Machinist

[Lawrence London <lfljvenaura@gmail.com>]

http://www.practicalmachinist.com/vb/general/open-email-o-bobjohnson-261472/

Open eMail to Seattle scraping class

Bob, you contacted Dave Feinberg about scraper sharpening equipment. He forwarded it to me for response. You have a spam blocker so I couldn't email you directly. Here is the body of the email I tried to send you. I think it will be of general interest to the scraper people.

Bob.

The "grit" is diamond lapping compound 9 - 12 micron abrasive in a `viscous vehicle about the consistency of tooth paste. This comes in syringes in several sizes: 1, 5, and 12 gram seem to be the most common. The stuff also comes in oil or water soluble vehicle (goo) but I prefer the oil soluble as it does not dry in use.

The 9 - 12 micron is actual abrasive size not screen mesh (translates to 1800 grit) as is common in US usage. Another parameter is concentration. 25% is the compromise I've chosen for scraper lapping.

The color of the vehicle is an international standard indicating abrasive size and green is the color I favor for lapping scrapers. You can find the cheap import stuff on eBay

1 PC 5 Gram Diamond Polishing Lapping Paste Compound Syringes 0 5 to 40 Micron | eBay

or spend real money for the name brand goods.

Buffing & Polishing Compounds - Buffing & Polishing | MSCDirect.com

Naturally the pricey stuff is better but only slightly and not really worth the extra cost for this application.

BTW, green is the color of the lapping compound intended to be smeared on the cast iron lap that looks like this:

Shop Stuff: Scraping Class

Look in the second image.

Do not confuse the green lapping compound with "green silicon carbide" grinding wheels touted for grinding carbide. I strongly recommend against green silicon carbide grinding wheels for scraper sharpening. While you can produce what looks like a sharp edge, the edge is rough and micro-fractured. While you can scrape with it a diamond sharpened and lapped edge is far keener and takes about 1/4 the physical effort to use and produces a far better finish.

If you want to grind, sharpen, and reshape scrapers and in general work with carbide in a meaningful way, use a diamond wheel on a face wheel grinder in the third image down in the "shop stuff" link. The grit size I favor here is 150 to 220. Import wheels work fine. I suggest those from Shars Tools.

If you balk at the first cost, consider: the lap and grinder will last more your lifetime and the diamond wheel many years if not abused (never grind steel on a diamond wheel, diamond is carbon and carbon is soluble ini ron.) I've used the same $46 12 gram tube of diamond lapping compound for five classes and its only half used up, My students slobber it on like it was apple butter. If you shop wisely you can acquire/scrounge/make what you need for keen edge scrapers for about $200.

Forrest Addy

> 				
> 					Last edited by Forrest Addy; 03-02-2013 at 09:32 PM.
> 				
> 				
>