Every time I have seen yet another 'power drawbar' thread, it has been here in the general section, so I'm starting here. If it needs to be moved, so be it.

I've been talking about this for a long time but there are a number of pices to the puzzle that have to come together and finally they are all in the sand box so its time to play.

Requisites were BT30, power drawbar (pneumatic only), Servo driven spindle, access to certain ATC parts. Ths big holdup was the Servo driven spindle, but I finally have that now. The new servo spindle drive is demonstrated in this 'intro to parametrics' video:  http://www.youtube.com/watch?v=Cu61oBY5-rw

I have had the pneumatic cylinder and some other stuff for quite a while from when I was contemplating doing an R8 power draw bar. I abandoned that idea after a lot of research. I recently found a source for the 'fingers' that go on an ATC and hold the BT30 adapters.

A stumbling block was the $500 stud gripper for the BT30. There are two types of gripper that I know of; finger type and ball type. Either needs to be hardened and properly tempered to perform safely. I know how to do that, but I no longer had a furnace (sold with the prototyping shop many moons ago). So I have aquirde a furnace. I still have to install the new automated controls, but the 50A 220V wiring is completed.

Meanwhile, I havecompleted the first amd most essential component, the above mentioned stud gripper. I went with the ball type. It works perfectly so all I need to do is turn doen the sleeve and harden the components. I have successfully completed my first scratch built spindle for my 4th axis, so I will use the same techniwues and new tools to make the mill spindle. Compared to making the stud gripper, everything alse should be fairly simple.



The center piece (hold the balls) is S7 tool steel while the 'sleeve' is A2. The sleeve OD is 'mill finish' as it has not been machined yet.




THe next two photos show the gripped in open and closed positions. I have colored the bals red becuase otherwise it was very difficult to see what was actually happening. This was the first round prototype and it works perfectly    so these actual parts will be hardened and used in the spindle.

Simpson,

Beautiful work, as always!  I will be very interested to see how this progresses, as I'm currently building an ATC for my mill - TTS/R8 for now, but still plan to switch to ISO30 down the road.

Regards,
Ray L.

Thanks Ray,

This will probably be the last post for a while on the BT30 spindle because I have other work to complete before I can re-focus on this project. The gripper is a central character in this play so really making and testing the gripper mechanism could be considered part of the feasibility study. Put in simple terms; no gripper - no spindle.  So I wanted to accomplish a working gripper before I dedicate the time and resources to a spindle/drawbar project.

Below are photos of the completed gripper parts. After heat treating the sleeve was tempered only slightly for max hardness. The ball holder was tempered to a typical 'cold work' spec for S7 After heat treat, the parts were pickled and blasted to remove scale.  Small S7 parts and A2 parts are air quenched and typically there is only minor distortion. The ball holder was fine and the threads still worked perfectly, so it just got a black oxide treatment and was completed

The sleeve, as expected, had minor distortion. The ID does not have to be perfectly concentric and the ball holder still slid nicely in the sleeve, so the ID was simply re polished for smooth action and low wear on the balls. However, again as expected, the OD had to be ground to bring it to size and concentricity for pressing into the spindle nose.

My research turned up a figure of around 600lbs drawbar tension for BT30, so I will design the mechanism for 600 lb pull and 1000lb release on 100psi shop air. The cylinder is rated to 200psi, so there will be plenty of headroom. I will test the gripper at 2,500lb pull so that in case I have to go up to 1,000 pull on the holding force, I will still have a 2.5x test.

Making the new mill spindle will be simple compared to the new 4th axis spindle that I recently completed. There are decisions to be made about cartridge vs open frame and of course the type of bearings.

OK, on to the photos. First is just a shot of the completed gripper pieces:



This is a shot of the setup for grinding the OD of the sleeve. The sleeve is bolted to the same custom made mandrel for both machining and grinding the OD.



This next shot is the 4th axis spindle getting 'cored', but a similar operation, albeit not nearly as big a hole, will be done on the new mill spindle. This is the toughext part of the job. Once that hole is thru the piece, the rest is relatively easy. I was only barely able to get this hole made.  The 4th axis motor stalled several times and the stress twisted the mill column. The final drive on the current 4th axis is 5.76:1 and the column is bolted together and not fully boxed (this is just a temporary mill). To resolve, the new 4th axis final drive is 12:1 and the column (now realigned) will be welded and boxed in. This will defeat my 'easy shipping' plan for this mill, but there are no other options.

Simpson,

Beautiful work, again.  FWIW - I recall some time back seeing a spec of 1300# for the drawbar tension on ISO30.

Not sure if you're aware of the power drawbar I did a while back.  I used a small stepper motor, driving through a high-ratio planetary gearbox.  Using current limiting on the stepper driver, I can control both tightening and loosening torque.  I'm currently running TTS, but this design let's me swap TTS or R8 tools, simple based on how many turns I turn the drawbar.  And, of course, it will work if/when I switch to a 30-taper spindle.  Because of the high ratio, with a 400 oz-in stepper, I can get up to 75 ft-lbs drawbar torque - far more than needed.  It has worked absolutely perfectly from day one.  I'm now working on a 10-tool carousel ATC.

Regards,
Ray L.

Beautiful work, again.  FWIW - I recall some time back seeing a spec of 1300# for the drawbar tension on ISO30.

Thanks for the kudo, Ray. One thing I have learned in researching power drawbars; if you want another spec on tension, just wait 30 seconds . .  LOL!  I can recall several people trying to do R8 drawbars with about 1,000 and I had not done any homework at that point, but it *seemed* pretty weak to me. I had one 4th axis buyer who has a power drawbar that he purchased somewhere and he told me that it would not hold the TTS tools. He had no idea how much force it used so I had him measure on of the Belleville springs and tell me the arrangement of the springs. . . .  turned out he had something like 500 - 600 lbs . . .  mystery solved. 

There are two specs on BT30; above and below a certain RPM (6k as I recall, but I'd have to pull my notes). I cannot speak to any other taper standard, but it seems logical that it would go up as the holders got larger. The reason the tension is so high on R8 and TTS is that they rely on friction to transfer torque. BT30 has drive dogs . .  which are something of a mixed blessing

Not sure if you're aware of the power drawbar I did a while back.  I used a small stepper motor, driving through a high-ratio planetary gearbox.  Using current limiting on the stepper driver, I can control both tightening and loosening torque.  I'm currently running TTS, but this design let's me swap TTS or R8 tools, simple based on how many turns I turn the drawbar.  And, of course, it will work if/when I switch to a 30-taper spindle.  Because of the high ratio, with a 400 oz-in stepper, I can get up to 75 ft-lbs drawbar torque - far more than needed.  It has worked absolutely perfectly from day one.  I'm now working on a 10-tool carousel ATC.

Yes, I am aware of your 'silk purse'. I almost folowed you down the R8 road on another project that seems to me was quite a while prior to your current working model. It did not seem doable to me because I *think* you were looking at a single pneumatic cylinder at the time. Not positive on that . . .  just a recollection. There a lot of bodies along that roadside. It would appear that you have now taken a novel approach to an old problem and succeeded where many have failed. Kudos to you on that!

Couple of FWIW back at you; pneumatic R8 drawbar IS doable, but by my calculations say it would require an articulated stacked cylinder. I have a box with all of the materials to make just such an animal; but ultimately I judged it to be a poor investment not because it would not work, but because it would not be compatible with any reasonable ATC design. I'll be able to use most of that stuff now when I revisit this BT30 project. Second FWIW; I am running both R8 and BT30 in the same spindle . .  I did not realize this was impossible until it was too late . .  just saying. 

Hey Steve. Interesting new project to follow. Always nice to see your work and the way you present it.

Dan

P.S.
If you could resize a bit your photos it would make it easier to watch them as they are too wide to fit on the screen.

Simpson,

The spec I saw was for ISO30, which does not have drive dogs, so probably 600 pounds would make sense with the dogs.  Mine will be ISO30, as that's the only spindle available for my machine.

Actually, I think I was the first to actually quantify the drawbar tension required for TTS, and wrote it up here about 3 years ago.  At the time, even Tormach did not know, and one of their engineers asked me to keep them posted on my findings.  They have since arrived at the same number I did - about 2500 pounds tension, or 25 ft-lbs drawbar torque (for a properly-lubed drawbar).  R8 can be somewhat less, due to the taper.  I did make a single-cylinder air drawbar with mechanical advantage, aid it kinda worked, but was far from ideal, so I scrapped it.  It can easily be done with a stacked, large diameter air cylinder - several people have done it on CNCZone over the last year or so, using triple-stacked 3-4" cylinders - but I always leaned towards and air-over-hydraulic system, for compactness.  Then, I came up with the stepper design, and never looked back.  It's handled the most aggressive cuts I could do on TTS with no pullout ever.  And being able to do TTS, ISO30 or R8 with the same PDB is a nice bonus.

Regards,
Ray L.

IF memory serves me I remember 30 taper drawbar pressures ranging from  from 900-1300 lbs depending on manf. and useage. The higher pressure were normally on machines that did a LOT of heavy hogging all day long.

You do realize that r8 does NOT have the same taper as 30 taper Just checking(;-)

(;-) TP

The spec I saw was for ISO30, which does not have drive dogs, so probably 600 pounds would make sense with the dogs.  Mine will be ISO30, as that's the only spindle available for my machine.

Perhaps ISO30 (which I know nothing about) and BT30 are similar enough that you could still use BT30 adapters by just adding drive dogs to the spindle. Just thinking out loud. I see lots of stuff for BT30 and I can't say the same for ISO30 . . .   of course, I never look for ISO30, so that may have something to do with it . . . 

IF memory serves me I remember 30 taper drawbar pressures ranging from  from 900-1300 lbs depending on manf. and useage. The higher pressure were normally on machines that did a LOT of heavy hogging all day long.

I think BT30 is around 1000 if you run over a certain RPM, but I would have to drag out my notes before I would argue the point . .  or I could just wait another 30 seconds     BT30 adapters are often balanced to some outrageous RPM, so I suspect running under 6K would put most BT30 guys to sleep. I have 7HP peak 3000RPM motor, and while I would love to run 10k, a 3.x overdrive is practically trying to make a speeder using belts . .  maybe not such a good idea. My bearing choices would also narrow considerably because of he seals. My motor is 140V rated and it is dining on 180V so I do get some extra revs. I will be making all new pulleys and still have a dual range setup, but I think a 2x overdrive will be adequate.  I will need to move to a 1" wide belt on the underdrive . . . . do not ask. 

I already have a die grinder mount for really fast stuff like routing PCB and so on. The die grinder runs 32k as I recall. Really I want a turbine, but I do not have enough air.

You do realize that r8 does NOT have the same taper as 30 taper Just checking(;-)

Yes, I am aware of the  . . . minuscule  . . . difference. I re-ground the short R8 taper to BT30. The BT30 does not get full support with the arrangement so it is not 'ideal' strictly speaking . . .  but neither is the TTS arrangement, in my view.

My turn; you do realize that R8 are spring collets??  10 brownie points if you know why that makes a difference.  Incidentally, I'm not recommending anything . .  I'm just saying   .