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Author Topic: BT30 spindle from scratch - with power drawbar and ATC of course  (Read 402212 times)

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #330 on: December 02, 2012, 03:24:01 PM »
"Real" machines don't have quills - the whole head moves up and down.  If you have a knee mill, you move the knee up and down to swap tools from a table-mounted wine rack, and you can define that tools can only be released at the top of the quill travel.  You'll want the knee automated anyway, for applying tool length offsets.  A tool changer without support for tool length offsets would be pretty useless.  Making a power drawbar that would work with the quill at anything but the full-up position would be a living nightmare.  Doing it on a BP would be darned near impossible, because there is so little space inside the pulleys where it would have to fit.  Besides, I really can't see any advantage to making it work that way.  Making one that works in the full-up position is quite simple.  With BT/ISO tapers, it's not a problem applying the drawbar load to the spindle bearings, as the load is relatively small (about #1300 pounds max for 30-taper), and the bearings can easily handle it.  But, making a drawbar that does not apply any load to the bearings is also quite easy.

Regards,
Ray L.
Regards,
Ray L.
Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #331 on: December 02, 2012, 04:01:08 PM »
HimyKabibble-

If I get you correctly, You are saying that if you have the wine rack style tool holder on the table, and you use the quill for descending to pick up the tools, and then have the knee do the actual z positioning?

or are you saying that the knee is only for z height offsets. and that the quill (once correct offset is established with the Knee) does all of the Z positioning?

Seems to me that one of the biggest shortcomings on the BP as a cnc mill is that the Z travel is not much. After doing my conversion, I think I get a total of 4.5"

When using the Knee for Z offsetting, do you have it set up to unlock the knee, drive up or down and then relock? is the repeatability/accuracy suficcient? I always thought of the knee as a gross positioning device, and never thought that once unlocked, moved and relocked that the XY position would be repeated better than .002"-.005"

A few years back I thought that if the air cyl had to travel with the quill, that this could be effected by having the CNC Z axis on a (converted) BP also carry some sort of outrigger setup on both sides that extended up and then over the top of the head. I also theorized that a clamping assembley could hold the last .750" of the bottom of the quill and this could extend out and then have risers that would extend up and then back over. this would create an external "frame" like structure directly tied to quill movement and could carry the air cyllinder. Thus tool changes coiuld be effected at any Z height.

Another thought I have had was to have a linear tool rack that travels in X just in front of and mounted to the sliding head dovetail on a BP. it could move back and forth to set locations in a 4th or 5th axis in mach3 and would have trays in the assembley that would slide out via air cyl extension to position under the quill in Y. thus X would be moved to position by an additional axis, Y would be by air cyl pushout and retract in Y would be via Spring pullback.

Z travel would still be extremely limited because it would take 2.5/3 inches of its travel to raise up and have the new tool position under it for capture. I suppose the tools could be pushed both out (away from the mill body) AND up, by air cyllinder thus eliminating the Z travel of the quill being used to change tools. The entire Z quill travel could then be used to do machining ops.
Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #332 on: December 02, 2012, 04:02:07 PM »
Additionally- Hood, thanks for chiming in. you have (for years now) always been MOST HELPFUL to me...
Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #333 on: December 02, 2012, 04:12:54 PM »
Hmmmm... just went to the basement and looked at my BP. Why not have the wine rack style tool holder located at a far extension of the table and have IT raise up and meet the quill for toolchange? this would save all quill movement for Z machining ops.

Like this.

Toolchange call.
full retract Z.
XY position to over current tool used empty position in wine rack.
Raise wine rack up to Quill that is fully retracted.
Tool unclamp and release into wine rack.
Winerack descend.
XY position to new tool location.
Winerack raise to quill.
clamp tool.
Winerack lower.
XY position to required program position for new tool.

With a box like construction with linear ways and a stepper and ballscrew, the winerack up/down could be easily controlled. it could also have a coolant/chip shield that moved out of the way when the winerack raised.

Thoughts?

Offline Hood

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #334 on: December 02, 2012, 04:32:28 PM »
Rays mill uses the quill for normal Z moves and he has the knee powered for tool offsetting.
No real need to lock/unlock the knee if you have the gibs adjusted correctly and the ways are good, its just another axis and you dont have to lock/unlock X and Y after all.
Hood
Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #335 on: December 02, 2012, 05:16:34 PM »
HimyKabibble-

If I get you correctly, You are saying that if you have the wine rack style tool holder on the table, and you use the quill for descending to pick up the tools, and then have the knee do the actual z positioning?

or are you saying that the knee is only for z height offsets. and that the quill (once correct offset is established with the Knee) does all of the Z positioning?

Seems to me that one of the biggest shortcomings on the BP as a cnc mill is that the Z travel is not much. After doing my conversion, I think I get a total of 4.5"

When using the Knee for Z offsetting, do you have it set up to unlock the knee, drive up or down and then relock? is the repeatability/accuracy suficcient? I always thought of the knee as a gross positioning device, and never thought that once unlocked, moved and relocked that the XY position would be repeated better than .002"-.005"

A few years back I thought that if the air cyl had to travel with the quill, that this could be effected by having the CNC Z axis on a (converted) BP also carry some sort of outrigger setup on both sides that extended up and then over the top of the head. I also theorized that a clamping assembley could hold the last .750" of the bottom of the quill and this could extend out and then have risers that would extend up and then back over. this would create an external "frame" like structure directly tied to quill movement and could carry the air cyllinder. Thus tool changes coiuld be effected at any Z height.

Another thought I have had was to have a linear tool rack that travels in X just in front of and mounted to the sliding head dovetail on a BP. it could move back and forth to set locations in a 4th or 5th axis in mach3 and would have trays in the assembley that would slide out via air cyl extension to position under the quill in Y. thus X would be moved to position by an additional axis, Y would be by air cyl pushout and retract in Y would be via Spring pullback.

Z travel would still be extremely limited because it would take 2.5/3 inches of its travel to raise up and have the new tool position under it for capture. I suppose the tools could be pushed both out (away from the mill body) AND up, by air cyllinder thus eliminating the Z travel of the quill being used to change tools. The entire Z quill travel could then be used to do machining ops.

No, you've got it backwards.  You use the knee for applying tool length offsets, and inserting and removing tools in the spindle from the wine rack.  You use the quill for all machining.  Unless you have single machining operations that require more than 4.5" of quill travel (I never have!), this works fine.  This is how I operate my knee mill.  Position accuracy should not be an issue if your knee is properly adjusted, and locking it should be completely unnecessary. 

Regards,
Ray L.
Regards,
Ray L.
Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #336 on: December 02, 2012, 06:28:04 PM »
More thinking out loud-

If a TTS type of tool holder were to be used, then a special drawbar could be made that (instead of screwing down and in, to effect pulling and retaining the R8 collet into the spindles taper) instead has a threaded portion at the bottom to interface with the R8 collet, but is not intended to ever rotate except when initially setup. this drawbar could be additionally long at the top so as to act as a spring holder, springs could be stacked above the termination of the bridgeports splined spindle top, probably with a spacer from the top of the splines on up to where it would have full clearance above the BP's head bearing retaining cap. The extension could be made like this-


Grind out 1.5 inches of ID from the top of the spindle (splined portion) suficiently large such that a spindle extension (spacer) could have a portion of it that slides into the newly enlarged ID protion of the top of the spindle. With a mild shrink fit, and a well made (hardened) spacer, I think that it would act as a part of the spindle and could be concentric enough to allow for it spinning unsupported above the top of the mills head. With the "spacer" extending above the head of the mill, above the portion of the spacer that needs to rermain small enough to pass into and out of the BP head asdsembley, a land could be created at the perifery of the spacer, and any number or combination of die springs held on the spacer with the (moving) drawbar acting as the retaining cap to the spring(s). A threaded cap could effect perloading of the spring(s) and could also be the surface that an air cyl interfaced with to provide the unlocking downward force.

Really, semi-simmilar to the Mach1 design, except no pullstud (TTS type holders) and the spring placement is out and above the head, allowing for more/better/different spring setups.

Obviously, this could get dicey at rpms above 4000, and could be dicey below that if not constructed with strength, concentricity and balance being paid very close attention to...

Downside? one would be that the release would have the force transferred through the bearings of the spindle. However... were the "spacer" discussed above pinned into place at its interface with the top of the spindle, then the "spacer could have a groove cut into it that a sliding u shaped fork could extend into via air cyl that would act as a brace and would keep the release load from being transfered through the bearings. This assumes however that the u shaped "fork" would be coupled to the air cyl assembley. Easy to imagine...tough to explain.
Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #337 on: December 02, 2012, 07:18:50 PM »
You're kind of re-inventing the wheel.  Power drawbars for every kind of toolholder imaginable have been done to death, and what you describe would be less than ideal in a number of important areas.  I'd suggest you wander over the CNCZone, and see what other people have done for power drawbars and toolchangers.  There's little new under the sun....

Regards,
Ray L.
Regards,
Ray L.
Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #338 on: December 02, 2012, 08:15:25 PM »
Sure, I have a membership at the zone, but I have to disagree, I think the design I propose has quite a few advantages.

Namely-
springs can be altered.
springs can be of various sizes.
Air cyl can be used instead of impact or gearmotor.
Sensors not needed. Only one hard stop for downward travel.
table mounted tools, no swinging arm, carosel etc...
no dependance upon the air ratchet type PDB engaging correctly.
No need to have a gearmotor with its drive, switches sensors etc, not to mention making sure it engages correctly before turning.

 
Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #339 on: December 02, 2012, 11:55:24 PM »
Sure, I have a membership at the zone, but I have to disagree, I think the design I propose has quite a few advantages.

Namely-
springs can be altered.
springs can be of various sizes.
Air cyl can be used instead of impact or gearmotor.
Sensors not needed. Only one hard stop for downward travel.
table mounted tools, no swinging arm, carosel etc...
no dependance upon the air ratchet type PDB engaging correctly.
No need to have a gearmotor with its drive, switches sensors etc, not to mention making sure it engages correctly before turning.

 
Pretty much all of those, with the exception of "springs can be of various sizes" can be accomplished with a more conventional design, and you won't have to deal with the VERY considerable issue of keeping a rotating spring stack and housing well enough centered and balanced as to not be a serious safety hazard.  Keep in mind that "extension" will rise above the end of the spindle by at LEAST the amount of quill travel, and it will, necessarily, be rather large and heavy, making perfect centering and perfect balance an absolute necessity.  MANY power drawbars use air cylinders, hydraulic cylinders, air-over-hydraulic systems, linear actuators or even motors (see mine, earlier in this thread).  Keep in mind, too, that if you're talking R8 or TTS, you need a BIG air cylinder - they are typically done with about a 4" diameter triple-stack cylinder to get sufficient force (required drawbar force for TTS is about 2500#, unless you're running a fractional horsepower machine).  Sensors are not needed for any of these.  And, even if you feel they are, what's the problem with using $3 micro switch or inductive proximity sensor?  A table-mounted wine rack tool rack can be made to work with any of these.  There are several examples on CNCZone.  Making sure a rotating PDB engages before starting rotation is also not necessary - I don't bother to do that on mine, and it works perfectly.  If it does engage on the initial downward movement, it will as soon as the socket and drawbar rotate into the proper position, so it's self-engaging.  In fact, I don't think I've ever seen a power drawbar that makes any effort to ensure engagement before rotation starts.  You're creating solutions in search of problems.

Regards,
Ray L.
Regards,
Ray L.