[igolfat8]

OK, Another noobie question. I am going to assume that the Acme is the lowest priced and least efficient of the three for linear transfer of energy? If so, please compare the rack-n-pinion to the ball screws. Which is smoother and more efficient? I am comparing a CNC9000 (ball screws) to a Keling (Acme lead screws) to a Shopbot Buddy (rack n pinion). The Buddy uses a moving table while the other two use a gantry style so this may dictate the need for R&P? Please enlighten me. Thanks...

[txcowdog]

It would take a considerable amount of time to give you all the info you are asking for. I would suggest you get a cup of coffee, sit down, and spend a couple of hours searching the board for those terms. You will get a thorough education far beyond what I could give you in a paragraph or two.

[LazyMan]

I would suggest the same as txcowdog but heres some VERY basic information.

Ballscrew is generally the "best" if you want maximum performance and have an unlimited budget, big ballscrews come with a BIG cost.

Rack&Pinion seems to be the most common on large format commercial routers in the 4x8 foot. Id say if you are planning on cutting mainly woods and some plastic with a large machine, rack&pinion is the way to go and as the most cost effective solution.

Theres no reasons Acme cant be used but I wouldnt use it on a larger machine.

[igolfat8]

I did a search for "rack and pinion" before posting this thread and did not get any hits in the thread titles. I will try again though. Thanks.

[harryn]

Hi - try searching for "rack" in a couple of areas
- wood working machines
- Engineering / linear motion
- build logs

And then of couse there are belt drive based systems.

There is also a useful forum on a design called "mechmate". It is almost pure rack and pinion based and has some nice short summaries on what / why / how that is used.

Just to help you get started a bit more (being a newbie myself) it will ultimately come down to a few decision areas, but try to make your decision based on the "system" not just on the price of a few of the parts. The precision you achieve in the end is the result of both the weakest link in the chain, and an accumulation of tolerances.

For a wood working machine, I am convinced that ANY of the approaches will work, just read a bunch of build logs to see the details.

Just a silly suggestion, but perhaps one way is to start a thread in the "build log" section. Write down not what machine you want to build, but what you want to be able to build with the machine, and equally important, what you don't plan to build with it. (wood work and steel are very different ) Also post what tools (hand tools only, table saw, etc) you have access to, and some examples of things you have built in the past. This will help a lot with providing guidance.

[igolfat8]

Thanks Harry. Great advice and well received.

[DonW]

Hi igolfat8,

Of course the obvious answers are that the ballscrews are the best, and then followed by R&P and Acme. Interesting, a friend of mine is also questioning all the same things, and similar machines right now. Hmmm.... ;-)
I have a luthier friend who has a ShopBot and it works very well, and I believe it uses R&P. Another luthier friend has a system built with ballscrews, and it works great. I think the two of those get you the smoothest of motion from what I've seen.

[ger21]

Why is R & P better than acme? Wouldn't acme be more precise? Acme will usually give you more resolution than rack and pinion, and multiple start acme can have up to about 75% efficiency. And it's easier to install, imo.

[hybidder]

Gerry,

I was wondering the same thing.

Harryn mentioned belt drives. What about a moderately high tension cable drive? I have been thinking about building a plasma table with a cable drive using drums at each end with multiple cable wraps. It might be the cheapest and simplest to setup and if done properly I would think it would give sufficient accuracy for a plasma table but certainly not enough for something like a milling machine or critical router work.


Just a thought...

[DonW]

I didn't say "better" I merely said smoother motion. Acme may be more accurate, although I doubt ShopBot would be using R&P if it wasn't accurate.

[ger21]

You said one was "best", which implies "better" than the others. And I'd have to argue that acme would be smoother than rack and pinion. No comment on the cable drive, but it seems too complex for me.

[BobF]

I am no expert, but from what I have seen R&P seems to be a choice when there is a problem with screw whip.

[haroldj]

I like the idea of cable drive. After all, very precise accurate movement is achieved with scanners and copiers and they both use cable drives to move the assy.
Just my opinion, replies welcome.

[harryn]

I like the idea of cable drive. After all, very precise accurate movement is achieved with scanners and copiers and they both use cable drives to move the assy.
Just my opinion, replies welcome.

Most copiers and scanners have quite different loads that a router and relatively little impact / vibration. They also often include self correction or at least a regular re-zero. That is not that common on routers, but could be done.

[hybidder]

Most copiers and scanners have quite different loads that a router and relatively little impact / vibration. They also often include self correction or at least a regular re-zero. That is not that common on routers, but could be done.

"impact" would be an issue with any drive type but as we already know it's more of programming/ operator issue. That being said, I'll confess I have slammed an endmill into vice in the past.

"vibration" would be addressed by the guiderail/ frame system and not the method of locomotion.

I don't have enough spare parts in my stash (yet) but I think a simple single axis protoype could be set up on top of miiling machine table. A stepper motor and driver, couple of timing pullies with the belt, two drum type pullies to allow multiple cable wraps and a linear guide with a dial indicator to check for repeatability.

If I can scrounge up some eBay bargains I think I'll give it a shot. It might be a waste of time but at least it will keep me off the streets....

[haroldj]

Hybidder, if you are willing to do the work, I can probably help with most of the parts, I have cable, pulleys and could even come up with the drum to fit the stepper motor for the drive.(Lots of copier parts:-), let me know what you need.

[hybidder]

Thanks Haroldj,

I think I might actually get pretty close to having what it would take to at least test the concept. I can unmount my Z axis stepper on my milling machine and use my mill control software. I have an old Stanley automatic sliding door operator (think Home Depot entrance) that I can use as a track/ guide. I think for trial purposes I'll make a drum type pulley to mount directly on the stepper shaft instead of setting it up with timing belt reduction to see how "repeatable" it is and if it works out we would know accuracy could always be increased with the timing belt reduction while at the same time increasing load capacity.