R8 clamping force by drawbar.

[Taheen]

I'm throwing around different plans on my own power drawbar but before I get too far into it I'm curious if anybody has measured how much force it takes via drawbar on to the collet of an R8 taper for it to have a substantial enough clamping force so I wouldn't have to worry about the tool ever slipping.

Would 1000lb of upward force on the collet be enough? 1500lb? Just curious because that will determine where I go from here with my tinkerings.

Ideally I would like to use the Tormach tool holder system with my own power drawbar so I would always keep Tormach's collet in the spindle and just hit a button to drop it down and again to pull it up to it's exact location.

Help would be greatly appreciated and would save me quite a headache in figuring out how much upward force on the collet is required to provide enough clamping pressure on the tool.

Thanks!

[timlkallam]

This is a pitcure of my power drawbar i made about 15 years ago .I can change a tool alot faster with it than my cnc mill. It sits on 4 small springs to change a tool i just pull it down and it unscrews
the draw bar . If the collet sticks i let it up and slap it down and the collet unsticks. I cost less than $40 to make it.

[dkowalcz]

Well, the drawbar thread is 7/16-20 UNF, so:

1000 lb is about 9 lb-ft of torque on the drawbar
1500 lb would be about 13 lb-ft.

I'd guess that I tighten to something resembling the bottom number, and it's fine on my 2 hp mill.

What timlkallam posted is great - if you pop the cover off a Kurt PowerLock(tm) it looks basically the same as that. I'd say that'd be an easy way to go.

[JFettig]

For good holding, you do need about 1000lbs, I tried about 400 on my mill and chatter made the tool slip out a little bit.

Tim, those kind of drawbars drop the whole collet out, I think what he wants is for the collet to loosen up and drop the tool holder out of the collet.

I started making a power drawbar like Tims, but when I realized that it'd drop the whole collet out I just kind of stopped.


Jon

[Taheen]

Awesome, thanks for two great replies. I was toying with the idea of going pneumatic.

A floating assembly ridding on the spindle much like the way you have rails/springs timlkallam. have a plate with a thrust bearing on either side sitting directly ontop of the spindle over that on the top thrust have a set of spring washers or similar concept to give 1500lbs of upwards pressure against another washer and then the drawbar. Over all that bolted to that sandwiched plate (though which the guide rails run through to keep straight and in position) would be a top plate with a 4.5" bore cylinder with say a .5" stroke. Adjust it all for proper clearances.

So a 4.5" bore on standard 120psi would be ~1908lbs of force against the drawbar's spring washers. I would use a single action spring return cylinder

I'm sure I'm describing a drawbar plan that has allready been designed and sold, but timlkallam's is pretty interesting. Perhaps anybody else who has a homebrew powerdrawbar wouldn't mind posting and we could get some different design ideas.

To me the pneumatic route, although maybe a bit more complex than it needs would provide a very quick and reliable way to loosen the drawbar it seems. Just apply shop air pressure to drop it, release to tighten. I would be able to retain the full range of quill as it would all be floating, and I would take a guess that a nice aluminum pancake cylinder and using 6061 plate for the frame that the entire system wouldn't be near heavy enough to affect quill return but perhaps just heavy enough that having a completely slack drawbar would drop the r8 collet down enough to slip in a new tool.

Sorry if I'm rambling, only had 5 hours of sleep last night.

[Taheen]

Exactly. That's why i'm thinking a floating unit that would apply no downward pressure on the spindle but would still be able to apply 1500lbs of upward force on the drawbar and 1900lbs of force to slacken the spring force and just let gravity drop it down.

BTW, you're name has been bugging me. Frequent ProForums often?

Originally Posted by JFettig For good holding, you do need about 1000lbs, I tried about 400 on my mill and chatter made the tool slip out a little bit. Tim, those kind of drawbars drop the whole collet out, I think what he wants is for the collet to loosen up and drop the tool holder out of the collet. I started making a power drawbar like Tims, but when I realized that it'd drop the whole collet out I just kind of stopped. Jon

[JFettig]

haha, yeah I frequent proforums waterblock design

With these power drawbar setups, its probably important that you dont put all that stress on the spindle bearings.

Jon

[Taheen]

Yeah, the only pressure to the spindle bearings would be from the weight of the components which if I cant find a lightweight pankcake cylinder I could always put springs on the guide rails like timlkallam has.

I suppose I would have to wait untill it is finished and fine tune it to have just enough weight that the downward pressure on the slack drawbar would drop the collet down while keeping the support/guide spring pressure just high enough to keep the weight low as possible against the spindle bearings when the air is released and the drawbar tightens against the spindle so all the weight rests on that plate sandwhiched between the two thrust bearings and thus on the spindle bearings.