Inconsistent tailstock clamping surface thickness

[Dirtmover]

I've had my mini lathe for a number of years and have always accepted that something did not seem quite right with the way the tailstock clamps. I can secure it near the head then when I go to move it I loosen the nut off a bit and slide it towards the end of the bed. After a few inches it will bind so I need to loosen the nut a touch more then I can slide it a bit further.

Clearly there is a taper on the clamping surface under the bed. To put some numbers on this I measure about 0.75mm difference in the thickness between the head and tail ends of the bed. Is this normal or even common?

There is no play on the carriage so the front and back surfaces under the bed do not follow this taper.

Why does it even bother me? Well I was planning a couple of upgrades, 1. a cam lock for the tailstock and 2. a carriage lock. Both are clamping on these surfaces and I feel that the taper is going to be problematic for both but especially for the cam lock which ideally needs to lock close to the peak of it's travel along the full length of the bed.

I'd like to remove this taper but it's not the easiest place to work. Any suggestions how to address this?

 

[C-Bag]

Like the lady said “you gotta live it, or live with it!”

My HFT 9x20 lathe has the same problem and that’s the main reason I modded a cam lock from the old bolt setup. Yeah, it clamps in different places as far as where the lever ends up clamping, but bottom line it’s easy to just clamp it wherever and it doesn’t run out of throw and not clamp. So to my mind it’s a win win. I hated messing with the bolted setup, the cam lock just works.

 

[Firstram]

My 7x10 was machined with a hatchet. If I adjust the tailstock cam lock for mid travel that's the only place it works. Someday I'll machine the bottom of the ways.

 

[whitmore]

One easy solution: add a stiff spring and washers in series with the cam.
Just enough yield in the spring to compensate for the irregularities,
you want good clamping pressure at all locations, but the cam still has to release to
completely loose when repositioning.

Maybe a neoprene washer would suffice as a spring.

 

[Dirtmover]

My 7x10 was machined with a hatchet. If I adjust the tailstock cam lock for mid travel that's the only place it works. Someday I'll machine the bottom of the ways.

Yeah, that's exactly what I was concerned about. The friction that holds the cam in position varies with it's rotational angle. Sounds like the problem is common but not sure how to machine the underside in such a restricted space.

One easy solution: add a stiff spring and washers in series with the cam.
Just enough yield in the spring to compensate for the irregularities,
you want good clamping pressure at all locations, but the cam still has to release to
completely loose when repositioning.

Maybe a neoprene washer would suffice as a spring.

Hmm, not sure about that but you triggered an interesting idea that I've not seen yet. Maybe I could work it back to front from the conventional approach with a spring keeping the lock constantly engaged and the cam pushing it downward to disengage. A bit like a valve on an engine. I've got some pieces of pretty stiff spring left over from an oven door repair that would probably be good to experiment with.

 

[Firstram]

Yeah, that's exactly what I was concerned about. The friction that holds the cam in position varies with it's rotational angle. Sounds like the problem is common but not sure how to machine the underside in such a restricted space.

mine is so bad I’ll have to mill it with a key seat cutter if I ever fix it.

 

[RJSakowski]

I had that problem with my Grizzly G0602. The underside of the ways aren't machined to precision. There is a .020" taper from left to right in the ways on the region where the tailstock ride. This required removing the wrench on the tailstock nut and repositioning it in order to fully lock the tailstock. I replaced the M12-1.75mm bolt with a 1/2"-13 tpi bolt and I can lock the tailstock with about a 120º rotation of the wrench from a position where the tailstock is free to traverse the entire distance of the ways. (https://www.hobby-machinist.com/threads/g0602-tailstock-driving-me-nuts.87166/page-2#post-782136, post #'s 13 &16)

This was the easy fix. However, if I wasn't able to increase the clamping range sufficiently, I would have made a bolt with a custom thread, e.g, 1/2"-10 tpi

 

[Dirtmover]

I had that problem with my Grizzly G0602. The underside of the ways aren't machined to precision. There is a .020" taper from left to right in the ways on the region where the tailstock ride. This required removing the wrench on the tailstock nut and repositioning it in order to fully lock the tailstock. I replaced the M12-1.75mm bolt with a 1/2"-13 tpi bolt and I can lock the tailstock with about a 120º rotation of the wrench from a position where the tailstock is free to traverse the entire distance of the ways. (https://www.hobby-machinist.com/threads/g0602-tailstock-driving-me-nuts.87166/page-2#post-782136, post #'s 13 &16)

This was the easy fix. However, if I wasn't able to increase the clamping range sufficiently, I would have made a bolt with a custom thread, e.g, 1/2"-10 tpi

Ah, good idea. Space permitting, a semi-permanent wrench could be OK. The key I'd missed for that solution to work for me is the coarser thread!

 

[keeena]

My Grizzly G4003G lathe also has some minor deviation, but b/c it's a cam system it's not quite as big of a deal.

A spring eliminates binding when sliding the TS back/forth caused by the shoe tilting/moving around. While won't help with your adjuster nut position, it's certainly worth doing on any lathe IMO.

 

[homebrewed]

I replaced the original tailstock nut on my 7x12 with a home-made version that has holes drilled in it to accommodate a tommy bar. I determined the best hole locations before drilling them.

In some cases the compound crank gets in the way of a straight tommy bar. For that situation I use the short end of an allen wrench since it gives me a different angle on the problem, so to speak.

I have seen some projects where the lathe owner used a flat file with a simple jig to flatten the underside of this type of lathe, but that was to improve the saddle fit. It contacts the bottom in different places compared to the tailstock clamp, and they are much easier to get to. It _might_ be possible to lap the regions where the tailstock clamp goes but that could be very laborious, depending on how bad the fit is.