r/machining • u/McFappy_69 • 1d ago
Question/Discussion How is thread milling physically possible??
Apologies in advance as I will have a hard time articulating my confusion here, but thread milling baffles me. Also sorry for potentially wrong terminology, I'm relatively new to machining. As far as I'm aware, the teeth on a typical thread mill are totally horizontal. If you are cutting a 1/4 20 interior thread using a 1/4 20 thread mill, I don't understand how this results in clean threads, when it seems like it should just cut a smooth hole. The width of the teeth on the thread mill, or at least the width of the portion of the teeth that engage with the material at any point in time, are wider than the cross section of the grooves of the thread that is being cut. Thus, regardless of your feed rate in any axis, you should be destroying the threads you just cut as soon as you move lower in Z. I can understand as you move to larger hole diameters with the same thread pitch this stops being the case, but with the 1/4 20 mill and 1/4 20 thread example the physics simply don't work in my head. Again, I don't feel like I have the right vocabulary to really communicate what my confusion even is, but if anyone understands what I'm saying, please explain how thread milling isn't just witchcraft we've all agreed to just accept.
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u/Geti 1d ago
I think there's a misunderstanding here. The tool doesn't just plunge cut. You move it in a helix up or down so just the teeth are in the wall.
It requires a cnc - or I guess any mechanism to achieve a helical path if you want to be pedantic.
The movement of the cutter through that path creates the threads, and there's a bit of art in exactly how much to engage to get the fit right. But there's a fair bit of tolerance in the amount of clearance as well. Bolts aren't that picky.
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u/zacmakes 1d ago
Yep! It also might help OP to know that a 1/4-20 thread mill is actually a 3/16" OD cutter, and all thread mills are undersized like that so they'll clear the inner threads at their rated diameter.
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u/EtDM 1d ago
But there's a fair bit of tolerance in the amount of clearance as well. Bolts aren't that picky.
That depends on the class of fit needed. I've seen threads milled to incredibly tight tolerances in aerospace work.
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u/Geti 20h ago
Sure. In absolute terms depends on the fastener as well of course. M12 coarse vs M1 fine we're talking orders of magnitude, and there's all sorts of tapered forms and blah blah blah 😉 but for a guy wondering how they possibly work just knowing there's some clearance with threads anyway might help
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u/Charming-Bath8378 1d ago
watch some videos of helical gear hobbing. that will really twist your noodle:) and also might help you visualize what is happening with a thread mill.
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u/JeepHammer 1d ago
Watch a video before CNC when it was all mechanical...
When I was 11 or 12 years old (like '72) we toured the Dana corporation and I couldn't take my eyes off the differential gear cutting & grinding machines, all mechanical.
They ran previous gear sets on an educational display, the gear whine, chatter, in cutaways then showed the 'Hypoid' sets in cutaway so you could see them work under load.
Being that 'Weird' kid that was all about gears & wires I think I had my first 'Nerd-gasm' right then and there...
The second one came the same day with the operating display of an oversized Torsen-Gleason torque differentiating differential...
I think my 'Gear-tisum' was showing... "Aspurger King" for lunch that day...
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u/Memoryjar 21h ago
What would really blow OPs mind is that the cutter for the involute gear form doesn't have an involute gear form shape. The cutter actually has a more squared off shape, and when the gear rotates against the cutter, it creates the perfect tooth shape to mesh with the cutter and other gears.
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u/CrazyTownUSA000 1d ago edited 21h ago
It's just an end mill with the thread form ground into it. A 1/4-20 thread mill is good for any 20 pitch 60 degree thread.
The threads on a thread mill are like grooves. They don't follow a thread helix like a tap does. If you made a straight cut with one along the side of a part, it would leave the tooth pattern.
When you use them, you'll have a circle move with a Z move.
Say our tool diameter is .18 for the following. I'm just going to put a simple code as I would if I was standing at the machine and just needed it done.
G0 X0 Y0
Z-0.75
G1 X0.035 F25.
G3 X-0.035 Y0 Z-0.725 R0.035
G3 X0.035 Y0 Z-0.70 R0.035
G1 X0
G0 Z0.1
It made a full circle in XY and moved one thread pitch in Z. Each thread tooth only had to cut one full thread along the 0.750 depth.
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u/THE_CENTURION 17h ago
The tool has clearance cut back from the cutting face appropriate to the thread pitch. And it only cuts on one side of the hole, unlike a tap that cuts on the whole diameter simultaneously.
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u/Dr_Madthrust 1d ago
Thread mills are made for specific thread pitches. The teeth are not "totally horizontal", they are ground to cut the correct thread form when ramping at a specific pitch. You can cut threads in any size hole as long as you stick to the pitch the tool is designed for.
They work on the same principle as lathe single point threading, just the tool is spinning rather than the workpiece.