Z 0 Float

[desellers]

Hello everyone.
This is my first post at CNCZone. I have 20 years of experience as a toolmaker - toolroom machinist but I am fairly new to CNC.

I am having a problem with the Z axis of my 4 axis Bridgeport clone. The system is a stepper motor system and the manufacturer told me that I am running the program with too high of feedrates. I am curious to find out what others think is too fast of a feedrate.

I have attached a program that the manufacturer and I have been discussing. The attached program runs on my machine without problem. If you kick the F12 commands to F15 then the Z axis 0 point begins to move to the minus side. The Z axis float is slow at F15 and on most passes there is no float. If you kick the F12 to F22 the Z 0 floats down by about 0.03" per pass.

What do you think is this normal or as I think extremely excessive.

[desellers]

Hi all

Have I posted this in the wrong forum? I see a lot of people looking at it but I am getting no replys.

Any advice will be appreciated

Dennis Sellers
Model Dimensions Machine LLC
model-dimenisons.com

[judleroy]

Do you have a link to the manufacturer? Also the machine model number and factory specs would help. It sounds like a grounding issue or a pulse issue but I don't know enough about the machine.
Muskegon

[desellers]

Sure.

The machine is a CNCMasters Supra. I have talked with the company and been told that I am trying to run the machine too fast.

I am looking to see what the consensus is. Do all stepper machines have this problem to this degree? What I find intersting is that the X-Y axis do not float, just the Z.

Thank you for the response

[judleroy]

Okay I checked out the machine. The specs say rapids of 100ipm which in my opinion is slow but not to bad. When the float happens what are you cutting? What material? I would think you should get reliable feedrates of 50-75 percent of the rapid rate. I think it's funny the manufacturer doesn't give feed rates on there website only rapid rates. That's a questionable business practice all by itself in my opinion. Just to see what happens run the x and y in a full motion pattern at the rapid rate of 100ipm for say 20-30 minutes. Note the z position at start and then let us know if the float occured at that rate while not cutting anything. Also if float occurs let us know how long into the test that it starts to occur. Good luck
Judleroy

[desellers]

Got a question on your test. Is Z suppose to be stationary during the test? Is the pattern to be a box or an inturpolated circle?

I want to be sure you understand when I get float. I find and set all axis at the (0,0,2)position. The 2 on Z is just tool change clearance. Then if I run a job that is running all three axis at one time I begin to notice the tool is cutting deeper. A return to (0,0,2) shows me that the Z = has moved by it self.

Answer my questions above and I will be happy to run your test.

Thank you,

Dennis Sellers
Model Dimensions Machine LLC

[judleroy]

Set the z at 2 like mentioned above. Leave it stationary. Move the x and y through the full range of travel and return to zero. No special shapes or anything like that. Say your travel is 30x12. Then Something like this.

G00 x0.0 y0.0 z2.0
X30.0 y12.0
X0.0 y0.0
X30.0 y12.0
X0.0 y0.0
Etc... Over and over continuously for the time frame mentioned above.

Hope that understandable.
Judleroy

[desellers]

Sorry it took so long but I had to get a job done.

I ran the test and came up with a few results.

1. The machine is incapable of running the XY axis at 100 ipm. The best I could get is 50 ipm. The Y axis motor stalls and the counter contiues. When I bought the machine the advertised max was 50 ipm. I have recently had the control box upgraded and it should now run at 100 ipm. Is this something I should take to the manufacturer.
2. The XY axis is stable at 50 ipm. The Z does not move during the XY axis test.

I appreciate your help.

Dennis Sellers
Model Dimensions Machine LLC

[judleroy]

I know your busy with other things but there's one more test that will let us know if it is a pulse or noise issue or a defect. That would be to run two more short tests. First try a similar program as before raising up and down on z for a period of time while the x and y stay stationary. Then check if the z floated while running by itself. If not that will eliminate a direct issue with just that axis. If there is no issues with that test try a similar program with x y and z. If still no issues you may need to get some info from the manufacturer on the max cutting speed from there point of view. Also how many amps are your motor drivers set to draw? Last how many volts and amps does the motor power supply provide? I will do what I can to help you work things out and others will chime in with other info that may help as more info is provided. Good luck.
Judleroy

[James Newton]

Since the Z axis has to fight gravity (unlike X and Y) it makes sense that it would have problems returning to the correct height when being pushed to it's limits for a given motor/driver combination. Adding a simple counterweight, as dorky as a rope over a pully on the ceiling down to a weight, to offset the weight of the Z axis may "solve" the problem, or at least allow you to verify if that is the real issue.

[desellers]

I have finished the prescribed tests.

I ran the Z axis 500 trips plus and minus 1 inch at rapid travel. XY axis was stationary. Z returned to 0 when finished. There was no float. I think this eliminates the need for a counter weight.

For the combined all axis test I refer you to the program in my initial post. The program interpolates a curve on the Z axis in relation to the simultaneous moves of X and Y. As stated if the cutting speed, F12, is raised to F22 then the Z axis floats about 0.03" per pass.

I have talked to the manufacturer and been told that it is normal. They recommended a servo system if I need to run the machine faster.

What I need to know to make a decision is the float in Z normal on stepper systems? Is there anything that can be done to eliminate the float? This costs me too many parts and too much time trying to work around the issue. I should mention that I am a toolmaker not an electronics engineer.

I really appreciate the help I am getting.

Dennis Sellers
Model Dimensions Machine LLC
Model-dimension.com

[judleroy]

I would be upset with that response from the manufacturer. 12 ipm for a sherline or taig maybe but not on a 10,000 dollar knee mill. I know steppers are slower then servos but that is not an excuse for a poor quality system. Did they say why this happens when all three axis are run together. If it is due to power draw you can get a power supply that gives more amps at the same voltage. If it is a computer issue you might want to consider hooking your drives up through a smooth stepper board or a k-flop/kanalog setup. The motors should be plenty powerfull enough. Maybe the acceleration values are to high also. First see what they say about why this is happening. Let us know what they say and we will try to find a solution that works for you.
Judleroy