Scaling Gears

[Brian HParticipant @brianh50089]

Some years ago I built a 3" Fowell-Box traction engine. I've now decided to make the same engine again but in 1 1/2" scale as I have retained my original (hand drawn) drawings.

The original gears were 5/8" & 1/2" circular pitch. To arrive at 1 1/2" scale, do I work in 5/16" & 1/4" CP or is that too simple?

I'm nowhere near being ready to start modelling and will probably go with DP gears but I just wondered.

Brian

[Brian HParticipant @brianh50089]

How to scale down gears

[Pete RimmerParticipant @peterimmer30576]

If you re-make a 1/2" pitch gear and re-make it at 1/4" pitch keeping the same number of teeth then yes, it will be half the diameter.

[JasonBModerator @jasonb]

Simple as that,

For DP you would double the number so 5DP on your 3" would be 10DP on the 1.5"

For Mod you half the number so 5MOD on a 3" would become 2.5MOD on a 1.5

5/8" CP on the 3" becomes 5/16 CP on the 1.5"

[Brian HParticipant @brianh50089]

Many thanks Jason and Pete, I worried about this all Christmas and now it will be so simple.

Brian

[Anonymous]

Hmmmm, agreed that the DP and circular pitch scale in proportion. But the numbers seem rather odd. If a gear has an integer DP then by definition the circular pitch is irrational. Yet in the original post the circular pitches are rational numbers. So conversely the DP must be irrational. Of course there's nothing to say you can't have an irrational valuer for DP, but it would mean that no commercial cutters are available. So one would either have to make a cutter, or model the gear in CAD and use a CNC mill.

Andrew

[JasonBModerator @jasonb]

I was assuming Brian had made/used CP cutters the first time around and would do the same again only smaller, the DP example I gave was just to show that the number doubles if halving the scale and just used the closest whole number to the CP size.

[Anonymous]

That's why I'm asking the question, best to clarify rather than assume.

Andrew

[Brian HParticipant @brianh50089]

Jason is quite right in his assumption about using CP cutters on a smaller scale, in fact I have already found a 1/4" CP cutter on eBay but did wonder about using DP if CP cutters were difficult to obtain. The 1/4" CP cutter will be of use on a number of gears so that just leaves the 5/16" ones. If all else fails then I shall have to make the cutters or find DP or MOD cutters that are near enough.

This is not as horrifying as it sounds as the 3" engine used gears cast from MDF patterns cut with DP cutters but using CP spacings. The gears are very slow running and as long as they mesh I will be happy but I will probably cut the new gears into CI blanks.

Brian

[JasonBModerator @jasonb]

The DP sizes are quite close when you get down to small teeth sizes so you could probably fiddle things to keep PCD and teeth numbers the same, doubt it will make much difference on a slow running traction engine particularly if it has a sprung rear axle that will move in and out of mesh.

This table is handy to see what is close to your CP sizes

[Brian HParticipant @brianh50089]

Many thanks (again) Jason, that's very useful.

I came across this site, **LINK** which explains how ro make a cutter to cut different sizes of blank using a single cutter and using whatever pressure angle you choose. This is useful for me because I'm modelling early engines with varing methods of gear design.

Brian

[Pete RimmerParticipant @peterimmer30576]

If you want to make your own cutters for one-off gears I saved this document by the late John Stevenson and put it on my dropbox for another forum member who asked for it:

GEAR CUTTING

Edited By Pete Rimmer on 28/12/2018 09:17:49

[Anonymous]

Interesting; the only CP cutters I could find on Ebay had CP values corresponding to integer values of DP. So by definition the CP values were not rational. The first one I looked at had a CP value of 0.4488", corresponding to 7DP.

Offhand the only cutter I can think of that would have a rational value of CP would be for roller chain drive sprockets.

Andrew

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