Machining 4140

The most important rule in Machining is:

RPM equals 4 times the cutting speed divided by the work piece diameter.

The crusty old school machinists way back in the 1800's developed this simple formula along with many others. And they gave us the cutting speeds in feet per minute for various materials and tool bit types.

It works both forward and back ward. Use it to calculate the work piece rpm, in the above postings, the speed for a certain diameter shafting.

Or use it for calculating the tooling speed, such as for a 1/2" end mill in the vertical mill.

It is easy to derive this formula, I will put it up if any one wants to see the methodology.

Find the old South Bend Book "How To Run A Lathe" and read. Everything is in the book!

Like General George Paton said: "Rommel, I read your )*&^%$# Book"
 
If you want to get more exact, use 3.14 instead of 4. Sorry.
 
60 to 200 sfm with HSS cutters. 3-4 times that with Carbide.
 
4gsr said:
If you want to get more exact, use 3.14 instead of 4. Sorry.
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You know, it is guys like you that make us old timers reluctant to post anything. Pi was rounded off to 4 many years ago as the old timers wanted simple calculations to do in the head.

Anyway, even if you use 3.14 (which is not an accurate description of Pi) you will not have a gear in your lathe that will match exactly the calculated RPM.

A 10 to 20 per cent error in speed calculation really does not make any difference. It is the gross mistakes that the formula helps to avoid, like running at 500 RPM when you really should be nearer 100 RPM.

For your reading pleasure I have deleted my posts and I have placed you on my ignore list.
 
bluechips said:
To me if the chips coming off are blue you're turning too fast. I like when they come off silver to slight brown and turn blue in the chip pan, then they are carrying heat away with them... I want the chips to look like sixes and nines. Big fan of positive rake tooling with an adjustable chip breaker. Just old school I suppose...
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That's interesting coming from a screen name of bluechips. ;)
 
bluechips said:
To me if the chips coming off are blue you're turning too fast. I like when they come off silver to slight brown and turn blue in the chip pan, then they are carrying heat away with them... I want the chips to look like sixes and nines. Big fan of positive rake tooling with an adjustable chip breaker. Just old school I suppose...
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With HSS tools that is true, with carbide positive rake tools, tools survive longer with blue chips right off the cut. I am old school too!
 
A Machinery’s Handbook is a must, either printed which works even if the power goes out or the online version. Anyone can use an online calculator like this one. See below.
As the diameter is reduced the rpm can go up. After making 21 pounds of chips, I only had the sharpen the HSS once. I was going from 2” down to 1.135” and 1.02” prior to finish grinding. At 4” diameter any where from 50 rpm to just under 200 rpm depending on where in the range of cutting fpm you want to try.

http://www.custompartnet.com/calculator/turning-speed-and-feed
 
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