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Brushless Motor Foam Cutter and cutting sheet foam

Discussion in 'Concepts and Ideas' started by dkj4linux, Jan 1, 2016.

  1. dkj4linux

    dkj4linux Journeyman
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    Sam,

    You need a bit of slop, needle in guide, I think. HOW sloppy? I don't really know but there's significant friction-heating taking place in the guide... there's more heat build-up here than anywhere else in the foam cutter system. And it becomes significantly greater with the higher rpms required for cleaner cuts in DTF. I suspect it'll also be greater the tighter the fit between needle and guide. Personally I'm zeroing in on a 0.025" MIG- or music-wire needle running in a 0.035" needle guide bore... close to the best compromise between adequate "slop" and needle "focus"? I'd suggest you just use what you've got or what's available and see how it does... and then share what you find out.

    The inflation needle guide has actually worked pretty well at lower rpms (4000-6000 rpm) but has relatively little mass, so the heat generated at higher rpm (8000-10000 rpm) builds up and has no place to go and the threaded throat-area simply gets too hot for the PLA plastic it's threaded in to (in my 3d printed versions). The greater mass of the carriage bolt needle guide and its different mount seems quite an effective "heat-sink" and doesn't allow the heat to significantly build up and cause problems... but there's the daunting task of boring it out. The MIG welding tip looks to be solid copper, already has an accurately-sized bore and thread, should be a similarly good "heat sink", and is considered a "consumable" in normal use... so it's relatively inexpensive; i.e. a card of 10 tips runs less than $7 at Lowes. IMHO it's by far the most attractive option we have at the moment.

    I'm also believing the copper tip should provide a superior "bearing" surface for the steel needle that rubs against it; i.e. most bearing surfaces/bushings are generally made of softer materials such as brass, bronze, copper, etc. And, in general, you'd rather have a relatively cheap bearing wear and require replacing versus having the usually more expensive machined shaft wear and break/fail catastrophically. That said, guide wear has never really been an issue in this application to date and the needles I've broken generally were because the distance between flywheel and guide throat has been too short and the needle heated and work-hardened at the throat and snapped.

    I'm not sure where on your machine you'll get the power for the foam cutter but since the power requirements are so modest, anything resembling 2- or 3-cell voltages (7-12 volts) and 2-3 amps should suffice... a power brick with those ratings shouldn't be too hard to find. Motor size/power really shouldn't matter since it's just loafing along in this application but I think 1000kv-1300kv rating gives best use of the range of the potentiometer when using a servo tester. I've not tried any other method of speed control. And IMHO you'll be doing well to get 8000-10000 rpm... it's a non-trivial task to balance the flywheel well enough to go too much higher. Besides, the needle will fly off at some point... probably something above 15000 rpm. -- David
     
    #31 dkj4linux, Jan 7, 2016
    Last edited: Jan 7, 2016
    David the swarfer likes this.
  2. gotswrv

    gotswrv Journeyman
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    The router is still in design phase, then I'll machine the brackets and things should go quickly after that. Got a buddy that is gonna do the anodizing, and everything else but a breakout board is in house. (need to find a good board for ethernet and mach3)

    Making whatever works best is easy for me, but buying is generally even easier.
    I would think that the longer the guide the greater the heat, also the closet to the flywheel the greater the angles, wear, and heat. However a short guide will become a pivot point rather than create linear motion. For being such a straight forward system it is somewhat dynamic in its requirements. I suppose everything is once near it's breaking point though. What part do you hope fails, or what parts should not?


    Part of my desire for high RPM stems from anticipation of poor EPP cutting and the need for melting it. Might be completely ungrounded and nonsensical.
    Would a crankshaft from a nitro motor do well for this app? could support high loads and rpm. but would require some sort of coupling or belt as well as mounting bearings. Not sure what they cost, but I may already have enough parts around to give it a try sometime. Seems like overkill..
     
  3. gotswrv

    gotswrv Journeyman
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    By in house I meant on hand or purchased and delivered. I don't mind DIY, but I've had trouble finding boards that have more than XYZ limit switch inputs. Looks like that board is a parallel port board. Trying to have either SD support or Ethernet, probably settle for usb though. Also looking to get more than 4 axis support. The router is likely to grow into a dual gantry machine. So far mach motion boards are looking like what I'll end up with.
    I tried to get a coworker to get involved with making our own board (he is a top notch programmer), but didn't take off.
    Maybe Artsoft will offer a Mach Pi with an LCD.
     
  4. David the swarfer

    David the swarfer OpenBuilds Team
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    I have a parallel breakout board that I plan to use for 5 axis, plus PWM spindle control. it has opto isolated inputs and outputs and will be driven by LinuxCNC, which is of course, free.
     
  5. dkj4linux

    dkj4linux Journeyman
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    Here’s where I am with my current foam cutter. I’ve incorporated Karl Tinsley’s foam cutter mount for HicWic’s MPCNC universal mounting system (Assorted Mounts for MPCNC Universal Mounting System by karltinsly) and Thurmond Moore’s MIG-welding tip needle guide. I used a spare flywheel, bored out to 1/4″, to make a needle guide holder that allows adjustment for varying material thickness, up to the full stroke length. -- David
     

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  6. Thurmond Moore

    Thurmond Moore Well-Known
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    Hey David,

    As you might imagine, I really like the re-purposing of the spare flywheel. I picked up an assortment of "Tips" at Harbor Freight today. 4 sizes (.023, .030, .035, .045) and 5 of each size. I will experiment with different needle and tip combinations. I am also looking at the possibility of a hand guided rather than computer guided implementation of this style cutter. ;)

    Thurmond
     
    #36 Thurmond Moore, Jan 8, 2016
    Last edited: Jan 8, 2016
  7. dkj4linux

    dkj4linux Journeyman
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    A hand-held/guided model! Why am I thinking of a sabre-saw? Or, a router? I suppose there's a place for a portable model that could run along a guide or free-handed/nibbled in tight spaces. Sounds like fun, Thurmond.:)
     
  8. Balu

    Balu Veteran
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    David,

    So what distance do you have between the guide throad and the flywheel?
     
  9. dkj4linux

    dkj4linux Journeyman
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    Balu,

    Flywheel center to guide throat is about 2.25" (57 mm) on the foam cutter pictured a couple of posts above and runs fine. Others I have are as little as 1.75" (45 mm) and are okay as well. The music-wire withstands bending/flexing quite well... it's the friction at the guide throat that can be a problem if the distance between flywheel center and guide throat is decreased too much.

    Notice the needle blur in the photo... the bearing offset is 6 mm, total stroke is 12 mm. If the flywheel center to guide throat distance is decreased too much, the angle at which the needle enters the guide is greatly increased... so is the friction, so is the heat. Eventually something fails. I only ran into that problem when trying to make the foam cutter as compact as possible. -- David

    20160108_170836.jpg
     
    Balu likes this.
  10. Thurmond Moore

    Thurmond Moore Well-Known
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    Yes exactly like a saber saw. I tried some pocketing on some scrap EPP today by hand and it worked great. I am running much lower speed though. I estimate about 3500 rpm.

    Thurmond
     
  11. dkj4linux

    dkj4linux Journeyman
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    Sounds really neat, Thurmond, but I'm having a little difficulty picturing your cutter and the pocketing operations you are doing. Any way you can take some pictures/video and help us out? I'm also dying to see how that first cutter turned out... were you ever able to balance it out well enough to get pretty good RPM's out of the needle? -- David
     
  12. Thurmond Moore

    Thurmond Moore Well-Known
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    I will see what I can do tomorrow afternoon. For pocketing think Crash Test Hobby "Fly More - Fix Less" EPP planes and pocketing for the gear / batteries in the EPP wing. The cutter is balanced and will run with minimal vibration at 4S voltages (approx. 15K RPM). I have not done any long runs yet though.

    Thurmond
     
  13. Balu

    Balu Veteran
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    I just had a thought for a change of the flywheel design, that I can't test because of a lack of - well anything I'd need ;). Why don't you make the flywheel so that the bearing can "snap fit" into the flywheel and feed the needle through the hole of the bearing, clipping it in place somehow?

    Main reason I thought about that is to avoid having to cut the groove into the outer race of the bearing. It'll also reduce weight, because you don't need screws and washers and since it's not offset that much to the front anymore, it might help with the balance too. And you don't have to twist the needle wire around the bearing that way.
     
  14. gotswrv

    gotswrv Journeyman
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    Curious as to how the other will reply to this, I personally am unable to visualize. Perhaps a simple drawing to help illustrate the idea?
     
  15. dkj4linux

    dkj4linux Journeyman
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    That actually sounds feasible, Balu. I played around a bit with eliminating the bearing altogether... making a very small offset hole in the flywheel and simply putting a right-angle bend in the needle and inserting it into the hole. It balanced far better and ran beautifully with no load but the plastic wouldn't last very long of course, without a small bearing insert of some sort. And I'm not really sure how long the needle would last with the applied forces at the top of the needle not being in line with the shaft. Using the bearing would eliminate the wear and tear on the plastic but would still need to be counter-balanced and you'd still have the non-aligned forces. But it's certainly worth playing around with... and that's what we're all here for, isn't it? :D:eek::D

    All that said, the groove really isn't the big deal it may initially seem... it only takes a few seconds. Same with the needle... a few minutes. Plus I've used so many different bearings it never occurred to me to try to make a recess in the flywheel for a particular one.

    I hope this doesn't sound like I'm making excuses... it's just that at the time I was working with what I had and just trying to make it work and not focused on finding the simplest way to fabricate it. -- David

    20151204_160503.jpg 20151204_160624.jpg 20151204_160933.jpg 20151220_131702.jpg 20151220_131811.jpg 20151220_131915.jpg 20151220_132008.jpg 20151220_132205.jpg 20151220_132243.jpg 20151220_132533.jpg
     
  16. Balu

    Balu Veteran
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    I was mainly suggesting to mount the bearing inside of the flywheel. It's kind of inverting the current version which mounts the bearing through the center and twist the wire around its outside. My version would have the outside of the bearing be flush in the flywheel and feed the wire through the inner hole, clipping it in place somehow :).

    I also thought about just feeding the wire through the flywheel directly, but expected it to melt quickly because of the friction. Since David has already tested that option, I don't have to think about that. The only idea I have for that is to use some kind of small metallic sleeve instead of a bearing. I wonder how much heat that'll generate or how quickly it'll wear.

    Looks like I need to build me a foam cutter just to experiment on it - even if I don't have a CNC yet.
     
  17. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    Would a teflon sleeve be of any value? Would it stand up to the wear?
     
  18. dkj4linux

    dkj4linux Journeyman
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    YES, yes, yes! Build a foam cutter just to experiment with. And Thurmond's idea for a hand-held version is really starting to grow on me. This thing already resembles a little saber-saw (Thurmond is already free-hand cutting with it... as is!) so I think it's time we all fire up TinkerCAD and get to work. We're a bunch of RC'ers so get a motor, ESC, servo tester and Lipo battery and start bundling/compacting and add a handle. I picture a cross between a sabersaw and electric scissors. What do you think? :):eek::):eek::)

    20160110_103547.jpg 20160110_111856.jpg
     
    #48 dkj4linux, Jan 11, 2016
    Last edited: Jan 11, 2016
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  19. gotswrv

    gotswrv Journeyman
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    Balu,
    That last description makes sense to me. Once I get one going I'll give that a try first. Should be able to press in a brass liner with a snug hole. An .032" wire is about 84 sfm at 10k rpm. Might last a decent while with some moly.

    DHL shows the printer arriving today!!
     
  20. Balu

    Balu Veteran
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    I thought about that too, but you'd have to mount it into the wheel somehow. A brass liner like gotswrv suggested can be pressed into a hole that's slightly smaller and won't move. I haven't worked with PTFE tubes other than feeding it into a 3d printer ;)
     
  21. gotswrv

    gotswrv Journeyman
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    Teflon is soft and Mashable. I would guess the little wire would just hammer it away.
     
  22. David the swarfer

    David the swarfer OpenBuilds Team
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  23. Balu

    Balu Veteran
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    Vesconite seems interesting, but it seems difficult to source for DIY?

    I wonder if a flywheel cut out of Delrin would work? Delrin is beeing used for bearings and bushings too?
     
  24. David the swarfer

    David the swarfer OpenBuilds Team
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    yeah, the minimum lengths are a bit expensive, but there are alternatives like nylon and others. I happen to have a big chunk of the stuff from which I cut pieces as needed :)
     
  25. gotswrv

    gotswrv Journeyman
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    I would guess that a US alternative would be nylatron or moly impregnated nylon. Although I'm doubtful that it will take the temps claimed by vesconite
    Delrin does well with friction an is reasonably hard, but not high temp. I've got plenty of delrin (acetyl) around I could try.

    I'll take a look at McMaster some time and see what high temp plastics they offer
     
  26. dkj4linux

    dkj4linux Journeyman
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    Here's something I thought about when I was playing with this before. There are all kinds of tiny beads used in fly-tying (which I love to do) for weight... sold by diameter and material (brass, tungsten, etc.). I don't have any of the really tiny ones but here is a 2.8 mm tungsten (brass would be better) bead, "pressed" into the hole where the bearing usually mounts on a 3 mm machine screw. I was thinking that 2 or more could be pressed into an appropriately sized hole (maybe an equal number in a hole opposite for counterweight) and then a right angle bend made in the needle. I'm sure it would last longer than running in bare plastic but who knows for how long?

    Another option might be to cut a 0.025" MIG-welding tip (I think it's solid copper) into a couple of short "slugs" that mount on opposite sides of the flywheel... for counter-balance and a spare needle mount when the first wears out?

    20160113_083359.jpg 20160113_091541.jpg
     
    #56 dkj4linux, Jan 13, 2016
    Last edited: Jan 13, 2016
  27. gotswrv

    gotswrv Journeyman
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    Wow, either those are high end fishing lures or tungsten is cheaper than I remember. I guess lead is more and more a no no. And tungsten is a bit heavier (requires less).

    If a press in plastic will work it would have the least impact on balance. I think followed by brass.
     
  28. Balu

    Balu Veteran
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    The best option would be to have the flywheel made out of the plastic itself :). If people already have CNCs that they are going to use the needle-foam-cutter for, they should be able to cut the flywheel out of the plastic too instead of printing it.

    The PLA + bearing / pressed in plastic option is for those who don't have that luxury.
     
  29. dkj4linux

    dkj4linux Journeyman
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    Ya know... any respectable CNC owner ought to have a 3d printer or two laying around anyway IMHO... ;):rolleyes:
     
  30. Balu

    Balu Veteran
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    Yes, but not every 3d printer owner has a CNC. :) My thought was that CNC owners can cut the flywheel directly out of more resistant plastic, so they don't have to use any bushing / bearing at all to mount the wire. Just a hole that fits the wire and off you go. Should help a lot with balance too :).
     

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