Need Help With Carriage Gib Installation Problem

[Nogoingback]

When I removed the carriage on my Logan 200 a while ago, I noticed an uneven wear pattern on the front
gib where it contacts the bottom of the way. When I got around to looking at it
more carefully, I measured the spacers that are bolted between the gib and the carriage and discovered
they weren't the same length, with a difference of about .016". I checked my Logan parts list, which
showed they were the same p/n. A follow up email with Scott Logan confirmed that they should be the
same. So, I assumed that the difference in length was the problem, and shortened the longer spacer to
match the shorter part, which, because of the size of the notch milled in it's edge, appeared to be the
correct part. They are now the same length to less than .001".

I bolted the assembly back together, and found that spacer on the right (unworn) side is loose,
when the spacer on the worn side is snugged up. I was also able to slide a .006 feeler gauge into a
gap between the bed and the gib on the right hand side.

Here's a pic looking up, under the carriage at the gib as it's installed.



This pic shows the parts on the bench. The wear pattern can be seen on the left side of the gib. The right
side shows no wear at all. The amount of wear on the gib seems pretty small, though I
didn't have a good way to measure it. The gib has three points of contact: at the way, and under the spacers. All
three points were machined when the part was manufactured.



I decided to measure the gap between the two small contact points on the gib, and the carriage. The
pic below shows the gib clamped to the bed with spring clamps. The only tool I had to measure the gaps was
a spring caliper, which I carefully slid between the parts on each side, and then measured with a micrometer.
Not the best method, but I think I was consistent about how I did it. The gaps measured .514" on the left
side, and .523" on the right. (The spacers are .516" long.) So, it seems that the gib itself is the problem
and, I assume, was machined that way at the factory. It may be that someone put the longer spacer in
at some point without really solving the problem. So, what do I do? The only option I can think of is
to have the gib ground correctly, and then figure out what length spacers I would need. I think I'll
run this past Scott Logan next week, but I'm interested in what you guys think of this.

 

[RandyM]

WOW! I am surprised that no one else has any comments on this. Unfortunately I am not familiar with your machine. It could be that you mentioned contacting Scott directly and we are all waiting to see what he suggests.

Now! Come on Logan 200 lathe owners what say you?

 

[wa5cab]

I have always been surprised that, unlike with several other lathe makes, the hold-down plates weren't designed with shims for the final fit and to take up for future wear.

 

[Nogoingback]

Thanks Randy! I called Logan, but Scott was out of the office for a few days, so I'm waiting for him to come back.

I have always been surprised that, unlike with several other lathe makes, the hold-down plates weren't designed with shims for the final fit and to take up for future wear.

I'm glad you mentioned that Robert. The design is interesting: the contact points only support a portion the spacer and they're offset, so I assume snugging the screws a little put some upward pressure
on the gib, allowing them to be adjusted.
In order to properly grind the part, I need to know the height difference between the two small contact points and the part of the gib that contacts the bed. Then I can use the original spacers. If I can't get
that information from Scott, or can't get a replacement part, then one possible solution would be to grind the top flat, make new spacers and use shims to make up the difference.



This end view is a little blurry, but the difference in height between the machined surfaces can be seen.

 

[Mister Ed]

Just thinking off the top of my head, but I think this is how I would approach the issue at hand:

• use the shaper (or grinder) to even out the gib
• add shims as suggested (although for our purposes we'd probably never need to remove any)
• trim the top side of the spacers a tad if needed

 

[Nogoingback]

I think you're pretty much correct Ed, but the exact approach I take depends on what information I can get from Logan. If I can get the gib dimensions
and the spacer length, then I could either regrind all the surfaces on the gib with their correct heights, and then use the stock spacers or, if I grind one
point of contact down to match the other, I'd have to lengthen the spacers by that amount, (unless I add shims). Because this design is adjustable and
never used shims, I'll only shim it if I can't get the dimensions I need.

My preference is to correct the problem, but assemble it the way the factory designed it.

 

[Mister Ed]

I missed something earlier. I missed the difference in the measurements on the two outward contact points (where the spacers go). I was referring to machining the sliding surface, in order to remove the wear on the one side.

So I will amend my earlier thoughts. I would probably machine the long surface flat and then machine the other two points so they are even and on a parallel plane with the long surface. Then you'd probably have to take a tad off from the spacers. It will be interesting to hear what Scott can tell you.

I have never really understood that design, but I have never had the apron completely out of the way to study it. To me you either have the screws for the gib tite or at some various degree of loose. I have always wondered why the raised (machined) portion on the outboard side of the gib and why the recess in the spacers. In the later models those spacers were a rectangular block, but the gib remained the same part number. Not sure if they had the notch or not.

 

[whitmore]

When I removed the carriage on my Logan 200 a while ago, I noticed an uneven wear pattern on the front
gib where it contacts the bottom of the way.

If the spacers were uneven at the factory, and held the gib at a slant, that explains uneven wear. Because of the wear, the
contact footprint is no longer flat (though the spacers are now even). One really wants to re-flatten the
contact area, if only to make the whole surface hold way oil, and check with the factory on the
correct spacer length (make or buy new spacers if the short one was the out-of-spec unit, because
cutting down the long one... means both are now the wrong depth).
I'm guessing that the bearing surface, and the (fulcrum) surface pressing on the two spacers, have to be parallel,
and that the spacer optimum depth depends on the distance between those two planes. Wrong
spacer height, and the uneven wear goes from front-versus-back to right-versus-left.

What ought one to allow for an oil film? Two mils?

 

[Nogoingback]

Ed, notice that the contact points for the spacers are higher than the bed contact surface. If I machined all those surfaces flat, I'd have to make the spacers longer, not shorter. If I did that, I'd have
to use shims to adjust since the designed in "adjustment" would be gone.

Look at the location of the spacer contact points: outboard of the bolts. When the bolt is tightened it has a tendency to tilt the gib, which tightens the gib on the bed. The notches in the spacers
are there to clear a step in the bottom of the carriage.

 

[Nogoingback]

If the spacers were uneven at the factory, and held the gib at a slant, that explains uneven wear. Because of the wear, the
contact footprint is no longer flat (though the spacers are now even). One really wants to re-flatten the
contact area, if only to make the whole surface hold way oil, and check with the factory on the
correct spacer length (make or buy new spacers if the short one was the out-of-spec unit, because
cutting down the long one... means both are now the wrong depth).
I'm guessing that the bearing surface, and the (fulcrum) surface pressing on the two spacers, have to be parallel,
and that the spacer optimum depth depends on the distance between those two planes. Wrong
spacer height, and the uneven wear goes from front-versus-back to right-versus-left.

What ought one to allow for an oil film? Two mils?

When I first took it apart, I thought the uneven spacers were causing the wear. What I discovered was that they weren't. I think that someone noticed the difference and added a longer
spacer thinking that that would solve the problem, but it didn't. The problem is that the contact surfaces for the spacers aren't the same "height" relative to the surface that contacts
the bed. The uneven wear has probably made the situation worse, since it has a tendency to "tilt" the gib. What I would like to do is regrind all three surfaces to the original factory
dimensions. Then I could use factory spacers. You're right, the spacer contact points have to be ground to a plane parallel to the bed contact surface and the spacer length depends
on the difference between those two planes. That's the info I want to get from Logan.