Did I do the wrong thing?

This topic has 6 replies, 4 voices, and was last updated 6 December 2013 at 11:38 by Ian S C.

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5 December 2013 at 22:44 #137125
Howard Lewis
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@howardlewis46836
I needed to make a replacement for a failed gear, from a Chinese wood bandsaw. When I put together the jigsaw of loose teeth , the gear appeared to be have 15teeth, but the OD was not correct for 1Mod, or for 1.25 Mod.

For starters, having only one 1.25 Mod cutter, I made a 1.25Mod, 15T gear. It was too big, so turned down the blank to the size for a 1Mod, and recut the teeth, (there was a witness of the bottom of the teeth.

Disaster! the teeth were were like a church steeple, cutting a couple; before starting again. This time I used a depth of cut which was the 1.25Mod depth of cut reduced by a factor of 1.25 to 1. Although cut with theoretically the wrong cutter, both in terms of Module and number of teeth, the resulting gear worked well.

Not being knowledgeable about gear cutting, especially where Module gears are concerned, was it correct to reduce the depth of cut by the 1.25:1 factor, to try to allow for the 1.25 cutter being used on a 1 Mod blank?

Any advice would be welcome; not that I wish to make a habit of doing things this way!

Howard
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5 December 2013 at 22:44 #22988
Howard Lewis
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@howardlewis46836
Gear cutting
5 December 2013 at 22:58 #137127
John Stevenson 1
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@johnstevenson1
You can't scale cutters.

Plus there are 8 cutters to a set and you need the one for 15 t gears. If you only have one cutter is it the correct one.

Edited By John Stevenson on 05/12/2013 23:00:23
5 December 2013 at 23:21 #137131
Howard Lewis
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@howardlewis46836
No it wasn't the correct cutter for 15T, (a No.7) and I knew that using a No.2 was theoretically wrong.

The main thing is that although the gear tooth form may well be wrong, if for that reason only, it does the job.

(Very little backlash between the rack and new pinion)

If you had seen some of the other parts that I had to remake, (supposedly central tappings at an angle to the axis on turned parts!) you would wonder how the thing ever worked at all. The gear exploded because it was a very loose fitting sinter on a shaft with a flat, (which acted as a cam and forced the gear apart. The shaft even had an un needed thread on it – ex a M8 bolt perhaps?). It was fun brushing the sawdust out of the pairs of teeth. By the time it reached me, the gear was in five pieces, from which the OD and number of teeth had to found.

I was doubtful that you could really compensate in this way; if it were, every man his dog would be doing it!

So, it was really the wrong thing to do; but where needs must and so on.

But it put son in law back in action again with his bandsaw, which was the object of the exercise.

Wasn't it De Bouton, ( or perhaps Leon Bollee) who said "It's rough, but it works" of the gearbox on his car?

Howard
5 December 2013 at 23:25 #137132
John Stevenson 1
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@johnstevenson1
For the odd seat of the pants job it often pays to free hand grind a fly cutter to fit a tooth space.

Surprising just how many special tools are hiding inside a broken centre drill or end mill shank
5 December 2013 at 23:52 #137134
Oompa Lumpa
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@oompalumpa34302
Posted by John Stevenson on 05/12/2013 23:25:50

Surprising just how many special tools are hiding inside a broken centre drill or end mill shank

This is Oh so true. I will remember this.

Thanks for that.
6 December 2013 at 11:38 #137146
Ian S C
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@iansc
John, you beat me to it, the only gears I'v cut have been with a fly cutter, and free hand ground piece of old centre drill.

Howard, a cutter like that should be ok for the bandsaw. Ian S C
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