How much pullback per diameter change on 16C collet

Dave K · Feb 19, 2018

Can't seem to find this info on the net, but it must be calculated somewhere. I have to face off some pins and hold the overall length to a .002 tolerance. That's +0000 -.002. My grinder wants to know just how tight he needs to hold the diameters from part to part, in order for me to still hold that length. There must be a constant that can be figured out, but I don't know where to find it. So, if the part varies by .0005, how much more or less will the part pull back?

Bobw · Feb 19, 2018

Simple trig.. 10 degree taper per side, so we are dealing with radius...

SOH CAH TOA. tan 10 = .00025(half of the .0005 D variance)/pullback.
Pullback = .00025/tan10
Pullback = .00141782"

Cutting it close.

litlerob1 · Feb 19, 2018

Bob's post is dead on, as usual. Had a very similar part, told our guy he needed to hold ±.0002 on the diameter all the time. THEN we found these; a Dead length collet system doesn't pull back on the collet, it closes over the top of it.

https://www.google.com/url?sa=t&rct...x?catid=7389&usg=AOvVaw2Ey7jodUX3FZAtF4iFKQlG

Safe link

Tom · Feb 20, 2018

rule of thumb is 1 to 3, .001" in diameter change is .003" change in length

plastikdreams · Feb 20, 2018

Not to sound like a moron...but is this dealing with the amount of linear movement that will take place when the collet closes based on the od of the part?

Dave K · Feb 20, 2018

plastikdreams said:
Not to sound like a moron...but is this dealing with the amount of linear movement that will take place when the collet closes based on the od of the part?

Yes, you got it exactly. Thanks Bob for that formula. Looks like as said, I'm cutting it close. I'll have to run it in a chuck. Thanks everyone.

Zahnrad Kopf · Feb 20, 2018

Dave K said:
Yes, you got it exactly. Thanks Bob for that formula. Looks like as said, I'm cutting it close. I'll have to run it in a chuck. Thanks everyone.

Dave,

Read LittleRob's post. Won't help you this minute, but plan for the future and your life will become much easier. Dead Length Collet Noses are perfect for exactly this kind of application.

Another thing I do, personally, is over the years of having run into these jobs I have accumulated several blocks that I use with little Vees or holes in them, used to drop the pins in after machining, and then dusting the ends for length on the surface grinder. Easy to hold tenths and in practice actually takes surprisingly little time. ( minutes )

michiganbuck · Feb 20, 2018

Don't know what grinder you are using but if one ups I would mount a test station and while one is grinding would test length to know the needed take..then see the start was at spot on for start of grind..
Thinking +-.001 is tight for a collet hold..

Used to run long run of parts with testing 50 or so and sharpie mark each the needed take so to know the touch was on the mark..Then run them under the indicator on the out end for Ok.

litlerob1 · Feb 20, 2018

You guys must be awesome on a Surface grinder. For $250 I wouldn't even think twice about a dead length collet. I mean, if you have the grinder, and are very fast and want to add another operation to the job, great.

Or easily hold .0005" OAL on a bar that has .005" diameter variance and be done.

R

Zahnrad Kopf · Feb 20, 2018

litlerob1 said:
You guys must be awesome on a Surface grinder. For $250 I wouldn't even think twice about a dead length collet. I mean, if you have the grinder, and are very fast and want to add another operation to the job, great.

Or easily hold .0005" OAL on a bar that has .005" diameter variance and be done.

R

Well, YES! Yes. We. Are. :cool:

LOL.

I was only pointing it out as an option for someone that doesn't have a dead length on hand already, as well as other times that the need comes up. Some times, it is not only easier to avoid fighting equipment nuances ( speaking generally, here ), but more economical, too.

plastikdreams · Feb 20, 2018

Dave K said:
Yes, you got it exactly. Thanks Bob for that formula. Looks like as said, I'm cutting it close. I'll have to run it in a chuck. Thanks everyone.

So to further the discussion...are there any variables that would be involved or is this a constant? Would this only be valid when using a collet stop? Does this apply to 5c collets as well or do they have a different taper angle?

litlerob1 · Feb 20, 2018

Zahnrad Kopf said:
Well, YES! Yes. We. Are. LOL.

I knew it.

plastikdreams said:
So to further the discussion...are there any variables that would be involved or is this a constant? Would this only be valid when using a collet stop? Does this apply to 5c collets as well or do they have a different taper angle?

The angle is the constant, yes 5C is 10º bet not every collet is. Easy to find the specs on Google.

adama · Feb 20, 2018

Zahnrad Kopf said:
Another thing I do, personally, is over the years of having run into these jobs I have accumulated several blocks that I use with little Vees or holes in them, used to drop the pins in after machining, and then dusting the ends for length on the surface grinder. Easy to hold tenths and in practice actually takes surprisingly little time. ( minutes )

Yeah strange you say that because i had to make a mass of spaces and length was the key thing, as a rule i don't take on sub 1 thou tolerance stuff, its just not what i do but got talked into it by customer, from the point of view they wanted them +-1mm in length, but wanted all 500 of them the same to under 1 thou or idealy a lot better still. it was kicking my ass in the lathe, secound i went to surface grinder the job became effortless. Literally did a whole chuck full at a time, you can fit a lot of 1" collars on a 6x18" surface grinders chuck and they come off more accurately the same than i can measure, half thou dti showed no movment. Customer was amazed i ground them for the price, had not got the heart to tell them apart from the first 20 that went in the bin the rest had just been sawn off on a bandsaw and then went for a double sided ride on the grinder!

michiganbuck · Feb 20, 2018

Agree with the right surface grinder set up perhaps 6 to 20 seconds each knowing none will be .001 off zero....and .0005 is not that difficult. One nice fixture is an old bushing split down the side set in a V block over a stop height..
Another handy is a diamond set up to zero-size so a skim and the target number stays the same for the whole load of 20 or 2,000.

SIM · Feb 20, 2018

I turn parts and hold +0/-.002 on length. Its a short part so holding batches within .0003 isn't too tough and leaves me a tad over a thousandth to play with. I can't see why your grinder couldn't hold diameter even tighter.

Dave K · Feb 20, 2018

plastikdreams said:
So to further the discussion...are there any variables that would be involved or is this a constant? Would this only be valid when using a collet stop? Does this apply to 5c collets as well or do they have a different taper angle?

I'm not positive, but if 5c and 16c share the same degree of angle, then the math should be identical.

Dave K · Feb 20, 2018

SIM said:
I turn parts and hold +0/-.002 on length. Its a short part so holding batches within .0003 isn't too tough and leaves me a tad over a thousandth to play with. I can't see why your grinder couldn't hold diameter even tighter.

Oh, he can definitely hold tighter, but why put the extra work on him if it's not needed. I'm running them in a 3 jaw with bored jaws, and it's holding length awesome.

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