A Handy Mnemonic ... And A Coincidence???

[Bill Currier]

After watching a Stuart Batty video on his 7 fundamentals, I started noodling on his comments about lathe speeds. As I understood him, you want to turn at a speed that perhaps makes you a little apprehensive, but definitely not scared, characterizing the difference as one that might result in a workpiece bouncing off a faceshield vs going right through the helmet. At one point he characterized that at about 40 miles per hour, surface speed, of course.

So, a quick spreadsheet later, I had surface feet per minute for 1" - 20" diameter pieces, 100 - 3600 RPM. I highlighted those in red that were over 40MPH ... which turns out is 3520 SFM! Now that's an interesting coincidence ... at least if you have a PM 3520 lathe. I'm not sure whether that's really a coincidence, but it's sure a handy mnemonic.

Anyone know the source of the PM model number itself?

(SFM = RPM x Diameter in Inches x PI / 12)
(SFM / 88 = MPH)

 

[Dave Bunge]

Anyone know the source of the PM model number itself?

Powermatic's lathe model numbering system is: first two numbers are the lathe maximum distance between centers (longest spindle you can turn) and the 3rd and 4th numbers are the swing (largest diameter bowl you can turn) both in inches.

So PM 3520 has 35" btwn centers with a 20" swing. The same system is used to name their other lathes.

Very interesting coincidence though, 40 mph = 3520 fpm!

 

[Joe Kaufman]

35" between centers, 20" diameter swing.

 

[Bill Currier]

Dave and Joe, that's actually funny because I knew that ... but forgot it while lost in the calculation clouds.

Thanks for the laugh!!!

 

[RichColvin]

You've given me a great way to remember it.