Ambrosia maple stays out of round

I've got some large pieces of ambrosia maple that I rough turned about 22 months ago. It is good and dry. I have about a dozen pieces. I've finished turned 5 of these bowls so far. I begin by doing the outside first, of course. I use a Stronghold chuck with a wooden jamb chuck that has an old sanding mat glued to it. 2 of the 5 bowls that I have done so far, I could never get the outside to ever run perfectly true. Cut after cut, these two bowls would stay out of round. One of them, I finally got it close enough not to cause a problem and it turned out to be a decent bowl. The one that I worked on tonight would never run true no matter what. I could measure it out of round by a 1/16. I would cut a 1/16 off and it would still be that much or more out of round, time after time. I just gave up on it. My chuck was plenty tight and I had a ton of pressure on the bowl with the tailstock. I even measured my spindle rounout with an indicator and it was barely over .0001". I attached a picture of my jam chuck and a rough turned bowl that I have not started on yet.

Am I doing anything wrong or is it just some times that a piece of wood won't cooperate? I've turned a lot of big bowls and it is only this ambrosia maple that has given me this trouble. Thanks

 

Even dry wood will move . As you turn down a blank it releases stresses in the wood. Sometimes you can just let the wood rest til next day, next week or next month and it settles down.So I would suggest just setting this one aside for a bit before trying again.

 

Interesting, I'm having a similar issue with ambrosia maple. I cut down a dead maple and found a little curl in cut offs pieces as I milled it up. I took those downstairs in the house in a room with the wood stove and threw them on a shelf thinking it might work. They've been down their about five years and this year when I brought them up and milled out some boards with the curl it all started to crack. I can't believe it wasn't dry. I wonder if the relationship between the hard maple and the ambrosia infested wood is the issue?

 

All good feedback above.

I agree with Al to make sure your pad isn't causing a less than solid mount. At this stage of roughing the rough-out, there's no reason you need to pad the contact surfaces as any marring will be cut or sanded away. Because you are seeing it develop while it's jamb mounted I think that is what's happening. If the mount is solid and the wood is moving that much I'm wondering if it's truly reached equilibrium moisture content. (Are you weighing the rough-outs or only relying on the passage of time?)

My primary goal at this initial jamb chuck stage is to true the tenon, take off the bulk of the out-of-round from drying, and reshape the profile as desired -- in that order. Don't stress out too much on achieving perfectly true running here. The fact of life is that when you turn the bowl around to re-cut the inside it is highly likely the exterior will not run true anyway and can be cleaned up with light cuts - or not, because you have to keep in mind that all wood will continue to move, during turning and after -- just a fact of nature. Large pieces are just more noticeable spinning on the lathe but on the table I highly doubt you'd see that 1/16".

(As a curious aside, are you really measuring to one 10,000th of an inch? That is serious measurement and phenomenal you have so little variance. Measuring to one 1000th is much more commonly done.)

 

I bet it's the curl. Figured wood is proof-positive the tree was responding to a stress while it grew. The cellular structure the tree put in there to counter the stress now has nothing to act against so is released and shows itself with movement. Was the beetle a stressor such that the trees respond with uneven growth? I don't know, perhaps a note to Andi Wolfe (botanist at Ohio State and AAW wood turner) would get an answer.

Alternatively, depending on how thick your stock was, it may have become "case hardened" if it didn't dry slowly and evenly. This has the effect of a dry exterior and moist interior.

 

First, totally agree with advice given. I've tried to true up a warped, thick turned dry bowl using a jam chuck and it seldom works. You can be near perfect and a slight tough spot or dig and the bowl moves just a little and then you start over. Getting a very slight warp out is harder for me than a big one. Too easy to have your gouge simply follow the shape rather than change it. I always jam chuck it in place, true up the tenon and then flip it. No sense fighting it. Also, turned some ambrosia maple a while back and it moves a LOT so once you get the shape done leave it alone - it will move more anyway.

 

Well, wood moves, and it never really stops unless you do the stabilization process. That being said, if you have 1/16 of an inch of run out, that means 1/32 +/- which is very close tolerances for an unstable material. If your finish cuts are with a bevel rubbing cut. then there is always some bounce, in part because of cutting into end grain and then side grain, and cutting with and against that end grain.. There are 2 ways that I have found to get a near perfect circle. One is with a negative rake scraper and the other is with a shear scrape. Both of these cuts are done without rubbing the bevel, so you are basically trimming down the high spot or spots. I guess we can add in lathe speed as well. Since the bowl is side grain, the wood can stretch more oblong as it spins, especially if you are turning at high speeds. All of these above answers are why when turning the outside of a bowl, you can almost always get a finish cut in one pass from base to rim. On smaller bowls, you can do it in one pass on smaller bowls, but in bigger bowls, say over 10 inches or so, it becomes more difficult, and in deeper bowls, it is pretty much impossible, and you have to do finish cuts in stages rather than one pass. The thinner you go, the more difficult this becomes.

Side note here, I have been turning a few platters lately. They tend to vibrate and seem to move more than bowl forms. The deeper curve seems to be more stable, and the flatter curve/form seems to be more wobbly.

robo hippy

 

Big thanks to everyone! All great advice and I have a lot more knowledge in my arsenal now.

My sanding pad could be a problem. It is fairly thin though. I may try the next one without the pad. I know this is hard to quantify, but how tight does everyone crank down on their tailstock when jamming a bowl? I feel like I go pretty tight, but I don't crank down on it with everything that I have. I don't want to put undue stress on the bearings axially.

I think stresses could have been a big issue also. I roughed turned this bowl in a spittoon shape, which was pretty ugly. This odd shape may have created some stresses in the wood. I ended up actually parting off the top 2" of the bowl and turned it more into a calabash style bowl. Cutting that much material off could have relieved a lot of stress to make the bowl move more also.

Owen - As far as equilibrium, it was dry all the way through. I checked the moisture content of the center of the piece that I parted off and it was the same as the outside of the remaining bowl. I measured my spindle runout on the inside taper. I used a dial indicator with .0005" graduations. So, technically, it really doesn't have the resolution for me to get a precise runout measurement down to .0001". But, I can tell you as I spun the spindle, I could barely see the needle of the dial indicator twitch off the zero mark, so I "interpolated" my reading a little. When I check the outside body of my mounted Stronghold chuck, it reads about .002" of runout, still pretty good.

 

How hard to crank down is a tough one to describe. I suppose I screw the tail center in firmly but not like the Hulk. If the bowl slips while re-turning, I tighten a bit more. I guess that's the answer: tighten enough to ensure the bowl isn't slipping when tool pressure is applied. I don't see any worry about it coming off since it's pretty well captured between the two parts of the lathe. I suppose if you are way over-tightening it, you could be "straightening" out the drying warp against the jamb chuck and it springs back once the pressure is off.

The .002 is good. Nothing that should be of any concern.

 

You haven't mentioned (or I missed it) what type of live tail center you're using. A cone center will keep pressing deeper and deeper into the wood with more pressure but becomes loose as you work on the bowl. A cup center with a small point works better for me when re-turning a roughed turning. You can put reasonable pressure and it will stay tight. I don't usually use any kind of pad between the bowl and the wooden jamb chuck but if I do it's a piece of that non skid fabric you can buy to put under rugs. When you're re-rounding and shaping a rough turned bowl, adjusting the pressure against the tenon would be the most likely culprit in the kind of movement you're experiencing.

 

If I read correctly you are finish cutting the OD using a jamb chuck on the ID. Two things work against this method 1) The jamb chuck is locating on a warped ID applying uneven forces to the surface, 2) As OD material is removed, wall thickness now varies, multiplying the uneven stress relief within the shape.

For these reasons, when jamb chucked I true the tenon, take of hi points (not trying to get perfect round) of OD and rim, then flip to chuck mount. Remove hi points in the ID to even up wall thickness, then go for finishing the OD. I found that using a thin piece of rubber allowed too much flex. I use a 1/16” thick piece of leather. As for tailstock pressure - I set the spindle lock, and tighten enough that I cant rotate the bowl by hand, and dont take heavy cuts.

 

I started a new ambrosia maple bowl last night. With everyone's tips, I got the outside nearly perfectly round. I had one narrow band that wasn't perfect, but I had a good shape overall with minimal tooling marks so I decide to stop it there. (Knowing when to stop is a skill that I have to constantly work on!)

Al's suggestion on working from a known round spot and rubbing the bevel in steps helped a lot. That will take more practice to really get good at it. I also used my scraper per Reed's suggestion to fine tune the tough spots into round. As Curt suggested, I took off my cone on my Oneway center to keep the bowl tighter. I may look into a bigger cup center. I also like Doug's suggestion for setting the pressure with the tailstock, didn't read that until now, but I will definitely try that on the next one.

Thanks to everyone again!