Thread chasing troubleshooting.

[Jack Freed]

I’ve been trying my hand at thread chasing and I keep running into the same problem with pretty much every attempt. I can never get the top and bottom to seat properly. The top always tightens with a slight gap on one side. I’m getting cleanly cut threads, and the top and bottom work smoothly with no binding or slop, but they never want to tighten properly. There’s always a gap on one side, anywhere from very slight to as much as 1/16”.
Any advice would be greatly appreciated!

 

[robo hippy]

I haven't done hand chasing, yet. Best video I can recommend is one by the late Allen Batty, father of Stuart. I would expect that having the male and female parts dead on parallel, and the shoulders spot on as well before you start to cut would be very important. That can make a big difference with thread cutting with the jigs and cutters. It could be that threads are 'bottoming out', which to me means that the male part doesn't have threads cut far enough down for the lid to seat, if that makes sense. I do cut into the bottom of the tenon a bit for 'relief' so I can screw the cap all the way down.

robo hippy

 

[Brian Gustin]

I dont do thread chasing (yet) as I don't have the tools (and my lathe ain't exactly great either) but I have looked into it - and one of the most important things as Robo said is the tenon & mortise have to be absolutely perfectly square to the body. Just like with flatwork, the more out of square (even .002 out of square) the worse the gap or alignment issue is at the far end - Likewise the further away from the 90 degree corner you go the more important it is to be dead-on square Given your gap of 1/16 inch over a , say, 4 inch box, it figures to being nearly a quarter inch out of square over 1 foot... which is a lot, so I'd suspect it might help if you had a small 2 inch machinist's square and practice cutting tenons (longer tenons) and use the square to see how far off your tenon is from square to the body - Same with your mortise - If your wood blank is perfectly square across , then cut a mortise or tenon it should be dead parallel to the centerline axis of the headstock.

just my 2 cents worth (as if 2 pennies would be worth anything...)

 

[GRJensen]

Take a look at Sam Angelo's Youtube channel ... he has several very good videos on thread chasing.

 

[hockenbery]

There’s always a gap on one side, anywhere from very slight to as much as 1/16”.

I’ve never had this issue. In cutting the male thread I always have an escape gap where I pull the cutter out before it jams and scrapes all the threads away. There should be nothing to bind where the surfaces mate.

It seems the only way to get a gap is for the one of the mating surfaces to be out of square with the threads.

When I cut threads I screw the parts together while they are on the lathe.
When you do this you could check for the gap.
The piece being screwed on is one that should be out of true.
You could turn a threaded waste piece screw the piece on and true the edge with a gouge.

My guess is you are not cutting the matting surfaces square or at a very slight angle so that a tiny groove shows when closed.
If you scrape or sand them that could make them out of true. The mating surfaces need to be cut on the same chuck Mount as the threads.


Maybe you are pushing too hard when you thread and pushing the work off center.

Be sure you have a good tenon on your pieces- no vibration.

 

[robo hippy]

For making sure the tenon and recess are parallel, with my Baxter and Klein threading jigs, I use a trick Bonnie taught me. I keep a machinist 6 inch steel rule handy. I lay it up against either surface, then eyeball it against the lathe bed. You can get it pretty much dead on with a bit of practice. I use a specialty home made NRS for the lid/recess, and a small skew, also used as a NRS for the tenon. I don't know thousandths of an inch, but with the 6 inch rule, I can get it within a hair from one end of the rule to the other..... The rule does rest on the tool rest, which is set at center height. Makes it easier to hold in place.

robo hippy

 

[Jack Freed]

Thanks for all the suggestions! I have always used the ruler trick to check for square, but thinking it through, one or both parts being slightly out of square is about the only way this could happen. I should probably be checking a little better. That, and making sure the threads themselves a square once they’ve been cut.
The one remedy I did come up with was to make the fit a little loose. That way there is a little play when the top and bottom come together. Not a great solution, but it helps.