# Machsupport Forum

## Mach Discussion => General Mach Discussion => Topic started by: Marmot_Militia on October 17, 2007, 11:30:00 AM

Title: Cutting a YZ arc: Where do I put the K?
Post by: Marmot_Militia on October 17, 2007, 11:30:00 AM
Machinists,
I'm cutting a crescent shape radially into 1" aluminum round. (http://www.lightningtimesavers.com/concave.JPG)

Have my start and end points. But for some reason the crescent is too flat on the bottom, not  smooth arc.

I think I have it narrowed down to not having K in the right spot. The instructions I'm using say the K is the center point of the arc.
What's confusing me is where to put the center of the arc:  Is it the radius (measured from the bottom of the arc up? or actual distance the center of the arc is above Z0?

Top of rod is Z0.  Since the depth is .2" and the radius is .551" would I set K to z.351" or z.551"
Does the ijk method only allow 90deg arcs max?

Thanks, MM
Title: Re: Cutting a YZ arc: Where do I put the K?
Post by: bowber on October 17, 2007, 11:58:36 AM
Hi

I'm no expert but I'll give the answer as I see it.

The IJK is the centre point of the arc relative (normaly, but some are from the zero point) to the start position I is in the X axis, J in the Y axis and K in the Z axis.

So if your bar is length ways on the X axis you need to be using J and K to set the radius in the YZ axis, the J would be the centre of the bar stock and the K would depend on the start point of your arc.
From what your saying I'd guess that you've got the K too high making a large radius that is giving a flatter curve than required.
A quick easy way to work this out is to draw it in cad and measure the distance from the start of the curve to the centre point, most cad systems will also give the XY measurment, the Y (K in your Gcode) measurment is the one you need as I presume you know the X (J in your Gcode)

Just done it using your sizes. J=0.3078 & K=0.1060

If you have the bar along the Y axis you'd use I,K

Are you sure your wanting to do it this way as it'll take a lot of cuts to get a smooth finish along the length, but then a cutter of the diameter required would be huge for a home workshop.

Hope this helps
Steve