Wrong RPM for G96 SFM

I am having a problem with the RPM two machines are running at in relation to the SFM command on the G96 line. It seems this is only happening with low SFM. For example when machining at say a 12" dia at 80 SFM my command line is G00 T0101, G96 S80 M08, G50 S350 M03. When I move the tool into the cutting position the RPM is running at 50 RPM. When I check the correct RPM by using this formula (12 x 80 SFM) / (Pi x 12" dia being cut) it should only be 25 RPM. I am getting round it by controlling the max RPM on the G50 line. This is ok when I don't have a huge variation from the starting point of cutting to the final dia being cut when using a G71 canned cycle but obviously doesn't work well when facing with a G72 cycle as the speed needs to increase the closer get to the centre of the job. Any ideas?

You probably have a fault with the spindle drive. Those things are big and nasty (high voltage) so get an authorized Fanuc repair guy to look at it.
if the RPM is wrong you could get around it by changing the SFM speed higher/lower.

I thought there would be a factor of 60 in the rpm formula. Am I missing something?

https://www.carbidedepot.com/formulas-turning.htm

if you put into the 2nd box (SFM Calculator) 80 SFM and D12 it gives us 25.465 RPM.
So his calcs are good....

I though you were an engineer with a Master's Degree and Doctorate who has written numerous books on CNC Programming....
hmmmm.
I guess you only copied and pasted the text without actually reading it.

You are actually right this time. I had not seen any formula. Was trying to calculate myself. Did not realize that 60 would cancel out because of per minute factor. This is high school math. Not a big deal.