Logan 820: How do I lock the cross-slide and compound?

You checked the replacement bolts aren't too long? (Not actually clamping.)
 
Tim, I mean no offense but your post is difficult to understand because of your terminology. It might be a good idea to use conventional nomenclature so we all understand what we're discussing. Movements along the longitudinal axis of the lathe is called the Z-axis. Movements across the lathe from the front to the back is called the X-axis. So, when we turn a work piece we do so in the longitudinal or Z-axis, and when we feed in or face, we do so in the X-direction with the cross slide or compound. I assume that when you say "longitudinal" you mean a turning cut and a "lateral" cut is a facing cut or you are feeding in a depth of cut. If this is not correct, please let us know.

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Issue 1. Over time, I noted replicability issues; there was/is a bit of slop in both longitudinal (what I call east-west) and lateral cutting. I measured carefully each time, but could not pin down the ability to replicate a cut.

Can you clarify this for us? Do you mean that you could not accurately dial in a specific depth of cut and expect the lathe to take that off the diameter?

Issue 2. The compound was just too difficult to adjust. I disassembled it, discovered that both swivel bolts were bent, clearly the cause of my inability to get the swivel to work well. This appeared to have been a traumatic injury, as if something fell on the compound or the compound was used to move the lathe; everything else around it was sound.) I replaced those through the actuator. [Issue 3.] When I got it all back together, it would no longer hold a cut, either turning or facing. It bucks off the work for a cut of more than 5-8 thou, and with the less resistance the tool just pushed away from the work. I noted that the cross-slide, when pushed, moved appr 1/32", you could jiggle it back and forth. Tightening down the gibs did not eliminate this (the 820 has no cross-slide lock, a project for the coming weeks with a friend with a drill press). I could not tell if the loss of lateral rigidity (what I call north-south) has the same cause of the inability to hold a depth of cut.

Not sure what the "actuator" is but you replaced the swivel pins (I assume you mean the swivel pins, part # 180, that lock the compound down to the cross slide) and subsequently noted that there was excessive movement in the cross slide or the compound (do you know which one is moving?), such that a depth of cut more than 0.005 - 0.008" pushed the tool away from the work.

Issue 4. Based on the comments here, lateral slippage was excessive. Based on advice here, the next step appeared to be disassembling the cross-slide more completely. I removed the QCTP and compound again, removed the cross-slide thread nut, and this time opened up the handwheel and collar assembly. It showed that the threads between handwheel and inside jam nut were damaged, perhaps from the same traumatic injury that bent the compound swivel bolts (one can imagine someone grabbing the lathe by the cross-slide handle and compound). With advice from here, I located a 3/8-24 UNF die nut, repaired the threads, removed the nut. As noted above, I was not able during this attempt to dislodge the woodruff key, so could not get the cross-slide screw out, but cleaned out a lot of old crap and reassembled. (Robert, thanks for the download reference; I have that doc here and can now see the woodruff key.) The cross-slide slippage appeared to be much improved, showing almost no free turning of the handwheel.

So, it appears that fixing the damaged thread on the part of the leadscrew that takes up slack between the cross slide leadscrew and nut allowed you to reduce the amount of play in that assembly but ...

I tightened down the gibs, and it held tight, no lateral movement. I stuck a test piece of steel into the chuck; it turned without slippage, but after releasing the gibs a quarter-turn and, with the first facing attempt it bucked, and now the lateral slippage is back again. In fact, it's worse; the entire cross-slide now jiggles both longitudinally and laterally. The carriage and saddle assemblies are firm and rigid. All of the motion is above, with the cross-slide. When I tightened down the cross-slide gibs again, it holds firm. Are the gibs that sensitive?

Which gibs did you tighten - the cross slide or compound? Assuming you mean the cross slide gibs, your issues resolved until you loosened the gibs a quarter turn and the issue of "lateral slippage" recurred.

So, from what I can gather here, it appears that the problem is that the cross slide gib is extra-sensitive, such that you have to run it tight to eliminate turning and facing issues. Is this correct? If so, it implies that the gib or the dovetail is not flat or is not adjusted correctly. I am not clear on how replacing the swivel pins that hold the compound to the cross slide affects this other than that they now work properly and are transferring forces from the tool to the wonky gib situation.

Do you agree with the problem statement? If so, then the guys can maybe help you work through the evaluation and resolution of this problem. As we discussed, it might not be a bad idea to use some Prussian Blue to see what the contact pattern looks like under there. You might also take a really close look at the gib adjustment screws to see if there is any deformation at their tips. With luck, this might all be resolved by replacing the gib.
 
The bent compound screws and the buggered keyway would indicate (as you mentioned) that some severe trauma has occurred. Considering all the adjustment work that has been done without solving the problem I am starting to suspect that the top of the cross slide might be cracked somewhere. Cracks in cast iron are not always easy to see so you need to look for them.
 
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This might be a good one to ask Richard King about.
 
Mikey,
Thanks, as always. Sorry about the bad terminology; I got in a bad habit with that from the start, will do it correctly from now on.
You do certainly understand the problem statement, however clumsily it was worded.
I got the Prussian blue, will do it today. And, I'll inspect the cross-slide carefully for a crack, hoping to not find one.

Also, the actuator is the Logan Actuator, lathe.com. I believe it's either the successor or descendent firm.
Tim
 
And yes, reading your response again: yes, my issue with replicating a cut was exactly as you note: dialing in a cut, after accounting for the dial slippage, stopped being predictible. For a few weeks, a cut I made, always in the .005-.010 range, was pretty close to that, usually over, .002 off. But then, the error range started growing, and then it stopped holding a cut altogether as described. Most of this was done on 12L14, at the .006 feed rate and 420 rpm, so should have been easy cuts.
The problems started after I changed the swivel bolts, and while one might think that that might have caused it, my growing belief is that disassembly upset something that was on the edge of going wrong. The compound swivel was clearly not functioning and had to be fixed, and that install was quite easy, and the compound is working fine now. On to the gib.
 
here are pictures of the cross-slide gib and gib screws, presented left to right as x+ to x-. Is it meaningful that screws 1,2,and 4 are flat but 3 is rounded? All four screw indents in the gib appear the same, cleanly rounded. Do you think the three flat end screws were inappropriate replacements? The indent in the gib screw hole is approx .09" across, which matches the rounded end of the one screw, but the diameter of the flat screw ends is .154", so they would not fit fully into the holes. Could that have created enough slack for the gib to slip during turning or facing if the gib screws were loosened enough to let the cross-slide move? If so, why did it cut well for awhile but then not? Was it maybe in a perfect gib adjustment that I haven't been able to replicate?

The smooth side of the gib has two scratches I can feel, just opposite the 1 screw hole and 2/3 of the way from screws 1 to 2. There is some wear on the x- end, with the sharp edge rounded down, almost like a radius on a cutting tool.

Many thanks,
Tim


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I'm not a Logan guy but I'm going to guess that the flat-faced gib screws are the wrong screws. If so, they would spread force over a relatively broad surface area instead of being concentrated in a tiny part of the gib like the round headed one does. This could also put more force on the upper edge of the gib strip because this is where they would contact the strip. Furthermore, it looks to me like those flat screws were made that way; the tips look like they were inserted into a hole in the end of the screw. Makes you wonder if the former owner did this.

If this was my lathe, I would either make or buy the proper gib screws. I would also replace the gib strip and hope that this works well enough that scraping the dovetails of the cross slide won't be necessary. Ideally, the gib screws would be long enough to allow a locknut to be placed on the screw so they don't move once you adjust them.

I defer to the more experienced Logan owners. I may be totally wrong on this.
 
I'm not a Logan expert and I doubt that this will be the root of the problem, however I can't help thinking about something mentioned in a few posts earlier. About backing off the gib screws ".. a quarter turn.." In my world, that's actually quite a bit to back off. I couldn't find the gib screw size from the Logan manual and parts list, but even supposing they're #10-32 which would be reasonable, say. A quarter turn on a 32 tpi screw backs that screw away from the gib nearly 8 thousandths of an inch ( 0.0078" ). That's quite a bit, and way more than ample to go from a snug fit to a sliding fit.

For what it would take, running the gib screws back in and then backing off less aggressively might yield some better results. Or not :)!

-frank
 
Mikey,
I just wrote to the Logan Actuator to order the replacement gib and screws, in the hope that that will create enough of a solution. If it does, it also answers the "why now" question, as in "why did it work before"? The answer is that I hadn't touched the gib screws before I discovered the bad compound swivel bolts. IIRC, the entire saddle assembly was moved intact when we brought the lathe here. So I'll bet that however they had been set before, to accommodate the flat screws, was still in place until I disassembled it to replace the swivel bolts. Definitely hoping it's a cure.

Frank,
Your point is well taken; my quarter-turn was actually an estimate, it might well have been less, but am pretty sure it wasn't more. I'm going to be more precise with the new gib screws and nuts in hand; these nuts are pretty worn and hard to work with precisely.

Tim
 
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