Accuracy in design and manufacture

• accuracy

  the amount of conformity of a measurement to the required value
• measurement

  the process of using tools such as rules, tapes and gauges to check existing dimensions
• marking out


  the transfer of designs onto pieces of material or parts to indicate where features such as edges, holes, slots and recesses and bends have to be made during manufacturing processes
• marking metal

  - use a high carbon steel scriber

  - it is harder than the metal it is marking so it scratches the surface

  - engineers blue can be applied to make markings more visible.

  - circles and arcs - dividers are used (similar to a compass)

  - beam compasses/ trammels - for larger radii

  - calipers - check dimensions
• marking timber

  - marked with marking knife or pencil

  - carpenters pencil - thicker lead - avoid breakages
• marking polymer

  - marker pens

  - scriber used if material is being cut rather than bent
• measurement for marking out

  - steel rule

  - measuring tape - measurements over a meter

  - metric international system of units (SI)

  used in every country
• laser devices


  - using for measuring and marking out when large and complicated fabrications are being undertaken

  - projects perfectly horizontal and vertical lines
• engineering applications

  - gauges fitted with digital or dial readouts.

  - angle plates and v blocks

  - establish datum surfaces
• marking gauge


  - used to mark out a line at a constant distance from an edge of a piece of timber.

  - commonly used to mark lines perpendicular (right angle) to an edge or a 45 degree (mitre) angle
• centre punch


  - when marking out holes for drilling

  - ensure drill centres itself correctly

  - prevent drill wandering
• high precision work

  - micrometers

  - vernier calipers

  - coordinating measuring machines

  - digital test gauge - signle axis measuring device
• impact of computer numerically controlled machinery

  - do not require marking out because g-codes are generated by CAD/ CAM software
• tolerance


  - the acceptable upper and lower limits of accuracy of a measurement
• the important of accuracy

  - important feature for modern product design to ensure that parts fit and products function correctly
• details of tolerances


  - expressed as X +_ Y

  - X = exact size required

  - Y - how much error can be tolerated
• testing and the elimination of errors

  consider the tolerance that has been specified in that particular situation
• using measuring aids

  - accuracy and speed can be improved

  - potential human errors eliminated

  - use jigs, fixtures and templates
• non-contact measuring devices

  - lasers

  - thickness sensors

  - laser micrometers

  - alignment systems
• Jigs


  - a device used to control the motion of a tool, such as a drill relative to a work piece

  - need for marking out is removed

  - does not need high levels of skill
• fixture


  - a robust frame with holding points and clamps to hold product components firmly in place for manufacturing processes such as welding

  - ensures consistency
• template


  - a rigid shape or pattern, used to transfer a design onto a work piece to ensure consistency

  - ensure consistency