Ballscrews or Rack and Pinion?

[freak_brain]

I'm very close to having to make this decision. I am making a 4'X8' plasma that may be used from time to time as a router. So now I'm wondering, do I use ballscrews or rack and pinion??? I know there are advantages either way so is money the only real consideration at this point? Ballscrews seem easier and their not that expensive. What about racks, how do they compare in price, how difficult are they to get on the machine correctly??

Thanks for any advice you may have
Allen James

[Al_The_Man]

8' is too long for a ball screw, it can be done but the dia. would be up there at that length to retain rigidity and cure whip etc. and precision ones are very expensive, a proper tension pressured rack & pinion would be the way to go and much cheaper, but due to the large travel/motor rev. some kind of reduction is required, either planetary gearbox or timing belt/pulley.
There are several post in these forums with examples.
Al

[Hi-Vac]

I also have a similar question...

Would a belt drive work in lue of a Rack & Pinion?

I was wondering the same question as "Freak Brain" and had decided that a ball screw for the 8' movemnet (on a 4 x 8 table) would create too much whip. instead of a Rack, I wanted to use a gear reduced drive motor with a belt drive. To eliminate "strech" or slippage, I was going to include an eight foot linear Encoder (Yes I know they are expensive) instead of a rotary Encoder.


Thanks...

[Al_The_Man]

I have used belt drives up to 8ft you need to use the high quality reinforced non-stretch belts, but alignment, support and tension is critical. The rack & pinion is usually the accepted way today by commercial tables, especially for plasma where working to really fine tolerances are not crucial. This method usually involves a low backlash planetary gearbox.
I would hate to see the cost of a 8' linear encoder.
Al

[freak_brain]

A non-precision 1" ballscrew (I assume rolled, not ground) can be purchase for around $250-$275 area with two ball nuts. Actually by the time it's all said and done would most likely be around $300. Thats with two nuts without flanges, you could save $25 or so if you only needed one nut. So, I have to wonder how much a person would save by using the alternative methods, rack and pinion or belt. Any idea?

-Allen

[Al_The_Man]

1" dia seems a bit skimpy for that length, If I was using a ball screw that length I would think at least 2"dia would be in order, especially if you are using anything like 500"/min or up for rapid.
Al

[freak_brain]

Al,

I called Allen at Roton and he told me that a 1" dia. screw for my needs would be fine. I'm figuring no more than 350"/min. and 8ft of travel. on my machine. Do you think that's bad advice? I'm kind of at the mercy of everyone else as I'm not very knowledgeable on this subject.

-Allen

[Al_The_Man]

For a .25" lead screw, thats 1400rpm at 350"/min, I would have thought whip would be a problem, maybe someone here who has used a long ballscrew will jump in.
Al

[balsaman]

Too long IMHO. I used 3/4" acme at 4" and it whiped at 1000 rpm.

Eic

[steveg]

Two possible solutions to ballscrew use are, either
use a fixed screw and rotating nut which makes screw
whip a complete non-issue or "borrow" Thermwood's
idea of spring loaded plastic dampers that press against
the screw at a couple of points and flip out of the
way when the carriage passes.

[freak_brain]

Kind of funny. I was setting at work tonight watching my machine run and I realized that my "X" axis screw doesn't rotate. I then remembered that I had talked about this before and I came up with the plan of belt driving the nut. (I'm sure I heard it here first)I like that idea better for "X" anyhow.

-Allen

[mwp]

Originally Posted by freak_brain Al, I called Allen at Roton and he told me that a 1" dia. screw for my needs would be fine. I'm figuring no more than 350"/min. and 8ft of travel. on my machine. Do you think that's bad advice? I'm kind of at the mercy of everyone else as I'm not very knowledgeable on this subject. -Allen

Hi, you might want to check out:

http://www.nookindustries.com/ball/BallCalculators.cfm

Especially look at the column strength calculator.

From what I can tell using a 1" diameter (~.8 root dia) screw is iffy in this situation no matter what type of end conditions or nut/screw rotation scheme you employ due to buckling danger. According to the calculator, shaft whip could also be a problem depending on what lead you choose.

So, if you are going to use a ball screw, you might consider a larger diameter one. Even the 1.5" dia ones might be pushing it if you go by the book, depending on how you fix the ends.

Another good resource for this sort of thing is the thomson ballscrew catalog (pgs 50&54 of the 2004 ed has charts that say the same thing as the nook calculators).

Good luck!

Mitchell