TechDraw Geometric dimensioning and tolerancing

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Revision as of 01:26, 27 December 2020 by Uwestoehr (talk | contribs) (→‎Datums: formatting)
This documentation is a work in progress. Please don't mark it as translatable since it will change in the next hours and days.

Concept

The main purpose of a technical drawing is that others understand what the designer created with what tolerances AND how to manufacture the design. Since most parts have to fit to assemblies, also the geometric relations to other parts are important. To achieve this, the system of Geometric Dimensioning and Tolerancing (GD&T) was developed.

Take for example this part:

In the application an axis should be put through the 2 holes of the part. That mean we have to specify the following things

  • The diameter of the holes and their positions.
  • That the holes are along a certain axis that is not perpendicular to the XZ base plane.
  • That the axis part must have a certain straightness.

The straightness is important because a bend axis would not fit through the holes. The hole alignment is important because just drilling each hole at the given position without taking care of the axis, would lead to holes through which the axis will not fit. So just specifying the hole dimension and position is not sufficient. The first point is "classical" dimensioning while the 2 others are geometric information, thus GD&T comes into play. The example also makes clear that GD&T is essential to assure that parts fit well to assemblies.

If you are not familiar with the concept of GD%T, we highly recommend to learn about it now. For example a quick introduction is given in this video. For more like the concept of the "maximum material condition" have e.g. a look at this video. For details to all sorts of GD&T features, look around in the Internet. For example this YouTube channel provides some good explanations.

TechDraws' capabilities to fulfill the common GD&T standards is limited (as of FreeCAD 0.19) but you can achieve already many things. This Wiki page explains what is implemented and what tricks can be used for non-implemented features.

Dimensioning

TechDraw provides several tools to create different types of dimension. Please have a look at the different dimensions on how to create and modify them.

Formatting

The default format for new dimension numbers depends on the preferences options Use Global Decimals and Alternate Decimals. This specifies the number of decimals, but in every case the dimension is a floating point number. For example if the setting is to have 2 decimals, the used format specifier is "%.2f" (floating point number with 2 decimals).

The number format can be changed:

  1. Select a dimension in the tree
  2. Go to the properties tab Data and there to the section Format
  3. The property DataFormat Spec contains the format specifier, so change it to what you need

The syntax of the format specifier is explained here. You can also use this online tool to see what formatting specifier will format numbers.

Examples:

  • You use 2 decimals but for an angle you want to have only one decimal use the format specifier %.1f.
  • You use 2 decimals but don't like that trailing zeroes are printed (you prefer 4 instead of 4.00). Then use this format specifier: %g. The 'g' will use the shortest possible output and thus it omits trailing zeroes. Moreover it will automatically switch to scientific notation if necessary.

With the property DataInverted you can make length dimensions negative and flip angles from the range 0 - 180° to the the reflex range 180° - 360°.

Setting the property DataTheoretical Exact to true will mark the dimension as theoretically exact by adding a frame around it.

You can use your own text instead of a formatted number by setting the property DataArbitrary to true. Then the content of the property DataFormat Spec will be printed as dimension.

With the properties DataX and DataY you can change the horizontal and vertical position of the dimension text relative to the view. Alternatively you can change the position by dragging the dimension number or text.

Tolerances

Creation

  1. Create a dimension in your drawing
  2. Select the dimension in the tree
  3. Go to the properties tab Data and there to the section Format
  4. Add to the property DataOver Tolerance the amount by which the dimension can be exceeded.
  5. Add to the property DataUnder Tolerance the amount by which the dimension can be smaller.

Deletion

To get rid of a tolerance, change the the properties DataOver Tolerance and DataUnder Tolerance to each 0.00.

Formatting

The default format for new tolerances is by default the same format than the dimension (property DataFormat Spec). It can be changed by changing the properties DataFormat Spec Over Tolerance and DataFormat Spec Under Tolerance.

You can also use your own text instead of a formatted number by setting the property DataArbitrary Tolerances to true. Then the content of the properties DataFormat Spec Over Tolerance and DataFormat Spec Under Tolerance will be printed as tolerances.

Tolerances appear by default like this: . However, in many cases you have equal over- and undertolerance, thus it should better be displayed like "7,87 ±0,1 mm" since this is the standard. The only way to achieve this at the moment, is to delete the tolerances as described in the section above. Then set DataArbitrary to true an enter the the dimension as it should appear: "7,87 ±0,1 mm".

Tolerancing

Tolerances are created by using the Balloon feature of TechDraw. Since for most features a frame is required, it is helpful to change in the TechDraw preferences the Balloon Shape to Rectangle.

Tolerance indicator

The tolerance indicator of GD&T is a frame, also called "feature control frame". It can be created by using the Balloon feature of TechDraw:

  • after adding a Balloon and if its shape is not already a frame, set its property DataBubble Shape to Rectangle.
  • add the corresponding Unicode character for the feature you need to the Balloon property DataText. (You can copy them from the reference tales below.)
  • by adding the character '|' to the property DataText, you start a new indicator field.

There is no rule that defined how and if the tolerance indicator must have a pointer line or not, so you can either:

  • set the property ViewLine Visible to false.
  • set the property ViewEnd Type to Filled Arrow or Dot.

Datums

Datums in the GD&T sense of the meaning are surfaces your tolerance is relative to. They are created as Balloon:

  • after adding a Balloon, set its property DataEnd Type to Filled Triangle.
  • if the bubble shape is not already a frame, set DataBubble Shape to Rectangle.
  • drag the bubble with the mouse so that the triangle points away from the datum surface.
  • since it is common to have a straight line for datums unless the datum surface is not perpendicular to X or Y, assure that either the properties DataX and DataOrigin X or DataY and DataOrigin Y are equal to get a straight Balloon line.
Example of a datum in a drawing

Symbol reference

As reference, here are lists of characters to be used for geometric tolerancing:

Geometric tolerancing reference chart (according to table 2 in ISO 1101-2017)
Type of control Geometric characteristics Symbol Unicode character Notes
Form Straightness
U+23E4
Form Flatness
U+23E5
Form Roundness
U+25CB
In older norms also called circularity.
Form Cylindricity
U+232D
Form / Orientation / Location Line profile
U+2312
Datum necessary when used for orientation and location purpose
Form / Orientation / Location Surface profile
U+2313
Datum necessary when used for orientation and location purpose
Orientation Parallelism
U+2225
Orientation Perpendicularity
U+27C2
Orientation Angularity
U+2220
Location Position
U+2316
Location Concentricity / Coaxiality
U+25CE
Named concentricity when used for center points and coaxiality when used for median lines. In the norm ASME Y14.5 it was eliminated since the version from 2018.
Location Symmetry
U+232F
In the norm ASME Y14.5 from 2018, the symmetry was eliminated.
Run-out Circular run-out
U+2197
Run-out Total run-out
U+2330
Symbols used in the feature control frame to specify a feature's description, tolerance, modifier and datum references
Symbol Unicode character Modifier Notes
U+24BB
Free state Applies only when part is otherwise restrained
U+24C1
Least material condition (LMC) Useful to maintain minimum wall thickness
U+24C2
Maximum material condition (MMC) Provides bonus tolerance only for a feature of size
U+24C5
Projected tolerance zone Useful on threaded holes for long studs
U+24C8
Regardless of feature size (RFS) Not part of the 1994 version. See para. A5, bullet 3. Also para. D3. Also, Figure 3-8.
U+24C9
Tangent plane Useful for interfaces where form is not required
Continuous feature Identifies a group of features that should be treated geometrically as a single feature
Statistical tolerance Appears in the 1994 version of the standard, assumes appropriate statistical process control.
U+24CA
Unequal bilateral Added in the 2009 version of the standard, and refers to unequal profile distribution. Number after this symbol indicates tolerance in the "plus material" direction.