EngineerBenchPlanning a drilled and reamed hole
Reaming removes a small amount of material from an existing hole to bring it to a controlled size. The predrill diameter is the intended finished diameter minus the diametral stock allowance. Diametral allowance is the total difference in diameters, not the amount removed from each side.
Choose the allowance for the process
Enter a reaming allowance appropriate to the reamer, work material, and hole size. This calculator applies the allowance you provide; it does not select one from a tooling catalog. Too little or too much stock can make the intended finish and size difficult to achieve.
Use limits to check the resulting fit
The smallest hole against the largest shaft gives minimum clearance. The largest hole against the smallest shaft gives maximum clearance. These endpoint checks describe dimensional fit; they do not account for temperature, shape error, surface finish, or the alignment of several holes.
Keeping allowance separate from finished tolerance
A stock allowance tells you how much material is intended to be removed during reaming. A tolerance tells you the permitted range of the finished feature. They describe different stages of the process. For a 10 mm finished hole with a chosen 0.2 mm diametral allowance, the predrill estimate is 9.8 mm; the radial removal would be 0.1 mm on each side in an ideal concentric hole.
The final hole limits should come from the functional requirement, not from the amount of stock left for the reamer. Compare those limits with the mating shaft limits to see whether clearance is guaranteed. If minimum clearance is negative while maximum clearance is positive, some permitted combinations can interfere and others can clear. Reaming does not automatically correct a badly located or misaligned starting hole. Tool condition, material, workholding, and the predrilled hole affect the outcome, so confirm the process can achieve the specified final size and geometry rather than treating the calculated predrill as a quality guarantee.
Formula
Predrill = final hole − diametral stock. Minimum clearance = smallest hole − largest shaft.