Imagine this one....I recently designed a part and had it machined. The part was very complicated, and dictated ALOT of material being removed from the blank. Doing this out of 304 Stainless like I wanted to would have been prohibitively expensive. So I did it out of aluminum, and had it nickel-plated for durability, since it pivots on another part.

Since it was near the direct path of a laser, I wanted a diffuse surface, so I had it bead blasted before nickel plate. The tolerance was also VERY tight, as it pivots on a shoulder screw, and I didn't want any play in it as it pivots. I had to calculate the material the bead blasting would remove, as well as the amount the plating would add back on. Miniscule, really, but every thousandth of an inch matters in something like that.

The point is that even on 6061 Aluminum, the blasting process doesn't take much off.

erm... maybe you use a different system than automotive? Many sites talked about the visual difference between a shot-peened rod and one that isn't. The pictures show a difference.

"Shot peening does affect the surface finish and can greatly affect the finish on soft metals such as aluminum. For the highest fatigue resistance, you want as much depth penetration as possible, which means large shot and high velocity. This will have the maximum affect on surface finish, which may or may not create problems"
-F. J. Diekman

there'* a reason will keeps telling you the ridge will be there

here'* some food for though. on a heavy duty 4L60, we shot peen the output shaft, after machining. if what you think is true, that it would take off a ridge of material, we'd be in a crap load of trouble, as we have fully machined splines there. likewise, our gears that are shot peened would no longer be gears at all if it was capable of taking that sort of a ridge off.

shot peening is nothing more than a surface treatment. that'* all, that'* it. it increases the strength of the part by allowing it to withstand higher stresses. i'm sure that page will get into the why'* of it, but just trust the two of us here who work with shot peening...

those rods could deff. be shot peened. they may not be.

and as for the rods themselves, and the casting process, it depends on how they were cast - die cast, green sand cast, or lost foam cast. each has different finish characteristics and pouring methods. of those, green sand and lost foam would possibly leave dimples, with the green sand being more pronounced. the lost foam would leave dimples of size relative to the quality of the foam being used. you don't see dimples like that in die casting.

soooo, it could be shot peening, or it could be casting. either way, the peen won't remove any sort of ridge like that. you can almost think of it as painting on a higher stress finish even though that'* not what'* happening at all - but just like you wouldn't expect paint to take off that ridge, neither will shot peening. if you're using a shot and velocity high enough to take that off, you're going to be changing your part dimensions, which is *not* what you want to be doing.

we'll need higher res pictures of the cleaned rods before we'll be able to tell for sure what'* going on with these. if this debate lasts until monday, I'll find out what kind of casting method is used on rods in GM. that'll help solve it right there...

anyway, good pics of the clean rods will be able to clear a lot of this up.

"there'* a reason will keeps telling you the ridge will be there "
Why?


Yeah I'm sure there are different levels of velosity. It would seem that everyone talking specifically about shot-peening rods says there will be a visual difference in it. I haven't found anyone yet who says there won't be. Makes me believe there will be. So I found pictures of the visual difference..Am wrong to assume there should be a visual difference?

Food for thought:

"The result of this process is a uniformly dimpled surface, the roughness being determined by the shot size and the peening intensity."
http://www.mrtrally.com.au/performance/testing-shot.htm

"It leaves a slightly textured finish upon the surface of the material."
Some forum guy

and as I just posted:

"Shot peening does affect the surface finish and can greatly affect the finish on soft metals such as aluminum. For the highest fatigue resistance, you want as much depth penetration as possible, which means large shot and high velocity. This will have the maximum affect on surface finish, which may or may not create problems"
-F. J. Diekman

BTW, and a big BTW, don't take the wrong view of this thread. I don't think I'm right. I said "i dunno" "maybe" and "i have no experience" tons. Not a debate, Not thinking I'm right, just thinking about it...so no pulling hair, k?

Doc, methinks you're imagining a much more aggressive finish. Increasing the stress capability of steel doesn't leave an aggressive finsish. They describe it, but in actuality, it'* not deep, nor does it need to be.

Maximum depth is wanted, sure, but it'* not going to be DEEP on steel. Especially forged rods. Deeper than aluminum? Sure. Deep enough to measure? No.

I think shot peening can be done to take away possible issues with the outside of a metal piece. It may be slightly softer on the outside or have some impurities, but if you shot peen it, think of it as small scale forging?

it had to sink in, but it is 1:40...

mmhmm...so is there anyway to tell if your rod has been shot-peened?

I can clear up where the casting flashing (those ridges) goes right now... (sorry I didn't earlier)

every time I've been involved with a performance engine machine shop and they are shot peening rods and cranks (cranks a lil bit more delicate due to the journals but it can and is done) THEY REMOVE THE FLASHING BEFORE SHOTPEENING

theres a few reasons for this... removing the casting flashing before shotpeening takes away a thinner "sharper" edge allowing stresses to be terminated over a nice uniform wide surface stopping the formation of tiny micro fractures

it also allows the part to be more uniform for the balancing process

and in the performance aspect it lightens the rods mass without reducing it'* structural integrity.... when your going for ever ounce of advantage ... it adds up

usually the only time any form of a "ridge" is left is when it'* a FORGED rod ... it'* ment to be there kinda like an I beam in a building.... there are a few cast rods I've seen out there that have that ridge on them like a forged rod might ...but it'* pretty easy to tell which "ridge" is ment to stay and which is to go when dealing with cast rods

these rods that DrJay posted up should have had the flashing removed


where as these ones (which he also posted...) have been machined to remove the casting flash, you can tell looking at the difference in texture between the sides and middle of the rod


another reason you won't see the tiny tiny shotpeen dimples is cause the rods are ususally polished afterwards to get rod of the dimples which can also cause micro fractures... the first of the two pics have been polished after shotpeen... but the cast flashing wasn't removed.