This seems too simple - differential screw adjustment, am I missing something?

[WobblyHand]

Thought I'd make a differential screw adjustment for my tailstock horizontal position adjustment. My tailstock is out of adjustment and I find it to be incredibly difficult to accurately adjust it. This is for a mini-lathe, which seemingly has been designed for maximum inconvenience in adjustment. My tailstock has shifted to about 0.5 mm offset right now, which has made it unusable.

My idea was to use a 9.5mm x 9.5mm x 62.5mm steel bar that spanned the tailstock casting. The end screws would be 5mm. The bar would have the end holes drilled for clearance for 5mm. The casting would be drilled and tapped for 5mm x 0.8 screws. The center hole would be tapped for 5mm x 0.8 in the bar, and 4mm x 0.7 in the casting. I would need to machine (single point thread) an M4 x 0.7 thread onto and existing M5 x 0.8 screw. This would give me 100 um of movement per revolution of the thread.

To hold the screw, I would use a 1/2" piece of round stock and thread it to M5 x 0.8. I will counterbore the head so I get a longer surface for my ER32 collet to grab. The only issue I see is how concentric I can get the screw in the rod, and the initial fit up of the assembly. And obviously, getting the metric threading done without mishaps. (I have an imperial lead screw, so I would leave the half nut engaged. Mini-lathes have no eBrake.)

Are there other gotchas that I am missing? I'd appreciate any insights. Thanks!

Picture for context.

 

[markba633csi]

Looks like a good solution to me- after all these lathes are kits- you put the features in that you need
-Mark

 

[MrWhoopee]

I like it! M4 is pretty small, but so is the lathe. Single-pointing it might be challenging, go ahead and order the die as Plan B.

 

[WobblyHand]

I think single pointing the thread WILL be a challenge, (for me) so having an M4 die is a good idea. Have an M4 die, although it's not a great one. The plan is to single point about 5-10 mm at M4x0.7. I think I can do that without a center. After all - I am trying to FIX the tailstock which holds the center. I'll have to take a lot of fine cuts.

M4 should be ok for positioning the tailstock. There are two lockdown screws, so the M4 adjuster does not take any load except for overcoming the friction and weight of the tailstock (unless I forget to loosen the lockdown screws). The tailstock moves freely, so I don't think it will be an issue. Diving in...

 

[homebrewed]

I like it! M4 is pretty small, but so is the lathe. Single-pointing it might be challenging, go ahead and order the die as Plan B.

The adjuster screw doesn't need to be all that strong. The tailstock base has a recessed and slotted hole for a bolt that engages the upper part, and that's what holds the two parts together.

@WobblyHand, keep in mind that parts of the differential screw will move .7 or .8mm/turn so you will need to make sure you don't run out of range (or a screw bottoms out) before you get the tailstock aligned.

 

[WobblyHand]

@homebrewed, yes, that's the part that may be fiddly on assembly, Thought I would screw in the 5mm portion about half way into the bar, and then rotate the screw and bar (locked together) into the bottom casting about 5mm. The M4 hole will be about 10mm. If I am thinking about this correctly, then the bar will be close to the surface.After that, fiddle until it works. There should be some combination that works. Right now the tailstock offset to adjust is about 0.5 mm. I'd bet that when the back surfaces are aligned it's close. I may have to shim the sides to get a little more travel. It should work out. I thought a lot about the travel of the screws - been thinking about for a couple of days now!

 

[WobblyHand]

Now that I think about it some more, for the top piece (bar) 9.52mm there's about 8T of adjustment, assuming an initial engagement of 4 threads. This gives me only 0.8mm of total travel adjustment. Might be tricky to get it to work. I might want to make the bar 1/2 inch thick, rather than 3/8". Maybe this bar will be a practice run...

8T in the 4mm x 0.7 hole is 5.6mm. I need to see how deep of a hole is possible. I'm hoping about 10mm. Back to looking things over again more closely... One loses adjustment range at the 4mm x 0.7 end, since one needs 3 threads of engagement. I'll work it out this afternoon.

 

[mattthemuppet2]

is there no boss inside the tailstock (usually on the bottom plate) that a horzontal screw presses against to adjust offset? I swear I saw one in someone elses thread. Usually you then have a screw on each side of the boss (usually threaded into the top casting) so you loosen one and tighten the other to push the top casting over, then lock the tailstock down to the bed.

 

[WobblyHand]

Not on this unit. There is a screw on the bottom, threaded into the top casting. This locks the bottom to the top. There is a set screw that comes in from the side of the top casting that locks against the key of the bottom casting.

I'm making a test piece. Still having a little trouble visualizing this. I think I have designed something with 2mm of travel, using about a 21mm thick bar. That should be more than enough. Prototyping it this afternoon.

 

[mattthemuppet2]

There is a set screw that comes in from the side of the top casting that locks against the key of the bottom casting.

that there is your side to side adjustment. Screwing it in or out should change the relative position between the top casting and the bottom plate that rides on the bed. Have you tried adjusting that to correct your tailstock alignment?