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Having trouble sharpening scraper blades using my jig

addman20

Plastic
Joined
Nov 24, 2020
Hi guys, so after doing some reading and research, I built a jig that would give my scraper blade a 60mm radius and 5 degree negative rake.

I am using a regular old bench grinder that has a diamond cup wheel on it.

Right now I'm not sharpening carbide and just using old metal files to test the geometry and make sure I get the 60mm radius and the two cutting edges with the Crest in the middle with the 5 degree negative rake.

My jig works perfectly it cut the file into exactly 60mm radius. However when I flipped the scraper over to give it another edge after I have it one pass through the bench grinder the Crest was not in the center of the blade. It looked lopsided and off like it had missed somehow.

I will be uploading pictures tomorrow of my jig and what the blade looks like. Just wanted to put this out there and see if anyone had similar issues.

I'm basically following this guy's setup, my jig looks different but used the same principal.

https://youtu.be/ZSGeDs0d8IA
 
Why do you care if the crest is centered? Centering has absolutely nothing to do with the cutting action. What matters is within a few thousands of the cutting edge, everything else supports the cutting edge.
 
Why do you care if the crest is centered? Centering has absolutely nothing to do with the cutting action. What matters is within a few thousands of the cutting edge, everything else supports the cutting edge.

Just reading a book on how to scrape and it mentions that the scraper blade should look like it has a distinct crown along the radiused edge. It includes a picture but I don't want to attach that for copyright reasons.

I'm really new to scraping, but if I can get away with just having one cutting edge that works for me
 
If the crown is more towards one face of the blade than the other, it just means that you have ground less off one face. Flip the blade over so you are cutting the smaller of the two edges and cut a bit more. Rememver that one thou of cut at 5° will move the crown by 11.5 thou.

If the crown is tending towards the diagonal, rather than along the centre between the two flat faces, then the pivot point if the jig is slightly offset to one side of the blade so that (for instance) you are cutting the left side slightly more than the right side, then flipping the blade over and repeating that on what had been the right side. This results in diagonally opposite edges getting ground more than the opposite diagoals, if you see what I mean.

If the crown is within the thickness of the blade, it'll cut just as well both ways up.

If the wheel isn't cutting too fast for easy control, then once you've got the blade roughed to shape, you can even things up by using the jig as a guide for the 5° without clamping the blade into it and using hand pressure on the blade and traverse speed to control the amount you are taking off.
 
When I cut and pasted your link, I immediately thought it isn't supported directly under the carbide and I bet it is vibrating so slightly. Then I was thinking why isn't he grinding the blade in the center of the wheel like all other blade sharpeners do? It is so important to have the wheel moving down at the negative side... I'm not sure if your grinder has a spindle reverse switch. If it doesn't then your grinding on the wrong side of the wheel. Most surface grinders the wheel turns clockwise.

The way your grinding, the wheel is grinding up and as the wheel leaves the top of the carbide blade it pulls off minuet amount of carbide. When you grind at the bottom I doubt your grinding off any carbide and burnishing it more then sharpening it. So move your fixture over to the right hand side of the wheel and slide the blade closer to the fixture so it's directly under the carbide. Center the blade at the 3-Oclock position so as the wheel turns it starts on the outside top edge and grinds off the center on the slight 5 deg-neg angle. I would also lubricate the diamond with some spray penetrating lube, not WD-40, to cool the blade.

Many Engineers try to re-invent the wheel(pun intended). At first glance it looks good,, but getting down to the nitty gritty it looks bad. Do those changes and I'm sure it will help. The radius isn't that important as far as it's close as Mark said. Also if it's 5 or 6 or 7 degree's it should work.

Another issue could be the file handle is probably to stiff and no pad at the end of the wood handle means your arm scraping with less control then body scraping. The Connelly book was written 65 years. Many things in the book are good but many are off based. I ground files to make scrapers 55 years ago. I now use cold roll or high speed steel as it flexes. Merry Christmas everyone!
 
Richard nailed it.
( I am also against breathing WD)

To improve the set-up most easily I would extend the vise jaws so a solid section, perhaps 1/2 thick would extend to be under the carbide blade, then I would have a set screw to hand turn up to support the underside of the carbide blade it about 1/16 from the edge making it tight enough to spring the carbide up a little.. the wheel should be 500 gt or finer, running with coolant. (not uncommon to use 1500 or so gt for such a job)

The cup wheel may not be the best choice. An OD wheel might be better with the fixture swing facing long ways. Yes, for long way you would need to make a feed screw to go to your long travel bump stop.

The edge should come off the grinding so to feel smooth running a sharp knife blade along the finished edge. Yes way finer/sharper than the fingernail sharp test one might use for a mill cutter..

The fixture swing joints should be very smooth, perhaps nylon or brass shim and greased.

Using a cup wheel you might shim the mount face to get the wheel running better than .002..and then grind a slug to work-dress the wheel to run zero at the face. (a sharpie mark on the wheel, wipe off can tell it is true.

Note, it is very tough to get a good carbide/diamond wheel edge running dry.
Note, XYZ axis is not for surface grinders movements are cross/long and down.
 








 
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