gear for an indexing head.

I am well into making a Harold Hall indexing jig and would like to ask which gear and tooth number would be best for all round general work.

I have all the standard Myford gears for the ML10 plust the conversion gears to cut Imperial threads.

I am thinking perhaps a 60 or 90 tooth gear, but would be interested in a general concensus.

Clive

Hi Clive

Why not sent a pm to Harold - he is often on this site?

Norman

That is a difficult question Clive, but if you press me I would say 60 teeth. This will give you divisions of 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, and of course 60. This will cover milling squares, hexagons and small numbers of holes on a PCD as often used on steam engine cylinder ends.

Have you made it to the drawings in my milling book, in which case, if I may give my Dividing book a plug, the dividing book gives all the possible divisions using a single gear and also three.

Interestingly, with three gears, numbers such as 96 and 125 are possible, both of which are not possible with the semi universal dividing head with it's standard plates. Ninety six being common in clock making and 125 for a dial for a 8TPI lead screw. For the 96 divisions it was not even necessary to count teeth between each division as it worked out to be each next tooth space.

Hope this helps

Harold

90 has more factors, but those of 60 are probably more useful.

60: 1,2,3,4,5,6,10,12,15,20,30,60

If you can index half teeth 120:1,2,3,4,5,6,8,10,12,15,20,24,30,40,60,120

90: 1,2,3,5,6,9,10,15,18,30,45,90

With half teeth 180: 1,2,3,4,5,6,9,10,12,15,18,20,30,45,60,90,180

So 90 with a detent that allows you to index both a tooth and a space is probably the most useful as it does everything the 60 can do.

Neil

Hi Harold

You say " 125 for a dial for a 8TPI lead screw." I recall, many years ago, wanting to make a leadscrew handwheel dial for my (then) recently aquired Myford ML4. I posed the question to the lecturer at a college night school course (Ah the good old days!) that I was attending. He went away and scratched his head working out compound gears for the dividing heads there but the best he could come up with was 124 divisions so that's what I engraved. The error was too small to bother about ( for the stuff I was doing on the ML4 anyway!)

Regards

Norman

Thank you for the answers so far and to you Harold for responding so quickly. It looks like a 90 gear will do most of what I will want.

I will look in the dividing book and read that part about the gear sets for other divisions. Thank you all.

Clive

Thanks Neil for pointing out the use of a forked detent, my drawings do have that included but in the hurry to provide the above I forgot about it.

I thought Norman that I was about to have to explain my method of multi turns of the workpiece. However, 125 is quite simple, Using 50 as the dividing gear and then with 30 driving 75 giving a ratio of 2.5 : 1 we get 125 divisions.

Having mentioned multi turns of the workpiece perhaps I should not keep you in the dark, but I have never seen it mentioned anywhere else. Someone is bound now to tell me that it is a common method.

Typically, for cutting a gear of 200 teeth the gear being cut goes round three times. Using a 40 gear as the division gear linked to a 30 tooth gear driving a 50 tooth gear we get a ratio of 66.6666. multiply that by 3 and the result is 200. The method cuts each third gear space, eventually after three turns returning to where it started.

This may sound a method fraught with likely errors, Actually, it is very simple as it uses ever next gear space on the dividing gear resulting in there being no need to count the number of teeth passed at each division. Even if you get distracted and miss one it is easy to get back and fill in the missing gear tooth space.

A similar situation occurs with 360 divisions using a 30 tooth dividing gear linked to a 35 tooth gear driving a 60 tooth gear. In this case the workpiece turns 7 times. This also uses each next tooth space on the division gear.

Harold

Errrr, I make the number of factors for 60 and 90 the same?

Andrew

.

Andrew,

It's the list of factors that's important, rather than their quantity.

For example:

6, 12, and 20 are directly available from 60; but not from 90.

Note that 90 with a half-tooth-detent is effectively 180 ... as per Neil's earlier post.

MichaelG.

Michael, we must have gone to different schools. At mine, six times fifteen equalled ninety .

The factors for 60 and 90 are similar; there doesn't seem much point in using 90T rather than 60T unless you are keen to divide by 9, 18 or 45. And as you point out, without a half-tooth detent you would lose 6, 12 and 20.

I suppose that if the construction was such that the 60T could be substituted with gears near in size when required, further divisions you would probably never need would be possible. 63T would get you 7 and 9, 64T would allow 8 (that might be useful), 16 and 32, 65T would give you access to 13 and 66T would cater for 11, 22 and 33.

Andy

I obviously dropped a few fingers there! 60 and 90 do have the same number of factors. I also missed 36 as a facror of 180.

The main choice between 9 and 60 is the availability of 8 or 9, the latter being more likely to be needed if you want to cut small pinions.

Neil

Its a OT, but a number of years ago, about the time I bought my 6" Vertex rotary table, I saw in a second hand tool shop a rorary table made from an automotive flywheel, using the ring gear for indexing the table, sorry don't know the numbers for the gear, or which motor it came off. Ian S C

Oops

Ignore 6

... I did post that rather early for a Sunday morning

MichaelG

Michael,

It was the assertion that 60 and 90 had a different number of factors that I was querying, not whether the factors were the same or different. Looking at the prime factorisation of 60 and 90, they have the same number of prime factors, so by looking at the possible combinations it is logical that the number of combinations will be equal.

60 = 2x2x3x5

90 = 2x3x3x5

Personally I think that 60 would be more useful for general dividing, as it contains a factor of 4, for squares.

Regards,

Andrew

Andrew,

The important point is that we're interested in the list of all factors, not just the primes.

For example: 45 is a factor of 90, but not of 60

MichaelG.

.

But I think you knew that.

Edited By Michael Gilligan on 14/04/2013 11:37:49

I do have a question about the dividing head of Harold's. Will there be a need for dimension changes using Module 1 gears, ie 25.4 DP. I am about to embark on construction having finished the boring head and on to the next project of the milling book. I cannot see that it makes a lot of difference but not being an engineer I may be in ignorance of the salient facts!

BW

Andries

Hi Andries

The difference between the gears will be the different diameter for the same numbers of teeth, Your 1mod gear will be approximately 80% of the diameter of a 20dp gear with the same tooth count. Also the space between the teeth on the 1mod wheel will be smaller than that on a 20dp gear so you might need to thin the end of the engaging detent a bit to get a better fit,.

Harold has produced several designs for dividing heads so without knowing which one your building I can't be more specific, but the design in "Milling a Complete Course" has an adjustable index arm that carries the detent, to accommodate different gears with different numbers of teeth (Harold designed this for Myford 20dp gears), so you should have enough adjustment there to fit a good selection of 1mod gears in any case. Any changes you do find you need will be pretty minor and should be fairly clear. In the case that it is the design in "Milling" check how items 2 and 8 will fit together when you use the 1mod gears, you might need to either adjust the locations of the holes for the connecting bolt or elongate them slightly to give you the extra movement that you need. They are both small items so making a couple with differing bolt hole patterns isn't a big job anyway.

Keith

Edited By Keith Long on 14/04/2013 12:47:11

If you make the detent adjustable, then the issue of which gear or how many teeth largely becomes academic. The indexing device I use on my lathe can accept any changewheel, so it can divide many different numbers.

Neil