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Having trouble getting true for chuck

S

Sam Beagle

Joined
Jan 7, 2017
Messages
24
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6
Location
Mount Carmel, Pennsylvania
for some reason I’m constantly having issues going from face plate to chuck. I’ve tried everything
I have 3 different chucks, I use recess method. I make a perfect recess and dovetail. But still have to make a few passes with the gouge to true up.

Any thoughts
 
Being the first to respond, I'm sure you'll get some more "expert" opinions. But I've found that wood is not a very exact material to work with. When you switch from faceplate to chuck, many things can happen. Mostly though it's just the fibers in the recess or a tenon compress unevenly enough from the pressure of the jaws of your chuck to cause a slight off center when remounting. I really only use two solutions. I either make those few passes you mentioned or if it's sanded and I don't want to mess up the finished surface I leave it alone and accept the very slight variation in wall thickness when I turn the inside of the bowl.
 
Many things can affect how true the piece runs when placed in the chuck.
Off a little bit is common. Easy solution is true the piece after goes in the chuck if it needs it.
I do this routinely on bowls that will have thin walls.

As Curtis said it is an imperfect media and you never get absolute perfect.

It could be your faceplate Mount. If you did not turn the faceplate mount then it probably has a bit of vibration in it that will transfer to a tenon or recess that is slightly off. If you screwed the faceplate to a chainsaw surface I would not expect to get a true tenon or recess.

For blanks with a chain sawn surface my first preference is a spur drive (good locking tailstock essential)
Second choice is a worm screw in the chuck and it doesn’t require a tailstock.

Also if you do tenons or recesses with a scraping tool they may not be truely round and can have torn fibers that throw the mount off.
I prefer tennis on bowls.
 
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I'm a tenon on bowls and recess on platter kind of a guy. When initially tightening (or expanding) I snug down enough to hold the piece, then give it a spin. If it is off a bit, I loosen the chuck and rotate the piece in the chuck a few degrees, tighten and spin again. A couple of tries will usually get rid of wobbles or close to it. Then tighten the chuck. Another thing to check is how true the faceplate is. With a dial indicator it's easy and you get a number. Without a D/I put the face plate on the lathe (clean bearing surfaces) bring the tool rest close and touch near the edge of the spinning face plate lightly with a sharpie. Hopefully you will scribe a complete circle and rule out your faceplate. Don't press too hard you will scribe a complete circle, lightly, you'll just hit the high spot. You can check your chuck the same way. If your chuck uses a threaded insert, that's another area to investigate.
 
Curtis Fuller said:
Being the first to respond, I'm sure you'll get some more "expert" opinions. But I've found that wood is not a very exact material to work with. When you switch from faceplate to chuck, many things can happen. Mostly though it's just the fibers in the recess or a tenon compress unevenly enough from the pressure of the jaws of your chuck to cause a slight off center when remounting. I really only use two solutions. I either make those few passes you mentioned or if it's sanded and I don't want to mess up the finished surface I leave it alone and accept the very slight variation in wall thickness when I turn the inside of the bowl.
Click to expand...


What Curtis said. I will add that when remounting I always assume that the piece will not be concentric on the new mount, even when I transfer the mount from one chuck to a second mounted in the tailstock. I try to plan the operation so that I can live with that slight eccentricity.
 
Some of the issue is that wood is a compressible non-homogeneous material that is forever moving even if just sitting undisturbed on a shelf. But, I think that the biggest part of the alignment shift comes from how we mount the wood in the chuck as well as not recognizing that woodturning scroll chucks, while very accurate, are not high precision devices.

First, how we mount the piece onto the chuck:
Let's just assume that we are using dovetail jaws. There is only one diameter for a tenon or mortise where the jaws make full contact with the wood all the way around. It's called the perfect circle diameter. At any other diameter there are either only four small spots on the jaws pressing against the recess or eight sharp corners biting into the tenon. In either situation at each point of contact the wood (being non-homogeneous) will compress a slightly different amount so the mortise or tenon will be shifted slightly one way or another from its original center. From an engineers perspective we can never have anything that is absolutely perfect ... everything has some error: our dovetail angles are never an absolute perfect match to the jaw angles and we can never make a tenon or mortise that is precisely the perfect circle diameter.
​
Lastly, the chuck itself:
There are super high precision scroll chucks for metal turning lathes that cost a few thousand dollars and attach to the spindle using a machine taper for alignment. That's a little too rich for my diet and besides we are working with wood (remember that we will cause it to no longer be round when we sand it and besides that the wood moves with changes in humidity). Let's assume again that our chuck has dovetail jaws set to the perfect circle diameter. At the heart of the chuck is a disk that has a ring gear on the back side and a scroll gear on the front that engages rack teeth on the bottom side of the base jaws.By necessity each of these parts has to have some free play. The disk floats around a central hub, but the disk shifts position slightly depending on which pinion gear is driving the ring gear. On the other side of the disk there is a bit of free play between the scroll and the teeth on the base jaws as well as between the base jaws and the guides in the chuck body. The bottom line is that the chuck jaws perfect circle center is never perfectly the same as the rotation axis center.
​
After all of these issues, it's impressive that scroll chucks perform as well as they do.
 
It is one of the great pleasures of turning when you turn over the piece, put it in the chuck, and runs perfectly true. Since I have been working with super dense woods like Boxwood, I have noticed that my success rate with denser woods is much higher than with softer ones like Koa and Milo. This tells me that the compression from the chuck throws it off center more than what we think.
 
Wood moves! Your best chance for the least runout when turning over the piece to the second side is to make sure your last cut is a finish cut on the tenon or recess.
 
Could simply be the shape of your recess vs the shape of your chuck jaws. Stewart Batty demos this well but I could not find that video. Basically the V of the dovetail is slightly smaller angle than the V of the jaws so that the jaws are contacting the wood in 2 places nearer the outer edges of the V. If the V is too wide or doesn't have a sharp point at the bottom the jaws tend to hit her and can rock as you tighten the jaws and not seat perfectly flat which throws the bowl out a little.
Another thing that I have found is to locate the jaws on the turning so they are hitting side grain and end grain the same. End grain doesn't compress the same as side grain so it can throw the bowl off. The easiest way to do this is to make an X. One line goes parallel to the grain direction and the other line goes perpendicular to the grain direction. When you put the chuck in rotate it so the X marks fall on the gap between the jaws. Now you have an equal amount of side grain and end grain on each jaw. I find this is much more pronounced when gripping tenons vs gripping a rebate.
 
Well, when you reverse, if you spin the piece and it is within 1/16 inch when you spin it by hand, that is pretty good. Only noticeable if you are turning down to less than 1/8 inch. Lots of things can make for added run out. Any chip or piece of wood can make for up to 1/8 or more run out, make sure the recess or tenon is free of debris. One chuck jaw screw can do the same. For the finish cut on the edge of the recess, I hold the tool very gently and let the lathe spin, just taking off tiny dust motes of wood. I prefer a shear scrape to an actual scrape. Some times just rotating the piece 45 or 180 degrees can make a big difference. I do have a video up on 'Mounting things on the lathe' that might help. Oh, like my Kung Fu teacher says, "10,000 more times!" But teacher, that is what you said last time..... "Well, 10,000 more times!" You do get better with practice....

robo hippy
 
Some very good points here so far . Bill I had never thought of the mechanics of the chuck in the manner you posted. Another factor is moisture. In green wood that tenon will compress more in the soft areas that it will when it is a dry blank and since the first post did not specify I would think that to also be a large factor.
 
Has anybody tested their chuck with round metal rod? Testing with wood seems meaningless given the characteristics of wood.

Chuck the metal to determine if it runs true. A dial indicator would be the best instrument, but I bet run out will be obvious. Mark the metal and chuck where the run out seems worse. Re-chuck the metal and compare results. Test with different diameters of metal too. That could indicate the accuracy of the scroll.

I suspect the bottom line here is the chucks are relatively low precision. Considering selling prices how could you expect precision?
 
Headstock alignment, tailstock alignment and chuck accuracy is pretty good considering cost but at the same time it's the use of the machine. Nothing on my wood lathes is as accurate as the cheap harbour freight metal lathe I bought if you start checking alignment with an accurate dial indicator. My wood lathe chucks seem to run around a thousandth or so on the vicmarcs and slightly more on the off brands. Still very fine for wood. My cheap metal was too lathe was less than that. Look.at the tailstock. The wood lathe slides between a gap. There has to be a little slop or it would not slide easily. The bed on my metal lathe has a dovetail shape so when the tailstock locks down it's dead on accurate and I'm sure the ways are ground much flatter.
Of course why would you need dead on accuracy when the wood has enough flex in it to throw it off .010" or more just by tightening the jaws. In fact if you take a few cuts down a bowl the stresses in the wood will throw it off a little at the very least. Then you throw in hand sanding g and it gets even more out of round. We simply cant achieve the accuracy of turned metal in a piece of wood. That's the nature of the beast.
 
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