Making a tap for an unusual thread

[rxh]

I recently posted in the Hand Tools section about a Broken Norris Plane Adjuster. In order to repair this adjuster I needed to cut a 7/32” x 40TPI left handed thread and was unable to find any suitable taps in this weird size to buy so I set out to make my own of silver steel rod.

I cut threads by “hand cranking” the lathe and then hardened and tempered the rod. Then I cut the flutes using the makeshift arrangement of a Dremel tool with cut-off disc held in cross vices on a drill press. I tried the tap but I couldn’t get it to start the thread. I cut the flutes deeper and made a second tap with more of a taper on the end. These taps worked but didn’t cut as easily as commercial taps. However, they did the job but ended up rather twisted owing to the effort needed to turn them. I suspect that by good fortune I made the tips harder than further back on the taps so I got away without them breaking.

I thought members might be interested to see this rather Heath Robinson little adventure. I hope any proper machinists reading this are not too offended by the methods shown

 

[TFrench]

See, you should have said you were making a spiral fluted tap then we'd have been none the wiser :lol: Nice job though!

 

[DTR]

Nicely done =D>
Sounds like a left handed ME thread.

 

[chaoticbob]

Interesting indeed - particularly the ingenious method of cutting the flutes. I guess you could also have done that on the lathe, holding the tap blank in the chuck and clamping a Dremel equipped with a bull nosed cutter to the cross slide.
I had a similar experience (ie homemade taps not cutting as freely as commercial ones) until I took advice from a pro who gave this picture:

I printed, measured and scaled to my application - massive improvement.

Obviously the OP has accomplished his aim, but this might be of use to someone else going down the same road.

Robin.

 

[rxh]

Thanks chaoticbob - that makes a lot of sense.

 

[Alan Smith]

Late to the party! The twist on those taps is Rt hand but I thought they were Le hand threads so were they jamming on the way out???

 

[MusicMan]

Puzzled why you needed to hand crank the lathe? Did it not have the right gearing or traverse? Most lathes that can cut screw threads at all will have a back gear to reduce the speed suitably, and a tumble reverse to allow the tool to be driven in either direction.

 

[rxh]

Late to the party! The twist on those taps is Rt hand but I thought they were Le hand threads so were they jamming on the way out???

Yes, they resisted unscrewing.

 

[rxh]

Puzzled why you needed to hand crank the lathe? Did it not have the right gearing or traverse? Most lathes that can cut screw threads at all will have a back gear to reduce the speed suitably, and a tumble reverse to allow the tool to be driven in either direction.

My Super 7 lathe has back gear, tumbler reverse and motor reverse. I just prefer the "feel" that hand cranking allows when cutting fine threads. It was not very laborious.

 

[MusicMan]

Thanks, fair enough.

Keith

 

[Rorschach]

Hand cranking is great for threading in tight quarters too, even though the clutch on the Super7 gives you a lot of control, it still gets bit scary when threading up to a shoulder.