Re: [CAD_CAM_EDM_DRO] Re: collets

Hi Doug:
You're absolutely right; the collets I describe are "spindle dependent".
To make them perfectly concentric OD vs ID requires, in my opinion, a means
of machining both features in a single chucking.
Soft collets can be bored at the same time they are turned, as can collets
up to maybe 50RC or so.
The secret is not to slot all the way through into the bore, so that the
segments can't flap around as you finish the bore.
This means that the hope expressed below by Chris is not likely to give good
results.

<Snip>
Indeed, I have often wondered about buying $6 import 5C's 1/64" under
the intended size and boring in my lathe. What is desired here is NOT
concentricity with the collet OD, but rather concentricity with the
spindle journals. I probably would try it if I had an ID grinder...
< End of snip>

Notice I commented that I don't recommend this as a way to pretend that you
are saving money; I can't buy the steel and heat treat it for the cost of
finished commercial collets.
But for "specials" which the initiator of this thread seemed to want advice
on, this method has worked well for me.
Fred Smith's comment on locating the orientation of the collet is a most
useful additional step if the collet is not self-orienting.

My two cent's worth on the best commercial collets: I swear by and only
rarely at, spring collets such as the ER series.
They are the most accurate I have found and have the best gripping strength
too.
5C's are not nearly as good, because they don't close up parallel like
spring collets do.
Oversize stock will be gripped at the back of the bore, and undersize stock
will be gripped at the front of the bore with any collets of this pattern,
and even 0.001" off the nominal diameter will make a noticeable difference.
As an example of this, try taking a heavy cut with an endmill that has an
undersize shank and is gripped in an R8 collet.
The cutter will start to walk right out of the end of the collet long before
the same cutter in an ER collet will budge.
All other things being equal, you can also push a cutter much harder when
gripped in an ER collet without having it start to scream.

Cheers

Marcus

-----Original Message-----
From: dougrasmussen@... <dougrasmussen@...>
To: CAD_CAM_EDM_DRO@yahoogroups.com <CAD_CAM_EDM_DRO@yahoogroups.com>
Date: Saturday, November 10, 2001 5:00 PM
Subject: [CAD_CAM_EDM_DRO] Re: collets

>Marcus & Fred,
>
>You've made good points regarding collet making. I still stand by my
>initial statement, I'm pessimistic about "home-made" collets.
>
>You've both mentioned the use of a spindle as a holder in final
>sizing the collet's ID. That's "cheating", since the collet then
>becomes spindle-dependent and not general purpose. IMHO, a collet
>should be concentric and accurate by itself, making it
>interchangeable between machine spindles. That's the part where I
>see the major problems.
>
>Fred, it's not my experience that the typical CNC shop is about to go
>out of business. Where are you located that you see this?
>
>thanks to you both for sharing your knowledge,
>
>Doug
>
>
>
>--- In CAD_CAM_EDM_DRO@y..., "Fred Smith" <imserv@v...> wrote:
>> --- In CAD_CAM_EDM_DRO@y..., "Marcus & Eva" <implmex@a...> wrote:
>> > Hi Doug:
>> > My strategy is to use prehardened steel, most often P20
>because
>> I use it
>> > a lot for moldmaking,
>>
>> In the US 4140/4150PHT (Pre-Heat Treated) is more readily available
>> than P20 and has nearly the same chemistry and physical
>properties.
>> It is about 30-32 Rc hardness and machines very nicely with carbide
>> tooling. You can file corners made with it, but it will make you
>> much more tired than brass. ;-)
>>
>>
>> >so I've got lots kicking around.
>> > I turn all the outside bits in one setup, and rough drill the
>bore.
>> > Then I cut the slots, but not all the way to the free end. I
>leave
>> a stub
>> > that is unslotted.
>> > I can then drop the collet into its seat and bore the hole in
>> position on
>> > the spindle on which it will be used if at all possible.
>> > Sometimes this is the hardest step, because the spindle might not
>> be easily
>> > set up for the boring operation.
>>
>> Also if the spindle socket is worn, you may find that you want to
>> mark the orientation of the collet so that when you remove it and
>> place it back, the contact is at the same angle as when you bored
>it.
>>
>> Use a die grinder NOT a dry marker and just put a small notch on
>the
>> collet face that lines up with a feature or similar mark on the
>> collet holder face.
>>
>> > Last thing to do is lap the hole if needed (rarely) and lop off
>the
>> stub.
>> >
>> > For hardened collets:
>> > Best steel I've found is H13; it is tough, goes to about RC52-54
>> and turns
>> > and grinds well.
>> > The secret to success for me with hardened collets, is not to cut
>> the slots
>> > all the way through into the bore before heat treat.
>>
>> The slots can then be finished all the way through with a .030
>> or .040 thick surface grinder wheel.
>>
>> > But most people don't need "Tenths" accuracy for most tasks
>anyway.
>> > I have made a few hardened collets for threaded production parts
>> that needed
>>
>> The commercial tolerance on collets is usually stated as TIR at a
>> given distance from the spindle nose. Typically .0005-.001 for
>> standard collets and .0005-.0003 for precision lines. The distance
>> that the measurement is taken varies by manufacturer and collet
>style.
>> The cheap imports often run out .001 or more.
>>
>> > -----Original Message-----
>> > From: dougrasmussen@c... <dougrasmussen@c...>
>>
>> > >closed position. With the exception of the rough machining all
>of
>> > >the operations would present a challenge to the typical machine
>> shop,
>> > >not to mention the home shop.
>>
>> Unfortunately, the typical machine shop today is a CNC production
>> shop, about to go out of business, without a doubt they would
>seldom
>> have a toolmaker on the payroll. However the typical tool or gage
>> making shop has all the needed equipment and skilled toolmakers to
>> make something as simple as a collet. If you want to make
>something
>> hard, try an HSK tool holder(rated for about 20-30,000 RPM). Shank
>> angles are ground with precise location to a ground shoulder and
>> tolerances inside of about .000050, as opposed to standard CAT40 or
>> CAT50 tapered holders with tolerances of only .000200 on the angle
>> and about "Who Cares?" on the location with respect to the flange.
>>
>> Best Regards,
>>
>> Fred Smith
>> IMService
>
>
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