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Adjustable Reamers
- Thread starter Tomc938
- Start date
From the look of the tap, it's a shop made item, rather than a commercial product.
About the only way to know for sure is to get a strong magnifer, some decent light, and a set of various pitch and form thread gages, and start comparing, among other things, pitch, thread form, angles, etc, as well as to round up a thread measuring wire set to get an accurate measure of the pitch diameter.
Personally, I think my FIRST step, would be to strip it out of the mess of old tape, and see if the maker actually engraved any useful info on the tool shank!
Seems pretty obvious that whoever stuck it away, wanted the respective drill kept with...
General rule of thumb that works for most of us, is that the drill be the nominal diameter of the thread, minus one Pitch of the thread. Bit of a pain to deal with the pitch on an inch size tap until you convert to decimal inches, mind, but it makes it dead easy in Metric, where the pitch rather than teeth per inch is used (eg 10mm x 1.0 pitch thread clls for a 9mm drill...)
There are no flutes the whole length so I was thinking a roll tap (form tap). But that short section at the front makes me think its a chasing tap for slightly damaged threads. I have something similar for shotgun screw-in chokes. But then why would you need the drill bit??
That has some interesting thread forms on it!
I note that it does not actually have the specs for the UNJ thread mentioned earlier, in regards to Machinery's Handbook.
FWIW, the UNJ spec is a high strength thread used in aerospace that calls for a radius root, on an external thread.
It WAS specced out on several of the parts we regularly had orders for in my last workplace, so, as it also had several other operations conducive to training, aside from opening an apprentices eyes to the fact that the 'bible' doesn't always have the answers needed. Finished training projjects, if not desired by the apprentice, would be used to fill the work order for them when such was ordered.
Not the finest example of an image, as it does not spec the radius, which, IIRC (doubtful! LOL!) is .25p
Edit again...: http://5168102.s21d-5.faiusrd.com/61/ABUIABA9GAAgotG3rQUo6IrLtgQ.pdf Leads to the ASME thread specification in .pdf format, or ...should...
And yeah, was wrong above! R is .18042p Max, .15011p Minimum.
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My nickel says it was a shop-made tap for something along the lines of steam fittings and pipework, as would be used on a live steam model, ferinstance. I see in some of the Model engineering suppliers catalogs 1/4-40 and 5/16-40 taps listed. https://pmmodelengines.com/pipe-fittings/
For as shallow a working depth as that, the foreshortened flutes would not cause any real grief, I think.
It was in my Dads tool chest. I don’t remember him ever being into models but he did work at the DND Dockyard in Victoria I’ll stick a gauge on it and see what it says.
Yeah. Outside Diameter (given the ranges of specifications available), as well as threads per inch, is a pretty good start!
We (us Airforce wogs!) had inch, Metric, and various Brit standards available to us (courtesy of the Rolls Royce Nene 10's in the T-33's) as far as taps and dies went. But that was just the off-the-shelf stuff.
I made several severals, of 'special' tooling over the years to get done various jobs, some of which were repeats, some of which never did...
I would suggest that this set of tools was just one such!