A good friend found an old toolbox on a property he purchased. He gifted me some of the contents, and one of the items was this neat old “Unbrako Socket Screw Calculator”. If you have really sharp eyes, you might have seen it sitting in the lid of my toolbox in this post. It’s a very simple device that provides a machinist with all the measurements they’d need regarding socket screws. Let’s take a closer look.

The Old Kind of Calculator

Unbrako Socket Screw Calculator, set to display data on 3/8" screws.
It’s currently set to give information on 3/8″ screws.

We modern folk think of calculators in the electronic sense – machines that figure out an answer from scratch, every time you ask for it. But this calculator doesn’t work that way at all. It can’t figure out anything. Really, it’s just a set of tabulated information, presented in a unique way.

A closeup of the data. Everything you need to make holes for a 3/8″ screw, right there.
The sleeve and card of the calculator separated.
Here it is completely taken apart. It’s just a plastic sleeve and a printed card that slides in the sleeve.

The blank squares in the sleeve are clear. The bar across the top of the sleeve is clear too, but has the screw sizes printed on it.

The white circle in the black stripe at the top of the card highlights the screw diameter. When you line up the circle, the correct numbers for all the measurements appear in the clear windows on the sleeve. It’s a darn clever little device!

The back of the calculator.
The fun continues on the back side, with data on two other types of screw, and a chart to convert drill sizes to decimal inch sizes.

Better Than A Book

The information that this calculator provides is available in a lot of other ways. I own at least two books that will tell you all of this stuff. But it takes some time to get the book, find the right page, and then find the right line on the chart. Really, it’s way faster to grab the little calculator and slide the card to the right size. I use mine regularly. Can you imagine how helpful it would be to a professional machinist?

Where Did it Come From?

You can’t really miss the fact that the calculator is advertising Unbrako brand screws. And the company still exists! When they made the calculator, they were part of Standard Pressed Steel of Jenkintown, PA. As far as I could tell they no longer are, but SPS does still exist. Even more surprising, both continue to make screws.

I don’t know for sure, but I suspect the calculator was a freebie that salesmen gave out when they visited shops. When I searched for it on the internet I found a bunch of them for sale. One site called it a “1960 model”, but I have no idea what they based that claim upon. Mine doesn’t have any date on it.

All in all, it was a wonderful gift, much used and much appreciated.