Computer Aided Design(CAD)Week 9 :: Involute Gears and Dimensioning

Gears - IntroductionSingle GearGear TrainDriver and Driven Gear Wheels

Types of GearsSpur GearRack & PinionHelical GearWorm Gear and WheelNote: Not an exhaustive list.

Some Gear Terminology

Gear Data and EquationsPressure Angle: Normally 20

Pitch Circle Diameter (PCD): Module x No. TeethA circle representing the two gears as tangent cylinders.

Addendum = ModuleThe part of the tooth above the Pitch Circle.

Clearance = 0.25 x Module (1/4 of Module)The gap between the tip and the root of two meshing teeth.

Dedendum = Module + ClearanceThe part of the tooth below the Pitch Circle.

Gear Data and EquationsCircular Pitch = Module x 3.14 (Pi)The distance from a point on one tooth to a similar on the next.

Tooth Thickness = Circular Pitch / 2

Fillet Radius (approx) = Circular Pitch / 7

Spur Gear Calculation & DrawingThe method used here to draw a typical spur gear is called Unwins Construction. It is also called the Approximate Method because it is only produces an Approximate Involute Curve.Tip: When drawing a Gear in AutoCAD make full use of Layers and Colours.

Spur Gear Calculation & Drawing #1

Spur Gear Calculation & Drawing #2a circle

Spur Gear Calculation & DrawingNow its your Turn ->Draw a Spur gear with the following details:Module: 2mmNumber of Teeth: 25

Dimensioning to BS308 StandardsEach dimension required should only appear once. There should be no more dimensions than necessary.Linear Dimensions are normally in millimeters (mm). The symbol may be ommitted provided the drawing carries a statement of the unit used.Angular Dimensions should be expressed in degrees and minutes, e.g.:20 30

Dimensioning to BS308 StandardsProjection lines and dimension lines should normally be placed outside the outline of the view.Crossing of projection and dimension lines should be avoided.Projection lines should start just clear of the outline of the feature and should extend a little beyond the dimension line.Projection lines should normally be drawn perpendicular to the dimension required.

Dimensioning Overview

Dimensioning OverviewDimensions should be placed near the middle, and above the dimension line.Larger Dimensions should be placed outside smaller dimensions.

Dimensioning MethodsParallel Dimensioning:Consists of a number of dimension lines originating from a datum feature.Chain Dimensioning:Should only be used where the accumulation of tolerances does not endanger the function of the part.

Diameter DimensioningDiameter dimensions of a circle or cylinder is shown by the symbol in front of the No.

Radius DimensioningRadii should be dimensioned by a dimension line that passes through, or is in line with, the center of the arc.The dimension line should have one arrowhead only, that which touches the arc.The symbol R is placed in front of the No.

Dimensioning FeaturesDimensioning of equally-spaced features may be simplified by using methids similar to below:

Dimensioning Holes etc.Notes:- The term spotface implies that the depth is the minimum necessary to provide a machined surface.- Chamfers at 45 should be dimensioned as below and not described by a note.

Dimensioning Holes etc.

Dimensioning Tapered Features

Dimensioning Screw ThreadsThe letter M, standing for ISO metric thread, is followed by, the nominal diameter and the pitch, both in millimeters. E.g. M8 x 1.The absence of a pitch (e.g. M8), means that a course pitch is specified. (e.g. The course pitch for an M8 thread is 1.25mm. (See Page 7 in AutoCAD Notes part II)Thread tolerance 6H is suitable for INTERNAL thread tolerance. 6g is suitable for EXTERNAL thread tolerance. E.g. M10 x 1 6g

Dimensioning External Threads

Dimensioning Internal ThreadsNote the hatch lines and line weights.

Dimensioning Threads

Typical Dimensioned Drawing