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Holding ring in lathe


#1

Greetings All,

I have a particular style of band that I created which requires some
machining on the ‘outside’ of the band. I made a little holding
device that I mount in a small metal lathe and do the machining. My
holding apparatus isn’t really stable and the rings often come loose
in addition to it not being true to center…

I can use the chuck to ‘expand’ within the ring but it is also
somewhat limited as you cannot access both sides of the ring without
your tool getting dangerously close to the chuck… Also, the ring
must be a size 7 or larger to even use the chuck.

Can someone with some lathe/machining experience please advice on a
way to mount the ring in a lathe? I enjoy utilizing my lathe in
certain applications and it’s frustrating not to have a solid way to
hold a ring while having access to all sides.

Please Advise…

Thanks so much!
Andrew G.


#2

Hi Andy:

It depends on how up-tight you want to get about it. (and whether or
not you’re working in wax.) Wax makes life harder, as it has no
strength.

To do it ‘right’ you need either lathe mandrels, or expanding
mandrels. Both very expensive. (MSC has them, page 1629 of the
catalog. www.mscdirect.com) Solid mandrels are.0005" undersize, and
taper up to about.0005" oversize. You slide your piece onto the
mandrel and gently drive it up a bit, then turn the piece with the
mandrel mounted between centers. In ring size ranges, they run
between $10-$20 each, and you’d need lots of them unless you only
do a couple of sizes.

There are expanding mandrels that’d cover a wider range of sizes,
but in ring scale, those run between $60-80 each, and you’d need at
least 5-6 of them to cover most rings. Again, mount ring on mandrel,
turn between centers.

My trick is just to make my own mandrels as I need them. It’s also
important that I have a lathe that uses collets to hold the work. I
can put the ring into a large collet, and grip it around its entire
surface without marring. Try that with a three-jaw chuck, and you’ll
shred your surface.

Step 1: Grab ring in large collet. Bore ring to size. (any size, but
ideally to final size, less about .005".) (a few thou for finishing.)

Make sure ring sits in collet with axis of ring in-line with axis of
lathe. Don’t let it tilt.

It’s important that the bore of the ring is actually bored to an
accurate diameter all the way through. That’s what lets it lock up
on the mandrel in the next steps.

Step 2: Measure bore of ring.

Step 3: take a hunk of aluminum rod, chuck it into the lathe. Leave
2 inches clear of the chuck. Center drill for support later.

Step 4: Turn aluminum to match the bore of the ring. When I’m feeling
particularly tricky, I turn for a shrink fit. That’s where I turn the
rod very slightly oversized, and throw it in the freezer to shrink
it a bit. Pull it out of the freezer, mount the ring, and they’ll
lock together as the rod warms up and expands. By ‘very slightly’ I’m
talking about.001"-.002" or so. There are tables for that sort of
thing in Machinery’s Handbook. Don’t try to wing the fit. Look up
the proper interference for your relative bore diameters in the book.
You’re looking for a very light interference fit, as you need to get
the ring off again later.

If I’m feeling like I might want to use this mandrel again, I make it
a little longer (3-4") and turn it with a very slight (about .25 deg)
taper, so that it starts out undersized, and ends up slightly over.
The ring will lock up somewhere along that length. Don’t to try to
fit the ring immediately: the rod will be hot from turning. It’ll
shrink as it cools, and the ring will loosen. That will be bad. Let
the rod cool, then try to fit the ring.

When you’re actually turning on the ring, remember to use the live-
center in the tailstock to support the end of the mandrel. (and
remember to be very gentle. That interference fit isn’t much to grip
with.)

Do your turning, then throw the thing in the freezer. The different
contraction rates should make it very easy to knock the ring off
once the mandrel’s frozen.

(I’ve also been known to bore out aluminum mandrels, on very thin or
lacy rings. Junking a one-shot mandrel’s a whole lot cheaper than
redoing a gold ring.)

Regards,
Brian.


#3

Andy - i have a 9x20 southbend lathe in my shop i use quite
regularly, iuse expandable mandrels i buy from ENCO what i know is
this most lathes run out about.003" -.005" at the chuck mine is
somewhere around .003" -.004"…

What you machine is actualy fairly round until you decide to take it
out of the chuck or move it in some way.

Since it is almost impossible to do jewelry without a reorientation
event in the lathe chuck i try to minimize the out of round or off
round problem in two ways.

I have a mark on one of my chuck jaws and a mark on my expandable
mandrels that i line up when placing the mandrel in the chuck. When i
find i have to work the back side of the ring which means i have to
take it off the mandrel to turn it around, i flip the ring so it
goes back on the mandrel as close to its original position on the
mandrel as i can get it which puts the work as close to round
positioning as this human can make it. in other words on my set up i
only need to move the ring on the mandrel and not the ring and
mandrel in the chuck, at this point its jewelry for me, and not lets
say an engine destined for a world record attempt 400mph + speed
trials at the bonneville salt flats during speed week.

With these sorts of problems you just have to get creative i like to
read tooling and supply cataloges and find out what all the gadgets
do then i runn across things that help me solve my machining
problems

best regards
goo


#4

Andrew,

I have a particular style of band that I created which requires
some machining on the 'outside' of the band. I made a little
holding device that I mount in a small metal lathe and do the
machining. My holding apparatus isn't really stable and the rings
often come loose in addition to it not being true to center... 

What you are looking for is called an ‘expanding mandrel’. Do a
search at www.use-enco.com and be sure to be sitting down, they are
not cheap and have a limited expansion range, you might need more
than one size. I have not used this particular product but have made
one using the same basic idea. Cylindrical piece which fits my lathe
spindle, slitted 4 ways and a screw on axis with a tapered shank to
expand. Still a very small range.

If you can do your lathe work in just one direction a longish tapered
mandrel would probably work. Easy to make and a greater range of
sizes. Use the tail stock centre in the small end and only cut
towards the head stock (gently !!) Leather mallet and maybe some
crazy glue to mount the ring.

jeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand


#5
Can someone with some lathe/machining experience please advice on
a way to mount the ring in a lathe? 

Andrew ---- A collet chuck. Those are expensive, with the collets,
and depending on what you mean by a “small lathe” you may not even
be able to get one at all. But that’s the best. You can get inside
collets that are machinable, and you can also put a collet stop
inside a regular collet so just the edge of the ring peeks out.

Another way is expanding mandrels, which mount between centers. They
can be difficult to get a narrow ring straight, and your tool will
cut grooves in it if you’re not real careful. More for turning the
outside…

Finally is a chuck spider. Those are metal plates cut into three
arms that meet in the center. The arms go in between the jaws of a
3-jaw chuck so you can close the jaws with the work having a backing
plate in the center. Many are magnetic. They are used either to
prevent work going through the center hole when cutting, or to block
work out beyond the ends of the jaws. Same thing - storebought ones
aren’t so cheap, and they tend to be made for 8" chucks and the
like. You can make your own, but then you need to take great care
with the precision - a few thousands out of flat (round doesn’t
matter much) will magnify into the work. That’s what I know of,
anyway…

http://www.donivanandmaggiora.com


#6

Here’s a simple way to make your own expanding mandrel. Take a piece
of mild steel and turn it as shown in the following diagram…

Drill and tap an axial hole a bit deeper than the shoulder, but be
sure to use only a taper tap down to about the centre of the ring.
Cut the two slots with a hacksaw, place the ring up to the shoulder,
and when you tighten the bolt it will expand the mandrel when it
reaches the tapered thread.

I’ve made several - they are easy, quick, and cheap to make, and
work a treat.

Regards, Gary Wooding


#7
Can someone with some lathe/machining experience please advice on a
way to mount the ring in a lathe? 

I’ve made quick-and-dirty expanding mandrels to hold rings in the
past.

What I do is, take a bit of aluminium barstock that’s considerably
fatter than the ring and long enough to poke out an inch or two from
the chuck.

Drill the end, down to roughly where the aluminium comes out of the
chuck, then tap it with a taper tap. I use a 5mm drill and a 6mm
tap. You want to have a few mm of full-formed threads in the hole.

Take it out of the lathe, and use a hacksaw to saw it into four
pie-slices. The cuts should go all the way along the length down to
where the mandrel comes out of the chuck.

Now, put it back in the lathe, put the ring blank on it, and put a
bolt in the end. As the bolt meets the tapering part of the thread,
it will expand the mandrel & grip the ring. I use an allen bolt.

Turn it down to a close fit for the ring ID at the end, leaving a
shoulder for the side of the ring to register against.

If you need access to the inside endge of the ring, you can turn
away parts of the mandrel to get there, even with the ring in place.

When the end of the mandrel gets too beaten-up, or you need to fit a
very different ring on there, you can just turn a new gripping
surface and shorten the mandrel a bit.


#8

i have not heard any one say this yet but i do have wooden expandable
mandrels for wax turning that i made out of dowel rod i use the
knurling tool on them to help keep a grip on the wax. you can use the
allen bolt expander method but will need a threaded insert that
should be epoxyed in place for mine i use a tapered rod in the center
nut on the backside

goo


#9

Thank you all for the good advice… I found something useful in
every suggestion. The suggestions you all gave would make for a more
accurate holding device… very similar to what I was using but
with more potential to run true I’m sure.

This is what I initially was using for situations like this… I
purchased one of these ring enlargers:

…and would simple place the ring in the graduated thickness,
against the shoulder, where it fit best. Then I tapped the one side
so I could secure it in the lathe. It does work to an extent but
doesn’t run true enough and the work comes loose at times.

I also found the suggestion for the ‘chuck spider’ very helpful for
a different problem/challenge.

Thanks once again… it’s much appreciated.
Andrew Gonzales


#10

Hi Andy,

I recently took a wax carving class at Jay Whaley’s studio in San
Diego, and he had a wonderful kit for mounting a ring on a mandrel so
you could use a #30 handpiece as a lathe. It includes a set of
graduated, expandable plastic sleeves which mount inside the ring and
fit a wide range of sizes. If you’re turning metal and not wax, it
might not suit your needs, but I thought it worth mentioning in case
it helps.

The product is called “WaxMaster Kit” and I tracked it down to
Sculpture Supply Canada. They don’t yet have it listed on their
website, but they do have a PDF of instructions for it posted, which
is how I found it through Google. The PDF is found heRe:

http://www.sculpturesupply.com/techinfo.php

You can order it by calling the company, the part # is 201006.

Regards,
Lorraine