In manufacturing, the reality is far from that abstract ideal
- [Instructor] In manufacturing, reality is far from the abstract ideal. When parts are fabricated, the physical fabrication processes introduce imperfections into the desired geometry. The degree of imperfection can be controlled up to a certain point depending on the machine or process used to create the part. A machinist in addition to figuring out how to create a particular part also needs to understand how much imperfection can be allowed before the part will be rejected. The degree of allowable imperfection will effect how the part is made, including machine selection, machine set up, machine alignment, clamping mechanism, tool bit choice, tool path generation, machine processes, and inspection processes.
These factors represent many of the trade offs involved in the manufacturing process, which effect speed, precision, and cost. The real world limitations that affect the ability to manufacture increasingly precise geometry fall into a set of four categories. These categories are imperfections with the shape of an object feature, imperfections with the size of an object feature, imperfections with the angle of an object feature, or imperfections with the location of an object feature. In order to make precise shapes, we must have measuring tools and processes which have a greater precision than the desired specification of the part we're making.
If a very flat surface is desired, the part might require a specific process to achieve that flatness where all of the points on the surface lie in the same plane with a high degree of precision. For example, a mirror in a laser optics system may need to undergo a surface grinding and polishing process to achieve the desired flatness. The greater precision desired, the more time it will take to achieve that precision and a greater cost. If the surface does not require a high degree of flatness, for example, a bathroom mirror, it may be sufficient to use the mirror directly as cast.
In order to create an object of a certain size, we need a way to measure and determine the distances between two points on that object. A variety of tools with varying degree of precision are available to take those distance or length measurements. For large distances or long objects, it may be sufficient to use a tape measurer marked in centimeters to check a length. At the other end of the size scale, weights or measurement devices may be required to check lengths with sub millimeter precision. Regardless of the method and tool used, measuring lengths in the real world introduces error.
For example if a ruler is maker with millimeter indications, measurements made with that rule can be estimated maybe within half of a millimeter or so. Thus, the precision of any length mentioned on a manufactured part is limited by the precision of the measuring device. Additionally, physical objects change size as they change temperature. So any deviating in temperature during fabrication and measurement can affect size. Although less expensive tools for precision measurements are becoming increasingly commonplace, those measurement tools may actually have better precision than the machines used to make the parts in the first place.
This would mean that parts would still be rejected for being outside the desired specifications. Angle dimensions are related to size measurements. While they are a reflection of a rotation around some axis or center point and they can be fabricated by rotating the plane of a part relative to the plane of the cutting tool, they're typically measured by evaluating the position of several points along the angled edge or surface. This means the precision of any angle measurement is related to the ability to precisely measure distances and lengths. Additionally, parts with features at multiple angles may be difficult to anchor precisely in a fixture.
This difficulty will introduce further errors in measurement. Locations of various features such as holes, edges, or surfaces are determined by length measurements. As we noted before, the precision of any length dimension of a manufactured part is limited by the precision of the measuring device. If locations of features are to be precise, the devices used to measure length must also be precise. Some of the geometric limitations are a consequence of the physical processes used to create parts whether they be additively manufactured, shaped, or machined.
The sharpness of tools can affect the flatness of a surface. The forces generated by some manufacturing processes can cause tools to deflect, wobble, or shatter. Environmental temperature changes or heat generated during manufacturing can cause parts or tools to expand, contract, or warp enough of the part geometries are affected. Manufacturing processes such as welding or cutting can affect a part at the molecular level, where strain energy within a part becomes nonuniform, causing the part to change shape after it's been machined or welded.
Sheet metal parts will spring back after being bent. All of these affects change of the shape and dimension of parts during or after manufacture.
- Where dimensioning goes wrong
- Basic GD&T rules and symbols
- GD&T from the engineering perspective
- Manufacturing design rules
- Interpreting GD&T in CAD models
- GD&T from the machinist perspective
- Understanding the tolerance limits of your shop
- Communicating your tolerance capabilities to potential customers