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Author Topic: BT30 spindle from scratch - with power drawbar and ATC of course  (Read 411887 times)

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Offline BR549

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #10 on: July 26, 2012, 06:18:18 PM »
3000 @7hp is really not a good combination, at 3000 rpm you are still limited to a certain chip load so the extra HP does not add much to the combination. You tend to just generate more heat that you cannot get rid of easily.

IF you set up the spindle with low drawbar pressure AND HIGH torque loads on the cutter you will probably RATTLE the tool holder in the spindle under heavy loads

Now gear it 2:1 then at 6000 rpm you are getting somewhere. BUT even that low your bearings, Sindle bearing alignment, grease and preload better be spot on(;-)

R8 collets now that is an exercise in anger management(;-)


Just a thought, (;- )TP


« Last Edit: July 26, 2012, 06:24:09 PM by BR549 »

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #11 on: July 27, 2012, 04:39:14 AM »
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.
Yes, I remember those threads. It was a bit like discussing religion. Required more patience that I can muster.
Quote
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.
I have a few questions about your machine if you don't mind sharing on a forum.
*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.
*How do you prevent the collets from rotating in the spindle nose?
*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.
*How do you control the torque on the stepper. Are there drives with that capability, or are you controlling the supply to the drive?   

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #12 on: July 27, 2012, 05:19:56 AM »
Just a thought, (;- )TP
Your thoughts are always interesting and welcome. What I am hearing is that there are a lot of practical examples to consider. It all boils down to 'where the rubber hits the road' or in context; 'where the tool hits the workpiece'. Everything before and after is academic.

7HP is peak, so I have to design for that amount of power, even though it is momentary. Chiploads would be calculated on continuous power, which is considerably less. The motor is rated in the neighborhood of 7 amp cont. 

The question of drawbar tension is also academic with regard to 'how much is enough', so why not just go high and 'play it safe'? Once again there are practical considerations, in this mechanism there is a 'jesus nut' which is the pull stud. These little guys are stupidly expensive in spite of the tiny bit of material and very simple geometry. The cost is in the testing of each part. Fatigue life is probably too big a topic for a hobby forum, but suffice to say that any metal part that is repeatedly stressed and then released has a fatigue life. Airframes are retired after a certain number of hours for that reason and so *should* pull studs.  The relationship between the amount of stress and fatigue life is not linear.

An analogy that most reader can identify with is fuel consumption. If you drive 60 instead of 50, you might expect to use 20% more fuel for the 20% increase in velocity, but that's not the case. And the delta in fuel consumption between 50 and 60 is not the same as between 90 and 100, even though the delta in speed is the same. Travelling at very high speed has benefits and costs associated with those benefits. There is a lot of focus on 'too low' drawbar tention, but there is also a 'too high' drawbar tension which ultimately can do much more harm.

Just a thought back at ya, (;- )   SS
Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #13 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.
Regards,
Ray L.

Offline BR549

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #14 on: July 27, 2012, 11:33:54 AM »
You can never have TOO much drawbar tension(;-). But you do have to consider design parameters. A real pull stud(stud only) is designed for up in the neighborhood of  100,000 lbs of drawbar pressure before failure. The upper end level depends on the material and design used before the stud stretches or the drawbar stretches and releases the pressure and the tool holder.

The other side of the design is releasing the high drawbar pressure. Also fitting in the Very TALL spring stack as high pressure calls for a very tall stack to get pressure and overall travel.

The overal weakness of MOST power drawbar systems is the SPRINGS that retain the pressure. The more you use them the sooner they loose tension or Break springs. Not unusual to change out stacks twice a year on 24/7 machines .

I would personally shoot for the 900-1000lbs level.

You mileage may vary, (;-) TP

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #15 on: July 28, 2012, 05:44:30 AM »
The current limit resistor is switched by a relay.

I am aware of the resistor set power limits, but a couple of your descriptions lead me to believe you were changing the force in real time. Some of the industrial drives have commandable parameters that can be set 'on-the-fly', but I recall that you were using Gecko (or equiv), which do not have this capability, so I suspected that you had something more up your sleeve. 

Relay switched resistors . .  very clever indeed.  8)

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #16 on: July 28, 2012, 06:15:17 AM »
You can never have TOO much drawbar tension(;-). But you do have to consider design parameters. A real pull stud(stud only) is designed for up in the neighborhood of  100,000 lbs of drawbar pressure before failure. The upper end level depends on the material and design used before the stud stretches or the drawbar stretches and releases the pressure and the tool holder.

An interesting statement considering the range of sizes these things have. My calculator has you off by an order of magnitude for the cross section of a BT30 stud . . . and that would be with a zero safety factor. This is one for the 'agree to disagree' file.

Quote

The other side of the design is releasing the high drawbar pressure. Also fitting in the Very TALL spring stack as high pressure calls for a very tall stack to get pressure and overall travel.

The overal weakness of MOST power drawbar systems is the SPRINGS that retain the pressure. The more you use them the sooner they loose tension or Break springs. Not unusual to change out stacks twice a year on 24/7 machines .


Working with only  'Catalog knowledge' of Belleville springs, and having no resources to draw from, I am in WAG mode for this aspect of the design. I am aware of the tall stacks of springs. I was thinking the travel could be had by simply using a few large diameter springs at the top of the spindle rather than a long stack of small diameter springs inside the spindle. However, I have only seen the guts of a couple of spindles, and so far none have taken this approach. Have to wonder why.  Other than redundancy, it is hard to find a major advantage to the tall stack.  It there a hole in this logic?

Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #17 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.
Regards,
Ray L.

Offline BR549

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #18 on: July 28, 2012, 10:20:39 AM »
One too many zeros I am afraid. Ray has it covered on spring stacks. To get  pressure AND needed travel it takes a STACK of bevels to get it all done.

(;-) TP

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #19 on: July 28, 2012, 04:55:00 PM »
Thanks again for the replies.

I had not considered the disc-to-disc friction on the Bellevilles . . .  certainly a consideration.

Although in the final analysis it may be the only workable solution, the whole belleville stack idea just does not appeal to me. Is there some reason that, considering the small size of the BT30, one could not use a valve spring. Not from Grandma's Chevy, but a racing spring or a diesel spring.  I have not looked at diesel springs yet, but Racing Valve springs are available at 750lb rate with over 1,100lb open pressure and 1" travel. These are meant for about 350lb seat pressure, but the gripper needs only about .23" movement to release so that would leave .77" compression on the spring for about 750*.77 + 350 or approx 930lbs constant pressure and 1,100lbs max at full release. This seems doable with a pocket in the top of the spindle to center the spring and just using the racing spring retainer that would normally be used with the spring.

The relatively small diameter and mass of the coil spring being held at center I do not *think* would pose a balance problem, but the spindle would be balanced anyway, so that's not a concern.  At that high compression, the coil would loose tension over time, but I would wager it would be an improvement over replacing bellevilles twice a year.

Reliable, predictable, balanced, frictionless linear pressure. For the little BT30, it seems very doable. Have I missed another bole in the logic?



« Last Edit: July 28, 2012, 04:57:02 PM by simpson36 »