Measuring RPMs…

[tmac1956]

david & all:

I need to get some RPM measruements on my min mill drill and lathe, so I’m looking at getting an analog hand held gage for this purpose. So… would one of the old Starrett devices suffice? Here’s one from eBay but they have plenty for these available at any given time:

http://www.ebay.com/itm/Antique-STARRETT-NO-100-Speed-Indicator-Pat-March-28-1905-Tool-/111241222917?pt=LH_DefaultDomain_0&hash=item19e67e5b05

Any advice?

Thanks!
Tom

[david pierce]

Tom,
I think Starret introduced that tool before electricity was invented. I always saw it in their catalog but never knew anyone who bought one. Also, I never knew or heard of a machinist who gauged the cutter speed that way. Even on the CNC equipment the final feeds and speeds were determined by the way the chips looked , the way the part behaved while being cut and the way the part looked when it was finished. In the pivot cutting video you provided a link for, did you notice how slowly the spindle was turning. That RPM was set through experience and learning from previous failures.
david

[arutha]

Tom,
it just so happens I have one of those! It came in a box of bits I bought at auction and I had no idea what it was until a friend told me. It does the job and lets you know what the rpm’s are but as David says, over time you will just know where you should be with regard to speed. In the meantime it will be quite handy and should speed up the learning process as long as its not too expensive.
Paul.

[tmac1956]

All:

Thanks for the great information and really, it makes sense. I already fell into that on my lathe, but I just figured that I was doing it wrong. On the lathe, I was mainly wanting to check my calcs, but the mini mill only has min./max. speeds or low and high so I’ve just been going by the size and types of chips that I’m getting. I’m doing that on the lathe too, but I do have an idea on that machine.

Being an x-musician I’m used to improvisation. I don’t mind going out on a limb – I just don’t like falling off.

Thanks guys!
Tom

[david pierce]

Tom,
Years ago when I would cut aluminum I would crank the RPMs way up and run the feed at 10 to 15 inches per minute. It would shoot a spray of chips out about 6 to 8 feet. This took a large sturdy machine with a lot of horsepower. It was almost like routing wood. Steel would run much slower at 3 to 4 inches per minute with a slower rpm.
david

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