Replacement part for my Meade mount (3D printed)

[furrysocks2]

I've got a broken Meade mount sitting in the garage, as well as a tripod and controller. The bit that's broken is shown here:

I tried a JBWeld repair, but it failed in my hands.

 

Now that I've got a 3D printer (! :D) I thought I'd try replacing it. I'm not going to be putting the full weight of a scope on it... just a little webcam and CCTV lens for some alt-az live-stacking. So I think a 3D printed part should do fine.

 

I've got some freedom here to design a modified/custom part to suit, particularly wrt the mounting holes etc, but I've had a play with OpenSCAD and a pair of calipers tonight and come up with the following, fairly faithful reproduction of the original part:

 

In terms of "printability"... I could probably do away with most of the recess underneath, but I'll try printing a first tester with fairly sparse supports tomorrow. Not sure how the circlip slot will come out, or how I'm going to thread the top end.

Interesting to see if I come close, anyway.

 

[upahill]

What are you printing with? ABS? I have successfully tapped 100% infill parts but I wouldn't trust too much weight to it. Not sure on the parts usage, but is there scope to use a bolt for the thread thats encased within the part?

[furrysocks2]

6 hours ago, upahill said:

What are you printing with? ABS? I have successfully tapped 100% infill parts but I wouldn't trust too much weight to it. Not sure on the parts usage, but is there scope to use a bolt for the thread thats encased within the part?

All I have is PLA at the moment. I could put through-holes for M3 or M4 bolts.

[furrysocks2]

First test print went "ok". This is my first print of a custom part, so I'm learning. Really good for size, though - just print quality that's the issue really.

I hand-drilled the central hole out to 6mm to accept the knob shaft, hence it being off-centre.

 

 

0.4mm layer height, perhaps a bit much for the 0.4mm nozzle.

Regarding print settings, I think I will try:

• increase infill density from 25% to 50%
• increase infill overlap from 10% to 20%
• reduce printing temperature from 220 to 210
• increase support from 8% "lines" to 10% "grid"
• change support from baseplate to everywhere to support circlip groove
• decrease layer height from 0.4mm to 0.2mm
• reduce printer speed from 125% to 100%

I've a few tweaks to the model I think I will make:

• increase height of travel stop arc
• chamfer the top end
• thread the top end, at least as a starter
• broaden the flat at the top end
• increase diameter of hole in the top end to 6mm

I will try to fit a fan duct to the printer as well, to improve cooling, particularly when bridging though increased support and lower layer height may help with this. First layer adhesion was a bit iffy, so I'll level the bed again.

 

Main concern is the strength of the join where the top end joins with the middle section - ie exactly where the original aluminium part failed. Having the cutouts extend up into the top end is down to the shape of the three-lobed gear clamp that locates in the cutouts. By redesigning and printing that part with additional relief/taller clamp pads, I should be able to extend the top end down inside the top face of the middle part, which would give it a lot more strength, even if just by a couple of layers.

[cjdawson]

I bet you can center that hole better on your next print.

Did you print the hole as a 5mm hole, then use the drill to simply widen the existing hole?

[furrysocks2]

It should have been 6mm already in the model (and definitely already centered ) - but print came out pretty poor. I drilled it first to 5, then to 6. At least the top end didn't snap off, even though the drill snatched a couple of times and it wobbled a lot as I was accidentally drilling off-center. Hopefully a good indication of strength.

Working with OpenSCAD has been fun. Very like programming - my model probably needs a refactor.

[upahill]

Can you fit a small radius to the seam where it broke without it fouling on the connecting part?

I think 0.2 layers will help alot here. 0.4 is great for drafts though.

[furrysocks2]

Just now, upahill said:

Can you fit a small radius to the seam where it broke without it fouling on the connecting part?

I think 0.2 layers will help alot here. 0.4 is great for drafts though.

If I chamfer the central hole of the gear, yes. I tried to fillet the JBweld a little, but it still failed.

[John78]

So if you are using a 0.4mm nozzle you should try to stick to 0.1 - 0.2mm layers, 0.25mm if its something for quick dimension try out.

Tapping plastic works fine perpendicular to the layers, you need to use a tap in a tap wrench, not a tap in a battery drill.

For anything that requires repeat assembly/disassembly, adjusting, lots of torque or isn't perpendicular to the layers then I use M3x5x5 brass inserts.

[JamesF]

I'd have thought an engineering place should be able to repair the original part, assuming you can find one locally.

James

[furrysocks2]

5 minutes ago, JamesF said:

I'd have thought an engineering place should be able to repair the original part, assuming you can find one locally.

James

Aye, perhaps - if I can't make a printed part work, I know a guy. Good exercise in design though.

[JamesF]

1 hour ago, furrysocks2 said:

Good exercise in design though.

Absolutely.  It's a challenge to try to reproduce something you already have in a way that can be printed and work as intended.

James

[furrysocks2]

After a 4.5 hr print, the one on the left...

I didn't chamfer or thread the top end, wish I had because otherwise, this looks great. I've removed all the internal support, just need to gouge out and clean up the circlip groove support filaments, find the right die for the top if I can and give it a go.

[JamesF]

Looks like that's come out very nicely.  Fingers crossed it works.

James

[furrysocks2]

Managed to clean out the circlip groove with a junior hacksaw blade - there's clear space behind the support before I hit the top end proper, so easy enough to rasp it away.

Threading the top end was a bit fiddly. I had to chamfer the top, file down the outer diameter on the top 1/2" or so at the flats, then used the lock nut with four slots cut in it as a die. I had the nut in a vice and was turning the whole part to thread the end, so all the torque was going through the material at the three-lobed cutout - it held for what I needed anyway.

Assembled in the mount - I may have made the bottom of the flats slightly uneven and/or filed/threaded the end slightly off-axis, as the dial indicator doesn't sit perfectly flat but it's fine. There's a bit of play between the inner diameter of the gear wheel and the shaft, and perhaps I didn't rasp the circlip groove planar as it sits a little cocked, but it's operational. There don't appear to be any limit switches to limit alt travel, so the motors will keep turning probably resulting in the gear wheel slipping against the clamp, and I can see the whole assembly torque slightly, so easy answer is not to drive beyond travel limits.

Now, I make do with the screw holes as they are and try to mount something to it.

I'm intending on using either the Runcam Night Eagle 2 Pro Astro or my Datyon T7M, with one of a few small CCTV lenses I've got.

 

Notes for v3, if there ever is one:

• narrow the flats slightly - alt dial didn't fit, filed the flats slightly
• increase top end diameter below the circlip groove - gear wheel is a little loose fitting
• narrow circlip groove (away from end)
• thread top end - if possible, otherwise chamfer and reduce diameter at top to accept M16 fine (?) 
• increase 6mm hole diameter slightly - had to hand-ream to size
• tweak mounting holes - ribs, different holes, inserts, etc...??

[furrysocks2]

https://www.thingiverse.com/thing:3109502

[furrysocks2]