Determining the proper rpm for a rotating tool for milling or drilling, or the proper rpm for a chuck when turning on cnc lathes, is a question many new CNC programmers are often faced with. Before calculating the RPM of the tool or part, here are two basic things you need to know:
- Material (Aluminum, Brass, Steel, etc.)
- Cutting tool composition (HSS, Carbide, Ceramic, etc.)
All cutting tools have an ideal cutting speed based on the material. Brass or aluminum cuts easier than steel for example. Cutting tool manufacturers have this information available through catalogs, tech support phone numbers, or possibly websites. If no other information is available to you, the Machine Tools Handbook will have some basic guidelines in the chapter devoted to “Speeds and Feeds” – http://search.barnesandnoble.com/booksearch.
Let’s start with an example for milling applications. Say you have a high-speed cutter (HSS) and want to machine 6061T6 aluminum. You can look in the cutting tool manufacturer’s catalog and find that you can use 250 surface feet per minute when you have a ½” (.500”) diameter end mill.
Here’s a quick and easy way to calculate the answer. The formula is:
Multiply 12 times 250. (Answer is 3000.)
Multiply the diameter of the tool times π (pi) ( .5 * 3.1416 = 1.5708 )
Take 1.5708 and divide that into 3000. Your answer should be 1909.85485
The proper RPM is: 1909
Tip!
If you do this a lot, enter this formula into a programmable calculator or an Excel spreadsheet as follows:
In cell A1 type the text “Enter Surface Speed”; in cell A2, type the text “Enter Tool or Part Diameter” and in Cell A4, type the text “Calculated RPM”. In cell B1 type the number 250. In Cell B2, type .500
Cell B4 is going to contain your answer. So type the formula in the cell like this: =(12*b1)/(b2*3.1416) and press the ENTER key.
You can “right mouse click” on cell B4 and click on “Format Cells” and choose the “Number” tab and change the decimal places to “0” so your answer is in even numbers.
That’s it! Save the worksheet and you can easily calculate proper RPM’s for yourself.
