So I'm sitting here assembling a copy of Wade's fine work and I've got a temporary 8mm bolt through the main gear to mark it for hobbing.

After getting it assembled mostly, I noticed that if the largest piece was a mirror image, then when you got the whole thing running, it would tend to tighten the bolt rather than loosening it. I'm guessing that may do away with the need for nyloc or thread lock.

Either that, or find some left-hand threaded hardware.

I don't have a working machine yet, so I can't do any further testing. Does anyone see any obvious issues?

(BTW - for the record, I'm against a nyloc fastener if it means I've got to toss what should be perfectly good hardware after only one use)

I use two bolts tightened against each other. It prevents it from working its way loose, but I can still quickly undo it for flossing.

tbfleming Wrote:
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> I use two bolts tightened against each other. It
> prevents it from working its way loose, but I can
> still quickly undo it for flossing.


Yea, with the hardware I've got, I was planning to go the same route (maybe add a lock-washer). But it occurred to me that the entire extruder might be improved by a slight redesign.

I've got the bolt and nut on there hand-tite (I've actually got a nut pressed into the gear because the only bolt I have is like a M8x1.25-40, too short), and I note that if I turn the gear in reverse, the nut tends to tighten naturally. Flipping this part as a mirror image should allow for the same action, except when moving forward.

I don't think mirroring will work very well, as you don't want too much tension on the hobbed bolt - you only want to keep it lined up with the filament slot and the gears. If you tighten it up, the skate bearings are probably going to bind, and you might even collapse the main block over time. Plus, if it tightens up over time, the alignment will shift. Better to leave it a little loose, and let it self center via the filament and the idler wheel.

I re-use nyloc bolts all the time, never had a problem yet. If you really don't like nyloc, just use 2 nuts, or glue the one nut in place.

That said, I think I saw a mirrored version on Thingiverse, two stack two of them on a single x carriage - you can always try that.

Wait till you try mounting it on the X carriage - use a hex head under the stepper motor to make it a bit easier. The next version should be easier to mount.

Wade

edit - fixed the double negative. Oops.

Edited 1 time(s). Last edit at 09/16/2010 05:31PM by Wade.

Yes I agree with Wade, you don't want to tighten it against the bearings as it gets harder to turn. I tighten it to that point and slacken it off a bit so it is free to turn but has no noticeable lateral movement.

Also I reverse the extruder at the end of every filament run so a single plain nut would likely come lose. I use a Nyloc and it works very well.

Despite what it says on the Reprap wiki, you can use Nylocs over and over again. They might not grip as much the second time you use them, but they still grip plenty enough to stop them vibrating lose. In fact I prefer them when they have been used a few times as they get easier to put on.

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[www.hydraraptor.blogspot.com]

Looks like I'll need to design and print it to find out. I don't think it will naturally tighten to the point of binding.

Toyota engines turn over counterclockwise, thus the threads on the nut of the main accessory pulley has standard threads. The natural turning force of the engine tends to keep the nut tight. (Edit: I forgot, they actually turn over clockwise, but the bolt that holds on to the main accessory pulley ("fan" belt pulley) is threaded into the main crank such that natural turning force work to keep it tight).

As a counter example, the left bicycle pedal threads on a bike are left-hand threaded for the same reason. The motion of turning tends to tighten the threads instead of loosing them and you hardly could redesign a bike to have two right pedals.

Thanks for the mounting tip! That should come in handy.

Wade Wrote:
-------------------------------------------------------
> I don't think mirroring won't work very well, as
> you don't want too much tension on the hobbed bolt
> - you only want to keep it lined up with the
> filament slot and the gears. If you tighten it
> up, the skate bearings are probably going to bind,
> and you might even collapse the main block over
> time. Plus, if it tightens up over time, the
> alignment will shift. Better to leave it a little
> loose, and let it self center via the filament and
> the idler wheel.
>
> I re-use nyloc bolts all the time, never had a
> problem yet. If you really don't like nyloc, just
> use 2 nuts, or glue the one nut in place.
>
> That said, I think I saw a mirrored version on
> Thingiverse, two stack two of them on a single x
> carriage - you can always try that.
>
> Wait till you try mounting it on the X carriage -
> use a hex head under the stepper motor to make it
> a bit easier. The next version should be easier
> to mount.
>
> Wade

Edited 1 time(s). Last edit at 09/16/2010 06:06AM by pica.

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My blog's Reprap feed: [blog.markbova.com]
I'm currently working on a stock Mendel build with a Seeeduino Mega and four Pololu A4983 stepper controllers.

[www.thingiverse.com] <--kludgineer's tweak for a captive mounting bolt

[www.thingiverse.com] <-- Domonoky's mirrored and tweaked for the BfB hot end. It looks like he did it to mount two of them on the carriage, not because of the issue I brought up.

[edit: thingiverse.com really needs to be redesigned to make it easy to find derived works. Finding this was a PITA.]

Edited 1 time(s). Last edit at 09/16/2010 06:48AM by pica.

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--
My blog's Reprap feed: [blog.markbova.com]
I'm currently working on a stock Mendel build with a Seeeduino Mega and four Pololu A4983 stepper controllers.

As an aside, the bicycle pedals are an interesting case - the left pedal turns clockwise relative to the crank, which would tend to loosen a left hand thread in a friction only situation, but due to precession from the rotating moment load, it actually tightens a left hand thread. I've used that same precession trick to assembly tiny nuts inside of watches where I could only touch the top of the nut with the end of a tweezer - gently spinning the nut the wrong way would precess the nut onto the end of the bolt.

My extruder runs backwards and forwards much faster for the the ooze control than the general extruding (100 to 200 mm/s vs 32 mm/s for regular extruding), so it has to lock in both directions.

I also had the main crankshaft bolt on a Mitsubishi fail on me once; the main pulley fell right off and I almost lost the timing gear too. Took me a week of work to get the broken bolt stub out of the crankshaft, which was complicated by a broken hand I sustained while riding my bike into town for parts. I always wondered if it was a case of over tightening due to precession or something involving too tight alternator belts, but then I have something of a long history of bolt failures and Mitsubishi's.

In any event, make sure you don't over tighten it.

Wade