Shaft Turning (Mis)-Technique

GBCues

GBCues

Damn, still .002 TIR!
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Gentlemen,

I have found on occasion that I am turning down or tapering a shaft and all is going well. But then I make one more pass at the same cut and the middle (or near middle) of the shaft will wobble or oscillate, causing a deep groove at that point which may or may not be recoverable.

SCDiveTeam in another thread mentioned "After I put the squares through the doweling machine there is a much greater degree of straightness from the 5/4 stock dowels when I roll them. Over 90% of my shafts do not oscillate in the center at 1500 rpm and many can go up to the highest speed when I put them between centers on my wood lathe for sanding and sealing. The ones that do vibrate are due to lighter density and I reject them."

I use a Porter Cable router and a slot cutting bit mounted to my cross-slide for turning. I have seen this occur on both maple and purpleheart shaft dowels.

So is my problem technique and lack of experience (most likely) or is it the shaft wood or some combination of all the above?

Thanks in advance,

Gary
 
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It might be that you are tightening it up a little too much in the lathe and as the wood is cut off the blank gets thinner and may bend in the centre.
 
Might help to know afew things about your set up. What type lathe, what rpm and feed are you using.

Larry
 
You could be causing a bend if you tighten up on the blank to much as it gets thinner. What type up lathe you have and how you set up for a pass and the feed rate might give some insight too. Especially if you say it happens with purpleheart as well as thats some stiff wood.

I had a similar problem when I first started, I was turning at to high an rpm and it caused chatter in the middle......alot of chatter. You want to turn fairly slow say 400 rpm or even 300 rpm
 
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At first read, I didn't quite understand what "Mase" was proposing.
By the 2nd read, I had it. Early Sunday mornings are like that.
What he's suggesting is excess tailstock pressure. That certainly will do it and his explanation is valid also. As the shaft gets thinner, it will bow easier (from excessive tailstock pressure).

In your case, this may or may not be the cause. There are many suspects.
Are your cross-slide ways sticking? Lube them.
Is your cutter-bit sharp? A dull bit can cause 'chatter' and cause the shaft to 'bounce', particularly at it's least supported section (the middle).
Even cutter-bit height and direction of feed will have an effect.
Have you dialed-in your turning-speed relative to your feed-rate? Each set-up is different. Each shaft is different.
How often does this happen and is it starting to happen more frequently?

Just a thought but maybe a pic would help with the diagnosis.
 
I also think tail-stock pressure might be the culprit. I use a spring loaded set-up to get just enough pressure to keep the shaft turning while cutting. I never have any problems with that set-up.
 
KJ thanks for the translation from Canadian to US. I knew what I meant but others probably didn't.:):wink:
 
First of all, my setup is a Taig based lathe on a 4ft aluminum dovetail with ideas borrowed from Chris Hightower and Lee Malakoff. The router mount is from Todd (TSP&B) as is the spur drive. I have been using a cheap 2-flute 1/2" or 5/8" flush cutter. I had tried one of the 6-wing 1/4" slot cutters like was discussed here a while back and had to agree with Dick Neighbors and others that it seemed to be a grenade looking for a place to go off!:eek: I have recently purchased a 3-wing slot cutter which I've not had a chance to try out thanks to daughter's wedding and business trips, yada, yada, yada.

Technique-wise I run the router at full speed and the lathe spindle up around 1500 rpm. Based on comments above, that is too fast. In my mind, 300-400 seemed to slow and might cause a rougher finish, but I now need to try that.
Pressure-wise I know in the past I had too much tail-stock pressure and now try to use just enough to keep the shaft from slipping.
I generally take a .025 cut (.05 total) per pass until I get closer to finish size and back that down to .001 (.002 total). I turn the carriage by hand at about a 1" per second feed rate - trying to keep it nice and smooth.
It's entirely possible that my 2-flute cutter is getting dull. I also wonder about the angle of the blades and whether they contribute to the chatter that I'm seeing.
I lube the ways when I feel they are beginning to stick - and try to keep the lead screw/rack gear cleaned up as well.
Based on the comments above I have some new things to try.
Based on the pictures, does anyone see anything else that might be wrong.
Thanks as always for the advice,
Gary
 

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I believe highest rpm cutting speed with slower spindle speed should get you the results you are looking for, as long as you keep the tension on the shaft to a minimum.
 
Power feed needed

I've never been able to feed smoothly enough by hand to get a good finish. Besides, it's a PITA. IMHO, a power feed will definitely improve the finish.
 
I'm still experimenting wiith speeds and feed rates on my saw machine, but lately, my finish cuts on shafts have been at 200 rpm with a feed rate of 3-5 inches per minute.
 
You're turning the shaft WAY WAY WAY to fast.
Put the 3-wing slot cutter on there, and dial your rpm down to about 200 or less. No more tailstock pressure than necessary either.
End of problem!
 
I agree with many of the previous replys. From your original post it sound to me like one problem is a little too much end pressure. You should have just enough pressure so the spur driver grips. I have also found that I get my best and most consistant results with a very slow rotation and a slow feed rate. In addition I have found that it is necessary for me to take very very shallow cuts as the shaft get closer to final sanding size. Good luck and let us know how it works out!
 
I'm not sure you need the spur center for turning shafts.
You'd be amazed at how little force or pressure it takes to make the shaft rotate. A dead center at the head and a smooth live center at the tail is all you need. In fact I turn all of my rounds using this set up.
If the spur center is spring loaded it may be taking unnecessary force to move the spring down far enough to lock in the spurs which are really not needed to turn the wood.
 
Sheldon said:
You're turning the shaft WAY WAY WAY to fast.
Put the 3-wing slot cutter on there, and dial your rpm down to about 200 or less. No more tailstock pressure than necessary either.
End of problem!
Click to expand...

Hi,

I agree with Sheldon, slow lathe speed & slow travel gives me the best RMS finish to my shafts. I also go 2 cuts on the last pass without changing the dial, it helps! Very very light pressure on tailstock.

It might also help to put your machine on a piece of neoprene rubber to stabilize the foundation and diminish some vibration.

Rick G.
 
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I use the six blade slot cutter and get great results. I have found that I cannot get my smoothest cut unless I slow down the lathe rpm and feed speed, and slow down the router until I feel, and hear no harmonic vibrations. That step seems to be crucial. It is easy to adjust speed with the vs quiet router. I have had good results with a standard 2 blade 5/8" router bit, have used a 3 and four wing cutter to. But the finest cuts have been with the six blade cutter. One thing I also noticed is: one can take a huge HOG-out initial cuts with the six wing bit with similar great results.
 
Bill the Cat said:
I've never been able to feed smoothly enough by hand to get a good finish. Besides, it's a PITA. IMHO, a power feed will definitely improve the finish.
Click to expand...

Bill,
A power feed is in the works, but right now everything is manual.
THANKS!
Gary
 
BLACKHEARTCUES said:
Hi Gary; are you dialing the cross feed in by hand, without a taper bar?...JER
Click to expand...

Yes, right now. So I'm only making conical (straight taper) shafts. On the back side of the lathe you can see the beginnings of the CNC taper add-on that I have been working (slowly) on.

THANKS!
Gary
 
Sheldon said:
You're turning the shaft WAY WAY WAY to fast.
Put the 3-wing slot cutter on there, and dial your rpm down to about 200 or less. No more tailstock pressure than necessary either.
End of problem!
Click to expand...

Sheldon,

Yep, that's what I'm hearing loud and clear.

THANKS!
Gary
 
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