R8 Collet pressure

[JFettig]

Hey guys,

I want to design a quick change system for my mini mill to open and close my collets, basicly has a spring on top that pushes the drawbar up to keep pressure and have a lever that presses on the top of the spring to release the tool, kind of like those lathe contraptions for 5c collets.

Heres the question, Does anyone know the proper pressure or how to get the proper pressure for this?

Or maybe know an alternative way of doing this? All I want to do is release the pressure so say an end mill will drop out but collet would stay in.

If anyone knows anything, let me know please.

Thankyou,
Jon

[Ken_Shea]

Jon, I have been around long enough to know that nothing much is impossible, but I think you've got your work cut out for you on this one. As an alternative how about considering a quick change collet and chuck set, about $80 from Enco for a starter set with collets or about $475 for a premium set.


Ken

[JFettig]

I looked into those collet and chuck sets, I do and dont like those for a few reasons, that being that its pretty much impossible to set offsets with them. I want to set this up so I can have my offsets set mostly acurately.

I have to figure out what kind of pressure I need on this thing and I think Ill get building, Ill talk to a few guys at work.

Jon

[CNCgr]

Why don't you make a pneumatic drawbar while you're at it? As for the holding pressure it can be calculated from the tightening torque (in a manual system). You have to now some things about the collet's thread to do that.

[JFettig]

the collet thread is 7/16 20, yeah, I thought about the pneumatic thing, but as crappy as it is I dont have a compressor, and I havent seen really how its done. what my idea is, is that the spring holds pressure, and theres a lever that all I have to do is pull on and the end mill or end mill holder falls out of the collet, then pull down again and insert and hopefully just be able to let go of it, I think that if I make it so I can tap up on it that it would hold better.


Jon

[HuFlungDung]

I am doubtful that this approach is a good solution. For one thing, the tool shank sizes vary, so you actually have to change the collet size anyways, just to accomodate this.

Secondly, actually removing and replacing the toolbit is likely a source of uncontrolled random error. This would be due to the collet pulling up more or less, depending on how hard you pull your lever. This is a problem with regular 5C collets in lathes, where the end location of the part is somewhat indefinite (unpredicable, unreliable). Special types of chucks overcome this by attaching the collet by its threads via a nut that is fixed in relation to the spindle. The collet is then closed by forcing the female tapered sleeve down the outside of the collet (in the direction of the tool tip). This is a so called "dead-length" chuck, and is what you would need to emulate to assure that your tools were always in the same Z plane whenever you exchanged them.

Really for the average hobbyist, a good method is to have plenty of collet chucks (or single size solid holders, with setscrew tool retainer), and a pneumatic drawbar wrench operating the drawbar bolt. Yes, you need a compressor, but that is not a great hardship. You don't need a huge lot of air.

[JFettig]

Huflungdung, I am making a few holders for a 3/4" collet, basicly Im looking to open and the holder will drop out, and put a new one in and close it. I know how you mean with the tension, thats kinda why I want this completely spring controlled. All these holders might just be for drills as I dont know how acurately I can make them. I made 4 of them and they seem like they will work pretty good, all I need to do is put the hole for the setscrew in.

and with drills, I only need them +- 10thou about. probably not even that.

I am hoping to be able to make hardened ones and acurately for end mills, Ill try making a few at work out of 1045 and see how well they work then get some drill rod and harden them.

I have some cnc lathes at my disposal for making these

Jon

[JFettig]

so how is it that I would go about calculating the the torque? I supose I would need to use a torque wrench to see how much torque I am putting on it, but how would that calcluate out?


Let me know,
Jon

[Ken_Shea]

JFettig I know of no one that torques the draw bar, just snug it up, and not all that tight either, a guesstimate is 20-30 FP, maybe less. On my mill even with the collet loose you have to pull the end mill out, I have never had one drop out of a collet as you are needing. Perhaps the collet could be ever so slightly pre-sprung. It will be interesting to see how you make out on the project.

[JFettig]

Im not worried about making it drop out I actually dont want it to necessarily drop, but to be able to pull it out. Here is a picture of my drill holders, I will try making a few end mill holders one of these next couple days after work. I dont have enough options for making them extremely acurate so Ill see what happens.
I still have to drill and tap the set set screw holes.

Jon

[Ken_Shea]

not worried about making it drop out

OK, I will take you as you mean and not as you say

I sent you a PM with a sketch, did not want to embarrass myself