1. Firstly, get yourself a drop saw with a blade that cuts through aluminium.
2. Set up the drop saw so that it is cutting square to the grate.
3. Ensure that the grate overhanging each end of the drop saw is supported to stop it from falling or cutting incorrectly.
4. Measure the length you require and make a precise mark.
6. Gently lower the drop saw with the Power OFF and have a look at where the blade is cutting on the mark. If you are trying to be precise with the measurement, then you will need to find where the widest part of the teeth on the blade is touching.
7. Set the blade just outside of the mark, leaving only a slither of the marker on the grate when cut.
8. Now you are ready to cut the grate, move your hands away from the blade securing the grate against the very back of the drop saw’s stopper holding it in place tightly.
9. Turn the drop saw on and slowly lower the blade until it ever so slightly touches the mark you have made.
11. When you have finished cutting through the grate, turn the blade off and let it come to a complete stop before removing the grate from the drop saw.
12. Finally, check for burrs. However, you should not have these if you are using a sharp blade.
As you can see it is not rocket science and more manageable than it looks.