Spindle RPM again! It finally dawned on me how to easily get accurate spindle RPM

I had initially used published charts, of course they were not accurate for "MY" pulleys etc....
I also found one well published chart from a tool manufacturer with "Transcribed Errors" BIG ONES!

I found at LOW rpms, 80-90 I could just put a finger on the chuck and count the passing adjuster for one minute but this did not help for the other higher rpms.
I could not count that fast!!!!!

That being said, I am a stickler for finding the true results of ANYTHING mechanical like this.
I wanted to KNOW the spindle RPM for ALL my gears SB 10K 12 speed.

If finally dawned on me and "I'm SURE" other but thought I would share anyhow.

If you have a gear box and can "setup" known THREADS PER INCH (revolutions per inch) you can easily and have fun figuring it out yourself!

My explanation and examples follow.


I set my gear change box to 20 threads per inch.
Engaged halfnuts and timed for a distance. Measured from tailstock.
Length of time does not matter but LONGER DISTANCE is more accurate!
If you CAN go for 1 minute then RPM's equals inches traveled X TPI!
Measure the distance
Apply Time vs Distance formula against threads per inch (revolutions per inch)

Formula:
(Measured Distance Carriage Traveled) X (Threads Per Inch) = Total revolutions
(60 seconds) / (Time Of Carriage Movement in seconds) = Time Factor
(Total Revolutions) X (Time Factor) = RPM

EXAMPLE:
Measured Distance Carriage Traveled = 24”
Time of Carriage Movement = 45 seconds
Threads Per Inch = 20TPI
24” X 20TPI = 480 revolutions.
60 / 45 = 1.33 (TIME FACTOR)
480 X 1.33 = 638.4 RPM

Or if carriage movement time was 2:00 min (120 sec)
24” X 20TPI = 480 revolutions.
60 / 120 = 0.5 (TIME FACTOR)
480 X 0.5 = 240.0 RPM

This is not complicated and after figuring a couple I could just do them on my phone as I tested.

I felt stupid for not realizing this early on.
Some people have used/bought instruments to get RPM live well..... no need really!

Hope it helps others.
Not that RPM is the end of the world but it's NICE TO KNOW "what gear to change to" to be in the desired cutting speed.

Thanks...

Why are we so wound up about spindle speeds? Machining velocities (linear velocity at the tool point) change with the diameter of the piece as you machine it. Machining speeds are specified as a range. Unless you have precise rev counting along with a feedback control system you can't hold the linear velocity to an exact number anyway. Your machine will tell you when you're going too fast.

you didnt factor in leadscrew error.
or temperature...that will affect the above.
load matters too...real world conditions...
the world is made of rubber...everything is constantly changing.



ps - good on ya for using your noggin...but yes, absolute spindle rpm is not critical...not on an old belt driven machine...the actual cut is what matters.

You do realize they make little hand held digital tachometers? If I'm feeling like re-living the 70's I break out the Strobo-tach and put on some groovy disco tunes

Or you could buy this for less than $6:



L.S.Starrett Tachometer RPM Indicator Gauge, Good Pre Owned Machinist Tool. | eBay



Starrett, Fowler, etc. tons of them out there. Been around for 100+ years. Range from $6 to $20 on Ebay.

Best Regards,
Bob

Dobermann said:

Why are we so wound up about spindle speeds?.....

Click to expand...

This and plus.
What is the "right" surface footage for what you are doing? Some chart or internet recommendation?
And why are these always round numbers like 80-300-600?
Is 62/282/673 better?

While a good idea your process is maybe flawed by the ability to accurately measure time and distance traveled.
Kiss your stopwatch and scale goodbye here.
It will put you in the ballpark so with no other reference as to where I am..... it is a great idea and very good thinking
Bob

rjs44032 said:

Or you could buy this for less than $6:

View attachment 278152

L.S.Starrett Tachometer RPM Indicator Gauge, Good Pre Owned Machinist Tool. | eBay



Starrett, Fowler, etc. tons of them out there. Been around for 100+ years. Range from $6 to $20 on Ebay.

Best Regards,
Bob

Click to expand...

How do these work? I was given one in a box of do-dads.

You use it with a watch second hand. As I recall, each revolution on the gauge is 100 RPM and individual RPMs are marked on the dial. So you can count the Revs and additional RPMs after a minute of time, or longer for more accuracy.

I don't remember if you just run it to zero or if there is a way to zero it. Either way simple and accurate. More info and vids on internet.

Best Regards,
Bob

Because it's there to solve for the fun of it!
Plus I have a VFD and it's nice that it reads the correct RPM

It is really quit accurate.
12 speeds all "individually" recorded and yet they all calculate to the same geared percentage by 0.02% That's close!

What fun is that?

Dont forget load vs no load speed on AC motors.I have two mechanical(Jones and Starrett) and an old Japanese made electronic tachometers. No load speeds are more than 10 percent higher than the plate speeds on my 13" machine,and it has a 3/4 horse motor.Smaller than recomended.The reason is known as the motor's slip.A four pole motor has a synchronous speed of 1800 RPM they are typically marked 1725 RPM.Thats what they do with full load.If you are modifying a little brass screw on a SBL the motor will probably be turning close to 1800 RPM.The extra speed is good for those little brass screws on the 13" machine.

Mr. Pete just had a video on this.
YouTube
I suggest you watch it. Good information.

Already had.
I always do my homework.
Not spending ANY money and can't get on a Sunday so I figured out a system that works.

Already understood.
If you use feed the feed slows with the load (rpm drops) What? Who type this "so feet per minute remains accurate"?

What?? That just doesn't make any sense. If RPM drops so does linear velocity at the cut so the cutting velocity slows with RPM. The amount of material removed per revolution remains the same but the amount of material removed as a function of time (horsepower) goes down as well. I fail to see the point of the discussion.