Marks from Vacuum Chucking

[George Van Beynen]

I have just started using my Vacuum system. I got the Oneway 6’ vacuum drum for my first and main one before I try to make the others. It came with a neoprene pad for the seal. I am using a Robinair vacuum pump. Most of the bowls I have mounted with this system have a ring on the inside from the chuck; I do not see the ring on the outside. Have others had this problem or is it unique to my system? My gauge is showing I am pulling 20†vacuum.

 

[Bill Grumbine]

Hi George

I have gotten two kinds of marks from the vacuum chuck I have. One came from various colored pads that I tried with it, black being the worst - at least on light colored woods. Probably orange was the worst of all since it was not natural. I solved that one by using the white closed cell foam that electronics are packed in.

The second mark is a compression ring that will occasionally show up where the vacuum bell contacts the wood. I usually sand that out with a little bit of my last sanding grit. Depending on what grit you stop at you may have to vary that a bit.

Good luck with it.

Bill

 

[Mark Mandell]

George,

Ye-up, been there, got that. Pretty much determined they were caused by the rubber seal or fine dust/grit on or in the rubber/foam seal ring or both. Pretty much eliminated the problem with a piece of plastic wrap as follows:

1. Vacuum clean the piece well before mounting.
2. Do the same to the chuck's seal-ring.
3. Just before you mount the piece, stretch a fresh piece of smooth Saran wrap over the chuck and put a rubber band on to hold the folded corners; like a drum. Make sure that it is smooth with no folds or crinkles over the seal/contact ring.
4. Cut a 1" hole in the center of the "drum" for air flow.
5. Position your piece and hold gently with the tail stock, then turn on your vacuum.
6. Do not reposition the piece [intentionally ] while under pressure. If after you've turned a bit, you need to do so, release the vacuum, remove the piece, and vacuum clean the surfaces so that you don't get dust or grit trapped under the seal. You don't have to use fresh plastic.
7. Keep your ts in place for added safety as long as you can.

Hope this helps.

Mark

ps: This assumes that any finish on the contact surface is dry and cured, and that you do not apply more finish to the working side while under vacuum pressure; doing so will force the liquid to migrate through to the pressurized side and "bloom" inside the chuck ring.

 

[Bill Boehme]

First of all, you are using too much vacuum if your drum chuck is 6" diameter (I will assume that the six foot diameter is a mistype). Assuming that the inside diameter of yor chuck is 6", the amount of force on the bowl is over 88 pounds. If it happens to be thin, you could "pop" it. Half as much vacuum is enough.

Bill

 

[MichaelMouse]

Is just a bit of rubbed-off neoprene, or does it look like you're bruising the fiber by slipping it against a strong vacuum?

If there's compression, do a wet set and hand sand the re-expanded stuff with the finest grit you've used on the bowl.

 

[laymarcrafts]

Bill states a 6" diameter equates to 88 psi in fact it is more like 288 psi, this would normally be ok for an Object with a reasonable thickness, whatever that equates too, but probably too much for a Thin Object which could easilly implode.

I have produced some Diameter to Vacuum = Pressure Graphs on my web site under the Vacuum System in the Hints and Tips section.

Wheras this is an exact Science the actual Vacuum you use is one of Trial and Error unfortunatelly the Error bit often ends in Tears.

Does anyone have a guide to Pressure v's Thickness? and I guess also that the Wood Type also comes into play?

Richard
http://www.laymar-crafts.co.uk

 

[Mark Mandell]

Richard,

Go to the "How To" section of this site and look up the vacuum chucking atricle. I think you'll find some links to what you're asking for in there.

M

 

[George Van Beynen]

Thanks for the diagnosis and solutions!

Bill G., I think it is a compression mark; I will have to watch on the next batch. I will sand this one out.

Mark, I will try the saran wrap on the next one, none of these had a finish on them fortunately so I can sand again. The warning about finishing will be well heeded. When I was sanding I could feel the airflow through a yellow birch bowl.

Bill, I will watch the vacuum, I was worried that it might fall off but will experiment to find a comfortable reduction. I will try 44# for a starter.

Michael, thanks for reminding me to wet set, I would have forgotten even though you have me doing it automatically now with the salad bowls.

Richard, what a great site. If I didn’t have 8 cords of firewood to cut and split I would spend more time there, bookmarked for a rainy day. I printed out your chart, thank goodness you have # & “ scale, I thought I was going to have to convert it but you already went through the trouble.

 

[Bill Boehme]

Richard,

You are right -- I had a brain fart and forgot to include pi. ........ Mmmmmmm .......pie. Multiply my answer by 3.14 to get (88.4 X 3.14 = 277.6 roughly). OK, this is mistake number 1 ...... the pressure is off now. That is waaaaay too much vacuum, dude. You are lucky that all you got was a little dent -- you could have busted that bowl. Make up a little chart to tell yourself how much force you have on your bowl as a function of the vacuum. For all practical purposes, you can say that PSI is half of the inches of mercury reading and a 6" vacuum chuck has an area of 28 square inches. Roughly speaking, 20 inches of mercury is 10 pounds per square inch. If you feel a strange need to be more precise, 20 inches of mercury is 9.823 pounds per square inch. If, at some future date, you get a 4" diameter chuck, the high vacuum will be much more useful because the area in a 4" chuck is only 12.6 square inches.

Use only a moderate vacuum and keep the tailstock up against the bottom until necessary to remove it. The vacuum chucking system is intended only for light finishing cuts just like most other reverse chucking methods.

Bill

 

[George Van Beynen]

Thanks Bill, I was planning to make a smaller chuck so the vacuum will not be wasted when I start to use the new chuck. As stated earlier, I will reduce the vacuum. I do use the TS till the end and then turn on the vacuum pump before removing it.

 

[Mark Mandell]

Mark, When I was sanding I could feel the airflow through a yellow birch bowl.

Now you also know the reason why we tell everyone to put an air filter on the intake side of your vacuum pump. Along with that air goes fine wood and ABRASIVE particles which will degrade your pump's performance through excessive wear.

Remember too, as nifty as the vacuum is for holding, you can use MUCH LESS "pressure" and work safer to boot if you use your tailstock right up until you need to pare off that last little nubbie in the center. Take it from one who's such the bottoms out of more than one bowl during the learning process.

M

 

[CTutor]

Try facing the neoprene with "foamies" You can get it at any craft store like Jo Anns or Michaels. Runs less than a buck for sq ft and won't mark things up..

 

[George Van Beynen]

Will do CTutor, bought some at Walmart and have yet to try them. I will give it a go the next time I run the vacuum.

 

[mbuckley]

my system

I built my system from parts I was able to get online
some came from wood working supply houses, some came from Ebay
The pump is a 5cfm vacuum pump I got New on ebay for $90.00

my main question is, I have a rubber plate mounted to a 1/2 inch piece of Baltic Plywood, and the plywood is mounted to a faceplate.

what else can I use besides the rubber, that will seal not so perfect bowls?

see pics

thanks
mbuckley

 

[Richard Baker]

Remember too, as nifty as the vacuum is for holding, you can use MUCH LESS "pressure" and work safer to boot if you use your tailstock right up until you need to pare off that last little nubbie in the center.

In fact if you are using the tailstock until you get down to that last little "nubbie", you do not really need a vacuum chuck. Except maybe to hold and spin the piece for that last bit of sanding where the nubbie was.

 

[Steve Worcester]

what else can I use besides the rubber, that will seal not so perfect bowls? ....mbuckley

Rubber is the best bet. Think about whatever you use must be soft enough to conform to the squish of the mate between the plate and the bowl. Then it must also be non-fowling to whatever the finish is. The point to that is that silicone would be low on my list because any little tiny bit with fowl a spray on finish.

So basically rubber, or a derivative.

 

[laymarcrafts]

For Bowls with less than perfect rims, i.e. Natural Edge Bowls I use a Cup Chuck that is of sufficient Diameter to sit inside the Rim of the Bowl this allows the Vaccumm to work on a solid base and not in thin Air.

Richard
http://www.laymar-crafts.co.uk

 

[waltben]

I built my system from parts I was able to get online
some came from wood working supply houses, some came from Ebay
The pump is a 5cfm vacuum pump I got New on ebay for $90.00

my main question is, I have a rubber plate mounted to a 1/2 inch piece of Baltic Plywood, and the plywood is mounted to a faceplate.

what else can I use besides the rubber, that will seal not so perfect bowls?

see pics

thanks
mbuckley

I've three Oneway chucks and replaced the relatively thin rubber seal on the middle sized one with 1/4" thick closed cell foam rubber disks cut from a large sheet I picked up somewhere years ago (over ten). Can't remember where it came from, but it works very well for warped pieces. It won't hold a regular piece as rock solid as some might like for precision work, but I've found I can deal with that.

 

[Steve Worcester]

I've three Oneway chucks and replaced the relatively thin rubber seal on the middle sized one with 1/4" thick closed cell foam rubber disks cut from a large sheet I picked up somewhere years ago (over ten). Can't remember where it came from, but it works very well for warped pieces. It won't hold a regular piece as rock solid as some might like for precision work, but I've found I can deal with that.

The thinner the rubber used, the less the piece can rock and it will be more stable. You may try some 1/8" or even 1/16". All you need is enough thickness to compensate for anything out of round or unevenness in the surface.

 

[john lucas]

Don't forget that some finishes need more time to cure and whatever you use on the front of the vacuum chuck can leave a mark. I use lacquer for many of my bowls and only put the first coat on before I vacuum chuck it. The later coats will melt the subtle mark left by the vacuum seal.

 

[Steve Worcester]

Don't forget that some finishes need more time to cure and whatever you use on the front of the vacuum chuck can leave a mark. I use lacquer for many of my bowls and only put the first coat on before I vacuum chuck it. The later coats will melt the subtle mark left by the vacuum seal.

Or the vacuum sucks the finish right out of the pours leaving a bubbled surface.
I would not recommend vacuum on finished pieces or at least a higher vacuum pull.

 

[Clifton C]

Walt,
I do the same thing, but as I only have one bell (so far) I just add the ¼ inch closed cell foam “donut†to the thin foam already on the bell, the vacuum, bowl and tail stock if needed, hold everything together. As you stated, it’s not quite as solid as when using the thin gasket by its self, but I guess that’s the point, it’s used when you have a slightly out of round bowl and the thin gasket will not hold…
cc