Fix For Play Between Spindle Housing & Column On Mini Mill

[Wireaddict]

I recently found another source of play between the spindle housing & the column on my LMS 3990 mini mill. Side-ro-side play can be eliminated by by tightening the gibs, of course, but it still rocked vertically when I grasped the top of the motor & the lower spindle. Long story shorter, I filled in the space between the back of the drive housing & the front surface of the dovetails on each side with 0.050 inch shims then added keepers & wiper/seals on the housing, top & bottom, to keep the shims in place. Don't know if all the mini mills have this problem or, if they do, if this clearance is always 0.050 inches but that's something you might want to check. Regards!

 

[countryguy]

Cool. a few pics would be great to see. Thanks for the tip.

 

[Wireaddict]

Cool. a few pics would be great to see. Thanks for the tip.

I can do that but a pic won't show much, I'm afraid. When I submit a pic I'll also include a sketch of the top & bottom retainer plates. It'll be a few days but stay tuned...

 

[NightWing]

How old is your mill? Did you contact LMS? They are usually very good in solving problems with their machines.

 

[Wireaddict]

How old is your mill? Did you contact LMS? They are usually very good in solving problems with their machines.

The mill's nearly 3 years old; I contacted LMS two weeks ago, just after I discovered this problem & mentioned adding these shims as a possible remedy plus I asked about an unrelated matter. Their reply addressed the other issue but never mentioned the vertical rocking condition which I took to mean that, since I found a reasonably simple solution & given its age, there was no need for their involvement [yet, at least] &/or that my idea was as good as theirs. I did share this info with LMS again after I installed these shims & retainers 2 days ago & haven't heard back yet.

 

[Baithog]

I should probably keep my fingers away from the keyboard, but I don't understand. I have a CNC Seig X2, which mechanically similar. I don't see a picture of the 3990 at LMS, but it seems to be the same dovetail column design used by many models.

The nature of a dovetail slide is that the gibs force the two parts together. The column and the spindle box base (SPD) will be drawn together until the base of the column dovetail contacts the base of the SPD, or until the tops of the dovetails contact. It is impossible to have both in contact and still have movement. The bases of the dovetails are usually the surfaces used, and are ground true or scraped true in high dollar machines. The tops are not usually used because of the increased surface related friction.And They are avoided because there are often things mounted to them,like leadscrew nuts and in the manual mill's case the rack.

Shims between the column and the SPD will work, but it is not the best solution. If it was me, as soon as the current project is done, I'd take the spindle housing off the SPD and the column off the mill and figure out why you aren't getting the contact you should. My wild guess is that the weight of the head has worn the bottom of the SPD dovetail on the downward side more than the top. So now the gibs tighten on the top side, but the bottom can still rock. You can get an amazing amount of wear over time. I made the mistake of putting a vice in the center of my table and making parts for 4 years. Now I can't get full X travel unless the table is floppy in the center. There always seems to be something to fix.

Larry

 

[Wireaddict]

I should probably keep my fingers away from the keyboard, but I don't understand. I have a CNC Seig X2, which mechanically similar. I don't see a picture of the 3990 at LMS, but it seems to be the same dovetail column design used by many models.

The nature of a dovetail slide is that the gibs force the two parts together. The column and the spindle box base (SPD) will be drawn together until the base of the column dovetail contacts the base of the SPD, or until the tops of the dovetails contact. It is impossible to have both in contact and still have movement. The bases of the dovetails are usually the surfaces used, and are ground true or scraped true in high dollar machines. The tops are not usually used because of the increased surface related friction.And They are avoided because there are often things mounted to them,like leadscrew nuts and in the manual mill's case the rack.

Shims between the column and the SPD will work, but it is not the best solution. If it was me, as soon as the current project is done, I'd take the spindle housing off the SPD and the column off the mill and figure out why you aren't getting the contact you should. My wild guess is that the weight of the head has worn the bottom of the SPD dovetail on the downward side more than the top. So now the gibs tighten on the top side, but the bottom can still rock. You can get an amazing amount of wear over time. I made the mistake of putting a vice in the center of my table and making parts for 4 years. Now I can't get full X travel unless the table is floppy in the center. There always seems to be something to fix.

Larry

Hi Larry, as far as I know 3960 was the model no. for this mill in 2012 when I bought it. Since then LMS/Seig replaced the old mid column torsion Z axis counterbalance spring with an upper column gas-charged unit & changed the no. to 3990 &, since I upgraded the spring to the new type, I described by the new model no.

I don't use this mill a lot, probably 4 hours/week average, so I'd be surprised if the dovetail has worn much & there was no difference in play between the upper & lower portions of the column & no noticeable wear on the column gib. My guess is that the head casting was machined about 0.040 in. too deep in front of the dovetails on both sides of the rack. And the rack mounts to the column [as I recall---I'm not near the mill now] between the dovetails. If I had noticed & reported that vertical rocking when I first set it up LMS probably would've replaced it for me. Unfortunately however, I got knocked off a ladder in a logging accident & fractured a vertebra in my back shortly after I set it up so I was unable to do much in the shop for over 4 months & I just discovered this play within the last 3 weeks. The spindle feed is stiffer now but not overly tight & it shakes a lot less when cutting steel than it did when I ran it. Regards.

 

[Baithog]

Yikes! I hope there's no long term problems with your back.

I was assuming that the mill didn't have the problem when you got it. Seig has never been accused of having high quality standards. We don't always catch their short comings until we put a load on the machine. You mentioned wipers and seals, are you getting enough lubrication on the shim/column interface? If your Z motor likes the added friction and it wears well, it's a whole lot better than having to take it apart to do a "proper" repair. And if you develop problems with the shims, you can always take it apart and play with the dovetails then. Great work-around.
Larry

 

[Wireaddict]

The dr. fused the back & it's 95% as good as new & pain-free [so far]! Yes, I oiled it & head's just snug with the column that way but not binding so it's OK, IMO. Also, the Z axis on this model is manual, either rack & pinion with 3 arms on a hub driving the pinion like a drill press or with a worm gear driving it for fine feed.

 

[Wireaddict]

Here's a pic of the upper retainer plate at the top of the spindle drive head also showing the top of the column dovetail & rack. I put a wiper seal between the retainer plate & the head assembly. It was necessary to add the retainer plates to the top & bottom of the head to keep the 0.50 in. shims in place between the back of the dovetail & the head. I also took a pic of this 0.050 in. gap but, unfortunately, it didn't turn out but there's a hint of this space in the area between the rack & the retainer plate. I also attached a sketch of the retainer plates.