What The World's Been Waiting For - Yet Another Power Drawbar Design

[HimyKabibble]

Himmy,

We're on the same page here. What I called an 'extension', you are calling a 'guide tube' . . same function . . even down to using the existing threaded pre-load as the base for the tube.

I'm not feeling the same concern as you and Vmax for the wobble. A machined concentric tube of less than 2" in diameter turning under 10k RPM that is under 12" long (all guesses) should not be trying very hard to get off the top of your mill, unless you cannot keep it aligned with spindle center for some reason. If it worrisome to you, just reduce the mass by making the guide tube thin and lightweight and rigid by using 7075. Keeping the draw bar centered in that tube would be an easy matter of a concentric bushing perhaps of a lightweight self lubricating plastic like Delrin, again very little weight and no balance issues.

It occurs to me that the draw bar extension would not need to be very large diameter beyond the collet threads. I find that pre-ground 'stressproof' steel rounds are very straight and I think the collet threads are 5/16" which would be more than needed for the rest of the shaft, methinks, so you could turn that down to reduce the mass above the spindle also, but I just am not seeing a problem with wobble since as far as I know, nothing is extending out from the tube or the drawbar shaft.

I guess you'll know soon enough! I'm anxiously awaiting pics of your next few pieces.

BTW, now that you have mentioned it again, I do recall you saying you had the CNC on the knee as well as the quill. Probably not many conversions are that extensive, one might imagine.

Simpson,

Well, due to an oopsie on my part, my first attempt at the guide tube did not seat properly, and ended up with 0.050" runout at the top - not good.  At 8K RPM, it made some pretty good vibration!  I corrected that, and I'm now under 0.005".  With a more suitable lathe, I'm sure I could do better.  But, the vibration is pretty small at this point, even at top speed.  Probably tolerable as it is.  Certainly good enough for now.

The collet threads are 7/16-20.  I'm using a high-strength all-thread for the moment.  Once the dimensions are all settled, I'll probably make something better.   The Bellevilles and their supporting "top hats" stick out of the tube when the drawbar is up.  They are partially exposed through most of the quill travel.   But, they are always guided by the tube, so no problem there.

I first converted the knee.  In fact, I still haven't done the quill.  But, the knee is too slow, and there's too much wear and tear on the gears, screw, etc. for that to be the long-term solution.  A quill drive is the next major project.

Terry,

My spindle is not a real BP, but is nonetheless identical to your picture.  The front pulley is supported by two bearings fixed to the top housing.  It is those bearings I am attaching the guide tube to.  They, too, have to be concentric to the main spindle bearings, or you get a lot of rattling from the dog clutch and other things up there.

Regards,
Ray L.

[HimyKabibble]

Hood,

I assume that a CAT-something spindle, right?  So the drawbar tension is lower, allowing a more compact arrangement that can easily slide up and down with the quill.

Regards,
Ray L.

[Hood]

Ray, it is a 40 taper, not sure what the clamping tension is  but that is a hydraulic cylinder running at 900PSI.
Hood

[Hood]

Just looked in the manual, no mention of tension but there are 112 bellevilles so would be a fair bit I think.
Hood

[HimyKabibble]

Ray, it is a 40 taper, not sure what the clamping tension is  but that is a hydraulic cylinder running at 900PSI.
Hood

Whoa, baby!   Where do you get 900 PSI?  I assume that's hydraulic, not pneumatic?

Regards,
Ray L.

[Hood]

Yep, 4 words before the 900PSI is the clue
 Its an air powered intensifier.
Hood

[HimyKabibble]

Yep, 4 words before the 900PSI is the clue
 Its an air powered intensifier.
Hood

Hood,

What, now you expect me to read ALL the words??  :-)  Hydraulics would solve a whole lotta problems, but, sadly, too expensive for me....

Now, that "air powered intensifier" is interesting.  I'd never heard of that. 

Regards,
Ray L.

[simpson36]

Oops, yes, I meant 7/16" on the drawbar thread . . . you certainly would not be turning a lot of meat off if you were starting with only 5 . . 

Hood, hard to tell from the photo how big that gizmo is, but I think you have big machines so at a guess the cylinder might be 3"??  that would be over 6,000 lbs of force. Prolly enough 
Any chance of seeing what's inside that thing?

Ray, If a wave of motivation overtakes me, I'll be doing an X3 mill. Oddly, what I am after other than the larger size is the quill. In my case, I would not CNC the quill as I would want it for the occasional drilling or tapping job. I have a Tapmatic head and that thing is  , but a lot of trouble to put up just to cut one or two threads.

I've read a few blogs and articles about CNC quills, but in each case, I just can't see how a non-adjustable rack and pinion is going to produce the required accuracy. If I was heading that way, I'd want to try to rig a ball screw even if I had to run it thru a lever of some sort. Another option that exists but I have not seen anyone use is to split the pinion and put a very stiff spring between them to take out all of the lash. Not to get ahead of you, but what is your plan for the quill?

[Hood]

Ray,
 of course I do, I like being the only one that doesnt

Simpson
 Its a bit under 3 inch, the OD is 70mm or so, ID will be  probably about 60mm.
 I have more pics of it dismantled at home so can post tonight if you want.

As to quills, usually you fit a ballscrew on the front, I did a manual Bridgeport as my first conversion and again have pics but they are at home.
Hood

[Hood]

Heres some pics of the setup.
First pic is the housing that the cylinder slides in, there are two dogs that fit in the cylinders groove to stop it rotating.

Next pic is the cylinder and the attached guide tube for the drawbar. There is a left hand thread inside the tube and the tube rotates with the spindle, there is a bearing  at the lower part of of the cylinder for this.

Next is an inside view up the guide tube and you can see the thread and the lower part of the ram.

Next is a pic of the spindle showing the left hand thread.
Hood