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Luke Maddux
10th May 2015, 10:39 PM
I'm a bit of a one-trick pony when it comes to mounting bowls. Unless something goes wrong, it's almost exclusively a three step process with a scroll chuck.

1. Mount in faceplate ring and turn the bottom, including a dovetail mortise
2. Mount in the dovetail using expansion mode and turn the inside, then sand
3. Mount in bowl jaws, clean up the mortise, and finish sanding

Ever since I got my new lathe, I've been noticing that after each remounting event in this process, the bowl just isn't right on track. Like it's very slightly spinning out of plane and there's just enough wobble that I have to kind of go over it to with some shear scraping to take it down to "true". I haven't noticed any wobble in the chuck, jaws, spindle, or anything mechanical. Just in the wood.

I think it's also very important, and possibly the reason for the "problem", to note that my new lathe is drastically larger and more powerful than my previous and I have been turning larger pieces by a several cm margin.

So how typical is this? Is it normal to see a very slight "wobble" (for lack of a better word) when you remount, and for that wobble to be more exaggerated in larger pieces? I remember it kind of happening on the smaller stuff I used to turn with my previous lathe, but it's just a bit further out of plane on the big lathe so I want to make sure that this is at least somewhat normal.

Thanks a lot in advance,
Luke

Mobyturns
10th May 2015, 10:49 PM
Luke the process you use is fairly typical of what most bowl turners use. Some fairing is always required and there are tricks to disguise and to minimize any minor misalignment from reversing blanks. The shape and size of your tenon and how it is mounted in the chuck jaws has a significant effect on blank alignment. Misalignment is harder to control if you open jaws wide and rely upon the 8 corners to hold the tenon. The corners may crush the tenon wood at slightly different rates. Jaws near ideal opening diameter seem to suffer less misalignment.

Luke Maddux
10th May 2015, 10:56 PM
Ok, good to know. I've been meaning to take the time to make some kind of tool that will mark the exact diameter of the mortise I need for perfect alignment in expansion mode. Maybe I'll make that a project for tomorrow.

dai sensei
10th May 2015, 11:03 PM
Keep also in mind wood moves. Temperature, humidity, seasoning, centre of gravity, grain orientation, internals stresses etc all cause movement that also varies from timber to timber.

Mobyturns
10th May 2015, 11:04 PM
Ok, good to know. I've been meaning to take the time to make some kind of tool that will mark the exact diameter of the mortise I need for perfect alignment in expansion mode. Maybe I'll make that a project for tomorrow.

Luke check out Glenn Lucas' on YouTube @ https://www.youtube.com/watch?v=Bsq3w--rMhU At the 1:44 mark it shows a gizmo he attaches to his live center in the tail stock to indicate the size of perfect tenons.

Luke Maddux
10th May 2015, 11:20 PM
Luke check out Glenn Lucas' on YouTube @ https://www.youtube.com/watch?v=Bsq3w--rMhU At the 1:44 mark it shows a gizmo he attaches to his live center in the tail stock to indicate the size of perfect tenons.

Very cool.

jay h
10th May 2015, 11:30 PM
Hi Luke, I had the same problem when I started turning (still a novice) it drove me crazy. Checking dovetails and recesses for saw dust etc couldn't find the answer. Every time I rechucked a bowl the outside needed returning. Then I checked the chuck, one jaw was proud,quite a bit. Purchased a vicmarc vl100. no problems any more. It was the chuck and what you are describing is very similar to what I experienced. I am not trying to say buy a vicmarc necessarily; any good chuck will do. Jay

Luke Maddux
10th May 2015, 11:35 PM
Thanks Jay.

I am using a Teknatool G3 for my described steps 1 and 2. Then I move to my dedicated bowl jaw chuck, which is a Vicmarc VM90 with the 385mm bowl jaws, for step 3.

Luke Maddux
10th May 2015, 11:36 PM
Thanks Jay.

I am using a Teknatool G3 for my described steps 1 and 2. Then I move to my dedicated bowl jaw chuck, which is a Vicmarc VM90 with the 385mm bowl jaws, for step 3.

I've checked the jaws on the G3 and they dont seem to be misaligned. Maybe I should have a more thorough look though.

Sturdee
10th May 2015, 11:54 PM
I've checked the jaws on the G3 and they dont seem to be misaligned. Maybe I should have a more thorough look though.

Close the jaws of the chuck and mount on the lathe without any timber in it and then use a scraper on the jaws (at low speed and with safety helmet on) to make sure that the jaws are even.

Also check the inside of jaws to make sure that they are aligned properly for 1mm on one of the jaws can cause big problems.

Peter.

Luke Maddux
11th May 2015, 12:09 AM
Close the jaws of the chuck and mount on the lathe without any timber in it and then use a scraper on the jaws (at low speed and with safety helmet on) to make sure that the jaws are even.

Also check the inside of jaws to make sure that they are aligned properly for 1mm on one of the jaws can cause big problems.

Peter.

I literally thought about doing just that, so it's good to hear someone say that it's feasible. Thanks!

Paul39
11th May 2015, 11:28 AM
I agree with the above suggestions.

I filed a mark on my #1 jaw on all my chucks and mark the bowl before taking it out, so when I put a bowl back it will be close. It rarely runs the same when remounting.

I also have found even with "dry" timber that a bowl started one day and left to the next day to finish will not run true even though not removed from the chuck.

Just the taking on or losing moisture and / or relieving of internal stress will cause the bowl to move.

turnerted
11th May 2015, 02:39 PM
Luke
Probably the most accurate way to align your bowl when turning it over is to use a transfer mandril . This is a gizmo that has a thread to match your chuck , a bearing and an M2 taper to fit your tailstock . The catch is you need two chucks or have your bowl on a faceplate when turning the outside .
In use, you fit the mandrill on your tailstock ,remove your chuck with bowl still in it and screw it onto the mandril . You then wind up the quill until the tennon or recess on the bowl fits a chuck on the headstock, then remove the mandrill . Despite this , as others have said the wood may move while turning and not run dead true . A transfer mandrill is available from Mcjings.
You can also try loosening your grip on the tennon or dove tail and giving the bowl a slight tap to true it up .
In practice , a slight run out at the rim of a bowl will not be noticeble to anyone but you on completion.
Ted

Skew ChiDAMN!!
11th May 2015, 07:05 PM
...and I've found there's always a little bit of misalignment no matter how accurate I think I'm being.

I've come to accept that after reverse mounting I only tighten the chuck enough that it just grips the piece firmly, then turn the lathe by hand, taking note of how the piece is misaligned. Loosen the chuck slightly, thump the piece with the heel of my hand to nudge it in the appropriate direction, retighten, rinse & repeat until everything is hunky dory.

With practice it's now only a matter of 30 seconds or so with even my most badly remounted pieces!

Mobyturns
11th May 2015, 11:12 PM
Close the jaws of the chuck and mount on the lathe without any timber in it and then use a scraper on the jaws (at low speed and with safety helmet on) to make sure that the jaws are even.

Also check the inside of jaws to make sure that they are aligned properly for 1mm on one of the jaws can cause big problems.

Peter.


Before you consider these actions,

Check that all jaws meet accurately in the fully closed position. If not -

open the chuck jaws slightly,
loosen all the cap screws holding the jaws to the backing slides
just snug or firm the cap screws so the jaws can move under a bit of force
close the jaws firmly to the fully closed position
tighten the cap screws.
Check for run out of the jaws.


If this doesn't solve the issue look for things like a poorly seated chuck insert or damage to the spindle nose threads or insert threads. If they are OK then try other things, but I would not be attacking the jaws with a scraper if they come from a "reliable" manufacturer.

ps - they jaws are not intended to be a perfect circle in the closed position. They only form a perfect circle when opened to the original milling kerf width. Truing the sides or face of jaws with a hand held scraper has its own hazards with partially opened jaw sets or one protruding jaw. All the jaws should be the same height as they should be cut from the one milled blank. If they are not I would suggest that the chuck may be damaged, or you do not have an original jaw set but one made up from bits from two or more jaw sets OR there is something wedged between the jaw and the backing slide.

Mobyturns
12th May 2015, 09:01 AM
This comes from the Nova chuck manual

"Attaching Jaws -
To mount the jaws to the Supernova chuck is another easy process. Firstly wipe clean all jaw slides making sure a clean contact will ensure. Repeat this with all four jaws you wish to attach. Jaw slides are numbered one to four and have a corresponding jaw of the same number. Place a jaw on its correct jaw slide and position into the groove (location ring out from the jaw into the groove of the jaw slide). The first few times you may need to GENTLY tap the jaws into the locater slot with a block of wood or plastic mallet. Place M6X6 countersunk screws in jaws and screw them on finger tight. When both screws are in, nip them up and back them off half a turn. Repeat this for the remaining three jaws, now using the 8mm T bar Allan key scroll all jaws towards middle until they all come together. Now with all jaws touching and exerting equal pressure on each tighten all screws up. This will ensure a perfect run out. Check to see there areno gaps between the jaws if this has happened it will probably be due to a jaw being placed on the wrong number (e.g. a #2 jaw on a #3 jaw slide)"

powderpost
12th May 2015, 12:01 PM
I really do wonder sometimes if it is wood turning we are talking about or are we in fact machining parts for a car engine. Does it really matter if the wall thickness of a bowl varies slightly? Who is going to notice when the bowl is finished. Enjoy the pleasure of wood turning. Admittedly if you are aiming for a 2mm wall thickness, there could be a problem.

Jim

joe greiner
12th May 2015, 02:44 PM
...and I've found there's always a little bit of misalignment no matter how accurate I think I'm being.

I've come to accept that after reverse mounting I only tighten the chuck enough that it just grips the piece firmly, then turn the lathe by hand, taking note of how the piece is misaligned. Loosen the chuck slightly, thump the piece with the heel of my hand to nudge it in the appropriate direction, retighten, rinse & repeat until everything is hunky dory.

With practice it's now only a matter of 30 seconds or so with even my most badly remounted pieces!

Plus one for this method.

Misalignment is easier to detect if you gently hold a pencil near the rim so that it touches only at the extreme runout. That's where you thump it. Choose a different location from the rim for subsequent trials. Erase all by sanding.

Cheers,
Joe