Engineering Drawing - Part 5

Tolerances, limits and fits

In order to ensure that assemblies function properly their component parts must fit together in a predictable way. As mentioned in section 2.5, no component can be manufactured to an exact size, so the designer has to decide on appropriate upper and lower limits for each dimension.

Accurately toleranced dimensioned features usually take much more time to manufacture correctly and therefore can increase production costs significantly. Good engineering practice finds the optimum balance between required accuracy for the function of the component and minimum cost of manufacture.

Dimension tolerances

If a dimension is specified, in millimeters, as 10 ± 0.02, the part will be acceptable if the dimension is manufactured to an actual size between 9.98 and 10.02 mm. Below are some examples of ways of defining such limits for a linear dimension.

To give you a feel for the magnitude of decimal values in mm, consider these facts:

The thickness of the paper this page is printed on is approximately 0.100 mm.

Average human hair thickness is approximately 0.070 mm.

The human eye cannot resolve a gap between two points smaller than about 0.020mm, at a 20cm distance.

If you raise the temperature of a 100mm long block of steel by 10ºC it will increase in length by approximately 0.020mm.

General tolerancing

General tolerance notes apply tolerances to all unspecified dimensions on a drawing. They can save time and help to make a drawing less cluttered. Examples are shown below.

Some examples of general tolerance notes

Limits and fits for shafts and holes

Basic size and shaft/hole tolerancing systems

The basic size or nominal size is the size of shaft or hole that the designer specifies before applying the limits to it. There are two systems used for specifying shaft/hole

tolerances:

Basic hole system: Starts with the basic hole size and adjusts shaft size to fit.

Basic shaft system: Starts with the basic shaft size and adjusts hole size to fit.

Because holes are usually made with standard tools such as drills and reamers, etc, the basic hole system tends to be preferred and will therefore be used here.

Fit

The fit represents the tightness or looseness resulting from the application of tolerances to mating parts, e.g. shafts and holes. Fits are generally classified as one of the

following:

Clearance fit: 

• Assemble/disassemble by hand.
• Creates running & sliding assemblies, ranging from loose low cost, to free-running high temperature change applications and accurate minimal play locations.

Transition fit: 

• Assembly usually requires press tooling or mechanical assistance of some kind.
• Creates close accuracy with little or no interference.

Interference fit: 

• Parts need to be forced or shrunk fitted together.
• Creates permanent assemblies that retain and locate themselves.

ISO limits and fits

Fits have been standradised and can be taken directly from those tabulated in the BS 4500 standard, 'ISO limits and fits.'

The BS 4500 standard refers to tolerance symbols made up with a letter followed by a number. The BS Data Sheet BS 4500A, as shown on the following two pages, shows a range of fits derived, using the hole basis, from the following tolerances:

Holes:	H11	H9	H8	H7
Shafts:	c11	d10	e9	f7	g6	k6	n6	p6	s6

Remember:

• Capital letters always refer to holes, lower case always refer to shafts.
• The greater the number the greater or wider the tolerances.

The selection of a pair of these tolerances will give you the fit. The number of possible combinations is huge. BS 4500 helps to standardize this and offers a range of fits suitable for most engineering applications.

Selected ISO Fits - Hole Basis. Extract from BS 4500, data Sheet 4500A.

Selected ISO Fits - Hole Basis. Extract from BS 4500, data Sheet 4500A.

ISO limits and fits, determining working limits

Consider an example of a shaft and a housing used in a linkage:

Type of fit:	'Normal' clearance fit.
Basic or Nominal size:	φ40mm

We will determine the actual working limits, the range of allowable sizes, for the shaft and the hole in the housing.

Look along the bottom of the ISO Fits Data Sheet 4500A and locate 'Normal Fit'. We will use this pair of columns to extract our tolerances.

The tolerances indicated are:

1st column	H8	for the hole   (upper case H)
2nd column	f7	for the shaft  (lower case f)

The actual tolerances depend upon the basic, or nominal, diameter as well as the class of fit. So, locate 40mm in the left hand Nominal Sizes column. Either the 30 - 40 or 40 - 50 range is acceptable in this case. Read across and note the tolerance values for the hole and the shaft, as shown below.

For the hole diameter we have a tolerance of: +0.039mm   -0.000mm
For the shaft diameter we have a tolerance of: -0.025mm   -0.050mm

These tolerance values are simply added to the nominal size to obtain the actual allowable sizes.

Note that this is a clearance fit. As long as the hole and shaft are manufactured within the specified tolerances the hole will always be either slightly oversize or spot on the nominal size and the shaft will always be slightly under-size. This ensures that there will always be a free clearance fit.

These tolerances may be expressed on a drawing in several ways:

1) Simply as the nominal size with the tolerance class.

This is not always preferred as the machine operator has to calculate the working limits.

2) The nominal size with the tolerance class as above with the calculated working limits included.

3) The calculated working limits only.

Tabulated guide to types of ISO limits and fits

Assembly drawings

Assembly drawings can be used to:

• Name, identify, describe and quantify all of the components making up the assembly.
• Clearly show how all of the components fit together.
• Indicate all of the required fasteners.
• Record any special assembly instructions.
• Record any other relevant information.

Here is an example:

Note the use of sections, item numbers neatly laid out and the parts list.

Drawing checklist

It is easy to accidentally omit various items when creating engineering detail drawings. Before passing on your work it is recommended that you work through the checklist below for each drawing:

The general drawing:

1. Do projections conform to the relevant conventions, usually 1st or 3rd angle?
2. Have you used the minimum number of views necessary to accurately show the information required?
3. Are the views laid out in appropriate positions relative to the size of paper?
4. Has the title box been completed, particularly: 
• Drawn by
• Name of component
• Date
• Projection (1st or 3rd angle)
• Paper size
• Scale
5. If required, has the material been specified?

The geometry details:

1. Check to make sure that there are sufficient dimensions to manufacture the component. Check that positions and sizes of any features, such as holes, are clearly dimensioned.
2. No dimension should appear more than once on the drawing, do any?
3. Have the dimensions been laid out in consistent and clear positions, so that they are easy to read.
4. Have all of the dimension lines been constructed with correct extension lines and gaps?
5. Are the arrow heads all in the same style and the same size?
6. Have dimensions relating to a particular feature, such as a hole, been grouped together on one view, if possible?
7. Have appropriate line styles and line weights been used?
8. Have any surface finish requirements been specified?
9. Have any explicit tolerance requirements been specified?
10. Have any required center lines, break lines, etc. been used?
11. Have any required general notes been added, such as additional general tolerances, finish specifications or specification of special manufacturing processes?
12. If sections have been used do they conform to drawing conventions?