Chucking a Small Octagon (Delicately and Accurately!)

I'm setting up as a violin bow maker and need to hold an octagonal part around 8.5mm across and 15mm long in some kind of chuck very accurately on a Myford Super 7 lathe. The piece (the hair tension adjuster for the violin bow) is made from silver sheet, formed into a soldered octagonal ring, around an ebony wood core.

A typical four jaw chuck seems too aggressive for holding such a part and I wondered whether anyone can suggest an alternative? Perhaps a collet of some kind? Perhaps made from something that won't mark the silver such as nylon?

Thanks in advance for any advice on the subject,

Mike

I think you are on the right lines already, Mike

Not necessarily this range: **LINK**

https://hitekasia.co.uk/metric-cable-glands-m8---m90-128-c.asp

but something of that ilk would make the collet ...

MichaelG.

.

There's an exploded view in the pop-up picture on this page.

https://hitekasia.co.uk/ncg-m16-10-black---m16-x-15-cable-gland-ip68-5-10mm-ul-nylon-66-811-p.asp

Edited By Michael Gilligan on 16/08/2018 11:03:30

Yes, a split bush made from nylon rod should do the job. What do you want to do to the part, drill it?

Mike

Depending on the accuracy required you could make a top hat bush from Delrin or similar material, the ID bored to suit the across points dimension of the part, slit the bush length with a junior hacksaw and use in the 3 jaw chuck to hold the part for turning. The bush needs to have reasonable thickness walls that will not distort when closing the chuck.
If you have a collet holding system the same method could be used, just use a thinner wall to the bush with an OD to suit a collet.

Emgee

Simon beat me to it !!!!

Edited By Emgee on 16/08/2018 11:07:44

Yes, the part would be drilled. It would also be faced, and a very small form tool would be used to turn a collar.

A similar button, with its screw inserted, can be found here:

https://www.internationalviolin.com/ProductDetail/17_violin-bow-screw-weyelet-3-part-button

I like the cable gland idea and may try that later.

Mike

An ER collet might be accurate enough. Down. to about 1/4" dia. they have 8 fingers.

Could 3D print a suitable collet from nylon or PETG. Either as a collet to fit in the headstock taper (needs drawbar) or as a split bush to fit into a 3-jaw chuck. If you don't have access to a printer there are online print bureaus.

John

You could make a Delrin or similar plastic split bush with an octagonal hole down the centre, in the lathe. Drill the hole first, then cut the 8 flats on it by using a tool like a boring bar with the tool bit rotated so it cuts like a shaper or slotter when you rack the carriage back and forth. Indexing could be easily achieved by a change gear on the end of the spindle (or geared to it) with spring loaded plunger, or piece of hacksaw blade, engaging with the teeth.

I've cut a few keyways in cast iron gears this way, so I'm sure it will be a lot easier to cut flats in Delrin!

This would have the advantage that you don't have to tighten up the chuck so much to grip the job as it locates on the flats of the octagon, not the points like a round bushing or collet would.

What Hopper said, I think - or the plain split bush if it gives enough grip without mashing the silver.

I'd worry about concentricity with the cable gland idea.

How about this?

Take a piece of material, at a little larger than the across internal flats dimension of the octagon.

Turn down to the "Across Corners" size of the octagon, for more than the required length,

Mill the faces of the octagon, to produce a snug fit on the workpiece..

(For consistency, could the octagonal workpieces be made by forming from a piece of tube? Implies making press tools of some type, but for multiple pieces worth doing. Possibly not feasible with just a ML7 in the workshop)

At the same setting, (i e do not disturb the material between turning the two diameters) turn down a short length for a short distance. This diameter will be concentric with the larger diameter from which the octagon "bung" will be made.

The original major diameter can be held in the 4 jaw, the octagon will provide the drive to to precious metal workpiece, and the minor diameter can be clocked to ensure that it is concentric.

Once set to run true, it is a fixture on which a number of workpieces can be processed

Presumably the object is to face the octagon to length, so the first one will result in removal of the corners of the octagon for a short distance. Facing the other end to length will remove some more of the corners, but once done, there should still be enough material left to provide the drive to the metal workpiece.

Howard

Edited By Howard Lewis on 16/08/2018 13:45:15

Can be quite lucrative if you are any good at it. I know one of my sisters paid "several" thousand tokens for a violin bow not so long ago. As an engineer (and in fact she is one herself), I'd have to wonder how such an investment could be rewarded with any tangible improvement in sound, given that the interaction is between a (consumable) bundle of horse hair and the string. So the bow is surely little more than a handle, albeit an elegant and comfortable(?) one.

I'm a complete musical philistine, as you can tell. However, any benefit would certainly have been lost on me - in my case the money would have been far better spent on machine tools!

Murray

Murray,

Do you not know that "Music hath charms to sooth the savage beast" ? [or something like that ]

There,s no hope for you !!!

One solution might be to get a suitable octagonal collect 3D printed, ideally in Nylon.

Another option is a wood turning chuck that has removable sub-jaws for holding different sizes of bowl. These jaws are screwed on to the movable base jaws equivalent to soft jaws on a metal lathe. Make your own wood or plastic jaws to fit gluing on felt faces if appropriate.

Why nylon?

It's an Engineering plastic.

It will last longer if repeatedly flexed, is stronger than ABS or PLA, softer than the silver and should be non-marking.

You could also use PETG.

Don't forget Acetal

Doh! I just realized looking at this, that a bog standard square collet as picutred would hold an octagon very nicely. Just get the square collet that is the same size across the flats as the AF measurement on the octagon.

If the collets are only readily available in steel, your piece of silverwork could be protected by wrapping a piece of paper around it before inserting in collets.

Just a thought.

Ya think??? 

Pete

Edited By Peter Krogh on 17/08/2018 05:20:17