Re: [CAD_CAM_EDM_DRO] Re: collets

Hi Doug:
My strategy is to use prehardened steel, most often P20 because I use it
a lot for moldmaking, 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.
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.
Specify a triple draw back and a subzero ageing step if you want dead-nuts
stability, but this is really overkill.
Leaving the collet on a stub (at the back end this time) allows it to be
finish machined all over in one setup.
It can be cylindrical ground or hard turned at RC 54 without difficulty.
If an ID grinder is available, the bore can be opened to final size on the
same machine for best concentricity; if not, lop off the stub, pop it into
the socket in which it will run, and bore it, lapping if needed.
Then split the slots with a skinny abrasive cutoff wheel on the surface
grinder, clean out the burs, and you're done.
Notice that you're talking about a fairly involved process, that just ain't
worth it if you can buy what you need at the store.
And you're right; it is beyond the equipment that most guys will have at
home.
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
to be gripped by the threads, but my most common material has been P20
because it's so simple to use.
They have lasted really well for toolroom applications, and can be easily
hacked for different jobs, just like commercially available 5C emergency
collets.
To measure runout, a gage pin and a tenths indicator does the trick good
enough for me.

To compare with commercial collets: I once bought a set of Lyndex collets
for a cylindrical grinding fixture I was making.
The difference in body diameter on these premium grade commercial collets
was of the order of 0.0004", which meant that the smallest collets rattled
in the bore that was a snug sliding fit on the biggest collets.
I had to convince the people at Lyndex that their stuff was no good for my
application, and even after "mix and match" sorting at the dealership where
I got the collets, I still wasn't doing much better than 0.0003" TIR.
Bored-in-place collets do better than that, almost without trying.
The whole secret is to bore the collet in place on the spindle on which they
will be run.
Cheers

Marcus

-----Original Message-----
From: dougrasmussen@... <dougrasmussen@...>
To: CAD_CAM_EDM_DRO@yahoogroups.com <CAD_CAM_EDM_DRO@yahoogroups.com>
Date: Friday, November 09, 2001 9:29 PM
Subject: [CAD_CAM_EDM_DRO] Re: collets

>Hi Marcus,
>
>For example, let's consider a 5C collet. It has to be rough
>machined....not too hard. Then heat treated such that the split
>sections all have the same amount of spring-open.....that might be
>fairly hard to do. Then the OD needs to be ground, presumably with
>the collet in a closed position.....could be fairly hard without some
>fancy fixturing. Once the OD is ground, then the ID presents some
>grinding problems.......it also must be ground with the collet in a
>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.
>
>I'd certainly like to hear about your collet making methods. .0001"
>or better concentricity? How did you measure that?
>
>
>thanks,
>
>Doug
>
>
>
>--- In CAD_CAM_EDM_DRO@y..., "Marcus & Eva" <implmex@a...> wrote:
>> Hi Doug:
>> Which part of the collet do you think is difficult to make?
>> I've made them with concentricity down to 0.0001" or better without
>any
>> trouble at all.
>> Some of my homemade wonders are better for runout than the Lyndex
>collets I
>> can buy.
>> Course they cost a lot more to make than to buy when you factor in
>your shop
>> rate, so I only make what I can't get any other way.
>> But it's really not a difficult job.
>>
>> Cheers
>>
>> Marcus
>> -----Original Message-----
>> From: dougrasmussen@c... <dougrasmussen@c...>
>> To: CAD_CAM_EDM_DRO@y... <CAD_CAM_EDM_DRO@y...>
>> Date: Friday, November 09, 2001 8:12 AM
>> Subject: [CAD_CAM_EDM_DRO] Re: collets
>>
>>
>> >--- In CAD_CAM_EDM_DRO@y..., Sven Peter <peteryco@r...> wrote:
>> >Sven,
>> >
>> >I'm very pessimistic when people tell me they have success making
>> >collets with the typical lathe/mill setups in home shops. Sorry
>> >guys, but I lump you into the same group as those who tell me they
>> >can sharpen drills by hand as well as can be done on drill
>sharpening
>> >machines.
>> >
>> >Doug
>> >
>> >
>> >> Hi Ian,
>> >> I have seen a lot of machines and a few big ones but nothing
>> >compared
>> >> with this. My tries always ran out of center without cylindrical
>> >> grinder. And even sometimes with it.
>> >> Have fun making collets.
>> >> Sven Peter
>> >>
>> >> Iain Ward wrote:
>> >> >
>> >> > I do not have to worry about drill wander over the 29" length
>of
>> >> > the collet.
>> >> > Damnedest thing I've seen, 20" draw bar on a 29" long collet.
>The
>> >> > spindle bore is trumpet shaped and has 16" of contact surface
>for
>> >> > compressing the collet.
>> >> >
>> >> > Sorry for the ramble, but hopefully it helps.
>> >> >
>> >> > Iain
>> >> >
>> >
>> >
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