Concentricity problems (-or- It ain't round!)

[Mike McDonald]

I was a beginner to start with and then took a break from turning for the better part of a year.. now that I'm trying to turn again it seems like nothing I turn is round. (This wasn't a problem a year ago.)

There was a recent thread discussing things like changing moisture content while turning which could cause a piece to get out of round, but some of the pieces I've tried are well seasoned.

I yet to turn a bit of MDF to make sure it's not the wood, but I'm hoping ya'll can answer a couple questions I have and/or suggest other things to look at.

If it matters, I'm turning on a Nova DVR-XP, though I think this is more about operator error.

First, to check concentricity (after roughing), I very slowly bring a pencil up to the spinning piece until it just barely touches, then turn the lathe off. If the pencil mark doesn't go all the way around the piece, I know it's not round in that spot. (I usually get the feeling it's out of round and then do this to check.)

Does this seem like a valid way to check concentricity, or have I missed something?

Second, I've noticed that when I'm roughing the gouge moves in and out like a sewing machine needle as I make the cut. As the piece gets closer to "round" this travel is reduced and eventually eliminated. Is this normal, or does it mean I'm trying to make too deep a cut?

Lastly, on the pieces that start out very out-of-balance, I generally start the lathe very slow and then increase the speed until I feel a little bit of, but not excessive, "sway" in the headstock. Is this an acceptable method, or should I slow it down to where there's no sway and slowly increase the speed as it becomes more balanced? Could this cause the concentricity problems I'm having?

I'm rapidly becoming frustrated, so any help ya'll can offer would be greatly appreciated!

-Mike

 

[Steve Worcester]

Even seasoned wood will move as you remove material and relieve stress from the wood. But how out of round is it?
The pencil will sort of work, if you take into account the pencil lead is wearing away and when the wood stops turning, is the lead still in contact with it? If the lead is not in contact on the opposite side somewhere, it is off parallel (or sorts). But if it is a few thousandths (or even hundredths) , that's wood. It will never be as round as metal will get because it does change shape.
As far as the bouncing gouge, during roughing that is common because you are holding the gouge in and the tips of the wood pushes it back out. When you have this problem when it is round it is because you are not riding the bevel, induced a bump and you keep riding over that bump as the wood turns.

As far as out of balance, turn at speeds you are comfortable with, it doesn't have to be fast if you have tool control. I used to say start up and when the lathe rocks, back down, but normally, I don't turn that fast. A sharp tool will cut just as well with a slow speed (it is more about the wood surface feet per minute)

 

[MichaelMouse]

Circularity. When you haul all that wood out of the center, the outside is going to relax a bit. Natural that a rough would not be perfectly circular.

Why is the gouge moving? You make things round by rotating them against an immoveable object. Anchor that gouge to the rest and let the wood come to it and jump off. Don't press in, press down.

I think the gofaster knob is a useless distraction. I rough mostly at 680, finish at 680. If things are too out-of balance and the lathe surges and drags, I select the 360. Why are you in such a hurry? Disaster happens faster when you crank up rpm.

 

[Mike McDonald]

Steve-I haven't measured it, but it's out-of-round enough that I notice it and stop to check it. Also it's enough that I'm concerned about what will happen when I remount and go to hollow the thing. Definately feels like more than thousandths or hundredths, but I could be wrong.

You make a very good point about the pencil lead wearing down.. maybe I should try a pen or marker? Or I could be scientific about it and use the concentricity gauge out of my reloading gear, but that seems like a bit much..

I don't have the problem of the gouge "pistoning" once it's round(ish), just when I'm taking it down to round.


Michael-This happens before I get to removing the center.

The gouge moves because the wood isn't round, so I'm turning air for a moment and then the wood hits the gouge, knocking it back. If the gouge is supposed to be stationary from the first time I start the lathe, I think I'm trying to take too big a bite.

I don't really care what speed the lathe is going, except that it's much easier for us beginners to make a smooth curve when the lathe is moving faster. Plus, I keep reading how-to articles that suggest a rpm for each step, and I'm not turning anywhere near those speeds. Probably doesn't matter that much, but I read these articles for ideas on the "proper" way to do things, and they're suggesting the "proper" speed for each operation/cut.


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Thank you both very much for your help!! I'll take your info to the lathe this afternoon and see if I can't do better.
-Mike

 

[MichaelMouse]

If you can see it on the outside passes, it's probably related to the poor gouge control. I find that when I get to the lowest rpm on my lathe I lack the patience to do what I should, which is place the gouge and let the wood come to it. I start trying to maintain contact with an irregular surface, which is not only tough, but dumb. I know better, but I have to keep reciting "no push" as a mantra.

Try what the author of the only book or tape or DVD I have on turning suggests, and swing the tool into and through irregular areas. Don't try to feed guiding on the work until it's mostly round, guide on the rest and swing so that the bevel would parallel the work near the middle of the arc you're working. That way you're consistent, because you will want to make entries even into fully round surfaces that way, and start moving ahead somewhere near the middle of the arc where you start a shaving.

Shaving by leading with the nose, flute facing away from the wood, right? No stuffing into the rotating piece, just cutting across the face and down?

 

[Richard Coers]

I can't stress enough how valuable your local AAW chapter can be. Find the club, and you will find a mentor. I've seen a lot of beginners (my personal experience included) that go at the wood white knuckled, Katie bar the door! Turning is a subtle dance and not a battle. Watch someone with experience and the whole learning process will shorten a lot. Watch a couple of DVDs, you will see that everyone has motion in the gouge when roughing. The experienced turner will not make this worse by force, they will settle it down by letting the gouge do the work which will minimize the amount of time the tool moves. Are you starting on the end of the work to round it, or on the outside of the blank? Starting on the end lets you get to smooth wood much more easily. Good luck, and find some hands on help.

 

[odie]

All good comments above......^^^^^

Up to this point, there is one aspect that hasn't been addressed yet.........Is your gouge sharp? I once saw a photograph of a woodturning instructor, and on the blackboard behind him was written, "The answer to your question is: Sharpen the tool"! This is some good advice. As the suggestions above indicate, there may be one, or several reasons why your tool is responding with that pulsating feeling......and the level of sharpness is among those major contributors that will add to the effect.

Good point about the pencil lead wearing down, Steve. Mike, a pencil mark will tell you if there is an out-of-round condition.......and it only takes one single revolution of the mounted wood to tell. You can also use a small sliver of wood, 1/8" dowel, or anything, for that matter......if you have a variable speed drive. Just place the dowel on the tool rest and bring it to just touch the wood spinning at very slow speed. When it touches, stop right there, and hold it in place. You can now see just how much your wood has warped, by observing the gap between the dowel and the slow spinning wood. I'm also a reloader, and your mounted dial indicator will also work.....but, you're right! There's no need for that kind of accuracy!

Also, as Steve pointed out, as well.......even stabilized moisture content will not guarantee warping won't happen. I've found that the vast majority of wood bowls will have varying degrees of warping.......with your tool work, there is a whole 'nuther set of physical properties (stress) applying themselves to the wood that is undergoing the shaping process.....the degree to which it will warp is anywhere from not really noticeable, to extreme.......moisture content, size, species, grain patterns and characteristics, wall thickness and shape.....all of these things will contribute to how your particular bowl will flex while being shaped.

ooc

 

[Dick Wilson]

Out of Round

Mike, I don't know if you headstock is permanently attached to the lathe bed or not ( My Jet 1442 headstock moves). If it does you should do a headstock/tail stock alignment. On my lathe I put a drive spur in the arbor and live center in the tail stock. I bring up the tail stock up so the points in the spur and live center just about meet. On my Jet, the two points may be off as much as an 1/8". I re-adjust the headstock and lock down. Good luck

 

[Mike McDonald]

Thank you, all, for your advice.

I didn't get out to the lathe yesterday, but I should be there most of today, trying to figure this out.

Michael- I had completely forgotten about that "swing-through" method. That might definately improve things.

I'm not sure I understand the last line of your message, though. When roughing, I was shown to set the gouge on the rest bevel-up, rotate it a bit in the direction of the cut (left/right) to protect the corner/shoulder, and I've taken to swinging the end of the handle a little to move the corner/shoulder a bit farther from the piece. Are you saying that the bevel should be rubbing even when I've just put the piece on the lathe and start to round it ?

Richard- I actually had two great guys helping me, one of them was actually invited to Disney's Festival of the Masters for his turning. Unfortunately, I lost touch with both of them when I stopped turning for the better part of a year, and now I'm a little embarassed to try to get back in touch after so long. Maybe I'll have to bite that bullet, though.

I don't think it'd ever occurred to me to start rounding on the end of the blank. I'll definately give that a try.

odie- yes, sir, my tool is freshly sharpened. I check the edge regularly and resharpen whenever I feel a change in it.

The dowel is a great idea, I'll have to try that. I really don't think we're talking hundreths or thousandths, but eighths or sixteenths. (Though, I suppose, sixteenths qualifies as several hundreths, doesn't it?)

Dick- The headstock swivels 360 degrees for outboard turning, and though it has preset 'stops' at 0, 22, 45, and 90, the 0 stop is imperfect. I have found that there is still around 3/16" play so I use the drive spur/live center method you describe to make sure they're lined up when I'm turning between centers. In this most recent case, though, I am/was turning on a face plate without the tailstock. (And I did make sure the headstock was locked down.)

On an unrelated note, did you receive a private message about your gallery from me a couple weeks ago? I think I might have cancelled it when I thought I was sending it - new to the interface and all.

--

Thank you all very much for your continued advice! Hopefully I'll be able to take what ya'll have said out to the lathe today and get this worked out.

If ya'll think of anything else I might be doing wrong, please let me know!

-Mike

 

[Dick Wilson]

Mike,

I don't recall getting a PM. Try again. Dick

 

[MichaelMouse]

Flute facing out keeps you from hacking and whacking. If the bevel touches nothing, fine, and when it follows the arc of the cutting edge it has to touch, however briefly. Once again, about halfway through the arc as an aimpoint.

Take a look at these for an idea. It's cutting inside, but if I showed outside cutting I'd get flak for using a forbidden gouge. Starting around 25 seconds and ending at 60, you can see nibbling through out-of-round maple. Three swings worth, I believe. http://s35.photobucket.com/albums/d160/GoodOnesGone/?action=view&current=Inside.mp4 It's like carving, where you pull one way with one hand and resist with the other to maintain control, the whole time anchoring firmly to the rest.

Outside, with a semi-forbidden gouge. Watch the swing and hear the surface develop. http://s35.photobucket.com/albums/d160/GoodOnesGone/?action=view&current=TrueBottom.mp4

As to lathe rpm, a lot of people make much ado about turning rapidly, but when you consider the radial velocity at 8" from center versus the same rpm 1" from center makes the same bowl, it's time to review Newton's second and third. Energy varies with square of velocity, and exerts equal but opposite. You don't want the wood to hit your gouge with any more force than is required to remove the shaving.

 

[Mike McDonald]

Thank you, all, for your help!

I was finally able to get the thing round when I anchored the chisel on the rest and swung-through.

I thought it was interesting that, even once the piece was round, as soon as I tried to run the bevel on the wood as a guide, rather than 'swinging through', it got out-of-round again. I finally worked out a balance between anchoring the gouge and following the countour I wanted, though.

Unfortunately, I ran into more problems when I tried to remount it in expanding jaws, but I've read enough on that issue I think I'll be able to figure out what I'm doing.

It is definately round at this point, though!

Being a beginner is so much...... fun?


Thanks again!!

-Mike

 

[MichaelMouse]

I finally worked out a balance between anchoring the gouge and following the countour I wanted, though.

For me it's the point during the swing where the shaving starts. After that, maintaining the same relative presentation with the bevel helping allows the tool to follow.

All of us get a chance to be beginners when we try a new cut, shape or tool.