Rack & Pinion vs Lead Screw

[E-Stop]

We are finalizing the design (for the umpteenth time) for an 8' x 4' CNC router. All of the previous designs used a 5/8 or 3/4 inch diameter lead screw to drive the "X" & "Y" axis'. Now we are considering replacing that with a Rack & Pinion system. We were afraid of the amount of "whip" there would be in a screw of such a small diameter that is over 9 feet long.

Any thoughts, pro or con?

[CHAD]

HOW ARE YOU GOING TO CONTROL PITCH ERROR??? A COUPLE OF IDEAS= GLASS SCALES VERY EXPENSIVE!!@@**## OR HAVE THE RACK EDM'D ALONG WITH THE GEAR. YOU COULD ALSO HAVE THE MACHINE LASER CALIBRATED, AND HAVE ERROR COMPENSATION IN THE CONTROL.

[E-Stop]

Haven't got that far into it yet. Like I said, this is a new "thought" that just came up yesterday afternoon. Our concern over the the whipping of a 9' long screw led us to consider alternatives. But that is the kind of feedback I'm looking for. Thanks

[CNCadmin]

I would go rack and pinon myself, it would allow for more adj. and would eliminate the need to support that lead screw. You can make a "L" shaped support for the lead screw, you make it so it can rock back and forth so it does'nt interfere with the nut.

[balsaman]

One thing you can consider is mounting the motor on the axis and turning the nut via a timing belt/pulleys. This nut/pulley would need to be located between two thrust bearings. Then the screw will be installed on the machine in a fixed position (doesn't turn). Now you have eliminated whip, and you have much less mass to turn.

Eric

[HomeCNC]

For a screw as long as you are planing I would not go smaller than 1". How will you prevent backlash with a rack and pinion design? Are you thinking of having two racks, one at each end of the gantry? What about using a long timing belt that goes over a timing gear?

[balsaman]

You may need a gearbox for that setup.

Eric

What about using a long timing belt that goes over a timing gear?

[HuFlungDung]

Yes, you'd surely need a gearbox for a rack and pinion or timing belt type drive because of the mechanical disadvantage the motor would acquire by being used in this fashion. Instead of one turn of the motor causing a movement of .2", you'd get a movement of 2 or 3 inches (without a gearbox). Not good, IMO.

What do the commercially made units do to solve the screw whip problem?

[keithorr]

There is a solution to avoid whip on long lead screws. Leave the screw stationary. Mount a belt drive nut to the carriage. Long Bobcad tables used the method.

"The Rotating Nut ball screw line can increase traverse speeds up to 200 m/min with efficiency over 90%. The RN series consists of standard diameter and standard lead screws around which a flange-type precision housing has been designed.

The housing enables a timing belt to drive the nut and contains the preloaded rotating ball nut assembly. The bearings in the housing are grease packed and sealed for life, so no additional lubrication is needed.

Available in ISO precision classes 3 and 5 in both Medium Duty (single track), or High Duty(twin track) configurations.

Thomson Industries, Port Washington, NY, "

[HuFlungDung]

Bet you won't find a rotary ballscrew nut like that at Walmart

[HomeCNC]

I also like the rotating ball nut. This is how I designed my Z axis movement in my drill-mill CNC conversion.

[bhdavis]

Backlash in rack and pinnion can be an over rated concern. I tested the backlash in my nearly 10 year old Digital Tool rack and pinnion router a few weeks ago. It was .002 in both the x and y axis. I find that to be a non-existant issue in woodworking. We keep the machine clean and tight, but I've never done anything to adjust the rack and pinnion.

Oh, and there is backlash compensation in the control softwre but I have it turned off.

BH Davis