Using A Steady Rest????

Chipper5783

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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 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?

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It might be the chuck jaws tweaking the part off axis because they are not completely true. You might try turning it between centers. The follower rest looks OK, but the tool location is not shown, and of course we cannot guess the pressure against the work.
 
You can look at this 2 ways: A. Lose sleep figuring out the follower rest situation and all the quirks, or B, Finish the job by running a die over it to uniform the thread diameter. I would go with plan B.
 
It might be the chuck jaws tweaking the part off axis because they are not completely true. You might try turning it between centers. The follower rest looks OK, but the tool location is not shown, and of course we cannot guess the pressure against the work.
I put the follower rest on with about 0.01o" to go. I was taking another 0.003" - recognizing that there may be challenges, I didn't want to wreck the part (i.e. play around in the scrap material). I appreciate that trying to take a small cut can cause problems, however I'm down to the point where that's all the metal that I have left (or start over).

The rest had a round cut out (obviously previously used) about 0.77 - close to the size I would need. I'll not so sure that having the work push back/up against the curved surface is going to be very predictable. I'll try flipping the pad and go against the horizontal/vertical supports.

derf: I agree with you about a simply running a die over it. I don't have one, but may be able to borrow one.
 
I am nowhere near an expert, but I think the three jaw chuck-runout (compared to the position of the tail stock is the problem). You can turn your own dead center in the three jaw, then adjust the tail stock (set screws for side to side and shims for z)to match the "new" dead center (see tubalcain video on that last part). If the two ends are out of alignment you will turn a taper. The follower rest will fight that.
 
I've only used a follow rest a few times so I'm no expert David, but I think its the geometry of the support pad thats giving you grief. The ones I've used have two supports, same idea as a steady, with one pad on axis behind the cutter and the other pointing down and slightly back contacting the work slightly toward the cutter from centre. In other words the supports are slightly more than 90 degrees apart, maybe 100. With your setup if the pad was cut for .77 it only touches the work at one point, when you add pressure from the cutter it will try and roll on the pad at the point of contact. The rolling action will take it away from the cutter till the load eases then spring back into the cutter.
I think if you recut the pad to a smaller radius it would have two small points of contact and behave better. Ideally where one point is behind the tool and one forward so the work can't dig into the cutter.
Again no expert just some thoughts.

Greg
 
I thank folks for their input. I'm still mucking around with this - figuring out how to cut a fairly uniform bar seems to have become a mission for me. Attached below are pictures of my next attempt with the steady rest (using the flats instead of the hole). I switch back to the diamond holder, this time with a "Crobalt" tool bit (a cast HSS, supposed to be a bit harder and handle heat better). The result really wasn't much better.

I recognize that small cuts and the material choice (316SS) are really working against me. However: A. I'm running out of material (only a few thou to go) and I'm trying to get better at working difficult materials.

Next effort was to get rid of the follower rest, map the size along the bar and advance the tool during the cut (compound swung way over). Started to get the "waves" worked out, but then ended up overcoming down at the chuck end (didn't provide enough compensation for back lash). OH WELL, call it education and round up another piece of metal!!

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Sorry thought you were at the threading stage David. Try a shear tool, I've had great luck with them cutting long slender parts. They are used on a shaper for the finish cut but work equally well on the lathe. A Google search should find you one.

Greg
 
I did try a shear tool on the 316 material - the result was no better. It shaved pretty nicely at first, but after about 5" of travel it started to just rub.

So the next piece of metal was a 400 series stainless (probably 416 pump shaft). Much nicer material to cut. Without the follower rest, I only had 0.002" increase in diameter at the mid point. Also, the Crobalt tool kept a decent edge for numerous passes (hand honed after each pass). Follower rest was much better behaved - no issue with skimming very small amounts. Ended up with about 0.7494" to 0.7503" - pretty happy with that. Now I'll be onto the threading.

Thanks again to all for input.

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The threading worked out 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 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 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 it).

I guess the whole exercise took a while, but it turned out great.

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