I don't think the backlash is in the screws. I think it is in the end support bearings allowing end play. Long story not related to the probe, but it looks like there is a collar to adjust the slop in the bearings, but it may be all the way in at this point. I am just going to have to tear it apart and see.
New machine is a bit drastic don't you think
? Other than this, the machine is in excellent condition with little wear and tear. Looks better than most few year old machines in a shop. Do people buy a new machine when it needs a couple bearings or a shim? Even a decent Chinese machine to replace this would probably be $25k. This one was 35k in 1978 when it was new (equates to about $137k in todays dollars). Yeah it is old, but low usage. I had it sitting around tinkering and upgrading it for probably 15 years or something and cut only about 6 parts.....before that it was just used at a college for training. It never was in production. I think I will go the route of fixing the slop instead of buying a new machine. That would kind of defeat the whole purpose of this whole mach conversion.....
Besides this, I am not even sure backlash is causing this. It may be, but It may also be the $100 probe. I don't really know what kind of results people typically get with these things. It just seems odd I can measure a bore to ten thousandths, but not edge find. I am not positive how accurate a Chinese edge finder is either. I found a couple references saying the trip point of an edge finder is pretty tiny, but it is a Chinese edge finder. However, even if the edge finder was off, the difference would be the same at each point and it is not.