Sunday, May 22, 2011

Cutter Radius Compensation

I do a lot of milling in my home shop with my CNC Bridgeport mill. This machine has a Centroid M40 control on it that works pretty well. It has a feature called 'cutter radius compensation' in it that is standard on most modern controls. This allows for variations in cutter size or cutting conditions such that the machine operator (me) can make the toolpath run closer or further from the part profile. Cutter radius compensation is a subject that is covered pretty well in cnc programming textbooks, so I won't get into it much here, except to mention a few things about programming to 'part profile' or programming to 'cutter centerline'. Over 20 years ago, I was taught to program a part to 'part profile' like this, when using cutter radius comp:

Using this method, it's easy to look at a blueprint and just write the program by hand. You can just enter the end points and center points of lines and radius' and add a G41 (or G42) to your code. Once the program is done,it's pretty intuitive to change the diameter value in the control to the actual diameter of the cutter and get a reasonable part milled. If your end mill is .499 inches in diameter, just enter that as the D value in your control. If you want to make the part a bit bigger all around, enter the value as .501 and 'trick' the control into moving the path away from the part profile. With just a little simple math, done in your head, it's easy to manipulate the size of the part.

After moving across the country, to the west coast, I was taught a different way of using cutter radius compensation. A part programmer can use a CAM system to make the cutter path the radius value of the cutter away from the part profile and then use very small values in the D value of the control to manipulate the size of the part.

The cutter value can be .000" to be 'on the line' and you can change it by .001" or -.001" to manipulate the part profile.
I guess you could call this the 'West Coast' method. Nowadays, I am using this method with my home shop mill. I've just gotten used to it. I'm using my Centroid post processor with HeeksCNC and things work well for this. In the post, there is a bit of code that adds a lead in line to the roll on radius. This is needed to give the control a bit of distance to move the cutter before it reaches an arc, while doing the cutter compensation. If I don't have a straight line on the first line of a crc move, the control gives an error. It works the same way on the lead out line just after the roll off radius. I don't think EMC2 works this way- I think you can start with a roll on radius with a crc command, without getting an error. This seems to be an exception in the cnc control world though. Most commercial controls expect a straight line.