Metric drawing for inch gear?

[Philrock]

I am reverse engineering a double helical pinion. 6 transverse diametral pitch, 1.9375" diameter shaft extensions, .5625" wide keyways, - clearly a part designed in the inch system. My client has asked me to make the drawing metric, to facilitate having the part made overseas. At first I agreed to this, but now I am having second thoughts. Without question, the pitch must be stated in the inch system. So then, having stated the pitch in the inch system, it would seem very awkward to give the pitch and base diameters, for example, in millimeters. As for the rest of the part, it seems like making the drawing metric would be comparable to drawing a 5/8-18 hex head bolt in millimeters; at best it would raise eyebrows, and at worst, it would cause confusion and mistakes.

I would appreciate hearing any comments you may have on this.

 

[Occupant]

If you don't want (or can't) to convert 6DP to let's say M=5mm including every thing else, then you realy don't have much choice. Even for the keyway - if it is broached - you would have a hard time finding a fractional millimeter tool.

 

[Philrock]

The reverse engineered gear has to be a drop-in replacement for the existing gear, so an approximate pitch match, for example, is not an option.

 

[gearcutter]

It is not unusual at all to show gear pitch in DP on a drawing which has been dimensioned in metric.
I would show all other gear geometry related dimensions in metric.
There is no problem with showing a metric base diameter, however, the dimension is quite useless, as is showing a PCD, when it comes to manufacturing the part.
It’s also not unusual to show a fractional keyway size in a bore dimensioned in metric.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[tbuelna]

Philrock,

In the end, it's just numbers. It is not uncommon to see drawings with dual dimensioning (metric and inch). With the gear data, you just need to do the conversions (carefully). In fact, most of the current AGMA documents provide data in both metric and inch systems.

 

[Philrock]

All,

Thanks very much for your help. I used mixed units, as you suggested. The drawings are done and the client is very happy with them.

philrock