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2" Durham and North Yorkshire Gears

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Jon Cameron18/09/2019 13:11:47
228 forum posts
87 photos

Hello,

I acquired a "started" Durham traction engine, and after checking it over I've found a number of things that need addressing. However I have been looking at the prices of materials/tooling I need to finish the model.

One thing I'd like to try is gear cutting and after 2years of looking at it and thinking it's rocket science the penny dropped. Looking at the gears, I need cutters that will suit the following sizes of 8DP gears, 11T, 14T, 18T, 43T, 50T, and 66T. When I've looked on RDG, the cutters have a teeth range, so I assume that I'll be needing pretty much each of the cutters in the set. (I'm a Yorkshireman and don't like spending money where I don't need to) am I better buying the full set and arbour, or is there certain ones I don't need?

With a veiw to the future is there any other traction engines that use the 8DP gears whereby the cutters won't be purchased for a single engine therefore pay for themselves in the long term.

If anyone in the northeast happens to have a set of cutters that are in good condition under the bench get in touch.

The gears for me are one of the biggest single expenses left on the traction engine, having the boiler, hornplates, smokebox, and wheels already. Also one of the biggest challenges that I feel I've have to conquer with it.

Thanks for any advice or pointers.

Edited By Jon Cameron on 18/09/2019 13:14:11

JasonB18/09/2019 13:20:49
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When I did my Fowler I found it was cheaper to buy some of the smaller gears and modify to suit than buy the material and cutters. That used a mix of 12.10 & 8 DP much like the full size used different size gears.

You only need 5 cutters not a whole set 2, 3, 6, 7 & 8

Jon Cameron18/09/2019 14:01:23
228 forum posts
87 photos

Hi Jason,

Thanks for clarifying that for me. I had a brief look this morning at different types of gear suppliers, and there was only 2 gears I think which I found. One was a 12T (for the 11T) which then changes the distance from centre to centre. I haven't yet found a supplier that could do all of them. I know they are availabke from blackgates as a machined set, but frankly id rather buy the tools to do it a second time for free. As said above I'd be interested to know if other traction engine designs carry the same 8DP to hopefully do that one day.

not done it yet19/09/2019 07:23:04
3357 forum posts
11 photos

I would think that 5 separate cutters is likely more expensive than buying a complete set (probably from China). Quality can vary, of course. One needs to take into account extra import costs, but these things often avoid taxes (especially if ordered at busy times of the year) for small postal packages.

Perhaps a visit to one of the second hand tool dealers at a model engineering exhibition might provide a good deal.

I’ve bought cheap gears of ‘just too large diameter’ for recutting teeth for a smaller gear. Altering, or even changing, the boss is easy enough - especially if they are not on view. Thinking more of change wheels for these.

Andrew Johnston19/09/2019 11:42:49
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4855 forum posts
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In theory the 11 tooth gear will need to be undercut, in which case it cannot be cut one tooth at a time with an involute cutter. The need for undercutting can be overcome by increasing the OD and/or pressure angle. But changing the OD will change the centre to centre distance, which has serious ramifications for the rest of the design. I'd increase the 11 tooth gear to 12 teeth, and reduce the tooth count on the mating gear by one. That way the centre to centre distance will stay the same, with a slight alteration in the ratio.

I think the RDG involute cutters are 14.5PA, which is prototypical, albeit rather out dated. I'd be wary of the RDG cutters. I bought a selection when I was mucking about experimenting with helical gears. They did the job, but eccentricity and wobble weren't always great.

Andrew

JasonB19/09/2019 11:51:34
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11T is availabe to buy which would be correctly cut for about the same price as a cutter. It would be 20pa so at least the mating gear would have to be the same pa or all. The rest of the sizes are all available but maybe not from the cheaper sources and even they are not that cheap for DP gears as China prefers to make metric ones at far more reasonable prices.

Jon Cameron19/09/2019 16:01:11
228 forum posts
87 photos

Ok Andrew, (new to this) what do you mean by undercut, not cut to full depth?

Not done it yet, it works out as £180ish from RDG, with a mandrel for the required cutters.

Perhaps start with the mandrel and one cutter for the smaller size, that way if it goes wrong then I can chuck the towel in and look at other options.

Jason, Pressure angle is that the angle at the top of the tooth, or the angle of the V of the tooth.

As I said all new territory, but now I've got my head around dividing perhaps it will be easier than I first thought to cut the gears.

Cheers Jon

JasonB19/09/2019 16:57:38
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A lower pressure angle gives a more parallel looking tooth that becomes more triangular as the angle increases

The lower 14.5pa is especially useful on models that have a sprung rear axle as the gap between the teeth (backlash) does not change as much as the two gears move in and out of mesh.

I'll wait to see if Andrew comes back to explain the undercut, if not I'll do so later. I think he has some photos of gears he has cut that illustrate it well.

Edited By JasonB on 19/09/2019 16:58:43

Jon Cameron19/09/2019 17:40:03
228 forum posts
87 photos

Ok so the pressure angle is the point of intersection between meshing teeth from the horizontal. (In basic terms). It's the contact point that transfers the force from one gear to another, hence "pressure" angle. Thanks for the diagram as that explains it better. Even if I'm rubbish at explaining what I see lol.

Andrew Johnston19/09/2019 18:56:34
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Although convenient the diagram above is slightly misleading as the pressure angle is not referenced to the horizontal. As correctly described it's the angle between the normal at the contact point and the tangent to the pitch circles at the intersection of the centre to centre line of the gears. The important point to note is that as the teeth roll past each other the pressure angle stays constant and hence the angular velocity is constant.

For a gear with a small number of teeth during the rolling action the tip of one tooth does not clear all the sides of the opposing space. So the bottom of the tooth profile has to be cut away to give clearance. Although a bevel gear these 10 tooth pinions illustrate undercutting:

Bevel Gear Pinions

Look at the top tooth on the left pinion. From the top of the tooth the curve each side comes down in a smooth manner until about half way down the tooth. At this point there is a sudden change of direction and the width of the tooth gets narrower rather than continuing to get wider. This is undercutting. If you think about it the undercut cannot be formed using a rotary cutter cutting one space at a time. Undercutting is not inherent to involute gears, but is a solution to interference problems with small numbers of teeth. The need for it is less with higher pressure angles. As mentioned there are also other ways of reducing the need for it by increasing the OD or tweaking other parameters. If a spur gear is hobbed with a standard hob you will get undercutting for small numbers of teeth, whether you want it or not.

Since the bevel pinions as designed are undercut they were cut on a 4-axis CNC mill to avoid the issues of using involute cutters:

After Final Cut

Andrew

Edited By Andrew Johnston on 19/09/2019 18:57:19

Jon Cameron20/09/2019 10:36:16
228 forum posts
87 photos

Thank you Andrew, I'll have to read through that again later to hopefully get it to sink in.

Thanks to all for their comments.

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