How to get 89 divisions from a dividing head

[Adam HarrisParticipant @adamharris13683]

In order to get 89 divisions from a dividing head (Elliott Universal), must I find a plate with 89 holes?

Edited By Adam Harris on 18/03/2015 22:03:10

[Adam HarrisParticipant @adamharris13683]

[Capstan SpeakingParticipant @capstanspeaking95294]

No. An indexer works directly but a proper dividing head usually has a 40 to 1 reduction. One handle turn is 1/40th of a turn of the chuck or 9 degrees.

40/89 = 0.45 turns.

Find a plate that can provide 0.45 of a turn. Perhaps 45 holes on a 100 hole circle?

The brass spreaders help keep count.

[Anonymous]

For simple indexing the answer is yes, as 89 is prime. It is not difficult to make division plates; here is one I made for my Eliott with 63 and 69 holes:

However, since you have a universal dividing head you can use compound, or differential indexing, to get 89 divisions using a standard division plate and the gears.

Andrew

[Adam HarrisParticipant @adamharris13683]

Thanks Capstan but finding a small enough plate with 100 holes is worse than finding a plate with 89 holes isn't it??

[Adam HarrisParticipant @adamharris13683]

So Andrew what would you recommend I do?

[John Stevenson 1Participant @johnstevenson1]

Posted by Capstan Speaking on 18/03/2015 22:18:53:

No. An indexer works directly but a proper dividing head usually has a 40 to 1 reduction. One handle turn is 1/40th of a turn of the chuck or 9 degrees.

40/89 = 0.45 turns.

Find a plate that can provide 0.45 of a turn. Perhaps 45 holes on a 100 hole circle?

The brass spreaders help keep count.

That will give you 90 divisions.

[John Stevenson 1Participant @johnstevenson1]

Adam,

Look in the thread a few below this called A New Dividing Plate for my Dividing Head

 

In that thread I did a quick and dirty plastic one, would this do for you to either make your one off ?? or copy onto a steel plate ?

If so PM me and tell me the size plate you can get in max and centre hole and I'll pop one out on the laser for you.

 

 

Edited By John Stevenson on 18/03/2015 23:44:24

[Adam HarrisParticipant @adamharris13683]

I have only one dividing plate for my Elliott Universal Dividing Head – 6 3/8" OD diameter, 1 1/4" bore, with holes 49, 43, 39, 29, 21, 19, 17, 16

[Adam HarrisParticipant @adamharris13683]

Thankyou for your very kind offer John. If I cannot use my existing plate (described above) what number of holes would be useful in a new disc, that could also cover 56 divisions?

[Capstan SpeakingParticipant @capstanspeaking95294]

Posted by John Stevenson on 18/03/2015 23:35:16:

Posted by Capstan Speaking on 18/03/2015 22:18:53:

No. An indexer works directly but a proper dividing head usually has a 40 to 1 reduction. One handle turn is 1/40th of a turn of the chuck or 9 degrees.

40/89 = 0.45 turns.

Find a plate that can provide 0.45 of a turn. Perhaps 45 holes on a 100 hole circle?

The brass spreaders help keep count.

That will give you 90 divisions.

No.

40/90 = 0.44 which is 44 holes on a 100 circle or 22 holes on a 50 circle.

It rarely works out to the last decimal place but it is usually close enough. If not a special plate can be drilled by co-ordinates.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Adam Harris on 19/03/2015 00:17:18:

I have only one dividing plate for my Elliott Universal Dividing Head – 6 3/8" OD diameter, 1 1/4" bore, with holes 49, 43, 39, 29, 21, 19, 17, 16

.

Adam,

Regarding your follow-up question: "If I cannot use my existing plate (described above) what number of holes would be useful in a new disc, that could also cover 56 divisions?"

[assuming that the head is geared 40:1 ratio] …

You can already get 56 divisions, by stepping 15 on the 21 holes on your plate.

40 x 21 = 840
840 ÷ 56 = 15

MichaelG.

 

Edited By Michael Gilligan on 19/03/2015 07:53:09

[Michael GilliganParticipant @michaelgilligan61133]

Adam,

Just for the sake of completeness …

Returning to your original question [and noting that Andrew has already given the correct answer]

40 x 89 = 3560
3560 ÷ 89 = 40

So you step 40 on the 89 hole perspex plate which that nice Mr Stevenson offered.

MichaelG.

[Anonymous]

Posted by Adam Harris on 18/03/2015 23:26:47:

So Andrew what would you recommend I do?

Personally I'd make a custom dividing plate, easy peasy on a mill with a decent DRO. Or take up JohnS on his offer.

Andrew

Edited By Andrew Johnston on 19/03/2015 09:18:42

[Involute CurveParticipant @involutecurve]

If anyone is interested I have a software program I came across years ago, its written in basic and dos based! but will run in XP etc, you input all your divisional plates and the gear ratio of your dividing head, after which you can input the divisions required the software outputs the resulting plate and hole movements of the detent.

pm me if you want it.

[Adam HarrisParticipant @adamharris13683]

Michael thank you for the 56 division method

[John Stevenson 1Participant @johnstevenson1]

Posted by Capstan Speaking on 19/03/2015 06:59:40:

Posted by John Stevenson on 18/03/2015 23:35:16:

Posted by Capstan Speaking on 18/03/2015 22:18:53:

No. An indexer works directly but a proper dividing head usually has a 40 to 1 reduction. One handle turn is 1/40th of a turn of the chuck or 9 degrees.

40/89 = 0.45 turns.

Find a plate that can provide 0.45 of a turn. Perhaps 45 holes on a 100 hole circle?

The brass spreaders help keep count.

That will give you 90 divisions.

No.

40/90 = 0.44 which is 44 holes on a 100 circle or 22 holes on a 50 circle.

It rarely works out to the last decimal place but it is usually close enough. If not a special plate can be drilled by co-ordinates.

.

Still no.

40/90 is 0.444444444 and not 0.44 and that difference is reoccurring every hole.

Dividing is a precise science and not an approximate and is possibly the last domain of the exact fraction as opposed to the close rounded up decimal.

End of the day 89 is a prime just like 41 and 43 but the tables don't tell us to fudge those, they tell us to use prime circles.

The difference between 0.44 and 0.44444444444 over 90 holes is 0.39999999 or point 4 of a hole or gear tooth which is not acceptable

[Anonymous]

Posted by John Stevenson on 19/03/2015 09:55:53:

Dividing is a precise science and not an approximate…………..

Precisely; integer arithmetic rules! – Andrew

[BazyleParticipant @bazyle]

To be self sufficient despite JS kind offer you can make an initial 89 plate using traditional Modellers techniques of a disc with something with regular holes wrapped round it. In this case a good option would be a length of Meccano chan with 89 links around a suitable size of wooden disc.

You don't have to make the outer diameter of a plate as big as 6 3/8 (are you sure it isn't 5 3/8) as only K& T clamp the rim (awkward design). It is better to make the plate smaller so it doesn't extend below the base of the head mut that problem might just be the smaller Elliott head.

[Adam HarrisParticipant @adamharris13683]

Thanks Bazyle. The dividing plate is indeed 6 3/8" OD and you are correct that it does protrude below the base of the head which is a big nuisance and means the head has to be positioned on the milling table in such a way that the plate overhangs the edge (or sits in a T slot if appropriately positioned and deep enough!) Maybe the previous owner used the head on top of a raising block? Very odd, especially so since the head is so beautifully made and the plate with matching arms looks original. The head is an Elliott universal with 5" centre height

Edited By Adam Harris on 19/03/2015 13:17:41

Edited By Adam Harris on 19/03/2015 13:19:28

Edited By Adam Harris on 19/03/2015 13:21:38

[Neil WyattModerator @neilwyatt]

Bear in mind any error will be divided by 40.

Say you have a 60 hole plate. That's 40×60 holes total or 2400.

That means 2400/89 or 26.9663 division plate holes per final hole.

27×89 is 2403, so if you do 26 holes you will be3 holes over at the end.

So, lose those 3 extra holes by doing the first step as 26 holes, followed by 29 more steps at 27 holes each and so on.

The three 'lost' holes will be evenly distributed and the maximum error will be 1/2400 of a revolution or 0.15 of a degree – 4 thou on a 4" diameter circle.

If that isn't good enough, use this process to make a division plate with 89 holes and then use that division plate to make the final item, the maximum error will be 0.15/40 or 0.0038 of a degree, which I bet is way beyond the accuracy of the process you are carrying out – just a tenth of a thou.

Neil

 

 

 

Edited By Neil Wyatt on 19/03/2015 13:54:00

[Anonymous]

Adam: Sounds like you have the same dividing head as me; a 5" Elliott universal. My division plates are 6.375" diameter, and are lower than the base of the dividing head. You must have a pretty big mill. The table on my horizontal mill is 10" wide and with the dividing head in the central T-slot there is a couple of inches clearance between the table edge and division plate. If you make a division plate that is much smaller you may have difficulty in spacing 89 holes of an appropriate diameter around the edge of the plate. For reference the three countersunk screws that hold my division plates to the dividing head are 60º included, not the more usual 90º.

Andrew

[Keith LongParticipant @keithlong89920]

Hi Involute Curve.

PM sent.

TIA

Keith

[Adam HarrisParticipant @adamharris13683]

Neil you have completely lost me on this one! I am scurrying over to John Stevenson and his kind offer I am afraid!!

[Adam HarrisParticipant @adamharris13683]

Andrew i have not yet used the dividing head and that is part of the reason I am excited by the prospect of cutting an 89 tooth gear with it! I have a Tom Senior M1 and will be setting it up soon on the table , I hope successfully. It is rather large and extremely heavy to lift, but I could not resist as it looks so well made and I thought to myself "get this and you will never need another"! I also have a baby Geo Thomas designed one for the Myford but I regret I have never used that either and am planning to sell that when the Elliott is proudly installed onto the M1. All I can say is it better darn well fit.

Edited By Adam Harris on 19/03/2015 19:14:28

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