Guard Screws

[3DOKIE]

I have a 1920s Lathe that I am cleaning up. The slotted head screws for the gear guards are not a 1/4" or a 5/16"standard thread. The threads are 18 TPI and measure .265" on the major dimension. Does anyone know what size these are or where I could get some more screws?

 

[texasgeartrain]

I have a 1920s Lathe that I am cleaning up. The slotted head screws for the gear guards are not a 1/4" or a 5/16"standard thread. The threads are 18 TPI and measure .265" on the major dimension. Does anyone know what size these are or where I could get some more screws?

Commie metric M8 x 1.25 is the closest thing in OD. Nothing in standard is quite as close. But I couldn't tell you tpi, as they measure in mm between two threads.

I think I'd heli coil to use 5/16" 18 tpi if it were me. Might be able to tap to 5/16" fine thread though, as its OD is slightly bigger.

 

[3DOKIE]

It is smaller than 5/16". I could redrill and tap all the holes to 5/16" and replace all the screws with standard ones but that seems to be a lot of trouble to go through if there is a source for the original screws.

 

[texasgeartrain]

My mistake, I was looking at the drill size chart, not thread size. Looks like the 1/4-20 is closest in OD though a little under. Numbered screws. . . #12 comes in smaller, and I don't see larger. Metric has nothing close in tpi, and the OD not as close as the 1/4-20.

 

[3DOKIE]

The best that I can figure is that it may be a #16-18. I have not found a screw chart for the old numbered screw thread sizes to verify though. Found a conversion chart at Comparison Chart for Screw Size, Inch Fractions, Decimals and Millimeters. It does say a #16 is a 0.2656. So now maybe I can find a source for them. If not this may be one of the first jobs for the lathe when I get it put back together. Thankfully OSHA does not come to my house so I do not need any guards to run it.

 

[sfriedberg]

It's possible they are (very obsolete) #16 machine screws, with a non-standard pitch to boot. (Non-standard pitch would not be very surprising.) #16 machine screws are nominally 0.268" major diameter, but the ASME standard that specified them was obsoleted in 1947. (!) This thread elsewhere may be helpful.

Machinery's Handbook, 27th Ed., doesn't list anything larger than #12, but apparently the larger numbered sizes were still listed in the 15th Ed.

I think it's very safe to say you will not find #16-18 machine screws available for sale, anywhere. If you are incredibly lucky, someone reading this forum might have a few 75-year-old NOS parts in a bin somewhere.

[Added in edit] I see 3DOKIE got this info before me.

 

[jim rozen]

....where I could get some more screws?

Umm - if you happened to have a lathe you might consider making them?

 

[Rudd]

I ran across 3 of those on an old book nipping press from around 1910 that came in for repair. That was my guess too, #16. 18 TPI. One of the screw heads was broken, and the customer wanted something they could buy if needed.
We -just- had room for 5/16, so that's what we did.

 

[3DOKIE]

I ran across 3 of those on an old book nipping press from around 1910 that came in for repair. That was my guess too, #16. 18 TPI. One of the screw heads was broken, and the customer wanted something they could buy if needed.
We -just- had room for 5/16, so that's what we did.

Problem resolved. It took me some time but I was able to make some screws. Tough to do with a worn half nut and poor tooling. Next project is the half nut.