I remember seeing this mod on a Stefan video. I guess our bigger offshore lathes could have the same issue depending on the design. I always assumed maybe my tailstock base was slipping back on the ways since they are usually nice and lubey and my clamping shoe is a piece of sh*t that will get replaced shortly. Could be the quill is prone to drifting. I'm not really sure how vital or necessary a secure lock is but anyways an interesting project.
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tail stock mod
- Thread starter PeterT
- Start date
I watched that video the other day. His Jet GW1340 if very similar to my lathe but mine has a different tail stock quill lock. The locking handle is on top of the tail stock. I havent taken it apart to see how it works but it seems very solid.
Yes, I think that's my issue. I'm going to make a larger & better fitting shoe. If it works I'll upload the sketch & pics. I was thinking of bonding something like brake pad material to the existing shoe but I think increased surface area & better fit will do the trick.
Interesting discussion gents... here are some of my observations regarding clamping shoes:
On some lathes, they are just a flat plate with a hole to receive the clamping bolt. Some have a better design with ribs on the bottom of the plate to improve stiffness and resist bending. It is this bending under load that actually is the problem of the TS sliding.
This is what most decent TS shoes look like - flat and coplanar clamping surfaces
and the assembly at rest
and under load (clamped)
due to bending of the shoe, there is only a small contact area. The harder you clamp, the smaller it gets and thus the easier the TS slides under load.
A better design would be to slightly incline the clamping surfaces so as you apply force, the bending actually increases the contact area and thus a greater axial force is required to slide the TS away.
I have not modified my TS shoes to this design yet. I would only mill about thou or so off the inside of the flats to begin with to give them a very small angle.
I have found that when my TS slips, I have overtorqued it and / or am using tools that are dull. At least one of my live centres has internal expansion compensating features that allow the part to expand without it pushing the TS back, thus maintaining relatively constant center pressure. I can’t find it right now, but an expanding part puts a huge axial load on any supporting center and easily overcomes most TS clamping forces. It is actually a good thing that the TS slides back (if you don’t have temp comped centres) otherwise you would bend the lathe bed while turning. And possibly overload your spindle bearings.
On some lathes, they are just a flat plate with a hole to receive the clamping bolt. Some have a better design with ribs on the bottom of the plate to improve stiffness and resist bending. It is this bending under load that actually is the problem of the TS sliding.
This is what most decent TS shoes look like - flat and coplanar clamping surfaces
and the assembly at rest
and under load (clamped)
due to bending of the shoe, there is only a small contact area. The harder you clamp, the smaller it gets and thus the easier the TS slides under load.
A better design would be to slightly incline the clamping surfaces so as you apply force, the bending actually increases the contact area and thus a greater axial force is required to slide the TS away.
I have not modified my TS shoes to this design yet. I would only mill about thou or so off the inside of the flats to begin with to give them a very small angle.
I have found that when my TS slips, I have overtorqued it and / or am using tools that are dull. At least one of my live centres has internal expansion compensating features that allow the part to expand without it pushing the TS back, thus maintaining relatively constant center pressure. I can’t find it right now, but an expanding part puts a huge axial load on any supporting center and easily overcomes most TS clamping forces. It is actually a good thing that the TS slides back (if you don’t have temp comped centres) otherwise you would bend the lathe bed while turning. And possibly overload your spindle bearings.
Here is a page out of the Cazaneuve HB500 / HB575 manual. I do not have an English version. My French is not the best but the gist of it is that their TS apply a clamping force of ~ 1000 kg.
Then they go into an example of how much force is applied to the TS: if you take a 500mm long piece, 50mm diameter, and heat it by 30*C above where it was clamped (by a machining operation, for example), it now applies an axial force of ~ 14000 kg on the TS!
If something does not move, something will bend.
Perhaps someone with better French can chime in and correct my interpretation, thanks.
Here is a link to “Why good Live Centres Go Bad”. Not 100% about Tail Stocks, but related none the less.
https://www.ctemag.com/news/articles/why-good-live-centers-go-bad
Then they go into an example of how much force is applied to the TS: if you take a 500mm long piece, 50mm diameter, and heat it by 30*C above where it was clamped (by a machining operation, for example), it now applies an axial force of ~ 14000 kg on the TS!
If something does not move, something will bend.
Perhaps someone with better French can chime in and correct my interpretation, thanks.
Here is a link to “Why good Live Centres Go Bad”. Not 100% about Tail Stocks, but related none the less.
https://www.ctemag.com/news/articles/why-good-live-centers-go-bad
I think you are right, something has to slip to prevent damage to the lathe bed or spinďle. My tailstock clamp can be made to slip but after reading your thoughts I'm not going to try to reduce that tendency. My gripe with my tailstock clamp is the spongy feel when applying the lock and I am alway having to fiddle with it to keep the engagement point adjusted. Having a look at the hardware revealed the cause of those problems. The bolt that fastens the shoe to the cam follower assembly was so soft I could see visible stretching along the threaded portion. Also the hole in the shoe is so big that the head of the bolt and the cheesy washer on it bends and trys to pull through the hole eqch time it is applied. My shoe is over an inch thick so I don't think bending is too much of a factor in the spongy feel. I decided to improve the hardware by replacing the crappy bolt with a stud made from a 1/2" grade 8 bolt. I threaded the end that screws into to cam follower to M12 X 1.75 and machined a 3/8" thick by 2" round nut with 1/2" fine thread to match the original threads on the bolt. I put a straight knurl on the OD of the nut and milled the shoe flat where the nut will tighten against it to improve consistancy of feel when adjusting. I actually had to remove .025" of material from the shoe face to get it flat. The result is much better, the spongy feel is gone. Time will tell if the adjustment changes by itself, I may have to add a little interferance in the threads to prevent the nut from migrating down the threads.
The bolt on the left is the original, you can see the grade marking on the head is 4.9. The bolt on the right had a grade marking 8.8 which if about the same as an SAE grade 5 so the original bolt is made from balsa wood or something about that strong.
You can see the cam follower with it's M12 thread hole in this pic.
Here is the shoe next to the one for the steady rest, pretty beefy.
This is the shoe face after .010" was removed.
Here is the nut and stud I machined.