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

[HimyKabibble]

(;-) Some very smart people have been working on this for about 50 years now and so far no apparent winner.

Making a cat type spindle would be a piece of cake, "BUT" making it work in the BP enviroment, NOT so easy. There are design interface problems working with the BP spindle design (;-).

BP did have a 30 series spindle with tool changer the BT30  BUT I don't believe it would work in a standard BP/clone machine

Running a spindle like an X2 (bed mill) would be  simple to do as well Just a mini type VMC you could probably use the iso 20 type holders(not cheap and hard to find)

(;-) TP

Terry,

I don't see what the difficulty would be.  I can see that perhaps the spindle nose *might* have to come down a bit, to make room for the pullstud retention mechanism.  But, other than that, what's the difficulty in doing it?  It sure would be a nice thing to have, but seems to me it would be $$$ to do right, since both the inside and outside would have to be precision ground.

I think in judging this kind of device, it's important to keep in mind the limits of it's utility.  If I can get the tool to reliably carry 30 foot-pounds without slipping (which seems to be no problem), I don't think it's possible to exceed that on this class of machine, no matter what you do.  I have only a 3HP spindle, and typically use 1/2" endmills for all "heavy" cutting, and work almost exclusively in aluminum - that's not a big load.  I'm not running large tools making deep cuts at 500 IPM in hard steel.

Regards,
Ray L.

[HimyKabibble]

Here's some good news!  I've been doing some calculations, and I'm liking what they're showing.  My "worst case" cut is a 1/2" wide, 1/2" deep, 30 IPM slotting cut in aluminum,turning about 5000 RPM.  By my estimates, confirmed using several on-line machining calculators, this equates to just under 2HP at the spindle, and a tangential force of about 100 pounds.  This works out to a spindle torque on the order of 30 inch-pounds, not foot-pounds.  So, my 30 foot-pound target is WAY high of what should really be required.  This is looking better!

Regards,
Ray L.

[vmax549]

GO get um RAY, (;-)

Ray the main problem is where the spindle at the midpoint NECKS down to the small splined section there is almost no room to get all the parts inside of the envelope and have a reliable design. Note I said almost.

HUM 1/2 wide 1/2 deep at 30 ipm and 5000rpm would just about stall my 3hp spindle so you are close (;-) ALSO to get to 5K I would be in overdrive so there is a reduction in Torque.


(;-) TP

[HimyKabibble]

GO get um RAY, (;-)

Ray the main problem is where the spindle at the midpoint NECKS down to the small splined section there is almost no room to get all the parts inside of the envelope and have a reliable design. Note I said almost.

HUM 1/2 wide 1/2 deep at 30 ipm and 5000rpm would just about stall my 3hp spindle so you are close (;-) ALSO to get to 5K I would be in overdrive so there is a reduction in Torque.


(;-) TP

Terry,

"where the spindle at the midpoint NECKS down to the small splined section there is almost no room to get all the parts inside" - Yup, that's why it *might* be necessary to extend the spindle downward just a bit - certainly no more than 1".  I actually think the design would be pretty easy - It's the fab that difficult....  Just as with mine, the spindle would have to get a LOT taller out the top, as it would require a mighty long stack of Bellevilles to get the required travel to release the pullstud.

Regards,
Ray L.

[simpson36]

The complexity of a release mechanism that travels with the spindle should be considered in the total project scope along with the complexity of the tool changing mechanism for a 'home brew' changer, methinks.

I have no idea at this point what you are contemplating for a tool changing mechanism, but since you are moving the quill into a fixed postion to release the tool it is a safe assumption that the tool changing mechanism will need to move the tools around.

With a release mechanism that travels with the spindle, then quill could be utilized to place and pick up the tools, eliminating the need for a tool changing mechanism. Aluminum air cylinders weigh practically nothing and the arms could be made from high strength materials keeping the physical size very small, so the weight of the release mechanism would be negligible compared to the rest of the mass being moved.

The scissor could probably be arranged vertically (I've not though that thru) so that the actuator could ride centered on top. The spindle extention tube could contain the travel stop and also center the drawbar at the top via a simple slip bearing. If the spindle has a threaded preload nut, the extension could be made to replace it and thread onto those existing threads. It would then serve the function of the preload as well as extend the spindle.

Again, I can see that what you have already looks fine and workable, I'm just enjoying the conversation here.

[HimyKabibble]

The complexity of a release mechanism that travels with the spindle should be considered in the total project scope along with the complexity of the tool changing mechanism for a 'home brew' changer, methinks.

I have no idea at this point what you are contemplating for a tool changing mechanism, but since you are moving the quill into a fixed postion to release the tool it is a safe assumption that the tool changing mechanism will need to move the tools around.

With a release mechanism that travels with the spindle, then quill could be utilized to place and pick up the tools, eliminating the need for a tool changing mechanism. Aluminum air cylinders weigh practically nothing and the arms could be made from high strength materials keeping the physical size very small, so the weight of the release mechanism would be negligible compared to the rest of the mass being moved.

The scissor could probably be arranged vertically (I've not though that thru) so that the actuator could ride centered on top. The spindle extention tube could contain the travel stop and also center the drawbar at the top via a simple slip bearing. If the spindle has a threaded preload nut, the extension could be made to replace it and thread onto those existing threads. It would then serve the function of the preload as well as extend the spindle.

Again, I can see that what you have already looks fine and workable, I'm just enjoying the conversation here.

Simpson,

Nope, the toolchange mechanism does not have to move at all - I have a quill AND a knee.  So, while the quill is fixed during a toolchange, I am still free to use the knee and X/Y axes to do the tool pick and place.  So, I get the best of both worlds.  The weight of the drawbar mechanism is a non-issue.  As I said, it's the extension of the drawbar above the head that is an issue.  I am "housing" the drawbar in a tube that replaces the preload nut on the top bearing inner race, but since I run over 8K RPM, it's still not a great idea to make this any longer than absolutely necessary.  The floating drawbar assembly would require extending this at least another 5-6", which is a LOT.

With luck, I'll get the guide tube machined this AM, and be able to run the first at-speed tests today.  Now that I feel I have a good handle on the required drawbar tension, I'm pretty optimistic about the whole thing.  Crossing my fingers and toes....

Regards,
Ray L.

[vmax549]

YEP that part above the spindle shaft is the real problem. It needs to run dead nuts true or you get a wobble. THat wobble creates havuc at speed.

That has been one of the main problems in the past. The extension above the splined portion needs to run in a bearing set to keep it true to the spindle but it would also need to float inside of the upper housing as well.(;-)



I remember seeing an R8 unit in the past that worked with a pull stud on top of the toolholder can't remember exactly where it was.  I remember it used a ball type claw to grab the stud and it used a Royal R8 quick change adaptor setup in the spindle.

I todays world getting the spindle made should not be a problem IF you can work out the upper details. This is where the problems have never been resolved (;-)

Just a thought TP

[simpson36]

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.

[vmax549]

Ray you must NOT have a standard BP type spindle (;-)

I can't see any way to use a long tube as a preload nut with a BP spindle. Picture enclosed of a BP spindle shaft with bearings installed.

(;-) TP

[Hood]

Heres the setup on my Beaver Mill, the beauty of it is the drawbar can be activated at any quill position.
Hood