How to grind precise radius on scraping blades and more - Stefan's YT video

Another fantastic, dow-to-the-point video from Stefan Gotteswinter.

This video is about grinding a precise radius on scraping blades (and anything similar) with a T&C grinder.
Stefan himself is the first to mention that it can be done fairly accurately free-hand.

The advantages I see in his setup are essentially two: once you make the simple fixture, the process is fairly fast and, the most important one, the blade is ground parallel and not perpendicular to the cutting edge.

As usual, Great job, Stefan!

Paolo

A nicely put together video by Stefan, although like many people I tend to grind the radius by hand. I used to drill a small hole in the blade and use a pin but over time it's become unnecessary.

Another way I differ from Stefan's method is that I do not grind the initial radius form at the final grinding angle as I see no good reason to grind the whole (or half) face back every time you sharpen when your scrapes are less than half a thou deep. If I want a -5 angle I grind the blade at say -2 degrees then adjust the angle to -5 for putting on the cutting edge. This means that you only grind the tiniest amount of the edge with the very fine disc, which increases slightly with each sharpen. It has several benefits in that it reduces the time, effort and wear on the fine grit disc. It's especially useful for the novices as it makes it near impossible to miss the edge of the insert when sharpening, and during the scraping class it helped prevent the blades clogging up the disc so much.

When I grind the radius freehand, I do it without any negative rake, since this way it is much easier to check it with the gage. I grind the negative rake only at the end of the roughing. Yes, I waste approximately half a cubic millimeter of carbide. But, in my opinion, it is worth, since it helps making sure that the curvature in the center of the blade is correct.

For the principle of not wasting carbide and wheels, Stefan's method should be very appealing to you: if you color the blade with a Sharpie before starting any grinding, you could stop just when the wheel starts grinding an edge at the center of the blade, leaving still a significant flat in the middle, between the cutting edges.

Paolo

Well, since you grind the radius with no negative you're in a perfect position to use my technique and actually save effort and wear to your grinding disc. There's no need to grind the whole face 5 negative, only the first few tenths - the rest can remain zero-rake until it's all taken up over many sharpenings.

I only grind my hand scrapers by hand to eye. The rake remains set at the grinder tool rest / guide. The radius thus far seldom changes as I swap scraper through the process - I learned the hard way that changing radius takes an age jigged or otherwise (on a bench grinder with a green stone) - from my limited experience its better to take the minimum amount of material off and in the shortest time at the grinder (whilst achieving the edge you want) so you get back to the scraping and maintain your rhythm.
I always found jigs cumbersome and generally not to hand or dedicated to each tool then requiring storage etc etc - on the other hand if you want an exact / precise radius Stefan demonstrates a great method - he is very methodical and accurate in his work. Alas I'm not so much ;-)

I posted before, I also made a grinding jig, but in reality, the radius only matters in proportions, anywhere near 30, 60, 90 and 120 R is just fine. The irregularity of the curvature on a decent hand grind has no more effect than bearing down harder or lighter. But Stefan will say

"aber das ist nicht wirklich ordentlich"



IMHO more important than anything else: Round over the outside corners so you do not end up digging in.
dee
;-D

Well that's just plain neat.
A swing grind fixture with so very little effort.
Plus the grind lines parallel to the top surface which makes a huge difference in edge condition for a given grit size.
I admit my first thought was "crude" and not tenths or even a few thou. level rad size control but it is just a plain Slick idea.
Simple, easy and way so much better that hand forming. Kudos.
Bob