JC Price
Aluminum
- Joined
- Oct 21, 2007
- Location
- Boulder, CO
I decided to replace the readout on my new-to-me Starrett/Craigslist HB350 comparator because the original Starrett scales were not reliable and they did not have as much resolution as can be gotten from this machine, even at 20x. Also I wanted to get some of the cool measurement features of more modern comparator readouts. Being the miserable cheapskate that I am, I bought new 1 micron scales and a DC-3000 comparator readout directly from HXX (Shenzhen Hengxingxing Precision Instrument Company) on Alibaba, for a total cost of $324 including shipping. They were easy to communicate with and the package arrived 7 days after I paid them.
So how well does it work? The photos above show the test set up. I used a 250 mm glass calibration scale that is marked "David W. Mann Precision Instruments, Lincoln Mass." I bought it on Ebay and don't know how accurate it is, but D.W. Mann was a high-class enterprise in its day and it looks like the sort of thing to use to test a comparator. The Mann scale is ruled in millimeters, so I put the DC-3000 into metric mode and wrote down the readout value with each ruling on the Mann scale aligned with crosshairs for every millimeter within the range of each stage. The plots show the results. The x-axis on each graph is the position on the Mann scale and the y-axis is the HXX readout value minus the position on the Mann scale, so it is the deviation from the value expected if the Mann scale were perfect.
The X scale deviation fits a line pretty well with an rms deviation of 2.5 microns around the line. That seems to be about how well I can reliably gauge when the image of a ruling on the Mann scale is aligned with the crosshairs. (I used a 20x lens, the only one I have.) But the slope is larger than I expected, about 40 microns over 190 mm, equivalent to 2.5 mills per foot. I don't think this slope is due to the accuracy of the Mann scale, since when I use it to test the Y scale I don't see the same linear deviation. Maybe I am doing something wrong.
Are these scales about as good as can be expected, or does it look like I'm doing something wrong, or could I get better scales (for reasonable cost)? The DC-3000 can use any scale with 5V TTL output. I have not played with all of the features of the DC-3000 yet, but as far as I can see it does what it claims to do.