Blade lapping and grinding

Perhaps the most critical piece in ensuring successful scraping (with a pleasing appearance) is that the cutting edges on the scraper are lapped to a very fine finish.  

For push scraping the edge can easily be formed with a resin bonded diamond wheel but all scratches must be removed by lapping with a rotary lap.

Pull scraping is generally sharpened using a different method, as the edge has no radius a large bench stone is frequently applied in drawing motion. 

In an ideal well funded world we would all just buy specialized sharpening and lapping machines such as those sold by Biax, I am sure they great but they are also very expensive and should one be spending money of that sort it would be much better spent on a power scraper. 

This leaves us with producing our own shaping/lapping machine. There are a multitude of ways to do this depending on available materials, desired results etc.

Two rotating abrasive  wheels are required, a resin bonded diamond wheel (perhaps a d150 100mm cup wheel), the other a cast iron or ceramic disk loaded with a fine (3 micron seems to work) diamond lapping paste. While the diamond wheel should turn at a considerable speed (4500 rpm for a 100mm wheel for example) the lap must turn at a much lower speed. Exactly how much lower is unclear but less than 1/10th the speed of the diamond wheel. This means producing one machine to perform both tasks is problematic as electric motors do not generally accommodate such a range of speeds and doing so with belts etc becomes complicated and bulky. 

If possible it is ideal to form the edge on a grinder already in the workshop (say a Deckel SOE single lip cutter grinder which works really very well) a simple bench grinder could be modified to do this as well. 

For the lapping machine it is time to get creative. 

If scraping  is not something you do with great regularity it would be simple matter to mount an iron disk on a metal lathe set to run at 250 rpm, attach a simple rest to the tool post and use that to lap the edges of a few scrapers. This would of course become inconvenient to leave setup for very long should you want to use the lathe as a lathe. 

Proposed Lapping Machine Design

Perhaps calling this a design is a bit strong but I will attempt to layout a way that the average person in a home workshop could build a lapping machine for a moderate cost using all new parts.

Motor, Nema 23 stepper motor with inexpensive driver. Stepper motors are well suited, not because of their positional accuracy but because of their excellent low speed torque and moderate cost. It is possible to get a powerful motor and drive for around 50 Euros/dollars/pounds... 

Control would be with a simple 555 timer board that could be tuned with a variable resistor (no adjustment is needed once it is set) 

Direct drive 

Spindle using a sealed double row angular contact bearing will provide more than sufficient capacity for this application. a simple spindle with a cast iron disk attached with loctite and turned on a lathe  

Structure can either be 3D printed or from machined flat plate. 

The angles would be set with a wedge of the desired angle (it would be easy to make a few with 3D printing) the same angle wedge could also be used with an added table on a bench grinder to make sure the ground angle matches the lapping angle. 

PLEASE NOTE FOR IDEAL EDGE CONDITION THE TABLE SHOULD BE ANGLED DOWN RATHER THAN UP AS SHOWN.  (SEE THE LAPPING PAGE)

WHEN LAPPING IT IS HIGHLY UNLIKELY THERE WILL BE ANY ISSUE BUT IF YOU ARE BUILDING A MACHINE YOU MIGHT AS WELL DO IT PROPERLY.