Build a surface plate from ribbed cast iron, stress-relieved and then scraped flat against two companion plates, or from a slab of fine-grained granite lapped flat with silicon carbide and water, and make it at least 300 by 300 mm. Keep the companion plates after the job: when the three are rubbed together later, the pattern of which pairs disagree tells you which plate has worn.
Part of Precision Tool Making
Making, supporting and maintaining the flat reference surface from which heights, squareness and flatness are measured.
A Plate Is Only A Reference While You Can Tell It Is Still Flat
Every height measurement and squareness check begins on the plate, so a plate that has quietly gone out of true corrupts everything measured from it. Two materials serve. Cast iron is the traditional choice: it can be scraped and repaired, and it holds oil, but it rusts. Granite (black granite is best, per the carrier) does not rust or burr and is thermally stable, but it cannot be repaired by scraping (Surface Plates).
The carrier’s own recipe for granite is to lap the surface flat with silicon carbide grit and water on a matching plate (Lapping for Ultra-Precision), which is consistent: lapping is not scraping.
What You Need
- For cast iron: a pattern and mould for a ribbed plate, flat on top with ribs underneath for stiffness (Sand Casting)
- A furnace able to stress-relieve the casting
- Two further blanks for the companion plates, and a scraper with blue (Hand Scraping)
- For granite: a fine-grained slab, silicon carbide grit and water, and a matching plate to lap against
- A stand that supports the plate on three points
Method: Cast-Iron Plate
Step 1: Cast a ribbed plate with a flat top.
Step 2: Stress-relieve it. The carrier says heat to 600°C and cool slowly to prevent warping over time. The Three-Plate Method article gives 400-500°C or six months of ageing instead. The sources disagree, so age the casting as well as heating it, and expect the plate to move slightly in the first months; check it again then.
Step 3: Rough-grind or machine the top.
Step 4: Scrape it to flatness against two companion plates by the three-plate method (Three-Plate Method).
Step 5: Support it on three points. Three points define a plane, so the plate cannot rock; four can leave it rocking and twisting.
Step 6: Oil it lightly, wipe before and after use, and never use it as an anvil.
Method: Granite Plate
Lap the slab flat with silicon carbide grit and water against a matching plate. Granite cannot be repaired by scraping, so a chipped or worn granite plate is re-lapped.
Worked Example
Keep the three plates A, B and C together. After some months of use, rub them in pairs with a thin film of blue and read the result.
| Result | Conclusion |
|---|---|
| All three pairs show even blue | None has drifted noticeably |
| A-B even; A-C and B-C show contact in the middle only | C has gone convex (the plate common to both bad pairs) |
| All three pairs disagree | More than one has drifted; scrape the set again |
The logic is the same three-plate logic that made them: errors that cannot be shared across all three pairs point to the plate common to the failing pairs. Plate wear is not even: a used plate develops low areas where work is always placed, so move work around (Surface Plates). The minimum practical size is 300 by 300 mm; a plate of that size has 144 squares of 25 mm by 25 mm to be scraped.
Common Mistakes
- Discarding the companion plates. Then you have an owned reference and no way to check it.
- Setting the plate on four supports. It rocks and twists.
- Placing heavy sharp-edged work down hard. It raises burrs.
- Believing the stress-relief figure. The sources disagree; age as well as heat.
Safety
Heavy castings, hot metal and dust
A 300 mm cast-iron plate is heavy: lift with help or a hoist and keep feet clear. Stress-relieving means heating a casting to hundreds of degrees; let it cool in the furnace and handle it with tongs. Lapping a granite slab makes silicon carbide and rock dust, and granite and sandstone contain quartz, a silicosis hazard; keep the lap wet and read Dust and Respiratory Protection.
What’s Next
- Learn next: Testing Flatness - proving the plate is flat
- Combine with: Straight Edges and Surface Plates - the companion reference