Center gauge


I've got this Starrett center gauge no. C391. A very common
item
in toolboxes everywhere. So, can anyone explain the use of
this
gauge in particular the double depth of thread and also the
odd
scale that apparently is related to various pitches of
thread with
an associated decimal measurement. The 60deg v-notches are
self explanatory but the scales? The starrett web site is of
no help,
they just say the scales are 14, 20, 24 and 32nd's of an
inch. ok....?
phil
Reply to
Phil Kangas
Loading thread data ...
if you get the Logan "how to run a lathe" book, it will tell you exactly how to use this.
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Reply to
William Noble
The "double depth" is to tell you how deep to make your final cut when threading on the lathe. It assumes that the lathe has a cross-feed which measures diameter removed instead of radius.
That is the actual table of "double depth" data. The scales on either side of the words "double depth" (plus other words) are for a quick sanity check after you have made your first scratch cut -- for the common threads on the edges of the scale. 32, 24, 20 and 14 TPI. you lay the scale edge for your thread (if it is one of those four) against the scratch cut and make sure that all of the scale lines match the scratch line. Only then are you sure that you have the right settings in the quick-change box, or the right gear train built up in the banjo. A simple quick check which keeps you from spoiling a potentially expensive workpiece with a lot of time invested in the other operations.
For checking 14, 20, 24 and 32 TPI scratch cuts.
And the V notches are both for checking the grinding of the threading tool (if you are using a shop ground one or are grinding it yourself, and for making sure that you have the tool at the proper angle to the workpiece.
I *think* that there is more detail in _the Starrett Book for Student Machinists_, but I don't have a copy to check. Or maybe the _Tools and Rules for Precision Measuring_. Both are listed in the back of the catalog, just before you hit the reference tables.
Enjoy, DoN.
Reply to
DoN. Nichols
Actually, it's so you can calculate undercut diameters, or to determine the configuration of a tool for a thread that would have to be machined to a shoulder, allowing for enough depth of tool without excess width. That provides the fullest possible thread near the shoulder (when no undercut is allowed). Minor diameter of threads isn't a reliable way to gauge depth, or pitch diameter. Wires come to mind, or thread gages.
Harold
Reply to
Harold and Susan Vordos
Thanks. I hadn't really looked at the C391 that closely.
The double depth table matches the numbers for root / minor diameter on my threading table spreadsheet which were calculated as 2 * 0.6495/ Pitch, assuming a tool tip width of 0.125/Pitch, both from Machinery's Handbook.
I usually leave the tip 'slightly rounded' and feed the compound in 0.75/Pitch from initial contact, which should be correct for the proper tip width but is oversize with the finer tip, then finish to fit or wire measurement.
I think I could approximate the tip width without regrinding the bit each time by cutting in 0.75/P, then moving the bit sideways by shifting the gear train a notch. I avoid the issue of thread root weakness by buying load-rated forged eye bolts for high tension loads like the recent I-beam trolley hangers.
Jim Wilkins
Reply to
Jim Wilkins
to determine the
machined to a
width. ...
Thanks. I hadn't really looked at the C391 that closely.
The double depth table matches the numbers for root / minor diameter on my threading table spreadsheet which were calculated as 2 * 0.6495/ Pitch, assuming a tool tip width of 0.125/Pitch, both from Machinery's Handbook.
I usually leave the tip 'slightly rounded' and feed the compound in 0.75/Pitch from initial contact, which should be correct for the proper tip width but is oversize with the finer tip, then finish to fit or wire measurement.
I think I could approximate the tip width without regrinding the bit each time by cutting in 0.75/P, then moving the bit sideways by shifting the gear train a notch. I avoid the issue of thread root weakness by buying load-rated forged eye bolts for high tension loads like the recent I-beam trolley hangers.
Jim Wilkins
So what I'm reading is the information given on this gauge is pretty much a waste of time. Why give double depth of thread when the compound rest only affects the radius. It would be far more handy if that chart gave the actual compound rest movement required for a given thread. And the four most common threads (14, 20, 24, 32) ? Thread pitch gauges are far more useful than the scales given. My curiosity has been satisfied. phil
Reply to
Phil Kangas
When you cut an ending groove with a different tool and dial readings, it's deep enough when your calipers read (OD - DoubleDepth). If I have several threads to do I'll leave the dials zeroed for threading and cut the grooves by measurement. A groove that's slightly too deep is far less important than getting the threads right.
The actual compound movement varies with the tip width if you measure in from initial contact. If you ground a sharp point you'd have to cut in the full Sharp Vee depth to get the pitch diameter correct. As I mentioned, the two numbers I found useful are the double depth they gave and the compound feed which is specific to the way I personally cut threads.
Those four pitch rulers work for fractional and multiple pitches as well. You can check 7, 10, 12, 14, 16, 20, 24, 28, and 32.
Jim Wilkins
Reply to
Jim Wilkins
There's nothing wrong with using the double depth as a guide, but to use it for sizing threads makes no sense. The tolerance for the pitch diameter is generally smaller than the tolerance for the minor, and that has relevance only when the tool is properly ground for the given pitch. The flat that is required on the tool tip gives cause for greater error than might be imagined. It's all too easy to go too deep with the formula, especially when you must take into account the starting pass, which, again, you're at the mercy of the major diameter. The tip radius you speak of keeps you out of trouble, but may well be out of tolerance for the geometry of a given thread.
Theory for threads does not translate well into operation without gauging. People that rely on compound movement for sizing threads are commonly known for making scrap.
Please bear in mind, I'm speaking from the perspective of one that had to satisfy independent, critical inspection. I also worked in places where there was no inspection, ever, and no scrap produced. Anything turned out was assembled and shipped.
Harold
Reply to
Harold and Susan Vordos
[ ... ]
a) *Some* compound feed dials read in diameter, not radius.
b) Way back when (when the thread gauge was designed) lathes did not always have a compound anyway.
At what compound angle? Even just sticking with 60 degree threads (since this is a 60 degree center gauge, not a 55 degree for Whitworth threads, or a 28 degree for Acme threads)? It is common to set the compound at 29 degrees or 29.5 degrees (both a little below half the thread angle) -- and each of those requires a slight adjustment of the compound infeed.
Back when it was designed those probably were the most common thread pitches.
Certainly -- but not everyone has thread pitch gauges in an early shop -- but everyone was expected to have the center gauge.
Enjoy, DoN.
Reply to
DoN. Nichols
Not for precise sizing -- no. But it can tell you when you are getting close enough to *start* measuring. (Granted -- the appearance of the thread crests can do that too -- once you know what to look for.
Or -- when you are cutting an original thread on a workpiece and expect to make a mating thread to fit it -- not to any official spec, but to make a working device -- such as when I was making a few cylindrical waveguide antennas (four total) with white Lexan rings threaded to match -- to hold on the metal plate at one end, and the thin plastic one to keep rain out at the other end.
For a project like that -- there is no point to cutting a unique thread to a precisely calculated and measured spec -- since you can't buy parts to screw into it anyway.
Or one-off projects? :-)
Yep -- that can be a problem too.
Enjoy, DoN.
Reply to
DoN. Nichols
When you're doing a one-off, you are gauging. By the only thing that matters, the mating part.
Pete Keillor
Reply to
Pete Keillor
"Pete Keillor" wrote in message
thing that
Interesting.....so what do you guys prefer to do when making a one-off set of m/f threads? Do you make the nut first or last? phil
Reply to
Phil Kangas
Let's see. I made some bushings threaded inside and out, but the outside was to fit a tapped hole, and the inside to fit the leveling feet. Made the nose piece to fit an L-00 spindle, big male threads, adapter for a broom handle thread, female 3/4x5 acme. I don't ever recall making both parts, male and female. DoN did, his example from earlier in the thread.
I think I'd make first whichever part was easier (smaller) to use to check the other part.
Pete Keillor
Reply to
Pete Keillor
Depends on the relative sizes. If the male thread is on the larger part, I might make the female first, so I could bring it to the male to test as I approached size. In the case which I described somewhere up-thread, I had put male threads on both inches of an aluminum cylinder of about 4" OD, and then turned white Delrin rings and internally threaded them to fit the ends of the cylinder. Both ends (of four cylinders) were threaded to the same depth, so either end could be used to test the rings. The only difference in the cylinders was slight changes in length to tune for the frequency of the wireless transmitter/receiver on different channels, and a corresponding shift of the location of the connector and radiating antenna which went in the sides.
If the cylinders had been heavier, I would have made the rings first (or perhaps a master ring of steel) and then turned the threads on the ends to fit, and turned the actual Delrin rings to fit a male master thread.
Enjoy, DoN.
Reply to
DoN. Nichols
snip----.
Depends on the part, Pete. Fact is, I rarely made mating parts----except in building tools. Consider that if you're doing defense work, you often make a single item, with no idea where it goes, or how it fits. That's the reason for establishing standards, and upholding them, even if they aren't obvious for given circumstances. If party A orders a part that specifies a given thread and class fit, you can guarantee the fit by upholding the specifications.
All too many treat threads with contempt. As long as it works for them it's good-----but that doesn't always fit in the real world, and it's not any harder to do it right than it is to do it wrong.
I try to educate in such a way that choosing the proper path becomes routine. Sizing pitch diameter by dial advancement is one of the things best not learned. It is not an acceptable procedure.
Harold
Reply to
Harold and Susan Vordos
Good point, Harold. At least as long as it's not to fit a mop handle, in which case the appropriate standard looks like 3/4" x 5 TPI, melted.
I'll go back and re-read your post, then save it. I have no intention of ever making a part for the gov., but who knows, I might make something for my own defense. So far, I've not made a thread where I didn't have the mating thread to compare, but I like learning other methods anyway. The mating thread might not be accessible as a gage if it's too big or unwieldy.
Thanks.
Pete Keillor
Reply to
Pete Keillor
IIRC the male thread first, fitted it to 3-wire measurement, then used it as a gauge for the internal threads. Usually I try to design so the nut can be bought or tapped and the male part is all custom, such as the axle example I gave above. Much depends on how easy it is to measure or try the fit. A shaft held between centers can be removed, checked and replaced while a part in a chuck has to stay there until done, unless it fits a small 5C chuck.
Reply to
Jim Wilkins
Came by a couple of 1/2" LH thread nuts, so decided to make shaft adaptors for motors with 1/2" shafts since all of the available ones are RH thread. I got to the point of cutting the threads, did my first pass and checked the scratch against the mating male thread - didn't quite agree; the original thread was 1/2 - 24, so now I have a funny spot every 6th crest. Gerry :-)} London, Canada
Reply to
Gerald Miller
snip-----
Heh! Can't tell you how many times I've done that. Nice thing is, the "funny spot" virtually disappears if you make a habit of making your first pass a shallow one. A thou or two is plenty.
In spite of my years of experience, or maybe because of them, dunno, I always take a shadow cut when chasing threads. Just deep enough to allow a match with the pitch gage. It's too damned easy to miss a setting, especially on my lathe, where I've messed with the ratio, having cut all the feeds and threads in half. I like light feeds for finishing and found my Graziano too fast in that regard. I've lived with the changed ratio since 1967----but still screw up occasionally. Maybe a function of getting old, eh?
Harold
Reply to
Harold and Susan Vordos
"Harold and Susan Vordos" wrote in message >
make shaft
available ones
threads, did my first
thread - didn't
have a funny
is, the "funny
your first pass a
them, dunno, I
enough to allow a
a setting,
having cut all the
finishing and found my
changed ratio since
of getting old,
A couple years ago I made a thread toy called a puzzle nut. It was cut at 9 tpi double start. Since this is an odd number thread and it was being doneon a SB model A 9 inch I used the numbers on the dial for one groove and the unnumbered marks for the other groove. Did both the male and the female this way. The nut was cut in half and red loctited to the ends of the male section. It worked nice and looked like 18 tpi but the nut moved fast from end to end. So then I decided to try a triple! For that I started with a longer rod with a dowel pin in the side so it would rest against a chuck jaw. Cut the groove at 9 tpi as before but for the next groove I moved the rod pin to the next jaw and then the third jaw to make a triple start thread! This one was really fast moving! Sure acted strange rolling than nut back and forth as the thread looked to be fine at 27 tpi but really hauled ass from end to end! That was fun......;>) phil
Reply to
Phil Kangas

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