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Bevel Gear

Need help with gears

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Lawrie Alush-Jaggs11/07/2011 09:03:35
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118 forum posts
32 photos
Greetings O Wise Ones
 
I have it in mind to built an indexing turret and just have a question or two about the gearing.
The gear I am thinking of using is shown in the piccy.

 
I am after a six to one movement but have never constructed a gear.
My Questions:
 
Can I cut the large ring gear straight or should it be cut at fifteen degrees and the pinion the same?
Is it OK to leave the ring gear (hope I am using the right term) plane and cut the pinion at thirty degrees.?
The ring gear will be about 60mm dia. I have no idea how many teeth I should cut, what the pressure angle should be nor wether I should use DP or Module. I prefer to work in Metric, so I guess that ansers the DP/Metric question but the more experienced may wish to suggest otherwise. I don't have any cutters at this point so suggest away.
The space for the gears is not great but it will be created to suit whatever eventuates.
Should I just make up a fly cutter that approximates something that looks just about like an impression of a gear profile? Or should I head out and buy a gear cutter of the appropriate profile?
The steel will be 1020 unless someone thinks I should use brass or PB or whatever.
 
 
Thanks in advance
Lawrie
 
John Stevenson11/07/2011 09:11:50
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5068 forum posts
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Lawrie,
 
It's obvious that you are a rank beginner [ nothing wrong with that ] and my advise to you would be to guy the Workshop Practise book by Ivan Law on Gears and Gear Cutting, number 17 in the series.
 
This book has a very good chapter on cutting parallel depth bevels which are easier to cut in the home workshop. The book is also aimed at the beginner and leaves out pages of calculations on subjects you have no control over anyway.
 
It will not be money wasted.
 
John S.
blowlamp11/07/2011 09:52:27
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1885 forum posts
111 photos
If it's an indexing turret, why does it need to be geared?
 
Most skilled people I know would shy away from making a bevel gear, so it's manufacture should not to be undertaken in too casual a manner
 
Can you tell us a bit more about your device to give our collective brain something to ponder over?
 
Martin.
Lawrie Alush-Jaggs11/07/2011 10:58:46
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118 forum posts
32 photos
Hulo Evrybardy (Hello Dr. Nick)
 
Blowlamp: Thanks for your reply. The reason for the bevel gear is show in the illustration below. As the handle is wound back, it engages a pin (not shown) and a pawl (also not shown). As the pawl moves the turret through 60 deg, the bevel gear rotates the shaft which indexes the depth stop to the next position.
I have read the most recent turret article in MEW and have started making one.
Many many years ago I worked for eight days at a repetition engineering firm. Eight days was all I could stand. I made 23/1's. Hundreds of them. I came to dislike 23/1's but really admired the small turret lathe I made them on. The reason for over engineering something as simple as a tailstock turret into a full on turret is that I don't want to have to make two manual adjustments to tooling, one with one hand and one with two hands. It will just slow things up more than I like.
 
 

 
I appreciate your comment about not undertaking the job lightly which is why I have been looking at the sites of various companies who make and have stock gears. The difficulty in purchasing a ready made set is that as far as I can tell, a gear set that works on 30deg is not a standard item and becuase of set up costs, the price will be far higher than I can afford or wish to pay.
I had thought of hunting around one of several second hand to junk class resellers and seeing if they had the turret off an old Herbet or Ward and still might. But I do like a challenge and thought this might just provide one.
 
John: Thanks for your reply. I had thought of geting Ivan Law's book and have for some time, it is just one more thing to obtain.
 
 
Lawrie
 
 
 
Dunc11/07/2011 13:39:42
139 forum posts
 
Searching "bevel gear" on youtube gets a lot of hits
 
Perhaps not too practical in GB but Kozo Hiraoka's "Building the Climax" had a very detailed method of cutting bevel gears - including making the cutters themselves - in the home workshop. The book is on the current list of Village Press in the USA (https://secure.villagepress.com/store/items/detail/item/827).

Usual disclaimers
 
Steve Withnell11/07/2011 18:03:02
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858 forum posts
215 photos
 
 
Lawrie,
 
I built the formulae (parallel depth method) from Ivan Laws book into a spreadsheet. The sheet is pretty obvious, allows you to play with DP, number of teeth. It's not productionised ie the formula's are not locked. The only "input" cells are the values of N = number of teeth and DP, everything else auto calculates.
 
 
Even if you don't cut your own, it will allow you to play with the design before deciding on a spec to order.
 
Steve
Andrew Johnston11/07/2011 20:13:26
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7061 forum posts
719 photos
Posted by Lawrie Alush-Jaggs on 11/07/2011 09:03:35:
 
Greetings O Wise Ones
 
 
Ah well, that's me out the picture then.
 
I have a bed mounted six way capstan unit for my lathe. I'm pretty sure it doesn't use bevel gears to rotate the stops. I say this because the action that rotates the head and stops is not linear when the handle is wound back. All the movement occurs in the last bit of the travel. That notwithstanding here are some thoughts on bevel gears:
 
For bevel gears the angles at which the teeth are cut are dependent upon the angle of intersection of the axes and the relative numbers of teeth on each gear.
 
If the large gear is cut straight then technically the gear set is not bevel, but a crown wheel and pinion. This is a perfectly acceptable way of doing things and is much simpler, as the pinion is just a spur gear and the width of the crown gear teeth is such that any taper can be ignored. Old windmills often use these type of gears. Most sets I have seen have axes that intercept at 90°, but I see no reason why any other angle shouldn't work, with a bit of a fudge on the pinion.
 
If you feel like having bevel gears then the parallel depth method highlighted by Steve is fine. And I can vouch for the usefulness of Steve's spreadsheet. If the face widths of the teeth were thin enough you'd probably be able to get away with one pass round each gear rather than the normal three.
 
Regards,
 
Andrew
Lawrie Alush-Jaggs12/07/2011 01:38:47
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118 forum posts
32 photos
Hulo Evrybardy (Hello Dr. Nick)
 
Steve: thanks for your reply. I have downloaded your file and it looks like the ducks nuts. I'll give it a go.
 
Andrew: Thanks for yor reply too. The small amount of research I've done on turrets shows one system from the late 1900's which uses a bevel gear to transmit rotary motion from the turret (Capstan) to the stop index and uses two fixed pins, one as a pawl against a ratchet which imparts said rotary motion and the other to withdraw the locking pin.
There is also this animation on You Tube
 
 
Your answer in comination with Steves pretty much give me what I want I think
 
Thanks
Lawrie
 
Stub Mandrel13/07/2011 19:59:23
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4318 forum posts
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Cutting bevel gears is not difficult, because even I can do it!
 
I just followed Ivan Law's methods and they worked.
 
There is one error in his book the calculations on page 106 should use COSINE not SINE. His examples work as COS 45 degrees = SIN 45 degrees.
 
These are a very tiny pair of 2:1 bevel gears:
 


Simples!
 
Neil

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