Threading on mini-lathe, what am I missing?

Use some small drill bits or sewing pins as thread wires and compare against a factory made thread. Doesn't matter what diameter the pins are, you just need to get the same measurement as the factory thread. Not the class fit you're looking for, but that will at least give you a proper measurement of how far off you are from your desired diameter, without having to buy wires.

You said you're using a dial indicator against the compound though. Assuming that's accurate, which is usually safe to say, then the issue probably lies in the part (or the machine) flexing during the deeper passes. Run a few spring passes at the end until it stops biting, and minimize your part stick out. Using a sharp HSS tool might be better than a carbide tool for these passes.

EDIT: By the way, as far as the 29.5 degrees thing goes, it helps with chatter for a mini lathe. Especially if it isn't braced against anything and has stock bearings, it needs all the help it can get.
 
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Lots of things here. I'm no expert, but I don't mess with the 29.5° stuff. Just feed straight in. Also, flex will be a huge thing. Are you doing any spring passes to verify you're actually cutting what you feed in? Even still, the last little bit might require further feed to get it to cut while have spring back if the cutter. Like ulma said, carbide will require more force.

Also, a 3a fit will be a tight fit anyways. I've cut some to extreme tolerance on both sides and even tho they measure correct by the book, they won't thread on with out lots of friction which might make you think it doesn't actually fit.

Try running a triangle file down the root of the threads to clean them up and then a at file across the tops. .498 is large on the major diamter and might be interfering with whatever female thread you are using if they were cut with anything less than a perfect 60° point.

You cam count numbers and measurements all day long, but in the end if parys don't fit, they don't fit. Cut them to fit and you're good no matter how much you dialed in on the cut or what the measurements actually read.
 
springer said:
Try running a triangle file down the root of the threads to clean them up and then a at file across the tops. .498 is large on the major diamter and might be interfering with whatever female thread you are using if they were cut with anything less than a perfect 60° point.
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I didn't even think of that, but yeah, nothing about filing the top has been mentioned. The male and female threads aren't supposed to come to perfect V's, they should have small flats. The cutter will never have a perfect tip, so the thread valleys won't have perfect angles.
 
Also, if this is in mild steel there is likely a burr built up on the OD that needs removed with that flat file across the top.
 
How are you supporting your work? Is there a center support via the tailstock? How many spring passes are you making? Does material still come off with your spring passes?

If you are NOT using a center, your work is probably deflecting from the tool pressure. Without a center, your threads will be anything but class 3A, since the PD will vary greatly as you near the headstock end of your work. You show no pictures, so we have to assume that you have a SHARP tool and it is set up properly.

Trying to cut class 3A threads on your first try is an exercise in futility. You should practice making 1A and getting it correct, then proceed to class 2A and getting that correct BEFORE you jump off to class 3A threads. You state that you are shooting for a minor diameter of 0.457". Minor diameters are dimensions that CANNOT be measured by most people (and, they don't really matter). What you HAVE to be able to measure is your Pitch Diameter (PD). How are you going about measuring your PD?
 
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