Calculating draw bar force on R8 collet

[h3ndrix]

Hey alll,

I am trying to calculate the right amount of force needed by the draw bar to hold a r8 collet.

Is there a place where I can look this up or can this be calculated?

What I am trying to do:

I am going to try and use an actuator on top of the R8 draw bar to pull the collet up. I am thinking about using the same pull tabs that are used on NMTB, CAT or ISO 30 taper collets on a R8 collet.

Thx for the help

H3ndrix

[Ken_Shea]

I re-read your post so I deleted my prevoius comments as they were not applicable.


Are you referring to retro-fitting retention knobs on a R8?


Ken

[h3ndrix]

Yes! I just couldnt remeber the damn name for those

I am talking about using retention knobs on R8's and I am looking for the force needed to seat an r* collet properly.

Thx.

[Ken_Shea]

OK,
A couple of things here real quick before I fall asleep at the mouse

I don't think this is going to be simple, the actual force required to hold up an R8 is minimal simply because the rod will not let it go down and there is also the inherent snugging factor that the R8 has in the taper. The BT/CAT holders on the other hand do not as the retention knobs carry the whole load. The whole purpose of the BT/CAT holders revolves around mostly the ability to function with auto tool changer so when the retention knob is released the tool holder just drops. With the R8 as you know, has to be encouraged to let loose from the taper with a little tap of a hammer. This probably could be remedied from relieving the taper a bit.

THE FOLLOWING IS PURLEY A GUESS:
I do not know exactly how a retention knob is held in place but it may involve a pair of mating fingers that mechanically move into the taper on the retention knob and are then locked in place by a sleeve. When air is supplied the sleeve is pulled away allowing the retaining fingers be backed away from the retention knob mechanically. If I were not so tired I most likely would not have even written this haphazard guess.

I think you will find that the air devices on BT/Cat equipped mills are designed to release and not hold the tool holder up.

Hopefully someone with some knowledge will feel sorry for us and let us know the facts

Ken

[h3ndrix]

Well, AFAIK, only R8 collets need to be tapped a little to make them come loose, I never had to tap a R8 tool holder to make it come loose ever (R8 collet: The ones with slots that holds the tool; R8 tool holder: No slots in the taper, the tool is held by a set screw)

I believe that the reason for this is that the R8 collets have their taper portion opened up a little, thx to the slots, so it is easier to slide a tool inside of them, so, naturally when you insert an R8 collet into the R8 spindle, the slots close up and create tension. The more the collet is forced up, the higher the tension and the more tapping is needed to take it out of the spindle. The tool holders never have this tapping problem (I never did anyway) because they do not have slots cut into them and therefore there is no tension that is created in the spindle.

And, yes, you are right about how the knobs work. There is another taper in the spindle behind the main taper, a small one, that closes the fingers of the drawbar when the drawbar is pushed up. that is what holds the knobs in place. When the drawbar is pushed down, the back side of the fingers push the knob (and the tool holder) out of the spindle.

But, I still have no idea on how powerful this pulling on the draw bar has to be to hold the tool holder in place succesfully.

Hope someone can help out

thx

h3ndrix

[ballendo]

Hello,

Get a hold of Roland Friestad. He's the owner of Cardinal Engineering, and he writes regularly on CNC topics for Home shop machinist magazine.

Last year (or so) he wrote about a cam operated R8 manual quick change device; it's an article in one of the HSM mags. Anyway, he had the same problem you do...

Here's how he proceeded. First, he tried what you're doing here; asking others

With no takers (but with a lifetime of experience with his mills); he used the empirical approach. First, he tightened the drawbar the same as he would at any other time (by feel). Then he loosened it until it "just" freed up, counting the turns of the nut as he did so. Repeat a few times, keeping a chart.

Looking at the chart, he was able to deduce that he had been applying about .015 (from memory;do NOT just use this value!) "stretch" to the drawbar. So he designed his device to also have about .015 stretch of the drawbar. And it was and is successful. (I would have suspected that if he was way off, that sopmeone who used the article figures would have had problems and written in to the magazine...)

Holpe this helps,

Ballendo

[Ken_Shea]

Well, AFAIK, only R8 collets need to be tapped a little to make them come loose, I never had to tap a R8 tool holder to make it come loose ever (R8 collet: The ones with slots that holds the tool; R8 tool holder: No slots in the taper, the tool is held by a set screw)

You did say R8 Collet in your previous post so that was what I was referring to, I do not have any experience with the R8 Tool holder but it does make sense that they will not tighten up in the taper.

It will be interesting to see (factually) just how much force is required, my guess is not all that much, after all, with a precision taper fit where can it actually go is my thinking.

That would be a nice retro on any R8 mill, keep us aware as you make progress.

This guy has a separate set of plans for a tool holder using retention knobs, they are for a tiny mill (Sherlin ?)but the concept may be of value as you design your own. I bought the complete set of plans for his auto tool changer which included the tool holder some time ago just because it looked interesting.

http://home.insightbb.com/~joevicar3/default.htm

Ken

[ballendo]

Originally posted by Ken_Shea It will be interesting to see (factually) just how much force is required, my guess is not all that much, after all, with a precision taper fit where can it actually go is my thinking. Ken

Where can it go?

As you mention the R8 is a "nearly" NON-locking taper.

Don't discount the cutting forces, especially with a spiral flute...

And though we don't really talk about it much; metal stretches and "moves" a lot more than we think. Which all adds up to the need for a decent amount of "holding" of the toolholder into the taper. (Some of that movement happens as a result of heating during cutting...)

FWIW, the r8 design helps, because the upper locating diameter helps bunches. ISO and CAT tapers don't have this luxury, and require high holding force into the taper.

Hope this helps,

Ballendo

P.S. The typical r8 drawbar is 7/16-20, and R. Friestad empirically determined that .015 stretch was what he was "using" when manually tightening. Anyone have their Machinerys handbook handy to tell us just what tension this is?

[JFettig]

I know this is quite old, but required force is over 600lbs, I have about that on my quick change. Another quick change I have seen uses that much too.

Jon