Help cutting Arcs in Mach 3

This forum is dedicated to those Hobbyists Interested in CNC machining in their home shops. (Digital Read Outs are also topical, as is CAD/CAM as it relates to CNC)

Moderator: Harold_V

Post Reply
User avatar
Dave_C
Posts: 945
Joined: Mon Apr 21, 2008 10:34 am
Location: Springfield. MO.

Help cutting Arcs in Mach 3

Post by Dave_C » Tue Nov 04, 2014 7:44 pm

Help!!!
For some reason I can not get it in my head how to program arcs in Mach 3 for my Lathe using a front tool Post.

I have the "Reverse arcs in front tool post" checked in the setup under port and pins. I can run the wizards that cut arcs and they work just fine but when I try to modify them, they don't work.

For example: Lets say we have a 3" od Stock in the chuck. And we want to cut an arc that makes a semi circle on the end (ball end).

I can see from the code that it uses I and K but I can't understand the logic for some reason..

I know the arcs start from the current tool position. So if i am at z-1.5 and I want to cut a radius of 1.5" to make a ball end the code looks like this:


G03 X0 Z0 I-1.5 K-0

Normally the X and X positions written are the "go to" locations, so I guess this is going to the end and the center. The K must be the offset from center for the arc end?

Is there a better way to learn this that can be explained in logical terms so I can write this myself?

Dave C.
I learn something new every day! Problem is I forget two.

User avatar
GlennW
Posts: 6648
Joined: Sun Apr 15, 2007 9:23 am
Location: Florida

Re: Help cutting Arcs in Mach 3

Post by GlennW » Tue Nov 04, 2014 7:48 pm

"I" and "K" represent the center point of the arc.

"I" represents the "X" axis pivot point and "K" represents the "Z" axis pivot point
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!

User avatar
Dave_C
Posts: 945
Joined: Mon Apr 21, 2008 10:34 am
Location: Springfield. MO.

Re: Help cutting Arcs in Mach 3

Post by Dave_C » Tue Nov 04, 2014 8:20 pm

Glenn,

Ok, that is what I am looking for!

So when we past the 90 degrees part of the arc the center point reference must change from I to K or vice versa.

Does Mach 3 let you use the R input and then just define an endpoint? (Instead of the I and K values)

Dave C.
I learn something new every day! Problem is I forget two.

User avatar
GlennW
Posts: 6648
Joined: Sun Apr 15, 2007 9:23 am
Location: Florida

Re: Help cutting Arcs in Mach 3

Post by GlennW » Tue Nov 04, 2014 8:26 pm

The center point needs to be referenced by both "I" and "K". They would remain the same through 360 degrees of arc.

I've never even seen Mach3, so I won't be of much help there.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!

User avatar
Dave_C
Posts: 945
Joined: Mon Apr 21, 2008 10:34 am
Location: Springfield. MO.

Re: Help cutting Arcs in Mach 3

Post by Dave_C » Tue Nov 04, 2014 8:45 pm

Glenn,

Ok, I think I got it! I and K are like X and Z as far as defining a point in the coordinate system.

So from the cutter current position I need to define the center of the arc and "I" would be the X coordinate number and "K" is the Z coordinate number making a "Pivot Point" from where I am at. Then the other part of the line is the end of the arc coordinates.

I might just be able to learn this (With a little more help)

Thanks,

Dave C.
I learn something new every day! Problem is I forget two.

User avatar
GlennW
Posts: 6648
Joined: Sun Apr 15, 2007 9:23 am
Location: Florida

Re: Help cutting Arcs in Mach 3

Post by GlennW » Tue Nov 04, 2014 9:00 pm

Looks like you've got it!

Really GREAT job on the conversion. Looks like fun!
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!

User avatar
Dave_C
Posts: 945
Joined: Mon Apr 21, 2008 10:34 am
Location: Springfield. MO.

Re: Help cutting Arcs in Mach 3

Post by Dave_C » Thu Nov 06, 2014 4:44 pm

Glen,

I just wanted to add to our discussion about "I" and "K" in CNC Lathe work. The simple explanation you gave me was what I needed but I still had trouble figuring out why "I" was always a negative number and then I remembered that Mach3 ARCs use "Incremental" positions and not Part Coordinate positions.

So I played with the math a bit writing some progressive arc cuts. It became obvious that my "I" stayed put while the "K moved because as I started with small cuts the "K" had to be a larger negative number to keep the pivot point of the arc in the same place.

So I came up with a formula based on my 3" OD stock cutting a ball on the end:

If I = 1.5" then K = 1.5 - the current Z position.

So if I make my first small pass .0434" back from the end of the part, then K has to be a negative number to keep the arc pivot at 1.5" from the end. So subtract .0434 from 1.5" and you get your K number of -1.4573.

It works for as many steps as you want till you get to the end and then K is 0 as you have the tool directly over the pivot on the Z axis and since you are 1.5" away from the center line, you still have an incremental value for "I" of -1.5.

Last line looks like this: G03 X0 Z0 I-1.5 K-0

Thanks for the help,

Dave C.
I learn something new every day! Problem is I forget two.

Post Reply