harmonic looking pattern in slender turning help needed

[Your Old Dog]

I'm trying to turn down from .590 to .561 a piece of crs about 10 inches long. It's between centers.

When working the ends it turns fine with a nice finish but when I get to the middle third I get a pattern that is what I feel a 60 cycle affect would look like. It chatters in this area.

I've adjusted both speed and infeed and kept an eye on the tailstock live center tension. I've tried several bits and they are all freshly sharpened. The toolholder is a 2.5 inch solid steel cube made by a machinst. It uses 5/16 bits and they're kept close to the holder, not extended out in anyway. Also using a follow rest.

I'm taking extremely light cuts, on the order of .001 per pass as I know this is a slender piece to be worked over 10 inches long. I thightend up both gibs and the lathe is a SB9A in very good condition. ( I know the history of it)

Because I'm making an arbor I wanted to work between centers instead of using the chuck. Any suggestions besides taking it to a machinist or calling a priest?

Thanks for any help you can give....


[This message has been edited by Your Old Dog (edited 07-27-2005).]

[Marcus]

<font face="Verdana, Arial" size="2">Originally posted by Your Old Dog:
I'm trying to turn down from .590 to .561 a piece of crs about 10 inches long. It's between centers.

When working the ends it turns fine with a nice finish but when I get to the middle third I get a pattern that is what I feel a 60 cycle affect would look like. It chatters in this area.

I've adjusted both speed and infeed and kept an eye on the tailstock live center tension. I've tried several bits and they are all freshly sharpened. The toolholder is a 2.5 inch solid steel cube made by a machinst. It uses 5/16 bits and they're kept close to the holder, not extended out in anyway.

I'm taking extremely light cuts, on the order of .001 per pass as I know this is a slender piece to be worked over 10 inches long. I thightend up both gibs and the lathe is a SB9A in very good condition. ( I know the history of it)

Because I'm making an arbor I wanted to work between centers instead of using the chuck. Any suggestions besides taking it to a machinist or calling a priest?

Thanks for any help you can give....


[This message has been edited by Your Old Dog (edited 07-27-2005).]</font>

I think you will find that taking such a small cut will not allow you to remove the chatter pattern. Try the different feeds/speeds on a new piece and see if it is different. Of course the good book would say to use a follow-rest which would provide support for the job while it is being cut.

Let us know how you go.

cheers, Marcus

[wierdscience]

Could be the length or could be stress in the steel trying to get out,or could be any number of things.

Do you have a follow rest?

[Your Old Dog]

thanks guys. Yes I have a follow rest. But now that I think of it, is it a "follow" or "leader" rest. Mine mounts on the right side of the apron. But, if you're cutting right to left then I just removed the metal the follow rest was going to ride on? Maybe I have something else to try tomorrow?

[J Tiers]

The choice..... follow behind or follow ahead.....

You take the first part of your cut, then "follow behind". Think about it, when rough turning, you'd have to, as it wouldn't run true before being cut.

If your toolholder will let you, you can follow "at" and have what amounts to a box tool setup.


Oh, yeah, unless you have a very rigid setup with follow rest, you won't get rid of that pattern easily.

Only way that has worked for me (other than support) was to run up to max rpm, then take light cuts. Gets the pattern frequency too high for the mass-spring system formed by the work etc to follow.

If your piece is whippy, that might not be a good idea.



[This message has been edited by J Tiers (edited 07-27-2005).]

[Your Old Dog]

<font face="Verdana, Arial" size="2">Originally posted by J Tiers:
You take the first part of your cut, then "follow behind". Think about it, when rough turning, you'd have to, as it wouldn't run true before being cut.</font>

I'll do that. I just bought some more crs and will try as you suggest. Now that I think about it, I don't think my follow rest was doing anything at all except going along for the ride! I suppose I could have tried cutting some of the waste off the piece. I was working with 10 inch piece to make a 6" part as I thought I'd have trouble with the threaded end.

thanks, I'll let you know how it goes.
ray.....

[Boucher]

There is a technique where you use a 4 sided tool post. Start with a tool on the left and cut toward the headstock. When the right side gets far enough out put a brass bar in it and move it into contact with the work and lock it down. This must be repositioned on each pass but it dampens the vibrations in the work.

[darryl]

Maybe you'd need three hands for this idea, but two hands will do it. If you take a heavy metal caster or some similar weighty roller, hold it against the workpiece opposite the cutting bit. It will roll as the workpiece turns, and will raise the effective mass of the work at that point and possibly prevent the problem. Play with the positioning of the roller against the work to find the sweet spot.

[bigmac]

Since you are only taking off 29 thou I would take 20 thou on my first pass then take the rest for a finish pass. Increase your speed and slow down your feed for your finish cut. Run your tool across your work again to clean it up because there will be deflection in the work. Along with such a light cut, deflection is probably what caused the chatter in the first place IMHO.

------------------
Chips Ahoy

[topct]

I'm curious as to what form the cutting tool has been ground to. I have trouble when I make the point too round.



------------------
Gene

[Your Old Dog]

<font face="Verdana, Arial" size="2">Originally posted by topct:
I'm curious as to what form the cutting tool has been ground to. I have trouble when I make the point too round.</font>

Topct you are right. I started with about a 1/16 radius and then went to a planer bit with 1/8th radius and just using the radiused portion on extremely light cut and it chattered even harder.

Boucher: Your idea brings lots of other ideas to mind. It wouldn't take much to redesign a tool rest that allows what you mention only from the other side of the work. Certainly warrents a closer look.

I hope to work on this tonight and I'll get back to all to let you know how it worked out.

[Paul Alciatore]

I don't know if it works as I have never tried it, but I read somewhere that a bag of lead shot with a hook that provides some friction, can be hung on the work near the cut to help dampen the vibrations. I guess you would have to move it along as the cut progresses.

From my experience I would second the suggestion of a small radius on the tool tip. And perhaps a greater rake angle.

Other thing is you might try a better alloy. But I am not sure which to suggest.

Paul A.

[J Tiers]

Chatter is normally directly related to the length of cutter edge in contact with teh work. Going from 1/16" to 1/8" radius would be expected to increase chatter, other things being equal.

Also, with a tool like that you have lots of inward force. Inwards force also increases chatter in many cases, unless you use a properly adjusted follow rest.

And, the radius produces a long length of cutter in contact at any given depth of cut. Yes, it may be the smoothest, theoretically, but if that is at the cost of chatter, you lose all advantage.

If you grind the cutter to cut on a relatively straight edge perpendicular to the work axis, the cutting pressure is lengthwise. I have found that such a grind, with lots of side rake, enables me to do things otherwise impossible, like turning down an inch of stock to 0.093 diameter with no work support, in one step.

It seems to avoid chatter in many cases. Maybe this is because it has the absolute minimum length of cutter in contact for any given depth of cut.

[Boucher]

J Tiers: I am not sure I understood what you were describing. There is a tool configuration that I read about and tried to construct and use. It is for taking a fine cut on brass. the tool is square across the front edge with sharp side rake like 30 deg. the cutter then has very short contact at the tangent point. I was trying to make a 18" accurate brass honing bar. I didn't have enough material to take the heavy then the fine cut. I never could get rid of the chatter and have never got back to the project, but still curious about the cutter configuration.

[jburstein]

I believe you want to use zero or negative rake with brass. Brass is very grabby, and a tool with positive rake will get sucked into the material, causing chatter or worse. I've had success with zero rake tools on brass, and have heard that negative rake tools work well, but have not tried that yet.

-Justin