Bolton ZX45 Conversion losing z position during cutting. Need ideas please

Hi all,

I recently completed a CNC conversion on the Bolton ZX45 milling machine. I converted to double nut ball screws and am running a Nema 34 1000 stepper in the y and x axis and a Nema 34 1800 or so in the z. I'm running Mach3 software. I am also using a 150lb pulling gas strut mounted vertically. The steppers are mounted as direct drive.

My x and y work flawlessly and repeat to .001 and only needed .003" backlash offset. The Z also does well and repeats within .001 with .005" backlash offset.

The issue is when I'm milling. When milling steel, the z axis will drop and the tool will dig in deeper than I want it to go. I don't seem to have the same issue when milling aluminum. I can take .25" passes with a HSS end mill on aluminum without any issue whatsoever. However, cutting mild steel anything over .050 deep causes the head to drop. And, obviously, Mach3 does not detect it. And it's not a little bit...it's dropped as much as .125 on a 3" pass. I have experimented a bit with feeds and speeds as well. Maybe I need some more direct advice on feeds and speeds?

I have validated that the tool is not moving in the collett and also made sure that the quill is not moving. It is definitely the z-axis on the column moving downward as I taped some masking tape over at the start of a pass with a line on it and you can see the whole thing shuffled down.

So, now, what are some ways to deal with it? I'd think that a 150lb spring plus the larger 1800 stepper would be enough...maybe it's not? I can easily adapt another 150lb gas tension spring on there as I got a good deal on a pair of them. I have to think that the stepper is plenty large? I don't think a Nema 42 will fit without offsetting and adding a pulley.

Any help is appreciated! The first steel part I attempted to make was ruined (a new tool post t-nut for my lathe) and i kept milling on it and other mild steel to see if I could work it out...I haven't...I still get drop and i'm starting to get pretty frustrated. Maybe someone has some advise?

Thanks

Fatcaaat,

Please post a image of the motor coupler on the Z axis of your machine.

Welcome to the Zone,

Jeff...

I'll try and get out to snap a picture, but the coupler is the aluminum type that has a wound coil on it. You put it over the shaft, clamp it, and turn the set screw in on both sides. I have examined it and there isn't any slippage of the coupling to the shaft. This is the type I'm using on all axis. https://www.ebay.com/itm/5-10MM-Flex...pler.TRS1.TSS0


What I actually suspect is going on is that the tool is grabbing the work. The combination of feeds and speeds may not be right. When it produces some vibration is when it happens and I suspect it is a matter of the vibration shaking things a little and the heavy head just walks down a little bit.

I tighten the gibs up just enough so there is no movement or shaking when milling, but if i do them too tight, the motor will lock up on rapids which clearly throws things off. Again...all in the Z axis...i have had perfect results in x and y.

The head, from what I understand, weighs 240lbs with the motor on it. I think it would probably be better to add another 150lb tension spring to bring the upward force to 60lbs rather than leave the single spring on there which leaves a 90lb force downward.

I'd also like to upgrade the motor if I could find a direct replacement. I suppose a 3650 220v would be ideal if I could find one to mount and go. I'll never need those super slow speeds on this mill so doubling it up since the majority of what I'll mill is Aluminum would be better.

Fatcaaat,

The coupler you linked to is not very good, they wind up and unwind when subjected to heavy loads.

See attached images of couplers that are far superior.

Jeff...

Very poor at maintaining position when subjected to heavy loads.

You may want to consider implementing some sort of counterweight system using pulleys and weights behind the column.

Weights can easily be added to fine tune balance.

https://www.youtube.com/watch?v=NklyVzcagtQ

https://www.youtube.com/watch?v=Jwoe8blk7pg

Thanks for the inputs folks. Really appreciate it! I wanted to provide a quick update on the progress.

After adding the second 150lb tension spring as a counterbalance, and watching things very closely, I seem to no longer have an issue of the machine losing steps or the head dropping while it is milling at normal feeds and speeds. That solves 1 problem, but brings up a new one.

On Z-axis rapids there is a motor stalling issue that screws up positioning. And it happens in both directions. I had all axis running 60IPM rapids through the machine configuration. It works fine on the y and x axis, but it does not like that on the Z axis. Periodically, (and only in rapids) what happens is that the stepper will not have the torque to get the head moving and stalls for a second and then resumes. Well, in doing so, it drops the steps so when it repositions on the way back down to take up any lash, it doesn't account for those lost steps, and the result is that it plunges deeper than it should. For example, If I take a .050" pass and do a rapid up at 60IPM so I can clear something, and then rapid back down to 0 before the next pass, it will sometimes stall the motor and lose a few steps. However, if I reduce that 60IPM to 30IPM rapid, I do not lose any steps, no matter how many times I have tried to do so.

I think this indicates that the motor I have (direct drive 1800 oz/in) is not powerful enough to do 60IPM rapids in the z axis but it is powerful enough for 30IPM in the z. What I may consider doing is reworking the z-axis mounts so that I do a 2:1 reduction belt drive or just dropping money on a Nema 42 with 3xpower. It will solve it either way. I can actually crank the x and y axis up to 120IPM without any issues, but really, I don't believe it's necessary.

One last question for you guys out there. I am using DM850 drivers and honestly, have not had any issues with them. One of the on/off dip switches is for half/full power. I have all of my drivers to 1/2 power because I felt they got awfully hot on full power. Should I just put that z axis dip to "on" for full power and not worry about the heat it generates? When I had it on full power, it was hot enough for me to think I was going to get burned.

Fatcaaat,

Unfortunately it takes a fraction of a second for the driver to come out of power (sleep) reduction mode, during that fraction of a second position loss may occur, I personally have witnessed such behavior and do not use the power saving feature on any axis. YMMV

Jeff...

If you only thought you might get burned, then the motor was not too hot. Steppers are commonly rated for case temperature upwards of 200 deg F, which WILL burn you in seconds.

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Originally Posted by 109jb

If you only thought you might get burned, then the motor was not too hot. Steppers are commonly rated for case temperature upwards of 200 deg F, which WILL burn you in seconds.

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Thanks for this. I'm going to try putting it on full power on the z and see what happens. I'll also take some temp measurements with the infrared gun to see what's what. Appreciate the input.

One thing that is missing from your description is your acceleration settings. What do you have them set to? Also, what is the lead of the ball screw? 1800 oz-in should be plenty to move the head (give or take 2500lbs of force assuming 80% efficiency, 0.2" lead), however at higher speeds those steppers do drop off their torque quite a bit. Definitely set the drive to full power. Also, what voltage are you driving the steppers at? Insufficient voltage can contribute to issues at higher speeds. Lastly what stepper motors specifically? You've got me interested - I want to look up their torque curve and other ratings.

Here is my Z axis stepper: https://www.omc-stepperonline.com/ne...s59-5004s.html

Here are my Y and X axix steppers: https://www.omc-stepperonline.com/ne...s46-5004d.html

I'm driving each one on its own individual 350W power supply and is wired in parallel. I'm driving each of them using the 5A settings on the drivers, which I guess translates to 60V.

The comment above talked about acceleration. Since I'm still kind of new to all this, my acceleration at 60 is 120. Think that's my issue? What do you think I should try?

You evidently chose those motors based on holding torque, which is on the high side. But the inductance is also quite high, at 11mH. Using the formula (sqrt)mH x 32 it wants about 106 VDC supplied to run at full speed. If you're only giving it 60v, that could be why it's performing poorly. Swapping the Z axis motor for one with lower inductance might give you better speed without the faulting. You could also give them more voltage, but you'd need drivers that could handle it. But before doing any of that, you might try reducing the acceleration by half, and see if things improve.

looking at the torque curve of your X-Y motors which are similar, they have about 1/8th the torque at 300rpm versus their max rating.
https://www.omc-stepperonline.com/do...rque_Curve.pdf
As mentioned by awerby try reducing your acceleration by half, that should help, but your top speed is limited by your motor, especially on Z where it has to do real work versus X and Y which only need to push themselves around, no gravity effects.

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Originally Posted by ninefinger

looking at the torque curve of your X-Y motors which are similar, they have about 1/8th the torque at 300rpm versus their max rating.
https://www.omc-stepperonline.com/do...rque_Curve.pdf
As mentioned by awerby try reducing your acceleration by half, that should help, but your top speed is limited by your motor, especially on Z where it has to do real work versus X and Y which only need to push themselves around, no gravity effects.

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The whole "build your own CNC" was all new to me and this project has definitely provided me a lot of good experience. I also appreciate all the help from the boards.

I went out and retuned everything based on some of the inputs here. I reduced the acceleration a lot and it now stops the jarring effect it has from the sudden stops in all axis which I find a lot better. I also spent time getting the axis tuned by ear, as some of the vibration at certain speeds I didn't like.

Here's how I ended up: X and Y axis I'm running rapids at 45 and acceleration is at 2. On the Z axis, rapids at 30 and acceleration at 1.

I machined my first "real" part last night which did circle paths and some "3d" in the z axis and it produced it perfectly. It certainly wasn't the fastest, but i have other limits on speed since the spindle only runs 1500 rpm. I'm going to do a motor swap so i can double that spindle speed. Maybe, at some point in the future, I will swap out the steppers...i don't know. I'm not all that concerned with the speed of part since i'm not planning on doing production work. Most of my stuff will be purely 1-offs. For the part in question, it took about 10 min to program what I wanted and it took about 8 min to setup and run with 3 tool swaps. First, I couldn't even do the work on a manual machine with out a rotary table and a lathe. Right there shows me that this is a winner going forward.

What are the units of your acceleration settings? in/s^2?

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Originally Posted by skrubol

What are the units of your acceleration settings? in/s^2?

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Inches/min^2

It runs way smoother with these settings. I'm still working on tuning things, but this has really improved everything. Only thing I don't like is during the jog it runs after you let go of the button.

The 120 in/s2 you had before was quite fast, but 1 or 2 is pretty slow. The acceleration (and deceleration) time is speed (in/min) / acceleration (in/s2) / 60 (sec/min). So in your case 45/2/60=.375 sec and 30/1/60=.5 sec.