Hi gmeast,

There are many reasons to upgrade TurboCAD but the accuracy of the gear contour tool isnâ€™t one of them.

The gear contour tool profile in version 19 and earlier is more accurate than later versions. Drawing a gear of say 23 teeth, 2DP and 20Âº pressure angle and explode the result allows examination of the lines making up the flank profile. Ignore the line adjacent to the root fillet as this does not form part of the involute profile. The start of the next line is on the base circle. There can be no involute profile inside the base circle.

In version 19 there are 10 lines making up the involute profile. The one nearest the base circle is the shortest and each successive line is noticeably longer than the previous with the longest line of all at the tip of the tooth. This suggests that the calculation of points on the involute are calculated using increments of 1/10 of the angle subtended by a line from the gear centre to the origin of the involute and a line from the gear centre to the point of tangency with the base circle of a line to the tip of the teeth. This is the most accurate method because the curvature of the involute is greater near the base circle and this methods provides more points in that area. Of course, smaller increments and more points will give greater accuracy still.

Repeating the exercise with version 20, 21, 2015 (multiple explodes required) reveals there are 5 lines of approximately equal length making up the profile. This suggests that the calculation of points on the involute are calculated using increments of 1/5 of the difference in radius between the base diameter and tip diameter. Even ignoring the fact that there are fewer increments, this approach provides fewer points where the curvature is greatest, and therefore, the profile accuracy must be worse than earlier versions (see attachment).

If the gear contour tool were to used a spline through points approach, then both curves would have much greater accuracy.

Present in v19 and earlier, but removed from v20 onwards, is an option to use polylines with arcs to represent the involute. This uses a three point arc alternative to an involute curve. The three points used to define the radius are at the base circle, on or near the reference circle and tip circle. This is the least accurate of the three methods because it flattens the involute curvature near the base circle and exaggerates curvature near the tip.

The question we have to ask ourselves is, does it matter? Generally, it does not. The gear contour tool is not there to design gear teeth. It does not have that capacity. Rather it is there to provide illustrative spur gear contours which, if you donâ€™t use numbers of teeth in the undercut zone or care little about root fillets, it does adequately. Compliance with gear tolerance standards doesnâ€™t come into it.

See attachment

David