Making a 575 bottom plate from a 500 series donor part


Hi Friends,

In the continuing saga of restoring my 1962 model 575 122ModC Curve Tracer, I have completed adapting a bottom plate from a donor Tek 500 series scope part. I'll be brief as possible. Here was the process to fit a standard 500 series bottom plate to a 575 Curve Tracer. Many thanks to George Lydeck for getting the dimensions for me.

1. 500 series scope bottom plates are 20 3/8" long.
2. 575 Curve Tracer is 18 3/8" long
3. Plate width is identical. Armed with this knowledge, I acquired a donor piece from a seller on eBay for $15.00.
4. The material is a soft aluminum. It is very easy to work and cut. It is also quite easy to accidentally bend, but it can easily be hand bent back into shape. I taped and papered it to protect from scratching during the cutting process.
5. I directly marked the taped plate by trial fitting it to the 575. I planned for a final .0625 clearance for the cut.
5. I jigged up the donor plate in a table saw, clamping it to my table saw miter gauge with a 1x6 poplar piece screwed to the gauge and catching the beveled edge of the cover. I equipped my saw with a typical 10" metal cutoff wheel (Diablo DBD100093L01F) Kerf is 3/32. They are easily available at big box stores and are cheap too.
6. I raised the wheel just about 5/32 above the table
7. I then ran the piece through the saw, using a second piece of 1x6 poplar cut to the width of the cover, as a block and force plate over the kerf. The 1x6 poplar allowed me to safely press directly down over the kerf with my fingers. This kept the piece firmly against the cutoff wheel. Very safe, cut clean in one pass, and absorbed all flying grit as well.
8. After making the cut, I retained the cut-off portion, for use as a marking jig to drill the holes for the Dzus fasteners
9. Using the cut-off piece, I marked and punched the two holes for the Dzus fasteners. The screw hole was 11/64th, the catch-tab hole was a #29 bit. I used Tap-Magic and 300FPS bit speed to cleanly cut the holes using minimal pressure.
10. NOTE: Make sure to properly phase the location of the catch-tab holes, so that the Dzus fastener is captive when tightened, not loosened. Using the original piece as a template made it easy to get it right, but don't make the mistake of "flipping it over" to make it easier to mark from the cutoff! If you flip it it is now "backwards" to original orientation, and the phase of the catch tabs will be backwards!
11. After drilling, I then trial fit the now correct length piece, and scribed off the curves found at the flanged bends, near the last 3/4" end of the plate.
12. I used a simple hack-saw to carefully remove most of the excess material there. I then finished the rounding and clean-off job with a fine mill file. I also lightly dressed the cut end of the plate at this time.

Total job was 45 minutes, start to finish. I chose not to repaint, as the damage to the original paint was so minor as to be unimportant on this working instrument. If I had wetted the surface and the poplar, there would likely have been no paint loss at all. Enough heat was generated to slightly discolor about a 1/4" band along the edge.

I would rate this as a pretty simple job for anyone with the right tools and reasonable metalworking skills.