screw threads

[tinyminds]

i've become a bit obsessed with making screw threads in wood. i have had some success. (tho thread chasing appears to be impossible!)

the kits for turning screw threads with a thread box and a tap are way too expensive for just messing around so bargain hunter that i am i've been picking up bits on ebay. i got as 1 1/2 thread box and found a matching tap for it without too much trouble. then i found a really nice old 3/8 thread box and i've just tried it out to find out what tap i need to go with it and it appears to be 3/8" 9 tpi.

i have had no luck looking for a 3/8" 9 tpi tap, they don't seem to exist. is there any way to have a tap made? who would do such a thing? or can i somehow make one myself?

 

[dickm]

If you have a metalworking lathe, then making a tap for use in wood should be relatively easy and is probably the only way it can be done. It does sound an odd size, so the chances of finding one off the shelf are probably zero, but maybe a hunt through a Zeuss chart would show something. If it is a known size, then you MIGHT be able to find a bolt of that size and file two slots in it to make a "sort of" tap.

But for wood, my advice would be thread chasers and lots+lots of practice!

 

[NickWelford]

But for wood, my advice would be thread chasers and lots+lots of practice!

.... On dense wood like boxwood if you can.

 

[tinyminds]

yeah thread chaser do look like the way to go for most things but even if i could get a thread going i'm not going to be able to make an internal one that small

i would love a metal work lathe but chances of getting one are slim i think!

thanks.

 

[J-G]

...i got as 1 1/2 thread box and found a matching tap for it without too much trouble. then i found a really nice old 3/8 thread box and i've just tried it out to find out what tap i need to go with it and it appears to be 3/8" 9 tpi.

i have had no luck looking for a 3/8" 9 tpi tap, they don't seem to exist. is there any way to have a tap made? who would do such a thing? or can i somehow make one myself?

A 3/8" x 9 would be impossible under normal thread profiles (Whit/UNC). If we assume UNC form, the core dia. of a 3/8" 9 would be 0.147" - BSW would be even smaller at 0.090". Even Stub Acme form, which is probably the shallowest, would have a core diameter of less than 1/4".

I suspect that you might have mis-typed 3/8" when you meant 7/8". Both UNC and BSW standards call for 9tpi on 7/8" dia. but neither are 'preferred'.

 

[tinyminds]

well it is 3/8ths not 7/8ths and the way i measured the tpi was to count the number of turns per inch and then test with my thread gauge. it does have 9 turns per inch but it does not really match the profile in the thread gauge. this would not be a major problem for wood threads as they are very forgiving and you can always sand/wax them. the problem obviously comes when you try to find a tap or bolt in this size which does not exist.

i am going to have to see if i can find someone with a metal lathe who can have a go at making a tap or give up on using this thread box. but that would be such a same as it must be over 100 years old and still works perfectly

 

[chipmunk]

Hi,
Wood threads often have weird pitches and are often coarser and with wider thread angles than metal threads to prevent short grain problems.

Because you have the threading box which is the hardest part to make, the tap ought to be a nice little project. It just needs to be close in thread angle to your hand-cut wooden thread and slightly over-size to allow for wood movement. If you were to supply a length of threaded dowel, I'm sure a hobby machinist could measure it and produce a silver-steel tap for you. I know I could but I'm quite a way away from you.

I bet an email to these folks might turn something up... http://www.hoveparkrailway.co.uk/

Your other option would be to take a wooden thread you've cut with the box, taper it at the end with a skew on the lathe very gradually to remove the threads and add a hacksaw scraper blade into a groove in the side to create a wooden tap. If you then file the hacksaw blade to the right profile using the threaded dowel as a guide - voila. I've never tried it but I think it ought to work. I use the same principle for a morse taper reamer/cleaner.

HTH
Jon

 

[BigWing]

This technique on YouTube looks to be worth a try: Making the large wooden bolt and nut

 

[tinyminds]

this is all really really making me want a metal lathe!

thanks for all the help

 

[AndyT]

Tinyminds, just in case you missed it, there's a good thread (!) in the general woodworking section, which among other things, shows two different methods of making your own metal tap.

https://www.ukworkshop.co.uk/forums/a-wood-thread-thread-t86785.html

Points to note: you don't have to harden the steel - mild steel will make a usable tap for use in wood. A metalworking lathe helps, but is not essential. You could do all the work by filing, having first laid out a spiral, using paper wrapped around the tap body, just like doing a barleysugar twist. (There is a good chapter showing this in Roy Underhill's book "The Woodwright's Workbook" but you don't really need it - I just mention it as evidence that it works.)

 

[Inspector]

My father played with wood threads and one tap was made from a lag bolt so you might want to look around and see what the thread sizes and pitches are that you can find. Maybe one is close enough to use. He also used metal taps and wood taps too but to make his biggest thread of 4" he used a die grinder and a router bit (three wing cutter to cut a "V" goove) to end up with a powered tap. He cut the male 4" thread on the table saw with a special blade I had made up and then hollowed out the side of the male thread to allow the die grinder to nest in, the cutter lining up with the pitch. The die grinder was held in with a hose clamp or two. Worked like a charm but you had to unplug the power cord and untangle it from time to time.

Pete