vacuum chuck

[Jason Ledbetter]

I just completed my vaccum system. I made my own spindle adapter, vacuum chucks, etc. and I am pleased. I am pulling 25/26 inches of mercury.

Here's the question, I was fooling around with it last night and I am having trouble "centering" turned pieces so that they don't wobble on the lathe. I had a couple bowls sitting around that I put on the chuck just to experiment and I couldn't get them trued up enough to finish them. How do you do this?

 

[Bill Grumbine]

Hi Jason

There are a couple of ways to do this. One way is to purchase an attachment that fits into your tailstock and will hold your chuck so that you can reverse the bowl keeping everything centered, and bring it to the vacuum chuck that way. I don't know how much they cost, and truthfully they fall into the category of "stuff I really don't need".

Here is how I do it. If things are going well, I have a dimple in the tenon of my bowl from the support of the live center when I was truing it up for final turning. If not, then I make a mark as close to the center as I can, and then tap the live center into that mark to make a dimple. Then I bring it all together on the lathe using the live center for support. Most of the time this works pretty well, but sometimes I need to fiddle with it a bit. A lot of the time there will be a little wobble, just from things not being perfect, the wood moving, etc. That is what sandpaper is for. It only takes a few seconds to blend any out of roundness to the point where no one will ever know except you and the people you tell.

Good luck with it.

Bill

 

[john lucas]

I do the same as Bill. I bought my adaptor from www.bestwoodtools.com. It's the handiest thing since sliced bread.
I usually start bowls out between centers with the bottom toward the tailstock. This leaves a center mark. I'll turn the tenon for the chuck and true up the bottom but I will leave a short tenon leading up to the tailstock. I make this small enough that it will go inside my chuck even after the chuck is attached to the larger tenon. Now I can hollow the bowl, true up the outside if necessary, sand etc. Then when I reverse the bowl to put it on the vacuum chuck I still have the little tenon with the dimple from the tailstock. I put the bowl on the vacuum chuck and bring up the tailstock. I usually start turning with the tailstock in place even with the vacuum on. I pull the tailstock away when it finally gets in the way. I guess I'm just too cautious.

 

[Andy Hoyt]

Jason - I guess I'm reading your question differently from Bill and John.

Are you saying that that when you set a freshly turned bowl in the vac chuck you're having a hard time re-establishing symetric concentricity? And that when you bring in the tailstock as John and Bill suggest you're still having trouble finding that sweet spot where the freshly turned rim runs true?

If so, I hear you! There's a knack to this that, for me anyway, comes from lots of practice and patience. Here's what I do.

Put the bowl in the vac with just a skosh of pressure holding it there. Bring the live center into the dimple (as John and Bill say) with equal light pressure. Then bring the end of the tool rest up to within a 1/4" of the rim. Rotate the piece by hand and note where the rim is closest and farthest from the toolrest (which is nothing more that a fixed point in space). I begin tapping the bowl blank with a tool handle at the appropriate far spots. Note that this also affects the near spots. As I get closer to "true" I add a bit more vac and tail stock pressure so my tapping doesn't make it go wonky again. Tap some more to further true it.

I actually rarely get it spot-on but in most cases close enough is good enough. It takes patience to get it right, and practice to get it quickly. But for me it works.

Hope this is what you were looking for and that it helps.

 

[Bill Grumbine]

I usually start turning with the tailstock in place even with the vacuum on. I pull the tailstock away when it finally gets in the way. I guess I'm just too cautious.

I do it this way too John, only I call it prudence. 😉 It is so exciting when a bowl comes loose and that big noise from the air rushing in happens. I have only had it happen twice, and that was enough for me.

Bill

 

[Jason Ledbetter]

thank you for the timely responses. All your suggestions were very well stated. With four little boys to help with, I don't have the time or money to travel anywhere for instruction or to chapter meetings. You're instructions from the website are very much appreciated. I learn a lot from the postings I read and enjoy them very much. Thanks again!

 

[Bill Boehme]

Jason,

I also just completed my vacuum system (sort of) this morning. I built my own chuck and rotary adapter (and I am very pleased with the result, but that is another story ....) and I have played around with the thing just a bit this morning to experiment with centering bowls, two of them having natural edges with bark at the rim (one is end gain and the other cross grain).

By hand, I could not get anything even close to being aligned correctly. The good news is that I bought an adapter from Oneway about a year ago (Kevin tried to talk me out of getting one) and it is a great solution to the problem. It requires having the Oneway live center (which has 3/4 X 16 threads). The adapter is then screwed onto the live center and the outside of the adapter will have threads to match the threads of your headstock spindle.

While the bowl (or vase or ?) is still mounted in the scroll chuck, mount it onto the tailstock using the adapter and Oneway live center. Next, bring it carefully up to the vacuum cup while it is set to pull a minimal vacuum. Carefully ease the tailstock up until there is full contact (you can tell this because the vacuum gauge reading will suddenly show more vacuum and the sound of the vacuum pump will change. Increase the vacuum to the desired level. Release the jaws of the chuck from the bowl and back the tailstock away (you may want to change to a point center and move the tailstock back into contact until nearly finished). To me this is the quick and easy method until I learn a bit more about vacuum chucking.

In the first paragraph, I mentioned building my own rotary adapter and vacuum cup chuck. Everything went very well on that, but there is one minor problem that need to be corrected. I measured the length of rod necessary to go through the headstock (at least twice) and still cut it too short because I overlooked the thickness of the MDF bottom of the vacuum chuck. The remaining length of threaded lamp rod is the shorter half so it is back to the hardware store for more lamp rod (at least it is cheap). I estimated the cost of the whole rotary adapter and vacuum cup chuck was less than $20 not counting the faceplate. I used one of the aluminum faceplates from Don Pencil for about another $20. I would have spent somewhere in the neighborhood of $150 if I had purchased everything already built.

Bill

 

[Steve Worcester]

One way is to purchase an attachment that fits into your tailstock and will hold your chuck so that you can reverse the bowl keeping everything centered, and bring it to the vacuum chuck that way. I don't know how much they cost, and truthfully they fall into the category of "stuff I really don't need".

Since I use the adapter (made by Bestwoodtools or Oneway, or the Tollys) for more than vacuum chuck reversal, it falls into the "stuff I can't do without". But as previously stated, it does require a Oneway live center (at $100+) or Bestwoodtools does sell a reversing adapter for chucks for less than the cost of the combines live center and adaptor.

 

[Dave Beringer]

Blow Off

I will make a comment and not assume you have a blow off value mount somewhere on or close to the headstock of your lathe. The value easily allows you to apply a slight vacumm and still jockey the bowl around with a few taps. I use the tool rest as a reference to detemine where the location of high spots to be jockey. I would suggest looking at the 2005 symposium tapes at Dave Lancasters demo for more ideas.