Tool for 5/16" 2 start internal thread

In desperation can anyone confirm that it is possible to cut a 2 start internal 5/16" thread. I've tried several times to grind a thread cutting tool from a 1/4" HSS
drill bit, but the cutting tip keeps chipping off.
The lathe is set up for the 10 tpi thread and the part is mild steel. any advice would be welcomed.
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phil wrote:

Try getting a real tool steel blank from one of the online suppliers, MSC, J&L, etc., and grind your tool from that. You can even get round blanks for certain types of HSS. I like Tantung blanks for internal threading, they seem to last a long time. Used to cost 2-3X as much but lasted 10X as long. I've done 2-start threads, but nothing that small. There's some fancy driver plates you can use that will let you index your work 180 degrees while leaving the gear train engaged. Instead of building one of those, I carefully marked the tumbler and headstock gears, cut the first thread, then disengaged the tumbler and flipped the work a half-turn and re-engaged. It worked. Make sure you have a male thread to gauge against.
Stan
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Thanks Stan..... sounds like great advice. Ive got the male thread so I'll get some tool steel and try that.
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On 13 Oct 2006 13:40:32 -0700, snipped-for-privacy@prolynx.com wrote:

I did a 4 start last month.
1"-18
Took me about 15 minutes to program the lathe <G>
Gunner
Rule #35 "That which does not kill you, has made a huge tactical error"
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A 10-pitch thread in a 5/16" hole has a pretty steep helix angle, about 5.8 degrees. If your cutter isn't ground with this helix angle, it will either chip or barf the thread.
Back the cutter well out of the thread before returning to start for another pass.
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Thanks Don. So does this mean that on the lead side of the tool the relief angle should be undercut by about 6 degrees more and with the trailing side about 6 degrees less. Hell I wish I could draw on this thing.
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phil wrote:

Wow, that sounds very difficult for single-point threading! The small diameter plus steep thread pitch means the tool has to be steeply releived on TWO sides at once. This may be a case where you have to do it with a tap-like tool. How about cutting the male version of the thread, then grind away some flutes to make it look like a straight-flute tap?
Jon
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There was an article in either Home Shop Machinist or Machinist's Workshop / Projects in Metal a year or two ago about cutting small double lead internal threads. If I remember correctly, the author used a straight flute tap with the proper TPI and gound off all but two adjacent points near the tip of one flute, ground some extra clearance on the leading flanks and cut both leads (starts) at the same time. The first tip cuts one lead and the second tip starts cutting half of a pitch away from the first making the second lead, no need to index the part 180 degrees or make compound / cross slide adjustments for the second lead. From your description it sound like your cutting a 5/16-20 double lead (2 start) thread. A 1/4-20 tap might make a good tool, grind as described above and thread with the lathe set for 10TPI. YMMV Good luck, Paul
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Since the lathe is set to 10 TPI, you're doing a 5/16"-20 2 start thread.
Depending on your lathe, you may be able to do each lead from a different mark on the threading dial. I do 4 start 36 TPI threads all the time, setting the lathe to 9 TPI and cutting each lead the same depth by using each of the 4 marks on the threading dial. Just happens to work well on my setup.
The tool needs extra clearance. Imaging looking straight on at an internal threading tool, pointing towards you. On most tools, you see the line come straight down from the cutting point. For a multi lead, the line you see should go down and to the right. That is, you need some extra clearance below the trailing side of the V. The amount can depend on the thread.
____ \ | / <- front view of regular internal thread tool
____ \ \ / <- front view for multi-start
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