Once you are proficient at scraping things flat it is time to start making things much harder. At this point it is time to put down the scraper and start making measurements to understand the wear patterns.

You will quickly learn that the hard part of scraping (and machine restoration in general) is not removing metal, but removing it in the correct location and in the correct quantity.

The challenges include but are not limited to:

Identifying datums

Preserving or restoring way angles

Aligning separate ways at right angles

Printing and measuring parts without adding artificial deformation

scraping at difficult angles

replacing or restoring gibs

fitting gibs

The best way to move to the next phase of scraping is to add another dimension. One option of doing that would be the angle plate I discussed earlier. Once you have one side of the plate flat with a good even pattern of points on it you can scrape the second face to be perpendicular to the first.

To do this you will need a surface plate, a right angle square and ideally an indicator stand with a foot that allows you to set it off the square to check perpendicular angles.

As a first pass you can simply place the scraped side on the plate and hold the square up to the unscraped face of the angle. You can use spotting compound on the square or just look for light to get a rough idea of the mismatch. The following figure shows an exaggerated version of one possible

Based on the initial assessment it would be a good idea to measure the amount of misalignment at the top or bottom.

Based on this measurement, lets say something reasonable such as 0.05 mm (0.002 in) over 100 mm (4 in) you would need to plan out the course of scraping to reduce this error. This will be done by "Step Scraping"

Another way to look at this problem is as a cross section of a part. A lathe bed for example will often be worn heavily in the middle (or near the chuck) but with no wear under the headstock and little at the far end of the bed. In most instances it is best to restore the original alignment and often is is possible to measure off of unworn sections of a machine tool to help understand where the original position and alignment was.

In a situation such as the one shown below the scraping will primarily take place on either end of the way until the entire section as been lowed to new constant level. Once that has been accomplished the entire way will be scraped for contact and to establish a suitable pattern.

It is important to remember this is a simplified example we are scraping planes, not lines so the transverse angle must be restored or maintained depending on the state of wear.