I was doing some work on my Mini Mill the other day and noticed that despite the fact that I had set a 0 position on the Z axis, the readout that came with my Chinese scales would randomly reset and as a result the 0 value was suddenly elsewhere.
This was not a huge problem for the task being undertaken at the time, as it was not hugely critical dimension wise, I just needed to be more careful with regards the bottom of the hole I was boring.
This re-piqued my interest in the
TouchDRO project and I finally built a board, the writeup can be seen
here.
I bought some shielded USB cables from ebay seller cnredceo for $0.99 (£0.63) each with free P&P.
I needed to cut off the outer plastic cover from the mini USB end, I then wrapped it in electrical tape before substituting it for the original cable on each of the igaging scales.
For the tablet I bought an A33 Cortex A7 8GB 10.1″ 1024×600 from ebay seller adornsource for £42.99 inc P&P.
The default cpi (counts per inch) value in the
TouchDRO app is 10,000.00 for each axis, this is way too high for use with my
igaging scales.
Yuiry talks about setting via measurement on his
calibration page, which is fine, however these scales are actually all the same and use a standard cpi.
Calculating cpi based on a feed-back calculation similar to that on
Josef Prusa’s site can be very useful if you have already converted to CNC and can jog your axis a ‘known’ distance which you then compare with reality and update the software based on the output from the formula.
The correct cpi value for these scales is 2560 as stated on the
Schumatech OpenDRO website for the 21bit Chinese protocol that they use.
In actual use, the scales exhibit some
backlash due to the gauge ‘sticking’ on the slide, but this can be compensated for with some care.
