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Author Topic: BT30 spindle from scratch - with power drawbar and ATC of course  (Read 411880 times)

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Offline BR549

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #260 on: October 07, 2012, 12:48:47 PM »
For those that have the R8 and a ton of tooling you may want to consider the MACH1 tool system. It is as close to an auto system as you will find for the knee mills. AND note it does NOT use beveled washers (;-) IT also uses your standard R8 tooling with a simple mod.

Probably could be used with CNC as an auto changer IF your spindle axis drive was strong enough.

(;-) TP
Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #261 on: October 07, 2012, 04:10:21 PM »
I'd like to offer one other variable that may play into this equation, and that is the thermal effect.
Loading a chilly holder into a toasty spindle.
The expansion / shrinkage of the mating materials immediately following the tensioning of the draw bar would vary the final values ... would it not ?
I've certainly seen this effect with the #5 Morse in a radial drill. Granted, the shank surface finish wasn't pristine but required substantially more effort to remove.
Discard if irrelevant,
Russ

Offline BR549

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #262 on: October 07, 2012, 04:18:11 PM »
They call that shrinkfit. I have seen one spindle that came close to being scrapped because the OP could NOT get the tool holder back out.  I had to wrap a heating strip around the end of the spindle/toolholder let it get hot THEN pack the tool holder with dry ice to shrink it and it THEN popped out of the spindle with a bit of persuasion.  Lucky it was an NMTB40 spindle with a through hole draw bar.

(;-) TP

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #263 on: October 08, 2012, 09:04:09 AM »
If you were looking for grip/slip value per spindle type you would use the available TORQUE value not HP. 

Torque and HP are essentially the same thing. HP is just torque over time, and the relationship is linear.

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #264 on: October 08, 2012, 09:09:18 AM »
I'd like to offer one other variable that may play into this equation, and that is the thermal effect.
Loading a chilly holder into a toasty spindle.
The expansion / shrinkage of the mating materials immediately following the tensioning of the draw bar would vary the final values ... would it not ?
I've certainly seen this effect with the #5 Morse in a radial drill. Granted, the shank surface finish wasn't pristine but required substantially more effort to remove.

Absolutely. The adapter will expand and the spindle contract as the temp equalizes, creating (or in this case increasing) the interference fit.

In fact, this is the most common method for assembling and disassembling press fit parts.

Offline Dan13

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #265 on: October 08, 2012, 09:42:58 AM »
Torque and HP are essentially the same thing. HP is just torque over time, and the relationship is linear.

Not exactly. Power is torque times rotational speed. And saying that the relationship is linear is a bit confusing as the speed (RPM) is not a constant itself. Take a stepper motor for instance - peak torque is at zero RPM, where the power is zero, while peak power is at some mid point RPM where torque is substantially less than its peak value.

Hence, the way Terry put it, is the correct way. You are essentially transferring torque. Power has absolutely no meaning here. I could have a 6kW high speed spindle putting out the rated power at 30,000RPM, which means only 2Nm torque, whilst a tiny NEMA 34, 25 watt stepper would have twice that torque at zero speed. Power is the amount of work you can do over time, so obviously the 6kW spindle would do more work in a given time at its lower torque than the high torque stepper.

Dan
« Last Edit: October 08, 2012, 09:45:19 AM by Dan13 »

Offline BR549

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #266 on: October 08, 2012, 10:23:14 AM »
YEP Dan has it right. Torque is the measure of FORCE. HP is a measure of work over time.   HP is a calculation and torque is a true measurment.

Another part of the spindle equation is RPM and balance.  The inertial value of the imbalance squares as the rpm doubles making balance very important for high speed spindles.  It can add to the drawbar pressure equation.

(;-) TP

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #267 on: October 08, 2012, 11:14:26 AM »
Torque and HP are essentially the same thing. HP is just torque over time, and the relationship is linear.

Not exactly. Power is torque times rotational speed.

That is correct, Dan. However, consider what rotational speed is. It is revolutions per MINUTE. It is time in the HP equation.

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And saying that the relationship is linear is a bit confusing as the speed (RPM) is not a constant itself.

Sorry to be confusing. I make the mistake of assuming everyone is on the same page with the terminology. A 'linear' relationship means that the calculation is the same at every point in the curve.  It does not mean that the power output of any specific device is linear. For example, the formula for HP is the same if the stepper is running at 2PRM, 20PRM or 200 RPM. 

Put another way, the 'torque curve' may be dropping like a rock with the stepper motor, but the relationship between HP, torque and speed is the same no matter where you measure or calculate it.


Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #268 on: October 08, 2012, 11:26:40 AM »
A 'linear' relationship means that the calculation is the same at every point in the curve.

Actually....  conventionally, a "linear relationship" means a graph of the relationship would yield a straight line.  For example, Y = 3 * X is a linear relationship, while Y = 1 / X is not.  Both *are* "linear equations", since neither contains any exponentials, logs, trig functions, or or non-linear terms.

Regards,
Ray L.
Regards,
Ray L.

Offline simpson36

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Re: BT30 spindle from scratch - with power drawbar and ATC of course
« Reply #269 on: October 08, 2012, 11:39:18 AM »
YEP Dan has it right. Torque is the measure of FORCE. HP is a measure of work over time.   HP is a calculation and torque is a true measurment.
This is semantics. You are just saying the same thing in a different way. I don't know how you define a 'true' measurment. If you put a motor on a dyno, you are measuring toque and you are also measuring speed (time in the equation).

However I will add that torque is also a static force. I doubt you have a way of directly measuring torque in a practical way with a machine spindle, However, you CAN use the motors HP power curve to calculate the torque at any point. Since torque is what generates the stress on the mechanism (which I think was your point), then you can assess the effect, but you would get the torque out of the HP calculation . . which is linear.

If the manuf provides HP curve and also the torque curve, then there is no need to know the formula and the relationship is irrelevant, but inquiring minds may still want to know how it all fits together.

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Another part of the spindle equation is RPM and balance.  The inertial value of the imbalance squares as the rpm doubles making balance very important for high speed spindles.  It can add to the drawbar pressure equation.
Agreed. Certainly the lateral forces rise exponentially (as opposed to linear) as the speed rises. Recall that I found two separate drawbar specs for BT30 differentiated by RPM. Incidentally I would point out that the drawbar tension is force and not pressure, but that would be nit picking, don't you think?  ;)