"Dave, I can't do that" wrote in news:7de866b9-
:

Hi All,

Must be a senior moment but I am stumped.

1/2" dia. two flute carbide cutting SFM 850, cut per flute 0.002" it
calculates out to around 6500rpm at 26-IPM

1/2" dia. two flute HSS cutting SFM 425, cut per flute 0.002" it
calculates out to around 3250rpm at 13-IPM

No surprises there. However, I don't like running the the spindle at
top speed (3000rpm) so I tried to figure the IPM for say 2000rpm. My
thinking for carbide...

6500 / 2000 = 3.25
26 / 3.25 = 8 IPM at 2000rpm

OK, still so far so good, but using that same thinking with HSS

3250 / 2000 = 1.625
13 / 1.625 = 8 IPM at 2000rpm HUH???????????????????

Where am I going wrong here? The HSS should be half the carbide IPM.

Dave


You started out with .002" per flute for both carbide and HSS. You
calculated IPM based on two diferent RPM's

Then you made the RPMS the same by multiplying by DIFFERENT ratios.
That's where the percieved magic comes in. Nothing special is going on
since you started with the same cut per tooth for both HSS and carbide.

When you made the RPM the same for both (2000) the cut per tooth is
still the same (.002") so the IPM is the same (8).