Calibration of Plotter and Overlays

C

chasf

#1
I am looking for input on how some of you calibrate your plotters and overlays. I am talking about overlays used on Optical Comparators.
 
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Hershal

Metrologist-Auditor
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#2
I am sure that you will have a reply shortly from someone more directly involved in this activity.
 

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#3
Hopefully. I don't remember anyone having discussed this type of calibration here before. I'm interested in how to calibrate Plotter and Overlays.
 

Jim Wynne

Staff member
Admin
#4
I am looking for input on how some of you calibrate your plotters and overlays. I am talking about overlays used on Optical Comparators.
It's generally assumed that mylar overlays aren't used in tight-tolerance situations, so the easiest way to calibrate them is to use the (calibrated) comparator to measure them. You can also use an optical CMM if you have one, but in the past I've just measured them with the comparator. As far as periodic calibration is concerned, they should be checked periodically for cleanliness, wear and legibility.
 
C

chasf

#5
What you have described Jim sounds very much like what I have been doing.
For the plotter I was thinking of printing a test overlay with a range of features on it that covers what a regular overlay would have on it. Then measure the overlay with a comparator and if it checks OK I could qualify the plotter to print overlays without the need to calibrate them. At regular intervals I would re-print the test overlay to insure the plotter is performing consistently.
Does that sound like sound reasoning and would it hold up in an audit?

The reason I want to “calibrate” the plotter is that we sometimes use an underlay which is basically a printed to scale overlay but on plain white paper. This is used to check the radius of a bend on bar stock. An underlay can not be measured on a comparator obviously so proving that the plotter is accurate might be the answer. The only other idea I have had is to print an copy of the underlay on Mylar and check it with a comparator then lay it over the underlay and compare the two.
 
C

councilface

#6
To verify the quality of our overlays I will draw a scaled circle (with cad) on the actual overlay and compare it to a calibrated ring gauge. This checks that the overlay is ok in every direction.
 
J

JHagani

#7
Or as it has been done for years, include 5.0" or 10.0" grids in your Mylar.
Plotters may draw a small circle nice and round, but the accuracy tends to decrease over longer distances.

The problem gets worse with an older plotter with worn out paper guides.
 
C

councilface

#8
Or as it has been done for years, include 5.0" or 10.0" grids in your Mylar.
Plotters may draw a small circle nice and round, but the accuracy tends to decrease over longer distances.

The problem gets worse with an older plotter with worn out paper guides.

I always ensure that the circle encompasses the whole of the A4 sheets that we use.
 
A

andygr

#9
First let me state that there are 2 parts. The first is to make sure that the data fle is correct. A whole seprate issue on validation of digital data.

For verifying the output my practice has been to show that the encoders of the plotter have accuratly ( with in tollerance) performed. This verification is most commonely done by outputing a scale for each of the encoders direction and then verifing the accuracy of the plotted scale with a calibrated instrument.

As stated by others if the plot is measurable you can calibrate it this way using a OC.

Once verified the recalibration is usualy limited to visual for witness lines, wear and damage.
:2cents:
 
C

chasf

#10
JHagani said:
Or as it has been done for years, include 5.0" or 10.0" grids in your Mylar.
Plotters may draw a small circle nice and round, but the accuracy tends to decrease over longer distances.
What instrument do you use to measure the grid?
 
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