Pneumatic drawbar, spindle bearings...

[altaic]

Hi all, I'm considering making a pneumatic power drawbar for my mill with a stack of belleville spring washers (in the ballpark of 1000lbs of force) at the top of the spindle (where the head of the threaded drawbar is currently), and a ramped ball-bearing gripper at the bottom to grab the pull-stud attached to the toolholder.

The simplest design involves locking the quill and mashing down on the drawbar assembly with a pneumatic cylinder, compressing the spring washers and releasing the toolholder. However, this also mashes the spindle, which I'm not certain is wise for its bearings. OTOH, the other designs I've seen appear to just mash down like that.

The "proper" design in my mind is a bit more complicated. The spindle would not be locked by hand (via the quill lock), but instead be supported by an assembly on the top of the mill head. Thus the compression force on the springs from the pneumatic cylinder is transfered to this support on the mill head rather than to the spindle.

So, assuming I want to keep my mill in good repair, is it necessary to support the spindle in this way, or should I mash on the spindle with abandon?

[pete from TN]

However I am getting ready to start on my own tormach system power drawbar setup for my Lathemaster/rf45 cnc mill conversion. I have had some conversations with some clever fellows on here about this and the best Idea I have heard was to create a kind of fork setup on the top of the millhead using a small throw but large force pneumatice cylinder actuating on a dual fork type assembly that uses a scissor action compressing the bellville pack between two large washers . This will allow considerable force and rapid opening of the drawbar to collet link for toolchange yet impose no added force to the spindle bearings. I have yet to build this so it is all pie in the sky right now but it is the best Idea I have seen or heard yet. Another nice feature is that it will also not put any undue force to the millhead itself whereas several designs I have seen require substantial mounts drilled and tapped into the millhead at varying points to allow for counteracting the force applied to the top of the drawbar. It seems like this could be machined into a compact form and be literaly held onto the top of the millhead by a pair of bolts that would only serve to position it and ensure the scissor mechanism retracts adequately to not interfere with the rotation of the spindle. Does that make any sense? Easier to to envision it than to put it into words really...peace

[RICHARD ZASTROW]

You may want to "Google" Kurt Power Drawbar". I believe they also have kits.

Dick Z

[Runner4404spd]

if you do a quick search to mcmaster car you find your spindle bearings. usually a 35-40 mm ID tapered roller bearing. the thrust load rating for this particular bearing is 2000 lbs. more than plenty to support a pneumatic drawbar thats putting out only 1200 lbs of force. ultimately you will have to change them eventually but i bet they don't break down for a long while.

[wwendorf]

I'm working on a drawbar for my X3 as well, and running into the same concerns as you. I've been dreaming up something similar that reaches up and sort of grabs underneath the belleville springs as something presses on the top of the drawbar. I haven't had much time to work on it as i'm finishing a large machining project, but i'm getting close to drawing it out in Alibre.

Wade

[BobWarfield]

So some sort of "pickle fork" (like the auto tool) extends under a shoulder, and force is applied that squeezes between the pickle fork and a lever pressing down on the drawbar?

Hmmm.

That'll certainly keep the forces off the spindle bearings, but there's no feature on the spindle to use as the shoulder. Are you going to be able to afix something tightly enough to withstand the 1100+ lbs? Probably. Needs to be thought through though. That's not a set screw situation I don't think!

Pete, I would think you need to machine your pulley collar to include such a shoulder feature.

I guess I'm starting to wonder with all this panic over stressing bearings whether this isn't just too much trouble. Why not build an impact wrench-style drawbar? True, they're not as nice working with R8 collets, but I hate the collets anyway. I'm going to have to do some repeatability tests on my ER32 collet chucks and the powered drawbar I have.

Cheers,

BW

[wwendorf]

The answer is quite simple really. I have the tormach tooling system, and I want to build tool racks to make an auto tool changer solution for my mill. That being said, an impact wrench style drawbar won't work cause I can't automate it.

My initial drawbar system will be the type where it just presses down on the top of the drawbar and compresses the springs. I need to get something functional now. I will likely work on the design of one with a "pickle fork" as you call it. I'm thinking that a regular washer under the beleville springs (a rather thick and strong one that is) would work more than sufficiently to allow for some upward force. Then I wouldn't have to modify my spindle or quill at all.

Wade


Wade

[wwendorf]

Hmm, now I just had another idea.

Using my idea of a washer, what if I installed a plate on the top of the mill that ended just slightly underneath the level of the washer, like .010" or so. Then, when the arm preseed down on the drawbar from the top, it would just go against the plate?

Something like this:

             ____

            |___|               Top of Bolt

           <      >               Set of Beleville Springs

           <      >               2nd Set of Beleville Springs

          =========           Flat-thick-strong washer

     -------------------

     |                |     THis would be a bar with a U shaped cutout to fit around the 

     -------------------     top of the spindle.

In fact, that bar or base, could even be the base the the pneumatic cyclinder attaches to.

Wade

[BobWarfield]

Originally Posted by wwendorf That being said, an impact wrench style drawbar won't work cause I can't automate it.

The impact wrench automates in exactly the same way as this other style--using air solenoids. It's trivial to do.

Cheers,

BW

[wwendorf]

I realize that, but you run the risk of the R8 collet coming unscrewed. I haven't seen any ATF projects on here that make use of the type of system you are proposing.

The tormach tooling system fits into an R8 collet. There isn't enough control there for me to want to try using it with an automatic tool changer.

If i'm just not understanding, please give a better description of how you propose the whole solution would work?

[BobWarfield]

Originally Posted by wwendorf I realize that, but you run the risk of the R8 collet coming unscrewed. I haven't seen any ATF projects on here that make use of the type of system you are proposing. The tormach tooling system fits into an R8 collet. There isn't enough control there for me to want to try using it with an automatic tool changer. If i'm just not understanding, please give a better description of how you propose the whole solution would work?

Go for it Wade, I can see your mind is made up!

Cheers,

BW

[wwendorf]

It's not that my mind is made up, it's just that I don't see how an impact wrench type system would work with the tormach tooling system. I'm totally open to other ideas and if you can show me how it would work, then it's a great way to go.

I've even got the plans at home for making an impact style system, but I didn't go ahead with them because I couldn't see any practical way to make it work with a tool rack or ATC?

I'm always open to ideas.

Wade