Expansion Mode for your Chuck

**robo hippy** · 19th July 2013, 10:44 AM

I think I started using the recess after reading a Richard Raffen book. It is argued a lot on which holds better, a recess or a spigot. I always say that if they are made correctly, they both work excellently. I hadn't thought of the 4 vs 8 points of contact. I do make my recess to size, using a two dedicated sets of dividers that are super glued into place, so they never change size. Other than that, matching the dove tail angle as exactly as possible is necessary. There are specialty dove tail scrapers for that. If the angle matches your jaws (I think 7 degrees, which is what is used on dove tail joinery for drawers and such), you plunge straight in, and eyeball the tool shaft with your lathe bed. The scraper moves to the side all by itself. Depth wise, I never go deeper than about 1/4 inch, which I think is around 5mm to you metric people. With dry wood, I have seen Mike Mahoney do an 18 inch platter with a recess about 1/16 inch deep, so, maybe 1+mm. With wet wood, I am usually in the 1/8 inch depth range, and this works fine with my big Vicmark with 2 5/8 inch wide jaws on bowls that are up to about 16 inches, and I core and do heavy roughing on them. I do leave a bit more timber on the shoulder to push into. If you go deep, you need to make sure the angle is almost perfect, otherwise, there is unequal pressure on one part of the shoulder, and that can cause breaking problems. Like stated, you can over tighten, and if you don't break it outright, any extra little bit of stress in turning, like a small catch, can cause failure. With the spigot, you can trim the spigot up to be part of the foot rather than having to remove it.

robo hippy

**Cliff Rogers** · 19th July 2013, 02:21 PM

I've had more break outs/stuff ups/looses using a recess than a spigot.

**Mobyturns** · 20th July 2013, 07:35 AM

Both expansion & contraction modes are safe IF you understand the strength of your materials and how much "meat" you require to hold the size of project you have. A simple calculation of the surface area (pi x r x r ) of the tennon or donut ((pi x R x R ) -(pi x r x r )) of material around the recess is a worthwhile exercise.

The grain, its direction and the shear strength of the timber in the grain orientation (face or spindle) you use has a significant influence on your choice. Next consideration is the shape and accuracy of the tennon or the recess to match your jaw set. Then how much pressure you use to tightening the chuck jaws. The 4 or 8 point holding is only part of the story, chucks perform in an ideal manner in only a very small part of their jaw movement range, every other jaw position is a compromise.

Mismatched or incorrectly shaped tennons / recesses are mostly the problem as they encourage turners to overtighten chucks to compensate or to improve the hold which then creates micro cracks in the timber from the edges or corners of the jaw sets crushing the timber, which starts a cycle of the jaws slackening as the timber crushes more, so the turner retightens etc. This is a significant cause of failures in face grain orientations for both tennons & recesses. How often do you retighten a chuck whilst turning? Have you ever noticed how the bowl may start a small vibration before it fails?

Raffan and others have used very small detail on the inside of pieces to hold and reverse turn, but the key is understanding the whole process & to use light cuts that employ the tool to also hold the piece against or cut "towards" the chuck and not "away" from it.

I much prefer contraction mode AND large jaw sets & mostly use the 90mm Vicmarc bowl jaws with glued on waste blocks and a tennon (Titebond, AVXL Plus PVA's or Bostick hot melt glue), only because it gives me plenty of flexibility and I prefer bowls with a minimal or no foot at all.

**countstefan** · 20th July 2013, 10:44 AM

Great video Brendan. I think you point about the diameter of the recess is really important in expansion mode. I always undercut my recesses, but that's the way I was taught to do it.
Any thanks for taking the time and I subscribed to your YouTube channel

**Bruce White** · 21st July 2013, 11:24 AM

Excellent Brendon! I do not feel the need for spigots at all the way I turn.

Thread: Expansion Mode for your Chuck

Thread Tools

Search Thread

Similar Threads

Why i use expansion mode

How much expansion do you need to allow for when a job is hot

Set up for chuck expanding mode

Expansion v. contraction mode

What is it with Timbecon? <<< RANT MODE >>>

Tags for this Thread

Posting Permissions