ballscrew vs rack advice

[eloid]

I planing on making a dual drive cnc router that can cut a clear 5 x 9 area
So the x axis will be almost be 6 feet wide with hiwin linear rail support.
The question is will 5/8 rolled ball screw have to much whip for 500 ipm rapids? I may not be cutting that fast but want quick homing movments.

Or would i be better to go with rack and pinion setup. The price of 3/4 ball screw is just to much $$$$$$$$$$$$$$.

The next question is what pa to 14 1/2 or 20? and which pitch 12 or 16
Im looking for wider face width (3/4x 3/4) so i need to stay with standard sized from stdsteel.com

[ger21]

If you look at the critical speed charts at Nook, you'll find that for a 5/8" screw, with double angular contact bearings at both ends, at 500ipm the maximum length without whipping is only about 36". http://www.nookindustries.com/ball/BallCharts.cfm

At those speeds, you need to look at 1 or 2 turn per inch screws, but those are probably expensive as well. If price is an issue, then rack and pinion is probably a better choice for you.

This .pdf should help answer your rack and pinion questions.

[dahui]

We have a 4x8 at work that has a rack on the X and Y. It seems to work reasonably well. It's used to cut molds for a vaccum form machine and normally runs with a .625x6 ball mill cutting Jelutong wood at 200ipm (75% depth and 75% stepover) and it works quite well. I was a bit skeptical about the thrust and positioning accuracy with dual drives and a rack, but it cuts perfect circles 3 feet in diameter.

[DaveDavies]

Hi Gerry,

Where would I find that pdf on rack and pinions?

Thanks

Dave

[MonoNeuron]

Have you thought of Timing/toothed belts. You get great rapids from them and zero maintenance. After using them on two of my machines I don't think I will ever go back to screw setups especially ballscrews because of the ingress of debris getting into them and being hard to clean. You just gear them down to what you want.
Rich.

[deerhunter]

Mono, what length is your x and y just wondering because of belt length

[ger21]

Originally Posted by DaveDavies Hi Gerry, Where would I find that pdf on rack and pinions? Thanks Dave

Oops.

http://bostongear.com/products/open/theory.html

[RICHARD ZASTROW]

Also, Martin Sprockets & Gears has 14 1/2 & 20 deg. pa racks & spur gears.

Dick Z

[MonoNeuron]

DeerHunter,
My longest length is 56" doubled in a continuous belt and 3/8 wide which is a fairly small belt in width. I plan on using 3/4" wide belts on my new plasma cutter table which will be 3 meters long but I will use open ended belts for it and attach them at both ends and have the Y axis motor on the X axis gantry. It's a simple matter to tension them at one end and to also align them which will eliminate racking of the Y axis. I am retro fitting another gantry router this way as I had no luck with a single ballscrew due to this problem and I won't have to lubricate anything either making for a cleaner surroundings.
I hope this helps you somewhat. I purchased everything from SDP/SI products as McMaster Carr don't ship overseas anymore to individuals. SDP/Si have a very easy to navigate site with a wealth of products at a good price which are also listed. I hate sites that don't have price lists and tend to move on to ones that have and I think a lot of companies miss out due to that fact.
Rich.
P.S. I may try and do a build of the plasma table but I still haven't finished off the build of the laser table yet.......but I will soon.......I hope. The belts are tensioned really tight like a drum and have no stretch in them and debris fall off them in use but in really dirty environments it always pays to cover them somewhat to stop premature wear of the drive gears.