Scraping Finally.

[Teenage_Machinist]

Well, yesterday, my Prussian Blue came in the mail. So I have been hand scraping.

Dykem Hispot

My scraper is very ugly. Its a dull file, with a chunk of carbide brazed on with Home Despot flux coated bronze rods. Done with Mapp gas, no oxygen. That took a while.


My first project was to scrape the bottom of my homemade, ugly surface gage so it will not rock on the surface plate. I put prussian blue on the plate, rubbed the piece a little bit, scraped off the blue spots, repeated.


Im a bit worried about how the blue spots are not growing much. THey are many, and a bit small, and plenty of them per inch, but they are small, and dont grow. Am I doing something wrong?

Next project is scraping my machinist's square, which is not straight, and possibly not paralel or square.

[tony ennis]

How will you know when you are done?

[lazlo]

Ian, what are you spotting it against? The scraper you made sounds fine, but if you're pinspotting (trying to get 20 spots per inch or better), you need a honed blade with a decent radius.

You can do a good job honing the blade with one of those cheap dipstick diamond hones from Lowes. That's what I used when I scraped the reference surface on my Millrite:



Hopefully Forrest will jump in...

[Forrest Addy]

Making a scraper is the fiirst step. Being able to sharpen it makes all the other steps possible.

Look at this link: http://prowave.blogspot.com/2007/05/scraping-class.html

You REALLY need the motorized diamond lap in the second photo down in the link. Charge it with a liittle smear of green (9 to 12 micron) diamond lapping paste. It's a 1/4 HP 1750 RPM motor bolted to a board with a disk of metal mounted on the shaft to form a lapping face. Most any metal will do but I used cast iron. The rest is a premanenty mounted wood block whose upper surface has been cut to a 95 degree angle to the wheel face.

The face grinder in the thir photo is a cheap Enco import euipped with a 220 grid diamond face wheel. These are handy for getting the initial angles and the end radius right but you can do it all with the lap if you have time to burn.

Contrary to urban legend you cannot effectively sharpen a carbide scraper for precision scraping with a green silicon carbide wheel. It microfractures the edge. It feels sharp and it may scrape but the results look like crap and it takes forever to get anywhere. The difference between a scraper sharpened of a diamond wheel is like night and day compared to one sharpened on a green silicon carbide wheel The difference between a scraper sharpened on a diamond lap os like night and day compared to one sharpened on a diamond wheel. Ask of my former students. That mirror keen edge will save you a lot of sweat.

The third and fourth from last photos show the famous white bearded bloviating belly with legs.

Wish I could drag you up to Seattle for next week's class.

[lakeside53]

170 rpm? Typo?

[lazlo]

Forrest means a 1725 (4 pole) motor. That's pretty close to the ideal speed for diamond lapping carbide.

I think the 1/4 HP is a typo too -- when Forrest posted the Grainger part number for that motor on PracticalMachinist, I'm pretty sure
it was a 1/2 HP motor, and eyeballing the motor chassis to the 6" lapping plate, that looks about right.

[.RC.]

already answered..

[.RC.]

Forrest means a 1725 (4 pole) motor. That's pretty close to the ideal speed for diamond lapping carbide.

But what size disc is that speed for....A 4" disc running at 1725rpm has a slower surface speed then a 6" disc..

Edit: I see you mention a 6" disc..

[Teenage_Machinist]

I used a green wheel to grind the radius on the carbide and then sharpened with diamond whetstone.


I simply CAN NOT AFFORD diamond. I had a thread looking for lower initial cost diamond sharpening, suggested green wheels and then plenty of hand lapping with the diamond stone. Which I had on hand as part of a knife sharpener.


I spot it against a chinese 9x12 surface pl8.

I am a bit unclear about precision of scraping- Do I need to be careful not to have any "spillover" off the blue spots? Do I want to cut off the spots, split them in half, just pare them down?

[dp]

Do a google search for "robert r wade art of hand scraping" and download the PDF file he's written (if you have not already done so). I'd give you the link but it's useful to see what else pops up on Google that is related.

The goal of scraping is somewhat subjective. A perfect blue surface is sometimes desirable but probably unachievable. After that you would seek a perfect distribution of blue spots. After that you seek an even distribution of evenly sized spots, and finally you seek an even density of evenly distributed evenly sized spots. The greater the density the smoother the surface (and the more work you have).

The relative size of the blue spots to the blank spots is useful to consider. If the blue spots are very small then most of the surface is below the contact points. On a wearing surface this will soon wear out.

If the blue spots are larger in one are or areas than another then you have in essence a wavy surface. Obviously large blue or blank areas indicate large high or low spots that require attention. Ideal is an even distribution of evenly sized spots. At this point the ratio of high to low spots is a measure of quality of the surface texture. 50% blue evenly spread around by evenly sized spots is a decent surface that taken on whole can be considered flat. It will be a surface that could be compared to an orange peel.

All blue is virtually no texture which may or may not be good. No texture on a sliding surface is not good because it cannot retain oil but it makes a great 1-2-3 block.

The things that are going on in scraping is the simultaneous achievement of surface quality, perpendicularity, and dimension. Miss any of these by much and you have an attractive door stop, or stock to make a smaller part.

The above is discussed in the Wade paper.

What I've picked up from the masters that is important is comfortable posture, consistent stroke, and consistent pressure. And of course, practice.

In practice I've found I get the best results working toward me than away from me as I can see the stopping point for the stroke. So I start on the far end of the work piece. I scrape diagonally to the sides of the piece, switching direction of the diagonal stroke between bluings to get overlap. Use an India stone after wiping the scraped surface to debur, and wipe again the surface before you put the work on the surface plate.

If you make a motorized wheel like Forrest's link shows (That is Forrest's) and use diamond paste you won't need a diamond wheel. The motor and wheel can be put together very cheaply it being nothing but a motor and a steel disk, in effect, a disk sander. The wood block tool rest as I recall is cut at a 10 degree angle to put the right slant on the scraping tool. Both the upper and lower surfaces are polished this way and allows you to flip the scraper over to use the other side.

Even if you don't have a perfect setup you can still do good work. People have used worse than what you've brazed up.

Edit:
This picture shows the first round of scraping of a new piece of Durabar and one of the scrapers I bought at the scraping class in Newport, Oregon. Notice a few things.

- Itty bitty strokes of inconsistent length and scour depth
- Cat claw scratches from the tool
- Chatter marks perpendicular to the cat claw scratches
- There is nothing consistent about tool placement and usage. I was doodling.
- At this particular moment my back did not hurt but would soon as my posture was bad

http://thevirtualbarandgrill.com/mac...g/DSC01241.JPG