2.7 Accuracy in design and manufacture

what is usually used to mark metals?

high carbon steel scriber which is harder than the material that it is marking. If the line is not very clear then ink called engineer’s blue is used to make it more visible.

If curves/arcs are needed, dividers are used which have two points and function the as a compass. Beam compasses are used for a larger radii.

To check dimensions, external and internal calipers are used.

Wood marking

timber is marked with a marking knife or pencil like carpenters pencils that are designed to be thicker to avoid breakages.

marking polymer

marker pen or scriber if something will be cut, not bent

what is used to measure when marking?

a steel rule, or distances over a meter is done with a measuring tape. Using the metric international system mm/cm/m

what is used to mark large or complicated pieces?

laser devices which allows for perfectly horizontal and vertical lines on an object

what are surface plates and gauges fitted with?

digital and dial readouts, angle plates and V blocks for accurate measuring and marking out. Necessary to establish datum/0 surfaces so you know where to measure from

what can be used to make other angels than right angles?

a simple protractor or a highly accurate vernier protractor

which tools can make precise measurements?

micrometers, vernier calipers, co-ordinate measuring machines, profile projector, digital test gauge.

digital test gauge

single axis measuring device used for very precise measuring. Has an easy to read display and potential for directly inputting to computers

impact of CNC machinery

CNC process do not require you to mark out anything since it is generated by the CAD/CAM software to co-ordinate the cutting path of the tool. You must select the right layout of materials to ensure as least waste as possible. You must also select an appropriate stock form of the material you are using

why is accuracy important?

to ensure that all parts can fit together which is very important since products have been miniaturised. The designer must decide a range of accuracy

Tolerance

the acceptable upper and lower limits of accuracy of a measurement. This is dependant of the material, size and function of the product.

how is tolerance expressed

X +- Y where X is the amount you would want and Y is how much error can be tolerated.

what happens when objects exceed the tolerance?

they are rejected as a part of quality control/assurance measure.

geometric dimensioning and tolerance

GD&T - a standardised set of symbols used on CAD models and engineers drawings that expresses the ideal distance and angles required in the product.

why is datum important?

in reducing cumulative errors - small errors that add up to be large errors

measuring aids

jigs, fixtures and templates, lasers

jigs

used to control the motion of a tool in relation to the workpiece. Most commo n are drilling jigs to ensure the hole is in the same place every time. Locked into the workpiece using a clamp. Removes the need for marking and does not require high skilled working

fixtures

robust frames with holding points and clamps used to hold the workpiece in place while machining, welding, etc. Very important that each piece is held in the exact same place every time.

templates

rigid shape or pattern made of a thin manufactured board or sheet. Used to aid marking process to ensure the the same design is marked every time