How Much Machining Allowance: Six Terms, Four Factors and the Table Method

Too much allowance wastes time; too little and the part cannot be made. But the amount is not a number picked from experience — it has a definite lower bound: this operation's allowance cannot be less than the error the previous one left behind. This article separates six commonly confused terms and unpacks the four factors that set that bound.

1. Six terms to keep apart

The last two cause the most trouble. When discussing allowance on an outside diameter or a bore, always confirm whether the other person means radius or diameter — the same "leave 0.3" means twice the material removed in one reading versus the other. Flat surfaces avoid this: their allowance is single-side by definition.

Also worth remembering is how operation allowance tolerance is composed: this operation's tolerance plus the previous one's. In other words, the looser the previous operation, the more this one's allowance varies — which leads straight to the next section.

2. The lower bound is set by the previous operation

Allowance is not free choice; it has a physical floor: this cut must remove every error the previous operation left. Four factors set it:

3. Only one of the four is yours

These four carry an important practical implication: the first three are debts owed by the previous operation; only the fourth is created by this one. So when allowance keeps coming up short — black skin or the previous operation's tool marks still visible after the cut — the thing to review is usually not that this operation left too little, but that the previous one ran too loose.

The fourth also explains why workholding affects allowance: both locating error and clamping error count toward it. For the related principles see choosing a locating datum.

4. Operation datum and operation dimension

The operation datum is the datum from which the operation drawing dimensions and position accuracy of the machined surface are stated; the datum used to measure them afterwards is the measurement datum. The two should normally coincide — otherwise what you measure and what you located from are not the same thing, and an extra layer of error appears from nowhere.

Where the locating datum does not coincide with the design datum, three criteria apply:

Operation dimensions themselves are found three ways: for simple diameters and bores, directly from the following operation's dimension plus or minus the nominal allowance (plus for enclosed surfaces, minus for enclosing ones), with tolerance taken from the economic accuracy of the method; where accuracy is secured indirectly across two or more operations, by dimension chain calculation; and for shafts and sleeves where the locating datum transfers repeatedly, by process dimension chain analysis across the whole route.

5. In practice: look up, then subtract

The working order is this: look up total allowance and the semi-finishing and finishing allowances in a standard process handbook, adjust for actual conditions, then obtain the roughing allowance by subtracting the semi-finishing and finishing allowances from the total — it is not looked up.

That order makes sense: finishing allowance is dictated by the accuracy requirement and has the least freedom; roughing allowance is simply what remains. Fix what cannot move first, and give the remainder to roughing.

For how many operations each surface type needs, see machining route selection; for how those operations sit on the timeline, see sequencing machining operations; for tolerance grades see tolerance grades and fits.

Compiled from a metal cutting technical handbook as general process principles and definitions; specific allowance values should come from the standard handbook in use, adjusted on the floor. No value tables are given here.

This article is part of Manufacturing Process: The Complete Guide - Set the Datum First, Then Sequence, Allowance and Dry Cutting; that guide shows how the whole topic fits together.