Allowance (engineering)
Encyclopedia
In engineering
and machining
, an allowance is a planned deviation between an actual dimension and a nominal
or theoretical dimension, or between an intermediate-stage dimension and an intended final dimension. The unifying abstract concept is that a certain amount of difference allows for some known factor of compensation or interference. For example, an area of excess metal may be left because it is needed to complete subsequent machining. Common cases are listed below. An allowance, which is a planned deviation from an ideal, is contrasted with a tolerance, which accounts for expected but unplanned deviations.
A tolerance is the limit of acceptable unintended deviation from a nominal or theoretical dimension. Therefore, a pair of tolerances, upper and lower, defines a range within which an actual dimension may fall while still being acceptable.
In contrast, an allowance is a planned deviation from the nominal or theoretical dimension.
operation that produces the final diameter may introduce a certain small-but-unavoidable amount of random error. Therefore, she specifies a tolerance of ±0.001 mm ("plus-or-minus" 0.001 mm).
As long as the grinding machine operator can produce a shaft with actual diameter somewhere between 9.999 mm and 10.001 mm, the shaft is acceptable. Understanding how much error is predictable in a process and how much is easily avoidable; how much is unavoidable (or whose avoidance is possible but simply too expensive to justify); and how much is truly acceptable involves considerable judgment, intelligence, and experience, which is one reason why some engineers are better than others.
An example of the concept of allowance can be shown in relation to the hole that this shaft must enter. It is evident that the above shaft cannot be certain to freely enter a hole that is also 10 mm with the same tolerance. It might, if the actual shaft diameter is 9.999 mm and the actual hole diameter is 10.001 mm, but it would not if conversely the actual shaft diameter is 10.001 mm and the actual hole diameter is 9.999 mm.
To be sure that there will be enough clearance between the shaft and its hole, taking account of the tolerance, an allowance is intentionally introduced in the dimensions specified. The hole diameter might be specified as 10.003 mm with a manufacturing tolerance of ±0.001 mm ("plus-or-minus" 0.001 mm). This means that the smallest acceptable hole diameter will be 10.002 mm while the largest acceptable shaft diameter will be 10.001 mm, leaving an "allowance" of 0.001 mm. The minimum clearance
between the hole and the shaft will then be 0.001 mm. This will occur when both the shaft and the hole are at "Maximum material condition".
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
and machining
Machining
Conventional machining is a form of subtractive manufacturing, in which a collection of material-working processes utilizing power-driven machine tools, such as saws, lathes, milling machines, and drill presses, are used with a sharp cutting tool to physical remove material to achieve a desired...
, an allowance is a planned deviation between an actual dimension and a nominal
Real versus nominal value
In economics, nominal value refers to a value expressed in money terms in a given year or series of years. By contrast, real value adjusts nominal value to remove effects of price changes over time...
or theoretical dimension, or between an intermediate-stage dimension and an intended final dimension. The unifying abstract concept is that a certain amount of difference allows for some known factor of compensation or interference. For example, an area of excess metal may be left because it is needed to complete subsequent machining. Common cases are listed below. An allowance, which is a planned deviation from an ideal, is contrasted with a tolerance, which accounts for expected but unplanned deviations.
Examples of engineering and machining allowances
- Outer dimensions (such as the length of a bar) may be cut intentionally oversize, or inner dimensions (such as the diameter of a hole) may be cut intentionally undersize, to allow for a predictable dimensional change following future cutting, grindingGrinding machineA grinding machine, often shortened to grinder, is a machine tool used for grinding, which is a type of machining using an abrasive wheel as the cutting tool...
, or heat-treatingHeat treatmentHeat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass...
operations. For example:- the outer diameter of a pin may be ground to 0.0005 inch (0.0127 mm) oversize because it is known that subsequent heat-treatment of the pin is going to cause it to shrink by 0.0005 inch (0.0127 mm).
- A hole may be drilled 0.012 inch (0.3048 mm) undersize to allow for the material that will be removed by subsequent reamingReamerA reamer is a metalworking tool used to create an accurate sized hole. The process is called reaming. They may be used as a hand tool or in a machine tool, such as a milling machine or drill press.-Construction:...
.
- Outer dimensions (such as the diameter of a railroad car's axle) may be cut intentionally oversize, or inner dimensions (such as the diameter of the railroad car's wheel hub) may be cut intentionally undersize, to allow for an interference fit (press fit)Interference fitAn interference fit, also known as a press fit or friction fit, is a fastening between two parts which is achieved by friction after the parts are pushed together, rather than by any other means of fastening...
. - A part may be cast intentionally too big when it is desired to later machine the surface. This ensures that the roughness that the casting process leaves is removed, and a smooth machined surface is produced. This machining allowance may be e.g. 1mm, but this depends on the size of the part and the accuracy of the casting process.
Confounding of the engineering concepts of allowance and tolerance
Often the terms allowance and tolerance are used imprecisely and are improperly interchanged in engineering contexts. This is logical because both words generally can relate to the abstract concept of permission — that is, of a limit on what is acceptable. However, in engineering, separate meanings are enforced, as explained below.A tolerance is the limit of acceptable unintended deviation from a nominal or theoretical dimension. Therefore, a pair of tolerances, upper and lower, defines a range within which an actual dimension may fall while still being acceptable.
In contrast, an allowance is a planned deviation from the nominal or theoretical dimension.
Example
An example of the concept of tolerance is a shaft for a machine is intended to be precisely 10 mm in diameter: 10 mm is the nominal dimension. The engineer designing the machine knows that in reality, the grindingGrinding machine
A grinding machine, often shortened to grinder, is a machine tool used for grinding, which is a type of machining using an abrasive wheel as the cutting tool...
operation that produces the final diameter may introduce a certain small-but-unavoidable amount of random error. Therefore, she specifies a tolerance of ±0.001 mm ("plus-or-minus" 0.001 mm).
As long as the grinding machine operator can produce a shaft with actual diameter somewhere between 9.999 mm and 10.001 mm, the shaft is acceptable. Understanding how much error is predictable in a process and how much is easily avoidable; how much is unavoidable (or whose avoidance is possible but simply too expensive to justify); and how much is truly acceptable involves considerable judgment, intelligence, and experience, which is one reason why some engineers are better than others.
An example of the concept of allowance can be shown in relation to the hole that this shaft must enter. It is evident that the above shaft cannot be certain to freely enter a hole that is also 10 mm with the same tolerance. It might, if the actual shaft diameter is 9.999 mm and the actual hole diameter is 10.001 mm, but it would not if conversely the actual shaft diameter is 10.001 mm and the actual hole diameter is 9.999 mm.
To be sure that there will be enough clearance between the shaft and its hole, taking account of the tolerance, an allowance is intentionally introduced in the dimensions specified. The hole diameter might be specified as 10.003 mm with a manufacturing tolerance of ±0.001 mm ("plus-or-minus" 0.001 mm). This means that the smallest acceptable hole diameter will be 10.002 mm while the largest acceptable shaft diameter will be 10.001 mm, leaving an "allowance" of 0.001 mm. The minimum clearance
Clearance
A clearance can refer to:* in chess, A positional move, where a player moves a piece occupying a certain square away, replacing it with an allied piece that will strengthen the player's position....
between the hole and the shaft will then be 0.001 mm. This will occur when both the shaft and the hole are at "Maximum material condition".