Reaming Prebore (Drill) Allowance

drill = reamer diameter - allowance, with Machinery's Handbook stock allowances on the diameter of about 0.010 in under 1/4 in, 0.015 in from 1/4 to 1/2, 0.020 in from 1/2 to 1, 0.025 in from 1 to 1-1/2, and 0.030 in from 1-1/2 to 2. A 1/2 in reamer wants a 0.485 in prebore (31/64 drill); a 3/4 in reamer a 0.730 in prebore. Too little stock burnishes and dulls the reamer, too much leaves an oversize, torn, or bell-mouthed hole. Hand reamers and thin-wall parts take less; the reamer maker's guidance and the material govern.

allowance = banded from the finished diameter (0.010 under 1/4 in, 0.015 to 1/2, 0.020 to 1, 0.025 to 1-1/2, 0.030 to 2 in) unless overridden; drill_diameter_in = reamer_diameter_in - allowance (on the diameter).

Machinery's Handbook (Industrial Press) machine-reaming stock allowances, by name; public-domain shop guidance. The reamer maker and the material govern.

The prebore relation is a first-principles subtraction; the diameter-band stock allowances are public Machinery's Handbook guidance, and the reamer diameter is user-supplied.

Estimate. AHJ and licensed professional govern.

Field names used by the API: reamer_diameter_in, allowance_used_in, drill_diameter_in

• Diameter-band allowance machine reaming: ~0.010/0.015/0.020/0.025/0.030 in by finished-diameter band, on the diameter; overridableMachinery's Handbook
• Machine reaming hand reamers and thin-wall or interrupted holes take less; the reamer maker and material governshop practice