Deciding what milling parameters to use can be tricky task, especially for less experienced people like me. Most of the information regarding CNC milling are related to professional equipment and may be quite detailed, hard to follow and not quite applicable to hobby machines. When you use small hobby machine, you are usually on your own. While investigated this I gathered some info that may be useful for others too, so I am sharing it.
The most simple way to deal with setting milling parameters is targeting optimal chip load. Chips are particles of material that milling tool cuts from the body of the material. For quality cutting and keeping tool wear to the lowest it is necessary to match proper chip load. It is read from tables according to used materials and tools.
Procedure is simple. You read from table what is optimal chip load for material you are cutting with cutting tool you are going to use. Then you find out that feed rate and spindle rotation give you that chip load. All values you get are just approximation. They lead you to ballpark, but to get exact values you have to do some test cuts.
For easier use I created spreadsheet with table and calculator.
You may download it here:
ODS – Simplified CNC Milling Calculator (ODS) (1257 downloads )
XLS – Simplified CNC Milling Calculator (XLS) (1337 downloads )
How to use it:
- use provided table to find out what is recommended chip load for material you are cutting and tool diameter. Use minimum value for better results. If you need to speed things up you may go up to maximum value given in table.
- use calculator to experiment with feed rate and spindle RPM to find out what combination gives you requires chip load
- do not forget to set number of flutes of milling tool
- calculation is assuming you are using cut depth that match tool diameter, as that is optimal. If you use larger cutting depth chip load reduces: 2x tool diameter gives 25% of chip load and 3x tool diameter gives 50% of chip load). Pay attention. if you use hard materials, especially metal, you may have to reduce cut depth significantly below tool diameter.
- optimal plung rate is no more than half of the feed rate.
Parameters you get this way are approximation. It is advised never to start with exact calculated chip load, but to go for example 50% lower and then do test cutting gradually increasing load until you get it right. Not all machines and especially hobby machines can handle calculated cutting parameters. You have to start lower and increase load until it is safe for your machine or tool you use.
Document contains links to two sites I used to get chip load info. You may look there for information about larger diameter tools, which I did not included.
If you find other sources please let me know, especially if you get info about cutting with tools less than 3.175 mm.