This is a 2826/2822 brushless RC motor version of the MPCNC foam cutter attachment (http://www.thingiverse.com/thing:1138627). In addition, an improved flywheel has been developed to better balance and handle the higher RPM's of a brushless motor. A symmetric and balanced clamp screw system is used that allows the bearing and mounting screw to be more simply counter-balanced by a weight that is easily calculated and placed directly opposite it on the flywheel. The flywheel needs to be balanced well enough to run at 6000-9000 rpm to allow feedrates of 600-900 mm/min, giving 10-15 strokes/mm which will result in clean cuts in DTFB with the paper on.
UPDATE 04/11/17: I've updated the needle cutter body to the latest version I use/recommend, which in addition to increased height, motor shaft to needle guide, adds sideboards to effectively reduce the number/intensity of any resonances. It also stabilizes and straightens the needle's motion before it enters the guide, and dramatically reduces the friction, and resultant heat, which had become an issue with some previous versions of the needle cutter. The red-bodied cutter in the photos is effectively obsolete and left only for reference. The V2 version operates exactly the same as previous versions but with smoother operation and longer needle life.
The development of this version of the needle cutter may be followed in my FliteTest forum thread
The prototype version was made of wood and shows that there are many ways to build a successful needle cutter. Other users' cutters are also featured in the above thread.
The sideboards are slightly slotted and adjusted to the angle of the needle in the 3 o'clock and 9 o'clock positions, close but not touching. These sideboards prevent the needle shaft from flying further outward with increased cutter RPM. I intend to eventually print the sideboards with NYLON filament but wooden or printed plastic sideboards will work quite well, especially if lightly lubed with a suitable grease.
UPDATE 11/18/16: The most current information about the development of this needle cutter (and variations, including a "no-flywheel" version) may be found at
Original development thread and earliest (mostly obsolete) version information is at
Check out also jhitesma's parametric (and improved!) version of this cutter platform at
Other related threads may be found at
UPDATE 5/13/16: An easier and better approach to the guide tip holder... get a thin piece of plywood (a small wooden mousetrap "plank" is great) and match-drill the holes for mounting it to the printed foam cutter body. Then thread either an inflation needle or welding tip (your choice) into the wooden guide holder. The wooden holder will most certainly be far more heat tolerant than the plastic one and less parts/complexity to assemble (https://www.youtube.com/watch?v=7yA7XSl-wnU).
UPDATE 1/25/16: The MPCNC foam cutter is now outfitted with a "floating" guide tip holder with 0.035" copper welding-tip. The "floating" tip holder uses a short section of carbon arrow shaft for a spacer/insulator and a 3-point set-screw arrangement to effectively isolate the hot (in use) copper tip from the PLA plastic.
UPDATE: added a 2822 version... different mounting hole pattern.
UPDATE 12/23/15: More info/pics about the design and construction of this foam cutter -- and more specifically balancing the flywheel can be found at
MPCNC Brushless Foam Cutter V2 in action:
Balancing the flywheel
Please exercise care and attention during this procedure. High motor rpms are involved so make sure everything is securely attached before running the flywheel assembly up to speed. And stay out of the plane of rotation... I've never had a flywheel come apart but small metal and plastic bits traveling at high-speed across the room is never a good thing.
After printing the flywheel, carefully "ream" the center hole and the holes through the raised "figure 8" on the backside with a clean sharp 1/8" drill bit. Leave all other holes alone... they are sized by design and hopefully printed such that M3 machine screws will thread into the relatively soft plastic by themselves and lightly snug up securely. DON'T OVER-TIGHTEN to the point of stripping the threads formed.
Mount the "raw" flywheel (no hardware at all) on a straight 1/8" rod (new uncut replacement motor shaft is good) and place it across and at right angles to two parallel "knife edges" a couple of inches apart. I use a plastic parts tray with smooth top edges. Find the heavy side... if gravity is still working at your location, it will be on the bottom. Mark it... it may come in handy later in the balancing process.
Take identical M3x10mm machine screws and nylock nuts (preferably) and populate the raised "figure 8" feature on the backside of the flywheel... it's the same as the "prop saver" configuration used to mount propellers on RC planes. Snug the screws lightly and equally to the shaft and check the balance. The M3 screws/nuts should settle into a horizontal position about center... it should be close.
If the balance is good to this point, note that there is a row of unequally-spaced holes all the way across the flywheel at right angles to the M3 screws/nuts used to clamp the motor shaft. The holes are spaced 6 mm, 11 mm, and 15 mm either side of the center hole. The outer holes all around the outer rim are 15 mm from center and can be used to add extra weight if necessary... they are not normally used.
Using a electronic scale in "grains" (gn) mode, weigh the bearing, mounting screw and washer you are going to use... mine weigh about 26 grains (gn). Mount the bearing, screw and washer at one of the 6 mm positions... on the FACE of the flywheel. The torque due to the bearing is calculated... 26 gns at 6 mm from the center yields 156 gn-mm.
To counter-balance the torque due to the bearing and mounting hardware, appropriate weights are now placed directly opposite and on the BACKSIDE of the flywheel... this is now the familiar "seesaw problem" from physics (and the school-yard). These weights can be M3 screws of varying type and length (insure they don't protrude through to the FACE of the flywheel), nuts, washers, etc... weigh and stack combinations that can be placed in the 6 mm, 11 mm, and/or 15 mm that add up to equal the torque due to the bearing assembly on the other side; i.e. 156 gn-mm, in my case. Possible simple combinations are 14.2 gn at the 11 mm location or 10.4 gn at the 15 mm location... I had some large-headed M3 screws that weighed 14 gn and placed one in the 11 mm location. Any combination that accurately counter-balances the 156 gn-mm torque due to the bearing assembly will work. This is where the flywheel's heavy side, marked in the first step, can also be used to help... swap the bearing and counter-weight positions, if necessary, to achieve best balance.
After applying the counter-weights, check the balance. As before with the shaft mounting hardware, the bearing and counter-weights should settle into a horizontal position across the knife edges when in balance. Add/remove washers to bring it to the horizontal as closely as possible. This is STATIC balance.
At this point, it is helpful to check the DYNAMIC balance by mounting the entire assembly into a variable speed Dremel tool or similar. Starting at the SLOWEST speed possible, start spinning the flywheel assembly and note the vibration in your hand. Hopefully it's not uncomfortable at the slowest speed setting... if it is, revisit the STATIC balance procedure until you can hold it comfortably. Start slowly increasing the speed until vibration again becomes uncomfortable... back off to a comfortable vibration level and check the rpm with an electronic non-contact tachometer if possible. Add or remove weight/counter-weight until 10000 rpm can be achieved and vibration isn't excessive and/or uncomfortable in your hand. Note that very small changes make a big difference at this point.
Once balanced, you're now ready to mount the flywheel on your foam cutter motor shaft, attach a needle, and go cut some foam.