vacuum chuck and discoloration

[Steven Forrest]

Hi, All,

I recently started with vacuum chucking. I like it, and would use it more, but a couple of times now I have had the vacuum result in a perfect circle of discoloration. Presumably this is moisture being pulled out of the wood. This has happened even with fairly dry wood, though, and after I have put on a finish, which means I'm stuck. I have tried to minimize the amount of suction, but it still happened. I have been a little gun-shy as a result, and haven't wanted to possibly waste the wood (or time) to really troubleshoot this systematically/scientifically. Anyone with similar experience, and any solutions? Much appreciated.

 

[Josh Stevens]

I found the same thing happened when I tried to use the vacuum on a piece that I had already oiled, so now I vacuum chuck pieces before I apply a finish. I'm not sure if all finishes react this way.

Josh

 

[hockenbery]

Most woods are pourous and most of the air loss of the vacuum system is through the wood if you have good seal between the Chuck and workpiece. As you have discovered the air moving through the wood carries dust with it. An oil will pull through the wood badly (or really well)

When you finish a bowl with a vacuum Chuck sand and seal the inside. If I'm using waterlox on the bowl, I put one or two coats on the inside. This eliminates the bathtub ring.

When I hollow balls in a vacuum Chuck I finish the whole ball before hollowing.
On my suspended form I hollow the raw wood usually green in the vacuum chuck. I sand after hollowing and have rarely had an issue with rings. maybe the moisture in the wood traps the dirt?

Al

 

[Owen Lowe]

Hi, All,
a couple of times now I have had the vacuum result in a perfect circle of discoloration. Presumably this is moisture being pulled out of the wood. This has happened even with fairly dry wood, though, and after I have put on a finish, which means I'm stuck.

Are you sure it’s not the gasket material rubbing off/staining the wood? The perfect circle sounds like the contact area of the gasket — especially when you see the issue with dry, finished wood — or is the discoloration over the entire area of the vacuum?

 

[Gerald Lawrence]

I would go with Owen's suggestion. Try a different gasket material . By the way what gasket material do you use? I us a self adhesive craft foam sheet from Walmart and it works well except when it is time to remove.

 

[Bill Boehme]

Most woods are pourous and most of the air loss of the vacuum system is through the wood if you have good seal between the Chuck and workpiece. As you have discovered the air moving through the wood carries dust with it. An oil will pull through the wood badly (or really well)

When you finish a bowl with a vacuum Chuck sand and seal the inside. If I'm using waterlox on the bowl, I put one or two coats on the inside. This eliminates the bathtub ring.

When I hollow balls in a vacuum Chuck I finish the whole ball before hollowing.
On my suspended form I hollow the raw wood usually green in the vacuum chuck. I sand after hollowing and have rarely had an issue with rings. maybe the moisture in the wood traps the dirt?

Al

I agree and one other tip that John Lucas has mentioned is to put painters tape or stretch plastic over most of the exterior of the bowl except for the foot that is being turned to final shape. This will help to minimize the flow of air through the bowl. When I started doing that, the results dramatically improved.

Use the vacuum level sparingly. Too much vacuum can cause the rim of the chuck to dent softer wood or bruise the finish.

Also, allow plenty of time for the finish to cure before using the vacuum chuck. A finish might feel dry to the touch within a day or two, but is very far from being cured all the way through. An oil finish might require a couple months, polyurethane varnish needs a couple of weeks minimum to be completely safe, and I would wait a week even for nitrocellulose lacquer to completely dry especially if you have applied lots of coats.

 

[Steven Forrest]

Hey, folks,
Thanks so much for contributing to this thread! To respond/extend the discussion (in order):

Josh, that's one possibility, and a good suggestion. I'm not sure, but it was likely Mahoney's oil/wax on the bowl when I first noticed it, and Bill, I probably had maximal vacuum that first time, because I hadn't yet experienced the problem, so that may represent a quick fix. But if I recall, I tried a raw piece and it still happened.

However, Al's remark suggests a different problem/solution. So the question remains, is the discoloration being caused by the finish itself, or is it being caused by moisture, or is it being caused by the dust/dirt being lodged in moisture/finish, or is it disruption of the cells due to excessive suction, or some combination of the above? If it's the finish, will fully curing the finish prevent it?

So Josh is suggesting vacuum chucking before finishing, and Al is suggesting sanding/sealing before chucking.

Owen and Gerald, to clarify, it's not a hollow ring along the rim of the chuck, but a solid circle defined by the chuck, so it's not the gasket material itself. Being a complete dunce re: mechanical stuff, I bought the Frugal Vacuum kit, and I have used the gasket that he provided, which I believe is basically the foam you can get from an art store. (I think he even says this on his website.) Mine isn't adhesive - I have it taped to the chuck - but it works great.

Bill, your suggestion about taping is intriguing. As it is, I commonly use clear packing tape around a bowl just to help stabilize it while I do the inside. I haven't noticed an effect one way or the other re: the discoloration with the vacuum chucking. Definitely using the least amount of vacuum now, rather than the most. And your suggestion about allowing the finish to fully cure might be definitive - it just takes so ungodly long.

So, I guess if I were a real scientist, I would take a few blanks, some wet, some semi, some dry, and chuck/finish some, and finish/chuck others, do some immediately, others after full curing, log all results...But I'm a hobbyist wood turner, and my time at the lathe is limited, and I really hate losing the time it takes to even get to the vacuum-chucking stage, so it's hard to contemplate doing all that. What I will do is try these various approaches gingerly, and record my results, and see if I can find something that definitively works or doesn't work.

Again, thanks so much for your time and comments. I'll be interested to see what anyone has as a followup.
Best, Steve

 

[Charlie Knighton]

Hi, All,

I recently started with vacuum chucking. I like it, and would use it more, but a couple of times now I have had the vacuum result in a perfect circle of discoloration. Presumably this is moisture being pulled out of the wood. This has happened even with fairly dry wood, though, and after I have put on a finish, which means I'm stuck. I have tried to minimize the amount of suction, but it still happened. I have been a little gun-shy as a result, and haven't wanted to possibly waste the wood (or time) to really troubleshoot this systematically/scientifically. Anyone with similar experience, and any solutions? Much appreciated.

this is worse than having 6 chucks........more $$$$$ in equipment, time to dry, frustration when blank flies off lathe (not sure if this happens all the time), safety

seems reverse pressure chucking is just to easy......I admit when I get the nub small, I stop the lathe and saw it, then sand whats left

to each his own way ......I applaud the willingness to overcome.......

 

[Mike Johnson]

There are so many ways to accomplish the task at hand, it is amazing how many jigs, tools, etc. we try
out to simplify the process and clutter the shop with. Can't tell you how many times i have worked on
a project and got down to the last couple of tasks and get in a hurry and have multiple hours of work fly off the
lathe. Using a vacuum chuck certainly requires taking your time and not getting in a hurry.
I like to use a recess on the bottom of a bowl to chuck up with and avoid the "Nub" issue later on.

 

[Owen Lowe]

However, Al's remark suggests a different problem/solution. So the question remains, is the discoloration being caused by the finish itself, or is it being caused by moisture, or is it being caused by the dust/dirt being lodged in moisture/finish, or is it disruption of the cells due to excessive suction, or some combination of the above? If it's the finish, will fully curing the finish prevent it?

So, Steven, can you add a few more details?

With what wood species have you experienced the discoloration?
What is the grain orientation — end grain or side grain bowl?
You mentioned Mahoney oil finish; do you thin it or use it straight?

 

[Steven Forrest]

So, Steven, can you add a few more details?

With what wood species have you experienced the discoloration?
What is the grain orientation — end grain or side grain bowl?
You mentioned Mahoney oil finish; do you thin it or use it straight?

Hi, Owen,
I have definitely noticed the discoloration with light wood - Bradford pear and I believe maple. Not sure about others. Like I said, I don't remember all the various permutations, and after it happened a couple of times, I got nervous about ruining a good piece and stepped back from vacuum chucking.
It's all bowls - side grain.
I use the Mahoney straight.
On the pear piece, I tried sanding it out with fine grit, but it was in deeper than that. I didn't probe to see just how deep. And I don't believe it faded (I don't have the piece anymore, but at least for the time that I had it, it didn't go away.)

 

[Tom Hamilton]

That used to happen to me with light colored wood. For me it was the gasket material. I changed to a different gasket material and the problem has never returned.

Happy turning - Tom

 

[Dave Fritz]

Hi Tom, happy smile on your new photo, that's the guy I know.
I have had this happen with a rubber chunky gasket. The inside was sanded smooth and sealed with sanding sealer only. Maybe I'm too strong on the vacuum?