screw whip lengths and limits

[ssutton]

What are the practical lengths that you can use a screw? I know that this depends somewhat on the speed, so lets say that you want to be able to cut at 150 ipm, maybe rapid at 300 ipm. Ball screws are out of the budget so we are talking precision acme with a zero backlash nut.

Thanks,
Scott

[CNC Lurker]

What are the practical lengths that you can use a screw? I know that this depends somewhat on the speed, so lets say that you want to be able to cut at 150 ipm, maybe rapid at 300 ipm. Ball screws are out of the budget so we are talking precision acme with a zero backlash nut.

Thanks,
Scott

I've heard on here that anything past 4ft is pushing it for an ACME 1/2-10 5-start.

If you want high speeds at a lower cost than ball screws, maybe rack and pinion would be the way to go.

Rack and Pinion Drive, Nema 23 LINK

[ger21]

It depends on how you mount them. If you use two bearings on each end, you can spin them a lot faster than with a single bearing on each end.

My 40" 1/2-8 2 start has no whip at 190ipm, but the 60" ones whip at ~140ipm.

[louieatienza]

What are the practical lengths that you can use a screw? I know that this depends somewhat on the speed, so lets say that you want to be able to cut at 150 ipm, maybe rapid at 300 ipm. Ball screws are out of the budget so we are talking precision acme with a zero backlash nut.

Thanks,
Scott

It depends on the number of starts too you use on the screw. I easily get over 400ipm on my y axis; then again I use 8-start ACME which gives me 1tpi effective, and .005" rmechanical resolution; which is probably what you'd get with a rack and pinion drive. To do that with 5 start you'd have to spin the screw at 800rpm; not going to happen, at least practically... On my x I get 250ipm reliably. But because of the weight, pulley system, and one screw being slightly bent, any more faster and it starts to whip.

My y screw is 36" oal and x screws are 42" oal. A lot is dependant on how the screws are mounted, their size, and alignment; so considerations should be taken as to allow for slight adjustments.

You COULD get faster rapids with leadscrews with a spinning nut design; though thos would probably add to the cost, whether you use a non-captive hollow shaft stepper/servo, or make a gear reduction unit, but this would help immensily with screw whip and ovverall length.

[rowbare]

Engineeering Product Selection Calculators - Nook Industries, Inc. see the Critical Speed calculator.

bob