[How-To] Power draw-bar: design question

[keeena]

So, I need to build me a power drawbar for my mill. I'm not planning on anything really fancy. After seeing a bunch of different designs I've been gravitating towards this one; I like its simplicity yet seems very stout and practical. YouTube

I have one very basic question though. On my mill I can completely undo the drawbar and the tool will remain wedged in the spindle. You have to give the drawbar a good rap to knock the tool out.

I assumed that power drawbars had a vertical limit which would effectively do the same thing. e.g. after a few CCW turns, the power drawbar stops moving up / draw bar can't move up any further, thus the unscrewing motion ends up pushing the R8 adapter down and out of the spindle.

The design featured in the video above has no such limit; it can lift right off the linear guides. I thought about the fact that holding the handle down might provide this resistance but it doesn't look like the operator is holding the lever tightly at all. What am I missing?

 

[hman]

You may be missing the fact that the impact wrench is pulsating. I'm pretty sure the vibration is what loosens the collet.

 

[keeena]

You may be missing the fact that the impact wrench is pulsating. I'm pretty sure the vibration is what loosens the collet.

Thanks for the suggestion! I gave it a quick test using a M18 impact (fairly powerful) both with and without some downward pressure: R8 shank remained firmly seated in the spindle. This was with a large jacobs chuck in the spindle.

So I started wondering about torque. It looks like I'm torqueing the DB to about 25-30ft-lbs when I use the combo wrench. The impact will not dislodge this. If I tighten the DB to 20ft-lbs or less: then the impact pulses do cause the tool to drop. So maybe that's my problem. But I couldn't readily find a agreed upon spec. Saw some folks quote the typical torque for the fastener size (7/16 IIRC) which is around 50ft-lbs. Others say just snug (which I guess would mean about 10ft-lbs?). Saw a bunch of folks say about 25ft-lbs.

So suppose this has to be part of the my question: what is the proper torque? I realize that there may be considerations if we're talking about a shank tooling vs. collet w/ 1/8" EM vs. collet with a 1"+ EM. I know that at the end of the day: shank and tool should not spin. And I know the locating pin is not an anti-spin device.

What do you guys use? Less than 25 seems a bit light to me.