Cutting wood screw threads?

[TomB]

I had ice off the roof go through a thermopane window. The window has been sent out, the glass fixed and returned but during the trip a brass track guide was lost. It is about 1 1/2 inch long with what might be a #10 or 12 wood thread on one end and a round ball on the other end that fits in the slide track. The manufacturer is long since out of business. The old design seals better than modern sliding windows so I don't want to just replace the window. There should be two pins on the top corners of the frame of which one is still there, so I have a pattern that I could follow. But how does one actually go about cutting a wood thread?

I'm strictly speculating but pretty sure I could put together a gear combination to match the pitch and since it does not go into a nut, close is perfectly suitable. Obviously the thread shape is different than a machine thread but lots of small cuts with carefully set starting points should rough out the thread groove so that a shaped bit could finish it. But wood threads seem to end at knife edge and not a flat so how do you machine to not break the edge? Finally how can one address the taper? I'd probably run the feed to cut from left to right (toward the point) but I expect the un-supported point would break off before it was fully cut. Not sure that would be a problem as I could pre-drill for installation. Are my speculations missing any points or otherwise ill advised?

I realize there are other approaches for example a thread insert and modified brass machine bolt but my preference would be to keep it original.

Tom

[mikeehlert]

How about looking at it from the other direction? Take a brass wood screw and add a ball on the end?

Crazy thought but it might just work.

Mike

[Glenn Brooks]

How about looking at it from the other direction? Take a brass wood screw and add a ball on the end?

Crazy thought but it might just work.

Mike

Second Mikes comment! Maybe Take a brass or bronze wood screw, cut off the head and run a die to thread the end a little bit, then drill and tap a brass ball and screw together. You could use locktite or maybe silver solder to make permanent. Only problem is, where to find a brass ball these days??

[SteveM]

Only problem is, where to find a brass ball these days??

Yea, there are lot fewer brass balls out there these days, but McMaster has them:

Unthreaded:
http://www.mcmaster.com/#brass-balls/=xe5ufx
Threaded ones here:
http://www.mcmaster.com/#threaded-balls/=xe5v1m

Steve

[Harold_V]

I would highly recommend one avoid doing any silver soldering on brass screws. Works fine, but the end result will be a screw that has been fully annealed---easily bent, even by finger.

Harold

[silence dogood]

What is this screw going into? You can tap wood and use machine threads. In the past, I've have been able to use machine screws in wood and it holds just fine. Mark

[dly31]

I would think a suitable tool and an improvised taper attachment along with manual rotation of the spindle would do it. A simple angle iron clamped to the bed at the desired angle and some sort of cross slide stop bearing against it could be used to make the taper by manually feeding to keep the stop against the angle. Should be easy with slow spindle rotation.

A sharp edge will be automatic if your tool taper is greater than the thread depth and you feed it deep enough.

It might be a good idea to leave the work long enough to support the pointed end until it is cut to the center. You could leave it a little blunt and saw off the support end after the screw is done.

[mcostello]

Could use a Stainless screw, avoiding brass.

[earlgo]

You can actually turn a woodscrew if you guide the taper by manipulating the crossfeed by hand. It is not a true taper but a bit of a curve. These threads were actually ground with a Dumore with a cutoff wheel in place of the turning tool because there was no outboard support possible and the profile cut is quite large. The spinning cutoff wheel was cocked a bit from perpendicular to give the elliptical thread form.
The pic shows a commercial #14 FH, an unfinished screw and a trial finished screw.


I made these because I needed a screw to emulate an "old style" butt plate screw for a pre-1900 rifle.

It took a lot of setup and a few tries to get the taper close, but it didn't turn out too bad. Considering one will only see the head, it may not have been worth the effort.

--earlgo

[redneckalbertan]

It took a lot of setup and a few tries to get the taper close, but it didn't turn out too bad. Considering one will only see the head, it may not have been worth the effort.

--earlgo

You might be right about that, in my experience though little projects like this make me stand back and smile to myself everytime I see them, or rather don't see them. I think the personal satisfaction is worth it!

[pete]

Certainly not my idea since I've read about it somewhere. Get some rough measurements of your wood screws thread length, small diameter, and then large diameter. That will give you the taper angle per inch. You then need your cross slides TPI. You then calculate the correct drum size that gets attached to your cross slides handwheel and then wrap light weight aircraft cable or fairly heavy weight fishing line around the drum. Fixture the other end to the lathe bed. Under power feed that's set up either by change gears or through a gearbox if you have one for the correct coarse TPI and depending on which way the line is wrapped on the drum the cross slide will either feed in or out automatically as the carridge advances towards the headstock.

And yes, far more time involved building the system than actually machining your needed screw, but it will reproduce a tapered screw thread.

Possibly a shop built steel screw head welded to a standard steel wood screw thread might work better or at least faster with a lot less effort?

Pete

[jpfalt]

Original wood screws were often not tapered. I have some mid 1800's muzzleloaders that had wood screws with the wood thread screw form, but a blunt end. The only difference in use was that the hole has to be predrilled at the minor diameter to install the screw and prevent spitting in the wood. It's also possible to cut a wood screw with nothing more than a triangular or wedge shaped hand file.

On the original (1600's) screw cutting lathes, the first thread was a shallow cut with a hand file and then the threads were cut with a hand held scraping tool with several threading points that followed the first cut thread and scraped additional threads ahead of the first one.