Buy because a constraint is measured
A second machine is justified when profitable work is delayed, outsourced or declined because existing capacity is genuinely full.
Separate machine bottlenecks from labor bottlenecks
If the operator cannot keep the current machine fed, a second machine can make the same labor problem worse.
Model incremental contribution
Use only the extra profitable pieces the second machine lets you complete. Do not count revenue the existing setup already earns.
Consider redundancy
Two independent machines can preserve partial production during service and support mixed jobs.
Do not ignore utilization
A machine that sits idle most weeks can still be justified for a short critical seasonal peak, but the payback calculation should reflect that reality.
Use your own measured data. These guides explain the decision structure. Run the calculators with your actual stitch times, handling times and batch sizes before treating a machine class as a fit.