How to use a curved engineer’s scraper?

29th February 2020


How to use a curved engineer’s scraper


	Shop for Engineer’s Scrapers


		Unlike flat scrapers, curved scrapers are not used to make a surface flat. Instead they are used to achieve a very accurate fit between two curved surfaces, such as a cylinder and a bearing, or a bearing and a shaft.

		Other equipment you may need
		Engineer’s spotting ink This is used to highlight the areas on the workpiece that are causing a sticking point and need scraping. There are many types of spotting ink that can be used in scraping, the most popular is Prussian Blue.

		Small roller A small rubber roller is best to spread the ink, although the foam sort you would use in decorating can also be used. This is needed to roll a thin, even layer of the spotting ink.

		Microfibre cloth The microfibre cloth lifts dirt from the surface of the workpiece, leaving it clean and ready to be scraped.

		Felt tip pen A felt tip pen is used to divide and mark up the sections to be scraped between the oil grooves of a Babbit bearing.

		Brush You will need a soft-bristled brush to clean off the scraped metal from your workpiece.



		Master shaft This is the shaft that will be fitted into the cylinder or bearing.

		Curved scraper technique
		Hold the curved scraper with your dominant hand at the top of the handle and the other at the base of the handle, as you would a large pepper grinder.

		You should stand with your feet shoulders width apart with one foot in front of the other, facing the workpiece so you are able to look down the bearing you will be scraping. You need to be able to see down the bearing to see the spotting ink marks that have to be scraped.

		Push the scraper away from you with your dominant hand, whilst rotating the scraper blade with the other.

		The curved scraper blade should rest on the workpiece and be rotated towards the direction of the scraping edge in contact with the workpiece.

		Babbit bearing scraping process
		Instead of using a reference plate, the shaft that is to be inserted into the bearing or cylinder is coated with the marking ink. As the shaft is pushed into the bearing or cylinder it leaves traces of the marking ink on all the contact areas (these are the equivalent of high spots).

		Step 1 – Clean workpiece and master shaft Begin by first checking that your workpiece and master shaft are clean. Use the soft-bristled brush to clear away dirt and then wipe over both the master shaft and workpiece with a microfibre cloth.

		Step 2 – Ink shaft Apply a couple of drops of spotting ink to the master shaft and use the roller to spread it out thinly and evenly over the surface.



		Step 3 – Position shaft in bearing Place the inked master shaft into the bearing. Do not force the shaft into the bearing.

		Step 4 – Rotate shaft Rotate the shaft in the bearing in the direction the shaft will spin. Then remove the shaft from the bearing.

		Step 5 – Divide up bearing Divide each section between the oil grooves up into six equal smaller size sections and mark them using a felt tip pen.

		This is how the bearing should look once the master shaft has been removed and the sections between the oil grooves divided up and marked with felt tip.

		Step 6 – Scrape contact points The contact points between the master shaft and the bearing will be highlighted by the spotting ink.

		Using the curved blade scraper, scrape the contact points.

		Step 7 – Clean inside bearing Using the soft-bristled brush, clean the inside of the bearing or cylinder to remove any scraped metal or dirt. Then repeat steps 2, 3 and 4

		Step 8 – Repeat scraping If the felt tip pen marks are no longer visible, repeat step 5, then repeat steps 6 and 7 until 80% of the four middle sections between the oil grooves (highlighted in yellow) are highlighted by the spotting ink.

		Step 9 – Scrape only outer sections Once 80% of the middle four sections are highlighted with spotting ink, scrape only the outer two sections nearest the oil grooves. Repeat steps 2-7 scraping only the outer two sections nearest the oil grooves (highlighted in blue) until there is not spotting ink visible in these areas.

		Once you reach this stage, the bearing should look something like this. All that is now left to be done is brush the inside of the bearing and wipe it clean with a microfibre cloth.

		Why scrape all the contact points away from around the oil grooves? By scraping away all of the contact points next to the oil grooves, oil dispersion around the bearing is improved. The 80% contact points between the oil grooves ensure the master shaft will be evenly supported within the bearing and so rotate true.