Imperial to metric: reading a drawing that predates the millimetre.

Convert a size

One inch is 25.4 mm exactly, so the millimetre figures in the table are exact, not rounded.

Thou to millimetres

A thou is one thousandth of an inch. Old drawings give tolerances and clearances in thou.

A tolerance of plus or minus 5 thou is plus or minus 0.127 mm. That is still the common commercial machining tolerance, as the machining tolerances guide explains.

SWG: sheet and wire gauge

Old UK drawings give sheet thickness in SWG, the Imperial Standard Wire Gauge. A higher number is thinner. The last column is the metric sheet usually sold in its place.

The metric sheet is not the same thickness. 16 SWG is 1.626 mm and the usual stand-in is 1.5 mm, about 8 percent thinner. Check that against the strength and the fit before accepting it. The sheet metal fabrication page covers cutting and folding.

Imperial threads: identify, do not convert

Sizes convert. Threads do not. No metric thread shares the diameter, the pitch and the form of an imperial one, so the thread on the drawing is the thread that gets cut. The table gives threads per inch for the four common series.

• BSW and BSF have a 55 degree thread form. UNC and UNF have 60 degrees.
• BSW and UNC share the same threads per inch at most sizes, but not at 1/2 inch: 12 for BSW, 13 for UNC. They should not be mixed at any size.
• If the mating part stays, the thread stays. Changing to metric means changing both parts.

Pipe threads on old drawings are usually BSP. The size names the pipe bore, not the thread, so the thread is much larger than its name.

Convert or keep: how to decide

• If the part mates with existing imperial parts, keep every interface dimension and every thread as drawn.
• If the bar size is imperial, ask the stockholder whether that size is stocked before rounding to a metric one.
• If a size is rounded to metric, carry the tolerance across too. 1/2 inch is 12.7 mm. 12 mm and 13 mm are both different sizes.
• If the drawing gives limits in inches, convert the limits, not the nominal size.
• If the sheet is called up in SWG, decide whether the metric stand-in is acceptable, and write the decision on the order.
• If there is no drawing at all, the part is measured and a new drawing made. The reverse engineering guide covers how.

A worked example: one line from an old drawing

A drawing from the 1960s shows a shaft of 1 1/4 inch diameter, plus nothing, minus 2 thou, with a 16 SWG cover and 3/8 BSF studs.

• The shaft is 31.750 mm, plus nothing, minus 0.051 mm. It mates with an existing bearing, so it stays at 31.750 mm and is not rounded to 32 mm.
• The cover is 1.626 mm. If 1.5 mm sheet is acceptable, that is written on the order. If not, the gauge sheet is sourced.
• The studs are 3/8 BSF, 20 threads per inch. They screw into the existing housing, so they stay BSF.

TrueNorth Engineering, Manchester, remakes legacy and obsolete parts from old drawings or from a worn sample. Where the drawing is lost, the sample is scanned and measured and a new drawing set is produced that the customer owns.