If you have basic metal working facilities you aren't far from being able draw wire and then you can make relays. It might take a while to replicate Konrad Zuse's machines but I think that it might be simpler to do that than go for purely mechanical computers, see https://en.wikipedia.org/wiki/Z3_(computer)
And of course the serious calculator enthusiast would have a Curta. Needs a bit more than basic metal working I suspect: https://en.wikipedia.org/wiki/Curta
But if you only want a calculator the there are plenty of examples from Blaise Pascal onwards: https://en.wikipedia.org/wiki/Pascal's_calculator.
A friend of mine in high school had a calculator like this one: https://www.computinghistory.org.uk/det/4584/Magic-Brain-Cal... that could be made quite easily.
And of course the serious calculator enthusiast would have a Curta. Needs a bit more than basic metal working I suspect: https://en.wikipedia.org/wiki/Curta