Increasing the capacity of bowl jaws

**turnerted** · 14th April 2015, 06:29 PM

G'day
I normally use a vaccuum chuck to hold bowls while finishing off the foot of a bowl ,but sometimes it just won't work ,as in holding an eccentric bowl , where it needs to be held by the edge .
I have a set of 285mm bowl jaws to suit a VM120 chuck and the biggest eccentric bowl I can hold using my home made extensions, is 280mm . See my previous posting on "bowl jaw extensions" 8/12/13.(sorry I don't know how to make the link).
To get the capacity I wanted, I would have to buy another set of jaws at about $140 and for infrequent usage it was not justified and in any case I like making tools.
I bought some 9mm MDF and cut out the largest disc that I could turn between centres then jambed this between my VL120 chuck with bowl jaws fitted and the live centre and trued it up.I then mounted it in reverse with the chuck on a reversing mandril on the tail stock and the live centre on the headstock . I opened the bowl jaws to their full capacity and used a clamp to make sure the relationship between the jaws and the MDF disc did not change . I also put wedges on the bed to make sure it wouldn't rotate . Then using the jaws as a template,I ran a 5mm drill through the 6mm tapped holes that hold the bowl holding mounts . I chose to use only the maximum and minimum adjustment holes . I then screwed 6mm bolts in through the jaws and into the MDF . The MDF is soft enough that the bolts tapped into it .Actually, when I had a few bolts in and I knew nothing was going to move , I removed the chuck from the lathe and drilled the rest of the holes with it on the bench.
I drew a line down the edges of the jaws then removed the disc from the jaws and cut down the extended lines to separate the four MDF jaws .
I drilled out all the locating holes to 6mm and counter sunk them to take new 6mm bolts and refitted the extensions to the original jaws .So each one is held by four bolts.
With the chuck back on the lathe , I marked the positions for the holding mounts ,removed them again and drilled the 6mm mounting holes then refitted them and fitted the mounting posts with nuts and washers on the back.
This now increases my capacity up to 410 mm .
A warning . Although I think this modification is quite safe , I am sure Vicmarc would not approve, so if anyone wants to copy my method they must maketheir own judgement.
Ted

**hughie** · 14th April 2015, 07:45 PM

Looks good to me Ted. I did something very similar a few years back and its still going strong on a SN2 chuck.

**Christos** · 14th April 2015, 08:02 PM

I agree sometimes we need something to get to finish a task. Nice going on the extendable jaws.

**powderpost** · 14th April 2015, 08:42 PM

Ted, I did the same thing, but used 6mm plate aluminium. Works well too.

Jim

**mick59wests** · 14th April 2015, 09:02 PM

Ted,

great post. Many of us (especially me) appreciate and benefit greatly from these 'how to' threads. Sometimes I had not even thought of doing it in the first place, let alone how to!!

thanks

Mick

**Optimark** · 15th April 2015, 08:27 PM

I have thought of that, but didn't think MDF would have the strength, but with 9mm and care, it would seem to be so.

One question I have, what do you believe your top speed should be with this increased diameter and/or capacity?

Thinking out loud here: the outside will start to travel quite fast as the diameter goes up.

Mick.

**cybergod** · 15th April 2015, 09:29 PM

Saw similar in a magazine, and thought this was pretty much for finishing work. Can you use this setup for actual lathe work?

If so, is it centrifugal force that makes it work?

Interesting looking, but a little above my Newbie skills level for now.

**Sturdee** · 15th April 2015, 11:51 PM

Originally Posted by cybergod

Interesting looking, but a little above my Newbie skills level for now.

You could buy the extension jaws from Gary Pye. I've got one and am very happy with what I got and the quality of the extension jaws.

Peter.

**Mobyturns** · 16th April 2015, 10:17 AM

Originally Posted by turnerted

G'day
A warning . Although I think this modification is quite safe , I am sure Vicmarc would not approve, so if anyone wants to copy my method they must make their own judgement.
Ted

These tips come up regularly & they are one solution to the problem at hand. I do not believe they are good solutions. They solve one problem but introduce new hazards with significant risk.

The risk from the hazards introduced by the jaw modification can be significantly reduced IF the user observes the original manufacturers recommendations about maximum speed and applies a significant safety factor to allow for the extra mass and radius of the extension.

Vicmarcs chuck manual - http://www.vicmarc.com/downloads/4%2...20Aug%2010.pdf

**cybergod** · 16th April 2015, 12:08 PM

Originally Posted by Sturdee

You could buy the extension jaws from Gary Pye. I've got one and am very happy with what I got and the quality of the extension jaws.

Peter.

Thanks Peter. The link is a good one, provides info I sought as well. As I said, I would class myself as "Newbie", so going outside the basics, per my l;athe, isn't going to happen . . . yet

)

Cheers

Greg

**Sturdee** · 16th April 2015, 04:12 PM

Originally Posted by cybergod

Thanks Peter. The link is a good one, provides info I sought as well. As I said, I would class myself as "Newbie", so going outside the basics, per my l;athe, isn't going to happen . . . yet

)

Cheers

Greg

But in time you will Greg and then this kind of setup of the 8" flat jaws and the extension jaws will be very useful and at a reasonable cost. Of course before use you have to check the tightness of the screws and only run at the maximum speed of 600 rpm, but that ought to be a normal routine in using these kinds of chucks.

You could also look at making donut chucks, a Google search should show how to make them. But here are some links and this.

Peter.

**DavidG** · 16th April 2015, 04:44 PM

Google "longworth chuck"

**turnerted** · 16th April 2015, 05:35 PM

Thanks for the comments.
Jim and Hughie It looks like I've just reinvented the wheel.
Peter I didn't know about the Gary Pye extensions but it looks like they are specific to the Gary Pye bowl jaws .
Cybergod They are principally designed for finishing the base of bowls, but I have occasionally used them for other things . Just recently I wanted to cut a circle out of the centre of a plywood disc and they worked well.
Mick Vicmarc rate the bowl jaws at 1000rpm so obviously I would be using them at less than that . I expect about 500rpm would be plenty .
I believe that the principal load on the jaws is dependant on how tight you do them up .A finished bowl is normally balanced or close to it , therefore the centrifical effect of increasing the speed would have very little effect on the loading of the jaws . That said , the main reason I made these is for holding eccentric bowls for finishing and these are not balanced ,so I will have to be more carefull . Although these extension jaws have a capacity of 410mm ,I can't see me ever making an eccentric bowl that big . I think about 350mm is about as big as I would ever want to go.
Ted

**Luke Maddux** · 16th April 2015, 09:47 PM

Can anyone confirm that those bowl jaw extensions work with vicmarc bowl jaws?

**Sturdee** · 16th April 2015, 10:46 PM

Originally Posted by Luke Maddux

Can anyone confirm that those bowl jaw extensions work with vicmarc bowl jaws?

I'll have a look tomorrow to see if they fit.

Peter.

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