Help with I and J please!

[aceofspades25]

Lets say I am cutting a radius with G3 or G2. I know that I(X) and I(Y). So I and J always equal the signed distance from the arc start point to the arc center point and it is incremental. But I'm not getting it. Can someone explain it to me in laymens terms so I can understand it better. I'm sure its something simple I am missing. THANKS ALL FOR HELP

[ger21]

There are two different I,J modes.

Relative I,J mode:
G1 X1 Y1 (move tool to 1,1)
G3 X0 Y2 I-1 J0 (move along arc to 0,2 center of arc is 0,1. I and J are relative to the start of the arc (previous line of code), so I is 0-1 = -1, J is 1-1 = 0

Absolute I,J mode:
G1 X1 Y1 (move tool to 1,1)
G3 X0 Y2 I0 J1 (move along arc to 0,2, I and J specify the actual center point of the arc)

Absolute is probably a bit easier to visualize.

[HuFlungDung]

So what don't you get, Ace? Your explanation sounds okay.

Do you understand absolute, versus incremental coordinate systems?

Do you understand how right triangles are used to derive the X and Y components of a hypotenuse (vector) lying at any angle in the Cartesian coordinate system? The XY table of your machine is a Cartesian coordinate system.

[aceofspades25]

I'm coming from x-.389 y.921

[aceofspades25]

next line is G3 x-1 y0. I am having a hard time figuring out what I and J I use from there. I am using a cnc simulator and everything I am using is coming out all funky lookin.

[aceofspades25]

Yes I understand absolute and incremental. What do you mean with the right triangles? Am I going to have to trig out the center point of the arc?

The way I see it I after g3 X.. Y.. for I and J I go the incremental distance from starting point to ending point. I don't know I need to get this soon because it is driving me crazy and making me ill. I'm sure when I figure it out I will kick myself in the butt.

[aceofspades25]

I went to this website and I understand it a little better now. Does anybody know a good website that will show it in a drawing and do it step by step.

[HuFlungDung]

Yes, you have to trig out the center point of the arc. Some controllers will alarm if the center point location is off by as little as .0001". And others will make a funky looking correction to get to the end position.

The current position of the machine is always the starting point. The XY point named in the G02/G03 is the end point after the arc has been traversed. The arc center has only one theoretically correct coordinate relative to the start and end point.

Incremental arc centers are easier to calculate because they always work from the current start point, and the named end point of the arc. Absolute arc center coordinates are a nightmare to figure out when your start point if anywhere except X0Y0 and most controllers do not use this option. If you have to use absolute arc center coordinates, you basically have to calculate ("trig out") the arc center in incremental, and then add in the X and Y offset from the part X0Y0.

[aceofspades25]

Thanks I got it now. Whew! I was almost ready to break something.

[oldjohn]

Originally Posted by aceofspades25 Yes I understand absolute and incremental. What do you mean with the right triangles? Am I going to have to trig out the center point of the arc? The way I see it I after g3 X.. Y.. for I and J I go the incremental distance from starting point to ending point. I don't know I need to get this soon because it is driving me crazy and making me ill. I'm sure when I figure it out I will kick myself in the butt.

Of course you will. You need trig to work out arc centres blending two lines that do not form 90 deg angles. You have to find the right triangles. But that's were the fun part of NC Programming is. Unless you use cam software that will do it for you.
John