Another PM-833TV thread!

[Juanca]

It took a few hours but I got the backlash fixed, or at least better. I was originally measuring about 10 thou on the x axis and about 15 on y. After a lot of measuring and fiddling I found it was due to a combination of preload on the lead screw bearings and backlash in the brass split nuts.

As shown in the picture below, the preload on the bearings comes from the #70 screw, there's no separate nut for it. After torquing the x and y handle screws to about 15 ft-lbs, using my calibrated arm, the backlash from the bearings came down to about 0.5 thou. I also noticed that these don't appear to be angular contact bearings, just regular radial bearings. After adding the preload the bearings feel a bit gritty and I don't think they will last all that long. If/when they do fail I'll try to replace them with some angular contact bearings.

View attachment 322823

The other large source of backlash was the split nuts. They both needed to be tightened a pretty good amount. I didn't need to tighten them all the way but they're not that far from it. They're made from casted brass (which isn't a bad thing, these are cheaper to replace than lead screws) so not sure how long they will last before needing to be tightened again. These will probably eventually get replaced too.

After making all the adjustments I'm down to about 2-3 thou for both the x and y and the table feels a lot less loosy goosy, although a bit gritty because these radial bearings aren't meant for axial load. I'm going to cut some mild steel on it this weekend and we'll see how she handles climb milling now.

Hellow Mr. M.T.Pockets. I am planning to buy a PM-883 MT for my home shop. My big concern (among others) is about the perpendicularity of the spindle with respect to the Table. I ask thid because correcting this problem is not an easy thing to do. Just by the use of shims is not the best way to do it, The column will lose rigidity by not having full contact with the base. What has been your experience with this. ? Appreciate your professional

 

[M.T. Pockets]

Juanca, this is indeed a problem with no simple solution. If you absolutely need the accuracy of having the spindle perfectly perpendicular to the table, my advice would be to get a regular knee mill, not a benchtop. There is no simple solution to this problem using dovetail ways.

That being said, if you're looking for that level of precision you're probably not going to be buying a PM Mill in the first place. The 833 models can be shimmed well enough to get more than close enough for any hobby work and the loss of rigidity is minimal if you do it correctly. You want to make sure you are able to shim around the bolts, not only in between. Even in the case of two un-shimmed and perfectly coplanar faces the loads are only going to be transferred at the bolt locations anyway, so make sure thats where you shim.

 

[Juanca]

Juanca, this is indeed a problem with no simple solution. If you absolutely need the accuracy of having the spindle perfectly perpendicular to the table, my advice would be to get a regular knee mill, not a benchtop. There is no simple solution to this problem using dovetail ways.

That being said, if you're looking for that level of precision you're probably not going to be buying a PM Mill in the first place. The 833 models can be shimmed well enough to get more than close enough for any hobby work and the loss of rigidity is minimal if you do it correctly. You want to make sure you are able to shim around the bolts, not only in between. Even in the case of two un-shimmed and perfectly coplanar faces the loads are only going to be transferred at the bolt locations anyway, so make sure thats where you shim.

M.T.Pockets, Thank you for the reply and advice. Yes, a knee mill could be the solution, the problem is space yn my garage, and these machines are quite heavy and the place where I live make it impossible for me to handle. There is a procedure to level the mill post until you get near to perfect perpendicularity, however is not an easy job to do it, but with some effort, it can be done.

 

[mksj]

I would expect the column alignment to be within a few thousandth, al least that has been my experience with similar types of bench top mills. Typically the head is locked in a fixed position when milling. Usually what is more of an issue is you can get some nod from the head as the gibs wear. I had my benchtop for about 6 years with a power Z axis and notice a small amount of nod but typically went away when locking the gibs. If you are fixed on having to align the column, there are a number of videos where one used epoxy to fill any gaps. They applied a release agent in case they needed to separate the column in the future.

 

[Juanca]

I would expect the column alignment to be within a few thousandth, al least that has been my experience with similar types of bench top mills. Typically the head is locked in a fixed position when milling. Usually what is more of an issue is you can get some nod from the head as the gibs wear. I had my benchtop for about 6 years with a power Z axis and notice a small amount of nod but typically went away when locking the gibs. If you are fixed on having to align the column, there are a number of videos where one used epoxy to fill any gaps. They applied a release agent in case they needed to separate the column in the future.

Hello mksj, thank you for your note. Yes, there are some videos on how to align the column/head to make it perpendicular to the table. Epoxy and collapsable round soft copper shims is a good one. You can make and install a SS thin plate between the base and the column to void "gluing' the base to the column in case you want to take the mill apart in the future. Is doable.

 

[Rodneyk]

I ordered this mill a few days ago, the one unanswered question I have it what type of outlet is required (there are several 220v standards in the world).

 

[7milesup]

I ordered this mill a few days ago, the one unanswered question I have it what type of outlet is required (there are several 220v standards in the world).

I think whatever fits your receptacle. I think I used a NEMA 6-20 if I remember correctly.

 

[Rodneyk]

Response from PM was that the machine does not have a plug installed so you can use whatever matches your building codes. So off to figure out what I want to install before the mill gets here

 

[StevSmar]

Thanks for documenting how you pulled the mill apart to get it into the basement.

Interesting you’re making telescope parts, that seems like a fine reason to get a mill!

 

[M.T. Pockets]

Thanks for documenting how you pulled the mill apart to get it into the basement.

Interesting you’re making telescope parts, that seems like a fine reason to get a mill!

No problem. My #1 concern with getting this mill was how to move it. Hopefully this helps some people who also have their shops in the basement.

I'll probably be updating this thread soon with some modifications I'll be doing to the machine. I've made a few dozen parts on the machine since setting it up and I keep running into one main issue, backlash. Backlash is always a problem on machines like this but the split nuts that come with the machine don't seem to last very long. I'm not yet at the point where I'm going to get ballscrews but a future project is going to be finding better nuts for the table and use some actual thrust bearings for the lead screws. I think they've got standard radial bearings stock and any reasonable preload makes them very grindy.