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Thread: Are Perfect Circles Achievable? Any tips on cutting Polypropylene

  1. #1
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    Default Are Perfect Circles Achievable? Any tips on cutting Polypropylene

    Does anyone have experience with cutting perfect small circles? I'm not having much luck, the quality is not impressive at all. I must be doing something wrong. I'm including some pictures that show the results. I have a 2009 PRSstandard 48x96 w/a 4HP spindle.

    I used a profile cut with a 1/2" 48-072 Onsrud bit with a 16k spindle speed, a feed rate of 102 ipm and plunge rate of 18.

    Another problem I encountered with the poly is melting. I was trying to drill some 1/4" holes through 1/2" sheet of poly, the holes were not very good quality mostly due to the melting. Unfortunately I don't have a Bot-drill only a spindle for this. Also the poly tends to attach to the bit and it gets very bad after maybe 4-5 holes drilled, then the poly on the bit starts to score the material.

    Do you think I will have the same issues with melting on type1 PVC?


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    43739.jpg

  2. #2
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    I never thought bits would make that much of a difference until I called Onsrud for application help. I cut a lot of plastic, and their O-Flute bits are AMAZING for plastics. They cut like butter. Go buy some immediately and enter my world of amazement! Slow spindle speed and medium speed make plastic effortless. I don't even want to cut wood anymore. Okay, enough ranting. But yes, go get yourself some O-Flutes...

  3. #3
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    Zeke,
    You are most likely cutting too fast (guessing you are cutting something like a .875" diameter pocket for the quarters?)

    1st try adjusting your VR settings - specifically your XY Move Ramp Speed. Change this to .2 - this sets the slowest speed that your tool will move. Then pull down your 102 ipm speed (102 IPM = 1.7 IPS) to .2 to .4 IPS (12 to 24 IPM) and see how the quality looks. If you get melting during cutting, you might want to switch to an Onsrud 63-series tool. If the bottom of the pocket's finish suffers, they have an O-flute that is made specifically for a clean bottom finish. Straight fluted tools generate too much heat in some plastics, causing gummy edges and rewelding of chips.

    In regards to drilling plastics, the best results are acheived using an inside profile strategy (circle vectors) using a smaller tool than the hole itself. A .125 or .1875" tool would be fine. In some cases, cutting the holes undersized (say .22" in diameter) and then coming back with a .25" tool to drill precisely should give you the best of both worlds.

    -B

  4. #4
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    Zeke...
    The geometry of that bit will not yield good results as it is designed for composites and wood. The attack angle of the carbide face chips the surface, rather than shears it off. I also think that you should use a smaller diameter bit to allow the tool to move faster.

    Both Acrylic and PVC solid are difficult but possible to cut with a little experimentation. I would use an Onsrud or Belin 3/16" single O flute at somewhere around 45ipm and 8-12K rpm. 1/8" diameter if the material is less than 1/4" thick. These bits do not SOUND good when cutting well so watch the cut edge for your desired results while you increase/descrease the move speed/RPM. It seems each brand of material has its own sweet spot, sorry I can be more specific.

    You may want to cut these with a .010-.015 final pass allowance for the best results. I sometimes remove the slug between passes to make sure there are no extra pieces of plastic to heat up when cutting the final pass. An air blower helps remove chips and cool the bit to help eliminate remelting of the chips.

    EDIT: Or what Brady says!
    Gary

  5. #5
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    Are these same bits recommended for any plastic including type1 PVC? Thanks gents, I'll work on your recommendations.

  6. #6
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    Zeke...
    I use them for both. The PVC seems more dense than acrylic, but like the acrylic it likes to melt. I cut the piece shown below from type 1 gray with a 1/4" O flute, including the reinforcement rings using speeds similar the above.

    Brady makes a good point about the VR settings. I set mine according to his article a while back and forgot to mention what a difference that it makes.

    43757.jpg
    Gary

  7. #7
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    Thanks Gary! An intriguing piece of work!

  8. #8
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    I talked to Onsrud. Gary, was that a 1/4" Onsrud 63-700 (hard plastics) or 63-750 (soft plastics) series O flute that Brady was referring to?

  9. #9
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    Onsrud 63-700 Series - Hard plastic and solid surface plastics

    Provides a smooth finish in hard plastics with upward chip removal. Helix angle=21°

    Usage: Acrylic, nylon, PVC, polycarbonate, and solid surface
    _________

    Onsrud 63-750 Series - Soft plastic, hard plastic, and solid surface.

    Gives a smooth finish in soft plastics with upward chip removal. Helix angle=30°

    Usage: HDPE, HIPS, UHMW, ABS, polycarbonate, PE, polystryrene, polypropylene, acetal, acrylic, PET, and solid surface
    _________

    Either of these tools will work fine for what you are cutting, with the -750 Series being a little more versatile. It appears that the -750 Series seems to do everything the -700 one does, but this starts to enter into the nitty-gritty of fine tuning after you have dialed your tool in for a given material. Note that the important thing here is the O-flute geometry. It is designed to cut away quickly, with the helix shoulder of the tool falling away quickly, to reduce friction, heat and melting. The helix angle is not as important, although this depends on the material. Bear in mind that whichever one you chose, it will be a night & day difference compared to what you are using now. They last a very, very long time if you only cut plastic with it.

    -B

  10. #10
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    Excellent, thanks Brady, I'll be ordering tonight. I was also thinking about purchasing some drill bits from Onsrud after talking with them. They said at a low speed these drill bits can be used to produce good quality drill size holes without melting the plastics. I would need to run my HSD 4HP spindle at 1-2k RPM's according to Onsrud. I'm still new at this, the spindle always start at 6k RPM's, so I figure I need to do some research to see if this will be an issue or if it is even capable of going below that range and without damaging the equipment.

    I would use the 63 series for routing, profiling, pocketing, etc. Do you feel the drill bit is a good quality option?

    I'm also going to purchase some 90degree bits, they recommended the 37-50 series.

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