Cutting foam sheets... with a needle!

[Rit]

What is the needle distance from the crankpin at the top of the stroke to the first bearing set?
How much needle is exposed at the tip at the top of the stroke?

Thanks.
Old people dont play with fidget spinners and eat laundry soap, they are too smart for smartphones.

 

[dkj4linux]

What is the needle distance from the crankpin at the top of the stroke to the first bearing set?
How much needle is exposed at the tip at the top of the stroke?

Thanks.
Old people dont play with fidget spinners and eat laundry soap, they are too smart for smartphones.

That's all pretty variable, Rit, but Jason's design, using Onshape, appears to be 53mm... motor shaft to top bearing center. Add 6mm of offset on the eccentric and you'll have 59mm, call it 60mm, at top of stroke. Keep it reasonably close to that and you'll probably be good. As far as trimming to length, I usually set it to top of stroke and snip the needle even with the very bottom of the needle guide... then remove and lightly sharpen to a conical point, like a pencil. I think Jason prefers the needle to fully retract... I'm not convinced it really make a difference. You'll want at least a couple of mm of clearance between tip of guide and top of material when cutting... and a couple of mm past bottom of material to insure full through-cuts. With 6mm thickness and flat material (not the norm) , you'll want a minimum of 10mm or so of exposed needle... with my trimming/sharpening method I usually have close to 12mm, exposed. YMMV... -- David

 

[Rit]

Xcellent thank you

 

[jhitesma]

Sorry I haven't been around to reply. Spent most of the past week helping my parents move out of their house of 33 years and 10 miles down the street to a slightly smaller house in a town with much lower taxes so they can afford to visit their granddaughter here in AZ more often (A granddaughter who is currently enjoying 3 weeks in Ohio with her grandparents and giving me and her mother a bit of time to relax!)

Thankfully David has pretty much said everything I would

I agree with about 10-12 mm of needle sticking out at full extension. I do trim so it's fully retracted into the tip at the top of the stroke - but not by much. David's method of trimming flush and then sharpening is about what I do, I may take a tiny bit more but since I tend to loose a mm or two while sharpening it's about the same.

I've never actually measured my needle (or at least not recently enough to remember doing so let alone what the measurement was.) I wrap it around the flywheel bearing (I didn't have much luck with the retainers) then install it and mark where it comes out when it's at the top of the stroke. Then trim it just above that mark and sharpen.

The motor I use is a 2212 850kv. The place it actually came from calls it a 2830 but they're measuring the OD of the bell not the size of the stator like most places do now. I used to run a higher kv motor but I found the 850 is nicer since the sweet spot for speed is closer to mid-throttle on 12v.

Your motor should work - you may need a small spacer behind it or a thicker flywheel since it's probably a little shorter (though that depends on the mount style of the motor.) The higher KV on yours isn't what I prefer - but you can always run it on lower voltage (2s power instead of 3s) or just keep the "throttle" very low. Keep in mind ESC's aren't very linear so you can't always just "do the math" to figure out a throttle point for a given RPM reliably as the ESC's usually have a curve to their throttle response. I was running mine way too fast for months and having all kinds of problems because I tried to do that - when I finally picked up a $13 tach I slowed it way down (then switched to the lower kv motor) and all my issues went away almost immediately!

 

[Rit]

ok, thanks for the reply, Will this motor work? (for 3S) I will have to design my own mount.
https://www.ebay.com/itm/Emax-MT3506-650kv-Motor-CCW-Thread/173342888046?_trkparms=aid=222007&algo=SIM.MBE&ao=2&asc=52475&meid=f10e89bf820447baa73383245135b950&pid=100005&rk=2&rkt=9&sd=173342886666&itm=173342888046&_trksid=p2047675.c100005.m1851

 

[jhitesma]

That should work but would need a bit of adjusting...kv is a little lower so you'd be higher in the throttle range. It's also wider but that shouldn't matter...but shorter so you'd need to add some kind of spacer behind (or use a thicker flywheel - but I'd go with the spacer since less rotating mass is usually better) to get the needle to line up.

The Sunnysky 2212 980kv for $12 is probably the best easy to find motor for my design:
https://www.ebay.com/itm/Sunnysky-X...029054&hash=item3399041ef8:g:7Y0AAOSwWCBasGjU

Specs on it are the same size as the motor I used and it doesn't have a protruding shaft which makes it even nicer since my flywheel design will bolt right on. (assuming the "snout" portion is the same size, I can't find dimensions for that.) But if I was buying a new motor today it's probably what I'd use.

Though if I was to redesign my mount I'd probably design it around a 2204 mini quad motor since I have lots of them laying around

 

[ironkane]

that would be a very good motor. I have a similar motor https://www.amazon.com/gp/product/B06W9HHDZW/
You can literally skip the flywheel and just attach a bearing with a needle wrapped around it. You'll just need to grind down protruding bearing shaft. I have some video's a few pages back if you want to see it in action.

 

[Rit]

that would be a very good motor. I have a similar motor https://www.amazon.com/gp/product/B06W9HHDZW/
You can literally skip the flywheel and just attach a bearing with a needle wrapped around it. You'll just need to grind down protruding bearing shaft. I have some video's a few pages back if you want to see it in action.

Yes, I think I saw that one, and I like the simplicity, and compactness of it, I had your setup in mind when I chose the motor that I did. About how fast are you running it? 1/2-3/4 throttle?

 

[bperc]

This is the motor I use with jhitesma's needlecutter. http://a.co/3VIv7Zb

 

[ironkane]

Yes, I think I saw that one, and I like the simplicity, and compactness of it, I had your setup in mind when I chose the motor that I did. About how fast are you running it? 1/2-3/4 throttle?

Top speed would be 8,400. I'm probably around 7,000. I mostly go by sound as well as visually looking at the needle's deflection in motion. A lot of torque in these motors. Gives some good punch.

 

[Rit]

Top speed would be 8,400. I'm probably around 7,000. I mostly go by sound as well as visually looking at the needle's deflection in motion. A lot of torque in these motors. Gives some good punch.

I agree, QC motors have great torque, and the large diameter acts as a flywheel for punching, how is your setup working, do you think it is worthwhile to add in the 4 bearings, or not to bother ?

 

[ironkane]

I agree, QC motors have great torque, and the large diameter acts as a flywheel for punching, how is your setup working, do you think it is worthwhile to add in the 4 bearings, or not to bother ?

I am of mixed feelings. I had created one for my MPCNC, but never put it into use. So I think it sounds good in theory, but I'm still a fan of K.I.S.S.
If I saw a lot of wear on either end of the MiG tip, I'd probably look at forcing the needle to be vertical sooner. But what if having some flex is beneficial to needle life?

The key elements is having a good needle that's free of any nicks and has been annealed to further reduce any stress from bending/coiling and then moving it in an elliptical fashion with as little stress to it as possible. The speed should be set as high as you can go before you start to melt the foam.

 

[Michael9865]

With my setup if I go above 5,800 rpm on the cutter I start having the foam melt.

 

[dkj4linux]

With my setup if I go above 5,800 rpm on the cutter I start having the foam melt.

Have you tried increasing your feedrate, Michael? Also, I can't remember, do you have "pre-guides" of any sort (bearings, sideboards, etc) on your cutter? Their main purpose is of course to constrain the needle-in-guide motion to near-straight line motion and reduce the friction (i.e. heat) in the main guide. IIRC Jason is running his newer pre-guided cutter at considerably higher speeds now than he did with the old...
-- David

 

[Michael9865]

Have you tried increasing your feedrate, Michael? Also, I can't remember, do you have "pre-guides" of any sort (bearings, sideboards, etc) on your cutter? Their main purpose is of course to constrain the needle-in-guide motion to near-straight line motion and reduce the friction (i.e. heat) in the main guide. IIRC Jason is running his newer pre-guided cutter at considerably higher speeds now than he did with the old...
-- David

I have toyed with the feed rate some, but I find that the needle starts getting hot at higher rpms regardless of the feed rate. My cutter does have guide bearings, see https://forum.flitetest.com/index.php?threads/cutting-foam-sheets-with-a-needle.24251/post-368418
I am traveling at the moment, but I think I am using .025 wire needle and a .032 mig tip.

 

[jhitesma]

Confirmed that I am able to run higher speeds with the guides than I did before them. I'd have to look back at my posts or break out the tach to get an exact number...but I do know it's a few k faster than I used to be able to run. Pretty sure I'm using the .025 wire and .032 mig tip as well.

But I also changed to a longer needle, changed from the crank to the flywheel, went with a lower kv motor, and changed from the air needle to mig tip at the same time. So...hard to say just which made the biggest difference. I wish I had the time to test the changes individually but not being retired, having a young child, and way more projects than I have time for already that's just not going to happen

I do also use the cotton ball oiler above the mig tip and I do notice a difference in cut quality and heat when I forget to oil it after it's had a lot of use or sat for a few weeks. So that really does seem to help.

I also know that on my previous cutter the mig tip did run hotter than the air needle guide when compared with all else being the same. But I prefer the mig because:

1) It lasts MUCH longer
2) the air needle can fail - some fail after very little use and some don't even last one use they're so cheaply made.
3) the mig tip doesn't need any special prep while the air needle needs to be trimmed.
4) You can solder on some heat sink fins if you're having heat issues with the MIG tip. (I've also considered chucking it up in my drill press as a poor mans lathe and using the cut off wheel in my dremel to cut some grooves in it turning it into it's own heat sink - but since I haven't had heat issues due to the other changes haven't had to resort to trying that or adding fins.)

I can think of a few reasons for the heat difference. The mig tip is a tighter fit (but I like that since it should give a more accurate cut), the copper of the mig tip may be a bit "stickier" giving more friction than the material the air needles are made out of. The air needles have way less mass so the can't hold much heat in while the mig tip's extra bulk can really hold the heat.

 

[Rit]

I am working on my needle cutter, and I see that the bearing does help to keep the needle strait, so, I will incorporate them, there is quite a bit of room in the tip for the wire, it will be interesting to see this thing run.
--Does anyone have any hints on hanging the needle on the motor bearing?
--Has anyone tried to harden the tip of the needle?

 

[Shurakair]

I am working on my needle cutter, and I see that the bearing does help to keep the needle strait, so, I will incorporate them, there is quite a bit of room in the tip for the wire, it will be interesting to see this thing run.
--Does anyone have any hints on hanging the needle on the motor bearing?
--Has anyone tried to harden the tip of the needle?

Rit,
Have you looked at my flywheel design? It holds the needle more securely. Since it sounds like you're designing most of this stuff custom for your setup, maybe it would work for you.
Shur

 

[Rit]

Rit,
Have you looked at my flywheel design? It holds the needle more securely. Since it sounds like you're designing most of this stuff custom for your setup, maybe it would work for you.
Shur

Great design, I will definitely keep it in mind if this contraption doesn't work out, it makes sense, I am trying to eliminate the flywheel with a large QCopter/Low KV/pancake motor like ironkane did

 

[jhitesma]

I had forgotten about Shurakair's flywheel...still want to try that style at some point...wish I had remembered it when I broke my last needle and then melted my flywheel and gave the retainers a try.

That said....the way I make my needles is I use a longer piece of wire than necessary, then wrap it three times around a drill bit a few sizes smaller than the OD of the bearing (I forget the exact size I use, I think David had a size in one of his early posts or thingiverse posts, or maybe it was in Tom's original instructable which I can't seem to find right now.)

Anyway, the drill should be small enough that once the coils relax they're still a bit smaller than the bearing. I then cut the wire in the middle of the last coil and bend it at the end of the other coil so it comes off the coil at 90 degrees. Making that bend is the trickiest bit. Use round nosed pliers if you can - regular pliers with their sharp edge can make too sharp of a bend and create a stress point. This is where most people have issues with their needles breaking.

Then just force the coils over the bearing. I go one step further and add a tiny tiny bit of CA glue. I don't really need it when cutting foam but when I was experimenting with cutting cardboard I found it necessary. I just dip a scrap of plastic in the CA then touch it to the wire to let capillary action pull it around the bearing. Use too much and it will get into the bearing which wouldn't be good.

Then I install the wire on the cutter and turn the flywheel to the top of it's stroke. At that point I mark the wire and remove it so I can cut it this is where David just cuts his flush...I just like to make more work for myself Well, actually I like to remove mine because I find it easier to sharpen that way. I just twirl it between my fingers while holding it against the face of my dremel cut off wheel for a few seconds. I sometimes do this with the whole cutter for a quick re-sharpen...but it's much easier to just remove the bearing screw and take the needle off to sharpen.