Bevel Gears - Parallel Depth Method (more)

Here is a spreadsheet I built to design bevel gears using the parallel depth method. I've tried a number of combinations and it seems to work just fine. I haven't cut any gears yet, once I get out of the armchair and into the shed, that's next.
If anyone has time to look through this and see if anything is broken that would be great.
http://www.btinternet.com/~steve.withnell/lathe/gears.xls
I've an article on parallel depth method recommending two cut's, but Ivan Law recommends three - are there any significant benefits of three cuts over two?
Regards
Steve

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Hi Steve I have always used 3 cuts as recommended for these reasons 1 the first cut 'up the middle' takes most of the metal away so that the other two cuts are loading the tool less 2 at the large end of the gear the root of the cut is cleaner if there are 3 cuts 3 When it comes to reposition the cuter and the blank I find it easier to find the gap at the small end if the first cut is radial. I am sure that the gear is fine if you only use two but I just find 3 easier. Peter

wrote:
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Hi Steve I have always used 3 cuts as recommended for these reasons 1 the first cut 'up the middle' takes most of the metal away so that the other two cuts are loading the tool less 2 at the large end of the gear the root of the cut is cleaner if there are 3 cuts 3 When it comes to reposition the cuter and the blank I find it easier to find the gap at the small end if the first cut is radial. I am sure that the gear is fine if you only use two but I just find 3 easier. Peter
Thanks Peter. I'm now in the process of setting up my first attempt at this, made an arbor today for the involute cutter and produced a gear blank. Hopefully I will get to play some more over the Easter weekend...

Hello Steve,
Took a look at your spreadsheet. I presume. Like me, I expect you had the method from Ivan Law's book "Gears and Gearcutting". In fact there is an error in Ivan's maths concerning the OD of the small end of the gear. He has :
OD small end = PD + 2 x Addendum x SIN pitch cone angle
This should be :
OD small end = PD + 2 x Addendum x COS pitch cone angle
Likewise the effective number of teeth on the back cone for selecting the No. of form cutter should be N/COS pitch cone angle not N/SIN pitch cone angle
It becomes obvious if you consider a gear of pitch cone angle = zero degrees which is of course a spur gear.
It makes no difference, for mitre gears because SIN 45 = COS 45
Otherwise the spreadsheet is fine.
Regards,