Thread: Why grip a spigot?

How is it safer Skew Chi Damn? I found that I had to keep tightening the chuck when I tried using a spigot - lost a piece off the lathe - scary! That is when I worked out that the centrifugal force works against the spigot and for the recess. I do check the chuck periodically in the recess mode but have never found that it needed tightening.

G'day Bruce.

In compression mode you can tighten your jaws until they start crushing wood fibres. (Not that you should but you can. ) In expansion mode, such forces would most likely cause the blank to split.

Also, if the blank has a slight flaw, compression mode can hold the blank together. There have been times when I've discovered a hairline fracture, decided to keep on turning and then, when finally dismounting, had the piece fall in two as soon as I released the chuck.

That's not really a good thing, but in expansion mode I'd have probably worn those pieces much, much earlier.


With your problems with the spigot, it sounds like your compression technique needs some work; there are certain rules that need to be followed for a secure grip.

Eg. the end of the spigot should not"bottom out" in the the jaws. The spigot shoulders should be sitting hard against the ends of the jaws, with no spigot visible between them and the rest of the blank. Otherwise it will work loose!

Also, for any set of jaws there's a "best holding diameter." With nothing in the jaws, close them fully and then slowly open them up until the inside diameter forms a perfect circle. Measure that diameter... and that''s the diameter the spigot should be for those particular jaws.

Hopefully it is just poor technique and not a mechanical problem with your chuck! ;:

Hate to rehearse old debates ... but:

With dry timber there's no reason not to go expansion, provided you match the recess wall angle to your dovetail jaws. It's a dovetail joint. That means it's a locking joint by its nature and you don't need much pressure outwards on the chuck. I just take up the slack and then add a bit. And test if need be by locking the spindle, grabbing the workpiece and trying to wiggle it.

Note: I'm only talking about jaws that have a dovetail. Not the stuff that one maker calls spigot jaws. And that same maker produces some beaut 3" heavy duty bowl jaws with a generally dovetailed x-section but the outside jaw face is rounded over so you loose some critical depth that's needed for the keying.

Originally Posted by rsser

Hate to rehearse old debates ... but:

With dry timber there's no reason not to go expansion, provided you match the recess wall angle to your dovetail jaws. It's a dovetail joint.

Yes. And the same dovetail should be used in compression mode too! Although it seems not many people bother...

I think you are right Skew ChiDAMN!!!. I can see that there are, as you describe, situations where a spigot is preferable. And yes, it is was, from your description, certainly my technique when it failed previously. Will give it another bash but still favour the expansion mode.
Thanks to all for your opinions/advice.

Agree Skew, if you don't want to return the foot. Get the diam and angle right and you're scott free.

There are other variables too Bruce.

With an expansion joint and a foot, your chuck may give some some design probs. You need say 3/8" of foot width, and on the rule of thumb that a foot should be about 1/3 the diam of the bowl, on smaller bowls you won't get there.

Another variable is how hard you go in with your gouges.

I see some guys with 3/4" gouges and go in like their life depended on it.

Raffan's an exponent of expansion mounting and not coincidentally every time I've seen him demo he's said 'let the wood come to the tool'.

The only times I use contraction mode on a spigot is when I'm coring large green bowls, sometimes when I'm green turning 20"+ bowls (and going in hard with the 3/4" or 1" bowl gouge...), and on end grain blanks. In those cases there is a lot of extra weight and leverage on the style of foot ring that I use, which can break off if held in expansion mode.

___/ /___________\ \__


If you use a recess without a foot ring there is not the same risk, but then you have to come back and return the foot if you like a proud foot ring, as I do.

/____________\

Otherwise I use expansion mode in all other cases.
.

Originally Posted by Skew ChiDAMN!!

Yes. And the same dovetail should be used in compression mode too! Although it seems not many people bother...

The Supernova II manual specifically states this should not be done (page 13). It says the spigot should be parallel, and the lip should not be recessed.

OK, I'll wear the charge of imprecision.

Gen. principle: the tool should match technique, wood and application.

By way of illustration, pic depicts my first solo deep hollowing effort.

Munro, gen. 1, hand-held. Wood: described by the donor as some kind of pine; turned green. Looks like Cypress to me. Total 40 cm. Hollowing depth: 20 cm. Recess 75mm. Recess depth (with Vicmarc jaws): 5mm.

No chatter, no drama. Will say that deep hollowing hand-held is best learned with medium density woods, and with the pines of various kinds doing it semi-green seems to work best.

That's my two bob's worth for the day.

Originally Posted by sjm

The Supernova II manual specifically states this should not be done (page 13). It says the spigot should be parallel, and the lip should not be recessed.

Nova Spigot jaws are not the same as dovetail jaws.

Originally Posted by rsser

Nova Spigot jaws are not the same as dovetail jaws.

True. Just thought I'd mention it, because when I started out, people were telling me I needed to shape a dovetail on my spigot, and I had all sorts of trouble. Different jaws need different spigots.

Originally Posted by sjm

The Supernova II manual specifically states this should not be done (page 13). It says the spigot should be parallel, and the lip should not be recessed.

True. However, I also have the precursor to their Precision Midi chuck and in that manual it states that it should be done.

Similarly, I've had slips of paper included with jaw sets that show the specs for the spigot/mortise for that jaw set. They usually show a slight dovetail in the shoulder. (Shark jaws, Powergrips and Teknatool's big "Spigot jaws" being the notable exceptions.)

I think that what was happening was people were tapering the whole spigot rather than just the shoulder, so Teknatool covered their butts against being sued for misinformation by taking the simple path and saying "don't."

But you're quite right that different sets of jaws require different spigots!

Originally Posted by sjm

Different jaws need different spigots.

Yes, the jaw profiles vary.

Look at the profile of the jaw being used and match the recess/spigot to that jaw profile.
.

You can overcome all the problems of matching dovetails, spigots, tenons or what ever. Screw a waste block to a smallish face plate, hollow it out and glue the job to the hollowed waste block. Turn to your hearts delight, inside and outside. Then cut the job off with a hand saw, through the waste block and clean the surplus of the foot with hand plane, sand paper. You can even rechuck in a longworth or jam chuck, to refine the foot. Very cheap and you can make the diameter of the foot as thick or thin or a diameter as you like, without being dictated to by a chuck manufacturer. Haven't lost a job yet, using this method.
Jim