Dimensional Tolerance

Chapter 1. Basic Machining and Tips

Dimensional Tolerance

Necessity of Dimensional Tolerance

It is almost impossible (and sometimes uneconomical) to maintain the strict degree of accuracy as listed on a plan. To accommodate this, it is normal to display measurements with a plus or minus (+/-) tolerance which allows for some margin of error. Care needs to be taken however when determining such +/- tolerance, particularly where there are mating parts. For example, a shaft which is machined to its maximum tolerance may not fit a gear center that has been machined to it minimum tolerance or an unsatisfactory loose fit would result from the shaft being machined to its minimum tolerance with the gear center machined to its maximum tolerance.

Usually, the dimensional tolerance is decided at the design stage and a Machinist must take care to apply the required dimensional tolerance and to ensure that discrepancies are not introduced as a result of poor workmanship of measuring techniques.

Dimensional Tolerance of a Shaft and a Hole

Figure 1 shows the plans of a fish robot's joint. In the plan, a shaft is inserted in the holes of Parts 1 and 2. The diameter of the holes are required to be on the plus-side of the dimensional deviation, and the diameter of the shaft is required minus-side of the dimensional deviation. Part 2 is inserted a slot of Part 1. Then the slot of Part 1 must have a plus-side dimensional deviation, and the size of Part 2 must have a minus-side dimensional deviation.

Additionally, holes of many "commercially available" mechanical parts, such as gears and couplings, are already finished to plus-side dimensional deviations.

Dimensional Tolerance of a Reamer

When an accurate hole is required, we often use the hand tool called 'a reamer'. A diameter of a general reamer has plus-side dimensional deviation. Therefore, when a hole is made by a reamer that has 12 mm of nominal diameter, the hole is finished plus-side dimensional deviation than 12 mm.

Fig. 1, Dimensional Tolerance of a Shaft and Holes

Installation of a Bearing

Many machines have bearings that support a rotating shaft. Various standardized bearings are commercially available and easily obtained. Generally, the outer diameter of a bearing has a minus-side dimensional deviation. The hole for the bearing must be finished to a plus-side dimensional deviation. On the other hand, the inner diameter of a bearing is a plus-side dimensional deviation, then the shaft to be inserted into the bearing must be finished to the minus-side dimensional deviation.

Fig. 2, Installation of a Bearing

A Slot for an O-ring

An O-ring is a mechanical component which is used as a seal device for various fluids. In order that it should work correctly, a slot of the O-ring must be finished to required dimensional tolerance. The values of the required dimensional tolerance are shown in typical O-ring catalogues.

Fig. 3, A Slot for an O-ring

Installation of an One-way Clutch

In some special cases, as exampled below, a minus sided dimensional deviation is required:

Figure 4 shows a one-way clutch fitted with bearings and a shaft. In order to obtain the correct operation, the outer ring of the one-way clutch must mate firmly with it mating hole. To achieve this, the hole needs to be finished on the minus-side of the dimensional deviation.

In this case, the hole was finished with a hand reamer of 11.98 mm diameter, though the nominal dimension of the one-way clutch is 12 mm.

Fig. 4, Installation of an One-way Clutch

Which is the Dimensional Deviation Plus- or Minus-side?

Much care and consideration needs to be given to this issue and the results of the determination needs to be clearly stated on the plans.

Surface Finish

In the case of general machining, we do not measure the surface finish in the machning process. However, when we consider the order of the machining process, it is very important that we know the required surface finish.

For example, the seal surface for an O-ring must have a high degree of accuracy of the surface finish as shown in Figure 5(a). If it has a rough surface, the O-rings are damaged during the assembly stage. Also the sliding surface as shown in Figure 5(b) must have high accurate surface finish.

In order to obtain a high surface finish with a lathe or a milling process, a slow movement of a tool and high blade speed gives better results.

Fig. 5, Examples of a Flat Surface

The Standard Surface

As described above, we cannot always finish the actual size to the same as that stated in a plan. In order to achieve the required degree of accuracy, we need to determine a "standard reference point" and plan the measurements from this point for an example of mating parts. In the case of mating parts (see Figure 6), these are where the parts mate is often used as the reference point as this provides a common reference point on both parts (see figure 7).

Care needs to be taken when planning reference points as any errors can be accumulative resulting in parts not fitting together. The reference point varies from job to job as the complexity of shapes provide many challenges to accurate measuring and setting-out.

Then it is important to decide the standard surface for measuring of the length or the machiningof the location. If the decided standard surface is not suitable, errors are piled up, and the completed parts often cannot be constructed as the completed machine. First, see the part plan carefully, and consider where is decided the standard surface.

The deciding of the standard surface of a part is different by the shape or how-to-use. When several boards are constructed, the surface touched other parts is very often decided as the standard surface (see Figure 6). When the we make holes in a circumference, the center of circle is very often decided the standard point (see Figure 7).

Fig. 6, Holes Inserted a Shaft

Fig. 7, Holes of a Flange

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