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Cutting foam sheets... with a needle!

Interesting notion. Have you tried this out? Sounds like it would tighten the orbit by 10mm (plus wire coil). That's what? 50% reduction? Looking at the specs for the motor, the prop holes are 12.5mm from center. It's something to consider.

With my flywheel cutters, I've always tried to equalize the needle flex to each side... striving for a symmetric needle "blur". In my experience it reduces the friction within the needle guide. With larger needle loops that surround the bearing race, I've always used round-nose pliers to center the needle's exit angle rather than coming off at a tangent...


the "blur" with needle coming off tangentially...


and coming off centered...


-- David
Thanks David, sometime you get a little tunnel vision and need a light shined. So I took my backup needle (still long) and put a bend in it and cut to size. Worked great on the first sheet. Then the motor wouldn't spin up. Wasted a whole bunch of time looking at the PSU. Turns out it was a fault in the ESC. Replaced the ESC. Cutting the 2nd sheet as I type. Better than a laser. Well faster at least. lol. Pretty close on the cut.
So that needle didn't last very long. I didn't see it break but since it came completely off of the bearing as well as breaking off by the mig tip, I'm hoping that it broke as a result of it coming off the bearing. I was worried about that. Looking at it after bending to center you can see that when the needle strikes it would tend to push even more on the coil.

I needed a retainer of some kind. Then I remembered these flanged bearing I had. Also 10mm in diameter. And I have 10 of them. Ran it through a sheet of DTFB and no complaints. Needle 1.jpg Needle 2.jpg Needle 3.jpg Needle 4.jpg
I really appreciate the work you are doing here, Steve. Keep it up!

A couple of things I might mention... not to argue but simply things I do differently to what you are doing here. I've never had problems with broken needles or needles coming off the bearing when I exercised appropriate care when fabbing and mounting them. I have bent/broken a few needles when torturing them while trying to cut cardboard and coroplast -- light cardboard actually cuts okay, coroplast not so much -- but in normal use with DTFB and fanfold foam I've never really had any problem with the needles. That said, I've never cut the large quantities of foam that some of you cut and I've not tried cutting any of the new waterproof foam board at all. I detailed the procedure I use to fabricate needles and mount them on the bearing way back in post #26 of this thread.

First, I do no annealing, or other heat-treating, of the needle at all. I trust the heat-treatment of the music-wire steel from the factory and feel the springiness of the steel helps keep the needle in place around the bearing. Another thing is that I always grind a shallow groove in the outer race of the bearing to help retain the tight loops of needle. And finally, I used a "mandrel" (a drill bit shank) about 2/3 the OD of the bearing to wind my loops around... about 1-1/4 to 1-1/2 turns and I usually leave a little tail/handle, which makes it easy to slightly spring open the loops to go around the bearing race. The loops spring back a bit when removed from the mandrel but, when fitted around the bearing, it "snaps"/settles and holds in the shallow groove in the outer race simply by its own spring pressure.

I recognize that many folks have had difficulty fabbing needles and grooving the bearing race as I've described... and have experienced many more needle-related failures than I have. But this is the procedure that has worked well for me. And, as always... YMMV :)

-- David


Some guy in the desert
I wasn't able to groove my bearing and seriously considered ordering some flanged ones instead (I didn't have the right size on hand.)

But...instead I tried a tiny drop of super glue (because I didn't want it getting into the bearing) and it worked great. Even using the needle on cardboard after that it didn't come loose.

I also haven't tried annealing yet, I've considered it right from the start - but the more I researched the properties of music wire and proper methods of heat treating/annealing....the more I found good reasons not to. I forget the details of what I found, but the short version was that due to the size of wire and the way it's treated from the factory it's VERY VERY difficult to anneal effectively and it's more likely to make things worse than better for our purposes. Not impossible...but...much trickier than annealing larger pieces apparently.
FWIW, I just use TWO regular bearings with a washer between them that is slightly less thick than needle wire - the needle wire loop sits in the 'groove' that is formed.
I've NEVER had a needle wire come off, and like David, never broken one either (although I only cut 6mm depron) - nor do I anneal them - they get enough use that I'm sure they've annealed themselves by now :)
Bearings are 8mm OD, 2mm thick, 3mm ID


Needle Bearing.png
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I've cut probably 150+ sheets of DTFB & WPFB. I've had a few needles break. Especially back when experimenting with some of the less successful methods like the crank wire around the motor shaft. I'm always looking to improve on my designs. But don't take this to mean that I'm any troubles that I haven't overcome.

Annealing the needle is a process where you're relieving the stresses you put into your needle from bending it into a coil. It's allowing the crystals in the steel to realign to its new shape. 500 degrees isn't all that hot. It's not like I'm putting it a forge until it's red hot. It's about 50% of a full anneal. So it's not going to remove the temper. If you don't want to do it, then don't. This thread is about sharing information, it's not a dick measuring contest.
There's absolutely nothing wrong with sharing what works for you and your use model. That is indeed what this thread is all about. That's what I'm doing... and have been doing all along. And I think that is true of most everyone who's contributed to this thread from the beginning. But there's never just one way of doing things and the testimonies/experiences -- and myriad successful cutters -- shared in this thread serve as clear proof of that. Please do what works for you... i.e. what works for me may not be suitable for you and your purposes.

I do understand what you are saying about relieving the stresses produced while creating the loops/bends (I used to make my own woodworking lathe tools/chisels from files and other metal bits...) but in the years I've been playing with the needle cutter -- as a hobbyist -- those stresses have never been so severe as to create a problem... so I've never sought to relieve them. Had I been cutting large quantities of foam board in a production environment, I'm certain my experiences would have been different, and the information I've shared would reflect those experiences. This thread is full of ideas and tidbits -- many different, some even contradictory -- that almost anybody, regardless of their experience level and interest in CNC and cutting foam, might find useful... and you never know which ones are going to grab the fancy of someone, somewhere, who'll take it, run with it, and come back to tell us all about it. What fun!

You've already contributed greatly and I, for one, thank you. -- David
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FWIW, I just use TWO regular bearings with a washer between them that is slightly less thick than needle wire - the needle wire loop sits in the 'groove' that is formed.
I've NEVER had a needle wire come off, and like David, never broken one either (although I only cut 6mm depron) - nor do I anneal them - they get enough use that I'm sure they've annealed themselves by now :)
Bearings are 8mm OD, 2mm thick, 3mm ID


View attachment 116521
Neil, what do you use that Servo for?