Backlash Or Just Poor Technique?

[pstemari]

... I have managed to get rid of it by setting the tool BELOW center. ...

Setting the tool below center reduces the rake and increases the clearance. It's also going to increase the inward force on the tool.

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[kingmt01]

I'm sure a follower would but....

After my last roughing cut I take a spring pas before measuring for my last pass. If it comes out tight I'll take another spring pass to see if it cleans up. Boring I usually take several spring passed to try to clean up a hole.

 

[JimDawson]

Setting the tool below center reduces the rake and increases the clearance. It's also going to increase the inward force on the tool.

That makes sense. I never could figure out exactly why it works. The inward force on the tool being increased thus offsets the spring force and the tool cuts exactly where you put it. I have noticed that when doing this it makes no difference if I take a cut at 0.100 or 0.005, it cuts exactly what I dial in.

 

[kingmt01]

So how far down do you drop your tool? I've played with cutting just below center some but then I always need to face the end off. So I put it on center most of the time but If I am making a pass to just get it concentric before moving it back in the jaw or the end doesn't get faced & the tool is a tad low I just run with it.

 

[JimDawson]

So how far down do you drop your tool?

You made me get out of my chair and go out the shop to measure my current setup. I just finished turning six 316SS 6 inch long shafts from 0.5 to 0.375, and my tool is about 0.030 low. As I was making roughing passes, I kept lowering the tool a little at a time until the spring went away. One time I was turning some 3 inch 4150, down to about 2.5 inch. When I checked the tool height on that one it was about 0.125 low.

I normally start on center (because I was facing), then just drop a little at a time until I get the results that I want.
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[kingmt01]

So the right answer is "it all depends" right?

I can remember my little mini lathe trying to learn to use it. You probably don't want to drop any on those because I had the work climb up on the tool a few times. Worse was when I tried to learn threading.

 

[pstemari]

So the right answer is "it all depends" right?...

Yep. It's going to be sensitive to the diameter of the workpiece. Dropping by 0.1" on a 4" diameter piece is equivalent to dropping 0.025" on a 1" piece. In either case you've reduced the rake and rotated the cutting forces by 2.86 degrees.

 

[great white]

This is going to sound "noob", but could someone define " spring" in the context of this thread?

 

[JimDawson]

This is going to sound "noob", but could someone define " spring" in the context of this thread?

Great question! Even though machines and tooling look ridged, they are really not. Spring is the movement of the various pieces of the tooling/machine system when placed under a load. Tool bits bend, and things move around a bit. Hobby class machines are more prone to this than a massive heavy industrial machine, but metal will always flex a bit under load. Generally the tool is pushed away from the work then springs back when the load is removed.

As an example, many years ago I watched a movie of a Cummins engine running taken with a high speed camera. The engine looked like quivering Jello, it was twisting all over the place.

 

[eeler1]

This is going to sound "noob", but could someone define " spring" in the context of this thread?

And really noticeable when boring or doing internal threading. Cutting tools sit out a long ways from th compound, depending on length of the bar. Think "overhang" and you'll get the idea.