Internal Threading

M

m.batz2

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I am making a set of Machinist Jacks for Machinist school, and when I internally threaded the base section of the jack, the threads were all jagged. I used the Machinery handbook to get the minor diameter, the tool was on center, and I took maybe .010" at most per cut, lighter towards the end. I wound up having to take a piece of round stock with some emery cloth to try and clean up the threads. The tooling was a carbide insert, and material was 1018 CRS. If anyone has any insight I would definitely appreciate it. This was my first attempt at internal threading by the way.

Thanks,
Mike
 
Mike,
What diameter roughly is the thread? Small screw holes = small internal tools. I am thinking maybe flex/chatter/vibration in your threading tool is the problem. Also tool overhang. Did you have the tool set back as short as possible. Were they deep threads?

Cheers Phil
 
12bolts said:
Mike,
What diameter roughly is the thread? Small screw holes = small internal tools. I am thinking maybe flex/chatter/vibration in your threading tool is the problem. Also tool overhang. Did you have the tool set back as short as possible. Were they deep threads?

Cheers Phil
Click to expand...

The thread is a 1"-16. I had roughly 2-1/2" of threading bar hanging out, the minimum I could, and the thread is about .080" deep. The jagged threads isn't in the thread itself, it's on the crest of the thread.
 
What did you make the minor diameter? Sounds like it may have been too small if the threads came to a sharp crest.
 
Which way was your compound pointed? The handle should have been at 7 o'clock for internal threading and 5 o'clock for external threading.
Chrispy
 
Not surprising with 1018. Also, .010 cuts are way too heavy for any but the first one or two passes. After that, start backing off, .005, .003, .002, .001, and finally a couple of "rubber cuts" (no advance of the tool at all). A well ground HSS tool, honed, will be sharper and produce cleaner threads than carbide. Not knocking carbide, just sayin'.......

Finally, a little squirt of the old dark sulphur based cutting oil (pipe threading oil), will really improve the finish.

Regards
Bob
 
The minor diameter was turned to. 935", right within the spec for a 3B fit. The compound was set at 45°, but that doesn't matter, since I wasn't using the compoud.
I did make a second one, and it turned out fine, so the only thing I can figure, is I was either taking too heavy of a cut or I hadn't used enough oil. I took many free passes on the second set, so that could gave helped also. I will post some pictures when they are done. Thanks.
 
If you were not using the compound you must have been feeding straight in in which case the chips would be coming equally off both sides of the tool and piling up, a better way would be to feed in at 30 or 29 1/2 degrees to get most of the chip coming off one side of the tool. Or better yet have the compound set parallel to the ways and for every .002 infeed of the crosslide advance the compound .001 (26.5 degrees) then when you reach your depth you can advance/retract the compound slightly to clean up each side of the thread until your gage fits.
 
basalt said:
If you were not using the compound you must have been feeding straight in in which case the chips would be coming equally off both sides of the tool and piling up, a better way would be to feed in at 30 or 29 1/2 degrees to get most of the chip coming off one side of the tool. Or better yet have the compound set parallel to the ways and for every .002 infeed of the crosslide advance the compound .001 (26.5 degrees) then when you reach your depth you can advance/retract the compound slightly to clean up each side of the thread until your gage fits.
Click to expand...

Yeah, the way the instructor has taught us to thread was strictly using the cross slide, not using the compound. I have used the compound for external threading, and find the thread actually looks good. I had never heard the method of placing the compound parallel with the work piece and feeding slightly. I might try that some time when I am just practicing internal threading.
 
I didn't see the mention of the rpm used. With carbide, it's doubtful you could run it as fast as necessary to produce nice, shiny threads. But I would guess if I were doing it on such a large, fine thread, 500-600 rpm would probably be about right. Straight in feed is fine also as long as your bar is rigid. Otherwise, you do need to use a slightly positive rake on some HSS and compound your way in.

Plus the material choice could be a major factor. 1018 is pretty tough to get a nice finish on. But speed helps. I also use the dark sulphurized oil, but on the last few passes, I just squirt some Tap-Magic in the hole. I would have preferred 4140 HT, myself, or 17-4 H1025
 
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