Function of this socket head screw (PM-833T)?

[Rhizome]

Hello,

In my attempt to reduce my PM-833T's backlash on the X and Y axes, I had to remove the X-axis lead screw. I confirmed with PM that the "C" (red label) screw is for adjusting the backlash of the X-axis. PM advised that target backlash is 0.005". Less than that may lead to excessive and premature wear. I have no idea what the "A" (green label) screw is for. There was this thread that discussed tightening this "A" screw to reduce backlash, but I cannot find it in the part diagram of my machine. PM couldn't find it either.

The B-1 and B-2 screws are for securing the the lead screw nut to the saddle.

What is the function of this mystery socket head screw ("A"), anyone? See illustration below.

 

[Firstram]

It should squeeze the nut to take up backlash, similar to a lathe cross slide nut.

 

[7milesup]

It is interesting that it is directly above one of the saddle locks. I wonder if it has something to do with the gib.

 

[Tipton1965]

I read that thread you linked to above. In that discussion the OP was trying to adjust excessive backlash in the Y axis. As it turned out that mystery socket head bolt actually helps secure the Y axis lead screw nut underneath. In that linked thread the OP's Y axis lead screw nut was actually moving because that mystery bolt was not tight.

 

[COMachinist]

It is interesting that it is directly above one of the saddle locks. I wonder if it has something to do with the gib.

Nooooop! It takes up some of the back lash in the lead screw. There should be an other one on the y axis as well just be careful how much camp pressure you put on the lead screw. The lead nut can break, if there is to much width on the squeeze slot. I don’t know why you got the lead screws out but if it was me I would put some good quality rolled lead screws and good nuts, in that machine. That is what I did to my PM932.
CH

 

[davidpbest]

Nooooop! It takes up some of the back lash in the lead screw. There should be an other one on the y axis as well just be careful how much camp pressure you put on the lead screw. The lead nut can break, if there is to much width on the squeeze slot. I don’t know why you got the lead screws out but if it was me I would put some good quality rolled lead screws and good nuts, in that machine. That is what I did to my PM932.
CH

I'm skeptical. Describe how that functions please. Looks to me like "C" will pinch or force open the split nut to adjust out backlash. "B1" and "B2" are the mounting bolts that secure the split nut to the saddle. "A" appears to thread into the saddle casting and has no relationship to the bronze split nut or how it mounts to the saddle.

 

[Firstram]

I'm skeptical. Describe how that functions please. Looks to me like "C" will pinch or force open the split nut to adjust out backlash. "B1" and "B2" are the mounting bolts that secure the split nut to the saddle. "A" appears to thread into the saddle casting and has no relationship to the bronze split nut or how it mounts to the saddle.

You are correct! I was on my phone and didn't see the "A" screw, "C" adjusts backlash, "A" is just a conversation starter!

 

[7milesup]

You are correct! I was on my phone and didn't see the "A" screw, "C" adjusts backlash, "A" is just a conversation starter!

David was responding to COMachinist, but your point is valid.

 

[Rhizome]

I believe Tipton1965 got it. That hex screw secures the Y-axis's lead screw nut below the saddle.

My mill came with a parts list, which also includes parts list of other similar model from the factory. The front page labeled GEM-400G as the model equivalent to the PM-833T. However, the GEM-450G parts list illustrates an entirely different lead screw nuts arrangement. Instead, the GEM-450GCT matches my copy of the PM-833T and looks like a parts list of a drill press. Part #40 is the hex screw in question and is not illustrated on the GEM-450G page. Older parts list not updated by manufacturer?

 

[Illinoyance]

Remove the screw and see what falls off the machine.