I’d start with big holes, because they actually reduce weight. But they don’t reduce it by much. If you drill a dozen 3" diameter holes in 1/8" thick material, you’ll get rid of a pound of aluminum. You’ll need 26 2" diameter holes to get that same pound. The smaller the holes, the worse it gets.
I’d be seriously looking at other things to change, and drill holes in the frame as a last resort. But that’s because I’m naturally lazy.
Agreed - hardware is where you can trim down a lot. Look for areas where bolts are longer than they need to be, areas where small screws/bolts can be replaced with rivets (or even epoxy in some cases), superfluous wires and cables that don’t need to be as long as they are, etc.
1> circles are better than squares/diamonds (usually) due to stress risers.
2> before we can truly answer this we need to know the amount of force expected on the part and the direction of the expected force.
I agree with the posters that say round holes are best. Also, smaller holes are better for stress (but the same for weight if you remove the same square inches), they’ll affect buckling loads less.
I’ve had good luck making 1 to 1-1/2 inch dia holes with a conduit punch–they look nicer than any drill and they go quickly. You mentioned diamond, so maybe you’re intending on NC machining
Also, all of the people worried about strength, I can’t think of many areas where you need the amount of strength a 1 x 4 x 1/8" aluminum rectangular section will provide. But I don’t know your loading.
Another way to shave weight from such a section would be to thin it using a table-saw with an aluminum blade. This would have the least effect on stress, less than any holes.
Another note: if the tube sees mostly bending, remove holes or thin evenly. across the faces. If the tube sees lots of torsion (twisting), remove more toward the corners and leave the centerline of each face intact. Highest stress in torsion is down the center axis of the face.
I’ll agree with what I see above except this. A step bit is like a crescent wrench - one tool does a lot of different stuff, but doesn’t do any of it as well as the right tool does. Using a step bit requires grinding up every bit of aluminum in the hole, increasing time, energy, and waste. If you are going to drill more than a few holes of a given size, get an appropriate hole saw. Faster, neater, less swarf, and most of the aluminum you remove can be tossed in the recycle bin in one piece. I have also noticed step bit holes tend to “wander” off the intended location farther than hole saw holes, especially if drilled in a hurry.
One of the easier ways to drop weight is to switch from CIMs to Mini-CIMs. You can drop 0.6 lbs. a motor. The two have very similar torque at 40 amps, the Mini-CIM just doesn’t have as high of a stall torque.
Might be easier than drilling that many holes in your frame.