Wierd things happening

[pastorbill1952]

Not sure if this is the correct forum, so please move if necessary.
I "seem" to be having a very unusual/ wierd occurance with my lathe. The knurled knob seems to be tightening up as I turn a pen. I finger tighten to turn but then I start noticing after a few minutes that the shaft is bowing, which causes the blank to be oput of round. I stop the lathe & have to use pliers to loosen the knob. I have not noticed if it one particular type of wood. I do use paste wax to coat the shaft & bushings for CA finish. I just changed out the tail stock tip but as of yet it is untested. anyone have similar experiences or know what may be happening?

 

[Rifleman1776]

Mine tightens too but hasn't caused bowing. The bowing may be due to your blanks not being properly squared before turning. I don't consider the tightening to be a problem.

 

[DCBluesman]

The tightening is due to centripetal (not centrifugal) force. (See Isaac Newton's "Principia" for a full discussion - no link given) On start-up, the arbor has immediate force applied resulting in its being placed into motion before the friction with the knurl nut gets them both moving at the same speed. Since the knurl nut stays at rest slightly longer than the arbor, the nut tightens. With a VS system, you can overcome this by always starting the lathe at its lowest speed, then slowly increasing it. The bowing comes from the range of possibilities often discussed here, including but not limited to having ends which are square to the tube and bushings, and bushings which are square to a straight arbor, etc.

 

[pastorbill1952]

Thanks, I will dbl check my squaring.

 

[ctEaglesc]

If I get a "catch" I notice the inertia will cause the knurled nut to tighten.
This could cause the madrel to bow especially if you need a pair of pliers to loosen it.(sorry Frank)
I did have an instance where a piece of palm was totally disintegrated by the compression of the nut and bushings on the tube.Ithe tube got stove in like a sqaushed beer can only smaller.
It wasn't a pretty site.

 

[JimGo]

Still another reason the knurled nut may tighten is friction. As you turn (especially rough turn) the blank closest to the nut, the blank slips slightly each time it strikes the tool - the harder the hit, the bigger the slip. With the nut resting up against the wood, the nut is also turned slightly with each hit. If you do a search, I believe it was Rich Kleinheinz (sorry if I mispelled your name Rich!) (aka ScubaMan on here) who suggested stopping the lathe after roughing down the blank and loostening the nut.

I only wish I could remember to do that after I start - I'm usually so excited to see how the pen will come out that all my best-laid-plans go out the window!

 

[JimGo]

Dang it Eagle! Had I just waited a few seconds, you could have saved me from thping all that out!

 

[alamocdc]

Bill, about everyother time I turn a pen I experience the same phenomenon, so you are not alone. The cause has already been explained. Once I get the blanks mostly finished with the rough turning, I loosen the nut, spin the blanks 1/4 to 1/3 turn and finger snug it back. Then I finish the pen, checking the nut every now and then to make sure it hasn't retightened. The first time this happened, it gave me quite a start. I'm just glad I figured out what was going on.

 

[Paul in OKC]

Originally posted by Rifleman
<br />Mine tightens too but hasn't caused bowing. The bowing may be due to your blanks not being properly squared before turning. I don't consider the tightening to be a problem.

I agree here. The mandrel should not bow if all is square, you are pulling straight out against the headstock. You would have to be pushing in on the end of the mandrel pretty good to bow it.
I find that the nut tightens most of the time when the blank catches while turning, which causes the friction to turn the nut.
And on a technical note, without an eccentic chuck, you really can't turn something oval. Out of concentricity, yes. (excuse my machinist brain[])

 

[pastorbill1952]

Thank you, I wasn't sure if I was loosing my mind as the fellas at WC thought today when I was asking. SO if I can summarize what everyone is sayinf, the "self-tighteniung nut" is not uncommon and can be corrected. It is the loose nut with turning tools in his hands that I should watch a little closer []

 

[jenamison]

thanks Eagle, now I have absolutely no eyesight left after trying to read the "bleached out" statement...[][]

 

[JimGo]

You are correct Pastor, it is not uncommon at all. I've also had a self-loosening nut, which I've found tends to occur when the mandrel is already bent, and I'm doing something that makes the whole thing vibrate like crazy. Very odd, but does happen occasionally.

 

[btboone]

I had to study the situation for making my own mandrels for the CNC lathe. I wasn't sure if I needed righthand or lefthand nuts. If I got it wrong, the machine would continue to feed while the part stopped turning, a condition that plays all kinds of havoc on the tools. At 7000 rpm on the spindle, I didn't want anything coming loose either. Like the other guys, I found that the lathe action tightened the right hand nut down on the part rather than loosening it. I have cracked a couple plastic pen halves because of it. I use a double nut system now where it doesn't self tighten.

 

[JimGo]

Hmmmm...I wonder if a washer made of UHMW might not work too, Bruce. The force from the nut should keep everything tight against the mandrel and allow everything to spin due to friction, while at the same time allowing the end bushing to slip some without the friction against the nut (if that makes any sense).

 

[btboone]

It makes perfect sense Jim. One thing I need to be concerned with is that I machine the long pen halves with the nib side toward the tailstock. I need to have a fairly long and thin nut to lock the part onto the shaft and for the turret to miss the tailstock. Plastic might get squashed in that scenario. I simply used two 1/4-20 nuts and allowed the lathe to eat through the one closest to the pen. It ends up like only .030" thick per side after the lathe turns it and the pen down. It still acts like a lock nut because I have another one wedged next to it. The system works pretty well.

 

[Rifleman1776]

Do we allow people here that say things like 'centripetal' and 'UHMW' ? [?][]

 

[BigRob777]

I am so glad I stumbled accross this post. I was having the same problem with my adjustable mandrel. It started vibrating while it was turning. Now I know why.
The end blank caught when I was starting it, I guess I got too excited.

 

[chigdon]

Originally posted by Paul in OKC
<br />

Originally posted by Rifleman
<br />Mine tightens too but hasn't caused bowing. The bowing may be due to your blanks not being properly squared before turning. I don't consider the tightening to be a problem.

I agree here. The mandrel should not bow if all is square, you are pulling straight out against the headstock. You would have to be pushing in on the end of the mandrel pretty good to bow it.
I find that the nut tightens most of the time when the blank catches while turning, which causes the friction to turn the nut.
And on a technical note, without an eccentic chuck, you really can't turn something oval. Out of concentricity, yes. (excuse my machinist brain[])

I can't speak to the specific physics involved here but from my experience I would have to disagree and say that it can cause bowing.

I start off much the same way as Billy by tightening the knob more tightly to turn the blank round. At that point I loosen it to a very light grip against the bushing. When I start turning again it is obvious it was not true before as the tool hits the blank intermitently until it is round again.

I can't explain this any other way other than bowing but someone else might know why this happens so consistantly (and my blanks are square).