i think with my 8.8 the spring pad and spring plate would sandwich the old hole and assumingly reduce the stress on it
It's nothing to do with "sandwiching with the spring pad & spring plate" (that would mitigate the stress by relocating the bending force, but only slightly.)
It's entirely to do with the fact that
any discontinuity in the material - particularly in
wrought material (like spring leaves...) creates a weak point and a stress concentrator. It's only a matter of degree relative to size (a 1/2" hole would be a larger stress riser - obviously - than a 3/8" hole, but only nominally. It would stay that way until you get to about .3-.5
w on the member.)
Drilling, gouging, cutting, or even nicking with a file is like putting a dashed line on the part that sez "Break here."
This is why even "lightening holes" on a structural member are a stonkingly bad idea.
Have you ever noticed how, on cast members with holes for access, the section around the hole is thickened? That's to reduce the loss of strength.
If you're got your heart set on modifying a leaf (instead of having a new leaf made - which would be best,) the_weirdo makes some good points. Let me give you the list of what I'd do, if I had no other option:
1) Use a
brand new, sharp drill bit. I'd probably throw it away after drilling the hole, or resharpen it to use for the next one. You want
fresh cutting edges (using a spade bit with an indexable or replaceable carbide insert won't go amiss here. It's painfully easy to wreck the temper on a spring in a small area - which is every bit as bad as drilling the hole to begin with. The two factors amplify each other...)
2) Keep the area
flooded with coolant. Any light oil will serve neatly - tapping oil, honing oil, air-tool oil - but keep it
flooded. Have a helper keep the coolant going for you, so you can use both hands to control the drill. "Flooded" means that there is a constant supply of fresh coolant/lubricant going over the cut (Hell, even water will help - just make sure you get it all off!) Drilling over a catchbasin will help to keep the mess down.
3) Recall the formula for tool speed - 4s/d. "s" is the cutting speed of the material in SFPM, "d" is the diameter of the rotating element (tool bit for a mill or drill, material for a lathe.) Cutting speed for spring steel can be taken as being between that for mild steel (~100scfm) and alloy steel (~80scfm) - the alloy is close to mild steel, but the temper throws you a slight curve. 3/8" = .375". Therefore:
(4x90)/.375 = 360/.375 = 960rpm. Use a drill press if possible, run it somewhere between 850-1000rpm - or as close as you can get. Double the speed if you're using a carbide insert spade bit - 1700-2000rpm for that. Keep flooded with coolant to protect the spring temper (if you wreck the spring temper, you've increased the zone that's going to give you trouble, and you've increased exponentially the likelihood that it's going to break on you.)
4) After you've drilled the new holes, find yourself a hardened ball bearing 1/2"-3/4" in diameter (the bearings that come out of FWD Rzeppa joints are good for this - I kept a few when I had to rebuild some years ago...) Lay the spring on a hard surface (garage floor, cement driveway,) put the ball in the hole, then give the ball
one and only one solid whack with a small sledgehammer (3-5# head.) Repeat for both sides, both holes, both spring leaves. This will "cold-forge" the surface of the metal at the hole, giving the hole itself a fighting chance.
Better options:
1) Have a spring shop make a new main leaf for you.
This is best.
2) Find a machine shop with a punch press that can go through the spring stock (~1/4") in a single shot. Punching the hole is better than drilling it, and they'll also have a shop press (and probably ball punches) that can peen the surfaces of the holes for you. If you mark
exactly where you want the holes to go, there's no reason that the whole operation on both springs can't be done inside of an hour. A spring shop should definitely have the equipment to punch and peen new holes.
DO NOT TRY TO HAVE THE OLD HOLE WELDED UP! Not only does it make the discontinuity worse (AFAIK, there's no such thing as "spring rod" for welding...) but you end up with the heat affected zone (HAZ) from the weld process - which is what you're trying to avoid in the first place! Welding up the hole is just
gagging for the spring leaf to snap on you!
