Rack-n-pinion vs ball screws vs Acme

[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.