[mwbridgwater]

I'm putting 1/4-28 threads into some 303 stainless fasteners I'm making. Blind hole in parts 1" long, so drilling .950 deep and minimum thread depth is called out at .750 (Why they need a 1/4" fine thread that deep, I don't know - but suspect it wouldn't be necessary if the engineer knew what he was doing...). I made a fixture to hold 10 of these at once for doing some milling on the ends, then drill/chamfer/tap each one.

Presently, I have an M1 just before the end where the tapping starts. When it gets to this point, I blow the soluble oil/chips out the holes and fill each one to the top with Mobilmet 766 Thread Cutting Oil. Using 2-flute, spiral point, plug chamfer, coated taps (I first tried spiral flute since it's a blind hole, but those began seizing almost immediately at very shallow depth and even when brand new/sharp.). Back to the spiral point 2-flute: Works okay, but I can only program .550 deep (That's including the chamfered end) before seizing might begin. I think the cutting edges just need oil on them again to keep going deeper - which doesn't happen when it just continues to be driven down.

So, after I've run them to this point, have to fill with oil, hold part in the lathe collet, pickup the thread with the tap (in the tailstock chuck), and power tap down to chips where the tap slips. Retract, blow out chips, and repeat down to the bottom of the hole. This gets me about 1/16" beyond the minimum depth specification (2 revolutions of the 28 pitch thread.) (I have the point of the tap ground off.). Having to finish them this way (640 pieces, manually) is too much trouble/time consuming.

Two thoughts:
1) If I have an M1 after tapping each part in the fixture to the .550 depth - to blow out and fill with oil again, can I program to go back in a second time (Two separate times actually, like I presently am manually.) and expect the machine to stay synchronized with the thread it put in previously? And then carry on through each of the ten positions in my fixture that way? By the way - using a 1993 Fadal VMC3016.

2) I've never done thread milling, but thinking that might be the best solution. Can someone share a sample program for that? Don't know if it would need to be Fadal-specific in any way...

Thanks!

[Jim Dawson]

Try using a #1 drill (0.228) rather than a #3 for a tap drill and try the spiral flute again. I deep tap (> 1 inch) 304 all the time, but normally 1/4-20, 5/16-18, and 1/2-20, never tried a 1/4-28. I just use Bassler coolant as a cutting oil mixed 7%, either flood or high pressure depending on the machine. Just spent the last 11 hrs tapping 304, 1/2-20, 1.5 deep, about 200 RPM as I recall. About 500 parts on that particular tap.

[djr76]

2) I've never done thread milling, but thinking that might be the best solution. Can someone share a sample program for that? Don't know if it would need to be Fadal-specific in any way...

Thanks!

Scientific Cutting tools, Inc, has a thread mill G code generator.