Foundations 2D/3D

Information dimensions, also called reference dimensions may in some occasions be necessary and useful to include in the drawing, but will not affect the function of the part or relate to the tolerance table (see section on tolerances below for more information)

Such information dimensions shall be enclosed in a parenthesis.

Dimensions are usually shown in millimeters when drawing according to the NS/ISO standard.

• Some international companies use dual dimensions to show metric and inch dimensions on the same drawing.
• Usually the inch dimension enclosed in parentheses as a reference, and placed behind or below the metric dimension.
• Properties for dual dimensioning are selectable in Alternate Units from the Dimension Style dialog box.

Dimensioning of holes can be clearly communicated by use standard symbols.

• Please note that the front view (upper drawing) shows superfluous dimensioning and are therefore displayed in parentheses, as the same dimensions are also visible on the top view (lower drawing).

When producing an inner thread, as displayed below, one first has to drill a hole in the material. After one has drilled the hole with the correct diameter drill bit, one can then tap it to create threads in it. The size of the drill bit used to create the initial hole is defined through standards. It is therefore not required to include information regarding the diameter of the drill bit used for drilling the hole prior to tapping it, as the standards defining it are available to the workshop. One also has to bear in mind that a thread will never be fully threaded as long as it has a defined depth. Due to the tapping process the last bit of the hole will not be threaded. The most cost efficient way display a thread is therefore by dimensioning the required depth of the thread, leaving the workshop to decide the depth of the initial hole. 

The image below displays a part where the initial hole prior to tapping goes through the whole length of the material. The threading on the other hand is only has a depth of 20mm. Because of this the diameter of the drilling hole is displayed (8,5mm).

• Please note that the front view shows superfluous dimensioning to illustrate the use of symbols from the top view.

Some drawings have a datum point, a reference point where dimensions and coordinates are measured from (e.g. Foundation drawings).

Datum Surfaces, are reference planes who that are used in conjunction with geometric tolerancing, to ensure greater accuracy on critical feature of a part. Critical features are elements that are vital for the functionality of the part. Datum surfaces are also important when inspecting the part after production.

Datum Surface symbols are capital letters (A, B and C) framed with a leader line with a foot shaped as a triangle who touch the specific line of the geometry, the datum surface.

Center of Gravity or CoG is the average location of weight of an object. The term is not to be confused with Center of Mass (CoM).

If you want to know more about CoG - se NASA's Calculations on the subject.

Single parts and assemblies with a weight above 20 kg cannot be moved manually, and will need a drawing showing lifting arrangement. Information of center of gravity, CoG, is always included for lifting arrangements and hanging objects.  

Dimensioning of CoG for a part or an assembly is normally performed from the datum point or datum planes.

Dimensioning of CoG in a lifting arrangement in normally performed from a single lifting point.  (E.g. typical where lifting slings are gathered in a lifting shackle for a common fixing point).

Even though the surface look smooth and shiny will a closer inspection identify rough structure and filets.

Information regarding the surface of a part is often vital information due to its functionality and quality. If there are no requirement for the surface finish, then the surface will not be inspected.

Preferred series of Roughness average rating are standardized. Examples are listed in Maskintegning, section 27.5 Valg av ruhetsverdier.

Information of surface roughness in technical drawings are described in section 27.7 Angivelse av overflateruhet på tegninger.

More information regarding surface roughness can be found in:

Maskintegning, Lundkvist, 1993; Section 27 Overflateruhet Maskintegning, Lien et al. 1995; Overflatebeskaffenhet.

The image shows dimensions with limited tolerances and standard tolerances.

Limited tolerances are easier to interpret:
Accepted length dimension at 23 mm can vary between 23,01 mm and 23,02 mm.
Accepted length dimension at 18 mm can vary between 18,0 mm and 18,1 mm.

Standard tolerances in the image here is indicated by a basic dimension with a tolerance symbols. Standard tolerances must be locked up in the applicable standard.

• H7 and g7 are passing tolerance classes for an axle; Limit dimensions for shafts
•  Tolerances for h7 for dimensions between 18 – 30 mm are +0 to - 21µm.
  The diameter of 28 mm can vary between 28, 000 mm and 27,979 mm.

Tables and reference guides are a necessary part of setting and reading tolerances.

On parts with less strict demands to accuracy and function, it's sufficient to declare the tolerance with a simple note at the bottom of the drawing:

All dimensions +/- 0.1 mm

It shows us that the one drawing hasn't made a consious decision of measure, as the dimension isn't accurate.

Always use fitting Snaps/OSnaps and/or Direct Distance.

We're not expecting you to choose the correct tolerance or decide allowed surface roughness. This is part of an engineers work practice, we are simply teaching you to recognize it and how this information is shown in technical drawings so that you can input it some day.

When a window is slotted into a wall the builder uses a Leveler (Vater) to gauge the direction, and ittings (kiler) to position the window.

Usual clearance is at least 20 mm on all sides of a window, which is then sealed and covered afterwards. It would be too expensive to let the builder work more accurately if the window, in our example, was a press fit; i.e. exact fit.

Se this assembly to get the idea - Innsetting av Norges Vinduet

Standard dimensions on windows and doors is therefore 10 mm less than the actual measure, while the opening for placement is 10 mm wider.

We are deliberatly overlooking any complications regarding the window being leveled, as the window could be fitted into the opening regards of weather, or if the builder was having a bad day - which happens.

On buildings you should always take into account deviations of approximately 10 mm on each side in openings.

The dimension of the door opening in the image should then read between 10-20 mm larger than the actual door width.

When the deviations being built (As Built) isn't being upholded the builder must use some extra time, and your money, press fit the window, or other.

This is usually a simple operation when dealing with wood materials, but it might not be as simple when the object is made out of aluminum and is to be fitted in a concrete wall opening.

A large part of added construction costs (may be 15% or even more) is due to corrections from incorrect tolerances, or deviations not being held.

The principals for use of tolerances between architectural features and mechanical features are the same, but mechanical tolerances have usually stricter requirements for accuracy. It is therefore common to state general tolerances in mechanical drawing through a tolerance table. The most common table to be used within mechanical drawings is ISO 2768. Information regarding the table is therefore often included on the drawing, where it also specifies which tolerance class it follows (fine, medium, coarse, very coarse).

Temperature and surface roughness will also have impact for mechanical parts.

For mechanical parts is it important to consider the effect of one tolerance to another tolerance.

As a rule, it is best to dimension each surface once, so that it is affected by only one dimension!

This can be done by referring all dimension to a single datum surface. The selection of the datum surface or baseline should be based on the functionality of the object.

When dimensions are specified as a chain, the tolerances for the part may add up. A chained dimension uses the end of one dimension as the beginning of the next.

Tolerance stacking refers to the way the tolerance for one dimension of a feature is added to the next dimension of the next feature in the chain, resulting in a large variation in the location of the last feature in the chain.

Reference; Technical Drawing with engineering Graphics, Giesecke et al., 2016, s 559.

Passing tolerances are used for parts who shall fit together as a window in a wall, or an axel in a hole. Passing tolerances indicates the clearance between the objects.

Tolerance for holes are indicated by a capital letter (e.g. H7) and axle tolerances are indicated by a smal letter (e.g. h7).

There are different passing tolerance classes for different use as loose fit, light interference fit and interference fit (press fit).

• For more information regarding tolerances look in Maskintegning, section 23, Toleranser og pasninger.
  Pay some special attention from section 23.10 including 23.17 regarding identification of tolerances.

• Also some excellent findings to discover in Technical Drawing with Engineering Graphics
  • Chapter 12: Tolerancing (p. 546 – 555)

Maskintegning, section 28 Form og beliggenhetstoleranser, shows tables with basic symbols and supplementary symbols for position and form.

Look at Maskintegning, section 28 Form og beliggenhetstolernaser for more Information.

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