View Full Version : Metric Goof in March/April HSM

Paul Alciatore
02-25-2005, 11:55 PM
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.

Paul A.

J Tiers
02-26-2005, 12:18 AM
Agreed.... I have suggested it here, and directly to the editor, maybe Neil, maybe his predecessor. It was kinda dissed-off as too much trouble and expense.

I would think that it wouldn't be hard to do, peer review is not uncommon in other forms of publication. And it isn't as if it would be that difficult, mainly overall looking for safety and sanity, and a check on any described procedures to see if they make sense.

Those matters are a big deal, not just an annoyance. No really unsafe procedures, or badly unsafe projects, please. The "cheap EDM" with a lot of the metal parts live at line voltage comes to mind as a particularly bad one.

The math errors are inconvenient, yes, but not devastating.

They can be found only with difficulty, and are easily correctible in the next issue. Most who try to follow the info will discover them rapidly, I'd think.

Some that are at the core of the article obviously could be checked.

It is true that technical proofreading is an art, it must be approached a certain way. Not everyone is good at it. Typically fast readers are very poor at it, for instance.

Forrest Addy
02-26-2005, 12:46 AM
Oops! You mean that the stuf I write for HSM has to be accurate consistant and subject to peer review?


Looks like I'll have to go back to graffitti.

J Tiers
02-26-2005, 01:43 AM
Naw. Your writing is fine. easy to follow, good info.

We aren't talking "schoolmarm" review, just a filter for really bogus and unsafe info.

I strongly doubt we are in any danger of that from you. And the April issue is already out..........so we are safe there too.

John Stevenson
02-26-2005, 04:25 AM
<font face="Verdana, Arial" size="2">Originally posted by Forrest Addy:
Oops! You mean that the stuf I write for HSM has to be accurate consistant and subject to peer review?


Looks like I'll have to go back to graffitti.</font>

Can you sign a cheque with an aerosol ?

Fred Prestridge
02-26-2005, 10:36 AM
A review for errors will not catch them all. In one article I wrote I included a drawing of a left hand Acme tap. When the article was published the drawing showed a standard thread. It had been redrawn and whoever did it did not notice that the original drawing was not a v thread.

Paul Alciatore
02-26-2005, 11:11 AM

I'm sorry, I meant to add a little praise for your work there but it was late and my brain cells were already half asleep.

The only complaint I have on your series is that it should have appeared about twenty-five years ago - for me at least. It has so much in it that I wanted to know and that I have had to pick up piece by piece. But, I'm still reading every word as one can always learn more. Please keep up the good work and thanks for shareing your knowledge and experience.

Frankly, I debated on posting the message in the first place. I don't want to degrade the author as I apppreciate his effort and understand that we all make mistakes. It helps keep the scrap bin full. I just wanted to reinforce the idea that mistakes do happen and that, perhaps, more measures are called for to catch them. I would really appreciate it if any error that I made was caught before publication.

Paul A.

02-28-2005, 11:38 AM
If anyone has an article they would like to have proofread, I would be willing to give it a go. I can overlook errors just fine, but catch some occasionally. Can give sugestions for rewording or asking what you meant by a particular statement. Sometimes an unbiased second look can catch unclear explanations or show the need for a simpler or more complete explanation.

Just send a .txt file under subject Machinist Article. Promise to reply within one month if the email gets to me.