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Vacuum chuck artifacts

Marc Banka

Marc Banka

Joined
Jul 27, 2020
Messages
349
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1,361
Location
Durham, NC
My practice for finishing bowls or platters is to grip the piece between a vacuum chuck and the tail stock. I turn whatever foot I want, blend it in and finish sand, apply a Walnut oil/wax finish. Then turn the vacuum on, remove the tailstock, and sand the interior of the foot.

There is then, a ring where the chuck seal is against the wood. In the worst case it’s been a black ring very difficult to get out. In other cases it’s just a small change in the surface, like a smudge or scuff. It’s very frustrating as remounting the piece to fix it is fraught, at best.

My findings so far:

The peal and stick gaskets for the oneway chuck can make the worst rings. Pressing the gasket against an already finished interior seems to soften the rubber.
Using closed cell foam doesn’t cause black rings, but imo can cause the piece to run untrue. It doesn’t compress evenly.
I’m now using a non-marking rubber from McMaster-Carr. It’s really fantastic. No black marks, but still a ring.

I turn on a Nova 2024 lathe, which I mostly enjoy. However, the fit and finish from Nova isn’t even close to ‘top shelf’. In this case, it’s the tailstock alignment that is surely one factor. When gripping between a chuck and live center, any alignment error results in scuffing the piece against the chuck. (I could easily hear the scuffing squeaking). I aligned headstock/tailstock with a double ended morse taper, then further fine tuned it with paper shims, etc.

All of this has reduced the problem, but has failed to eliminate it. At present I suspect the alignment problems may be what’s responsible.
Do I have to replace the lathe? A friend with the same lathe is struggling with the same issue.
Have you experience this?
How did you remedy?
Do you chuck against bare wood, finished wood?
What lathe are you using?
 
I vacuum chuck bowls, most often with unfinished interior. I do eave the tailstock engaged as long as possible.

I use a ring of white "craft foam" (it's about 1/8" thick) between my Rubber Chucky gasket and the wood. Mainly because I find it improves the suction/fit. If I have a bowl that's no longer true, I might add a bit of the foam somewhere around the ring to get it to run how I want around the foot.

I usually get the vacuum on right away, even with the tailstock engaged, because the bowl will move as the suction pulls it into the chuck. So I like to get that all resolved (running true) before I start turning the foot.

I do see a ring sometimes. But if it happens, all it takes is a bit of hand-sanding with my last grit (usually 400) to clean it off.
 
What Dave said

Marc Banka said:
The peal and stick gaskets for the oneway chuck can make the worst rings. Pressing the gasket against an already finished interior seems to soften the rubber.
Using closed cell foam doesn’t cause black rings, but imo can cause the piece to run untrue.
Click to expand...
I use the thin craft foam. There is a trade off. The spongy foams seal better on something out of round but the sponginess is felt while turning - not good.
The thin foams need a pretty round surface to seal well. The turning is great.


Marc Banka said:
Do you chuck against bare wood, finished wood?
Click to expand...

The inside surface is usually bare wood. I will seal the outside on woods that leak air.
Leaking air can pull dirt into the wood leaving a ring. It’s more of a problem with wet wood. Sealing the outside fixes this.


Finally the tailstock gets me at a starting point for the vacuum chuck. I fine tune the alignment with the tailstock backed off.
I don’t need a tailstock for vacuum chucking. Many times I don’t use the tailstock at all for vacuum chucking.
 
A few thoughts:
  1. Apply finish off the lathe. Don't use vacuum - which pulls the finish through the pores and is probably reacting with your black foam seal.
  2. Use thin closed cell (white) shipping foam over the black Oneway seal. It comes as thin as approx. 1/16" - just slap a square piece over the black seal and punch or cut a slight vacuum hole.
  3. Don't use a drum type chuck (i.e. Oneway). Use a flat disc vacuum chuck which only touches the rim and increases the volume area for suction. Of course, that won't work for a NE bowl !!!
 
Finish is the last part of the process and one without lathe attachment or vacuum.
If I get the post correctly, you are applying finish and then reverse chucking to remove the tenon.
The vacuum is sucking the finish through the pores of the wood and that area will get way more penetration of oil than anything else.

A colred ring, not from oil, would seem to be the chuck slipping and causing the foam ring to abrade the wood
 
I built a vacuum chuck out of a disc of mdf a couple inches smaller then the swing of my lathe. I glued and screwed it to a block of hard maple that i tapped for my spindle. The face is lined with a sheet of high density foam and the bowl seals with the entire rim. It does not work for every bowl but the ones that have level rim and no holes hold well in it.
 
l learned vacuum chucking from David Ellsworth. His chucks are all shop made from wood and use vinyl shelf liner from Lowes for the gasket. Glues it on with contact cement. The stuff lasts forever and doesn't mark. It can also be used as a jam chuck. It does not have much give, so the bowl needs to be fairly round/smooth. Since this can sometimes be a problem, I also use commercial gasket tape made for clamping. It has a little give but hardly any squirm. Next step is the closed cell foam, but it squirms and the piece never runs true.
 
I have had good luck with the gaskets from Rubber Chucky ... made my own chucks out of schedule 40 PVC.
 
Karl Best said:
I don't recall seeing such a thing. Suggestions?
Click to expand...
Probably made my first one about 25 yrs. ago. Don't have any photos.
Think sanding disc - except instead of sandpaper use a vacuum seal (neoprene, closed cell foam, etc.). Better to not use anything too thick. Drill a small hole in disc (3/8" - 3/4") so vacuum can be drawn directly through the spindle. Any size will work depending on the size of the bowl, but if you make it to almost the full swing of your lathe you can mount any size bowl with just one disc. You can also get some fancy details if you mount the bowl a little off-center.
 
Most of the time I use my Oneway vacuum chuck like a friction pad. Bring up the tailstock and clean up the bottom until only a stub remains. Cut that off with a carving chisel and a touch with the sandpaper. If I was to make a vacuum chuck, I would buy some big o-rings and build to that size. Farm supply stores are the place to get big o-rings. You can get chemical resistant o-rings at McMaster Carr up to 6.5" diameter.
 
I learned that seating the bowl on the chuck with pressure from the tailstock before turning on the pump gets me better alignment than letting the vacuum seat the piece. Also I never use the tailstock while turning as any misalignment is going to cause the seal to scrub the surface its in contact with. Be sure to clean the seal surface before each use too to avoid imbedding dirt in the finish.
 
I've noticed the vacuum pulling oil through the wood and this may be part of your problem, but I wonder if it's actually happening when you are turning without the vacuum on. That would allow the bowl to slip on the surface of the chuck, and rub or grind the seal into the wood. Why not try getting things set up as you normally do, except turn on the vaccum before you start turning off the tenon/making the foot to see if this fixes the problem?
 
Dean Center said:
I've noticed the vacuum pulling oil through the wood and this may be part of your problem, but I wonder if it's actually happening when you are turning without the vacuum on. That would allow the bowl to slip on the surface of the chuck, and rub or grind the seal into the wood. Why not try getting things set up as you normally do, except turn on the vaccum before you start turning off the tenon/making the foot to see if this fixes the problem?
Click to expand...
Would never ever recommend applying finish while using the Vacuum chuck . Any reasonably thin bowl will have finished pulled thru.
 
From reading all of your helpful comments, thanks so much, I’ve decided that there are two separate issues.

First, with vacuum on and no tailstock, is the problem of pulling either dirt or finish through the bowl. Al’s method of sealing the outside first seems reasonable, but doesn’t quite work for my workflow. Perhaps there’s a way.
I’ve certainly seen oil pulled through, and in the case of walnut, have seen a bit of the color move out with the oil. Not a good thing!
The steps I‘ve taken to minimize the migration are minimizing the time vacuum is on (meaning more time with tailstock support), and limiting the area exposed to vacuum. So, a smaller chuck. Nothing unsafe, but no real need to go with maximum. Besides the smaller footprint, the smaller chuck avoids (largely) pulling on end grain areas, the sides of bowls. This is true for bowls oriented pith down. Don’t know how it works pith up.

Second, and I think the biggest culprit is scrubbing when using a pressure chuck with tailstock. There are a lot of opinions on which material to use for a seal. The thicker, softer seals have more resilience, might give laterally enough to mitigate the effects of misalignment. The trade-off is that they are squishy, and the bowl may even shift or tilt a bit when vac is turned on. Thinner, and/or harder materials last longer and run true, but are less forgiving as relates to scuffing.

If there were zero misalignment, headstock to tailstock, there should be zero scuffing, right? ( I’m dealing with the biggest culprit first.) Given my work experience as a mechanic/machinist for a lot of years, I expect my equipment to give me just that ability. That the only errors would be the human factor. Not at all true for my Nova lathe! What is the experience for those of you with top shelf lathes, Robust, PM, oneway, etc? Do they come with this precision, and/or ability to dial them in? Will spending upward of 10k give me something with machine shop tolerance?

A few other thoughts:
The vacuum disc is interesting. Probably great for fixing a finish on an existing piece, once recentering is figured out. Being able to run the piece shifted a bit could be cool. Not sure how, but neat to know it can easily be done. For larger bowl, that must be a lot of pressure! How does one keep a plywood or mdf disk that size running true?

I bought into vacuum chucking as a way to finish a bowl with no tell-tale, as a very last step. To allow me to sand my work glass smooth and to apply a wax/oil finish, and to friction set the wax and be done.
I’m loathe to re-sand the interior after a perfect finish. I think it’s difficult to beat the surface created from abrasives at lathe speed, and a finish buffed at 1000 rpm.
Finishing ‘off‘ the lathe and adopting a Beal buff workflow achieves this, or gets close, at least, so I understand . But that’s a lot of time and work that I don’t do, nor have to do with my current methods.

Maybe I’m asking for the world here.
 
I turn on a Powermatic and use a oneway vacuum chuck. I use thin closed cel foam and almost always can see where the bowl contacted the chuck. A quick hand sanding takes care of the problem.
 
Karl Best said:
I don't recall seeing such a thing. Suggestions?
Click to expand...
Precisely because of "dust hickeys" inside of my bowls from when I used a cup-style vacuum chuck, I switched to plate-style chucks. I bought a cheap aluminum faceplate and screwed a 3/4" plywood disc to it. I trued up the plywood on the lathe, and then drilled a small center hole through the disc. For gasket material I use 1/16" white or beige craft foam, lightly attached to the plywood plate with double-stick tape. As other folks have noted, the bowl contacts the plate only at the rim so you don't get those rings inside the bowl. I still use my shopmade cup-style chucks of my bowl has an irregular rim or when I need to mount a bowl with the base toward the headstock, but the flat chuck takes care of most of my vacuum chucking. I made a few different sizes, too. To help center pieces that no longer have their center point, you can draw a few light pencil lines on the foam gasket with the lathe running.
 
Gary Beasley said:
I learned that seating the bowl on the chuck with pressure from the tailstock before turning on the pump gets me better alignment than letting the vacuum seat the piece. Also I never use the tailstock while turning as any misalignment is going to cause the seal to scrub the surface its in contact with. Be sure to clean the seal surface before each use too to avoid imbedding dirt in the finish.
Click to expand...
I just found this out . I also learned to use the tail stock to align and turn the object. Prevents flinging.
 
I guess its a question of which devils are you willing to tolerate. I finish turn the inside of my bowls - outside is finished except for the tenon/foot - then apply a light oil fnsh. I typically let that finish cure as lonng as possible - 4 to 6 hours minimum. The bowl then goes on the vacuum chuck (JT Turning tools hub with sched 40 fittings) for completing the final lower form and the foot. I use Rubber Chucky seals or, if movement is a roblem, I true the rim of the chuck lightly and use 1/16 closed cell foam after removing the Rubber Chucky seal.

When done the finish is applied to the outside/foot off the lathe. Once in a while there's a very slight abraded area where the chuck seal rode on the inside - 30 seconds with the last grit used or 0000 synthetic wool takes care of it. One final coat of the oil, ren wax overall and done.
 
It is amazing how many different ways there are to reach the same point in the end. My only use for a vacuum chuck is to be able to remove the last little stub after the tail stock is removed. I don't use it for finishing at all as I don't like having my lathe tied up for that purpose. I use 600grit wet or dry sand paper to wet sand my first and second coat into the bowl. I do the inside and outside and have a drying closet with an bathroom exhaust fan in it. I have a table and shelf with small sheets of plywood that have three screws run up from the bottom. The top of the screws are filed down slightly to remove the sharp point and the bowl (s) are set on the screws that are in a triangle pattern. I can dry 8-10 bowls at a time this way and never smell the finish in the shop or worry about dust. This allows air to circulate all around and only three tiny tips are making contact. I put on another coat the next day and perhaps a third on the following day. I have a free standing buffer from Grizzly that has long arms to get the bowl away from the motor.
 
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