Thread: Aust Wood Review article on bowl mounting

I read that article a few days ago Ern - while standing in the newsagency!

You correctly point out the problen of the angle, and this can be difficult for most people

I think there have been special tools made to cope with this problem.

The fellow who taught me will not hear of this method. I do as he taught me and it has only evr been through my own carelessnee that I have had problems - twice.

we cut a recess with perpendidular sides that is an exact fit for the jaws of the chuck when closed. The Chuck is seated firmly on the base of the recess and tightened.

As I said, unless you are careless, no problems

Well done Artme, a low cost way of getting info!

IMO vertical sides will only work reliably with ribbed jaws and pressure.

Excess pressure I would say would be the cause of most problems.

And the ribs just leave marks that need cleaning up and so more work.

Yes, you can shell out $$ for so-called dovetailed scrapers, but the angle has to match your jaw dovetail, and anyway, you can shape your own out of a bit of HSS or an unused scraper on the tool rack. If anyone's interested I'll describe how I've done this.

Originally Posted by rsser

Yes, you can shell out $$ for so-called dovetailed scrapers, but the angle has to match your jaw dovetail, and anyway, you can shape your own out of a bit of HSS or an unused scraper on the tool rack. If anyone's interested I'll describe how I've done this.

I just use the skew as a scraper. Or should that be "back beveled scraper"? One trap for young players is measuring the top of the dovetail then transferring it to the top of the foot ring. You actually want the bottom of the foot ring to match the top of the jaws. a lack of outward pressure only results in the bowl spinning in the jaws rather than a UFO, if it is a dovetail rather than parallel sides!

Its interesting to note that the Leady chuck that was designed for secondary schools and I gather has been in service in schools for many years. Uses expansion and keying via a ridge or raised grip area.

The inventor claims that when used in conjunction with supplied cutter, failure rates a no greater than the experience mentioned here.

So what does this all mean? wrong sized cutter, over tightening, to little 'meat' to hold by etc

heres a previous review on the chuck

https://www.woodworkforums.com/f8/key...g-chuck-36662/

Don't mention this to Ken!

I'm with Tea Lady - I just undercut the recess a little with the long point of the skew used as a scraper. The angle of the skew is a bit steeper than the angle of the taper on the shark jaws on my Vicmark VL100, and the undercut serves to pull the square bottom of the recess hard against the end of the jaws.

I use the same technique to hold the body of my pepper mills while I bore them from the top. There are two potential problems:
1 if there is not enough meat around the recess, over-expanding the chuck sometimes splits the timber collar;
2 if the recess is deeper than the pitch of the "teeth" on the shark jaws (ie more than about 9mm) I need to taper out the neck of the recess to make sure it's only the end of the jaws making contact.

(Hope all that makes sense - it's really hard to describe in words what is easy to demonstrate with the hardware in your hands).

I've attached a couple of pics that might help the explanation.

cheers, Colin

Originally Posted by rsser

............Some folk bag this method of mounting................ .

I've heard of this phenomena Why??? I've never had a problem with (tapered) recesses but I have snapped a spigot or two off going the other way

Yes, you can shell out $$ for so-called dovetailed scrapers, but the angle has to match your jaw dovetail, and anyway, you can shape your own out of a bit of HSS or an unused scraper on the tool rack. ****If anyone's interested I'll describe how I've done this.****

Okay. I'm interested. I've not been able to do this correctly yet.

I don't imagine there is a link where I can read the article is there? I have been doing this for years. If you drill a recess with a big forstner bit, you never have to take the chuck off.

I won't use expansion chucking on end grain, too much chance of splitting.

robo hippy

RH: fraid not. Yep, Martin starts with a saw-tooth bit as well, one that closely matches his 50mm jaws, for mounting the top for outside shaping. then undercuts the recess wall with a spindle gouge.

Colin: yep, clear to me anyway, as a shark jaw user. Maybe 'ribs' is a better term than teeth.

Martin emphasises the importance of matching the recess circumference to the jaws'. And of having some bulk to take the pressure. But he shows 25cm plates, 6mm thick turned on a recess 47mm x 1mm.

Allen, I'll try to explain it and bear with me as I'm not much of a technical writer. My scraper stock is 1/2" wide, 3/16 thick. The scraper angle (long side on the left to the end) has to match the jaw dovetail angle (face to side). I grind a bevel both on the left side and the tip since I open the recess with a parting tool and want to clean up both the wall and the floor of the recess. But I relieve the tip so that not all is in contact, maybe just the left third, to reduce the likelihood of catching. First step then is to go in scraping the wall til the floor is contacted and then gently clean up twds the centre.

I used to start the recess with the long point of a skew vertical, so cutting not scraping, to leave a better wall surface, but found tool control difficult. It needs a pronounced skew angle for clearance in small diam recesses and there's little bevel support.

Raffan talks about recess depths of about 4mm.

I usually have a foot on bowls so the recess goes into this and often with not much bulk then to take any great pressure from the jaws, and have rarely had a failure. Course a deeper recess can help here but that can compromise the form, and with small bowls, sticking to a foot diam of c. 1/3 the bowl's, a 50mm recess is just too big so another method of mounting is needed.

EDIT:

Pat, Ken is right to warn about this method. If the wood is unsound and you apply too much pressure, it can split at speed with ugly consequences. You also have to moderate your hollowing method to suit. Big cuts with a 5/8 gouge with a piece in a shallow recess ain't gunna cut it, just lever the piece out.

To set the pressure I snug the jaws up and then add a bit of a turn on the key. With experience you can feel/hear it when it's right. You can always start with light pressure, try to move the piece with a hand either side, and if more is needed add some.

HTH

to get the angle of the dovetail jaws - lay the chisel blank across the end of the chuck (side of chisel touches jaws) and the angle of the end of the chisel should be a continuation of the angle of the "dovetail" on the jaws.

Attachment 131511

Then you grind the releif that Ern desribed and you have what you were after

or you use the Skew as a scrapwe and adjust the angle (you cut at) to suit the dovetail. The point of the skew also gives a good clean corner to allow the dovetail to seat right in against the bottom of the recess.

The correct size of the recess is most important, it needs to be when the jaws are a perfect circle so even pressure is distributed arpound the whole recess. This perfect circle is usually when the jaws have a 2 to 3 mm gap between them. (just open)

Just to be a bit different from Calm, I shape my 1/4" round skew this way.

Attachment 131525

Ok, I hesitate to offer this pic as I'd reground the thing for another job.

It still works for the recess and I'll return it to a better shape by making the angle more obtuse and the top edge less rounded.

As you can see, it's pretty crappy steel and will soon need the top lapped again.

Here's a pic of Nova's job. Click. I'd reshape the tip so that only a third or a half contacted the floor while the side was scraping. And of course your dovetail angles may not be the same as Nova jaws'.

The tightening part took a while to figure out, and the proper amount of shoulder to leave.

Too tight, and you can blow it out, but that again can depend on the wood. Some woods you can really tighten, especially if you have a big shoulder. The problem with over tightening, if you don't blow it out first, is that if you do have a catch, the recess can fail because the extra stress of a catch can be just too much. You can get away with a narrow shoulder if you don't tighten too much, but you have to be dainty when you turn it out. I prefer the keyed chucks, and tighten a bit in one key, then rotate to the other, then back again, never getting it too tight.

I mark the recess by using a dedicated compass or dividers, which are super glued in place. First cut is just inside the line, then I just barely take the line. If the side of the dove tail is too fuzzy, I will use a small pointy spindle gouge and clean it up. I also let the scraper rest, just barely touching the wood, like a finish cut, to remove the run out you always get from going through end grain and side grain. This helps a lot when reversing the bowl.

As to depth (apologies, I never learned millimeters) I usually am about 3/16 of an inch. 1/4 inch is over kill, and proper fit is more important than more depth. This works fine for heavy roughing and coring. I saw Mike Mahoney turn a nice platter, about 14 inch diameter with a 1/16 inch deep recess. You need jaws that have never been hit by your gouge, scraper or coring tool, which for me is brand new ones. I can't go that shallow as I prefer green wood, the wetter the better, then turn them to final thickness, and let them warp. Some bottoms will warp so bad, that I can't remount them for sanding on any jaws, even with 3/16 inch deep recess. I do use extended jaws which are smaller, and they will get enough of a grip for sanding, which for these bowls is at a very slow speed, like 10 to 15 rpm.

robo hippy