12 TPI vs 13 TPI

Some time ago I bought a compound cross slide from a junk dealer. After
disassembly and cleaning off the rust and gunk its now back together. Heres
the problem. One lead screw has 13 TPI and the other 12 TPI. For 1
revolution, the 13 TPI screw will move the slide .077 inch which is the
thread pitch, right. The 12 TPI screw moves the slide .166 inch which is
twice the thread pitch. WHY????
My guess is these are metric threads. Don't know how I'll ever convert
these numbers to something useful. Neither screw has a graduated dial.
Maybe it really is just Junk.
Reply to
Loading thread data ...
If you are going by markings on dials (which you say these don't have), it could be that one is calibrated to read how much is removed from the diameter in a lathe, instead of the radius. Some lathes are marked one way, some the other. Even on the compounds, which is a bit more problematical, since sometimes you are using the compound parallel to the axis, and need it to be direct reading, and often you have it at an angle for thread cutting, or cutting step tapers.
Metric? Maybe just the difference between old British and US 1/2" threads -- one is 12 TPI, the other is 13 TPI, just so they are different. :-)
If it were metric, the 12 TPI would work out to a 2.1167mm thread pitch, and the 13 TPI would work out to 1.9538mm. That one is sort of close to a 2mm pitch, but still enough of an error so one would not work well for the other. (But -- a 2mm thread might measure close enough to look like 13 TPI with an imperial thread gauge, and a 2mm pitch would be fairly reasonable.
Enjoy, DoN.
Reply to
DoN. Nichols
Two lead thread?
Mount cheap digital calipers to use as readouts? For small movements use dial gages? Gerry :-)} London, Canada
Reply to
Gerald Miller
The way you describe the threads, I suspect what you have is a 13 pitch thread on one, but a 6 pitch double entry on the other. Check your thread to see if it has two starts on the end. If so, that's the reason it advances double the pitch. In threads, there's two things to consider, pitch, and lead, which are only one and the same on single entry threads.
It doesn't really matter how much the screw advances the compound aside from perhaps how sensitive it may be. As long as you know the amount of advance per mark, you can always use it. Give some thought to making a dial that is in keeping with the marking system you may find convenient for your personal use. When using a compound, one generally is less concerned with marks, anyway, unless you intend to trig every movement, something most machinists don't do.
Reply to
Harold & Susan Vordos
In pre-WW2 days, the old British Whitworth standard for 1/2-inch diameter coarse thread series was 12 tpi. Early in the war years (maybe around 1939 or 40) the Unified Thread Series was adopted so that both British and US thread systems were compatible with each other. So, the old 1/2-12 was dropped in favor of 1/2-13. The British Whitworth profile was then 55-deg and the American thread profile was, and still is, 60-deg. All of this comes from my memory which may be questionable? If so, maybe some other old timer can confirm or deny the above. Dave
Reply to
David Anderson
You're absolutely correct Harold. I just rechecked the screw and upon closer examination it is a double entry. So I really have a 6 instead of a 12. In all these years this is my first experience with a double thread.
Thanks again guys
Reply to
Sounds to me like one is single lead and one is double lead.
Ron Thompson On the Beautiful Mississippi Gulf Coast USA
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'Don't trust anything that has no moving parts. Especially if it's a relative' Red Green
Reply to
Ron Thompson
Engineman and I were down at a local train depot park/etc - that had 2/3 scale trains. The shop had some nice big lathes and mills....
When we walked in for a look see, the Drillpress was having one mounted on the main spindle.
Seemed logical if one had to drill down different depth and the manual locks were to much work for each one...
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