[tbaker2500]

I've been experimenting with some higher-power cuts recently, and lost a tool or two. But I'm curious about horsepower requirements as reported by G-Wizard, and what people actually see at their spindle.

1100 Series 1 machine, early 4-digit serial.

Problem cut: 1/2" Atrax carbide end mill, 2 flute 30deg, uncoated. 6061 T6. 0.225" depth of cut, full width. (Starting a pocket)

G-Wizard recommends on setting "3" , 4500RPM (max) at 36ipm, resulting in a horsepower of ~0.84. The motor stalled out after about 1" of cutting. I'm running flood coolant, and the chips seemed to clear just fine.

It seems to cut good at setting "2", 4500rpm at 27ipm, resulting in a horsepower of ~0.635.

This implies that either:
A. I'm getting about 1/2 rated horsepower at the spindle, or
2. Something else that I didn't see was overloading that cut.

Is this similar to other PCNC owners' experiences?

Thanks!
Tom

[philbur]

Horse power calcs are very approximate. They just put you in the ball park.

Phil

PS: Whats the available horse power at the cutter when running at 4,500 rpm.

[Doubleoh9]

I have never had any luck using GWizard on 2 different series machines. The math is cute, but so many other factors play into feed rate. I tried using it for a while, using conservative settings and broke tools. It always seemed to calculate feed rates that seemed too fast based on the old fashioned hand calculated methods. I would have to see the HP curve as a function of rpm to trust that I am getting the rated value. It seems to come down to tool quality, rigidity of work holding, etc.

[Scott_M]

You will make rated horse power at 60hz. On my series 1 machine with spindle upgrade that is about 2200 rpm in high range. Anything over 60 hz will result in less horse power. 4500 rpm is probably up around 120 hz.
I would plugin 2500 rpm for you max spindle speed and see what it says then. It will make the rated HP there. Your feed rate may go done but you will be able to take a deeper cut so MRR may improve.

Scott

[philbur]

I thought VFDs basically gave constant torque below the rated speed and constant horsepower above the rated speed?

Phil

[tbaker2500]

I have never had any luck using GWizard on 2 different series machines. The math is cute, but so many other factors play into feed rate. I tried using it for a while, using conservative settings and broke tools. It always seemed to calculate feed rates that seemed too fast based on the old fashioned hand calculated methods. I would have to see the HP curve as a function of rpm to trust that I am getting the rated value. It seems to come down to tool quality, rigidity of work holding, etc.

Actually, I've had nothing short of fantastic success with G-Wizard over rolling my own out of the Machinery's Handbook. It takes into account so many other factors that contribute to tool breakage, especially on small, like 1/8", bits.

But horsepower is something I've not dealt with in great depth before, I've been using small tools with light cuts.

[tbaker2500]

You will make rated horse power at 60hz. On my series 1 machine with spindle upgrade that is about 2200 rpm in high range. Anything over 60 hz will result in less horse power. 4500 rpm is probably up around 120 hz.
I would plugin 2500 rpm for you max spindle speed and see what it says then. It will make the rated HP there. Your feed rate may go done but you will be able to take a deeper cut so MRR may improve.

Scott

This is new to me. I should learn more about how a 3-phase motor works, but I was guessing that max RPM=Max HP.

This is a series 1 machine, is this a VFD drive?

Thanks
Tom

[pete from TN]

I had always found that G Wizard also gave me much faster speeds than I was comfortable with using my belt driven 2hp sensorless vector driven rf45 machine. I know that you can actually set it for a specific max HP. I find that if you set it at 1hp for my machine I get some relatively reliable numbers and so far most of it I can use having tested it in aluminum, stainless, and even tool steels. It can be a bit scary at times but as long as you are clearing chips and running clean with a rigid setup on your machine it USUALLY works. I have no idea what a Tormach puts out for HP but if you play with the actual setting on G wizard I am sure you will find a way to make it work for most situations. Peace

Pete

[Scott_M]

My series 1 motor is rated at 1.1kw at 60Hz @1715rpm. 1100w / .746(746w per hp)=1.47 hp. If you go over or under 60Hz you will lose something. You can't get something for nothing. The curve below 60Hz is flatter but over 60 it drops off faster.
With the added ratio of the pulleys 60 Hz is 2200 rpm. I only have anecdotal evidence to back this up but I have done quite a bit of testing with my 2.5" facemill and at 2200 to 2500 rpm I can take some pretty serious cuts. At 3000 rpm and up I really have to start backing off or it will stall.


whoops forgot to say that on the VFD upgrade( series II and up) there is an LED readout on the VFD to show the frequency it is supplying. At max rpm it is up around 140Hz. Again the motor is rated at 60Hz.

Scott

[philbur]

Here's a quote from the Tormach White Paper on VFDs'.

All drives behave differently above the motor’s base (normal) speed than they do below base speed. Below base speeds the motor provides constant torque but above base speed the motor acts as a constant power device.

Above rated speed you get constant horsepower, so by definition you lose torque.

Below rated speed you get constant torque, so by definition you lose horsepower.

It is my understand that this is the accepted, basic definition of how VFDs work.

With your anecdotal evidence did you increase the feed rate with the increase in rpm. This would up the material removal rate which would require an increase in horsepower. If you where in the constant horsepower zone stalling would be a possibility.

Also I guess even if you kept the same feed at the higher rpm you would be chopping the chips into smaller pieces so possibly this would require some additional horsepower?

Phil

My series 1 motor is rated at 1.1kw at 60Hz @1715rpm. 1100w / .746(746w per hp)=1.47 hp. If you go over or under 60Hz you will lose something. You can't get something for nothing. The curve below 60Hz is flatter but over 60 it drops off faster.
With the added ratio of the pulleys 60 Hz is 2200 rpm. I only have anecdotal evidence to back this up but I have done quite a bit of testing with my 2.5" facemill and at 2200 to 2500 rpm I can take some pretty serious cuts. At 3000 rpm and up I really have to start backing off or it will stall.

Scott

[Scott_M]

With your anecdotal evidence did you increase the feed rate with the increase in rpm. This would up the material removal rate which would require an increase in horsepower. If you where in the constant horsepower zone stalling would be a possibility.

No. Same feedrate. But to get it to finish the cut I had to reduce feedrate or rpm.
I also have the load meter on my spindle and it agrees. The above cut in question @ 2500 will run in the green but the cut @ 3500 will be in the red and stall rather quickly.

Scott

[Andre' B]

No. Same feedrate. But to get it to finish the cut I had to reduce feedrate or rpm.
I also have the load meter on my spindle and it agrees. The above cut in question @ 2500 will run in the green but the cut @ 3500 will be in the red and stall rather quickly.

Scott

Every load load meter I have see on a CNC machine is based on motor current which is a measure of torque and not horsepower.
Also the higher the speed the more power gets used up in the bearings, so less is available at the tool.

[Zetopan]

Here is some information that may assist people trying to understand the HP
and torque ratings of AC Induction Motors. Note that I have a late Series I
PCNC 1100 with the spindle drive upgrade, so some of these below numbers
will differ for those who have the original Series I spindle driver. The actual
spindle speed calibration also has a small influence.

At 60HZ and 240VAC RMS drive the Tormach spindle motor is capable of its
maximum HP rating. This rating also requires about 4.5A RMS of current and
it produces a spindle speed of about 2400 RPM (the actual value depends
on your spindle speed calibration). When the spindle is not producing the
maximum torque at 60 Hz, the current automatically reduces because the
slip factor reduces (I will not explain why here).

The current is a function of the frequency, voltage, winding inductance,
and the load. At slower speeds both the drive frequency and voltage are
reduced. The reason why the voltage is also reduced is because as the
frequency drops the voltage must be reduced to keep the same current
in the windings. Keeping the voltage constant would result in exceeding
the current ratings of the motor (and eventually the motor drive).

Since torque is proportional to the current, when the motor is operating
below its base frequency (60 Hz here) it is running in a constant torque
region. Since HP is torque times RPM (times a constant, ignored here)
the HP output of the motor drops as the motor speed is reduced below
the base frequency.

When the AC motor is operating above its base frequency the winding
voltage must be increased proportional with the frequency to keep the
current constant, but doing that would exceed the maximum HP ratings
of the motor. As a result the voltage remains constant and the motor
operates in a "constant" power region.

However, as the frequency increases the motor (and bearing) losses also
increase, and if we do not increase the voltage to offset those losses we
experience a loss in HP as the speed increases above the base frequency.

Since the motor drive is limited to the 240 VAC RMS supply voltage we do
not have additional voltage available to offset the HP losses in the system
and as a result the HP will only be approximately constant above the base
frequency, actually reducing as the speed increases.

This is why at the minimum and maximum RPMs AC Induction motors that
are driven by Variable Frequency Drives will always have less HP available
than when operating at their base frequency. There is nothing here that
is specific to Tormach, it is simply how AC induction motors operate when
driven by VFDs.

[tbaker2500]

Thank you, Zetopan, and thanks to everybody who is trying to help me understand this. This is quite informative.

Reading Tormach's document here, they state that with the VFD they tune it to limit the max speed right around when the back EMF starts to reduce HP.
I think it's time I went back and calibrated the VFD again, in case I have it spinning the motor too fast.

It sure would be nice to have publish HP curves for these machines, but I guess experimenting is the next best thing. :-)

[tbaker2500]

Just a follow-up on this:
I backed the cut down to 27ipm @4500rpm, and had enough power. G-Wizard calculated that at about 0.635hp. The RPM bogged slightly in the middle of the cut, but not bad. This leads me to believe that for this cut, I need to keep the G-Wizard calculated HP under 0.75hp. Interestingly, This correlates to a 1" BN cut I've been tweaking, at around 2200rpm, it seems to not bog down when under the G-Wizard calculated hp of around 0.75 also.

So I'm sure different cuts and calculations vary, But I'm going to keep my cuts under the calculated hp of 0.75. Until I get adventurous again. :-)