Benchwork
What this domain tests
Benchwork tests hand fitting, filing, sawing, chipping, scraping, lapping, thread repair, and assembly at the bench. The exam looks for controlled removal of material, correct fits, and protection of finished surfaces.
Core rules and concepts
- Filing: choose the file for the finish, use chalk on a fine file when filing soft aluminum, and cross-file to check flatness. Draw filing improves surface finish but removes little stock.
- Sawing: start the cut with a light stroke, use the full blade length, and let the teeth cut. Relief cuts and correct vise height prevent binding.
- Chipping and scraping: chip with a sharp cold chisel and eye protection; scrape bearing surfaces using bluing to show high spots. Scraping produces spots, not a mirror finish.
- Lapping: use a lapping plate and abrasive compound to refine flatness and finish. Keep the part moving in a figure-eight pattern to avoid a dished surface.
- Fits: a sliding fit, interference fit, and press fit are selected from the drawing or standard. Use an arbor press, not a hammer, for press fits; align the part before applying force.
- Thread repair: use the correct tap or die, start square, and check with a thread gauge.
Worked example
A key is too tight in a shaft keyway. Measure the keyway width and key thickness; if the key is oversize, file the sides evenly, checking with the keyway and bluing. The goal is contact on the sides with a small clearance at the top. Do not file the key's top to force a fit; that reduces driving contact and can cause the key to shear. Recheck the fit and deburr.
Traps that cost marks
Filing only in one direction and creating a rounded edge; using a pin punch to align a part under press force; scraping without bluing; forcing a press fit with a hammer; and deburring after assembly instead of before. Benchwork marks often hinge on process, not speed.