Messages

321

General Mach Discussion / Re: BT30 spindle from scratch - with power drawbar and ATC of course

« on: July 28, 2012, 05:42:23 PM »

I agree about Bellevilles, which is why I gave up on that whole approach.  I expect for your sub-1000 pound target tension, you *might* could find a valve spring or die spring that would suffice.  For the 2500# I was aiming for on my attempt, I couldn't find anything close that wasn't really huge OD.  Even at 1000#, most seem to up in the 2-3" diameter range.  Up to about 5000 RPM, balance will be fairly forgiving.  Above that, I think despite a perhaps small-ish diameter, you'd need to at least provide very positive centering for the spring to prevent vibration.

Regards,
Ray L.

322

General Mach Discussion / Re: BT30 spindle from scratch - with power drawbar and ATC of course

« on: July 28, 2012, 09:19:51 AM »

When I tried to do a Belleville system for my knee mill, I could not find springs with characteristics that would allow me to get the force and travel I needed without a fairly long stack (about 30 springs, about 3", IIRC), even when I went to pretty large diameter (~1.75", IIRC).  And, unless you're only running very low speed, even a fairly short stack will need to be contained within some kind of housing, to prevent it from whipping, with ruinous results, when the spindle is on.  That housing will then need to be balanced.  Perhaps you can find more suitable springs, but I could not.  I also found such a short stack of springs, for all practical purposes, could not be compressed beyond their rated force.  The friction goes up exponentially beyond that point, making any additional travel come at the expense of extremely high incremental force for a very small incremental travel, even when the springs are well greased.  Using a very tall stack allows the response to be nearly linear, as each spring need only move a few thou.  VMCs seem to use stacks of about 150 springs, I assume for this very reason.

Regards,
Ray L.

323

General Mach Discussion / Re: BT30 spindle from scratch - with power drawbar and ATC of course

« on: July 27, 2012, 10:23:39 AM »

*How do you accomplish releasing the collets?  On my R8 design, I provided an adjustable gap to effect an 'impact' load to release the collet.

The entire PDB mechanism moves up and down.  When activated, it is pushed down by air cylinders, applying a few hundred pounds of down-force, which is enough to pop the collet free, provided the taper is kept well-lubricated.  Once the taper is popped, the TTS holder simply falls out.

*How do you prevent the collets from rotating in the spindle nose?

With R8, the alignment pin in the spindle takes care of that, though I suspect just the friction on the taper would do it as well.  I'll find out someday when I get around to removing the pin.

*Do you turn the drawbar itself or a nut at the top of the drawbar? With TTS, I can conceive of either method, but with R8, I would imagine there is only one choice.

I'm using the stock drawbar, with a 3/4" 12-point socket on the PDB spindle.

*How do you control the torque on the stepper. Are there drives with that capability, or are you controlling the supply to the drive?

Any decent stepper drive will have current limiting that is controlled by the value of a resistor. Reduce current, and torque reduces more or less proportionally.  I set the current limit while tightening to a value that results in 25 ft-lbs drawbar torque, which is about 2500 pounds drawbar tension, when the drawbar and taper are lubricated (with a thin coat of anti-sieze), and the TTS shank and collet bore are clean and dry.  When loosening, I use full current, which give about 75 ft-lbs torque with the 400 oz-in stepper I currently have.  The current limit resistor is switched by a relay.

Regards,
Ray L.

324

General Mach Discussion / Re: BT30 spindle from scratch - with power drawbar and ATC of course

« on: July 26, 2012, 02:51:18 PM »

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.

325

General Mach Discussion / Re: BT30 spindle from scratch - with power drawbar and ATC of course

« on: July 26, 2012, 09:12:06 AM »

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.

326

General Mach Discussion / Re: Spindle speed sensing on a Bridgport

« on: July 25, 2012, 09:33:02 AM »

You can also do it quite simply by using a reflective optical pickup on the timing belt.  Just paint a section of the belt white.

Regards,
Ray L.

327

General Mach Discussion / Re: BT30 spindle from scratch - with power drawbar and ATC of course

« on: July 21, 2012, 09:50:25 AM »

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.

328

General Mach Discussion / Re: Help - Pendant for Indipendent Axles

« on: July 19, 2012, 02:48:35 PM »

Given that the MPGs effectively issue jog commands, and, AFAIK, you can't issue a jog command while a jog command on another axis is already in progress, I fail to see the utility of such a setup....  You'd still only be able to move a single axis at any time.  Sounds to me like a complicated, expensive solution in search of a problem....

Regards,
Ray L.

329

General Mach Discussion / Re: Australian suppliers (CNC)

« on: July 17, 2012, 12:03:11 PM »

Another vote for Peter Homann here.  I've been using his stuff for years, and it is first-rate, as it his support.

Regards,
Ray L.

330

General Mach Discussion / Re: Can someone explain how a capacitor acts as a filter?

« on: July 07, 2012, 07:54:49 PM »

The other thing you can try, which does not slow down the signals at all, is to add a stiff pull-up resistor at the BOB input.  On the order of 300-1K ohms should work well.

Regards,
Ray L.