Cutting Threads

[thestelster]

No, near certain the profile is parallel to and through the works axis. You cannot for example cut a 60 degree very fast thread (say a multi start) by taking a 60 degree tool and tilting it. You grind the 60 degrees on a flat top then grind the required side clearance to accommodate the helix

But isn't that exactly what the shims do? Tilting the thread insert.

 

[Mcgyver]

But isn't that exactly what the shims do? Tilting the thread insert.

Maybe I don't understand what is being tilting how, but if you get the correct thread profile with zero tilt, then by tipping the insert X degrees, you will not longer have the correct thread profile. I think at one degree it won't matter, but if you keep tilting it, it will start to matter a lot.

 

[PeterT]

the profile is parallel to and through the works axis

We might be saying the same thing with different words, but not 100% sure. Only the center line of thread profile intersects the work axis. The thread section is parallel to work axis (example the flat of a buttress thread is parallel to axis looking down from top view). But the thread profile plane is perpendicular to the helix termination in 3D space = pierced relation = some angle relative to work axis normal plane The larger the pitch, the more the plane tilt. If the pitch were zero (a circle not a helix) then the plane would be 100% perpendicular & fully coincident to axis. The thread cut is constrained along (but perpetually perpendicular to) the helix path. This workflow mimics the thread geometry. The piercing bit is at ~2:20. Now this negates side clearance issues because its just a shape cutting through 3D space.

 

[Susquatch]

Maybe I don't understand what is being tilting how, but if you get the correct thread profile with zero tilt, then by tipping the insert X degrees, you will not longer have the correct thread profile. I think at one degree it won't matter, but if you keep tilting it, it will start to matter a lot.

Nah, I'm on @thestelster 's page.

Of course, you are right about how the single point tool gets distorted as you tilt it. But for a grinder you just form the grinder profile as it is when it's grinding.

In my mind I see it a wee bit like differential equations. As you tilt the grinding wheel and correct the profile, tilt the wheel correct the profile, etc and take the limit of the corrections needed till you get to infinity, the grind stone doesn't know its tilted to the helix angle. It just cuts the tilted profile into the thread.

Said another way, the profile in the direction of rotation of the grinding wheel isn't the same as the profile on the plane 90 degrees to the shaft axis. But if you take a section of the wheel at 90 degrees to the shaft axis, it is the same!

 

[Susquatch]

See what you started! LOL!

 

[thestelster]

by tipping the insert X degrees, you will not longer have the correct thread profile. I think at one degree it won't matter, but if you keep tilting it, it will start to matter a lot.

Yes, I agree, and perhaps that's why Sandvik make shims only up to 4°. Perhaps up to 4° the change in thread form is inconsequential in the real world applications.??

 

[Mcgyver]

Nah, I'm on @thestelster 's page.

Of course, you are right about how the single point tool gets distorted as you tilt it. But for a grinder you just form the grinder profile as it is when it's grinding.

In my mind I see it a wee bit like differential equations. As you tilt the grinding wheel and correct the profile, tilt the wheel correct the profile, etc and take the limit of the corrections needed till you get to infinity, the grind stone doesn't know its tilted to the helix angle. It just cuts the tilted profile into the thread.

Said another way, the profile in the direction of rotation of the grinding wheel isn't the same as the profile on the plane 90 degrees to the shaft axis. But if you take a section of the wheel at 90 degrees to the shaft axis, it is the same!

Right, but that's the question, you can't just dress it say sixty degree. What's the formula on how to dress the wheel, so you get the angle you want, say sixty degrees, in the plane through the works axis. I've just never come across a good account of how the man doing the thread grinding does so. Something I wanted to try at one point.