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Using a Bowl as a Friction Chuck (Read 1,835 times)
 
Vaughn McMillan
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Using a Bowl as a Friction Chuck
Jul 11th, 2011 at 4:20am
 
I wrote this up to answer a few questions on another forum, so while I have the photos out, I figured I'd share it here. This is old hat to some of you guys, but hopefully it'll help a few of the newer members...

Here's the basic idea...the shallow bowl that's mounted in the chuck is a roughout that I don't plan to finish. It's got some hairline cracks that I don't think are worth the effort to fix. The bigger bowl on the right is the one I need to finish the bottom on:

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I put a sheet of rubber shelf liner/router pad in between the two, and used the tailstock to hold the bowl in place:

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At that point, I'd typically turn the tenon down to a nub, do any finish turning to the bottom of the piece, then turn the nub down to a cone and remove it, doing any final sanding of the foot off the lathe. I've used this method for a lot of bowls, and have been happy with the results. You can use a similar method for finishing the bottoms of hollow forms. To do that, the "chuck" bowl needs to be bigger than the hollow form, so the vessel fits inside of the bowl.

But with this particular pair of bowls, I didn't really like the way the big bowl fit over the smaller bowl. So I went back to the scrap bowl pile and picked out another, larger bowl to use as the friction chuck. If you look closely at the photo, you can see I've re-trued the rim of the "chuck" bowl:

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Here again, I used some beat-up rubber shelf liner as an interface pad:

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And used the divot in the tenon from when the piece was first roughed out to center the bowl on the friction chuck:

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Giving me this:

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And from another angle. I should point out that the blue line near the bottom of the bowl shows me where the inside bottom is. Before reversing the bowl, I use the laser pointer on my hollowing rig to accurately determine the bottom thickness, so I know how much I can (or should) remove from the bottom as I finish it:

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Then I do any final shaping of the bottom 1/4 of the bowl, as well as start whittling away at the tenon on the bottom:

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Once the tenon was down to about 3/4" in diameter, I decided to take out a little extra insurance in the form of plastic stretch wrap. I wrapped it tightly around the joint between the two bowls, making sure to wrap in the same direction as the lathe rotation. (I don't want the lathe's spinning to unwrap the plastic.):

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Here's another view. You can see that I've done some initial sanding of the concave foot area (by hand, with the lathe running):

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The I go about turning the nub down to a cone. Here's a spinning shot:

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And another photo showing how I hold the detail gouge to get a shearing cut on the cone. I'll point the gouge the other direction to make similar shearing cuts on the foot itself. It was a bit exciting taking this shot one-handed, while holding the gouge in place right next to the spinning wood:

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And, the "free at last" shot. Notice that the bowl is still turning but the cone is not.

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Here, I've stopped the lathe and moved the tailstock back a bit. You can see that the cone stays stuck to the live center:

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At this point, any of the work I've done on the bottom of this bowl could have been done without the stretch wrap. As I mentioned earlier, it was there just for insurance. But since it was there, and since it was holding the piece securely, I decided to do a little bit more tooling on the foot after moving the tailstock out of the way. Here's the finished bottom after the extra tooling and final sanding:

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Although I didn't get any decent photos of it, the inside of the finished bowl did show some marks (they looked almost oily) from the rubber pad. Those marks sanded away easily with 400 grit sandpaper after I took the bowl off the lathe. The stretch wrap, however, leaves no trace on the finish-sanded raw wood.

This isn't the only way to finish off the bottom of a bowl or hollow form, and it's not even one that I use very often (I usually skip the stretch wrap part), but it's one of many ways that works well.  smiley=thumbsup.gif

Questions and comments are welcome and appreciated. Wink
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« Last Edit: Jul 11th, 2011 at 4:28am by Vaughn McMillan »  
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Robert Harper
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Re: Using a Bowl as a Friction Chuck
Reply #1 - Jul 11th, 2011 at 6:17am
 
That's pretty much how I do mine only I don't use a bowl but generally use tenons from parting off goblets and such. Sometimes I have to make a longer one to get out of the way of the motor so I have some that I've made long for that purpose. One also has some sandpaper taped to it for a sanding drum. I use some old mouse pads I picked up from work for the interface.
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Vaughn McMillan
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Re: Using a Bowl as a Friction Chuck
Reply #2 - Jul 11th, 2011 at 8:33am
 
Robert Harper wrote on Jul 11th, 2011 at 6:17am:
That's pretty much how I do mine only I don't use a bowl but generally use tenons from parting off goblets and such...


I used to use various solid stubs, and still have a bunch in the drawer, but I've gotten to where I prefer either a wood bowl or the cup from my vacuum chuck. Using something close to the rim to support the bowl instead of something close to the middle seems to keep things more stable and less likely to get off-kilter. It also gets rid of the potential problem of having the contact point being right in the center, which can happen if the radius on the face of the stub is smaller than the radius of the inside of the bowl.
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« Last Edit: Jul 11th, 2011 at 7:29pm by Vaughn McMillan »  
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Re: Using a Bowl as a Friction Chuck
Reply #3 - Jul 11th, 2011 at 7:20pm
 
Thanks for the info Vaughn.  you should do it up as a PDF and give it to Ron for TPT.
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Re: Using a Bowl as a Friction Chuck
Reply #4 - Jul 11th, 2011 at 7:31pm
 
Great tutorial Vaughn, I will have to add this to my tool box.  Now if I only had two blanks that big.
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Re: Using a Bowl as a Friction Chuck
Reply #5 - Jul 11th, 2011 at 9:59pm
 
You mentioned that you want to stay off the center of the bowl bottom.  Why is that?  I've been burning the bottoms of my bowls lately.  Is this why?  Should I be turning a depression in the face of my jam piece?  Still learning.
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Vaughn McMillan
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Re: Using a Bowl as a Friction Chuck
Reply #6 - Jul 12th, 2011 at 12:00am
 
Rev. Doug Miller wrote on Jul 11th, 2011 at 9:59pm:
You mentioned that you want to stay off the center of the bowl bottom.  Why is that?  I've been burning the bottoms of my bowls lately.  Is this why?  Should I be turning a depression in the face of my jam piece?  Still learning.
CoolRev. Doug Miller


If the bowl is only resting on the center spot, there's more opportunity for it to wobble. (Think of sitting on a one-legged stool, as opposed to sitting on an overturned bucket.) The wider the support, the less chance for wobble. This is especially important as you're making the final parting cuts on the tenon stub. Unless the jam chuck face exactly matches the curve on the inside of the bowl, you're probably better off turning a bit of a depression in the middle of it.

The burnt bottom is another symptom of the center spot. That's happening because the surface area that's making contact between the bowl and the chuck isn't enough to overcome the torque of the lathe, so the friction chuck is spinning out. If there's a 1" diameter spot in the middle making contact, that's only about 0.8 square inches of contact. On the other hand, if you have a 6" diameter bowl rim as a contact point, and it's making contact 1/4" wide all the way around, that ends up being about 4.7 square inches of contact area. Much less likely to spin out and burn.

Does that help explain things?
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« Last Edit: Jul 12th, 2011 at 12:02am by Vaughn McMillan »  
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Re: Using a Bowl as a Friction Chuck
Reply #7 - Jul 12th, 2011 at 7:12am
 
Yep.  Helps me and a lot of other folks understand what in the world is going on.  I've been a proponent of this method for final turning for a long time.  Just had not heard of using a bowl.  I may have to go turn a couple for some of the projects I have in process. 
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Re: Using a Bowl as a Friction Chuck
Reply #8 - Jul 12th, 2011 at 8:03am
 
Maybe I'm not envisioning this right, but doesn't the wrap need to go in the oposite direction to lathe rotation, eg, clockwise around the piece (when viewed from the tailstock end) to keep it from unwinding when the lathe rotates counterclockwise?
Except for that point of confusion, that was a very instructive slide show; a picture really is worth a thousand words (give or take a few)!
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Re: Using a Bowl as a Friction Chuck
Reply #9 - Jul 12th, 2011 at 8:26am
 
You are right on the direction of the wrap.  Whether you call it the same or opposite direction is a matter of how you are looking at it.  Imagine holding the wrap still and turning the spindle the direction it would turn with the motor running.  That is what you are after, like you have said. 
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Re: Using a Bowl as a Friction Chuck
Reply #10 - Jul 12th, 2011 at 8:27am
 
Vaughn McMillan wrote on Jul 11th, 2011 at 8:33am:
Using something close to the rim to support the bowl instead of something close to the middle seems to keep things more stable and less likely to get off-kilter.


Great write up and pictures Vaughn.
I use small sections of 2X and try to have the transfer of power just outside of the foot. If necessary it only takes a small cut if the friction chuck needs to be re-trued when reused.  The bowl would work well also but I assume less pressure could be used for fear of cracking the bowl that far out.  If the "bowl/cup chuck" warped between uses would it limit the number of reuses?  A 2X4 would provide support for a foot up to 3.5" and a 2X6 for up to 5.5" and still keep the transfer of power concentrated at the bottom of the bowl where there should be less flexing?
A 2X is not even necessary for smaller items, a 1X should suffice.  Or if you don't have 2X material handy, glue up some 1X and alternate the end grain?
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Re: Using a Bowl as a Friction Chuck
Reply #11 - Jul 12th, 2011 at 11:58am
 
Doug nailed it about the wrap. I hold the wrap in place and turn the lathe in the direction it normally spins.  smiley=thumbsup.gif

Mike, you're right about the bowl/chuck having a finite lifetime, although once they're dry, they don't tend to warp much. I do end up re-truing most of them every time they go on the lathe, but that's mostly because I don't chuck it in the exact same place each time. Plus, they're scrap anyway, so if not used for something like this they just end up in the burn pile. You're also right about using lighter pressure from the tailstock. It doesn't take much pressure to hold the workpiece, and on a thin-walled bowl, I'd be more inclined to use a friction chuck that wasn't right at the rim of the bowl. Wink
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« Last Edit: Jul 12th, 2011 at 11:59am by Vaughn McMillan »  
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Re: Using a Bowl as a Friction Chuck
Reply #12 - Jul 26th, 2011 at 3:36pm
 
Hi Vaughn, great tutorial. I haven't tried this yet as I'm still new to these things but how about the plastic wrap over the top of the router mat? Would that stop the marks on the inside of the bowl?
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Vaughn McMillan
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Re: Using a Bowl as a Friction Chuck
Reply #13 - Jul 27th, 2011 at 11:57am
 
Keith Padwick wrote on Jul 26th, 2011 at 3:36pm:
Hi Vaughn, great tutorial. I haven't tried this yet as I'm still new to these things but how about the plastic wrap over the top of the router mat? Would that stop the marks on the inside of the bowl?


It probably would, Keith.  smiley=thumbsup.gif  On the other hand, I think it would reduce the 'stickiness' of the mat, making it more possible for the bowl to slip. I'll have to give it a try to see.
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Re: Using a Bowl as a Friction Chuck
Reply #14 - Jul 27th, 2011 at 1:02pm
 
You know I never had good luck doing it this way. It always seemed to wobble.
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