3/4" diameter cutter in MDF with Shapeoko 3

No no, it’s fine. The CA-50 you chose is one of a long line of very good Korg tuners.

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So far I’ve only been published by the European Commission and Wiley on the subject of data centers but I’ll be sure to let you know if I talk Wiley into a book on hobby CNC mechanics :wink:

Indeed, and counter-intuitive is when work gets interesting :wink: I didn’t have a good argument either way when I started measuring and building up the simplified model.

Basically the excess tension forces on each side of the moving axis balance out to zero as the belt pulls between the anchor points. If this wasn’t the case the stepper motor would have rather a hard job moving the axis at all. Things don’t improve with extra tension because the belt forces increase linearly with extension, there’s no rising rate until you’re well beyond the forces a Shapeoko belt clip or stepper can handle and the belts wouldn’t last long either.

If we were trying to hold the axis up with the belt as well then more overall tension would help, but we’re not, the axis wheels or rails do that and we’re only interested in the extra force required to deflect this point on the belt.

I’m also somewhat weak on metallurgy but we have seen quite a few people on the forum here have snapped motor shafts after cranking on the belt tension. It’s a wearout type failure as it happens as the shaft flexes under belt forces with rotation of the stepper.

The stepper motor manufacturers specifically warn about excess shaft radial loads and this failure mode so, yes, I think more belt tension = faster stepper death both in shaft failure and bearing failure modes.

So, yes, as you say, the least belt tension required to react motor forces and that seems to be about 80-100N.

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That awkward moment when you’re supposed to be at the more geeky end of the Shapeoko users spectrum, and folks out-geek you by a significant margin while discussing belt tension. C’mon, if it goes “twang” it’s probably fine, isn’t it ? :sweat_smile:


Jeff - thanks for the speed info. 18,000 rpm, 2 flutes, and 60 ipm works out to 1178 sfm [359 m/min] and 0.00167 in. [0.042 mm] chipload, both of which jive with my previous online research.

The belt tensioning theory / mods are an interesting topic. I will have to mess with at some point when I have time. I do have a decent Boss guitar tuner that may come in handy.

Lost foam casting will be a good fall/winter activity. Too hot out right now. I’ve been saving my decent-sized scrap / skeleton pieces of blue foam (where I’ve been experimenting with it on the machine) so they can become sprues.

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Machine and workholding rigidity definitely make a big difference as you noted. I come from more of a metalworking background where things get cranked down on in vises, chucks, step clamps, or bolted into a fixture. It’s been interesting playing with foam that is only taped down with masking tape.

It looks like you’re doing a great job with your machine. Keep up the experimentation and tuning.

I haven’t done any casting in a while. I have a bunch of used oil saved up for the oil burner. And some Litex wall texture mix that I need to try using as coating on the foam (supposed to help the dry sand that it’s buried in from embedding in the surface of the casting). One fun thing you can do with lost foam is embed steel fasteners or other elements in it to make knobs, handwheels or pulleys with steel hubs, etc.

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Mildly related, since it’s another design containing troughs, here’s a toothbrush holder I did in lost foam. Pattern was hand cut using a 1:1 dxf printed out and glued to the foam. It was supposed to be 4 troughs, but the end one didn’t fill, so I lopped it off. Need to take it back off the wall some day and smooth it out more and maybe powdercoat. Kinda like the blasted aluminum look, though.



Slightly more back on topic, I ordered a dust cyclone for my Shapeoko for MDF and other cutting. I have one on my blast cabinet (a converted 250-gallon oil tank) and thought about just moving that one back and forth, but for as cheap as they are, it’s not worth the hassle.

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Pics of blast cabinet here in case anyone needs a welding / fab project:



I’ve got a bunch of Pi Picos laying around. @jepho, Maybe we could do this with your tensioners?


I wonder if it would be helpful to have a belt tensioner that would be similar to a guitar tuning peg/machine. Something where the belt wraps around a worm-gear driven cylinder. The goal would be to keep the effective length of belt constant independent of tension. You could either have a belt with one fixed end and the other end goes straight to the tuner or use an idler bearing (like the ones already on the Shapeoko) to act like the headstock nut on a guitar.

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The motocross rider in me immediately thought the first foam picture was a foot peg for an off road bike. I still like it as a holder though.


I have the Fusion models (posted here somewhere too) Were you going to do the CAM in Fusion?
I might would redesign if I were to use aluminum. Those were designed for 3D printing.

Are they available somewhere for others to download and print?

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The experiment @jepho referred to is here. In place since then and it works perfectly. Improving the overall design and machining more compact versions is on my (rather large) to-do list.

Ha - I know exactly what you mean!

KDX450 in Wyoming, early 2000s:

CRF250X, in 2008 (not a great pic; a video grab):


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Spiffy! Now your other picture with the digital tuner makes more sense. I have some spare Ibanez electric guitar tuning pegs. I’ll have to pull those out and see if they’d be large enough to fit the belts.

Got a dust cyclone set up near the machine and finally cut some MDF. The scrap piece of MDF was from the packaging that my new kitchen table came in (what the MDF is sitting on in the picture). The little hole next to the trough is from a staple that was in it.

3/4" 2-flute carbide ball-end router bit
10,000 rpm
10 ipm plunge
30 ipm feed
0.0625" DOC, 0.250" total depth

through slot:
1/4" 2-flute carbide upcut bit
18,000 rpm
20 ipm plunge
60 ipm feed
0.0625" DOC, 0.770" total depth

I hit the top surface with some fine sandpaper after taking it off the machine.

I need to make a proper dust shoe / boot. Holding the vacuum hose near the cutter leaves a lot to be desired in the way of dust collection. Thinking about making something like David Johnson’s DustJet. Also need to read up on anti-static measures. The space where I have my machine is about 45% humidity and the vacuum seems to generate a good bit of static and I don’t want to zap anything (including myself).

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DustJet threads:



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