Machsupport Forum
General CNC Chat => Show"N"Tell ( Your Machines) => Topic started by: Bodini on November 07, 2013, 08:30:58 AM
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The problem was that when Estop was enabled, the Z axis would come crashing down to the mechanical stop.
After looking into how rather expensive CNCs are designed, I found that some use a nitrogen charged counterbalance system. So I decided to mimic that with compressed air.
I used a double acting air cylinder from McMaster-Carr (#6498K861). The air is pressurized on the lower portion of the cylinder (pushing the cylinder closed, essentially). The upper part of the cylinder has breather vent (#4456K11).
There is a 5 gallon air tank under the machine with a regulator on it. I tuned the regulator to offer just enough air to the valve to keep the Z suspended.
I can turn the ballscrew with my fingers in either direction with very little effort. The machine has no problem working against the air pressure during Z- moves. I've been using it for over a year like this and it came out way better than I could have hoped for. So now when the Estop is hit, Z doesnt go anywhere, it just stays where it is (encoder DRO proves that) :D
Hope this helps someone.
-Nick
(http://i1082.photobucket.com/albums/j364/nlapensee773/Speedy%20Z%20air/air1.jpg)
(http://i1082.photobucket.com/albums/j364/nlapensee773/Speedy%20Z%20air/air2.jpg)
(http://i1082.photobucket.com/albums/j364/nlapensee773/Speedy%20Z%20air/air3.jpg)
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Very good system Nick but I dont know why the Z will crash down on E-Stop
Never had this problem with mine
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Must depend on the weight of the spindle or whatever is mounted on Z.
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Not a particularly safe system if Z drops at all. How is it moved and held in normal use?
You may want to set the air pressure a bit high so that in the event of estop or losing the axis for any reason it rises out and clears the cut. Just my thoughts ;)
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Not a particularly safe system if Z drops at all. How is it moved and held in normal use?
You may want to set the air pressure a bit high so that in the event of estop or losing the axis for any reason it rises out and clears the cut. Just my thoughts ;)
Huh? I think you missed the point. The Z does not fall because of the air cylinder holding it up.
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Does your E-Stop cut controller power and this is why Z can drop if unsupported?
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Estop cuts power to the motor, which puts it in free fall. The air cylinder keeps it suspended. It's balanced just right too. I know that because estop does not kill power to the encoder board and the position continues to be monitored even during estop.
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Which motor? The spindle motor or the Z axis motor?
I can't see why cutting power to the spindle motor would affect Z position. Equally confusing is why power to the Z axis would be cut on E-Stop as this would cause an unsafe drop and potential crash of the spindle.
My mill E-Stop uses one contact set for VFD spindle E-Stop and one contact set for Mach3 E-Stop, neither of these would allow Z to go out of control, it sounds wierd!
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neither of these would allow Z to go out of control, it sounds wierd!
What kind of "mill" are you talking about that you have? This is a gantry router I have. Doesnt sound like you are familiar with the concept.
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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.
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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?
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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.
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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.
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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
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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.
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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.
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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.