I've watched some on Youtube with commercially obtained boring heads using all 3 gib screws to unlock and re-lock the slide. That's simply not necessary. Looking at the majority of the small 2"-3" 50-75mm offshore heads, I'm just about positive the design was directly cloned off the Criterion heads. I know my first off shore head was because I was able to compare them side by side when I finally upgraded mine. And Criterion are quite specific in the user manual about just using that center gib screw as the slide lock. In fact there newer heads are now marked with the word lock just above that center screw. Using all three gib screws upsets what should be a carefully set gib adjustment and allows highly undesirable and variable movement when your trying to make very precise and small movements on the tool slide. So if your currently using all three screws, that could well be a major part of having imprecise adjustments and unpredictable hole size results.
Yes those heads with differential thread systems allow extremely fine adjustments, but it needs to be well understood the manufacturer's would be using sophisticated methods to obtain screw threads with extremely low lead and lag errors in each screws thread pitch. At a minimum, I would guess there being thread ground on specialized equipment that's built to produce those very low error thread pitches. Plus there's a pretty good chance there then being precision lapped to reduce and average out those errors even more. All of that is just part of there very high price that's well outside my budget. If the work is important and precise enough to justify it, I can still measure my boring heads adjustments, admittedly a bit slower, but just about as well using an indirect method. A high accuracy dial indicator to show the amount of slide movement. And no matter how accurate your boring head and tooling is, it's still only half of what's required. Even more expensive is a method of accurately measuring what that hole size being produced really is, any taper, or out of roundness once you start getting below + - .001" / .0254 mm. Luckily and for most of us, that's seldom required other than maybe for bearing fits.
Not to throw Nerd1000,s thread OT, but since those differential threads were brought up. The majority of this PDF would be well above what most of us would ever be doing in a home shop.
https://ia800104.us.archive.org/20/...curacy/Foundations_of_Mechanical_Accuracy.pdf But it does detail how much time, effort and investment Moore Tools did trying to obtain those perfect thread pitches for the feed screws and nuts on there jig borers and grinders. They got very close, but could still find and measure those errors no matter how much effort they put into the project. I also happen to think that understanding some of the physical accuracy limitations in what we might be buying or making is still one of the fundamental pieces of information to have for anyone with a home shop. In addition, Moore's clear diagrams about the possible 6 degrees of inaccuracy that may be present in various amounts or even multiple combinations of each one of them was a light bulb moment for my understanding of machine tool alignment and/or misalignment. Its not quite as intuitive or easy as some seem to assume.
.... to me.
