What size bowl with chuck handle?

[DennisM]

I would appreciate comments / and or information about what are reasonable size bowls to attempt with the various size chucks currently on the market. All chuck specs will contain information about how wide and narrow a foot the chuck will hold, but I don't find information about what reasonable size bowl blank is appropriate.

What I am finding on my Delta 16" lathe with a Stronghold chuck with a standard set of jaws (#2 I think), that when the bowl gets over 12" diamenter and +-6" depth, I start running into wobble problems. The first time this happend, I assumed it was caused by hidden void in the bowl blank. But it has happened a couple of times since and I am beginning to think that it may be the chuck or the chuck jaws - or may be the lathe. Not sure which. I have tightened the chuck, tightened the head stock etc.

So--- as I try to remove the variables, are there rules of thumb about the maximum reasonable size that a specific chuck and jaws will handle?

P.S. I am consisently amazed at the depth of the experience and expertise that the AAW website brings to bear on questions like this. My thanks and compliments to all.

 

[Bill Grumbine]

Hi Dennis

There are some other variables you should check first. If I understand correctly, you are getting the bowl into the chuck and it runs true for a while, but then starts to wobble. This could result from not having a square tenon and shoulder for the tenon to index against the chuck jaws. Make sure your tenon is straight with a square shoulder for the top of the jaws to rest against. Make sure that it does not bottom out in the chuck on the bottom, but rather rests against the top of the jaws as described above.

You should be able to turn bowls of the size you describe with the #2 jaws. I regularly core 12" bowls out of larger bowls being held by the #2 jaws.

Bill

 

[DennisM]

You are correct - the bowl runs true and then several minutes later it doesn't run true. I then retrue it and couple of minutes later it is out of round again.

I will add double check the foot - no bottom out and true - as part of my check list in the future.
Any other rules of thumb about how large on a Stronghold with #2 jaws?

 

[John Taylor]

Check the Chuck adaptor's bolts too

Dennis,

I had simular problems with a new chuck and a vacuum chuck too. I took the time to re-torque the adaptor bolts back into the rear end of both chucks my mystorous wobble was gone.

You might want to check the chuck's insert to see if the bolts are tight enough. If the tenon on the bowl doesn't solve the problem.

John Taylor

 

[DennisM]

John - excellent thought. Never struck me to check the adaptor. That will be first on my list. But the basic question remains, does anyone have rules of thumb about how large for a specific chuck and jaws. Or is it - Just when it doesn't work?"

 

[MichaelMouse]

What I am finding on my Delta 16" lathe with a Stronghold chuck with a standard set of jaws (#2 I think), that when the bowl gets over 12" diamenter and +-6" depth, I start running into wobble problems. The first time this happend, I assumed it was caused by hidden void in the bowl blank. But it has happened a couple of times since and I am beginning to think that it may be the chuck or the chuck jaws - or may be the lathe. Not sure which. I have tightened the chuck, tightened the head stock etc.

The manufacturers have recommendations, of course, but they're pretty much for insurance purposes. It depends mostly on how you hold and how you cut. A case of how much you can push it before it starts to wobble!

Bill mentioned the importance of the shoulder if you're gripping outside. It's important that you get that tight up against the front of those jaws, and that they're true. In this case, however, the Stronghold works against you. Those serrations sort of destroy what they held, and gripping it the same way again when retightening becomes difficult if not impossible, because what was there isn't any more. What you want to try is using that tailstock to jam the bowl back tightly against the shoulder as you tighten. Symmetry in the serrations makes them as likely to wedge away from the shoulder as toward. Another thing which might help is to make a bit of a relief cut at the root of the tenon so that fibers pushed toward the shoulder will have a place to go without pushing back.

Then there's my preferred - smooth dovetails. Match your mortise or tenon closely to the circular point on the jaws, and make a flat surface for them to bear against, inside or out. They wedge the face of the jaws into the shoulder or the bottom of the mortise as you snug them, and since you don't honk on them, the whole thing remains pretty well circular in case you need to remove and reset, rather than chewed on. Even with wet wood.

Now a "number" answer. I know you can swing 12" on a 1" recess 3/16 deep if you cut as I do, which is pretty gently, and with support from the tailstock until your bowl is at its lightest and truest. I will say that a bowl that size looks a bit dumb standing on an 1 1/2" OD base, though. Sixteen will go on a 2" set. Both are well out of warranty, unfortunately.

 

[DennisM]

Dennis,

I had simular problems with a new chuck and a vacuum chuck too. I took the time to re-torque the adaptor bolts back into the rear end of both chucks my mystorous wobble was gone.

You might want to check the chuck's insert to see if the bolts are tight enough. If the tenon on the bowl doesn't solve the problem.

John Taylor

John;
You win the prize - I checked the adaptor bolts on the back and one was loose! Be a couple of days before I can test if this is full solution, but your diagnosis was right on.

Many thanks

 

[C Edward Moore]

reseat the adaptor

Dennis,

1. I recommend that you don't simply tighten the loose screw. Rather, remove the adaptor and clean it and where it seats very carefully. Then reseat the adaptor by tightening each side a little until both sides are torqued equally and the adaptor is secure. Actually, I put a THIN coat of lithium grease on the adaptor before I insert it into the chuck body. Make sure there are no burrs on either surface.

2. Consider getting some #3 jaws for your Stronghold if you want to turn larger bowls.

3. If the wood is not dry you may get wobble as you near the end of the turning process. This is especially true if the bowl is natural-edged and the bowl ovals and goes out of round.

 

[Bill Boehme]

Here is my $0.02 worth as a controls system engineer. The moment of inertia of the spinning piece of wood on the lathe along with the cumulative moments of inertia of the chuck, spindle pulleys, motor armature, and the effects of system compliance in the belt and other components that allow torque wrap-up will have a natural frequency, which when perturbed at that same natural frequency or some even harmonically related frequency will produce an oscillation that is self-sustaining or divergent.

OK, now in English for those who are not control system engineers. That spinning hunk of wood will want to vibrate all to Hell at certain speeds. You set the speed where it is running smooth and begin turning. As you turn, what are you doing to that hunk of wood? Answer: removing mass and, thus changing the speed at which it would just love to vibrate. So then ... what is the solution to this dilema? Right you are --- change the speed!

This may not be the main cause of the problem that you are having, but it is valid for all closed-loop control systems (i.e., your lathe) even if you have not been particularily bothered by the problem. Some of the better designed and well engineered lathes make efforts to minimize structural resonances by various means referred to as de-tuning the system. Having a drive sysem that is very robust and able to overpower these resonances is one solution. Another is adding lots of mass to the lathe body. Designing the lathe body such that resonances are decoupled between various parts of the lathe (can we say "white whale"?) is another method that works very well.

Bill

 

[Jake Debski]

Simple check...........

Everyone has assumed, my self included, with the exception of Bill(engineer), that it is a problem with the chuck. To make sure, why not start a bowl on your face plate? That will eliminate all doubts about your chuck. :cool2:

 

[C Edward Moore]

speed changes

Bill's analysis may have a lot to do with the problem. One can continue that line of thinking and consider what happens as wood is removed while turning a natural-edged bowl. Harmonics may be involved as the outside of the bowl is shaped and speed changes will affect the wobble, but the interior of the bowl still holds things together.

Now consider what happens as we remove wood from the inside - and we assume that a final thickness of 1/4" is achieved as we work inward. Now the bowl can flex near the "rim" because fibers that held those points at a fixed distance apart have been removed. And the more we hollow out the more flex can occur. Now consider speed changes. To have the bowl almost retain its roundness, we would have to slow the bowl down due to the centifugal force. If we sped up the lathe, then the highest points of the rim would get farther apart, accentuating the ovaling that will occur with drying.

Now consider the bowl in the chuck and the turning is complete so that the thickness of 1/4" is achieved everywhere (sure!). Keep your face shield on. If you slowly increase the lathe speed and watch the ghosting, then you will be amazed at the changes as speed is increased. Don't increase the speed unreasonably or you will be wearing parts of your bowl. I turned a 3/16" bowl and decided to use lathe speed to help it dry out a little quicker. Yes, water did start to emerge, especially in the end grain, but I'm glad that I was standing aside when the bowl exploded due to excessive centifugal force. 😱

No, this isn't off topic because ghosting and wobble are closely related. 😀

 

[MichaelMouse]

Engineer's analyses aside, take a trip back to that unbalanced mass called either a missile or a bowl. It seems a bit out of the normal, even after the fix is reported, that the insert-initiated wobble should become more pronounced as the piece is rounded and presumably achieves better balance. Unless, of course, it's taking a few strategic hits in the rounding process. Tailstock help would be nice.

A lot of people love to talk about how badly out of balance a piece they can mount and still turn, when they really should be concentrating in my opinion, on getting the best balance possible. I'm as guilty as the next on that account, attempting to trade some quick stock removal on the lathe for the same at the bandsaw or drill press. All the strains of that mass swinging are still there even if they're not felt, it's just that they are being resisted (Newton's Third) adequately. Puts a lot of strain on the chuck itself, whether to the hold on the piece or insert, the spindle, or what have you through those things you paid for. What it doesn't stress is the sand, stuff you can pick up outside for nothing. Is that worth it?

The number of responses based on the primary interface is understandable. That has to be firm against both balance and the stress of the turning tool, both of which pose a greater hazard with increased speed. I know that some folks automatically increase speed, but it's normally unnecessary, and likely to bring on other problems, as Ed has mentioned. Before someone jumps in, yes, I know you can get a smaller chatter pattern with greater speed, and greater force can compensate for a dull or poorly presented edge. Just ask yourself if you believe it's worth it, or if you can't get by with less stress.

On centrifugal drying - I like it, but longer and lower speed will do as well. Some have also been observed blasting air from the inside of the bowl to speed that unbound water's way out. Nope, it won't do a thing to minimize drying degrade, but it can help keep the mildew at bay, and that's worth something, after all.

 

[Steve Worcester]

Figure that #3 jaws will allow a compression on about 5" and I have done several bowls in the 20"+ range without too much trouble.

Figure also, that as you cut, you are freeing up tension in the wood and that can cause it to go out of round, as well as warpage if it is a wet piece of wood.

 

[pfduffy]

Chuck size

I follow the simple rule that, with green wood, the chuck needs to be used after the bowl is in balance. That means starting with a face plate and and and the tail stock in place! Adherence to the tailstock in place theme will save you all kinds of problems. Face plates come loose because screws come loose in green wood. Took a bowl off the lathe Friday that needed longer screws to hold it tight. Also, when I reverse to the chuck I tighted all the torque holes and then stop and retighted at the point where I think it might be safe to take away the tail stock. But, by then half of the interior of the bowl has been gouged out and the bowl is running smooth as glass. = = 54 cents of free advise Philip