Follower rest tips?

[Chipper5783]

I am making a lead screw (not for a machine tool) and struggling. The target size is 0.750, the blank is about 19" long and material 316SS - then 15" of 3/4-10 thread. I started with 1" bar (being what I had on hand) and when simply cutting with only the chuck and tailstock for support, found the vibration excessive for any decent cut. So I took it down to 0.77" by cutting advancing 5" at a time out of the chuck (still with tailstock support) - getting it roughed out was pretty quick.

Now I'm trying to get it down to finish size and finding it "not so easily done":
- if I use carbide (positive rake inserts), I can take a pretty small cut (few thou) - the ends are the same size, but the center is 0.003" larger. Not a big surprise that the work deflects.
- if I use a HSS tool ("Diamond" holder), then I can hone it up real keen - but with a small cut the tool edge has started to turn by the time I get to the other end - so it starts to cut a little larger by the time I get to the chuck.

- I tried to set up the follower rest. I have never used the follower rest on this lathe. I used a cobbled together one on my other lathe when Acme threading - pretty dodgey but got the job done. The follower rest looks pretty straight foreward, not a lot of moving parts. I chose to put the support pad on the new cut (since the shaft was slightly barrel shaped) - the just cut surface would be the reference. I cut about 1" long to size from the tailstock end, set the pad about 3/4" behind the tool. All seemed to be good. Then it started to set up a wave - feeding back to the support pad (with nearly 0.005" variation in diameter).

- I can go back to my carbide inserts, get the ends right and tweak the cross slide as I'm feeding - then finish to size with a file. That seems rather defeatist!

- Threading will probably be okay, I'll just use the rest on the last few passes. I'll put the rest infront of the tool, and since the reference surface (the top of the thread) is not changing all should be good (which is what I thought when sizing the bar!).

Anyone have tips / pointers on using a follower rest?

[Harold_V]

I question your choice of material. Stainless is not known for its bearing qualities (the 300 series galls easily), and the majority of the 300 series alloys are not pleasurable to machine. Further, they can become troublesome when you must take a few light passes, as they work harden easily. Don't take any passes with an even remotely dull tool for that reason.

You've have been better served choosing Stressproof.

Harold

[earlgo]

Harold, in my opinion, forgot to capitalize WORK HARDEN.

--earlgo

[Carm]

Best to have the follower opposite the tool, if possible. It can be mounted to the toolpost or compound.
Make sure your depth of cut exceeds the hone on the insert, otherwise you are rubbing and as said above, work hardening.
Typical non-aluminum inserts have a radius (not the nose radius) and are not dead sharp, even though positive.

[Russ Hanscom]

I recently made a long 5/8" dia screw with about 10" of acme thread. A follower rest was essential to getting a constant diameter. My wear tips were brass, and the thread tended to cut into them a bit; so resetting the follower after every pass is necessary. I took a lot of final passes with either .001 or zero infeed to get a uniform diameter. The follower was almost directly behind the cutting tool. I was probably using annealed 4140 stock.

Agreed that stainless is not your friend if you want to very fine cuts and if the application does not require stainless, look for a more friendly material.

[liveaboard]

I'm a real deadbeat when it comes to buying things I can make myself; but ready made Acme or metric leadscrew is very cheap.
Even here in overpriced overtaxed Europe.
Just saying...

[spro]

I wasn't familiar with Smart & Brown until I saw your pics of the 1024. Super Great machine and further than that, a window into the engineering. While I cannot offer any advise, the search of Smart & Brown does clear up some questions we and other sites have had. The models prior to this of a series SA, Sabel, were a type similar to a South Bend 9. I remember questions about a tailstock or identity of some part and it wasn't Logan, Sheldon,. anyone except South Bend. I didn't know about this history and how they would be so similar. Great post showing a model "A" of the 1024 when they were "styled".

[wsippola]

Nice lathe! Is that a Smart and Brown follow rest? I have one just like it, no idea what it came from. I'm reserving it for my 10EE, but it will need a minor mod for the mounts. I don't have any advice that's not already been given, but I'll agree the material should be something that machines well, or you will have serious finish problems which will quickly wear the nut.

[Harold_V]

but I'll agree the material should be something that machines well, or you will have serious finish problems which will quickly wear the nut.

That's generally not a problem with 316, which machines with a nice finish, assuming sharp tools. The real problem is what I mentioned----it galls easily, and work hardens. If sharp tools are used with acceptable depths of cut, the resulting finish tends to be very good.

An EE, eh? I spent more than a year on one, although many years ago. Best lathe I ever operated.

Harold

[Chipper5783]

Thanks to all for their input. The choice of stainless steel material was because of the application (it is the advancing screw for an old meat slicer). I did not go with the option of simply purchasing threaded rod, as the securing end is to be smooth and is secured by two snap rings on either side of the frame. Certainly one could develop a redesign, however the intent was to replace the component in kind.

Carm probably makes a good point about having the follower rests opposite the tool and to even mount it on the tool post or compound. I don't quite follow that, since if the pads are opposite the tool, then they are sitting on a ridge.?? I have not seen a works built follower rest intended for mounting on the compound or toolpost - but many that mount on the carriage - so I was hoping to use the attachment supplied with the machine. Again, I have not used a follower rest before, so I don't doubt Carm's wisdom (I'm just going off posts, pictures, books - the usual sources to learn something).

Yes, the machine is a Mk1 S&B 1024 (not a model "A" - that is another product S&B offered, 2" smaller swing). A real classic English machine, and a joy to operate. Though a 1963 vintage, it offers change on the fly spindle speed - even while threading (since the constant mesh gear box is before the threading take off). Quite easy to bring threads right up to a shoulder. Not as good as a high speed threading unit, but still nicer than my gear jammer that is 20 years newer.

Anyway, I attempted to map out the change in diameter, then advance the tool (using the compound swung way over) as the carriage travelled. That may have worked out okay, but I didn't pull the tool back enough, so ended up over cutting as I got close to the chuck. Oh, well - start over.

For the next attempt, I scrounged up another piece of scrap metal - a 400 series stainless (probably 416 ?). Much nicer material to cut. The tool stayed sharp (I still honed it after every pass), I could take small cuts, and the follower rest performed as one would expect. I was looking for 0.750" and achieved 0.7494 - 0.7503. I'm quite satisfied - hope the threading goes as smoothly.

I'll further my 316SS education on some other project.

[Harold_V]

If the material machines nicely, and is magnetic, then it most likely is 416, which is the best of all stainless grades for machinability.
Thanks for your explanation of material choice. It makes sense when one understands the application.

Harold

[Chipper5783]

The threading worked out well (you are certainly right Harold about this material machining well). I forgot to take pictures of the threading set up - pretty straight forward. I took it to about 95% complete supported only by the chuck and tailstock. I could not take much of a cut or it would start to chatter (probably made 40 passes?). I did not use the follower rest because I thought the chips would foul the follower rest pads, any burr kicked up would chew the pads and the little cardboard shield would be a pain.

I then set up the follower rest to finish it. I still ended up miking the thread every inch or so, and had to tweak the depth along the thread to get it to land on the published thread pitch +0.000 to -0.0015" the whole way along.

I kept the little cardboard shield between the rest and the tool to reduce chips getting under the pads (and at that point of the job there weren't a lot of chips). I also glued some real hard plastic onto the pads (found the bar had worn into the pad, perhaps the thread would have really chewed on the brass).

I guess the whole exercise took a while, but it turned out great. Thanks to all for their input. David