Jam chuck versus Straka chuck, question

[Jon Murphy]

I'm thinking of making a Straka chuck, and have read the posts on this forum as well as Alan Lacer's article - but I see a potential problem with its use. I have been reasonably successful using jam chucks for both natural and cut edged bowls, but it is a bit of a pain to get the centering just right. I can see the greater flexibility of the Straka chuck, and that one can make a more exact centering - but the donut prevents one from blending the outer surface of the bowl on the lathe. With a jam chuck I can shape the bottom then lightly shear scrape the outer surface to blend it with the upper part, then finish the whole outer surface with light sandpaper. That seems impossible with the Straka chuck.

It's possible that the accuracy of centering is so good with the Straka that the blending isn't necessary, and that the final sanding can be done off the lathe - but if one is just moving a problem to another part of the process I don't want to make the effort to make the Straka. I'm on a PSI 12" midi and make both small forms (particularly Ellsworth style "spirit forms") and bowls up to the practical limit for the 12" lathe of about 10". I mainly use green wood off the tree, but sometimes use kiln dried "plank wood". BTW, I don't reverse chuck my "spirit forms", I use a glue block and once it is hollowed I finish the bottom at the headstock end with a detail gouge. That leaves a small "flat" to be made so it will sit upright, and I do that with a small disk sander.

So I ask this fine forum to tell me why I should make a Straka chuck - I think you will have good answers and that I'll end up making one. I can see that it is a lot easier to re-center a natural edged bowl with the PVC extention, although I do the same with the jam chuck. The problem with the jam chuck that the point on the revolving tail center (and I use a cup tail center) chews up the wood as one readjusts it - making it very difficult to get an exact recenter. If the recentering with the Straka is so exact one doesn't need to blend the outer surface on the lathe then I'll make one.

[John Keeton]

Jon, I have used a donut chuck - I presume the same thing by a different name, and it does allow much better access. However, I am not sure it is a failsafe way of avoiding the "blending issue." Personally, a vacuum chuck is much preferable. I use the Holdfast system, and have been very pleased with it. Others have built a vacuum system using a pump. Those also seem to work well, but the cost can easily be as much or more than the Holdfast and there is a lot of "stuff" that is always present unless it is placed on a movable cart and disconnected each time as is done with the Holdfast.

I would think turning green wood would nearly always require some blending of the outside curve, as it will warp/move on the lathe.

[Thom Sturgill]

+1 on John's comment. I have made two donut chucks - one for each lathe. I very rarely use them since building a vacuum chuck, which gets a lot of use. Unless you make a lot of bowls the same size, or adjust the hole size for each bowl, the donut eventually gets in your way too much. JMHO.

[Darryl Hansen]

to solve the centering situation there is a fitting that has your chuck threads at the end of a morse taper. You leave the piece in your chuck and removethe chuck from the headstock as a unit. Then thread the fitting into the chuck and turn it around and put it into it into the tailstock. Run the piece up to your jam or vacuum chuck and release the chuck from the piece. Perfect entering if your tailstock and headstock are in alignment.

[Brian Brown]

Jon,

I have had poor success with the doughnut chuck because of the issues you mentioned. I thought I just couldn't get the piece centered. Sometimes that is the problem, but usually the problem is wood movement. Green wood almost always moves during the turning process, and even dry wood will move some, as the internal stresses in the wood are released. This causes "centering problems". Blending saves the day for this slight off center, but the doughnut chuck is always in the way where you need to blend. This is the problem you described.

I am too poor/dumb/lazy/space challenged/time challenged to make a vacuum chuck, so I found another solution. It involves using Darly's reverseing technique above. I'll try to describe the steps here.

1. Finish turn and sand the bowl interior to completion.
2. Using acetone, clean the interior of the bowl thoroughly.
3. Wait until the acetone has completely evaporated, and cover the bowl bottom with short strips of blue painters tape.
4. Press the tape into the bowl well. Be certain you have good adhesion with no bubbles.

Now the hard part, which I found is not really so hard.

5. Turn a scrap piece of hardwood into a PTC. (painters tape chuck I don't know what else to call it.). This is a short round disc that has a tenon on one end to fit your chuck, and the
other end contoured to match the curvature of your bowl. The contour does not have to be exact, but very close. The closer it is the better adhesion you will get. The diameter
of the contour end should be about 1/2 the diameter of your bowl, or just a bit smaller if necessary to work with the form of your bowl.
6. Reverse mount the bowl as Daryl described.
7. Mount the PTC in your chuck. Apply hot glue to the contour end of the PTC. Quickly side your tail stock (with bowl mounted) up to the the glued end of the PTC, and using the
quill, tighten to get good contact with the PTC. You are gluing the PTC to the blue tape in the bowl, not the wood of the bowl. Wait a few minutes for the glue to cool and harden,
then remove the tail stock chuck from the bowl, and tail stock. You should now have a firmly mounted bowl. similar to a vacuum chuck mount.
8. I finish my bowl bottom, then I use a small disk of wood with a rubber disk glued to one end as a pad between the tail stock center, and the bowl just for insurance, while I finish
and blend the bowl sides.

Some extra thoughts:
1. I can't tell you how much glue to use. I used too much the first time, and I didn't think I would ever get the PTC loose from the bowl after it was finished. It is amazing how well
this holds. I used less the second time, and it was better.
2. If you make the contour disk extra thick (deeper cylinder), you can use it many times by just turning a new contour each time.
3. If your contour disk is not a perfect contour match the glue provides some measure of gap filling, but you should be diligent in getting the contour as close as possible.
4. As with all tools and jigs, use at your own risk, and use common sense. YMMV.

Good luck if you try this. I was amazed at how well it worked.

[Bernie Weishapl]

I use the same system as Darryl does for both the donut chuck and vaccum chuck. It is about as perfectly centered as you can get. Once secured you can release the chuck and finish.

[Michael Mills]

I had thought about making a Straka/Donut chuck but never have. Just too quick and easy to knock the nub off and sand. I did buy the 1” disc and pad from Vince’s for the few times the 2” is too large. Probably not any more than the wood, bolts, and wing nuts to make a Donut chuck.
Just semantics but you say a jam chuck. The first picture here is what I have always heard as a jam chuck.
http://www.cumberlandwoodturners.com/Tips/Methods%20and%20Jigs%20for%20Reverse%20Turning%20B owls.pdf
Some people do call a friction chuck a jam chuck and I assume this is what you are using since you use it with natural edge bowls.
I really have no problem at all centering due to the following (or the Gods or Turing are with me).
The friction chuck should be flat or concave on the end. If convex like a baseball bat there is almost no friction provided and it is much harder to try to center as it just wants to wobble about.

I cut mine slightly concave and round the outside edge about 1/4”. This provides plenty of friction the same way the end of a pvc pipe would. I have 2-3 different diameters so I can sorta match the drive to the diameter of the base. It may make no difference probably but I feel more secure knowing the friction point is right below what would be the heaviest cut. With my Nova I can turn about 6” deep with just a ¾” thick friction drive in my chuck before the wood would hit the headstock. Deeper than that and I would need a thicker face friction drive or the pvc pipe it the item is really deep.

I always leave the original divot from the tailstock. I leave the center about ½” diameter with the divot and up to ¼” long. It always fits in my chuck with the divot in the throat of the chuck or not even reaching the bottom of the chuck jaws. The same divot is there from the start of the rough turning to the last of cleaning up the base. I just bring the tailstock point back and it centers the bowl on the friction chuck.

I may give it one or two rotations by hand to check before I turn the lathe on but I have never repositioned the point, maybe just adjust the friction seating a hair. I use a scrap of leather over the end of the chuck and it provides plenty of friction.

All that said a Straka/donut chuck is probably nice especially if you want to apply finish to the full base before removing from the lathe. I don’t think it will help with centering over the friction/divot method.

[Thomas Heck]

I ended up buying a holdfast system to help with these problems. It works great for me. It is small, doesn't take up much space, and works off an air compressor. Thanks John for recommending it to me.

[Thom Sturgill]

...
Just semantics but you say a jam chuck. The first picture here is what I have always heard as a jam chuck.
http://www.cumberlandwoodturners.com/Tips/Methods%20and%20Jigs%20for%20Reverse%20Turning%20B owls.pdf
Some people do call a friction chuck a jam chuck and I assume this is what you are using since you use it with natural edge bowls...

just picking nits, but since you brought it up - jam is something you spread on bread, or a group of musicians making impromptu music I always thought it was 'jamb' chuck since it acts much like a door jamb, but then some call it a 'door jam'. What Brian describes is better called a friction chuck, but i often hear it called a jam chuck also.

BTW thanks for the link.

[Thomas Canfield]

I have made Straka or donut chucks for my Powermatic 3520 and also for mini. That said, I normally today just use a jam chuck and rubber shelf liner to turn the bottom of most bowls and hollow forms using a dimple in the tenon or bottom prior to reversing to turn the inside and the live center. That allows turning down to a small center nub that I cut off with a flush cut saw with a plastic shield against the bottom. I have broken the bottom out a couple of times trying to "snap" off a nub and decided it was not worth the gamble. That also works for a recess if a dimple is left in the center. One has to be careful to not use too much tail pressure when the bottom is thin as the tenon is removed or you can press through the bottom. The Straka chucks I made had multiple rings for different size pieces to expose the bottom area around the tenon. I will sand the exterior before working on the bottom and the blend area is usually small, or just on the underside which is recessed and out of view.

My jam chucks are also usually made from MDF scraps and contoured to give about 1/2" of surface contact at point about 1/2 of piece diameter.

[robert baccus]

I appreciate the workmanship and engineering I have seen in donot chucks but this looks like a very difficult and confusing way to do a bottom compared to a pipe/disc jam chuck. To avoid tearing up the bowl bottom I use an aluminum oneway tailstock cone turned down to a square 1/4" point to hold the bottom tight. Easily sanded off.

[Jon Murphy]

This is a test message to see if it lines up left, a reply to myself as there are so many good answers that I don't want to single out one.

[Jon Murphy]

Excellent answers from all. I have to write off the vacuum chuck suggestions, my shop is in my bedroom in a condo - the space for the vacuum pump and the noise precludes my using one. BTW, there should be an article in the AAW Journal sometime this winter on the use of a midi-lathe and a small shop for other things than pens and bottle stoppers - it is accepted, all I have to do is clean the shop enough for photos and write the details.

As to jam chucks (or jamb chucks) I used the wrong term - I gather I'm using friction chucks. But they aren't actually pure friction, I shape the chuck to the bowl. I keep a stack of different sizes, some built up from Home Depot thick multi-plies, but I've found that hard maple is a good choice - and I'm lucky enough to be near a hardwood supplier that has "cutoffs" of his boards I can get cheaply. I take two layers of 8/4 maple (a true 2" board) and turn them to size - then shape for the particular bowl if needed. One of you mentioned a Morse tenon on the "jam/friction" chuck, I blew for a Beall Spindle Tap a few years ago, it it is expensive but a one time expense. I can thread my wooden chucks onto my spindle and shape them at will.

Jam or Jamb? I think it is jam, as in jamming something into something - a door jamb is a passive frame. Friction versus jam? Both involve jamming and friction, it seems to be six of one and a half dozen of the other - but I'll use the accepted terminology in the future. The origin of words is always a problem - I could say "I don't give a dam what you call it", and someone might think I was avoiding saying "damn" - but the phrase came from the tinker's dam - a bit of waste material that a tinker would use when fixing a metal pot to hold his solder in place to fill the hole. And even that may be wrong. I will say that I'm probably the only wood turner who was quoted in Wm. Safire's book "On Language" forty years ago.

OK, back to the topic. My centering problem isn't a big one, I can get very close to perfection. Sometimes I use a blunt tip on my tail stock revolving center so I can make very small adjustments - it depends on how much friction I have inside the bowl. When I need it I use a needle and cup center on the tail stock, a bit more difficult to adjust but a bit more stable for the turning. I pad my jam/jamb/friction wooden chuck with either half inch single cell foam (doesn't break down) or quarter inch "rubber", again depending on the shape and fit. The object is to support as much of the internal shape as possible.

I'm going to have to look into Brian's "PTC" chuck, there are possibilities there. As to Darryl's suggestion, I have the revolving spindle Morse center for the tail stock and have used it. There is a small problem in that my PSI midi isn't perfectly aligned - although I've just realigned it so will have to check the reverse mount again. Good idea, and it could be a better test of alignment than trying to line up the points of centers.

I'm going to have to look at Michael's posting in detail, but I think we have a different approach. I'm reshaping my friction chucks to the internal shape of the bowl and he is using direct pressure at a point (radial) on the bowl. They may be complimentary techniques - I do a lot of very thin wall work, decorative pieces that couldn't handle the pressure of a flat or concave friction chuck. But I also do more practical pieces, and I could probably save a lot of chuck reshaping by using Michael's technique.

If I've left anyone out I apologize, all the answers were excellent - including those on vacuum chucks, although I can't use one for local reasons. I think I'll forget the Stratka/donut chuck and stick with my form of jam/friction chucks, with some experiments with the PTC and the concave.

[Michael Mills]

I brought up the difference mainly for new people. Since you mentioned natural edge I think most readers knew what type you were referring to.

I like the tip from Thomas about using a plastic shield on the bottom when sawing off the nub. I’m sure I have a little scrap of counter top laminate to drill a 1” hole to go over the nub.

I had not really considered thin or wet turned bowls but I can see where more contact will be a definite advantage.

For you entrepreneurs: How about a soft plastic disc to turn to fit your chuck tenon or recess. This will have a valve through the side for a needle (think basketball). It would come with 3-4 durable friction bladders which could be swapped out depending on the size/shape of the turning. Air up with your turning over it and deflate for storage. Drawback would be using a hand pump to insure you did not overinflate and break the item.

[Thomas Canfield]

The piece of plastic that I use for bottom protection with flush cut saw was made from a piece of plastic dealer advertisement/license plate filler on new car about 12 years back. You need a flexible piece of thin plastic to be able to bend into the recess on the base. Some of the thin plastic notebooks would work well since the flush cut saw is not supposed to mark the surface. I also use a notch cut into one side rather than a hole in middle of the plastic to get more flex inside the recess.