can your employer legally not pay you for work done?

[silverfoxcc]

Work for a firm that has fitted an electronic sign in/out machine by means of a touch card.they are now not paying us for shifts if we fail to clock in or out, and have to sign a form and the held back pay comes in the next paycheck However we have to sign in and off on the vehicle tachometer and tracker so they still have a record.although we regularly work a 9 hour day we are contracted for 8 and never get the o/t. They are now threatening to not pay us at all if this happens in the future. Can this be legal?