Linear gauge for saddle position readout

A second extension to my C-2 is a readout for the saddle position. For me, that has been a very useful enhancement, because the saddle drive is the only one that has no scale reading like the other axes. using the top slide is possible, but annoying or impossible if you have to feed larger distances.

The maximum travel of the saddle (with the tailstock removed) is about 350mm, so I acquired a digital caliper-type gauge with 400mm range - also at the online shop at RC-Machines for about 70 EUR.

Since the overall length of the linear gauge is about 500mm, it was too long to be mounted to the machine bed properly so I had to shorten it for proper mounting. You do no harm to the scale if you shorten the ruler part of it to any length. There is just a printed circuit board laied into the steel ruler with a periodic structure that repeats itself every 10mm or so. It can be cut anywhere and will keep working. The hard part was in fact to cut it, since it is hardened steel. I have a small Proxxon disc saw, that was on the edge when cutting through the steel with a diamond disc. It took me about 15 minutes to get all the way through, so don't even bother using a hecksaw with an HSS blade - the teeth will be round after a couple of strokes!

There come two mounting  brackets with the ruler that allow to fix it to the machine bed. I mounted the ruler on the back of the bed - there is no space in the front because of the saddle drive gear and leadscrew. So I threaded two M5 holes into the feet of machine bed and mounted the ruler to align it in parallel with the bed. The easiest way of doing that is to use a dial gauge and fix it to the saddle. Point the tip of the gauge at a vertical face of the ruler, and run the saddle along. You will see exactly how much your misalignment is. I used some thin sheet metal spacers for fine adjustment to line up the ruler with the bed to to a few hundreds of a mm. The adjustment of the horizontal face will be necessarry once you assemble the whole system in the end, but once you got the spacers right, this is easy because the mounting brackets have long holes and all you have to do is loosen up the mounting screws, adjust the angle with the gauge, and tighten everything back up.

The next step is attaching the slide of the digital scale to the saddle, so that it moves precisely along the ruler without significant friction. For that purpose I took a piece of 3 mm aluminum sheet metal and bent it by using vice and hammer. On the bottom I made two holes, which are used to attach the metal to the back of the scale (there are two M3 screws supplied with the scale).
To make the digital part of the scale slide smoothly, I bent the metal so that it is approximately in parallel to the mounting surface on the saddle. I left a 1 mm gap between the saddle surface and the Aluminum.

Next, I demounted the aluminum part again, and drilled two 5 mm holes into the top part of the aluminum plate. These holes should be at a position where the saddle can bear being drilled in and tapped. Think about that for a moment before you start! Then, I tapped four M3 threads around the 5 mm holes. I attached the plate and the scale again, and indexed the two 5 mm holes on the surface of the saddle. Then I also tapped M3 threads there. The idea is that the Aluminum is fixed by the two M3 screws in the middle. The other four M3 screws are just to align the metal plate with the saddle at it's natural position given by the digital slide gauge. So before tightening up the two center screws, I turned each of the four outer ones into the taps, until they just touched the saddle metal. Once this is done, just tighten up the center screws, and you'll have a perfectly aligned guide. The fact that I drilled 5 mm holes for the M3 center screws allows also for some lateral misalignment of the threaded holes in the saddle. Use some washers to make the screws sit securely above the 5 mm holes.

In the end, I added a guard that keeps metal chips from landing on or in the linear gauge. some plastic piece (white) and some screws did the trick:

Saddle on the rightmost position. The travel is limited by the leadscrew bearing block on the front of the machine.

So hurray! Now, I have a digital gauge on the back of my lathe, which is very hard to look at without bending over the rotating chunk while cutting :(
This is why I built a digital readout unit for up to four scales, but on that project I will elaborate on another day...