Testing Optical Limits

This isn’t really a tip, but it is pretty cool, so we decided to throw it in this week.

When it comes to the homing operation repeatability is everything.  So we had to come up with a way to test the repeatability of the limit.  I thought for a while about just throwing an indicator on a mill and doing some homes and see what I got.  But what would that get me?  An overall test of the homing ability?  Limit to PC and everything in between?  Not to mention my indicator is good only to 0.0005 (half-thow’).  We needed something that tests JUST the Optical Limit.

Here’s what we came up with.

As you can see in the picture we started with an aluminum plate and mounted the Optical Limit and a cut off micrometer to it.  For the power supply we used an adjustable +5 volt supply (from the Hot Wire kit) and we measured the output with a Fluke multimeter.

In operation it was pretty straightforward to use.  Just slowly (VERY slow) turn the thimble until the output goes to a logic HI.  This works fine because the limit will stay at a logic low until it’s tripped then it “flops over” to a HI. 

I laid the assembly at the end of the bench with the end of the mike hanging over and started adjusting.  It didn’t take long to figure out that the limits were fairly precise.  I was getting repeatability numbers in the 0.0001 (ten-thousandth) range, and was pretty amazed (down right shocked).  I figured half-thow’ at best, but a tenth’ seriously blew me away. 

Everything was going perfectly fine; I was up to about 15 readings of some serious numbers, saying “No Way” the entire time then everything changed. All my numbers were off by about a half-thow’.  I stopped and looked over everything, everything was fine.  Then I realized that I had picked up the assembly at the midpoint to get a good look at the veneer scale, I did it for just a moment, 10 seconds tops, but that was enough.  The aluminum base had expanded due to the heat from my hand.  I started saying “No Way” again, and ran to get an ice cube.  Then I nudged the mike up to 0.0001 shy of the trip point and touched the base with the ice.  It took less than a second to trip the limit.  I was able to do this like clock work.  0.0001 shy of trip, hit it with the ice, say “No Way” and laugh a little, and warming up the base a little by pressing my thumb on the same spot.  I put a circle on the base to show where I alternately put the ice and my thumb.  (BTW if you think this is exciting you have issues too)

Final numbers

I was able to get solid numbers in the 0.0001 range, and I was truly impressed.  That being said I will state the “Official” repeatability of the Optical Limits is 0.0002.  With some more test equipment and a bunch of lab time I would probably be able to state the number is a solid 0.0001, but the cost and time that would add to the project isn’t worth it. 

Not to mention who is going to put their mill in a temperature controlled environment?