whipping screw?

[15mgtar]

Can someone explain to me what is a lead screw whipped?
What does it look like when that happends?

[irving2008]

any long body when spun will have a natural resonance that will cause the unrestrained end to 'wobble' about centre (think of a ruler clamped end on to a desk and 'twanged', but rotating). If both ends are trapped then the centre starts to wobble (like a skipping rope held between two schoolkids)...

obviously when that happens there is a considerable sideforce on the leadnut leading to high wear rates plus vibration...

[15mgtar]

I am using 2.1 meter leadscrew and I think it is whipping in the middle and causing alot of vibration on my cnc router. How can I fix this?

[Andre' B]

Use a larger diameter screw.

[ger21]

Also, use a screw with a higher lead so it'll spin slower.

[irving2008]

Originally Posted by 15mgtar I am using 2.1 meter leadscrew and I think it is whipping in the middle and causing alot of vibration on my cnc router. How can I fix this?

what dimensions are the screw - diameter, root diameter, pitch and how is it supported at each end... free (obvious), supported in a sliding bearing, able to move axially) or fixed (rigidly held axially as well as radially)?

It is possible to calculate the max rpm before whipping occurs...

Originally Posted by Andre' B Use a larger diameter screw.

Originally Posted by ger21 Also, use a screw with a higher lead so it'll spin slower.

While both are true, neither may be practical... larger diameter = more inertia = more torque to maintain drive speeds, higher lead = less resolution, may not be available... another thing to consider is how the screw is supported... moving to better bearing arrangements can double the critical speed for a given screw...

[ger21]

Originally Posted by irving2008 While both are true, neither may be practical...

Right, screws must be part of a matched drive system. Going with larger screws will possibly require changing other parts of the system as well. Everything needs to work together.

[Andre' B]

Originally Posted by irving2008 While both are true, neither may be practical... larger diameter = more inertia = more torque to maintain drive speeds, higher lead = less resolution, may not be available... another thing to consider is how the screw is supported... moving to better bearing arrangements can double the critical speed for a given screw...

I give you practical in a hobby machine, but for production it just depends what you want/need and are willing to pay for.

The company I work for is currently looking at a new mill, it has ball screws about 16' long and 2" diameter the pitch must be 1.5 or 2 inches, and rapids at 1500IPM. Resolution is easy, expensive but easy, just put an encoder on it with several 100 thousand counts per rev..
As far as inertia, if you have ever played with model airplanes you know that if you put enough HP on it, almost anything can make a good airplane.

If you put 2 bearings on each end of the screw, spaced few inches apart all properly preloaded and in line it will work much better then bearings at a single point on each end.

[15mgtar]

I am using 2 units of Hiwin 5mm Lead, 25mm diameter and 2.1 meter long ballscrew with BK / BF 20 support blocks for the Y axis. At the middle I would say it is whipping good causing vibration. To solve this problem should I change the BF 20 to BK 20 so it's BK/BK20?
or getting a larger diameter and higher lead say 10mm lead 35mm diameter ballscrew (which is to cost me dearly) and BK/BF 25?
my stepper is size 42 nema and about 3200 oz/ in.
Please advice.

[BillTodd]

You could re-design the axis, so that the nut is rotated and the lead-screw kept stationary (i.e. mount the motor on the moving part and fix the nut in a pair of bearings)

http://www.cnczone.com/forums/showpo...03&postcount=8

[irving2008]

Originally Posted by 15mgtar I am using 2 units of Hiwin 5mm Lead, 25mm diameter and 2.1 meter long ballscrew with BK / BF 20 support blocks for the Y axis. At the middle I would say it is whipping good causing vibration. To solve this problem should I change the BF 20 to BK 20 so it's BK/BK20? or getting a larger diameter and higher lead say 10mm lead 35mm diameter ballscrew (which is to cost me dearly) and BK/BF 25? my stepper is size 42 nema and about 3200 oz/ in. Please advice.

You dont say what speed you are trying to achieve but the critical speed for that screw in a fixed-supported (BK/BF) configuration is approx 3,825rpm, so whipping is very unlikely to be the issue.. even in a support-supported (BF/BF) is would be approx 2,467rpm (thats >12m/min and your motors wont even get close to that!)

However what you describe sounds like some sort of resonance in the system.
Is the vibration speed dependent? Is there a speed it starts to appear or one it peaks at?

Are you using one or two motors to drive the 2 screws and what configuration?

One thing I would suggest is removing the ballnuts (tricky i know) and spinning each screw in isolation... does it still vibrate then? Maybe one of the screws is slightly bent (over 2.1m even 1mm out of line will give a significant vibration).

[15mgtar]

Originally Posted by Andre' B If you put 2 bearings on each end of the screw, spaced few inches apart all properly preloaded and in line it will work much better then bearings at a single point on each end.

Putting additional bearings? Can you explain in detail please?
Will these solve my whipping problem?