[johnyac]

A Simple Power Drawbar - YouTube

[concombrefrais]

Here is a plan for one very similar:
POWER DRAWBAR KIT
This one even has a pneumatic cylinder which pushes the impact wrench down for you! I have it installed in my 770 and it works great. A few issues: most likely uses more air than the tormach one, louder (though not very), sometimes the collet comes off and I have to put it back, does not work with tormach tool changer and his likely more difficult to adapt to you tool changer if you want to go this way.
Here is a video of me using it:
Impact Wrench Power Draw bar in Tormach 770. - YouTube

[SCzEngrgGroup]

That kind of drawbar has been around forever, and there are a number of sets of plans out there for them. I built one many years ago, and HATED it! I tossed it after just a few weeks of use. It is INCREDIBLY rude, crude, and noisy, and a really brute-force approach to the problem. And the drawbar head really takes a beating. Some people may consider it better than nothing, but I wouldn't ever put one on one of my machines again.

Regards,
Ray L.

[Scott_M]

Or a shameless plug for my Lever operated drawbar http://www.sdmfabricating.com/LOD.html

And a thread here at the zone about it http://www.cnczone.com/forums/tormac...d_drawbar.html

Scott

[handlewanker]

Hi, I did a test set-up to try out this design for a simple power draw bar, the reason being that when the tool is removed and another inserted the danger is from a crossed thread under power.

So I set up a nut in the bench vice horizontally.

Now, when you enter a screw to a threaded hole you always turn initially anti clockwise to hear the click as the thread crosses the start point, (well I always do), and then proceed to turn the screw in with a clockwise turning action, be it with a screw driver or by hand.

The set-up was with a power screw driver with a cap screw and a hex driver, and after 50 ins and outs the thread had not crossed once......I did another 50 just to make sure.

I always reverse turn the draw bar when manually inserting the bar to the tool anyway, purely to feel the click as it crosses the start.....then turn it in the normal way.

In my opinion, if this design is used, MAKE SURE YOU REVERSE TURN THE MOTOR....WHILE APPLYING LIGHT PRESSURE before entering the draw bar thread to the tool and tightening it.

The important point is.....do NOT apply heavy pressure to the draw bar when running the thread into the tool at the start.......light pressure is needed......you will cross the thread if you apply heavy pressure.

I have an ISO 40 taper in my spindle and when inserting the tool the pressure for insertion is hand pressure, so re-engagement is not a problem.

With the power set-up you would hold the tool in your hand anyway while applying the up pressure.

In manual mode, I hold the tool up to engage the spindle drive dogs in the tool collar slots and push the draw bar down while turning it.

The starting reversing motion tends to centralise the threads before insertion......if this does not happen and cross threading happens the draw bar is in poor shape and would cross the thread anyway no matter how careful you were even with a manual insertion.

It also pays to make sure the final torque setting of the motor/drilldriver is sufficient to tighten the drawbar without being too much.

The final word too is to make sure you have at least 10 threads engaged in any tool to offset thread stretching on the bar.

As a matter of interest, when I bought my Ajax mill. (similar to a Bridgeport but bigger) the drawbar was missing, and the spare Bridgeport drawbar I have, had almost all the thread stripped.

I cut off 75mm from the stripped end and butt welded a hi tensile steel 7/16" UNF bolt with the head cut off to the end.

I might go and make a similar set-up to post #1 as it's so simple, and I'm tired of getting up on a small steps to reach the draw bar end every time.

Sometimes you need to give the drawbar end a light tap to break the "non stick" taper grip, but I find a light tap to the tool body works too and would be necessary if the power draw bar was fitted, especially if the spindle was R8 which is a bit more tricky.

UNDER NO CIRCUMSTANCE SHOULD THIS POWER DRAW BAR DESIGN BE USED WITH MORSE TAPER SHANKS.
Ian.