Using Chinese Scales

 

I was looking for a low cost way to get a digital location of the X and Y tables. I settled on the use of the Chinese calipers because they have a habit of selling cheap. I have bought 6 inch calipers from Homier for 14.95 and sometimes, on sale, from Harbor Freight for $9.99 and been very happy with their accuracy and stability. For longer ones the best choice is Harbor Freight, but watch for sales. For the Mini-Mill I initially used a 6 incher for the Y table and a 12 incher for the X movement. I milled suitable mounts for the rails from blocks of aluminum and did a bit of drill and tap on the table beds to mount them. I used .010 brass shim stock to mount the scale readouts. The individual scales details are given below.

General:  The duck bill heads were cut off the calipers using a Dremel with a cut-off wheel. The printed scale on the back of the readout (R/O) was taken off with a razor blade scraper and the retainer screws removed. The stop plate was removed from the end of the rail and discarded. The back plate with one of the duck bill halves was removed carefully laying aside the little neoprene rail scrapers that are on either side of the R/O housing. They must be replaced when the rail is placed back into the R/O housing later. Note that one of the screws is threaded differently from the other three. The R/O was now slid off the rail. The tensioner strip will fall from inside the R/O and can be discarded. It is not useful for an attached scale. At this point, the depth strip is broken off from its spot welded location and discarded. Clamped in a vise, the duck bill could now be cut off the back plate. The one on the main rail was now cut off also. The back plate was now replaced noting that one of the retaining screws is different from the other three. The rail is now slid back onto the R/O housing in the same orientation as it was before removal. We now have a rail with readout. All that remains is to carve out suitable mountings.

X Table:  This one was done with a 12 inch caliper. I milled two blocks from aluminum for the ends of the rail and drilled and tapped holes in the X-table end caps to mount them. The rail drops into a space milled out for its size, but a bit less than its thickness. A small paddle screwed over it clamps it into place. The scale has a piece of .010 brass shim stock Gorilla glued to the back of it and then drilled to match the two rightmost gib adjusting screws. It is placed over the screws and secured with two nuts. It should be mentioned here that the reason for the shim stock mounting is that the mounting of the rails is unlikely to be perfect. As the R/O travels from end to end there is likely to be a bit of difference of spacing from the moving surface to the rail position. The .010 brass shim stock will push the R/0 from side to side rather rigidly, but will flex in and out with the minor position errors. This has worked very well for me on my installation.

Y Table:  This one was done with a 6 inch caliper. I milled two mounting blocks for the rail ends, clamped to the rail similar to the x-table, and screwed them to holes drilled and tapped into the mini-mill base. I drilled and tapped holes on the moving table base and mounted a small aluminum block to it. A piece of .010 brass shim stock was Gorilla glued to the back of the R/O and screwed to the block.

Z Axis:  A 12 inch caliper was used for the head depth (z axis) because my mill has been modified with the air spring kit and the head set to run to the very top of the back column. This allows a max travel in excess of 10 inches. The rails were mounted in blocks using the clamping method as above. The blocks were drilled through from the side and mounted into drilled and tapped holes in the side of the back column. The R/O was mounted by .010 brass shim stock to the power head between the two lower gib adjusting screws into drilled and tapped holes. The shim stock from the mounting screws to the 90 degree bend at the side of the scale was reinforced with a thin strip of steel glued to the stock. The only flexing allowed is for minor errors in vertical alignment of the mounted rail.

Connection:  I soldered flexible, flat, 4-conductor telephone cord to the connector contacts of the R/O heads and cut clearance out of the covers with a wire cutter so they could be placed back over the cords. The points of entry of the cables were then sealed with hot glue. The other end of each cord has a Mini-DIN plug attached to connect to the Shumatech DRO-350. All cables are routed along the base and rear column and held close by small tie wraps through stick-on tie wrap mounts. This keeps electrical interference to a minimum. This setup has worked very well for me and I dearly love the use of the mini-mill with the DRO-350 in place.

Comment:  I have seen several complaints about the stability of the Chinese scales, but I suspect there may be some issues about the mounting and/or cable routing on these. I have been working with electronic applications and cabling for many years and the proper installation and shielding precautions are second nature to me. My scales have given me none of these problems. They have been very stable and locations are very repeatable. Plus or minus .001 are the norm. There is a possible issue with the DRO itself depending on the stability of commercial power in your area. The DRO-350 has minimal power supply filters. If you have many glitches in your commercial power feed it can cause unpredictable problems with the DRO. Mine would shift into slow update mode on scales at random, one or two at a time. Sometimes it would lose a scale entirely and cause a good deal of heartache getting it to display again. The problem was solved with a 350VA UPS I picked up on sale at Office Depot for $19.95.

 Click to see mounts