Thanks for that idea. And thanks for the encouragement. I know they aren't ready for prime time work product, but I'm not disappointed with them - gotta start somewhere before you can get better!
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Learning to Cut Threads
- Thread starter Todd3138
- Start date
That's actually what my friend showed me and is apparently how he does it, too. He said he likes to go a thou under for the reason you shared. Something I did was obviously too far under, whether it was the initial turning (I believe I got a measurement of .740 just using calipers - I know they aren't the same as using a mic which didn't seem too undersized to me) or cutting the threads too deep and taking some off my major diameter in the process. I definitely waited too long to do my first fit check with the nut and should have been a little less aggressive after the first several passes. As has been said, practice and learn.
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Yhere is another device for measuring threads for pitch diameter, called thread triangles, they are easier to use than wires, measured by ordinary micrometers.
I've seen those used in a couple videos. I need to read more about them to get a better understanding of how they work. I get that they fit into the threads, but not much beyond that.
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The little grey cells kinked in! You count the number of teeth on the QC box input, set the QC box for that number and substitute a 30 tooth change gear for the one that was counted. Bob's your uncle, and Tillie's your aunt!
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Most of what I thread is much smaller than what you would use for learning. And the advice given herein on this site is on point. I wanted to bring up a couple of points that might be useful to a novice. Once you have a good understanding of threading, much of the details will become second nature.
Theoretical threading of standard 60 degree threads is called a "V Sharp" thread where the OD is exactly the diameter of what you are working toward. And the bottom of the thread is a sharp "V" or 30 degree angle. But as is often said in many fields, "in theory, theory and practice is the same, but in practice they are not."
In Practice, the tooling will "dull" a whisker so that the V Sharp is no longer sharp. This is outlined in Machinery's Handbook, which has editions going back to 1916. I don't know the current standards for threading, they have changed several times over the years. But the final touch up of threads involves taking a whisker off the OD after the threading pass(es). And the bottom of the thread is actually a slightly rounded form for the same reason. In production, tooling will dull after a while.
In practice, the slightly undersized OD is a practical solution for the outside of the thread. When I cut a 32 TPI thread on a 3/16 rod, I start at 3/16 inch and cut to a sharp profile. Then following the threading cut I touch up the edge with a whetstone to the point that the sharp edges are gone. Essentially duplicating the "standard" given for a Unified Thread. Unified Threads are a post WW2 phenomenum. where, among other things, Whitworth 55 degree threads were reconciled with 60 degree threads. I only understand about 10 percent of all that, it's a deep subject.
The bottom or inside of a thread is a function of the nose radius of the particular tool used. It is normal for the novice to use carbide tooling to learn. Either brazed or insert types. If you look closely at the specifications for a particular insert, there is a given size for nose radius. When grinding HSS tooling, this is less of an issue. For a threading tool, the target for grinding is a sharp point. But grinding tools is as much an art as a science.
The point is that a carbide insert for a "large" thread, say 10 TPI, is in no way suitable for a smaller thread of 56 TPI. It simply doesn't cut deep enough. Leaving the bottom of the thread too shallow or rounded. 56 TPI requires a much smaller nose radius. As grinding tools is an artform, so too is threading. There are many details to be learned, but the final parameter is "touch". Just how much to make that last pass where you don't adjust any deeper, just make another pass to smooth up the surface. That's the art part of threading. I highly recommend acquiring a copy of Machinery's Handbook. Which edition you acquire is as much a matter of what you will be doing. I have several copies, including a 1942 version and a 2005(?) version for when I need to see what the current practices are.
Theoretical threading of standard 60 degree threads is called a "V Sharp" thread where the OD is exactly the diameter of what you are working toward. And the bottom of the thread is a sharp "V" or 30 degree angle. But as is often said in many fields, "in theory, theory and practice is the same, but in practice they are not."
In Practice, the tooling will "dull" a whisker so that the V Sharp is no longer sharp. This is outlined in Machinery's Handbook, which has editions going back to 1916. I don't know the current standards for threading, they have changed several times over the years. But the final touch up of threads involves taking a whisker off the OD after the threading pass(es). And the bottom of the thread is actually a slightly rounded form for the same reason. In production, tooling will dull after a while.
In practice, the slightly undersized OD is a practical solution for the outside of the thread. When I cut a 32 TPI thread on a 3/16 rod, I start at 3/16 inch and cut to a sharp profile. Then following the threading cut I touch up the edge with a whetstone to the point that the sharp edges are gone. Essentially duplicating the "standard" given for a Unified Thread. Unified Threads are a post WW2 phenomenum. where, among other things, Whitworth 55 degree threads were reconciled with 60 degree threads. I only understand about 10 percent of all that, it's a deep subject.
The bottom or inside of a thread is a function of the nose radius of the particular tool used. It is normal for the novice to use carbide tooling to learn. Either brazed or insert types. If you look closely at the specifications for a particular insert, there is a given size for nose radius. When grinding HSS tooling, this is less of an issue. For a threading tool, the target for grinding is a sharp point. But grinding tools is as much an art as a science.
The point is that a carbide insert for a "large" thread, say 10 TPI, is in no way suitable for a smaller thread of 56 TPI. It simply doesn't cut deep enough. Leaving the bottom of the thread too shallow or rounded. 56 TPI requires a much smaller nose radius. As grinding tools is an artform, so too is threading. There are many details to be learned, but the final parameter is "touch". Just how much to make that last pass where you don't adjust any deeper, just make another pass to smooth up the surface. That's the art part of threading. I highly recommend acquiring a copy of Machinery's Handbook. Which edition you acquire is as much a matter of what you will be doing. I have several copies, including a 1942 version and a 2005(?) version for when I need to see what the current practices are.
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”Oil field thread technique”
I’ll have to check that out.
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You bring up some interesting points Bill.
I have wondered if there is a range of thread sizes your typical carbide threading insert is designed to cut.
Mine have broken teeth. Not a big fan of carbide when threading. I have gone to HSS,
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To my knowledge, there isn't a specification as such for thread sizes. There is however, a specification for nose radius. Looking at the less expensive tooling I use, the "kits" or assemblies of tooling will have a fairly large nose radius. That larger radius is more suitable for general turning, having a stronger tip. But for threading small screws like I do, a 1/64 inch nose radius is too large. Think it through, a 1/64 radius equates to a 1/32 inch diameter. Try cutting a 64 TPI thread with a piece of 1/32 rod and you can see the problem. The thread becomes a spiral "U" shaped groove around the screw rather than a "V" shaped thread. In most cases, I grind threading tools from HSS. Carbide is a fair, general, turning tool. But if I'm doing threads, I need more "right" than mass produced carbide can provide. At least in my price range. There may be finer tips at a higher cost. . . I don't know. It's a lot easier to just grind it.
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