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Spring Loaded Follow Rest

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Robert LaLonde

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#1
I was just thinking about the fact that some metals seem to finish better if you take a bigger depth of cut. 4140QT and most stainless come to mind.

Now, I have some more modestly long tapers to cut. I can't really use a follow rest, and no matter how careful I am using a steady rest in the middle always seems to leave me worry away at a slight mismatch where the first and second half of the turning meets up. If I could cut it all in one pass from your stock a follow rest would work, but the diameter difference is enough that just won't work. If I try to cut the full length with enough DOC to get that good finish I'll get a little chatter in the middle. Would a spring loaded follow rest that engages the work pieces a little before the chatter chows up be able to give enough to finish to the end of the cut and have enough spring force to prevent chatter in the middle of the cut? Ideally it would be finished based on measurements taken from previous heavy cuts, and the follow setup based on chatter marks from previous cuts.

The only other thing I can think of is taper turning fixture paired with some sort of scissor mechanism that moves a follow rest the opposite direction the same amount as the cutting tool. I'm thinking still spring loaded to take up an variance, but then it would be constant back force the length of the cut.

Or is this just another one of my overly complicated dumb ideas?
 

benmychree

John York
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#2
I don't think it would work, the springs would bow the work; sometimes I have used a lead hammer held against long work behind and above the work to deaden the chatter; a guy could invent some gadget to retract the follow rest jaws as the tool moves along, but would the time spent be worth it? If I had to do such a job, I would finish them on a cylindrical grinder with the table swiveled and use a back rest to support the work in the middle; back rests travel with the table and function the same as a follow rest on the lathe.
 

Robert LaLonde

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#3
That's perfectly fine if you HAVE a cylinder grinder, but if its possible to get a finish quality cut on the lathe why wouldn't you. One less setup means its more cost effective to make the part.

Now think about this. There are several ways to tackle such an application.

Remember:
1. A follow rest also exerts force against the work piece, and the work piece can bow away from that force more in the middle than at the ends.
2. The only stated goal is a finish quality cut. I didn't say anything about accuracy or precision, although it might not be as bad as you think.

Options:
1. The most complicated would be a mechanism using a constant force spring. Add on that constant force springs typically have a finite life and cost more it hardly seems worth it.
2. Regulated pneumatic cylinder. In theory it could be pretty good, but in practice a regulator that will be closer force tolerance than option 3 would be quite expensive.
3. A simple compression spring in a tube that is easily adjustable relative to the axis of the work piece. (two actually)

Now #3 is most likely what you thought of when you "pooh poohed" the idea, but it might be the easiest and have the least amount of variance if you think it through. Here is a simple example. An 1/8 inch of taper travel gives you a 1/4 inch of total taper. That's not the most extreme taper you might see, but its a lot of taper. Now if you have the small end of your taper on a center in the tail stock the spring pressure is lowest at that point. At the middle where you are most likely to get chatter the spring pressure increases, and as it approaches the chuck where the work piece is thickest and most firmly held against deflection the spring pressure is at its highest.

(Theatrical interjection) AURGH! SEE MY POINT EXACTLY SPRING PRESSURE CHANGES ALONG THE WORK PIECE !!!!!!!!!!!!!

Um, yeah, a little, but think about it a little differently by putting specific numbers on it. If you use a 10lb compression spring (it can be any practical length) the total variance in pressure is only 1.25 lbs over the entire length of travel. If the spring is 3 inches long you can vary your overall pressure from zero to 30 lbs, and its a pretty simple mechanism anybody could build. Certainly any "real" machinist. Add something like a rubber wheel (using the term rubber broadly) to roll along the work piece and it will help level out and dampen the vibration a little more I think.

You might have to play with adjustment to get it just right, but on the type of part you are gong to need this on you will have to make multiple passes anyway. If you could do it in one pass you could just use a regular steady rest.

I would note that a fellow over on Home Shop Machinist responded, when I asked, that he had made such a mechanism for turning tapers on rifle barrels.
 
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Bamban

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#4
Bob,

How about a pneumatic valve mounted on the follow rest with the rest fingers attached to the stem. Regulated air in should give you constant pressure on the workpiece. I would think a 2 inch diaphragm Bimba with shop air might suffice.
 

rgray

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#5
Skip the follow rest.
Here's why I think this.
My experience is with rifle barrels and It is quickly noticed that dampening is the biggest thing you're up against.
I tried turning with offset attachments and none are ridged enough to give a good finish. That was the first step offsetting the tailstock and gaining rigidity there.
Then all is great till you get into the middle of the barrel where it is the easiest to flex away from the cutter and start the chatter that once started is very hard to machine out.
We would make light cuts and hang onto the barrel dampening the chatter. This gets tiring and boring, but worked.
Happened to have thanksgiving dinner last nov with my sons girlfriends family. One of the guys there worked for Lilja http://riflebarrels.com/ So we got to talking guns and I mentioned the taper turning problem.
He told me of an air cylinder actuated scissor roller affair that Lilja had on their lathe. Like a scissor clamp knurler but air cylinder applying pressure to 2 rolling wheels that were urethane he thought. Mounted with the tool or opposite side of tool post and riding along the barrel same as cutting tool.
He said it worked good and that they had since updated to a 3 roller system.
A barrel is flexible and that can lead to chatter but I imagine it as a tuning fork and mostly it needs dampening as the hand squeezing on the barrel work well all though tiring.
Feeds , speeds, sharpness of tool I'm sure all play their part. The dampening is like the big rubber band for OD of a brake rotor to keep it from "singing" and affecting the finish.
 
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