Questions about ACME screw assembly

[Albrow]

I've been having a great time learning about DIY CNC on this site and others. Mostly this site. It's a truly impressive community and knowledge store.

Now that I've been bitten by the bug I'm planning on building my first machine. It will be a JGRO but with upgraded screws. It seems that so many people immediately upgrade from threaded rod to ACME screws because the advantages are huge, even in a basic 'entry level' machine. And I'd like to choose the other drive components to maximize the potential of the design. I'd also like components that can be reused when/if I build my second CNC. That also seems to be a fairly common occurrence.

One area that is hazy to me is ACME screw attachments - nuts, spacers, bearings - needed for the different types of end fixity (never encountered that word before I came here). I get that there are a few issues to deal with that determine the choice of components: whip, side loading of the screw, preventing screw from transmitting forces to the motor bearings. So what's the recommended way to go with a JGRO? In fixity terms is it SIMPLE-SIMPLE or SIMPLE-FIXED? The fixed end seems to be nothing more than two bearings spaced twice the diameter of the drive screw apart. Is that overkill for a JGRO?

For keeping the screw from loading the motor bearings it seems like you need a nut-spacer-bearing-spacer-nut combination. The nuts are fixed to the screws and 'capture' the bearing. So, as long as the motor and the bearing are attached to fixed members of the frame they can't move relative to each other. Is this the right idea? I've seen quite a few pictures and read quite a few descriptions of screw assemblies, but there always seems to be variations: thrust bearings, angular thrust bearings, etc. I'm guessing that is to deal with non-axial loads placed on the screw? I'm still looking for diagrams that show all of the components relative to each other. I'm a picture guy so that would help tremendously. One vendor of components had an exploded drive diagram that actually created more questions than it answered!

So, I guess I don't know a whole bunch of stuff! That's par for the course! But I do know that I want 1/2 inch 8 tpi 4-start or 10 tpi 5-start screws. I think I'll be using flanged anti-backlash nuts to attach to the X/Y/Z axes. Do I need to tension the screw? What does that accomplish?

I'm also pretty good with woodworking, have a router, table saw, drill press, and have been doing pattern routing for years so the table construction iseems pretty straightforward.

So, can anyone deliver some elightenment to this perplexed newbie? I'd like to be to experiment with various pieces, but as money is an issue, I don't really have the luxury of buying things only to not use them. I'm sure many of you have been there...

[ger21]

In fixity terms is it SIMPLE-SIMPLE or SIMPLE-FIXED? The fixed end seems to be nothing more than two bearings spaced twice the diameter of the drive screw apart. Is that overkill for a JGRO?

Where are you getting your terminology from? Nook calls a fixed and a rigidly mounted end, allowing no rotation.

For keeping the screw from loading the motor bearings it seems like you need a nut-spacer-bearing-spacer-nut combination. The nuts are fixed to the screws and 'capture' the bearing. So, as long as the motor and the bearing are attached to fixed members of the frame they can't move relative to each other. Is this the right idea?

Basically, yes. But it's more common to use two bearings, one on each side of a fixed mount. This is because angular contact and tapered roller bearings can only handle thrust in one direction, so two bearings are needed.

but there always seems to be variations: thrust bearings, angular thrust bearings, etc. I'm guessing that is to deal with non-axial loads placed on the screw?

Actually, they are to deal with axial loads. Which are loads along the length of the screw.

This .pdf has some exploded views that show the proper (but very expensive) way to do it. http://www.nookindustries.com/pdf/NookEndsmounts.pdf

Loads on a JGRO and similar are rather light, and you can actually get away with using skate bearings instead of angular contact bearings. This is how I did mine (not a JGRO, though) http://www.cnczone.com/forums/diy-cn...s_without.html

Be aware that you can't use the nut - bearing - bearing - nut method on a multi start screws, as multi start acme nuts won't tighten. So most people use some type of clamp on each side of the bearings.

This image uses needle bearings, which can't handle radial loads, so the additional radial bearing is needed.
CNCRouterParts
Notice the clamp on collar which holds everything tight to the bearing block.

[jckstrthmghty]

Something from my current experiences with the JGRO build and power transmission.

In reality you have only ~17 x 32" of movement on the y and x so you don't have to be so picky with leadscrews. Either ones you stated are plenty provded your electronics are ok.

You should take a look at dumpster for acme threaded locking collars. I can tell you from experience they are excellent. Far better than any standard nut can be. While you are there look at the couplers as well. Once again far better than any set screw could ever be.

So lets consider the build. Difficult to square. Plenty of deflection. I almost feel I could've skipped the support bearings for the leadscrews and used only thrust bearing with the limited cutting speeds. Ofcourse it's important to keep your tolerances as tight as possible and you don't want forces acting on your motor bearings.

Since you are experienced with tools consider the joes2006 build. Like the JGRO the plan are free for personal use. Supported rails are the way to go. Probably 3x as hard to construct but the end product would be much better. Don't get me wrong. My JGRO is cool and was a great path into CNC but I knew halfway into the build that the main purpose of my JGRO was to build a better machine.

[Albrow]

Where are you getting your terminology from? Nook calls a fixed and a rigidly mounted end, allowing no rotation.

Interesting. The site cncroutersource.com says that fixed is either a fixed screw with rotating nut as you describe or two bearing supports at least 1.5 times the diameter of the leadscrew apart. The rockfordballscrew.com site describes fixed as "spaced support points" but doesn't say that the screw is not rotatable.

I'm going to check out the Nook Industries link you gave, though I won't be using that arrangement if it is expensive. This is a budget project where possible and advisable. But I do like to know how these things work.

Thanks for the guidance and other links as well. I have some more studying to do!

[Albrow]

You should take a look at dumpster for acme threaded locking collars. I can tell you from experience they are excellent. Far better than any standard nut can be. While you are there look at the couplers as well. Once again far better than any set screw could ever be.

So lets consider the build. Difficult to square. Plenty of deflection. I almost feel I could've skipped the support bearings for the leadscrews and used only thrust bearing with the limited cutting speeds. Ofcourse it's important to keep your tolerances as tight as possible and you don't want forces acting on your motor bearings.

Since you are experienced with tools consider the joes2006 build. Like the JGRO the plan are free for personal use. Supported rails are the way to go. Probably 3x as hard to construct but the end product would be much better. Don't get me wrong. My JGRO is cool and was a great path into CNC but I knew halfway into the build that the main purpose of my JGRO was to build a better machine.

I've read many recommendations for dumpster...

You know, I considered doing a joes2006 by hand, almost decided on it, but then found a build log from someone who did a hand build and ended up having alignment problems here and there. He corrected them, and the end product was quite nice, but it helped me reach the conclusion that the whole process would be more labor intensive than I wanted it to be. So I am taking the approach that many others have - build a JGRO in order to build a Joes2006.

Will I be able to reuse the bearings and clamps? I figure I can turn the JGRO X and Y screws into a Joes2006 Y and Z, and get away with buying a new X. For steppers, drivers, and power supply I'm thinking of buying one of the G540 based kits with 380 oz. in steppers. They seem to be highly recommended.

[jckstrthmghty]

I've read many recommendations for dumpster...

You know, I considered doing a joes2006 by hand, almost decided on it, but then found a build log from someone who did a hand build and ended up having alignment problems here and there. He corrected them, and the end product was quite nice, but it helped me reach the conclusion that the whole process would be more labor intensive than I wanted it to be. So I am taking the approach that many others have - build a JGRO in order to build a Joes2006.

Will I be able to reuse the bearings and clamps? I figure I can turn the JGRO X and Y screws into a Joes2006 Y and Z, and get away with buying a new X. For steppers, drivers, and power supply I'm thinking of buying one of the G540 based kits with 380 oz. in steppers. They seem to be highly recommended.

It's generally my plan as well. Build a joes2006 with my jgro. Something to be aware of. The jgro cutting bed is too small to cut some of the parts in one pass. Fortunately those pieces are the easier to cut on a table saw or cut in two phases on the jgro.

Bearing yes but you'll need a few extra. You might need to make new assemblies though. Good plan with using the x and y for the joes y and z. You'll need more leadscrew for the x. More gas pipe as well.

I use the g540 with the 380 stepper and 48v 7.3a power supply for keling. Far more power and speed than the jgro requires. Quite happy with that part of the setup.

I know it seems I'm knocking the jgro design but the truth is for many this is the only way to start. All the parts can be made with simple hand tools and the cost is reasonable. Having a blast with mine.

Why is it everyone wants me to make the custom coasters?

Good luck with your build. Looking forward to seeing your progress.

[Albrow]

Basically, yes. But it's more common to use two bearings, one on each side of a fixed mount. This is because angular contact and tapered roller bearings can only handle thrust in one direction, so two bearings are needed.

Loads on a JGRO and similar are rather light, and you can actually get away with using skate bearings instead of angular contact bearings. This is how I did mine (not a JGRO, though) http://www.cnczone.com/forums/diy-cn...s_without.html

Be aware that you can't use the nut - bearing - bearing - nut method on a multi start screws, as multi start acme nuts won't tighten. So most people use some type of clamp on each side of the bearings.

Your method of turning the ACME screw ends is really clever. Nice job!

So the multi-start acme nut won't tighten because at that thread angle (relative to the axis of the screw) you just can't load the nut enough to lock the threads together by friction? So acme nuts are only used when the nut is to "travel" on the screw? I thought that anti-backlash nuts were indicated in almost all cases?

[ger21]

Multi start screws can be backdriven, so no, you can't tighten a nut on them. As far as I know, they don't even make regular nuts for multistart screws, other than leadnuts (which aren't always anti backlash, depending on the application).