Cutting Left-Hand Acme Threads on the Metal Lathe 

I said, "and...that's how it's done!"

You forgot to add single point threading...

I'm sure Adam will post pics when he get is back on his instagram - blacksmithtools

@@CAPFlyer And / or he could be a member of OWWM.org?

We had a design engineer replace an 8620 cold-headed part with 12L14 on a screw machine. The part should have only been loaded in compression so the change was so much of a no-brainer that it didn't even need testing (or so they thought!). It turned out that dynamic inertial loading put the part in tension and they broke in the field, requiring an expensive field campaign. That experience caused the banning of 12L14 throughout the company, regardless of usage. No, I was not the design engineer that caused the problem. I was the test engineer that had to find out why it failed. 50 years later, leaded steel still causes bad memories. For what Keith was doing, it's a perfect material, making the turner's life a lot easier without compromising the actual use.

The leaded steel is a major factor in making this kind of awkward thread cutting possible. If the specifier will permit then always choose a piece of free cutting leaded. It makes the job so much less difficult.

@@kensherwin4544 That isn't a surprise, it was also banned in the shop where I did my apprenticeship.

Dave Calabretta confused the hell outa me too.

@@hodgepodgeenginerd1258 Same here.

I also agree that was confusing. Would like to know how those numbers were arrived at.

I will be cutting an acme thread for a restoration, really like the acme fishtail gage, but do not understand the comment about 1/2 the included angle being 12 1/4 deg. Thanks for your great video!

Acme included angle is 29 degrees, so half included is 14.5. .i am confused too.

@@littleworkshopofhorrors2395 That makes sense. The HALF angle is 14.5, making the 12.25 on the compound correct.

I think Keith meant to say 14.25 degree, not 12.25 degree. That would be 14.5 degree minus .5 degree. If you are cutting a 60 degree included angle thread, you set the compound to 29.5 degree.

Paul Topolski Nope, 14.5 minus 0.5 is 14.0. I’m confused too.

@@paultopolski1978 I had to watch it twice. I think you are write and it was just a slip of the tongue?

The thread makes contact on the flanks. There is clearance between crest and root so the runout will only vary the clearance, not affect the fit.

I was taught the same here in the States to break the chip. Never worked with leaded steel, perhaps that's the difference?

@@sunny71169 So are you saying the lead makes it softer and threads go without binding and possibly snapping the tap?

I learned tapping that way also. Always "Good Practice" in any material. Half turn, break the chip, another half turn. But I am an Engineer so we are kinda picky about things. Oh and "Wie Gehts!"

Maybe that tap is big enough that it can resist torsional loads and the chip channels are large enough for evacuating the chips?

It's just a beefy tap. And also the stops and starts while turning the tap tends to break the chip in leaded steel. From my experience at least just keep going till the tap feels clogged then take it out completely and blow out the hole then go back in. I'm guessing that's what was cut out in the video.

I was going to mention this myself. Another way of achieving a thread with the same lead as this 2 TPI is to cut a 4 TPI with 2 starts. The method I use for this is mount the shaft to be cut in a drive dog and mount on a centre in the headstock. A drive plate or catchplate with 2 drive pins spaced 180 degrees apart is used to drive the dog. Workpiece and dog rotated 180 degrees to cut the second lead. This thread is classed as 4 TPI 2 start. One revolution will give the same linear lead as the 2 TPI. The depth of cut on the twin start is far less deep than the 2 TPI we've seen Keith cut here.

gd walters Mr. Pete has some very good videos of multi start threads.

@@howardosborne8647 that's interesting. I have worked on overhead cranes with load brakes. They had 4 lead thread were very quick acting.

William Garrett Me too. And I’ve been doing this stuff for 40 years.

Yes. it does, now let's see if this quote is posted!