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Thread: Soft plastic milling help needed (video to demonstrate the problem)

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    Default Soft plastic milling help needed (video to demonstrate the problem)

    Seems like the nature of the material. Try cutting something a little different like acrylic or PVC and see if you still get the same issue.

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    Default Soft plastic milling help needed (video to demonstrate the problem)

    I am new to milling and just an amateur, so please excuse my perhaps basic question. I am currently building a CNC of my own design, and the initial tests seems to be promising. I have done some CD writing and that seems fine. However, yesterday I thought I'll test milling some soft plastic and run a surfacing program but I am not very happy with the results. I made a short video, please have a look at it and tell me what you think, why the plastic is not cut, more like sliced.

    Am I having too high, or too slow feed rate? I had it set to 220mm/min (8in/min) and also tried with 180mm/min (7in/min) but it made no difference.
    Am I having too little overlap? I am using a 2mm end mill and the overlap is 0.5mm, in other words, I move the mill 1.5mm at each turn around.
    Am I having too high or too slow RMP on the spindle motor? I run it with 36V and that supposed to give me about 9000-10000 RPM. Have not measured it, so that's just a guess from the motor performance.

    Could any of you give me an advice about why the cuts look like they do in the video?



    I made some hand wheels (also can be seen in the video) out of the same material before, and those were made using a bench drill machine with a circular cutter in it, and even those cuts look ugly. I really don't know why.

    Thanks.



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    Default Re: Soft plastic milling help needed (video to demonstrate the problem)

    First ensure that you have a very sharp endmill, endmills used for aluminum might work but best to get specialized plastic endmills.

    I would try increasing the feed rate, also use a smaller stepover.
    I cant tell form the video if you are using a ball or flat mill ?

    If the endmill is a ball you need a much smaller stepover I would start at 10-20%



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    Default Re: Soft plastic milling help needed (video to demonstrate the problem)

    Thank you very much. So, you think that the feed rate is actually too low? I would have guessed the opposite, but that maybe shows my inexperience. The end mill was never used before for anything, it is a flat type. The decrease of stepover to 50-60% crossed my mind as well, but not tested. I will try both of your advises to see if it helps.



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    Default Re: Soft plastic milling help needed (video to demonstrate the problem)

    Definitely decrease the stepover and increase feed rate. Failing that, get a bigger end mill ! I cut acrylic at 2000mm/min 16,000rpm 0.5mm DOC with a 3mm single flute



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    Default Re: Soft plastic milling help needed (video to demonstrate the problem)

    Ya, that feed rate is way to slow for 10krpm. We cut a lot of soft plastic , the guys usually run a fairly slow rpm to.



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    Default Re: Soft plastic milling help needed (video to demonstrate the problem)

    Take a look at the Onsrud web page here: Chiploads | Feeds and Speeds | Metal | Plastic | Composite | | Documents

    Like the others said, your feed rate is probably way to slow and you need a sharp mill. For high speed router type spindles I really like the Onsrud single flute cutters. They are not cheap, but they work great and on some machines single flute is the only way you can get the feed rate you need.



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    Default Re: Soft plastic milling help needed (video to demonstrate the problem)

    Onsrud is great, but Chinese single flute eBay bits would work as well. Not as long lasting and onsrud has (too) many types to dial in great cuts for production, but 20 bucks for 10 is less than a single onsrud will cost and makes the pain of breakage much less!



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    Default Re: Soft plastic milling help needed (video to demonstrate the problem)

    OK, thank you all.

    Anyway, with my lead screw I can't get feed rates much higher than I have. Remember that this is a small machine, so 2000mm/min is not an option. The cuts I done in the video are made with a 4 flute end mill. I will get a single flute, increase the overlap and reduce the rpm next time.



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    Default Re: Soft plastic milling help needed (video to demonstrate the problem)

    Also, if you can use coolant that will help too, especially good if the ideal rpm has too low of torque.



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    Default Re: Soft plastic milling help needed (video to demonstrate the problem)

    hi,
    Forget about coolant, not necessary for plastic.
    I'm assuming you are cutting HDPE. it will be really hard to get a perfect finish. you should however get chips, not shavings. definetly switch to single flute, use 2 flutes only for bits over 5/8.
    I cut only plastics at my place. I've tried MANY onsrud cutters, upcutting, downcutting, from 1 to 4 flutes: I was never able to get a finish free of chatter marks, the way I can get it with styrene, ABS or acrylic. I use an 5'x12' AXYZ router, maybe it's over the machine's limits.
    but if you are getting these fines, the filmy stretched cuts, you need to increase the chip load. a good sharp knife will allow you to slow down the motor considerably without harm.
    BTW, drilling is a different story: you will often get these bird's nests wrapped around the bit as you plunge. this will often ruin the finish of the rest of the cut and prevent chips to properly evacuate. either clean it by hand after the plunge, ramp the cut sideways, have the bit retract often during drillng (peck drilling), slow down the plunge rate A LOT, or - for plunging operations alone- onsrud 70-500 plastic drill bits are beautiful. basically they are simple drill bits ground to a very sharp point (maybe like a 60degree?). these will give you some really satisfying curls...

    good luck!



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    Default Re: Soft plastic milling help needed (video to demonstrate the problem)

    Quote Originally Posted by studioso View Post
    hi,
    Forget about coolant, not necessary for plastic.
    I'm assuming you are cutting HDPE. it will be really hard to get a perfect finish. you should however get chips, not shavings. definetly switch to single flute, use 2 flutes only for bits over 5/8.
    I cut only plastics at my place. I've tried MANY onsrud cutters, upcutting, downcutting, from 1 to 4 flutes: I was never able to get a finish free of chatter marks, the way I can get it with styrene, ABS or acrylic. I use an 5'x12' AXYZ router, maybe it's over the machine's limits.
    but if you are getting these fines, the filmy stretched cuts, you need to increase the chip load. a good sharp knife will allow you to slow down the motor considerably without harm.
    BTW, drilling is a different story: you will often get these bird's nests wrapped around the bit as you plunge. this will often ruin the finish of the rest of the cut and prevent chips to properly evacuate. either clean it by hand after the plunge, ramp the cut sideways, have the bit retract often during drillng (peck drilling), slow down the plunge rate A LOT, or - for plunging operations alone- onsrud 70-500 plastic drill bits are beautiful. basically they are simple drill bits ground to a very sharp point (maybe like a 60degree?). these will give you some really satisfying curls...

    good luck!
    Thank you for the many, and very good advises. Regarding coolant, I am not sure how gfacer meant by that, I have never heard about using coolant on plastic but I am a newbie so I am willing to learn. Anyway, I have no plans in using any fluids but perhaps air at some stage. I am planning to build in a small fan and directly concentrate the air on the tool, similar to using compressed air, just not that compressed. I don't know if that would help or would make a difference, but I will try. I have worked on some acrylic and since it melts so easy, the results were not very good, so that will definitely need some cooling.

    Regarding increasing the chip load, you mean I should slow down the spindle motor? Since my feed rate is already at around the maximum, I can not increase chip load by increasing the feed rate, so my only option is to decrease the spindle speed. Currently running at 9000 rpm, perhaps I will try to decrease gradually and see what happens. I am not expecting a perfectly smooth surface, free of chatter marks, for me that is OK if there are some chatter marks, that gives a sort of nice, machined look, which I actually like.

    Another thing I learned is what nobody here mentioned yet, is the milling directions. My spindle rotates clockwise, and I read on this page that milling direction using a CNC should also be clockwise.

    "Unless specifically recommended by the tool manufacturer for the material being milled, always use climb milling on a CNC. Climb milling produces far less cutting pressure and heat, leaves a better surface finish, and results in longer tool life."

    This maybe explains why the cuts are cleaner when moving towards positive, increasing Y direction compared to negative movement, which produces the shaving in my video. Basically, if I would have run the same surfacing program and moved from the outside towards the center along the outer edges. This would result in less heat, the cuts would have been cleaner and there would be no "shavings", or considerably less, with better surface as result. I will try that as well.

    So my summary is:

    1. Use single, or maximum dual flute cutters.
    2. Increase feed rate or decrease spindle rpm to get higher chip load.
    3. Cool, using air.
    4. Cut from the outside, moving towards the center in climb milling.

    I will try this next time and see if the results will be better, but I am quite sure that there will be a noticeable difference.

    Since I started this thread I have also done some cutting on much harder plastic and that turned out really nice, but I guess even that can be improved with the above four points. Apart from that, I did a whole lot of engraving and that went well also, except in acrylic, which I think needs some cooling, or many delays to let the tool cool down before the next plunge.



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    Default Re: Soft plastic milling help needed (video to demonstrate the problem)

    If you are melting plastic it is from too low a chip load and no amount of cooling air will fix that. Check the Onsrud web site for feed and speed recommendations for their cutters for plastic. Even if you are using a different brand this will get you close. Using bits specifically made for plastic (and there are bits made for hard plastic like acrylic and soft like ABS) will also help a lot.



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Soft plastic milling help needed (video to demonstrate the problem)

Soft plastic milling help needed (video to demonstrate the problem)