Newbie- Seeking advice on linear motion components..

[Zouave]

So, a little background is in order...

I have not built a CNC machine before, I am very much a newbie in this field, so please bear with me if I ask what may be obvious questions to more experienced people. However, I love reading, so if there are extensive tutorials that you found particularly useful when starting out, please, please, PLEASE link me to them, it'll help me ask better questions in the future.

My project: A CNC router, cutting primarily wood or plastic, with a working surface area of just over 4' x 8'. I am strongly considering the MechMate design, but am not yet completely sold on the linear motion setup. I don't want to reinvent the wheel, I want some honest feedback on what people's experiences have been.

My considerations: I started out my adventures in a high school machine shop, so I have a bit of a fetish for being overly accurate. Sometimes I have to remind myself that I'm doing woodworking, not precision metalworking. That said, accuracy for what I'm doing will be fairly important, repeatability as well. Speed, as always, will be a major factor in my final decision, as will cost (isn't that always the biggest part?)...

My questions:

For a project of my size (8'x4' table), are there major drawbacks to any of the primary systems people are using for linear motion (ballscrews, leadscrews, rack & pinion, etc)?

For a fairly heavy gantry, as it will be primarily steel, in line with MechMate's design, are there specific bearing systems that I should avoid or gravitate towards for something of that length? (Vee-groove, supported shaft ball bearings, supported shaft plain bearings, linear rails/guide blocks, etc.) I am assuming that over an 8' length, unsupported linear bearings will have too much flex to be even considered for my application.

For a system that is highly repeatable, what drive systems work best, and are they worth the increase in cost? As I understand it, ground ball screws have the lowest backlash and depending on the quality, the highest accuracy, but are rather pricey, rack and pinion appears to be fairly repeatable, but not as accurate, and has more problems with backlash typically, and Acme style leadscrews are somewhere in the middle. Am I fairly on the ball in that regard?

Now, while I like to keep my shop clean, I am under no illusions that this machine will not accumulate its share of dust, grime and grit. I will be taking whatever precautions I can to prevent that, but if one of those systems breaks down significantly in that environment, that would be a consideration as well.

If you've made it this far, thank you very much for reading through my lengthy questions, and any help you can offer would be very much appreciated. Thanks!

(Back to my reading...)

[Jesse B]

Can't help too much not having built a machine yet myself, but I will say one thing. Avoid unsupported rails like the plague. You'll want either supported rails with pillow blocks, or some rails/trucks or similar.

Ballscrews tend to be the "best", but as you mentioned, are quite expensive.

One thing you have to decide is which is more important, accuracy or speed. Accuracy will lower your speed, while a higher speed will lower your accuracy (generally speaking). This makes choosing your lead screw, etc. a pretty vital decision.

Hope I helped a bit,


- Jesse

[jsheerin]

Calculations are your friend. You can really go anyway you want, but how you get to the final result you are shooting for will vary a bit. For example, you could use "unsupported" rails if they were big enough to give you an acceptable deflection with your gantry weight.

I'm currently working on designing a similar size router as an upgrade to my current one and will be using THK linear bearings, rack and pinion drives for the X and Y axes and a ball screw for the Z. Either of these methods can be made to work. There are several ways to eliminate backlash in a r&p - split pinions and spring loading the pinion against the rack are two typical ways. Also, you can buy racks that are very precise if you want to spend the money - same as with ball screws. The advantage of r&p is that you don't have whipping of the screw to contend with and you can get a better force / speed curve out of your motor depending on what the motor is. Of course you can do the same with a screw using some sort of reduction gear to get the same overall ratio, but I think the rack makes it a bit easier.

All these systems eventually break down in the presence of dirt. You should design shields to keep your linear motion components clean.

[Zouave]

Thanks for the comments and suggestions. Is there a particular reason that you chose THK as your supplier for the linear bearings? Cost/quality considerations?

[jsheerin]

I got a huge box of new, sealed in plastic and oil carriages for basically nothing at an industrial auction. So I just had to buy the rails. But they are known for being high quality and a bit pricey. Ultimately it will be up to you to evaluate the cost/performance tradeoff.