General formula using I and J for a circle, please.

[greybeard]

Could anyone give me a general case using the diagram I have attached for the values of I and J in coding for a circle, if the tool start position is Xb,Yb ?

As my software seems to run into problems if I enter zero for the value for I, I've located the tool start position at 45 degrees from the circle center.
The value of sq.rt. 2 is permanently etched in my brain, so I have no problem in locating the tool, given the position of the center.

I'm looking for a general formula primarily to help me get a better grasp of this command, as I have found a lot of 'explanations' ambiguous.
Many thanks,
John

[Geof]

Half the square root of two comes into it.

If the radius is 1.0000 the X and Y distances from the start point at 45 degrees are 0.7071 which is half the square root of two. Your I and J would be the same and the +/- sign is determined by the direction of your 45 degrees; in your diagram J is positive and I negative.

This presupposes your software treats the I and J as incremental distances from the start point to the center; I believe some controllers treat the I and J as the absolute position of the center.

[greybeard]

Originally Posted by Geof ......This presupposes your software treats the I and J as incremental distances from the start point to the center; I believe some controllers treat the I and J as the absolute position of the center.

Hi Geof, and thanks. This is where I think my problem lies, and perhaps just trying each would show me how my controller reacts.
Is ther any possibility that it might treat I and J differently, one as absolute and one as incremental ?
Sounds a nightmare, but I need to consider it !
John

[jimalb]

Mathematically, you can calculate I and J as:
I = XArcCenterPoint-XArcStartPoint
J= YArcCenterPoint-YArcStartPoint

0, 90,180, and 270 degree arcs always have either I or J as zero.

Hope this helps,
Jimmy

[Geof]

Originally Posted by greybeard ...... Is ther any possibility that it might treat I and J differently, one as absolute and one as incremental ? Sounds a nightmare, but I need to consider it ! John

You never know with pointy headed software types.

[greybeard]

Thanks Geof. Now I know why my hat stays on in high winds.

Originally Posted by jimalb Mathematically, you can calculate I and J as: I = XArcCenterPoint-XArcStartPoint J= YArcCenterPoint-YArcStartPoint

Thanks Jimmy. So on my diagram, I = (Xo - Xb), and J = (Yo - Yb).

Originally Posted by jimalb 0, 90,180, and 270 degree arcs always have either I or J as zero.

Unfortunately this is a problem for my set up - the controller seems to have a problem with a vaue of zero for I or J, and just tries to drill a hole radius = 0

John

[ger21]

Jimmy's giving you incremental IJ. Is your control looking for absolute? Absolute IJ specify the actual center coordinates.

[greybeard]

Hi Gerry,

Originally Posted by ger21 Jimmy's giving you incremental IJ. Is your control looking for absolute? Absolute IJ specify the actual center coordinates.

I quote from the user manual - "G02 Arc clockwise, X,Y, ending coordinates, I,J, center offsets from beginning location."

So do I assume this for incremental or absolute coordinates ?
Regards
John

[Geof]

Originally Posted by greybeard Hi Gerry, I quote from the user manual - "G02 Arc clockwise, X,Y, ending coordinates, I,J, center offsets from beginning location." So do I assume this for incremental or absolute coordinates ? Regards John

I interpret this as incremental. Have you done a successful circle using non-zero values in both I and J?

[ger21]

Looks like incremental to me too. Not sure why it won't work with a zero coordinate.

[HuFlungDung]

What comes to my mind, is that the TAN90° or TAN270° are undefined. So perhaps whatever formula the controller uses incorporates this trig function somehow, so it craps out when these particular angles are involved.

[Geof]

Here is a technique to test.

Position the tool at what will be the hole center.

Move the radius distance in Y and something like 0.1 in X.

Now program the G02 using I-0.1 Y-(radius distance).


EDIT: Your radius comes out wrong of course with the mov e of X0.1 but if this works see if it will accept an X move of 0.001 with a corresponding I-0.001; this will have negligible affect on the true radius.