DIYEFI.org Forum

For all those who thought you should buy your shims when you need a thinner one, you've been wasting your time!

While sanding takes about an hour per 0.001", and putting them on the lathe at best makes grooves while your bits smoke and break, and this last one... It does remove material, but not much before the stone loads:


Anyway, no time for preamble today. Here's what I did, an it worked. It's what I'd call "emergancy repairs only", but it does work for grinding hardened stuff materials in circularly symetric ways.



This lets you get a feel for how much you took off, since the the stone goes away pretty quick



The problem is the stone seemed to wear as I went from center to edge (doing it again I might run the lathe slower). This gives a few mills of "cupping" which is actually nice, since you can set the final dimension by sanding. I used a vavle lapping suction cup stick thingie for the sanding, and you have to wet sand (squirt bottle of water or alcohol) to keep the paper working. I used 80 grit, then 200-400-600 to polish it up, and face that side down.

Post grinding:


Final product.


Other notes: One shim makes a good backing plate for another, but you have to tighten it a lot. You can crack the shims if you bite on their edge. Something I did, I forget what, made it bite much better than in this video, but this at least gives you an idea for what it's like:

http://video.google.com/videoplay?docid ... 4346&hl=en

Dude, this shim :

http://abefm.smugmug.com/photos/348832882_xR9Lh-L.jpg

Looks obviously not flat. I mean, it seems to be thinner on one side than the other. Perhaps your lathing was off and the sanding brought it back to be correct? If not, the fact that it's thin on one side and thicker on the other (even a TINY bit) will mean that A they don't rotate properly to generate even wear B they immediately end up facing "down wind" (highest bit being pushed away from cam movement) C once the high bit is down wind there will be some side load from the cam movement, more than usual, and quite possibly what caused your other issues

Fred.

Prolly want to get a new shim Abe, and yea I know, you want to too.

The thing is, the hardened shims are blanchard ground. Blanchard grinding with a convolute wheel keeps the material stress even.

Surface stress from other forms of surface grinding can deform or curl the overall shape of a softer part, like a flywheel or rotor. With a fully hardened part like a shim, the stress can cause a full thickness crack.

And that'll leave a mark.

- Jim

heh - sorry, I should have made it a bit more clear - this was in the hopes of running a junk head for one more 4 hour run the next day to take the girlfriend to the drive in.

I checked the thickness in a few spots around the edge, and it was within 1 thousandth. The hope was to put it in, machined side down, and drive it for a day, maybe a week. Turned out all for naght since the head was goofy.

I never ran it in the motor.While it has a low spot in the middle, I thought it was interesting it worked as well as it did.

Blanchard grinding is used to quickly remove stock from one side of a large part. Typically, ferrous metals are subjected to the Blanchard grinding process, since the part is often held in place by a magnetic chuck as it is ground. It is, however, possible to use Blanchard grinding on non-ferrous metals and even plastics, but production rates tend to be lower and costs are higher


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hot rolled steel plate

Er, is that blanchard grinding?

Edit: NM.

Wow ! A nice work of precision machining. I have also done a lot of work but currently I am unable to post pictures because I do not have one. Soon I will demonstrate my work to you guys. I am working on Precision Machining in a small firm and ready to switch over.


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For cutting shims you should use a surface grinder with a magnetic table.

at the very worst, if you have to use a lathe.
set it up with a carbide tip and run the thing up as fast as it will can with still cutting, even if the streamers come out blue, then use the auto feeder as slow as possible.

the just face it off with some 400grit water sand on a piece of glass.

Yeah - the issue was I had no magnetic table, no surface grinder, no bits made out of anything harder than solder, and I still have no auto-feed on that axis.

Now I have a couple tips which might actually bite in. They might not. What I *do* have is enough time to buy new shims.