Brushless Motor Foam Cutter and cutting sheet foam

Balu,

Not to argue, I've got to speak out of my own head/heart/experience here... it's all I've got. It's okay to disagree... I'll respect you either way.

Here's the progression I went through... I built several CNC machines. I built foam cutters for them all. I bought/built a 3d printer several years ago. I discovered/built a neat CNC machine that is built with 3d printed parts. I bought/built another, more modern, 3d printer to go with the first. I used both 3d printers to build 2 more copies of that CNC machine. Along the way I updated my foam cutter design to use 3d printed parts to go on my 3d printed CNC machine. It turns out that that neat CNC machine actually uses a very common board set used with 3d printers as well as the firmware (Marlin) to run it. In fact, the CNC machine itself, with an extruder mounted, can actually "natively" be used as a very capable 3d printer as well... it is, after all, a 3d printer at its core. Check out www.vicious1.com to see it in action. [No affiliation.]

So the line between CNC machine and 3d printer has been redrawn IMHO. I've discovered that the 3d printer has at its core an embedded CNC machine. And for about $300 a decent 3d printer makes sense and is within reach of almost anyone with a CNC machine. It's just another tool in the toolbox.

I understand why one might want to simplify the flywheel design... I've messed with that idea myself. And I think it's a good idea to keep looking at other ways to improve it or simplify its construction... that's one reason I've chosen to make it public and offer up ideas to go along with yours. But until we find a better way, I'll just continue using the ones I've got... they're not that difficult to make and are doing a reasonable job for me.

My first foam cutters didn't use 3d printed parts... they were simply discs of aluminum I turned in my lathe. I followed Tom McGuire's lead (post #4) and used a small, lightly grooved, ball-bearing mounted eccentrically on the disc. I found balancing them a bit tricky at first but not at all impossible... and from the first I got some pretty good results with my foam cutters. Maybe it's all the practice I've had but I don't find it at all a daunting task to groove and mount the ball-bearing, balance the flywheel "good enough", fabricate a needle, and go cut foam. It's really not as difficult as it first seems.

I rarely use my CNC machine with anything but the foam cutter nowadays... it's relatively quiet/clean and does what I love to do. I live alone and keep my machines in the house with me. I hate the noise and dust created by rotary tools and vacuums and though I have used my CNC machine to cut parts out of MDF for other CNC machines, I choose not to anymore. I'd rather 3d print them.

My $0.02.

David

 

Good deal, Balu. We're fine. And I love to brainstorm... but, unlike the deep pockets we have to work with in industry, we all have different experience/budgets/living conditions/junk boxes/etc. that's going to limit what we each can do and what makes sense for us individually.

Let me address a few of your concerns...

1) Bearings are everywhere. They are in old PC fans, RC motors, hard-disk drives, etc. You can even buy suitable bearings new from HobbyKing and other vendors as replacement parts for your RC motors... they are quite inexpensive, usually come several in a package, and are made to turn at high rpms.

2) Check the FliteTest forum thread (post #26)... I recently addressed grooving the bearing and fabricating the needle. The trick to grooving the bearing is to allow it to "free-wheel" against the cutoff wheel and use very light pressure. Don't allow the cutoff wheel to stay in contact long enough to spin the bearing too fast; i.e. touch it, let off, touch it, let off... repeat. When a "scuffed" line appears, you've probably gone deep enough.

3) It doesn't take much of a groove for the needle to stay in place by its own spring pressure. There aren't any side-forces trying to pull the needle off the bearing. Virtually all the significant forces involved are in line with and/or parallel to the plane of the face of the flywheel. I have only thrown needles when I've gotten up to 17,000 rpm or so... much faster than what you really need to cut foam.

4) Weight is weight... a little or a lot, it will probably have to be dealt with; i.e. counter-balanced. On my printed flywheels, I use a balanced "prop-saver"-type mounting system (symmetrical and self-balanced) so that the flywheel is firmly affixed to the motor shaft and everything is in balance. The bearing is 6 mm offset from the motor shaft center-line and gives a 12 mm stroke length. I weigh the bearing, mounting screw, and washer that I will use... my current one is about 26 grains (a little electronic scale is invaluable). I can now compute that the torque at the bearing location is (26 gn) * (6 mm) = 156 gn-mm. Now you have the "see-saw problem"... you have holes opposite the bearing at 6 mm, 11 mm, and 15 mm. Weigh all your available screws/nuts/washers and find a combination that, when placed at one of those three fixed locations, will produce the same 156 gn-mm in the opposite direction; i.e. just like the little kid sitting near the end of the see-saw plank can counter-balance the big kid on the other side, sitting in closer to the middle.

5) I could have grooved the bearing and fabricated the needle in the time it's taken me to type this far into my response. Again, check out post #26 in the FliteTest thread. It is not difficult and does not take long to do... it takes longer to explain than actually do it.

6) Your version will come soon enough. We've all learned from history -- we study what others have done, what worked, what didn't work, and we start experimenting ourselves. That's when the "creative juices" start flowing... you start trying stuff, pushing the envelope, and soon get a good idea as to whether you want to be an engineer (or not) when you "grow up". Just don't make the mistake of trying to "pencil-whip" the problem and for-see every conceivable gotcha for so long that you never get started actually playing with it and getting your hands dirty.

I'm sorry if this sounds "preachy" and philosophical but this is what has worked for me. I'm nearly 70 years old, a retired electrical engineer, and I have been doing this stuff - both professionally and personally - for over 50 years. -- David

 

All,

I just read my last post and I want to apologize to everyone here, especially Balu. I'm just a hard-headed, opinionated, old man who DOES NOT have a monopoly on good ideas. The last thing I want to do is throw my weight around and put anyone down, intentionally or otherwise... or squelch their voice. Please keep coming with the good ideas... I really do like to brainstorm. Please just remember that we all have to work out of our own experience/budget/circumstances/junkbox/etc and just because a method/material is good/best for me doesn't mean it will always be good/best for everyone else. Please accept my sincerest apologies. -- David

 

Thanks, Balu. Dementia/Alzheimers runs in the family and I'm trying to stay "sharp" for as long as I can.

 

Welcome back, Thurmond! Things are starting to move quickly around here... especially with Mike's success with his first foam cutter run. Have you made any progress on a hand-held version?

 

As most of you know, the foam cutter is being discussed on a couple of forums... for the benefit of those who don't frequent them here's one of the early success stories from MikeJM over on the FliteTest forum. -- David

 

Getting ready to deliver this last MPCNC to my buddy, I've gone back to the hard-mounted MPCNC foam cutter for a little more rigidity and mounted a 2822 1200kv brushless motor. The lower kv rating of the motor better utilizes the servo tester's potentiometer range. I've used the 0.035" copper welding-tip in my latest 3-point floating needle guide holder. Feed rate is 609 mm/min and cutter is running a more relaxed 6000-7000 rpm. This is a successful attempt to stay below the point where the guide tip heats excessively and the needle begins melting the foam. All in all, this seems a good combination... good quality cuts in DTFB (paper on) and everything running pretty stress-free. -- David

 

Shurakair over on the FT forum posted this great version of the needle cutter... very rigid and precise. -- David

 

All, I guess I've taken my foam cutter just about as far as I care to. It works quite well and there are now a number of them -- built by others -- in existence. So, I'm spending most of my time nowadays playing with the MPCNC itself.

As you know I highly recommend the MPCNC -- designed by Ryan "allted" Zellars (www.vicious1.com) -- for folks wanting a quick and affordable way to break into the CNC "game". I doubt there's a more flexible or economical way to accomplish that at the current time. I've used a couple of different MPCNC's during the brushless motor foam cutter development and even did a pictorial build log of an MPCNC I built for a friend, documented in posts 114-125 on my thread over on the RCPowers forum (Mostly Printed CNC and cutting foam). Of particular interest to RC'ers, this machine can be easily built in a larger form factor to handle the foam sheet goods popular with the RC crowd.

As MPCNC is not Openbuilds specific, I'll start a new thread here in the "Concepts and Ideas" forum -- if there's any interest. But for now here's the direction I'm going with the MPCNC. Though not terribly heavy these larger-format foam-cutting MPCNC machines are definitely awkward to handle and I'm always in fear of racking and cracking parts when I try to move them around by myself. So, I'm looking at down-sizing it and making it at least semi-portable... and more suitable for laser or router.

So here's my latest laser/router CNC machine... I've fitted a 24"x24" base made of 1"x4" lumber and used half-lap joints to *hopefully* avoid racking when moved. It's still a bit large/heavy yet at this size to be classified as truly portable but I figure I can always make it smaller (similar to dimensions of Handibot)... this'll be okay to start playing around with anyway. Gcode files can be created or cut up into "tiled" format so I'm thinking maybe this -- and a way to physically index a smaller machine over the workpiece -- is the way to handle larger "artwork".

Any thoughts/ideas/interest?

-- David

 

Hmm..mm...

 

GOING FORWARD...

Looking back over this and other forum threads at the foam cutters that Thurmond, malukk, russbean, moebeast, MikeJM, Shurakair, and others have built there can be little doubt the foam cutter can cut and cut well. Thanks to all for the pictures and videos. Now everybody go build something nice, post the results, and allow everybody else to see why they really need to build one of these things.

Enough cutters have been built and demoed by folks (besides me) that it should be pretty well established by now that the cutting method works. I satisfied my curiosity and need some time ago so now I'd like to try to simplify the build and start breaking down the barriers to entry for the masses.

The lack of a CNC machine is obviously what holds most people back and the step into CNC is a big one. But this being a scratchbuilders' "hands-on" forum, with a fairly high level of computer expertise and/or technical competence for many/most, this would also seem IMO the perfect time and place to encourage some more brave souls to make the leap. While there is a fairly steep learning curve for beginners and novices (as with almost anything technical) that's one reason forums such as this one exist. And $500 and up for a truly usable CNC machine is a lot of money... but not necessarily out of line with the cost of entry into most any worthwhile technical field of endeavor. Good news, it can be a lot less if you are "handy", have a well-stocked junk box, a spare 4'x4' work area/surface, and have (or have access to) a 3d-printer.

[DISCLAIMER: I've built more than a dozen CNC machines from scratch over the years and IMO the MPCNC (absolutely no affiliation... satisfied customer... blah, blah...) is simply the easiest, most affordable, and flexible way to make the leap into CNC that I've found. And it's absolutely perfect for this foam cutting stuff... even without industrial-grade rigidity it's more than adequate for the tool forces involved with cutting foam and pen-plotting. Building it is like putting together a large erector set project and it can be built in larger sizes (for virtually the same cost as smaller) to cut foam sheets of almost any size common to RC. I've built four MPCNC's (www.vicious1.com/specifications) for less than $300 a machine (I 3d print all my own parts), not counting my time... which I've gotta spend doing *something* anyway. ]

For the foam cutter itself: Cost is not an issue... the modest RC parts are the major expense (and most RC'ers will already have most of them on hand) so it's pretty cheap almost any way you build it. Possibly it's the 3d printing stuff... NONE of this requires 3d printing. My first foam cutters were conventional construction and common materials... crude but effective. And the majority of foam cutters others have built since I re-introduced it late last year in the RC forums have been NON-3dprinted versions, so I'd like to come up with a super-simple construction that anybody can go to Walmart and pick up most of the necessary bits and pieces, build it in less than a day, and stick on any CNC machine to start accurately cutting foam plane parts.

Finally, the most critical part of the foam cutter -- the flywheel -- is the most difficult part to manufacture and prepare for use. What's needed is an easy-to-build, easily-balanced, NON-3dprinted flywheel that requires minimal machining. In truth, that's where the 3d-printed flywheels have shined... once printed, construction is dead-simple and it is readily balanced to 6000 rpm and above, allowing for reasonable feedrates -- 600 mm/min and above -- to cleanly cut DTFM with paper still on.

Here's my initial swag on something that should work nicely...



Ideas? Thoughts? I'd love to hear what you think. -- David

 

Thanks, David, for the suggestion. I've tried on numerous occasions to "be the wire" inside the guide tube and I really think there must be a "damped sine wave" sort of action going on in there and I'm not sure treating just the ends would be enough... blah, blah...

Heat was never an issue until I started 3d-printing the foam cutter. The metal needle and guide can take the modest heat... it's the material the guide threads in to that is the problem. The PLA plastic the foam cutter is printed with is extruded at a relatively low temperature (195C-200C) and the heat in the guide tube is significant enough to cause softening and deformation at the metal-to-plastic interface; i.e. the guide tube will start to "wallow" and make further cutting impossible. The only way to avoid it is, as you suggest, lessen the heat generated (somewhat difficult, more expensive) and/or insulate the plastic-to-metal interface (easier but more parts)... or, simply make the platform of a material that can take the heat. Hence, this "better mousetrap" edition.

Actually, the majority of the foam cutters others are building (that I know of) are for CNC machines other than the MPCNC (which I used during development). And many folks, even those with CNC machines, don't have 3d printers. So this is an attempt to simplify the build (minimal number of parts and least amount of machining) and use cheap, readily available materials that don't require you to have a machine shop. If you do happen to have one, though, I think Shurakair's metal foam cutter with custom-built Oilite needle guide approach is near-ultimate. -- David

 

In search of a better (or *no*) flywheel and simpler needle attachment, here's something that might have potential. It hasn't been tested yet -- actually cutting foam -- but it's not unlike the zillions of "prison tattoo guns" that have been built and [successfully?] used to apply body art to untold numbers of folk.

Still in the current -- NON-3dprinted -- "better mousetrap" ideology, the springs (there are two) on this particular trap (from our local DollarGeneral... 4 for $1.75!) actually fit pretty nicely around a 3mm (loosely) or 3.17mm (tightly) motor shaft. Add a prop saver to space it out on the end of the shaft and bend it around the prop saver screw to lock it into place and the needle can be slipped on to the bent out end of the spring. A needle with a few turns, formed around a small mandrel, might accept and hold a small drop of lube. The direction of motor rotation (clock-wise in this case, viewed from the end of the shaft) should probably be such as to tighten the coils around the shaft

Not sure it'll have the rigidity to pierce 1/4" foam board or not but we'll see... -- David

 

Here's a brief video of the "better mousetrap" foam cutter. I was running off a lab power supply set to 7.4 volts (approximately 2-cell voltage) and current maxed out at just less than 0.75 amps (5.6 watts). I didn't actually measure the rpm but should have been in the 9000 rpm region... and it did sound like just about that to my trained(?) ear. Best of all, this thing requires no 3d-printing, no balancing, no bearings, and... it aggressive pierced a piece of DTFB I held up to it. Now, I need to outfit one to mount on my MPCNC and try cutting a sheet of foam. So far, so good! -- David

 

Hey, guys! Check out what Bob Neitzke is doing with his BEAST of a foam cutter...

RC Groups - View Single Post - Laser Woes

 

All,

Here's my prop-saver/clothespin-spring needle attachment retrofit on a MPCNC quick change mount with Emax motor. I found that, with a 3mm diameter motor shaft, the loose (though not sloppy) mousetrap spring allows just enough play at speed to start throwing the needle. I didn't have that problem with the 3.17mm shaft of the RCtimer 2822 mounted on the wooden "better mousetrap" version shown earlier. So I robbed the spring from a clothespin and found it had to be sprung slightly open to fit over the 3mm shaft. Bent it up similar to before -- and mindful to keep the "dog" end 180* out from the needle axle end -- and it operated smoothly all the way up to 11000 rpm with absolutely no other balancing required. -- David

 

Alright! For the sake of completeness...

I made a wooden guide holder for each, welding tip and inflation needle. Using exactly the same setup, with only the guide holder difference, and running max rpm at 7.4 volts...

Inflation needle: 7447 rpm, 0.77 amps

Welding tip: 7355 rpm, 0.81 amps

It does appear there is a slight bit more "drag" using the welding tip vs. the inflation needle ... costing nearly 100 rpm and 1/4 watt or so. I'm not sure why... guide bore diameter, friction - copper vs. brass(?)/stainless(?), excess needle length due to guide length differences, ???

Cursory investigation complete, I'm going with the inflation needle for now. If for some reason down the road that doesn't seem to be working, the welding tip is always there. -- David

 

Such the procrastinator I am, here -- finally! -- is the first full-sheet DTFB cuts using the "no flywheel" version of my foam cutter. Running at about 8500 rpm or so on the cutter (under load) and 609 mm/min feedrate the result is a relatively clean-cut CheapNEasy plane (that I'll probably put off gluing up indefinitely!).



The 0.025" music-wire needle (overly long here) is attached to a simple offset crankpin (bent clothespin spring) that is light enough to require no balancing. A precautionary move, I added a small keeper (short length of insulation stripped from cheap extension cord) just in case the needle loops should tend to slip off the end under the vibration while cutting. -- David

 

Here's Bob Neitzke's (Hoosier Cutout Service) latest needle cutter job in MPF -- 0.75 IPS (1143 mm/min) at 4000+ cutter rpm. He's obviously working to beef up the cutter and make it more suitable for *production* use. Sweet! -- David

 

Now you're talking! Get those creative juices flowing and join the party... you'll have a blast -- David

 

All,

I got to thinking some more... about lube in the needle guide. My previous attempts just made a mess... lube migrated down the needle to the material and/or just baked/crusted up and disappeared in short order. I've always thought Shurakair's custom-machined Oilite (oil-impregnated bronze IIRC) needle guide is almost ideal but most of us have neither the Oilite material nor the machining skills/tools to make it. I've also personally come to prefer the inflation needle for a needle guide... maybe it's purely sentimental since that's what I used way back in the beginning, has been almost trouble-free, and has served me well and cut many sheets of foam.

But, looking at it again, the inflation needle guide has this ready-made "reservoir" above the entry to the guide bore that should be put to use. I'm thinking "old-school" here... that a little oil-soaked wad of cotton, felt, balsa, or leather could be packed tightly into the "reservoir" and used to continuously wipe and lube the needle shaft without the lube migrating to places it shouldn't go. Just an occasional drop of light machine oil should be adequate to keep the needle lubed and the cotton/felt/balsa/leather wadding, in addition to holding the lube in place, might also serve to dampen/stabilize the needle movement within the guide.

It should be easy to test... I may try later today if I get extra ambitious/energetic.

Simple, effective solution... or delusional thinking of an old man?

-- David

 

Speaking of needle guides and lube, Shurakair shared an updated video of his foam needle cutter with us in my thread over on the FT forum

Cutting foam sheets... with a needle! - Page 42

His foam cutter was built with the help of a machinist friend and features relatively heavy aluminum construction and custom-machined flywheel and Oilite (oil-impregnated bronze IIRC) needle guide. It's very much in its near-original condition, seems quite trouble-free, and he indicated he is extremely happy with it and the cutting method.

Here he is cutting DTFB at 900 mm/min and 9000 rpm on the cutter. He also indicated he feels he gets a cleaner cut when he does two passes on through-cuts. It's hard to argue with his results! -- David

 

All,

I'm posting this to the several relevant RC and CNC forums where I've started foam cutter threads, to acknowledge and express my appreciation for all the nice folks who've commented, encouraged, shared their ideas, and contributed in any way to the development of my foam cutter attachment... and to let you see an example of just how far we've come over the past year.

First, I want to apologize for being so inactive lately and basically taking the summer "off". I've actively kept up with all the great posts but contributed very little. Basically a lazy slob and living alone in my grandparents' old farm house with no a/c in the Texas summer heat (hey! my grandparents did it...), I've been pretty un-energetic and hunkered down most of the time under a fan. But, now, the heat is starting to subside, fall is in the air (sort of...), and I'm starting to feel a bit more energetic again. So I'm now trying to un-clutter my path back to my machines and hoping to start being more active and useful again with all this foam cutter stuff.

Of note, I promised one of my two 3d-printers to my son-in-law when he got settled in a bit more, so I have recently looked into getting another one. I've been rather spoiled having TWO printers to work with during all my MPCNC and foam cutter builds, so I really feel I NEED two printers going forward. I've been entirely pleased with the $300 FolgerTech 2020 All-aluminum Prusa I3 kit I ordered and built last year, so I looked there first... it's very well supported, there is a lengthy thread dealing with it on the FliteTest forum, and IIRC Jason (jhitesma) also has one and seems quite pleased with it. Anyway, it is indeed still available (google it... it's the ebay listing) so I ordered another one and now have it to build. It is another of my motivations for getting my workspace cleaned up and de-cluttered so I'll have the room to actively get back to building stuff.

Lastly, I can't begin to tell you how excited I am about all the foam cutter developments of the past year... and now -- cheap! -- VACUUM hold-down to boot! I think the magnitude of it all really hit me when Curtis (CartCurt), over on the FT forum, posted this picture of his very impressive, relatively inexpensive, and complete MPCNC-based foam cutter setup... that wouldn't exist today were it not for the collective contributions of all the nice folks in the several RC/CNC forums where all this development has taken place. What hobbyist RC'er in his right mind wouldn't love to have a setup like that? Thank you all! -- David