Rack & Pinion X Axis Slipping/Binding on Big Used Mystery Machine

[dgolden]

Since everything seemed to be mechanically sorted out after the last post, I was gearing up for a little test to figure out settings for a good surface finish last weekend, and had a full Z-Axis crash during a text v-carve pass. Something broke, and the whole assembly fell down like a rock. Luckily I was watching it, e-stop in hand, and caught it immediately.

Image: Crash Result

Turns out what had happened was the threads had completely broken out of a (brass??) reducer which the lead screw goes through, which was screwed into an aluminum block that bolts with a plastic spacer directly onto the X-Axis bearing plate assembly or whatever you call it. Turns out this has happened before, as there were a bunch of those reducers included with the various surplus hardware/junk that came with the machine, and at least 4-5 of them where the threads were stripped out in exactly the same way....

Image: Trashed Threads

After taking the whole thing apart it seems like a pretty sketchy setup. The motor connects directly with the screw by means of an aluminum coupling with 2 pairs of 2 set screws, 1 pair for the motor shaft, and one pair to tighten the coupling onto the screw, which is somewhat messily ground down to make flat-ish spots for the screws to dig into. At the time of disassembly there was only 1 set screw present on the screw side, which put the screw off center, since it's a 3/8" screw in a 1/2" coupling going to a 1/2" motor shaft.

Image: Motor-Screw Coupling

Is there a better type of setup I could put together relatively simply with the same overall configuration of the machine? Coherent mentioned oldham couplings for the motor to screw connection, and that seems like a good idea. I'm guessing I could get a new lead screw, since this one is worn and slightly bent anyway, and use the clamping type hubs as shown here https://www.mcmaster.com/oldham-couplings without having to grind the screw down?

What about the reducer/block thing. Is there a better way of doing that? Or is using a static threaded brass block to keep a screw holding a 40lb Z assembly and router in place a common practice? It just makes me really nervous about leaving the machine unattended, especially for long 3D passes, since I know this failure has happened multiple times in the past.

[he1957]

Acme screw for the Z axis is not all that conducive to longevity and accuracy. Have a look at ready made/assembled ball screw Z axis assemblies (or kits). The photos don't really give a good perspective on sizes and so forth but there are a lot to choose from and it should be fairly easy to attach a ready made one to your gantry.


Cheers,

HarryE.
===

[ger21]

There's nothing wrong with using an acme screw for the Z axis, if done properly.

The proper way to install the screw is to have it machined for bearings, and a support block that holds the bearings support the weight of the Z axis via the screw. Retrofitting would likely require custom machined parts, so a ballscrew set as mentioned above may be a cheaper option, if it will fit.

[skrubol]

The brass acme nut is an acceptable way to do things. Just looks like the screw/nut combo is undersized for the job.
Everything else leaves something to be desired though. That spacer and coupler are not the right way to do things..
The coupler should have a slip fit on the shaft. As mentioned above, the screw should have the threads machined off where it goes into the coupler, but that'd make it even smaller.
I wouldn't bother with an Oldham for that screw. You really need a better screw, either large acme or cheap ball screw.
One plus of oldham and lovejoy couplers, because they're 3 pieces, it's easier to get a set that will fit both shafts (rather than a rigid or spiral coupler that would have to have 2 different diameters bored.)

[dgolden]

Acme screw for the Z axis is not all that conducive to longevity and accuracy. Have a look at ready made/assembled ball screw Z axis assemblies (or kits). The photos don't really give a good perspective on sizes and so forth but there are a lot to choose from and it should be fairly easy to attach a ready made one to your gantry.

Thanks for the idea. I think this is something I definitely want to do down the road. The ones I'm seeing are a little expensive for right now though, in the $300-$500 range for decent looking ones. Maybe I don't know where to look though. I think I am going to modify the machine eventually to raise the gantry to get a clearance over the current 4". I think that would be a good time to get a new ball screw Z-axis, since I can make sure it will accommodate the new height.

There's nothing wrong with using an acme screw for the Z axis, if done properly. The proper way to install the screw is to have it machined for bearings, and a support block that holds the bearings support the weight of the Z axis via the screw. Retrofitting would likely require custom machined parts, so a ballscrew set as mentioned above may be a cheaper option, if it will fit.

The way it is now does seem to be a little weird, since I think the weight of the whole assembly is basically supported on the motor shaft directly, and held up on the linear bearings by the threaded block/flange. If I'm understanding you correctly, the motor shaft wouldn't really have any weight on it, is that right?

The brass acme nut is an acceptable way to do things. Just looks like the screw/nut combo is undersized for the job.
Everything else leaves something to be desired though. That spacer and coupler are not the right way to do things..
The coupler should have a slip fit on the shaft. As mentioned above, the screw should have the threads machined off where it goes into the coupler, but that'd make it even smaller.
I wouldn't bother with an Oldham for that screw. You really need a better screw, either large acme or cheap ball screw.
One plus of oldham and lovejoy couplers, because they're 3 pieces, it's easier to get a set that will fit both shafts (rather than a rigid or spiral coupler that would have to have 2 different diameters bored.)

For the time being, do you think it would make sense to get a bigger diameter screw, like 1/2" or 5/8" and machine down the threads to fit maybe a 3/8" oldham hub, and connect that with the nylon or acetal disk to a 1/2" oldham hub for the motor shaft, and then drill out and tap the aluminum block for a 1/2" or 5/8" acme nut instead of the smaller ones I have now that seem to strip out? Seems like I could do that for under $100 if I buy a 1018 carbon steel screw.

[bhurts]

Try something like this for 100$ + whatever shipping is. The next length up is 120.

https://www.automationtechnologiesin...ers-duplicate/


Ben


Sent from my E6810 using Tapatalk

[dgolden]

Try something like this for 100$ + whatever shipping is. The next length up is 120.
https://www.automationtechnologiesin...ers-duplicate/

That looks pretty decent. I guess I would have to do some modifications to get that to work because of the way it's set up now, but it wouldn't be too bad probably. About as easy as attaching a new Z axis, and quite a bit cheaper.

I started learning Fusion 360 recently, so I'm hoping to create a fairly accurate model of the mechanical aspects of the machine before I do any real modifications. That way it will be easier to show things, and possibly make custom parts in the future.

[skrubol]

Rolled 1605 ball screws with machining and mounts are pretty cheap. If you can make them work (will require at least significant modification of the nut mount,) it would be the better way to go in the long-run.
The motor shouldn't have much axial load on it. The bearings aren't built for it, it will wear them out and they may have some play. Even a cheap ball screw kit usually will have a mount with a couple AC bearings in it (though getting the preload right can be a challenge.)

[dgolden]

Rolled 1605 ball screws with machining and mounts are pretty cheap. If you can make them work (will require at least significant modification of the nut mount,) it would be the better way to go in the long-run.
The motor shouldn't have much axial load on it. The bearings aren't built for it, it will wear them out and they may have some play. Even a cheap ball screw kit usually will have a mount with a couple AC bearings in it (though getting the preload right can be a challenge.)

Yeah I'm definitely going to have to figure something out to get a ball screw system to work and take load off the motor, and be more accurate; but for now I decided to be lame and just buy a new screw and coupling and see how it goes for a little while. I haven't gotten to actually make anything finished yet, and I would rather prioritize doing some projects for a little while and get the basics in place, and hopefully with a new nut and screw it will run for a while without crashing. I'm just not confident enough yet to make real modifications. I do appreciate all the advice though, and will definitely be using this thread as a resource as I go.

But speaking of crashing, one of the last things I need to get set up are the limit switches. I just feel safer with actual switches as well as soft limits since I'm going to let people other than myself use the machine for their own projects. I think I want to re-do the wiring for all of them, since they were previously using those little plastic microswitches that were pretty much dry rotted to the point that they basically explode under the pressure of their own springs when you touch them, and the more heavy duty ones I got won't fit in the same spots.

Now, sadly I know nothing of electronics yet, though I'm trying to learn. I found this diagram for my C10 R10 breakout board that shows where the limit switches and e-stop can hook up. I got the e-stop working last time I was at the shop, and while I thought it would be just as simple to re-wire the limit switches in series I took another look at the board, and had to stop when I saw this configuration which, instead of having one end of the series start at the pin labeled 5V and the other end come back to pin 12, there's some kind of soldered component thing bridging those two ports, and only one wire it looks like going to pin 12.

This seemed a little weird to me, but of course I know nothing about it. Is there a reason this is different than in the diagram? Can I safely remove it, and wire it the way shown in the pdf, or is it there to compensate for something? If so, where does the wire from the other end of the loop come in? Is this something to do with having a normally opened configuration before, whereas I want to use normally closed? I just don't want to do anything to screw up the board, since the electronics is the area I know the least about currently (which is saying something...).

Thanks again to everyone who drops by to help out. I hope to be able to pay it forward and help people out in the future when I have some more knowledge and experience with this stuff.