Basic Procedure (getting started)

The goal of scraping a simply to make a surface flat*

The "Ingredients"

So you think you have all the tools ready and want to start, first let's review what you need:

Some cast iron, ideally finely milled, planed or ground. It should be about the size of a paperback book (if you remember those, a rectangle slightly better than your phone and at least 25mm/ 1 inch thick)

Do not try to start with a badly worn machine tool slide or any other other part that is not as flat as possible already. If you need a project to start with one face of an inexpensive iron angle plate will work, you can also buy short sections of continuously cast iron bar although those will need to be machined flat before starting. You want normal soft grey cast iron, not cast steel, not SG iron, aluminum, plastic etc. With experience most metals can be scraped but there is no reason to make things harder than necessary to begin with.

A hand scraper, surface plate, spotting compound, a slip stone, a permanent marker, a chip brush, good lighting, white spirits (mineral spirits in North America), rags, a sturdy workbench and perhaps a vise.

Start with the practice piece of iron securely attached to your workbench (a milling machine table can also work quite well as you can adjust the level and even the angle and they generally have a vise and or hold down clamps)

A note on gloves, I wear then in the workshop at all times other then when performing careful measurements. Not everyone will agree with this but if you don't like them don't wear them. I really like MaxiFlex Ultimate gloves as they are thin, breathable and very durable.

The "feel"

As you start out you need to get a feel for the correct scraping motion, there are a number of variables, the blade angles, the angle of the scraper to the work, the scraping force. This is not simple to explain with words and quite simply you will know when it is working. The cut should be clean with minimal chatter, deep enough that it isn't just dust coming off the metal. This might be a good time to watch some online videos. If the tool is sharp enough, and if you are unsure about that, it is probably not, the edge will not cut but will skip over the surface, or chatter and generally misbehave. The first thing to check, provided the radius is reasonable is that the edges are well lapped so go and work on the edge some more. Under magnification it should be mirror like on both faces.

Stroke length

Controlling the length of the cut is not easy and will require practice. Keep going until you can make rows of separate scrape marks of a range of lengths. A useful range would be on the order of 2 to 10 mm (3/32nds to 3/8ths).

Scraping for real (the first stage)

Once you have some approximation of control over the scraper it is time to try scraping with a purpose.

Flip your block over or remill/grind it and lay down a layer of blue or yellow contrast compound. (a pass with a dirty hand will also provide sufficient contrast so you may see what you are doing)

Start laying down diagonal rows from one corner to the other. Ideally each scrape mark should be distinct (not touching other marks to the side or ends) although at the beginning this is not critical.

Now brush off the chips and then lightly stone the surface. (and clean off any marking compound that remains)

Take the deburred block to the surface plate that has had a medium to heavy coating of marking compound applied.

Wipe the surface plate clear of any debris with your hand, check the compound is evenly applied and gentle set the block onto the surface plate. If you hear or feel crunching etc pick up the block and clean both surfaces. Once the block lays nicely on the plate, you may print the block by moving it back and forth a few times in perpendicular directions (or in a figure 8 pattern) but be careful to apply an even pressure and do not move more than about 50mm (2 inches) in any direction. Once printed, lift off the block and inspect the resulting print. It will probably only touch in a few places near the edges to start.

Go back to the scraping bench and without consideration for separate strokes remove all traces of marking compound from the face of the block with scraping cuts, deburr and return to the surface plate to print.

This process will go on for some time, quite a long time for the beginner, but at some point you will have the face of block covered in a more or less even pattern of large islands of marking compound.

At this point you have reached the first scraping stage.

A note on high centers, One must be on guard against parts which are high in the center as they can roll back and forth and falsely show contact where there is none. One way to find them is to "hinge" the part, this works best with parts which are considerably longer in direction than the other and are of uniform section (weight). If you move one end of a part slightly back and forth it should pivot about a third of the way from the opposite end, a behaviour which should be repeated by moving the opposite end as well. If it just spins about the center then you must endeavour to remove this material in middle before concerning yourself with the overall pattern

The Second stage

Now that you have the part mostly flat it is time to start working on contact. There is much fuss made about "points per inch" by some people and while it is a useful metric it should not be your primary focus at this point. Worry more about establishing an even pattern covering the entire surface.

To start improving your contact try to remember your original practice with the separate marks, you should shorten the stroke slightly from the initial roughing phase and apply a thinner layer of blue to the surface plate as you refine pattern. You should beginning the cycle again of separate rows of separate makes across the face of the part. the exact location of the scrape marks it not critical but try to hit the blue areas more than the unblued areas on average. With a few cycles of scraping, cleaning, stoning, and printing you will begin to see a finer pattern.

Third Stage

At this point you can begin to apply more specialized techniques to "improve" the pattern. With practice you should be able to cut the blue (high) islands into smaller and smaller points. The balance between high and low areas should be around half and half although this can be manipulated with stoning and or different thicknesses of blue and should not be relied on any more than counting points per square 25mm is the guarantee of a good scraping job. Alignment is far more important than patter or even coverage, to a certain degree you can't check the alignment until you have the surface reasonably flat but it is the average level that is going to matter at the end of the day.