I am threading a lid for an urn. Sorby 16 tpi threader has a thread dept of 1 millimeter. I am concerned the short thread will be too easy to cross thread because of the large lid. Is the thread depth of the 10 pti cutter significantly deeper than the 16 pti cutter.
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Sorby thread chaser
- Thread starter Dave Smith
- Start date
I did the math ( I think correctly) the 60 degree thread will form a 60 - 60 -60 triangle and the open side is the pitch length (PL) therefor all three sides are the same so you can then construct a right triangle with c = 1/2 PL, a = PL & b = depth then solve for b using the formula b = sq root (a sq - c sq). The result is 16 TPI b = .054" or 1.37 mm and 10TPI b = .0867" or 2.2 mm.
I have done many urns with threaded lids (including my avatar) and living in a state where the inside humidity drops way down in the winter and rises in the summer I have learned that fine pitch threads are bad. The problem is that you can not have enough slop or loose tolerance to account for the seasonal changes, without the lid either jamming or falling out so I would say that 10 TPI is the fine thread limit. Also note that any crumbling of the thread points will aggravate the problem. The method I use is based on a metal lathe such that I can cut threads down to 4TPI if I so desire. The problem where some woods will not cut clean threads I then soak the area with CA. The thread being cut in the picture is 11TPI.
I have done many urns with threaded lids (including my avatar) and living in a state where the inside humidity drops way down in the winter and rises in the summer I have learned that fine pitch threads are bad. The problem is that you can not have enough slop or loose tolerance to account for the seasonal changes, without the lid either jamming or falling out so I would say that 10 TPI is the fine thread limit. Also note that any crumbling of the thread points will aggravate the problem. The method I use is based on a metal lathe such that I can cut threads down to 4TPI if I so desire. The problem where some woods will not cut clean threads I then soak the area with CA. The thread being cut in the picture is 11TPI.
Thanks. I need bigger threads. Rats.
Great explanation, Don. Thank you.
How often is anyone going to open the urn? Once to check it out when they received it from you, and once to put Grampa in there. Maybe once more if they're going to scatter him across the south 40. It's not like a coffee container that would be opened every single day. Coarser threads would be more desirable, but maybe it's not enough of a concern to buy or make new cutters.
If you make the urns because you like doing it but are challenged in the marketing area it does not help if you can't get the cover off when you do manage to find a buyer or want to donate to a friend.
The urns that I have sold and were filled at the mortuary the mortician usually seals the threads so they can not be opened.
I have a chefware kit threader. It uses a blade similar to Don's, except mounted in the headstock a spun at high speed. The work piece, mounted to a chuck, spirals into the bit on a threaded insert equal to the desired threads. The inserts are changeable for different threads. I have the 16 and the 10 TPI. I think they make a couple of more coarse sizes too. It is not a cheap jig, but is pretty fast and easy to use. Excellent quality in it.
My experience with the 16 tpi is just like Don's: So tight in the summer you can't loosen or so loose in the winter they almost don't engage and will slip like a stripped thread will do. I think the 16 tpi is really for small openings, like no bigger than an 1.5" in dia or so.
I've read where some guys buy brass thread inserts they glue into their urns. No idea where they get them. I guess you could also turn brass ones yourself.
And the best threads must be cut in really dense materials or they will crumble. CA glue helps, but no substitute for tight grain, dense woods. And like with the brass, you can make dense wood inserts/threads tenons and glue them into the urn. I've done this several times and it works pretty well.
I've even thought about the tap and dies used to make wooden screws for vises and was wonder it they would work. I think they are like 3" in diameter. They are pricey too. Of course, the bigger the thread size the more shoulder you'll need in your urn.
So, after all of this, the brass is probably the way to go
My experience with the 16 tpi is just like Don's: So tight in the summer you can't loosen or so loose in the winter they almost don't engage and will slip like a stripped thread will do. I think the 16 tpi is really for small openings, like no bigger than an 1.5" in dia or so.
I've read where some guys buy brass thread inserts they glue into their urns. No idea where they get them. I guess you could also turn brass ones yourself.
And the best threads must be cut in really dense materials or they will crumble. CA glue helps, but no substitute for tight grain, dense woods. And like with the brass, you can make dense wood inserts/threads tenons and glue them into the urn. I've done this several times and it works pretty well.
I've even thought about the tap and dies used to make wooden screws for vises and was wonder it they would work. I think they are like 3" in diameter. They are pricey too. Of course, the bigger the thread size the more shoulder you'll need in your urn.
So, after all of this, the brass is probably the way to go
Thanks everyone. I used a home made threader for an other urn for a family member. I thought I would up grade the job. I decided to make the threads with the 1/2" bolt, 13tpi cutter I made. This urn has a 12 segment teak collar and produced decent threads. However, a teak board did not do so well. I figured a real threader would work better. But I decided to use my home made cutter and get a better blank for the male piece. I am in AZ for an other month and wanted to get the urn finished before heading home. I have plenty of honey locust and mulberry at home so I will wait. I do have some cocobolo and becote with me but those are not going in the ground no matter how much I love my sister.
The best threads can be cut in just about any wood, the key is to have a sharp bit, a solid mount for the piece being threaded and if needed a multistep application of CA. The CA only penetrates maybe at the most about 1/32" so therefore cut only a partial thread first and recoat with CA, then cut another step etc. and even after the final cut to size you may want to recoat and run at the same setting. The CA method may seam like a picky process but it it is a lot less problems and time then making and gluing inserts of dissimilar wood or material such as metal or plastic.
There was a mention of cutting threads in 8 segment rings and since the thread is cut into side grain that should be flawless except for minor chip out at the joint between segments.
This walnut urn was made before I had a digital camera I think about 25 years ago and it has a lid made from the same piece with 8TPI thread that has never jammed or fallen out
About 1990 I had a request to duplicate a bench screw that was 2 1/2' diameter and 2TPI so I came up with a setup using a 4TPI acme screw driven by a 2 -1 chain drive linked to the lathe head stock then a router carriage on the lathe ways moved by the acme screw to cut both the male and female threads.
john lucas
AAW Forum Expert
I wrote an article for one of the magazines years ago on making your own thread chasing tools. One method was to braze a tap onto the metal in two different configurations for the Male and female thread. Worked great. I also showed how to just file your own but that is quite challenging and takes a lot of patience