What did you use for the thread cutting tool bit ? Did you grind it yourself from a high speed steel blank, or did you use a factory ground threading tool bit ?
Grinding a 60 degree threading tool bit freehand is something many of us have done many times. However, a freehand ground thread cutting tool bit can be off slightly in terms of the profile of the thread it actually cuts. For most work, this little bit of error in thread form will work since the class of fit for the thread being cut may not be all that close. With a freehand ground thread cutting tool bit, it is sometimes necessary to "ease off" the thread being cut by a few thousandths to get things to make up. This easing off of the thread being cut addresses the differences in thread forms from cut thread vs whatever that thread has to make up with. The actual contact of the threads is less than "full flank contact" as a result.
The other issues which could be keeping your "dummy spindle" from making up with female threads in backplates, dog plates, or similar could be burrs or "spring" of the tool holder. A cut thread often has burrs, and these can prevent the cut thread from making up with whatever it has to screw into or onto. Running a fine lathe file over the work while it is turning in the lathe will knock off these burrs which often form at the corners between the flanks and crests of the threads. Spring occurs when the tool holder deflects slightly from the load put on it by the cutting process. Often, when a job is "nearly finished" it not make up and is a bit tight. Despite mike readings and corresponding movement of the cross feed or compound, a "spring cut" the final depth of the thread may be a bit shy of what it needs to be. This is due to the deflection of the tool holder and tool bit. A spring cut is taken by setting the tool bit to the final or last depth of cut and running another pass or two. Often, if a person looks closely at the tool bit, they will see a very fine chip coming off the tool bit's cutting edge. This is the "spring cut". Sometimes, two or three spring cuts are taken, each removing a very small amount of material.
The other question is whether you pulled the toolbit back (backed the tool bit slightly with the compound set at 29 degrees) and infed the cross feed an equal amount. This will cause the tool bit to cut on the RH side of its point, cleaning up the flank of the thread opposite the way the toolbit is travelling.
My hunch is you are most likely dealing with burrs, or possibly needing to take a spring cut or two, since the dummy spindle starts to screw into backplates or similar female threads. Start with running a lathe file (single cut file, long angle) across the crests of the threads while the work is turning in the lathe. A few light strokes with a lathe file are generally all that is needed. Try the fit of the dummy spindle after the filing. If things do not make up, take a spring cut if you still have the job in the lathe. If not, you will need to set the job back up in the lathe, "catch" the thread with the tool bit as we discussed in a previous thread (sorry about the pun) when you asked about that, and take a very light cleanup or scraping cut. A few passes with the tool bit not quite touching the threads, advancing 0.001" each time on the compound until the tool bit just begins to shave the least bit off the work is all we would be looking for.
A slight difference in the profile of the tool bit can cause this sort of problem when cutting a thread for a close fit. Add burrs and "spring" of the tool holder and the result is a thread that will start to make up but seem to bind after a very few threads.
