This is a PM-833 conversion with a Heavy Metal CNC kit just like Dr. D-Flo’s. My kit came with double ball nuts, so I don’t need to worry about that upgrade later.
I completely disassembled the mill which was a good opportunity to remove the vast quantities of cosmoline that shipped with the mill.
I started with the Z-axis and the only modification that was required on the mill was to drill and tap a hole to relocate the oil line manifold to accommodate the new nut block. No other modification to the cast iron was required on the Z-axis. I located the oil line manifold to require minimum re-bending of the oil lines.
The Y saddle did require a little grinding to provide clearance for the nut block assembly. I also had to do some grinding under one end of the table to clear the X nut assembly when sliding the table back on.
I did have some issues with the less than precise placement of tapped holes in the mill that others have noted. There was some mis-alignment with the holes in each end of the table. You can see the worst of it in the photo. That is the total misalignment that you see. I was able to use a round file and relieve it enough on both sides to get it to fit. There was just enough counter-bore to still fit the socket head screw.
The only significant problem I had with the kit was with all the screw brackets that were machined to carry the bearings. The minor bore diameter on those brackets was smaller than the major diameter of the inner bearing race. That caused the inner race to ride on the aluminum when assembled, resulting in a bunch of friction. I was able to grab the parts with the lathe and take care of that issue. Bruce at Heavy Metal says that bore has been enlarged now so you would not have this issue with any recent kits.
Overall, I’m happy with how this conversion went and would definitely buy a Heavy Metal kit again. I finished the assembly this weekend and ran some g-code to put the mill in motion. WooHoo! Now I’m on to the probe and tool height setter. I have some questions for Dr. D on that.
After observing the Z-axis at work, I think I’m going to up the priority of unweighting the Z-axis assembly.
I’m thinking some gas springs for my first attempt. Anybody done this yet? I’m thinking of starting with about 200 lbs of force and seeing where that gets me. There seem to be some gas springs available through McMaster-Carr that have the right force and stroke.
I missed my opportunity to weigh the Z-axis assembly when I had it apart. Anybody have a guess at what the Z-axis assembly weighs? Any feedback would be appreciated. thanks!
Good job. I’m thinking of getting the same mill and doing the same thing you are. I was looking at gas springs at McMaster Carr. I was also contemplating closed loop steppers. Looking forward to more insights.
@Sean I really appreciate you documenting your conversion. Showing your problems and solutions will be very helpful to those following along.
Regarding the gas springs, I have gone back and forth on this issue. The one drawback that I see is that if you unweight the Z-axis then you have to worry about backlash. But I see you have the double nut ball screws, so that shouldn’t be a problem
I got the gas strut project wrapped up. I was unsure if the engineering was going to work out so I designed such that no additional holes needed drilled in the mill. The extra bracket on the Z saddle also had the benefit of keeping the struts clear of the rubber:
Thanks for the nice review of your CNC conversion. Nice Job! I too am considering doing a conversion of this mill. Just curious, what kind of stepper/servo did you use for the Z-axis? Are you satisfied with the action of the Z-axis now that you added the gas strut?
I used a NEMA42 2830 oz-in 6.0A stepper on the Z. I’ll likely upgrade to servos later, but it works fine for now.
The main reason I wanted to add the gas struts was to take the load off the double ball nut. The Z axis was not sounding smooth while in motion as compared to the X and Y. I was concerned running with that load on the double ball nut was going to lead to premature wear or failure of that mechanism.
It sounds better now, and of course the Z axis should accelerate easier in the Z+ direction.
Thanks so much for you reply. I have been debating buying the PM-833VT or the PM-728VT for some time. I had received several post on other forums from folk that bought the PM-728VT and said that it works very well in aluminum, but does not have the rigidity for tools steel and harder materials. I am leaning towards the PM-833VT at this time, I am a bit concerned with the need to get a hoist to jocky it around while converting it to CNC. Thanks again… Richard
I used an engine hoist. I won’t lie, I didn’t like that part either. You should plan each lift, be careful and get help. I removed the head and then the column, and put it back the same way and those were manageable lifts.
Take the time to build the straps you need to secure and position what you are lifting. Oh and did I say “be careful”?
Just a quick question. You mentioned that you used a NEMA42 2830 oz. stepper. Did you use a closed loop stepper? I can’t seem to find a 2800 oz. closed loop stepper anywhere
Quick question. I am very interested in installing the gas strut that you used. The “Z” travel on the PM-833 is claimed to be 17 1/2 inches. The stroke of the gas strut is indicated at 14.3 inches. Does this mean that you lost a bit of Z travel when you installed the gas strut? Am I missing something?
No you didn’t miss anything. When I’m bottomed the spindle nose is about 2-1/2 inches off the table. It’s pretty rare I need to put the spindle nose on the deck, and I usually have a vise and tool setter in the way. If I do need to get lower, I can put another mount point on that side bracket to shift the range down.
Now that I know it works, I might work harder at finding a strut with a little more stroke someday…
Thanks for getting back to me so quickly. Funny, just after I messaged you, I checked out McMaster Carr and found a few very similar options with a stroke of 16.14 inches. They have 100 lbs of force.
They are available with various connection options. Here they are:
I am planning on using a Nema 34 1700 oz. closed loop stepper for the Z axis. Since you have tested the 90 lbs gas strut, I think these may work perfectly for taking some load off the stepper and stopping it from crashing into the table when power is removed from the stepper. BTW, do you think the 150 lbs option would be overkill?
I really can’t say about the 150# option. It might leave you with too much load the other way…
If I had been smart, I would have remembered to weigh the mill’s head while I had it apart, but I didn’t. I asked the folks at PM if they knew and they “guessed” about 200lbs.
I’m not sure the Z screw cares which direction it’s preloaded. Maybe if it’s already pre-loaded up what little lash is there is already taken up when it takes feed pressure? But I think if you go from 200 lbs down to 100 lbs up you are probably not in as good a place as if you are just a little short of the head weight with your strut force.
If anybody has better info on the the actual head weight on the PM-833 that would be useful.
Thanks Sean. I think the fact that you have had good luck with the 90# struts great. I will start there. When I take apart the machine I will weigh the head just to see what we are dealing with.
I don’t know if you’ve heard or not but Bruce passed away right before Christmas. I have been following David’s project pretty closely up to ordering the same servos he did. Unfortunately my Heavy Metal kit is missing the timing belt and pulleys. If anyone has the dimensions, I would really appreciate it. I’m not sure I’m going to get any response from Bruce’s widow under these tragic circumstances.
