How to get 9° .38′ on a dividing head?

[mark costello 1Participant @markcostello1]

I have an 15-16-17-18-19-20,hole plate. Maybe others. I have to make a handle that has 9 divisions and need to offset the first cut. I could scribe a line but was wondering if it had to be precise how would it be done.

[mark costello 1Participant @markcostello1]

[BazParticipant @baz89810]

What ratio is your indexing device 40:1, 60:1, 90:1?

[Pete RimmerParticipant @peterimmer30576]

Set your DH for 110 divisions and index 3 divisions from that 110, you'll be 1 arc minute out.

 

EDIT: nomenclature

Edited By Pete Rimmer on 09/12/2022 20:47:56

[Peter Cook 6Participant @petercook6]

Assuming a 40:1 device, 9 degrees is one rev of the handle. 38' is 25.333 degrees on the handle. One hole on the 15 hole plate is 24 degrees. Using that would give an error of 2' i.e. .36'. If you need more precision I would mark a disk of paper with a 25.33 angle and use that to position the handle.

[DC31kParticipant @dc31k]

Posted by Pete Rimmer on 09/12/2022 20:42:01:

…you'll be 1 arc minute out.

Please could you clarify if a typo. has crept in there.

3*360/110 = 9.8…. Alternatively, in minutes, 3 x 360 x 60/110 = 589 and change.

9 degrees 38 minutes is 9.6…. In minutes, 578 exactly, so the difference is 11 minutes.

Thanks.

[JasonBModerator @jasonb]

Do you have a rotary table, if so just wind in the 9deg 38min and then reset handwheel dial to zero and do the 9 x 40deg with plates or off the handwheel.

Are you building a radial?

Alternative is to mark the 9'38 on the work with a sine plate etc and then line that up vertical or horizontal as needed when mounting in the div head and then just do the 9 equal divisions from that starting point. Could also clamp something to the work rather than mark out and then you can run a dti against it.

Edited By JasonB on 10/12/2022 08:56:40

[Kiwi BlokeParticipant @kiwibloke62605]

The question piqued my interest and resulted in a bit of simple spread-sheet bashing. No doubt someone like 'Silly Old Duffer' will come along and explain a proper, analytical way to do it, but my maths has withered over the years, and laziness makes spreadsheets an attractive alternative to deep thinking…

So I now have a general-purpose spreadsheet that can (sort-of) answer questions like this. It's something I meant to do, long ago. Thanks for providing the impetus. Perhaps you might find it interesting to do the same, and (re-)discover the underlying maths, etc., it's quite satisfying.

A 57 hole plate, on a 40:1 head gets you 37.895 minutes, which should be adequate…

Edited By Kiwi Bloke on 10/12/2022 09:36:10

[Pete RimmerParticipant @peterimmer30576]

Posted by DC31k on 10/12/2022 08:31:29:

Posted by Pete Rimmer on 09/12/2022 20:42:01:

…you'll be 1 arc minute out.

Please could you clarify if a typo. has crept in there.

3*360/110 = 9.8…. Alternatively, in minutes, 3 x 360 x 60/110 = 589 and change.

9 degrees 38 minutes is 9.6…. In minutes, 578 exactly, so the difference is 11 minutes.

Thanks.

Thanks for the correction, not a typo but a mental arithmatic mistake. If you use 112 divisions however the error is halved to 34 secs.

I like Peter Cook's solution though, one turn and one hole on a 15 hole plate .

[DC31kParticipant @dc31k]

In the original post, he specifies he has 15, 16, 17, 18, 19 and 20 plates available.

Peter Cook 6's suggestion of 16 holes around a 15 plate falls within that specification and is the best that is possible using the plates he has.

Kiwi Bloke's 61/57 does not fall within that specification and needs a 57 plate to make the 57 plate he suggests using.

Pete Rimmer's 15/14 also does not fall within that specifcation and needs a 14 plate, which cannot be made using the plates the OP has.

If we want to improve on Peter Cook 6's suggestion, and still remain true to only using what we have, please consider this: use 8 holes in the 17 plate to make an 85 hole plate. Use 91/85 to get very close.

Assuming we can solve the practical issue of how to fit 85 holes around its circumference, we can use the plates we already have to achieve a 17-fold decrease in error.

Maths follows:

The exact fraction of a turn on a 40:1 ratio to achieve 38 minutes is 19/270.

1/15 is 18/270, so error is 1 part in 270.

19/270 is 1615/22950 and 6/85 is 1620/22950, so the error is 5 in 22950 or 1 part in 4590.

[mark costello 1Participant @markcostello1]

I have to make 9 rotary splines(?) on a handle and it needs to start 1/2 of a tooth from a feature. Don't know the proper name. The teeth will be cut horizontally with an end mill tapering to the center. 40 to 1 ratio.

[Roger WebsterParticipant @rogerwebster39184]

I have a Soba HV-6 dividing head DP1

I need to cut a 68 tooth gear but this is unavailable with the supplied A B C plates

Is there an easy solution please

[Tony Pratt 1Participant @tonypratt1]

There are online calculators just for this purpose.

Tony

[JasonBModerator @jasonb]

The chart with my 6" Soba says 1 turn and 11 holes of a A34 plate

11/43 = 0.3235294

.3235294 x 4deg (one turn) = 1.2941176

4 +1.2941179 x 68 =360

[Nigel Graham 2Participant @nigelgraham2]

On an ordinary dividing-head with 40:1 gearing, or a rotary-table, 9 divisions is straightforwards but to ensure that step from the "feature" you may have to set the work-piece in the collet or chuck by measuring between it and the machine table or an angle-plate.

However, re-reading, if you mean the first spline is centred half a spline from the feature, centre the work on that feature, then rotate the dividing-head or RT head by the half-pitch (9 splines = 40º angular pitch, so by 20º ); set that new angle as 0º.

Since 9, 40 and 20 are simple factors of 360 this task is possible by using any even-numbered division-plate: rotate to the half-way hole for the initial offset. Then each step would be a single, full turn of the handle starting from there. (Set the dividing fingers to indicate it.)

Using a rotary table with a fixed scale, turn the table clockwise from well back, to (360 – 20) = 340º. Centre the work-piece datum on that. Now turn the table, still clockwise, to 0º. That is your starting-point and each step – always in the same direction – is a multiple of 40º.

I don't use my RT often but for a task like this I write the angles needed, and mark them on the scale with a pencil or fibre-tip pen (easily cleaned off afterwards).

Might be advisable to test the setting by very light witness-cuts before chomping out the spline grooves.

Edited By Nigel Graham 2 on 04/05/2023 12:31:18

[Peter Cook 6Participant @petercook6]

Nigel, Roger asked a new question.

As Jason says you need a 34 hole plate, but looking at the online manual for that dividing head, you don't have a plate with that number of holes.

Using the 17 hole ring on the A plate you could make a 34 hole plate, then use that to cut the gear. If it's for a one off and you have access to a 3D printer it is possible to print a plate with 34 holes which would probably be adequate.

[JasonBModerator @jasonb]

Ignore my comment above that is for the rotary table not a div' head like me.

[HopperParticipant @hopper]

Or to get 68 divisions on a 90:1 div head, you can use 1 turn plus 22 holes on a circle of 68 holes. If you have a 68 hole circle. If not, probably machining an aluminium disc with 34 holes would probably be your easiest solution as suggested already. Then you have the rest of the disc to fill in with any other oddball circles you may need in future.

[JasonBModerator @jasonb]

What holes are in the plates you have? 17 hole will work on a 40:1 dividing head

10/17 x 9 = 5.2941179 x 68 = 360

Can you also confirm what you have a sHV6 is a rotary table and DP1 a plate neither is a dividing head

Edited By JasonB on 04/05/2023 13:13:29

[Roger WebsterParticipant @rogerwebster39184]

Thank you all for the very swift advice.

I will convert the 17 hole A plate to 34 holes.

Simple and cost effective solution

[Ian PParticipant @ianp]

Posted by mark costello 1 on 10/12/2022 23:22:46:

I have to make 9 rotary splines(?) on a handle and it needs to start 1/2 of a tooth from a feature. Don't know the proper name. The teeth will be cut horizontally with an end mill tapering to the center. 40 to 1 ratio.

If Mark is making a shaft with 9 splines why are minutes and seconds involved?

360 divided by 9 is 40 ordinary degrees

Or have I missed something?

Ian P

[JasonBModerator @jasonb]

1 you missed it is an old post

2 you missed he wants to offset the splines by that angle from the handle.

[Ian PParticipant @ianp]

I also missed that the question was only for a theoretical reason.

Ian P

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