RPM range scaling

[pontiac428]

Dan,

I got my motor installed in my band saw, and it's fantastic. Smooth, powerful, and quiet- I'm pleased.

I put some tape around one of the wheels and counted 10x revolutions with a stop watch, 3x each, and averaged them at 3 different speeds.

This is what I got: (RPM/SFM), (500/24), (1000/97), (1500/198)

Which means the RPM being reported is not true RPM, and the relationship between displayed RPM and SFM is squared.

I do not expect to ever need to run my saw faster than 1500, so what happens to the steps in RPM if I change my scale to 500 RPM min and 1500 RPM max?

The part that gets me is the servo is equipped with a rotary encoder, why wouldn't it report true RPM? Why would you add a discrete RPM sensor, as you mention in the black box thread?

There is no question at this point that the conversion is going to work, I just want to make a reliable conversion chart for setting RPM. The square is hugely different than a linear regression, so how is the RPM signal handled in the control box, and how are the display step increments scaled?

 

[The Custom Crafter]

Dan,

I got my motor installed in my band saw, and it's fantastic. Smooth, powerful, and quiet- I'm pleased.

I put some tape around one of the wheels and counted 10x revolutions with a stop watch, 3x each, and averaged them at 3 different speeds.

This is what I got: (RPM/SFM), (500/24), (1000/97), (1500/198)

Which means the RPM being reported is not true RPM, and the relationship between displayed RPM and SFM is squared.

I have checked many of these motors with an optical tach, all have been within 2%
On the other hand, most digital displays on the lathes I test are off by 30+ percent. Usually related to missing magnets.
I have also seen measurements way off caused by secondary reflections from shiny black out paint as well as welds on blades and such.

I do not expect to ever need to run my saw faster than 1500, so what happens to the steps in RPM if I change my scale to 500 RPM min and 1500 RPM max?

If you set it this way:
500 is the slowest the motor will run
and
1,500 is the fastest the motor will run.

Something to consider when setting it at 300 to 900:
If you set it at 300 you may experience low torque due to variances in the resistance of the pot and the fact the torque seams to be adjustable in these ranges.
Instead set it at 380. It should still run at 300 but at full torque. Try setting it at 370 and 390 until it works correctly
Same goes for 500 versus 580 and as such until over 1,000 at which time there are no more incremental settings.

The part that gets me is the servo is equipped with a rotary encoder, why wouldn't it report true RPM? Why would you add a discrete RPM sensor, as you mention in the black box thread?

Adding a rpm sensor is for the spindle of the lathe. This is because most are set up at 1.6 to 1.
The first display shows motor speed.
The second display shows spindle speed.

There is no question at this point that the conversion is going to work, I just want to make a reliable conversion chart for setting RPM. The square is hugely different than a linear regression, so how is the RPM signal handled in the control box, and how are the display step increments scaled?

Without your machine in front of me, I cannot determine what is really happening.
However I use a different method to calculate blade speed Feet Per Minuet.
Its pulley ratio and circumference.

 

[den-den]

Band wheel rpm and blade surface feet per minute have a linear relationship, multiply the rpm by the circumference (in feet) to get the surface feet per minute.