To All:
There is an article in the December issue of Cutting Tool Engineering about thread milling, that IMO, is very informative. I've copied some excerpts from the long article below:
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Where taps fear to thread
Heat-Treated Steel
Thread milling internal threads in heat-treated steel parts from 40 to
55 HRC has gained ground in recent years as part of a wider trend among shops to machine harder, more resilient materials.Carbide Thread Mills
The tool of choice for threading holes ¾" in diameter and smaller in heat-treated steel is the solid-carbide thread mill. The indexable carbide model is recommended by many toolmakers for larger holes.
Milling Stainless
Aside from material hardness, the amount of clearance at the bottom of a hole can be the deciding factor for choosing whether to thread mill or tap. If the hole is too shallow for a tap?s lead thread length, a thread mill makes more sense. ?You hold size better [with the thread mill] compared to a bottom tap, which has a tendency to cut oversize,? said Tworek.
Climb milling, as practiced by Micic and Tworek, is essential to effective thread milling, said William Durow, senior milling specialist for Sandvik Coromant Co., Fair Lawn, N.J. ?When you climb mill, you ensure a thick-to-thin chip,? he said. ?Carbide thread mills work very well in compression; when you conventional mill you create tensile stresses, which carbide does not like. [Conventional milling] also creates more heat, and when you are dealing with difficult- to-machine materials the conditions are multiplied.?
Tapping vs. Thread Milling
While X-L Engineering?s Tworek noted that tapping and thread milling run nose to - nose in terms of threading time, tapping has the clear advantage when it comes to certain-sized holes in hardened material. Using a tap is the prudent option when the depth-to-diameter ratio is about 3:1 or greater. Thread mill deflection becomes a greater risk in those cases.
And while a thread mill can cost five times or more than a tap, it?s much more likely than a tap to remain intact for multiple threading jobs on hardened material. (A tap for the hole Micic threaded costs about $16, whereas the price of a 6-32 thread mill can run from about $57 to $97.)
Thread mills also provide deeper, stronger threads. ?When you tap, you drill the hole first, generally 25 percent larger [than the minor diameter] to make the tapping operation easier,? Halas said. ?That means that the thread is not 100 percent; it?s only 75 percent because you lost 25 percent drilling oversize. With thread milling, you do the opposite. You go in undersize, take the material off and get a 100 percent thread on there. What you gain from that is 5 percent to 7 percent more thread strength.?
Troubleshooting common thread mill problems:
- Thread mill is showing accelerated or excessive wear.
Causes:
a) Incorrect speed and feed selection. Solution: verify the correct speed and feed was selected from the speed and feed chart.
b) Excessive tool pressure. Solutions: Decrease the feed per tooth; perform tool change at quicker intervals; check the tool for excessive wear?beginning threads will wear the fastest.
c) Incorrect coating creating built-up edge. Solutions: Investigate other coatings; increase the coolant fl ow and volume.
d) Spindle speed is too high. Solution: Decrease the spindle speed.
- Cutting edges are chipping.
Causes:
a) Incorrect speed and feed selection. Solution: Verify the correct speed and feed selection was selected from the speed and feed chart.
b) Th read mill moved or slipped in its holding device. Solution: Use hydraulic clamping chuck.
c) Lack of machine rigidity. Solutions: Verify the workpiece is being properly clamped; retighten or increase stability if needed.
d) Insufficient coolant pressure or fl ow. Solution: Increase the coolant fl ow and volume.
- Steps in thread profile.
Causes:
a) Feed rate is too high. Solution: Decrease the feed rate per tooth.
b) Ramp-in is programmed as an axial move. Solution: Make sure the threadmill is arcing in the major diameter instead of making a radial move.
c) Excessive thread mill wear. Solution: Perform tool change at quicker intervals.
d) Tool is sticking out of the holder too far. Solution: Make the amount of overhang in the holding device as short as possible.
- Gage difference from part to part.
Causes:
a) Tool is sticking too far out of the holder. Solution: Reduce the overhang in the holding device as much as possible.
b) Incorrect coating creating BUE. Solutions: Investigate other coatings; increase the coolant flow and volume.
c) Excessive thread mill wear. Solution: Perform tool change at quicker intervals.
d) Workpiece moving in its fixturing. Solution: Verify workpiece is being properly clamped?retighten or increase stability if necessary.
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