maths operations using "CELLS" with "IF" statement

The 1st and last Cells references were not qualified with Sheets(tab1)

If Sheets(tab1).Cells(i, 6) = Sheets("Machining").Range("j10") And Sheets(tab1).Cells(i, 12) = "As wound" Then Sheets(tab1).Cells(i + 1, 16) = (Sheets(tab1).Cells(i, 16) * (Sheets(tab1).Cells(i + 1, 14) - Sheets(tab1).Cells(i + 1, 18))) / (Sheets(tab1).Cells(i, 14) - Sheets(tab1).Cells(i, 18))

Or you could use something like this...

With Sheets(tab1)
If .Cells(i, 6) = Sheets("Machining").Range("j10") And .Cells(i, 12) = "As wound" Then .Cells(i + 1, 16) = (.Cells(i, 16) * (.Cells(i + 1, 14) - .Cells(i + 1, 18))) / (.Cells(i, 14) - .Cells(i, 18))
End With

Allow me to elaborate just a bit. Using Cells with no qualification will default, but what it defaults to depends on where this code is located. In general, it is better to qualify it as AlphaFrog has demonstrated.

• If the code is in a Sheet module, then Cells refers to that sheet.
• If the code is in a Module module, then Cells refers to the active worksheet.

From your code, it is not absolutely certain where it is, or what you want Cells to refer to.

Also test if both of these comparisons evaluate to TRUE.

Sheets(tab1).Cells(i, 6) = Sheets("Machining").Range("j10") And Sheets(tab1).Cells(i, 12) = "As wound"

If they are comparing strings, the comparison is case sensitive e.g; "Hello" is not equal to "hello".