There’s a surprising number of useful phone apps for a welder. However there’s a general thought that a smart phone and a workshop are generally not compatible, grinding dust and dirt don’t go well with a touch screen.
Welding and cutting company ESAB have their own app for setting up the parameters on a MIG Welder. However it’s not as flexible as the online calculators from the likes of the Miller Welding Calculator .
The Weld Design app also has a welding calculator for MIG, however it only covers flux cored wires at the moment. It includes some technical guides for selection of electrode type and thickness for TIG, explaination of welding symbols and tables with amperage settings for Arc and Flux Cored Arc welding.
The Machinist Toolbox allows calculation of cutting speeds for your drill or mill and M Starr who wrote this also do a range of other milling calculators.
There are a good range of angle measuring apps you can download and also calculators and convertors to switch between metric and imperial measurements. One of these that looks interesting is the visual geometry calculator that allows you to setup various shapes and view the angles between the edges.
My final recommendation would be to get some kind of ereader so that you can read or listen to books when not working.