Calibrated Spindle Speed - WAAAY Off!

[jderou]

After a very frustrating weekend trying to machine steel (first time with anything harder than aluminum), out of curiosity I thought I would check to see how close the spindle speed is on my BOSS 5.0. My setup involved a piece of plastic chucked in a fly cutter, and another piece in the vice (see pic). With the spindle running (two pieces of plastic hitting each other) I recorded the audio with Cool Edit Pro. Analyze the wave (count the peaks, after reducing the noise) and my spindle was off by a factor of 2!! i.e. 600 rpm was actually 1200! Anyone with a Varispeed I suggest checking this! To calibrate all you have to do is loosen the nut on the front of the dial and rotate the dial to the measured rpm.
No wonder my first endmill ended in a bright glow.
If you don't have an audio editing program, at low speeds (~100 rpm) you can just listen and count. Hope someone else finds this useful!

[zcases]

jderou - great idea. I was contemplating using a bicycle computer when I saw this......I used "Audacity", a similar freeware sound recorder/analyzer. I just verified the technique on my 8 speed bridgeport, and tomorrow I can fine tune my CNC machine, which uses a VFD to drive a varispeed head (Boss 5).

Thanks again
Scott

[widgitmaster]

Actually, on top of the head between the drawbar & motor is a screw with a lock nut!
It moves a leaver up & down to increase or decrease the spindle speed.
But be carefull, as the drive belt MUST have 100% contact with both pullyies!
I say this because it is possible to adjust the speed so high or so low that the belt starts to exceed the pully outer diameters! This will wear out the belt in no time flat!

I have used several different tachometers to test the speed range, and have also had to move the front dial to match the actual RPM!

Neet method you used tho!

Eric

[jderou]

Eric, you are right, since posting this I learned that the right way is to use the set screw at the top. I found this out because I did have the problem you mentioned.
The first time I used the audio technique we were trying to measure model plane propeller rpm. We were able to measure around 20,000 rpm without expensive equipment.

Joe