Slot Dimensioning – Basics, ASME Y14.5M

David
Etukudo

Proper dimensioning of a slot helps to provide size control when designing a part or equipment. In this article, we will review the basics of slot dimensioning, recommendations from ASME Y14.5M, and slot callout on some CAD software.

Basics of Slot Dimensioning

Slots are narrow holes on machine parts that serve a variety of purposes. Moreover, to be effective in their deployment, proper dimensioning is key when making design drawings. Properly located dimensions enable fabricators to accurately locate the slots, as well as punch, mill, or burn holes to the required size. Although there are several ways of dimensioning slots, the common practice is according to its function and form as the options below show:

  • Full-Length Dimensioning: One way of describing a slot is by specifying its full length alongside its end radii. This is the preferable option when it is critical to know the full span occupied by a slot.
Full length dimensioning of a slot
Courtesy: HCCS
  • Dimensioning Length of Centers: Another way of defining a slot on a design is by specifying the length between its centers. Usually, either this method or dimensioning the full length of the slot is used, depending on which length is viewed as critical when fabricating.
Dimensioning length between centers
Courtesy: HCCS
  • Slot Width: Generally, the end radii of slots are identical to their width. However, some designs have their radii larger than the sloth width. As a result, it is necessary to specify the width alongside the full length and end radii.
Dimensioning slot when end radii is larger than width
Courtesy: HCCS

ASME Y14.5M

This standard defines geometrical tolerances for design parts with wide adoption of its recommendations by American companies. For slot dimensioning, ASME Y14.5M recommends three possible options.

  • The first method provides the width of the slot and the distance between the end radii center points.
ASME first method of slot dimensioning
Courtesy: SWL
  • The second method simply states the width and overall length of the slot. In addition, an arrow should point out the slot’s object line for proper positioning. Although the ASME Y14.5M initially reserved this method for punch operations only, recent versions of the code recommends its use for any simple slot.
Second Dimensioning Method
Courtesy: SWL
  • Finally, ASME Y14.5M recommends a third method that provides the width and overall length of the slot in linear dimensions. It refers to it as the ideal way of dimensioning a slot that has positional tolerances using the boundary method.
Third Method
Courtesy: SWL

Slot Dimensioning on CAD

Design software plays a critical role in most engineering designs. So, it is necessary to review how these platforms go about slot dimensioning. Initially, drawing slots on CAD platforms involved drawing one aspect of the feature to the other. But recent versions have included the slot or hole command to produce a variety of slots as the following sections highlight.

Slot Callout on SolidWorks

SolidWorks supports all three methods of dimensioning according to ASME Y14.5M. In addition, the software offers different types of slots including straight slot, centerpoint straight slot, three-point arc slot, and centerpoint arc slot. All of these are plotted by selecting the slot command and subsequently its center points. Sometimes, a designer wants to indicate the presence of these center points due to their importance in fabrication. This can be setup to show automatically as follows:

  • Select ‘options’ on the standard toolbar.
  • Then, select ‘detailing’ from the ‘document properties’ tab.
  • Finally, select ‘center marks – slots – part’ or ‘center marks – slots – assembly’, which is under ‘auto insert on view creation’, and click ‘OK’.

When using the slot callout, dimensions can be added automatically. Most designers prefer custom dimensioning using the ‘smart dimensions’ command so that only necessary measurements are visible.

AutoCAD Slot Callout

Like other design software, AutoCAD offers the option of a variety of slot types. It also provides several options to customize the slot dimensioning and improve its appearance. Some of these include using the same baseline for multiple dimensions and adjusting space between nested dimensions. Moreover, it is easy to dimension unbounded slots such as in the figure below by specifying only the radius or diameter.

Dimensioning only radius or diameter of an unbounded slot
Courtesy: PTC

Some standard practices to follow when dimensioning slots on AutoCAD, or any other CAD software are:

  • Dimension only parts that are easy to measure. Also, dimensioning should be only for functional parts. Else, the cost of fabrication will increase without necessarily improving the quality of the part.
  • Avoid dimensioning hidden lines. Rather, provide different views where the feature is visible and dimension there. For example, two views are provided below to show all useful dimensions of a part containing three blind arc slots.
Different views of part to enable proper dimensioning
Courtesy: AUTODESK