plenty of stock to make a test run. if you didnt buy one I would say get a 1" or 3/4" dia tool.

    the ballnose will cut fine and give you good finish, if your using it for step overs (using it to surface your angle)you will need to make them very close together then stone the surface or wet sand it to get rid the CUSP

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    Quote Originally Posted by carbonbl View Post
    If cutting dry, route a shop vac near the cutting tool...suck up the chips before they go everywhere. 2.5" Loc-line works great.
    That may be difficult to do. The machine I am working on is enclosed. And besides its only a 2 pc order. Ill bite the bullet and do a cleanup at the end of the job.

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    Quote Originally Posted by Delw View Post
    if you have the endmill already try it, you have plenty of stock to make a test run. if you didnt buy one I would say get a 1" or 3/4" dia tool.

    the ballnose will cut fine and give you good finish, if your using it for step overs (using it to surface your angle)you will need to make them very close together then stone the surface or wet sand it to get rid the CUSP
    Thank you for jogging my memory. Just realized I have a 1 inch carbide insert ball mill. That probably will work for most of the scalloping.

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    Your machine is much larger than any I have, but what I do when cutting plastic (or even wood) is....

    I take an old bed sheet (or even an old fitted sheet), and I cut a hole in the middle that's just enough to sneak it over the vise. I hold the sheet down to the table with magnets all around the vise, and then a combo of magnets and spring clips to hold the sheet part-way up the walls of the enclosure all the way around.

    Usually it takes a few tries to get the sheet attached in a manner that still allows the table travel to make the part without pulling the sheet off the walls (though if you're on a bridge mill, that might be a non-issue, which would be way easier).

    When I'm done with the job, I just carefully remove the sheet around the walls and kinda just roll it all up toward the center. Remove it from being over the vise, and then either shake the whole thing out in the dumpster, or toss the whole mess.

    I still use the shop vac a bit if I'm doing any heavy stock removal, just so the sheet doesn't have to catch as much.

    Your mileage may vary.


    PM

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    The delirin I machine is smaller than yours, but I use a single flute 1/4" carbide endmill with 2.5" stickout. I rough machine everything, pause, then do 0.020" finish pass. Also, you can bead blast delrin then wipe it with Armor All. It leaves a really nice finish.

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    Quote Originally Posted by asraraleem View Post
    Thank you for jogging my memory. Just realized I have a 1 inch carbide insert ball mill. That probably will work for most of the scalloping.
    If you go the insert route get the alum high positive inserts( I used my 3" face mill and r390 sandvick cutters and they work fine with the alum cutting inserts). with regular inserts you will have to take less stock and beware of size as it tends to compress the delrin a tad. so you maybe have to let it sit for a while to spring back( why I suggest roughing the sit over night out of vises unrestrained
    I get a nice finish with both the ones made for alum and generic ones on delrin.

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    We do more acetal copolymer that homopolymer (Delrin) but we have seen significant size swings with humidity or temperature in the shop; I didn't nail it down but we had some standard (+/- 0.005") tolerence stuff that was hard to keep in range over a few days. We were actually running one of those parts unattended on a lathe when I left the shop last night.

    We get great surface finishes with aluminum inserts in some Seco inserted cutters we use (very much like an AKPT style 90* insert).


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