75 BBC Stingray

I've been looking at all of the posts showing custom brackets and suspension parts, and have been envious, because my fabrication skills are not so hot... However, one thing I can post about is what I do every day at work.... Superfinishing
I needed to drop off my piston pins to the engine builder. I decided to superfinish them first. Below you can see a photomicrograph of the surface at 100X magnification before and after. The grind on these pins was quite good, and the form (straightness and roundness) was excellent, ~40µin/1µm, which made my job easier.... The end result was that the surface finish was reduced from a Ra=4µin/0.10µm to Ra=0.5µin/0.12µm. While this won't give me more power, it will help with motor longevity and faster break in. I gave a similar treatment to my camshaft journals, and the crank was already polished when I bought it.

Enjoy....

GROUND SURFACE


SURFACE AFTER SUPERFINISH



[Modified by 75 BBC Stingray, 8:35 AM 3/23/2004]

norvalwilhelm

Nice job. :thumbs: How did you superfinish them???

ZD75blue

Jeeze, everyone knows how ya do that...

Ya chuck it up in a lathe, spin it at a couple thousand RPM while holding a wet paper bag up beside it... :jester

Nah seriously, thats pretty slick! :cool:

Eddie 70

Wow that does look slick. If you did the crank journals and rod journals, hmmm....

norvalwilhelm

Ya chuck it up in a lathe, spin it at a couple thousand RPM while holding a wet paper bag up beside it...

If you spun it in a lathe wouldn't all the sanding lines be in one direction??? There is a slight cross hatch pattern??

Freepop

Looks very good. I am familiar with some finishing techniques. One thing to remember though, is it is possible to get things too smooth. Oil will set in the valleys, provide lubrication, and a ciruclation path for the oil.

Cali,68,L-79

Whose Piston Pin?

:D

norvalwilhelm

I just looked at a factory pin from a 3.8 or 3800, the bonneville motor at 100 times and 200 times and your original pin is alot rougher. While the factory pin certainly doesn't look as good as your polished pin it is somewhere in between the two.

ZD75blue

It was a yolk... :jester

NHvette

Wow - pretty smooth.
I'm guessing it's a chemical/mechanical polish .... much like that used to planerize integrated circuit wafers.

Looks good. :seeya


[Modified by NHvette, 10:04 AM 3/23/2004]

turtlevette

how much material did you remove?

if a surface if too smooth will it hold oil? ie. will oil adhere to the surface?

75 BBC Stingray

The process is mechanical and in this case I used abrasive tape. The workpiece rotates, while fresh tape is indexed in at a consistant rate. The abrasive tape is oscillated parallel to the workpiece axis of rotation.

For my piston pins, I ended removing approx. 1µm/40µin of material. I believe most OEM's are superfinishing/microfinishing their piston pins after grinding. Norval, I would expect the piston pin you saw was already polished.

As for the surface being too smooth, it can be a concern, but not with this process. We have many, many machines sold to automotive manufacturers, and they are setup to deliver similar finishes. In fact, several companies market their pins to high-end race applications, and they have a finish slightly better than what I achieved.

The process does not use high speed. The piston pins were only spinning at 100RPM!

If you want more information on the process "superfinish", you can look at the following link for some basics:

http://www.supfina.com/en/index.htm
Click on "Technology" then on "Superfinish" underneath it.

Turttlevette, when the parts are finished this smooth, often it is impossible to wipe the oil off of it. The oil forms small droplets that cling to the surface and can be pushed around, but not easilly wiped off.


[Modified by 75 BBC Stingray, 11:42 AM 3/23/2004]

norvalwilhelm

Norval, I would expect the piston pin you saw was already polished.
Yes these are from a finished 3800 with full floating bronze bushed rods. I regualary tear new motors down fresh off the asssembly line.

75 BBC Stingray

Norval,
Now that sounds like fun. I do enjoy working on the automotive parts we finish, but I never get to work on assemblies, only the the parts...

Someone mentioned spinning the workpiece very fast, and while that is one way to get a smooth part, I think it is better to use the correct abrasive. In this case I used a 3M product from their "lapping film" line. It has a graded aluminum oxide abrasive of 9µm size.

Freepop

75 BBC, I agree. At first look some would think that a perfect mirror finish is what you need on everything. I just wanted people to know that there was such a thing as too smooth and that it can be a bad thing.

75 BBC Stingray

Freepop,

I don't get into the engineering of appropriate surface finish for a particular application. It is certainly a complicated field, where no simple rule like "the smoother, the better" could every hope to apply.
In this isolated case, I am confident in the finish only because of the trends in customer's demands for finish on piston pins that I've seen over the last 5 years. I certainly agree that there can be a "too smooth" finish for certain things. I've seen a lot of concern in this on surfaces where seals ride especially. I'm curious if you are involved on the manufacturing side or the machine tool side of things???

adam

Do they do the same thing to the cam and crank journals? Is that what the ads are
saying, microfinished camshafts... ?

75 BBC Stingray

You can have the same thing done to the cam journals and pins and mains of a crankshaft. Whether this is what you can get from the machine shops or not is something else... There are some "polishers" for cranks out there that can not come even close to what a microfinished or superfinished surface is. The pins on a crankshaft are very difficult to finish unless the shop has a quality machine. The following link is to an interesting article about this:

http://www.babcox.com/editorial/ar/eb20232.htm

Microfinishing is another trade name for a similar process. Superfinishing is a tradename for the process used by the company that I work for.


[Modified by 75 BBC Stingray, 2:36 PM 3/24/2004]