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Author Topic: Zero gravity Z  (Read 10809 times)

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Offline Bodini

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Re: Zero gravity Z
« Reply #10 on: December 02, 2013, 09:50:21 AM »
I had a similar problem on my home built mill. With the new spindle and the Y axis mounted to the Z the total weight is about 50 lbs and the Z started slowly drifting down. I just installed 2 50 lb gas springs hose clamped together to give an 18" stroke. About $40 total from Mcmaster Carr.

Gas springs have about 2000 psi of nitrogen in them. It takes the rated load to start compressing them. Then the load increases by only about 10% over the full stroke. So the 50 lb cylinders put out 55 lbs of force at the bottom of my Z stroke. They are also filled with oil so provide damping too.

I considered going that way at first but thought the lifespan of the gas springs may be limited (i thought of the tailgates of cars that have to be propped up because the spring(s) was worn out).  How has it worked for you?
Re: Zero gravity Z
« Reply #11 on: December 02, 2013, 05:13:29 PM »
Lifespan is qouted at about 100,000 full strokes.  Mounting position should be shaft end down so seal gets oil. One of mine is up and one is down, and my arrangement puts some side load on the shaft. So I expect the life of one of them to be quite short. So we'll see, but a replace once a year or so wouldn't be too awful.

I used to build pneumatic packaging machinery using cylinders like you used.  I did away with lubricators because they made more mess than they did good. However I always shot a bit of air lube oil in both ports on the cylinders before attaching the lines.  You can get 3 to 5 million stokes on those cylinders. We easily put 5 million strokes on in a year.

Offline Bodini

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Re: Zero gravity Z
« Reply #12 on: December 02, 2013, 05:22:59 PM »
Lifespan is qouted at about 100,000 full strokes.  Mounting position should be shaft end down so seal gets oil. One of mine is up and one is down, and my arrangement puts some side load on the shaft. So I expect the life of one of them to be quite short. So we'll see, but a replace once a year or so wouldn't be too awful.

I used to build pneumatic packaging machinery using cylinders like you used.  I did away with lubricators because they made more mess than they did good. However I always shot a bit of air lube oil in both ports on the cylinders before attaching the lines.  You can get 3 to 5 million stokes on those cylinders. We easily put 5 million strokes on in a year.
I did put a squirt of oil in there when it was new, as I had read that somewhere (was it you? :-) ).  Yes, I can see a lubricator making a mess all over the work under it as I have the cylinder mounted.  I just looked and that cylinder was $39.97 from McMaster (http://www.mcmaster.com/#6498K861) so like you said, replacing it wouldn't be a big deal.  5 million strokes a year;  I use mine quite a bit, but will never see that in a lifetime!  Thx for the input.
Re: Zero gravity Z
« Reply #13 on: December 03, 2013, 07:01:00 AM »
This is a gantry router I have.  Doesnt sound like you are familiar with the concept.

I am familiar with the need to counterbalance heavy weights which need to be moved vertically by steppers, my mill has a 1Hp 3 phase motor and VFD mounted on the head and is counterbalanced 50/50 by a counterweight and a gas ram.
Part of the process of upgrading the motor and adding the on-board VFD was to adjust the counterweight such that the head could be moved up or down by rotating the ballscrew with my fingers, thus ensuring that the stepper did as much of it's work as possible to move the head and as little as possible to hold it up against gravity,

  - Nick
Re: Zero gravity Z
« Reply #14 on: December 08, 2013, 10:23:41 AM »
I can see the problem now, the weight of the head is backdriving the stepper when it is unpowered. Took a while to see whay that could be, it's not a problem a regular mill would suffer from as it's just the weight of the quill against a fairly highly geared ballscrew. I certainly can't backdrive any of my axes  :D

The air cylinder looks as good a solution as any that doesn't add mass to the arrangement like a counterweight. Gas struts also for the same reason.

Offline frasej

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Re: Zero gravity Z
« Reply #15 on: December 10, 2013, 12:43:19 PM »
I've got a gantry system and no matter how hard I press down on the Z (spindle) I can't get it to move, with or without power.  Your Z motor must really be loose. It does seem to be a good solution though.
« Last Edit: December 10, 2013, 12:46:52 PM by frasej »
Jay
Re: Zero gravity Z
« Reply #16 on: December 10, 2013, 05:57:47 PM »
If you have really good quality ball screws (efficient) then they backdrive very well once you overcome detent torque on your motors if they are steppers.  My Z never moved with a Sherline spindle mounted.  When I changed to a mini mill R8 spindle with a treadmill motor it always drifts slowly down when the motor is off.  A 50 lb gas spring has it in almost perfect balance now.