Click
here to read more
about our BEST-SELLING CNC Programming
Courses. They cover just about everything you'll
ever need to know, whether you're advanced or a beginner...
Click
here to read more about them NOW -- |
Verify CNC Program
Correctness - STEP 3
Author: Mike Lynch
Now, let's move on to step three:
verifying the correctness of the first workpiece.
The objective of this step is to
carefully run the first workpiece in such a way that it will pass
inspection. For our purposes, let's assume this is very important
to the company that justified the first workpiece.
As long as the program is correct--it
is a proven program, or all programming mistakes have been found
in steps one and two--there is no excuse to scrap the first workpiece
during step three. No method is completely failsafe, but if you
follow the suggestions we make, then each tool will be forced to
machine properly. Thus, when the first workpiece is finished, it
will pass inspection.
Each tool approaches the workpiece
at the machine's rapid rate. Many programmers use an approach distance
of 0.1 inch or less (about 2.5 mm). It may be impossible to confirm
that cutting tools approach properly during step two, and a small
mistake with an offset setting can be disastrous.
Performing step three for a new
program presents another concern. Cutting tools will often make
rapid movements during machining. While you can confirm that none
of these will collide with fixture and clamping components during
step two, it may be impossible to tell whether all cutting tool
rapid movements will avoid workpiece obstructions during a dry-run.
For reasons such as these, the
operator must be careful when running each tool. Unfortunately,
machine tool builders vary when it comes to how easy they make it
to confirm approach motions and rapid motions internal to the tool.
The best method we've seen involves
the single block switch. When the single block is turned on for
this type of machine, the feed rate override switch controls the
motion rate of rapid movements. When each tool is approaching (because
the single block is on), the setup person can easily control the
motion rate for the approach. If anything looks wrong, he/she can
stop the motion by pressing the feed hold button.
During each tool's machining motions
(for a new program), the setup person will leave the single block
turned on and step through the cutting tool's motion. Whenever a
rapid positioning movement is commanded, he/she will have control
of the motion rate with the feed rate override switch.
If your single block switch does
not behave in this fashion, you'll have to use another method to
take control of the motion rate during the approach movement. Some
machines allow adequate control of rapid motion with the rapid override
switch. By providing a switch that is similar to the feed rate override
switch, they allow precise control of rapid motion down to a crawl.
This function is almost as good as the single block switch method
just described.
If you don't have these functions,
use the dry-run switch to control approach movements. While cumbersome
(you'll be turning it on and off during each tool), it enables the
user to take full control of the motion rate (usually with the feed
rate override switch) during each tool's approach and during rapid
motions within the tool.
As each cutting tool comes into
position and the machine stops (single block or optional stop is
on), one must consider what the tool will be doing. If you are concerned
that your initial offset settings are not accurate enough to stay
within tolerance limits, you must use trial machining techniques.
This involves making an adjustment that forces the cutting tool
to leave a little extra material on the critical surfaces.
Once the cutting tool has machined
the surfaces, the machine will be stopped and the surfaces will
be measured. Another adjustment will be made based upon the measurement,
and the tool will be re-run. This time the surfaces will be right
at target values.
About
the Author
See Mike Lynch on MODERN MACHINE SHOP Online at: www.mmsonline.com/experts/lynch.html
CNC
PROGRAMMING COURSES
|