If you have an old machine you will know what I mean. Those little annoying marks where the machine changes direction. If CNC Machines were underpants then these would be the skid marks.

Circle interpolationexamples

When milling a full circle each time an axis changes direction there is a chance you will be left with a visible mark on the circle. These points are North South West and East on a compass.

G13 does everything. It’s the an all inclusive holiday of circular milling except you don’t gain weight and you’re not pissed everyday.

Circularinterpolationprogram example

For my American readers in the UK we call those annoying brown marks at the back of your underwear skid marks. I don’t think you get them in America.

Oh by the way if I saw a tangential move that big I would think it was a wasted move. (It’s just to demonstrate). You only need a short tangential move to do the job.

Circle interpolationcnc

The reason is that there is no axis change of direction and no animals are injured or hurt in any way during the process.

Just a note. If you do use circular milling to produce a bore just remember that if your cutter is not parallel then your bore won’t be either. So don’t use that crappy old re-ground cutter. Try n get the boss to buy you a shiny new one.

Circularinterpolationcalculator

Circle interpolationcalculator

One said he broke the circle into two halves. One said he copied some code he already had in the machine. The other one said he got the bloke who did it in two halves to do it for him. Oh and the other one used I and J and X and Y.

The easiest way to programme a tangential approach is like the ones above. Coming in from North, South, East or West on the compass.

G41 X20. Y0 D1 F200.    (Apply Cutter Compensation) G3 X-20. R20.                   (First Half) X20.                                     (Second Half) G1 G40 X0 Y0                   (Cancel Cutter Compensation)

Circle interpolationformula

How cool is that and you even get a tangential approach. At my age you’re glad of all the tangential approaches you can get.

CircularInterpolationprogram example PDF

No there is no sexual innuendo here it just means you approach the circle using a circular move. You sort of creep up when it’s not looking.

I have been working on a 12 year old Mazak Horizontal machine recently and was amazed at just how good a circle it produced. It beats the shit out of using a reamer. Oh and you can easily adjust the size with your D offset.

If you watch the movement above and watch carefully what the X axis is doing. It changes direction before it is in contact with the circle. And the same thing on the way out of cut with the Y axis.

Watch below to see where these points are. If you have a new machine there should be no visible mark on this change of direction.

Well the older and more inaccurate your CNC machine is the more likely it is that your machine will leave a mark at the entry and exit point. This is also dependant on the material and the size of the cut you are using.