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- Apr 19, 2011
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Is there a limit to the diameter and depth of the thread that a tool can cut without bottoming out?
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threading tool sizing
- Thread starter knifer-one
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- Joined
- Jan 22, 2011
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- 8,016
OK, we'll start with some assumptions. Let's say that you have a 3/8 square HSS bit ground to the 60 deg of a UN thread. Xero radius on the tip. Now, what happens if you set up a thread, let's call it a 2 1/4 - 8 for the sake of this discussion. You can start cutting, moving your threading tool deeper and deeper. Thread depth for that thread is 0.0677. What will happen if we go deeper than that? At some point, the crest of the thread will start becoming more narrow, and more narrow, until it becomes sharp. After this point, the OD of the thread will begin to shrink as you thread. That's because you are making an ever wider thread, or trying to, but the material is not there because the 8 pitch is forcing the tool to cut wider than the 0.125 you have to work with. You will still end up with an 8 pitch thread, but the OD will be small. Eventually, it could be a 1"-8, with awfully sharp crests. Thread crests should not be sharp, so if you were to make a 1"-8, then top the threads, the OD would probably be too small.
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There has to be *some* limit on depth...
The thread's pitch diameter's not a problem, in fact the larger the diameter relative to the lead of the thread (how far a nut moves per turn) and so the smaller the helix angle the better as it needs less relief on the leading edge of the tool, meaning it'll be stronger, and the top face's rake will be closer to the angle needed for a clean cut.
The depth of thread is limited, I guess, by non-cutting parts of the tool getting in the way? If you had a (f'rinstance) sharp-pointed 60-degree triangular tip to the tool, with the flanks nicely sharpened, you'd be able to cut threads with a lead up to the width of the triangular section.
The issue would come with the root of the thread - most are specified with either a flat or a radius at the root (deepest part of the groove), and a *sharp* root will make the threade part weaker because of the "stress raiser" formed by the sharp vee. The radius is pecified by the thread pitch / lead, so a tool ground with a radius suitable for a (f'rinstance) 32 tpi thread will cut a root radius far too small for an 8 tpi thread (although a nut would spin on nicely due to the extra root clearance). The thread's helix angle could also be greater than allowed for by the tool's relief, leadng the tool to rub one flank rather than cutting with the edge, leading to a huge increase in cutting force.
Conversely, a tool ground for an 8 tpi thread cutting anything much over 12 tpi will leave too great a radius and thin the crests of the thread too much leaving sharp crests too weak for use. If advanced by the recommended depth of cut for 12 or more tpi the result would be pretty poor, leaving the thread major diameter way undersize...
Just my ha'pennorth,
Dave H. (the other one)
The thread's pitch diameter's not a problem, in fact the larger the diameter relative to the lead of the thread (how far a nut moves per turn) and so the smaller the helix angle the better as it needs less relief on the leading edge of the tool, meaning it'll be stronger, and the top face's rake will be closer to the angle needed for a clean cut.
The depth of thread is limited, I guess, by non-cutting parts of the tool getting in the way? If you had a (f'rinstance) sharp-pointed 60-degree triangular tip to the tool, with the flanks nicely sharpened, you'd be able to cut threads with a lead up to the width of the triangular section.
The issue would come with the root of the thread - most are specified with either a flat or a radius at the root (deepest part of the groove), and a *sharp* root will make the threade part weaker because of the "stress raiser" formed by the sharp vee. The radius is pecified by the thread pitch / lead, so a tool ground with a radius suitable for a (f'rinstance) 32 tpi thread will cut a root radius far too small for an 8 tpi thread (although a nut would spin on nicely due to the extra root clearance). The thread's helix angle could also be greater than allowed for by the tool's relief, leadng the tool to rub one flank rather than cutting with the edge, leading to a huge increase in cutting force.
Conversely, a tool ground for an 8 tpi thread cutting anything much over 12 tpi will leave too great a radius and thin the crests of the thread too much leaving sharp crests too weak for use. If advanced by the recommended depth of cut for 12 or more tpi the result would be pretty poor, leaving the thread major diameter way undersize...
Just my ha'pennorth,
Dave H. (the other one)