Author Topic: Mandrels, Gearcutter & Gears  (Read 23830 times)

Offline chucketn

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Re: Mandrels, Gearcutter & Gears
« Reply #45 on: August 05, 2012, 08:49:52 PM »
Again Arnold, I thank you for a great presentation. Well documented. As soon as I can get some silver steel big enough I intend to follow your methods.
I need a 55 tooth gear to repair a lift chair, some more change gears for my lathe, and am working on a gear reduction for a leadscrew drive.

Chuck

Offline EmanMyford

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Re: Mandrels, Gearcutter & Gears
« Reply #46 on: August 06, 2012, 06:35:31 AM »
Hi Arnold,

I echo Chuck, thanks for a great detailed thread.

Kind Regards.
Ewald

Offline jonesie

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Re: Mandrels, Gearcutter & Gears
« Reply #47 on: August 06, 2012, 02:30:29 PM »
thanks arnold ,nice post.  ihave some gear cutters but not all so will be making some like you did. thanks again jonesie

Offline arnoldb

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Re: Mandrels, Gearcutter & Gears
« Reply #48 on: August 07, 2012, 12:29:37 PM »
Chuck, Ewald & Jonesie, thanks gents  :cheers:

Kind regards, Arnold
Building an engine takes Patience, Planning, Preparation and Machining.
Procrastination is nearly the same, but it precludes machining.
Thus, an engine will only be built once the procrastination stops and the machining begins!

Online mklotz

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Re: Mandrels, Gearcutter & Gears
« Reply #49 on: August 07, 2012, 04:17:16 PM »
Your thread has the honor of being my first bookmarked thread in my MEM folder.  Well done.
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Regards, Marv


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Offline Alan Haisley

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Re: Mandrels, Gearcutter & Gears
« Reply #50 on: August 07, 2012, 05:19:33 PM »
Great thread, Arnold.


Seems like I always have "one last question" though:


Early on you mentioned that the tooth profile was faceted by this method. My question is do the number of facets, and hence the degree of imperfection, depend upon the number of passes and the depth of feed for each?


Thanks for a ton of good info in this thread.


Alan

Near Raleigh, NC, USA

Offline arnoldb

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Re: Mandrels, Gearcutter & Gears
« Reply #51 on: August 07, 2012, 07:16:22 PM »
Thanks Marv & Alan  :)

Alan and anybody else reading this - please do ask questions if you want to; it's for everyone's benefit.

The number of facets depends on the number of the teeth on the gear - not at all on the amount of infeed or number of cuts taken on the same tooth spacing.

With reference to the following photo.
I set things to cut the tooth gaps - and the third-from-bottom tooth on the cutter is bang on center line to cut the gap.  The teeth on the cutter immediately above and below the center line one cuts the first facet, and the ones further out from those cuts the second facet.  This is because the cutter actually looks like a rack gear to the gear blank when presented this way. 


With a low tooth count for the gear, one can get to the point where it will only cut the tooth root and one facet because of the tighter curvature on the gear blank.  I tried to draw up a quick C-o-C of what I mean with that:


One thing one could try to get more facets, and hence a closer-to-form tooth profile, would be to cut the gears like I had done on the "gap", and then offset the dividing head/rotary table by one half of the offset used for each tooth.  Then by raising or lowering the cutter by half of the tooth spacing as well and taking another pass through the gear, the amount of facets will be doubled.  Such a second pass could be done quite quickly as well; there will only be a minimum of material removed to form the extra facets.   :facepalm: Now, why didn't I think of that a bit earlier! - I'll have to test that out - unless someone beats me to it  :)

So thank you Alan for asking the question :NotWorthy: ; it's good to make one think a bit  :ThumbsUp:

Kind regards, Arnold
 
Building an engine takes Patience, Planning, Preparation and Machining.
Procrastination is nearly the same, but it precludes machining.
Thus, an engine will only be built once the procrastination stops and the machining begins!

Offline IanR

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Re: Mandrels, Gearcutter & Gears
« Reply #52 on: August 07, 2012, 08:52:58 PM »
I thought you'd have 7 facets per tooth flank, as that seems to be the number of cutter or rack teeth that will engage with a gear with an ordinary number of teeth. If I'm right, each facet would be about 16 thou wide on a 20 DP gear.
I've just reread the article by J A Radford, he made things more difficult for himself by cutting the hob from a dirty great lump of HSS, and did the indexing round half a tooth and along half the pitch, partly to get the necessary undercutting on a 25 tooth gear.
He said his 20DP gears were a little noisy until they'd had about an hour's running, so I'm not sure that his results were any better than yours. So I'll be trying it your way when I get round to making timing gears, instead of making 3 different cutters.

Offline JohnC

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Re: Mandrels, Gearcutter & Gears
« Reply #53 on: August 12, 2012, 07:45:16 PM »
Hi Arnold,
A bit late to the party, but you did say:
'If anybody's interested in a video of setting up the dividing head and operation while cutting, please shout up; I have to go through similar motions for the 21 tooth gears, so I'll be happy to oblige'.....


Well, I'd be very interested - if you haven't already done the job without the camera present! 

Thanks for a very informative post.
Rgds,
John
John
York, UK

Offline arnoldb

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Re: Mandrels, Gearcutter & Gears
« Reply #54 on: August 12, 2012, 08:44:40 PM »
Thanks Ian & John  :)

Ian, sorry; I thought I'd replied to your post  :-[
The number of facets depends on the amount of teeth on the gear; for smaller tooth-count gears, less teeth would be engaged with rack teeth. 
Well, technically this is not quite correct either; in the case of a straight-cut rack and pinion gearing setup, or with two straight-cut spur gears running together, there is only one tooth from each of the gears in contact at any point, with a momentary double-contact during operation as the gears move.  Strictly speaking, the spacing between the teeth of a gear must leave adequate clearance for the non-engaged teeth - otherwise the drive train would just lock up.
The width of the facets will depend on the DP/Module of the gear in question, and also on which part of the cutter actually cut it.  The widest facet is closest to the root of the tooth, and the narrowest facet closest to the tip of the tooth.

John, I've finished cutting the gears, but I will do some more work using the dividing head this coming weekend or the next.  I'll take a video then and post it up  :ThumbsUp: - When I first used it, it seemed a daunting thing to do, but is actually quite simple to use.

Kind regards, Arnold
Building an engine takes Patience, Planning, Preparation and Machining.
Procrastination is nearly the same, but it precludes machining.
Thus, an engine will only be built once the procrastination stops and the machining begins!