Gear Cutting

[David NobleParticipant @davidnoble71990]

I don't have any experience of gear cutting. I wondered that if I need to cut a gear with a number of teeth that the dividing head doesn't have an index plate for I can drill another and that if my drilled hole are out by a few 'thou' the error will be negligible due to the worms reduction.

Now, if this is correct, then if the said gear has a large number of teeth, will the 'negligible error' be compounded?

Hope this makes sense, many thanks, David

[David NobleParticipant @davidnoble71990]

Newby Question

[John HaineParticipant @johnhaine32865]

I think that's right, why not knock up a simple spreadsheet to check?

[David NobleParticipant @davidnoble71990]

Ahh! didn't think of that.

[Howard LewisParticipant @howardlewis46836]

Hope that it has been corrected, but I found that the chart with my HV6 contained errors and omissions,.

These only came to light when I started checking why things were going wrong, and made up my own spreadsheet to find out what the turns/holes should be.for any required number of divisions.

Had i done that first, would not have wasted a week wondering what I'd done wrong!

If your plates do not contain the number of holes that you require, some time ago an article was published on making accurate Division plates by a process of nteration, using old CDs, from memory, until the final accurate steel plate was drilled.

One day, may use the existing three plates to make one with the number of holes to fill in some of the blanks on the chart. Probably then, will be able to make even more plates.

"Greater fleas have lesser fleas upon their back to bite 'em and so on ad infinitum!" and so on.

Howard

[Neil WyattModerator @neilwyatt]

In short, yes. With a 60:1 reduction the 60 hole disk will give you a resolution of 1/10 of a degree.

If you use a spreadsheet to get the nearest place to index too, that means the maximum error per tooth will be 1/20 of a degree or 3 minutes.

For a 2" diameter gear that's an error of 0.0009" which is good enough for drive gears.

I have never had problems with gears I have made using my rotary table as a dividing head with 60:1 reduction and 60 divisions on the index on the handle.

Obviously if you need high precision gears you may want to get closer than this.

Neil

[Anonymous]

Posted by David Noble on 19/12/2018 14:12:38:

I don't have any experience of gear cutting. I wondered that if I need to cut a gear with a number of teeth that the dividing head doesn't have an index plate for I can drill another and that if my drilled hole are out by a few 'thou' the error will be negligible due to the worms reduction.

No need for a spreadsheet; the basic premise is correct. Although I could have used differential indexing I chose to make my own division plate when I needed 63 and 69 tooth gears for my traction engine. Here's the division plate:

The holes were done using the bolt circle function on the DRO, but that's a convenience, not a necessity. The division plate in place:

And gear cutting in progress:

It is correct that any errors in individual holes are reduced by the worm ratio of the dividing head. Overall the errors are not cumulative. Let's assume a worm ratio of 40:1. For one turn of the work the dividing plate handle needs to make 40 turns. So the holes on the division plate representing the number of teeth on the gear will be spread over a linear length equal to the circumference of the division plate times 40. But the gear teeth will only be spread over a length of the gear circumference, usually a much smaller distance. So any errors in the position of the holes are reduced.

The errors are not cumulative. After 40 turns the detent on the division plate handle will be back in the same hole that it started so, in theory, no error overall. Any errors in the other holes may wander about but will cancel out over the whole cycle.

Andrew

[Howard LewisParticipant @howardlewis46836]

With a 90:1 ratio, such as on the Vertex HV6, the resolution becomes 0.066666' degrees, which is probably good enough for most purposes. Even better with a bit of judicious interpolation!

Again, Neil, you have inspired me!

Howard

[David NobleParticipant @davidnoble71990]

Being new to this site, I am so impressed with the knowledge and helpfulness of you all. Thank you.

[Neil WyattModerator @neilwyatt]

Posted by Howard Lewis on 19/12/2018 15:11:51:

With a 90:1 ratio, such as on the Vertex HV6, the resolution becomes 0.066666' degrees, which is probably good enough for most purposes. Even better with a bit of judicious interpolation!

One of our weaknesses in this hobby is not to put the effort into calculating the scale of errors and then assessing their impact on the job at hand.

People tend to assume that any error has to be bad and strive for perfection. Introducing extra complexity to remove an error you can't even measure is pointless.

If industry worked like that, they would achieve very little!

Neil

[Brian HParticipant @brianh50089]

I seem to remember that someone on here had made a corrected chart. I'll dig out my copy (unless someone beats me to it).

Brian

[Brian HParticipant @brianh50089]

Here is a link to the piece on here about the corrected chart.

Brian

**LINK**

[Anonymous]

Posted by Neil Wyatt on 19/12/2018 15:49:27:

People tend to assume that any error has to be bad and strive for perfection. Introducing extra complexity to remove an error you can't even measure is pointless.

Ah well, I'll get me hat 'n' coat and leave.

At least I can do the maths if required.

Andrew

[John ReeseParticipant @johnreese12848]

I need to make a 127 tooth gear. The plates on my dividing head are not large enough to hold a 127 hole circle. I will have to use my rotary table. I have already made the spreadsheet for the 127 tooth spacing, including conversion to deg-min-sec.

I am considering making a decimal degree collar and vernier for the RT so I don't have to mess with the deg-min-sec conversion again.

[Alan Johnson 7Participant @alanjohnson7]

My current project is a 127 tooth gear for a Colchester. I am using a Hercus 127 tooth gear as the master, mounted on a common arbour mounted on the Dividing head, as it has a tailstock. Seemed the easiest solution, as 127 is a prime number and cannot be made without a universal (differential) dividing head – which I do have.

[HopperParticipant @hopper]

Another alternative might be to make yourself the "micro-adjustment" accessory from GH Thomas's Versatile Dividing Head. Using his basic design it could be adapted to fit any existing dividing head. In essence it consists of a small 60-tooth gear attached to the back of the indexing plate, driven by a suitable worm with a graduated handwheel (100 divisions).

So you turn the conventional handle the required number of turns, plus number of holes in the index plate, then use the micro-attachment to advance the indexing plate by a fraction of a hole, which takes the main handle with it and rotates the main spindle by that tiny extra amount.

Using a 60-hole circle on the index plate plus the micro attachment, you can make any division within one thousandth of a degree.

Details in GHT's book "Workshop Techniques".

Some pics below of the VDH micro-attachment in action making the full set of three index plates for itself, generated from scratch using the single 60-hole circle that was itself direct indexed from the main 60T gear on the main spindle.

Edited By Hopper on 21/12/2018 06:36:19

Edited By Hopper on 21/12/2018 06:41:48

[not done it yetParticipant @notdoneityet]

If one does not have a comprehensive DRO on the mill, just find someone else who has. 127 holes in a disc is not too arduous! Andrew’s 63 and 69 division plate has more holes than a 127.

[Micky TParticipant @mickyt]

I've just cut a 127 tooth gear for my Boxford in aluminium, I used the arduino / rotary table method discribed here https://www.model-engineer.co.uk/forums/postings.asp?th=122870

I also used the sketch from this threadhttps://www.homemodelenginemachinist.com/threads/arduino-rotary-table-for-dummies.26744/

Michael

 

Edited By Micky T on 21/12/2018 12:55:58

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