Metric Goof in March/April HSM
Anybody catch the metric goof in the "Inch Precision For The Unimat Lathe" piece? I have a Unimat and still use it so I just had to read anything that relates to it.
He first states that the Unimat has a 1 mm pitch lead screw, which is true. He has a full page table for converting many measurements into turns and marks on the Unimat handwheels. Not sure I like the approach but it can work. His table only goes up to one inch and then he states that for measurements greater than that you should turn the handwheel 20 turns and 8 marks for each whole inch and them find the fraction or decimal in the table and do that.
Now, the 8 marks is right on as the handwheels have marks every 1/20 mm so that's the 0.4 mm of the 25.4 mm per inch. But 20 turns is only 20 mm and you are going to come out 5 mm or about 1/5 inch short. It should read 25 turns and 8 marks.
He also states that the conversion is off by a small amount. "Each twenty turns, plus eight marks, virtually equals one inch (0.999998") of carriage movement."
Now if I am right, the international standards bodies have standarized the definition of the inch at exactly 25.4mm. No 6th or 7th decimal place differences any more. So, this is another goof. Of course, there is no way that the Unimat lead screw is anywhere near that accurate. Frankly, I doube that it is any better than 0.0005" over it's length with local variations as you go and it may not even be that good. Anything past the fourth decimal place just isn't even worth talking about. His table shows numbers out to eight and nine decimal places - nano inches. On this machine, heck on any machine I have or ever will have, such numbers are totally meaningless. (With the possible exception of my knife edge tester for telescope mirrors).
As for the technique of resetting the zero point of the handwheel over and over again while cutting a single part, I would think that that is just a invitation for error. Whatever small errors you would make when setting the zero once will just add up as the process is repeated.
I think I will stick with just converting inches to mm with my handy Radio Shack calculator. I stick 25.4 into the memory and convert back and forth at will with simple multiplication and division.
I am also coming to agree with the idea that the magazines need some kind of accuracy and reality review before publication. I have submitted one and am writing another and would welcome some form of review to prevent me from such embarrasing errors.
Make it fit.
You can't win and there is a penalty for trying!