Power drawbar operation??

I'm aware of power drawbars being fitted to vertical milling machines but not seen one in the flesh myself. I'm puzzling over one aspect of their operation that doesn't seem to be described and doesn't make sense to me.

Generally, there is an impact driver above the drawbar (either electric or pneumatic) and a piston to bring the impact driver down onto the splined end of the drawbar. Once engaged with the splines, the impact driver is energised and the drawbar is tightened. The bit I'm not clear about is what if anything stops the spindle turning while the impact driver is turning? If it's free to turn, how do you ensure the tool actually gets tightened. None of the fitting instructions eg Align or Kurt make any reference to a spindle brake, nor is it obvious from any assembly drawing if the spindle is locked in some way.

For my situation, I have an impact driver and an electric spindle brake (part of the motor). Do I need to prevent the spindle from turning when I am changing the tool with this system?

Hopefully I'm just missing something simple - I'm puzzled but hopefully somebody will be able to explain!

Murray

I made an air powered power drawbar with a small impact driver. The inertia of the spindle is sufficient to "prevent" it turning, it does move a little. I do not engage the spindle brake. THe tigtening is quite rapid and I have never had a lose toolholder. R8

For some reason I cannot add to my album, so no photo to show.

Edited By KWIL on 28/04/2017 15:45:46

Hi, I would say the same as Kwil...the hammer action will tighten an loosen the bolt with ease, should u worry holding the quill in ya mit will be enough........Mind the bigger impact wrenches, on my mill the quill bolt is only 3/8UNF.....the threaded part won't let go but I feel this long rod will wind up and snap eventually, I suppose u could make a heavier shafted bolt.....I'd like the system myself but for two reasons, not enough head room and I'd need a ladder......yes, yes, I know u can have the controls lower down but there's enough going on around the quill already......still need room for the mug of tea !!!!!!!.....clogs

What confused me was the only impact driver I have apart from the one on the mill. It's an 18V cordless Makita hand tool that's really intended for driving screws etc. So it acts as a fairly powerful driver until it reaches a fairly high torque, at which point it starts impacting. It seems fairly clear that a more normal impact driver doesn't actually turn unless it is almost completely unloaded. Looking at the videos of power drawbars in action, it seems that there is almost no inclination to turn at all, eg Ray's Garage.

I'm going to lash up some connections to mine in a minute and confirm my suspicions....

....and sure enough, with a large socket mounted on the impact driver (to allow me to manually load it through a piece of tissue), it spins reasonably fast until a moderate torque is applied (my wrist isn't calibrated), then starts to impact. Feels as if it should work well in this application - which of course we suspect it does, given that it was original fit. So I need only think about how to engage and activate the driver and not worry about braking the spindle.

Thanks for the thoughts.

Murray

Edited By Muzzer on 28/04/2017 16:49:43

Photo now in Album.

How come these air impact wrench based power drawbars don't seriously over tighten things? Bought one of the usual small butterfly impact wrenches several years ago intending to do a DIY on my Bridgeport. Chickened out when i saw it ran up to around 80 ft lb. Figure I'm more in the 20 - 30 ft lb region when I pull up the bar by hand which has always seemed more than plenty. I see later models have a pressure adjustment on the input which, presumably helps.

Seem to recall Sir John S advising that the relatively inexpensive mains electricity powered impact wrenches were a sensible alternative to the little butterfly air ones. Simpler to hook up the controls and no compressor running. However judging by my Lidl (again) special these are pretty serious creatures. I guess mine tops out something north of 140 ft lb. Somewhat excessive for 7/16 UNF R8 drawbar.

Clive.

Yes, the Kurt and Align ones seem to manage around 130lbft, which seems rather excessive for an M16 thread and 40 taper tooling but what would I know. I think the plan is to control the tightening "manually" ie by timing it roughly in your head, to get enough tightening, whatever you think that needs to be. But as noted above, it's likely limited by the inertia of the spindle to some extent.

My impact wrench, as fitted to the original machine, is basically a doctored Bosch mains powered hand held device. Doctored as in the handle was hacked off and the 4 wires brought out.

Murray