G.H. Thomas Retractable Topslide for Atlas 12" Lathe

[cjtoombs]

I've had the G.H. Thomas book "The Model Engineers Workshop Manual" for quite a while now, and I've read it completely a couple of times, and sections of it more than that. One section includes a design for a retractable topslide for a Myford 7" lathe. I like the design, and the concept, which makes threading much easier. I have wanted to incorporate one on my 12" Atlas/Craftsman "Commercial" lathe for some time. Doing the model is the first step towards that end state. I first did the G.H. Thomas retractable topslide (partialy) so I could get a more detailed idea of how it was designed than the text gives. To that end, I have completed the model in Fusion 360. There were some changes I made to the originals design intent that I will go into here. This design directly uses some of the parts from the GHT design and uses the handle and handle nut from the Atlas. The changes I made were as follows:

1. The leadscrew uses deep groove ball bearings instead of bushings to allow some preload to eliminate once source of backlash. It should also give a somewhat smoother performance.

2. It has the provision to have a 2:1 gear ratio so that you can halve the speed of the topslide movement relative to the input shaft. This was done mainly to make the micrometer dial easier to read, as it is only slightly larger than the factory one. This has the negative effect of requiring twice as many revolutions to move the topslide as before. The design is such that this can be easily changed to 1:1 by swapping the 40:20 tooth gears for 30:30, in case I don't like it.

3. It uses 3/8" thick gears instead of the 1/4" that the GHT uses, this gives more engagement when the topslide is retracted. The design of the gearcase is has been changed to allow for the additional thickness

4. It incorporates a backplate. I wasn't sure that the bearing sleeve and nut would clear the internals of the topslide, so I designed this to eliminate any problems that might have occurred. The design of the custom leadscrew takes this into account.

It is, besides the aforementioned changes, functionally the same as the GHT version. One other thing that could be done is to add a bushing in the gearcase and make the input shaft eccentric from bronze to give better bearing surfaces to the input shaft. Below is a render. I still need to do a little work to clean up the internals and complete the mechanism that locks the micrometer dial.


It may be a while before I get around to actually making this, as I have a lot of other stuff in front of it. I'll probably 3D print the more difficult parts just to make sure that I haven't done something with the design that I will regret in use. It's easier to reprint prototypes than it is to machine them. Take care.

 

[benmychree]

Leaves me wondering how it works ---

 

[cjtoombs]

Leaves me wondering how it works ---

The handle on top turns a Scotch yoke mechanism that pushes that bearing housing that the leadscrew rides in back and forth, which pushes the topside forward and back. There is a lot of adjust ability built into the design, which adds some complication. Based on what I could gather from the book, you adjust it so that when the handle in the rear position (topslide forward) it is slightly over center so that it locks the topslide in that position. I expect that if it adjusted properly it should be repeatable in the forward position to a fairly small dimension. The main purpose of the gear set is to move the input handle out of the way of the tailstock when doing certain operations on the lathe. I used it as an oportunity to change the gear ratio and increase the size of the micrometer dial as well The same chapter in the book has a geared topslide without the retractable feature for this purpose.

 

[markba633csi]

Nice renderings- I see what looks like a glitch on the gear teeth in the bottom picture- do you see it also?
-Mark

 

[cjtoombs]

I see what looks like a glitch on the gear teeth in the bottom picture- do you see it also?
-Mark

Yeah, I fixed it. Looks like another extension took a bite out of it. One of the problems I have with Fusion is that when you need to go back and fix something that was done early in the process, it sometimes has bad effects on parts that were made later. Probably my process, as I'm mostly self taught, but it's annoying.

 

[cjtoombs]

A better view that goes to the explanation about how it works above. I removed the sleeve that goes around the crankshaft. The crankshaft is attached to the handle. The sleeve is eccentric, which allows the adjustment needed to ensure you can get it to lock over center. I changed the block in the scotch yoke to bronze so it will be easier to see. The part has the slot that it runs in is the bearing carrier for the leadscrew and has 2 deep groove metric bearings that the leadscrew runs in. Unfortunately, I can't do cutaway views easily in the render environment. The bronze cylinder in the background is also ecentric, which allows adjustment of the backlash in the gears. Overall I think it's a pretty clever design, I wish I was the one who came up with it. On a side note, I would also be nice if I could just hit a "Render" button and get those kind of surface finished in the shop in about 2 minutes



Added the rear view on edit showing the eccentric sleeve. The expectation is that little movement will be needed, so the front is adjusted by allowing the gear case to move slightly to account for movement of the eccentric.