securing ballscrew bearing / pictures of my homebuilt cnc

[RICHARD ZASTROW]

Colin, The couplings in your post #12 will absorb shock, compensate for minor misalignment etc. They will also contribute to lost motion. They may work fine for a router but you said you wanted to design for .0001" tol. in small metal parts.

We've identified .0001-.0008" in bearing end play. The coupling will contribute (minutely) more "slop" and we have not yet determined accuracy of screw/nut. This is only one axis. What happens when we accumulate the error potential of 3 or more axis?

If you really want to hold .0001"'s tolerances you must plan for it. There's always the "law of compensating variables". I'd bet on Murphy's Law, "whatever can go wrong will".

BTW, did you ever find i.d. info for the screws/nuts. We might be able determine the error potential there as well. C0-C7 ?

Keep working at it.

Dick Z

[guru_florida]

I was just rereading this thread to go over some of what Richard was saying again. I realized I typed .0001 tolerance when what I meant was a thousands'th of an inch, 1mil or 0.001! Thats a big difference when you are talking that small a tolerance!

It's been a long wait on ebay for the right ballscrews but I finally have a set of C3 ball screws coming with the bearings already present. These should be what I am looking for.

Richard: If you are still around, what makes those flex couplers I got susceptible to lost motion? I assume its the flex part will compress under load? I thought those were metal couplers when they turned out to be cheap plastic ones! Sometimes you get burned with ebay. I should have been smarter there.

I saw a metal flex coupler on another ball screw I got for the z-axis. The idiot who took it out didnt take the time to remove it and instead just ripped the motor off the coupler so it was damaged, but the idea was it was two flat plates and some thin ribbon like metal strips attaching the plates and the motor end and the screw end. I can see how it would flex in the misalignment axis while staying rigid in the rotational axis.

[RICHARD ZASTROW]

Colin, Yes, I'm still here, the grim reaper hasn't got me yet.

Even to attain a .001" level, you must consider the error accumulation of all components.

The flexible coupling will compress to some degree. The more force applied the more it will compress. Granted it may be a minuscule amount and acceptable.

How much lead error and backlash are in the leadscrew/nut combination? Again, it may be an acceptable level.

Is the geometry of the machine in question in alignment within acceptable error limits?

Add these together and Murphy's Law say it will all go wrong at the same time. Fortunately, there is also the Law of Compensating Variables where the errors will cancel each other out. LOL

The equipment I work with is unforgiving and requires extremes in accurate components so I tend to be over critical. Sorry 'bout that.

I'd still recommend the metal bellows type coupling. Actually, a good belt drive might be better than a direct coupling. I've had bad experience with direct coupling with very small misalignments causing motor bearing failures and encoder problems from vibration transmitted thru the servomotors. Problem went away when direct coupling was replaced with a belt drive.

Good Luck

Dick Z

[Pres]

I'll 2nd the Dick Z observations.

Belt drive has a number of advantages: backlash, easier inertia/resonance mods, can be more compact, and adjustable ratios.

The attainment of 0.001" on home machines is attainable, but difficult and tedious.

Getting 0.0001" borders on ridiculous and is, for all practical purpses, unattainable on home machines.

By accident, maybe on one surface of one part.

Nice to think about achieving it however.

Pres

[guru_florida]

Originally Posted by RICHARD ZASTROW Even to attain a .001" level, you must consider the error accumulation of all components.

Thats what I am concerned with now. Actually making the numbers work. I have the table working now with temporary parts and I have spent some time learning different software tool chains. Everything from design cad, controller software, conversion utilities, etc, and using this to cut some things such as isolation routing a pcb board, some wood cuts and very light metal. It has worked great so far, even with a bad temp Z-axis. Now that I've actually seen cnc in action, I feel like I can begin to understand the design caveats. So I am back to the drawing board to complete a rigid z-axis and complete the base.

The flexible coupling will compress to some degree. The more force applied the more it will compress. Granted it may be a minuscule amount and acceptable.

I will look into this more.

How much lead error and backlash are in the leadscrew/nut combination? Again, it may be an acceptable level.

This is the problem with ebay, hard to get specs. I just about freaked right now. I was checking the part number on my NSK JAPAN 92A4 W1406FS - 1 - C3T's that I have on order and never noticed the T typically meaning Transport. I must have gotten caught up in the excitement with finding a set of rare C3's on ebay (damn). A google search finds an axial play of 0.005. Luckily this is mm not inches so its <0.0002.

I dont know the bearing part# but being C3 I hope the ball bearing they already contain are from the equip and likely thus the right match.

Is the geometry of the machine in question in alignment within acceptable error limits?

It appears to be in good alignment now and I've put little to no effort into proper assembly alignment, must be that Law of Compensating Variables working for me, lol. I have however put a lot of work into the cad design and the assembly mates. I use SolidWorks and it complains if mating doesnt match with even the smallest misalignment. My SolidWorks model rebuilds well even with a deep change. What I mean to say is I think my patience with the cad design has helped my final assembly in keeping parts in agreement. (kudos to cad.)

I have oversized certain holes appropriately to take up small misalignment with the ballscrew/nut combo mounts. Yes, I'm counting on the Law of Compensating Variables here but I am anticipating a little Murphys law since some of my parts are used and from ebay! During final assembly I will use the micrometer and a pre-defined order of assembly ensure alignment.

Am I on the right track here?

Add these together and Murphy's Law say it will all go wrong at the same time. Fortunately, there is also the Law of Compensating Variables where the errors will cancel each other out. LOL

I initially thought Law of Compensating Variables was a joke, though I understood the concept. Google can only find this reference, "...Makes just about as much sense as the law of compensating variables, aka: it all works out in the end." (it also finds this thread!)

The equipment I work with is unforgiving and requires extremes in accurate components so I tend to be over critical. Sorry 'bout that.

Thats why I liked this thread, you're a man in the know. Please dont apologize.

I'd still recommend the metal bellows type coupling. Actually, a good belt drive might be better than a direct coupling. I've had bad experience with direct coupling with very small misalignments causing motor bearing failures and encoder problems from vibration transmitted thru the servomotors. Problem went away when direct coupling was replaced with a belt drive.

Though I knew belt drives were good, I didn't know they could compete with direct drive with proper couplers. I figured I would upgrade to servos instead of steppers in the future, perhaps I will switch to belts then.

Thank you,
Colin

[RICHARD ZASTROW]

Colin, Keep trying for .001". Realistically, as Pres stated, a homebuilt DIY machine made with extrusions and without the proper equipment is not going to get you there. You should do your best but don't be discouraged if you can't reach that target.

It's like that old adage, "speed costs money. How fast do you want to go?". In this case it's more like "accuracy and precision are expensive and time consuming. How much time and money do you want to spend?"

One of my "cohorts in crime" and I undertook a project. (his idea) This was a (6) axis CNC gear hobbing machine. The first machine took 3 years to design; produce and purchase proper components; build, test and tweak the first machine. It was and still is more accurate and faster than any machine of it's type. A number were built and are all in one place successfully producing gears commercially. Of course, it took twice as long and cost twice as much to produce as originally planned. I'm working with him again on other projects and loving it. LOL

Keep on truckin'

Dick Z

[guru_florida]

Hey, Im pretty excited with what I can do with it now and Im at about 1mm accuracy!!! hehe. (due to z-axis, not the table.) Even 10mil would be good for 90% of what I want to do --- mostly robotics, prototype pcbs, and perhaps a few moulds for custom electronic boxes.