Lapping Scraper Blades - Which Way Up?

[loply]

Hi folks,

I've done quite a lot of scraping over the last few years, and recently started getting back into it as I have a few things that need realigning.

One thing I've always wondered is whether I'm lapping my Sandvik carbide scraper blades the right way up or not.

I have a cast iron disc running on a bench grinder, with a 5 degree block of wood to rest the blade on. The disc has diamond paste embeded in the face of it (by means of smearing it in with a cloth).

When I lap the blade I tend to push it into the iron with the cutting edge at the bottom - so the diamond starts at the top and runs downwards towards the cutting edge and then away.

Should I be doing it this way round? Or should I flip the blade over so that the diamond hits the cutting edge first?

I have felt like I always had problems getting a total mirror finish on my scrapers, which makes them chatter a bit especially during the early stages when the stroke is wide.

Thanks for any advice,
Rich

 

[4GSR]

You want to run the diamond paste into the cutting edge of the blade. Your table should be tilted in, or at 85 deg. angle from the face of your lap. The result is a negative 5 deg. cutting edge on your blade. If you grind your blade correctly, you should have two edges facing you when looking at your blade. I'll try to take some pictures and post later today.

Ken

 

[loply]

I've been doing it the other way around in that case!

I'll modify the block of wood tonight and try again. I guess I'll just have to flip it 180 so that the 5 degree angle is pointing the right way.

 

[Bob Korves]

I have been using a shop made ~200 rpm 4"/100 mm diameter diamond grinder that belongs to Ulma Doctor (Mike Walton), my scraping mentor (knowledgeable and nice guy!). We have been using about a 2-3 degree grinding angle. The slow speed of the grinder makes it easy to control and not over grind or facet the blade. I tried to use a 3400 rpm bench grinder a couple times, but it is just too aggressive and difficult to control. Perhaps yours is easier to control with the diamond paste. Nice but pricey slow speed lap/grinders are made by Glendo/Accu-finish. The one Mike built mimics the Glendo grinders. It is much slower than a high speed grinder, but completely controllable, and leaves a nice finish. Still only takes a minute or two to dress the scraper. There is documentation of the grinder on this site. When Mike is not around I just use my EZ-Lap L-PAK diamond hand laps to dress the blade, slow but cheap and works fine. I am looking for a slow speed motor to steal Mike's grinder idea...
http://accu-finish.com/
http://www.ebay.com/sch/sis.html?_nkw=EZE+LAP+L+PAK+DIAMOND+SHARPENER+SET+EZLPAK
http://www.hobby-machinist.com/threads/shop-made-diamond-tool-sharpener.34775/

Edit: Noticed my post was a bit off topic. I agree with grinding from edge toward center of blade for best results.

 

[4GSR]

I rough grind the blades on a setup on my K O Lee T & C grinder using a good diamond wheel. then i go over to my Glendo/Grindolap machine and finish lap using a 800-1000 grit face wheel. Once you get the profile on the carbide the way you want it, you only need to re-lap the edge while you are scraping. And I always use the "finger nail" test to check how sharp the edge is.

 

[Highsider]

I've been doing it the other way around in that case!

I'll modify the block of wood tonight and try again. I guess I'll just have to flip it 180 so that the 5 degree angle is pointing the right way.

The direction you were lapping can result in microchips in the cutting edge.