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Torsion twist while turning spindles

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Forum topic by PASs posted 02-16-2013 08:21 PM 1489 views 0 times favorited 14 replies Add to Favorites Watch
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PASs

566 posts in 1784 days


02-16-2013 08:21 PM

Topic tags/keywords: question trick tip lathe turning arts and crafts modern traditional chuck technique spindle spindles baluster balusters

I’ve been playing on the lathe with double chucking spindles.

Was turning a piece of scrap pine (2×4) 1 1/2 inches square by 40 inches long seeing how thin I could make the spindle.
I learned not to put too much pressure on the tailstock because that tends to bow the spindle which can increase the tendency and severity of any vibration that may develop.
To make sure I didn’t have any pressure I left a slight gap between the end of the wood and the tailstock, only about 1/16” just visible in the picture.

Everything went fairly well until I got the thinnest section to 1/2” diameter.
At that point I got a different kind of oscillation, torsional…the wood began to twist and untwist, in effect the tailstock end was alternately slowing down and then speeding up again.
I stopped going any smaller because I felt the piece was likely to twist itself in two.

I realize that soft woods are more likely to twist, but does anyone have an idea how to prevent or minimize it?

-- Pete, "It isn't broken, you just aren't using it right."


14 replies so far

View Wildwood's profile

Wildwood

1093 posts in 821 days


#1 posted 02-16-2013 11:01 PM

Just a SWAG try turning lathe speed down, turn entire spindle thinner and add more steady rest.

Saw a video of man turning extremely long skinny spindle with many string steady rest. If can find it will post.

-- Bill

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PASs

566 posts in 1784 days


#2 posted 02-16-2013 11:20 PM

Wildwood,
I found it, the string steady for turning trembleurs.
I don’t think the string is causing too much heat due to the low speed at such a small diameter.

They sure look cheaper to make than buying a bunch more Oneway or Carter steady rests!!!

And a learned a new tool word…bedan.

-- Pete, "It isn't broken, you just aren't using it right."

View lew's profile

lew

10088 posts in 2441 days


#3 posted 02-17-2013 12:48 AM

Pete,
Don’t know if you saw this video, but it may give you some ideas-
http://www.youtube.com/watch?v=t4JdA9zdUeI&feature=youtu.be

-- Lew- Time traveler. Purveyor of the Universe's finest custom rolling pins.

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PASs

566 posts in 1784 days


#4 posted 02-17-2013 02:36 AM

lew,
Thanks for the link…I drink way too much coffee to try that.

-- Pete, "It isn't broken, you just aren't using it right."

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Wildwood

1093 posts in 821 days


#5 posted 02-17-2013 12:17 PM

Lew that was the video was talking about. Went searching for it on You-Tube and gave up after several pages.

Thanks for posting!

Pete, use my Bedan on for cutting beads, spheres, and wide parting tool. Unlike those French woodturners revert to using my skews for other cuts. Can use a Bedan as a skew, just takes to long for me.

-- Bill

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Roger

14851 posts in 1490 days


#6 posted 02-18-2013 02:53 PM

Tis a really nice steady rest

-- Roger from KY. Work/Play/Travel Safe. Kentuk55@bellsouth.net

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PASs

566 posts in 1784 days


#7 posted 02-18-2013 04:00 PM

Hey Roger,
It’s TWO steadys.
At the headstock end is the Oneway spindle steady with a couple of modifications (hand knobs replace some of the jamb nuts.)
At the tailstock end is a Oneway bowl steady with the same modifications.

They are both beasts…thick bases, thick risers, thick support arms.

But the spindle steady’s weak point is the rollers. The softer o-rings deflect under pressure at speed. Will try to post a video.

Will post a review on them when I get around to it.
r/
Pete

-- Pete, "It isn't broken, you just aren't using it right."

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stefang

13260 posts in 2020 days


#8 posted 02-18-2013 04:32 PM

It seems to me that there will always be a loss of power/speed from the headstock to the other end of the workpiece. So if it’s held by a chuck it will be rotating a tiny bit slower causing the twist. I am wondering what your purpose is for the double chuck arrangement as I can’t figure out any reason for it, although I am sure you have one.

-- Mike, an American living in Norway.

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PASs

566 posts in 1784 days


#9 posted 02-18-2013 05:00 PM

stefang,
Thanks for the comments.
The twist was not so much an issue…the friction caused by the bearings in the steady rests and tailstock cause a slight amount of torsional stress.
However as the soft pine became thinner in cross-section it became more sensitive to that stress.
It reached a point where (my assumption here based on observation and a lack of equipment) slight changes in twist created a harmonic effect….slight changes in resistance to the drive force by the rotation of the bearings in the races caused the tailstock end of the spindle to speed up, then slow down.
I only began to suspect it was this effect because I could see a change in the appearance of the spinning piece similar to using a timing light.
It was fun to adjust the lathe speed to watch the light effect on the wood.
As I have no way to measure the tailstock RPM I won’t even guess what the actual torsion effect in degrees lag/lead were relative to the headstock. But when I stopped the lathe the end attached to the tailstock would oscillate for a second or so, twisting back and forth.
I still have the spindle and will try to get some video.

As for the double chucking…
Making canes I have always fought chatter as the diameter of the piece is reduced (nothing new there).
Steady rests are completely effecting in damping the chatter.
But I also noticed that the compression of the blank caused by tightening the tailstock would visibly bow some pieces.
I tried tightening the live center less, but sometimes it seemed that allowed the piece to vibrate at the center requiring increased pressure on the center.
So I have played with live center pressure and adjust it as needed while turning.
The idea occurred to me that if I could hold the blank with no longitudinal compression it would eliminate the tendency to bow the piece.
Hence the double chuck.
The headstock chuck holds the square drive end. The tailstock chuck holds the driven end but with no longitudinal pressure…there is actually a gap at the end of the spindle.

There are some issue I still have to work out…the tailstock chuck is screwed on tightly but I am concerned that vibration, especially torsional, may work it loose.

I’m actually thinking about turning a tenon in the driven end, pinning the live center into the tailstock, and turning a piece under longitudinal TENSION.

But the whole purpose of this is to experiment and learn.

-- Pete, "It isn't broken, you just aren't using it right."

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stefang

13260 posts in 2020 days


#10 posted 02-18-2013 05:43 PM

Sounds great Pete. The more you know about the forces you are working with, the better you will be able to control and use them. I have yet to turn a cane. In fact I bought one a couple of months ago to help me when my bad knee was at it’s worst. It helps when I have to shop with the wife for clothes, that usually takes awhile.

-- Mike, an American living in Norway.

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hairy

2062 posts in 2218 days


#11 posted 02-18-2013 06:22 PM

I think it has something to do with weight. All the torque is focused on the place where the small center section of the spindle meets the larger square section in the back half. I’m not certain what chucks you are using, but I’d say the weight of 1 chuck could be more than the workpiece. My Talon is 3 and 1/2 lbs, Stronghold is 8 lbs.

I do like using 2 chucks to reverse mount a piece, you can’t get off center that way.

If I did a spindle like that, I would start at the tailstock end and work back to the headstock. That could eliminate a lot of the need for a steady. You will always be cutting the thickest wood that way.

Just my dos centavos…

-- the last of Barret's Privateers...

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Harry Montana

46 posts in 681 days


#12 posted 02-19-2013 12:55 PM

wood that tends to twist or some other defcormation (cupping, warping, cracking) is always due to insuffient drying. Since I understand that this occured more towards the center of the wood I believe this is the cause. Drying does take away the natural tensions of the wood if quartersawn etc.
Since wood always work we:
1) first phase is cutting the boards in a saw mill with an extra margin (thick 1/8”, wide 1/8”), these 1/8” is not wood for free but this is exactly to mold out the distortions
2) dry the wood, the wood starts to ‘work’
3) we mold out all the distortions.

-- With regards from Harry Montana http://www.hardydeck.com

View rum's profile

rum

148 posts in 1272 days


#13 posted 02-19-2013 08:09 PM

I’m with Hairy on this one.

The chuck on the tailstock is probably adding to much resistance. I think whats happening is that the torsion or “whipping” action is caused by the tailstock not moving at the same speed as the headstock (i.e. its slowing down/catching up some) due to the added friction.

Try going back to a live center on the tailstock and chucking the piece on the headstock. Don’t turn the whole thing to size in one pass but only turn about ~3”-6” (try and see what works here) at a time leaving lots of meat on the headstock side. Move the steady rest to after the cut part.

Using basically that technique I’ve been able to turn down to ~1/4” out to around 14” long with no steady rest at all. For the last ~1/2 of the piece I don’t even need the tailstock engaged at all, I just have it barely touching.

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PASs

566 posts in 1784 days


#14 posted 02-19-2013 09:31 PM

Rum,
I agree that the weight of the chuck, a Woodriver midi chuck, is part of the cause.
I’ve got the spindle laying on the bed of the lathe right now…will video the effects tomorrow when I’m back in the shop. It’s pretty neat to see how much the tailstock end wiggles back and forth when I turn the lathe off.

And certainly turning from the tailstock end is the only way to go.
In this case the damage had already been done with the old routercrafter…and mostly I was in the experimenting mode.
Video to follow.

-- Pete, "It isn't broken, you just aren't using it right."

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