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School me on tip radius.
« on: January 12, 2011, 07:38:11 PM »
I'm making some parts now with LT. I love free CAM!!! However, I'm having a little problem with tip radius and how to use it. My parts are coming out to big. The radius of the parts are always the correct diameter of the part plus the tip radius.

Perhaps I'm zeroing the tool incorrectly. I'm zeroing the tool so that the tip of the tool touches the stock. I measure the stock and put the radius of the stock as the position that the tool is on the X axis.

I'm new to CNC turning so if I'm missing something obvious, please be gentle.

-Jim
« Last Edit: January 12, 2011, 07:41:28 PM by Kingjamez »

Offline RICH

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Re: School me on tip radius.
« Reply #1 on: January 12, 2011, 10:24:23 PM »
Have a look in the manual, appendix "F" section 5.
LT assumes that the center of the tool tip is related to X & Z=0 and pre-compensates the code.
RICH

Re: School me on tip radius.
« Reply #2 on: January 23, 2011, 12:04:48 PM »
Hi  Rich, thanks for the help. I'm now able to get accurate diameters by offsetting the tool the radius of the tip.

Another issue has come up though. It seems as if LT compensates for tip radius on the final cuts, but not on the first pass.
To demonstrate, here is how to duplicate my issue:
Import a straight line that will result in a .5" diameter cylinder. Now set the stock size to 1" and use a tool with a 1/64" radius. Then set a roughing tool path with  0.025" per pass. The first roughing pass will be at X0.475 which would be correct with a 0 radius tool, but not correct with a 0.0156 radius tool that ends up making a first cut pass depth of 0.0406" in reality because of the offset of the tool radius.

My work around is to specify a diameter of my stock larger than it actually is by 2 radii of the cutter. That seems a little kludgy.

So now that you see what I've done, what am I doing wrong in my set up?

Offline RICH

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Re: School me on tip radius.
« Reply #3 on: January 23, 2011, 02:32:25 PM »
Jim,
Post your dxf.
See attached, maybe i am missing something.
RICH
Re: School me on tip radius.
« Reply #4 on: January 23, 2011, 03:17:05 PM »
Hi Rich,
Thanks for the exceptionally complete reply. I'm known to be dense to please bear with me.

It seems that your are making the argument in the PDF that the final pass is offset correctly in regard to the tip radius. I 100% agree. The point I was making in my first part of post #2 is that when I touch off on my work piece, that is NOT the zero point (as it is on a Mill). If I touch my tool to the work piece in the X direction, and I'm using a tip radius of 0.0156", then my tool center is actually at a point 0.0156" from the radius of the stock.

 Example: I have a 1" bar. I touch my tool in the X direction to the stock, the actual location of the center of the tool is 0.5156". At least that is what I got from the manual (and your well made pdf). That works for me and I'm getting accuracies of about 0.0005" in finished diameter.

I believe that we are in agreement on the issue above.

Now to the issue I wrote of in post #2. Attached is my .dxf and .tap. I'm in radius mode. Note that the first cut is at X=0.475" when (from my understanding) it should be at 0.475+0.0156=0.4906 for a true 0.025" first cut.

When I run a program like this, my mill (lathe mode) nearly comes apart because the first cut is so deep. All subsequent cuts are smooth.

I'm quite certain that I'm just not getting it...

-Jim
« Last Edit: January 23, 2011, 03:28:15 PM by Kingjamez »

Offline RICH

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Re: School me on tip radius.
« Reply #5 on: January 23, 2011, 06:56:03 PM »
Jim,
My bad....
LT assumes that the center of the tool tip is related to X&Z=0.
Well you can't really put a radiused tool at X & Z = 0 as it would be into the material.
An even worst case would be a tool which is turned on an angle such that the tool side hits the material.
There is an initial move in the code ( look at the first few lines ) and is discussed in section 6.5 of the manual.
So how does one deal with the compensated code?
That is explained in Appendix "F"  section 6.2 by using a G52. Then read 6.5 again and make note of the last sentence!
But you may as well read the complete section as each section builds on the previous section such that in the end you will have the whole picture.
If it's hard reading then imagine writing it?  ;) Your comments would be appreciated.

RICH


 
Re: School me on tip radius.
« Reply #6 on: February 07, 2011, 01:15:11 PM »
Rich

I am having the same problem as Kingjamez. I would appreciate a simple example, complete with gcode. The pdf file helps a lot, but I don't understand the need for the G52. Yes, I understand keeping the tool off of the part at the start, but when do I turn it off? An example using G52 would be great. Why aren't the g52s automatically placed in the code?

Question 2: Do I or don't I need to put offsets in Mach tool table when using LazyTurn? Section 5.2 seems to imply that I don't (as does 6.1?)

Thanks
Thom