Kielwood shafts snapping during turning, strange oscillation.. can't seem to figure it out.

T

tg_vegas

Well-known member
I have a batch of Kielwood square blanks that I'm turning now. About 25% of them are failing on the lathe. The rest are fine.

I am turning them from 1.5 x 1.5 x 31 squares. I took off the corners on my router table like I always do.

Then the rough turns. 1" Freud 2-Blade straight bit, brand new. 4-5 shafts turn clean, no issues.

Then the last two. I get down to about .650 on the tip end during the tapering passes. Turn starts out fine, but once I get to mid-shaft, I can feel and even hear a slight harmonic oscillation start to build that I can see in the shaft, like waves. Once I start to get closer to the headstock, it goes away.

However, on the two I did today, the oscillations got so bad that it drove the shaft into the router bit and snapped the shaft. WTF?

I'm on a Cuesmith Deluxe. 145-150 rpm rotation speed, feed rate of <4" per minute, total pass is 8 minutes. .050" cut.

I've turned a 100+ shafts with no issues in the past few months but the latest ones have me at a loss. I can't feed any slower and even at smaller cuts, I have the same harmonic situation. You can actually hear it as a humming sounds that builds up to a vibration you can see in the work.

Any clues from the experts??
 
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It's all about speeds and feeds. Try reducing RPM of router, height of cutter, feed rate, spindle RPM.

No wood is identical and you will have to vary parameters to eliminate the harmonic vibrations. Every cuemaker has experienced this at one time or another.

Sent from my Pixel 8 Pro using Tapatalk
 
I'm at 150 rpm rate, the slowest i can go without stalling and I'm at the slowest feed rate I can get. I takes 8 minutes to go end to end.

Router is wide open, per Chris's recommendation. I'm cutting from the side so height is of no consequence.

I tried a smaller cut, .025, same wavy pattern but didn't break the shaft. However, the "waves" were so deep that they produced an ID smaller than my finished diameter, so into the trash they go.
 
Rodney said:
Did you put a lot of pressure on the shaft with the tail stock? I’ve never broken one but when I accidentally put too much pressure I get chatter/push off near the middle.
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Hmm.. you might be on to something there! I keep mine pretty tight to eliminate the backlash in the spur driver. I'll keep that in mind as I turn the next ten tomorrow.

THANKS!
 
I switch to a spring loaded center from Chris after they’re turned round. As long as I rely on the spring pressure I don’t usually have any problems.
 
tg_vegas said:
Hmm.. you might be on to something there! I keep mine pretty tight to eliminate the backlash in the spur driver. I'll keep that in mind as I turn the next ten tomorrow.

THANKS!
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You’re welcome. Hopefully it helps.
 
Rodney said:
I switch to a spring loaded center from Chris after they’re turned round. As long as I rely on the spring pressure I don’t usually have any problems.
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Tailstock center or headstock center? Are you using his Spur Driver? I use his but the runout is always an issue to deal with since you have to leave it hanging out so far from the Headstock so that the router doesn't hit the headstock.
 
Just because you're cutting from the side, the height absolutely can change the way it cuts.

I prefer mine slightly below center. Also ensure shaft is held between centers with the correct pressure. You don't want it too tight. I can pinch my shaft to stop it from turning.

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Rodney said:
You’re welcome. Hopefully it helps.
Click to expand...

I've been thinking about this since your comment. I'm betting you nailed it. The pressure I put on the larger diameter shaft caused no deflection but as I reduced the diameter, that same pressure likely caused some deflection, which I wrongly thought was vibration, and the more material removed, the worse it got. You'd think after hundreds of shafts I would have learned this lesson already! Thanks again. That's why I will never call myself a cue maker!
 
pip9ball said:
Just because you're cutting from the side, the height absolutely can change the way it cuts.

I prefer mine slightly below center. Also ensure shaft is held between centers with the correct pressure. You don't want it too tight. I can pinch my shaft to stop it from turning.

Sent from my Pixel 8 Pro using Tapatalk
Click to expand...

You can pinch your shaft to stop if from turning? So, that tells me you are not using any kind of spur driver but just dead centers on both ends?
 
tg_vegas said:
You can pinch your shaft to stop if from turning? So, that tells me you are not using any kind of spur driver but just dead centers on both ends?
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I move my height up and down over the life of the bit to move the wear point and extend the life of the tool. Never notice much difference. I even tried cutting from left to right instead of the other way, again, no noticeable difference..
 
I use a dead center in the chuck and the spring loaded one in the tail stock. There’s not much pressure, I’m sure I could pinch it to stop it as well. I typically only use the spur driver on octagonal stock.
 
tg_vegas said:
Tailstock center or headstock center? Are you using his Spur Driver? I use his but the runout is always an issue to deal with since you have to leave it hanging out so far from the Headstock so that the router doesn't hit the headstock.
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I have a 3” dead center and a 4 jaw independent chuck on my Hightower. I had a machinist make the center, pair that with the chuck and zeroing is pretty easy.
 
tg_vegas said:
You can pinch your shaft to stop if from turning? So, that tells me you are not using any kind of spur driver but just dead centers on both ends?
Click to expand...
Correct, I don't use a spur driver.

Sent from my Pixel 8 Pro using Tapatalk
 
I pull my shafts between centers with very slight tension.

I use a collet in the headstock and a live Jacob's in the tail.

Faster cutter speed is going to produce lower chip load.

Harmonics can be caused by the combination of the turning frequency and router speed.
 
tg_vegas said:
I'm at 150 rpm rate, the slowest i can go without stalling and I'm at the slowest feed rate I can get. I takes 8 minutes to go end to end.

Router is wide open, per Chris's recommendation. I'm cutting from the side so height is of no consequence.

I tried a smaller cut, .025, same wavy pattern but didn't break the shaft. However, the "waves" were so deep that they produced an ID smaller than my finished diameter, so into the trash they go.
Click to expand...
instead of using the full side of the cutter, you can try only using the corner. it should provide less contact surface and might help some. Maybe the torrification process makes the wood more abrasive and your cutter has dulled already. I haven't cut any torrified wood so this is only a thought and not based on experience.
 
Slower feeds are not always better. I get harmonic vibrations when my feed it too slow. All that depends on the cutter you are using. Try speeding up your feed rate on one of your junk shafts and see if it helps.

Alan
 
As others have stated, I have found that it's a combination of feeds, spindle rate, and cutter speed. I run my lathe up around 600 rpm as opposed to your 140-150.
Something else for you to try.
HTH
Gary
 
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