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Cutting foam sheets... with a needle!

So I have my mpcnc working great. Vacuum table made with it and the dealt works great. I really want this cut foam board for FT planes to use for STEM and intro to flying. The machine makes great cuts but like most, the needle breaks between the bearings and the motor. So a couple of questions. Which cutter head are people having the most success with? What are people using to measure the rpms? Here is a video of the system running. This is before the vacuum table was up and running.
 

2jujube7

Well-known member
So I have my mpcnc working great. Vacuum table made with it and the dealt works great. I really want this cut foam board for FT planes to use for STEM and intro to flying. The machine makes great cuts but like most, the needle breaks between the bearings and the motor. So a couple of questions. Which cutter head are people having the most success with? What are people using to measure the rpms? Here is a video of the system running. This is before the vacuum table was up and running.
I know this probably isn't the best method to test the rpms, but I just set the speed of the motor to 20% (using throttle hold on my tx) and I was able to get a good cut at 20mm/s. (using a 3s battery) Any more rpms than that, and the needle starts bending because of the centrifugal force acting on it between the flywheel and MIG welding tip. I'd recommend either buying an optical tach from somewhere, or just trying my strategy and going slow. :D Oh, and I'm using a stock flywheel that Moebeast has on his Foamripper's Thingiverse page.
 
I know this probably isn't the best method to test the rpms, but I just set the speed of the motor to 20% (using throttle hold on my tx) and I was able to get a good cut at 20mm/s. (using a 3s battery) Any more rpms than that, and the needle starts bending because of the centrifugal force acting on it between the flywheel and MIG welding tip. I'd recommend either buying an optical tach from somewhere, or just trying my strategy and going slow. :D Oh, and I'm using a stock flywheel that Moebeast has on his Foamripper's Thingiverse page.
Thank you. I just bought a Tach. That should help. I am using a power supply and a servo tester to set the motor speed. There is a good chance I was going way too fast. Thanks
 

Guy S.

Well-known member
I know this probably isn't the best method to test the rpms, but I just set the speed of the motor to 20% (using throttle hold on my tx) and I was able to get a good cut at 20mm/s. (using a 3s battery) Any more rpms than that, and the needle starts bending because of the centrifugal force acting on it between the flywheel and MIG welding tip. I'd recommend either buying an optical tach from somewhere, or just trying my strategy and going slow. :D Oh, and I'm using a stock flywheel that Moebeast has on his Foamripper's Thingiverse page.
I’ve had good luck with a phone app that pulses the flash like a strobe tachometer. It won’t go up to the 8500rpm that I want to run, but setting it at 4250 it works perfectly.
 
I have a 2212 1000KV through a 12V power supply. In was running close to Max. The needle is fatiguing about the bearings. Tach should be here soon.
 
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I just put up my needle guard* on Thingiverse. It fits onto MoeBeast's FoamRipper, and (should) protect from a broken/flung needle.
2jujube7,
I have built the MPCNC and now I am starting to build the FoamRipper. Since you have just built one I was hoping you could answer a question about the X carriage plate? I can't find anywhere that says how deep to cut out the square for the stepper motor to mount. Also, did you use 60mm or 69mm wheels?
Thanks,
Britt
 

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2jujube7

Well-known member
2jujube7,
I have built the MPCNC and now I am starting to build the FoamRipper. Since you have just built one I was hoping you could answer a question about the X carriage plate? I can't find anywhere that says how deep to cut out the square for the stepper motor to mount. Also, did you use 60mm or 69mm wheels?
Thanks,
Britt
I actually didn't cut out a square for the stepper motor, as I thought that it was just an outline to show everyone where the stepper motor goes. I just got some longer m3 screws that went through the whole plate. It was just simpler to make like that, but you can do whatever. I'm a poor lad, so I just 3d printed the wheels. 60mm is smaller, so I just did that. Here's a link on Thingiverse btw, it fits 608 bearings, or whatever they are called. Just let me know if there's anything else you need to know, i'm happy to help. :)
 
2jujube7,
I have built the MPCNC and now I am starting to build the FoamRipper. Since you have just built one I was hoping you could answer a question about the X carriage plate? I can't find anywhere that says how deep to cut out the square for the stepper motor to mount. Also, did you use 60mm or 69mm wheels?
Thanks,
Britt
That end plate you show won't allow adjustment of the height of the gantry rails. I'm not sure why there are multiple DXFs still on Mark's TV page... but you'll want the one with the bolt slot centered on the conduit slots if you want adjustment of height. Here's a couple of test plates I did when I built my FoamRipper... note the three slots on left side of the test plates (foreground plate is correct).

306576_69d427e4d18f23649cd86b316efc49b1.jpg


192647_2f46768a419adbffa105709849c97e7e.jpg


I documented my FoamRipper build earlier in this thread... and you might find it helpful. The depth of the pockets for the motor will depend on the thickness of the material you use for the endplate but if you leave 4mm-6mm of material, the shaft of the motor should extend far enough to properly align the belts. You could also probably chop off the lower slot if you're building this for needle-cutting/laser/penplot/etc... the gantry has more than enough weight on its own; i.e. I found a lower wheel, running under the worksurface, unnecessary.

-- David
 
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That end plate you show won't allow adjustment of the height of the gantry rails. I'm not sure why there are multiple DXFs still on Mark's TV page... but you'll want the one with the bolt slot centered on the conduit slots if you want adjustment of height. Here's a couple of test plates I did when I built my FoamRipper... note the three slots on left side of the test plates (foreground plate is correct).

I documented my FoamRipper build earlier in this thread... and you might find it helpful. The depth of the pockets for the motor will depend on the thickness of the material you use for the endplate but if you leave 4mm-6mm of material, the shaft of the motor should extend far enough to properly align the belts. You could also probably chop off the lower slot if you're building this for needle-cutting/laser/penplot/etc... the gantry has more than enough weight on its own; i.e. I found a lower wheel, running under the worksurface, unnecessary.

-- David
Thank you David,
Do you think the X carriage needs to be 3/4" material. I was thinking of using 1/2" MDF. Also, have you posted the STL files for your wheel spacers and belt guides?
Britt
 

2jujube7

Well-known member
The X carriage plate is just a plate that the 3d printed parts and wheels (and I guess stepper/idlers) screw into. As long as you have the right length screws, you can make it any size that you want. I'm not quite sure what the density/sturdiness of MDF is, but just make sure that it would be able to support the needed stresses. BTW, for wheel spacers I just used some (1/2"?) old nuts that clearance fit over the 5/16 bolts that hold the wheels in.
 
Thank you David,
Do you think the X carriage needs to be 3/4" material. I was thinking of using 1/2" MDF. Also, have you posted the STL files for your wheel spacers and belt guides?
Britt
Britt, I think 1/2" MDF would probably be fine... I just used what I had on hand. I haven't posted an exact set of STLs for spacers/bushings and bearing guides on this project because I find it simpler to measure, CAD, and print exactly what I need... as opposed to spending a lot of time trying to find something I may have already done and that may, or may not, fit. Hopefully it will be the same for you. Though I prefer Onshape for more serious CAD work, I find TinkerCad can be extremely useful for designing simple parts such as spacers and bushings, which is all I needed to add to what Mark provides with his Foam Ripper "thing" on TV. -- David
 

2jujube7

Well-known member
I'm been having a problem with my needle. At a notch above idle or even up to 10% throttle input, the needle isn't bent very much and works great (except for the slow feedrate). The minimum throttle that I haven't been having this problem is at 20%, so I've been cutting there at 20mm/s. At that speed you can still see the individual holes that the needle cutter makes, so I don't think I'm running the motor too fast. The cutting at that speed has been going fine so far, but today I had a needle break (about 5mm from the attachment onto the flywheel Luckily I had the needle guard I designed installed, so nothing was hurt. I think the needle was just stressed out from all the bending it's been doing. This first picture is a touch above idle, so nothing funky is happening there.
Needle1.jpg
The second picture though shows what it looks like right before the needle starts flapping around. I added in what the needle looks like above ~30% throttle with the red lines. (not an exaggeration, it actually looks like that. I couldn't get a picture of it above around 25% throttle because it was moving too fast and inconstantly to get a good picture.)
Needle3.jpg
I'm using the stock Foam ripper setup, except I'm using a 0.025" MIG welding tip instead of 0.035". There is no friction with my current tip, as the tips are clearance holes for the specified size. That's the only thing I can think of that's causing this problem, should I try and drill the hole out a little? I'm stumped.

Thanks
 
I'm been having a problem with my needle. At a notch above idle or even up to 10% throttle input, the needle isn't bent very much and works great (except for the slow feedrate). The minimum throttle that I haven't been having this problem is at 20%, so I've been cutting there at 20mm/s. At that speed you can still see the individual holes that the needle cutter makes, so I don't think I'm running the motor too fast. The cutting at that speed has been going fine so far, but today I had a needle break (about 5mm from the attachment onto the flywheel Luckily I had the needle guard I designed installed, so nothing was hurt. I think the needle was just stressed out from all the bending it's been doing. This first picture is a touch above idle, so nothing funky is happening there.
View attachment 191267
The second picture though shows what it looks like right before the needle starts flapping around. I added in what the needle looks like above ~30% throttle with the red lines. (not an exaggeration, it actually looks like that. I couldn't get a picture of it above around 25% throttle because it was moving too fast and inconstantly to get a good picture.)
View attachment 191268
I'm using the stock Foam ripper setup, except I'm using a 0.025" MIG welding tip instead of 0.035". There is no friction with my current tip, as the tips are clearance holes for the specified size. That's the only thing I can think of that's causing this problem, should I try and drill the hole out a little? I'm stumped.

Thanks
You really need a tach. ESCs are notoriously non-linear so 10% or 20% *throttle* tells us nothing about the rpms you are really running at. I use a "rule of 10" for a *ballpark* ratio of feedrate and cutter rpm... 600 mm/min for 6000 rpm. This results in 10 perforations for every millimeter of travel... and cranking the cutter speed up to 9000 rpm (15 perfs/mm) is about as high as I would ever go. Typically, as I use a servo tester in conjunction with my tach, I pass through 6000-8000 rpm in the first 1/4 - 1/3 of the servo tester's dial markings, using a 1100KV motor at 12 volts (theoretically 13200 rpm, max).

I don't know what motor and voltage you are using but the needle "blur" in your 2nd photo shows you are definitely running too fast... and the mass of the unsupported needle is being forcefully thrown outward with each change of direction, resulting in the "bulge" in the blur. Further, if you are running at 20 mm/sec (1200 mm/min) and nearly getting a decent cut, you are running double the values I recommend... probably close to 12000 rpm on the cutter.

I, too, used 0.025" piano wire for years on cutters dimensionally similar to Mark's Foam Ripper cutter and generally found the "sweet spot" for the cutter rpms to be in the range of 6000 to 8000 rpm. I know some people run faster/hotter than I've recommended here (especially in the ERC Timsav cutter "camp") but I feel it puts undue stress on the cutter's bearings and needle and probably shortens their life. It also results in more friction heating in the Mig-tip needle guide. So my recommendation is getting a tach like the one linked above... but at the very least I'd slow down back down to something in the 600-800 mm/min feedrate and ~10% "throttle" setting you were using and get more of those "works great" experiences. A little patience goes a long way... :cautious:;)

-- David
 

2jujube7

Well-known member
You really need a tach. ESCs are notoriously non-linear so 10% or 20% *throttle* tells us nothing about the rpms you are really running at. I use a "rule of 10" for a *ballpark* ratio of feedrate and cutter rpm... 600 mm/min for 6000 rpm. This results in 10 perforations for every millimeter of travel... and cranking the cutter speed up to 9000 rpm (15 perfs/mm) is about as high as I would ever go. Typically, as I use a servo tester in conjunction with my tach, I pass through 6000-8000 rpm in the first 1/4 - 1/3 of the servo tester's dial markings, using a 1100KV motor at 12 volts (theoretically 13200 rpm, max).

I don't know what motor and voltage you are but the needle "blur" in your 2nd photo shows you are definitely running too fast... and the mass of the unsupported needle is being forcefully thrown outward with each change of direction, resulting in the "bulge" in the blur. Further, if you are running at 20 mm/sec (1200 mm/min) and nearly getting a decent cut, you are running double the values I recommend... probably close to 12000 rpm on the cutter.

I, too, used 0.025" piano wire for years on cutters dimensionally similar to Mark's Foam Ripper cutter and generally found the "sweet spot" for the cutter rpms to be in the range of 6000 to 8000 rpm. I know some people run faster/hotter than I've recommended here (especially in the ERC Timsav cutter "camp") but I feel it puts undue stress on the cutter's bearings and needle and probably shortens their life. It also results in more friction heating in the Mig-tip needle guide. So my recommendation is getting a tach like the one linked above... but at the very least I'd slow down back down to something in the 600-800 mm/min feedrate and ~10% "throttle" setting you were using and get more of those "works great" experiences. A little patience goes a long way... :cautious:;)

-- David
Thanks David, that really helped. I knew that ESCs were very unlinear, but I didn't know how much. I was thinking that the recommended 6-8k rpm would be around 1/2 throttle, and it would have more of a curve shaped like a cubic graph (like expo on flight controls). I just thought that I had done something wrong while building it. :ROFLMAO:

I'll invest in a tach and get the settings dialed in to a safe level at the rough speeds you've suggested.

Thank you for taking the time to write back to my questions I had. :)

-- Jude
 
You really need a tach. ESCs are notoriously non-linear so 10% or 20% *throttle* tells us nothing about the rpms you are really running at. I use a "rule of 10" for a *ballpark* ratio of feedrate and cutter rpm... 600 mm/min for 6000 rpm. This results in 10 perforations for every millimeter of travel... and cranking the cutter speed up to 9000 rpm (15 perfs/mm) is about as high as I would ever go. Typically, as I use a servo tester in conjunction with my tach, I pass through 6000-8000 rpm in the first 1/4 - 1/3 of the servo tester's dial markings, using a 1100KV motor at 12 volts (theoretically 13200 rpm, max).

I don't know what motor and voltage you are using but the needle "blur" in your 2nd photo shows you are definitely running too fast... and the mass of the unsupported needle is being forcefully thrown outward with each change of direction, resulting in the "bulge" in the blur. Further, if you are running at 20 mm/sec (1200 mm/min) and nearly getting a decent cut, you are running double the values I recommend... probably close to 12000 rpm on the cutter.

I, too, used 0.025" piano wire for years on cutters dimensionally similar to Mark's Foam Ripper cutter and generally found the "sweet spot" for the cutter rpms to be in the range of 6000 to 8000 rpm. I know some people run faster/hotter than I've recommended here (especially in the ERC Timsav cutter "camp") but I feel it puts undue stress on the cutter's bearings and needle and probably shortens their life. It also results in more friction heating in the Mig-tip needle guide. So my recommendation is getting a tach like the one linked above... but at the very least I'd slow down back down to something in the 600-800 mm/min feedrate and ~10% "throttle" setting you were using and get more of those "works great" experiences. A little patience goes a long way... :cautious:;)

-- David
do you happen to know what post the recommended tach is listed in. I would like to order one.