We'd call them "boltcutters" - not "a boltcutters" or "some boltcutters", just "boltcutters" (like trousers (although I am led to believe (by the Guardian's fashion chappies, who may well be winding me up) that in certain very on-trend circles, it is correct to refer to "a trouser", as in "he cuts a very fine trouser")).
If you were asking where they were, you'd say "Where are the boltcutters?"
By way of additional detail to my answer: although I'd use "boltcutter" myself (by analogy to can opener), I wouldn't think it weird if someone said either "boltcutters" or "pair of boltcutters" (by analogy to scissors).
The workshop downstairs call these shears (a pair of shears as they are like grown up scissors). They are sometimes referred to as boltcutters (as people above have commented) but this is frowned on as if you use them to cut high tensile steel bolts it deforms the blade and you have to throw them away. I
I'd usually refer to them in the plural, and usually as a pair or a set. I can imagine situations where I might say that I need "a bolt-cutter", in the sense that I need a device that will cut a bolt, irrespective of whether it looks like your picture.
I might also refer to them as bolt-croppers, should the fit take me.
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If you were asking where they were, you'd say "Where are the boltcutters?"
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If I was looking at it, or had a mental image for some reason, then I'd think it was like a pair of scissors, and thus call it a pair of boltcutters.
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'the' if there is only one set/pair in view, my/yours if there is a choice.
I would use 'some' interchangeably with 'a pair of' if I was asking for them in a hardware shop rather than looking for them in a diy warehouse.
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I might also refer to them as bolt-croppers, should the fit take me.
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Henry, if I was told they had been named.