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Connecting the GM3 to a drive screw

Posted by Joshua Merchant 
Connecting the GM3 to a drive screw
August 03, 2008 05:14AM
Using non-RP parts, what's the best way to attach a GM3 to a M6 drive screw?
I'm not quite clear on the output shaft's shape, etc, and [www.solarbotics.com] seems to be experiencing some technical difficulties.

The five types of drive screws I'm considering are:
1) M6 studding, with no head (#93805A333)
2) M6 hex bolt (#93635A241 or #91287A346)
3) M6 machine screw, philips head (#92000A444)
4) M6 machine screw, slotted head (#91800A448)
5) M6 socket cap screw, hex socket (#92125A254)
[Note that the part numbers are for [www.mcmaster.com].]

If I was using RP parts, I'd probably go with the hex bolt (option 2) because I would make a plastic piece that coupled the hex head to the GM3 shaft. This appears to be the standard extruder's design. Alternately, I suppose I could somehow couple headless studding to the shaft (I don't quite know how, though). For the machine screws and socket cap screw, I figured I could somehow attach a screw driving bit (or similarly shaped object) to the motor and press it into the screw head.

I did however see some sort of hole in the output shaft in the pictures of the GM3 I've found, which, in the picture at [store.rrrf.org] has a screw in it (at least that's what it looks like when I click the picture to get a larger image). Is that what it is? Does this mean the output shaft has a threaded hole in the center?

[Edit: I'm gathering info before I actually go ahead and buy the motor because there are a lot of parts that depend on each other, and I'd like to not have to pay shipping twice from the same supplier in a short time period, so I want to sort the majority of it out before I start purchasing.]

[Edit2: At the moment, I'm thinking I'm most likely to get the hex bolt and make my own coupling between the motor's output shaft and the bolt out of wood or something. I'd rather not glue it so I can disassemble the extruder and replace parts, etc.]

Edited 2 time(s). Last edit at 08/03/2008 06:43AM by Joshua Merchant.
Re: Connecting the GM3 to a drive screw
August 03, 2008 11:40AM
You can buy a shaft coupler from the likes of McMaster that will couple two different-sized shafts like that generally using set screws. You'll want to cut a flat into the screw before using it first, or the set screw might slip.
Re: Connecting the GM3 to a drive screw
August 03, 2008 11:41AM
I would go with the hex bolt or put a hex head on all-thread with a hex nut and epoxy. Then to drive the hex head you can fabricate a coupling from a 1/4" drive socket and some epoxy putty.
Re: Connecting the GM3 to a drive screw
August 03, 2008 11:49AM
Yes, the shaft has a hole in it. Don't really know if it is threaded or not. It's just a hole. Would probably have trouble mounting with that.

I'm not exactly sure how you would couple those outside of rp parts or without a machine shop or something...I'll think about it.

Re: Connecting the GM3 to a drive screw
August 03, 2008 12:37PM
The bitsfrombytes design uses a small axial screw to hold their drive gear on. You could check their partlist?

When fitting grubscrews onto the shaft radially, it is good to file a flat. As mentioned above, this helps grip, but also helps you when you want to remoce the pulley/coupler. The tip of a grubscrew raises a burr on the shaft. Without a flat this burr will snag in the bore of the precision fit bore ! Much swearing can ensue.
Re: Connecting the GM3 to a drive screw
August 03, 2008 01:27PM
Joshua et al,

If the solarbotics site is still down, the .pdf files with the dimensions are named solarbotics_gm3_metric.pdf and solarbotics_gm3_imperial.pdf -- Google has these cached, and they're on sourceforge as well: [reprap.svn.sourceforge.net]

FYI, Nophead posted the design of his GM3 coupler (27 October 2007
Extruder dimensions) in [hydraraptor.blogspot.com]

His design mates to a hex-head bolt; very nice, but the hexagonal pocket is hard to make without CNC milling. Bruce W. has an almost identical coupling, though I *think* he's using a 10-24 UNC screw instead of M6. It seems to me that this coupling can be simplified, so it can be made with only a non-CNC mill. I think a slot to accept the GM3's flatted shaft will be more robust than trusting that little, axial screw (thread not specified in the drawings either.) Some accomodation for mis-alignment is a good idea; it might as well be at the GM3's shaft. Bruce's part measured ten thou wider 0.193 inches than the drawing's imperial dim for flat-to-flat width. Though this adds slightly to the backlash in the drive system. Oh well, if we wait for perfection, we'll be waiting a long time....

However, if there is sufficient accomodation (of misalignment) on the motor side, then one could attach the coupling *rigidly* to the extruder screw, by tapping the non-slotted end of the coupling for the M6 studding (or whatever thread you're using.) This is basically what I'm going to try, as soon as my GM3s arrive. Note that (assuming a right-hand thread on the extruder screw) it'll be turning counterclockwise (looking in the direction from the motor to the nichrome'd section) to push the filament down the barrel. So, to keep the drivescrew from unthreading itself from the coupling, one would need to add a nut and tighten it *firmly* against the coupling. A second nut, tightened against the first, afterward is probably cheap insurance.

If I get this working, I'll post some pictures and dimensions.

-- Larry
Re: Connecting the GM3 to a drive screw
August 03, 2008 02:43PM
Lot's of interesting ideas.

I've decided that I'm not going to order any special parts for the coupling (yet, but I don't think I will eventually either), as this is a bootstrap extruder and I'm trying to make it reasonably simple.

I'll take a small piece of scrap (wood, probably) and cut it to a square cuboid (square prism). On one side I'll cut a [sloppy] rectangular slit for the GM3's output shaft, and on the other side I'll cut a square hole for the hex bolt, such that two sides of the hex bolt touch the sides of the hole. I'll probably do both holes using a drill and a jigsaw. The sloppiness of the process means that the play between the shafts and the coupling might lead to problematic amounts of backlash. If necessary, I may apply some sort of putty or wrap tape (or something) around the shafts to make them fit better.

Alternately, if, after trying to make such a single-piece coupling, I find that it is too much hassle/not feasible, I'll probably take 2 or 4 pieces, cut them individually, and then use wood glue or spare fasteners to connect them. This would probably be a bit easier (and allow me to make more accurate hole shapes for the shafts), but probably not as stable.

Once I get the extruder working with this makeshift shaft coupling, I'll probably make an RP replacement (I can't see why I wouldn't).

Thanks for all the input; it did help.
Re: Connecting the GM3 to a drive screw
August 03, 2008 04:36PM

the play between the shafts and the coupling might lead to problematic amounts of backlash

That shouldn't really be a problem in this case... the drive shaft only ever turns one way, and you can always use nophead's strategy of starting the extruder and wiping the nozzle before a build is actually begun to the drive has a chance to spin up and remove any 'slack' in the connection.

The main problem people have mentioned, I think, involves coupleing or bearing prblems that result in a varying speed of rotation of the drive screw.
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