Myford dividing head

[NilochParticipant @niloch]

Those of you who are familiar with the Myford dividing head will know that each division plate is secured to the spindle/body of the head by three countersunk screws.  Having just made a custom division plate I need to know what the pitch circle diameter of the holes for these screws are  –  urgently please!! (In case it is of any consequence, which I doubt, the screws are .1365 in diameter.)

Thank you.

[NilochParticipant @niloch]

PCD of 3 csk. screws

[mgjParticipant @mgj]

Well if you know the od of the boss or fitting it fits on —-. stick a screw in a mounting hole, measure from the outside of the boss/platform to the edge of the screw and add 1/2 the screw diameter. Subtract that from radius and then  x2 and you have your PCD.

 

As a check, it will probably come out at some round number in decimal inches, or an accepted fraction. (One of the benefits of the imperial system. )

 

If it had 4 screws you could measure from outside screw to outside screw and subract screw diameter and thats PCD

 

Does this go in hints and tips?

Edited By meyrick griffith-jones on 22/11/2009 15:51:19

[John HaineParticipant @johnhaine32865]

I just measured the distance between the screw holes on one of my plates – slightly tricky to do, I used a vernier caliper between the “insides” then the “outsides” of the holes and averaged; also they are not all that accurately drilled.  As far as I can see they are on an equilateral triangle with side 20.64 mm centred on the plate axis.  If my sums are right the PCD must then be 23.83 mm.  or 0.938 inches.  However I also tried scribing a one-inch circle on a piece of ali and laying the plate over it with the holes positioned as closely as possible over the circle and it looked like it fitted pretty well!  From experience I know that Myford seem to like whole inch measurements where they can so I wouldn’t be surprised to find that 1″ is right.  When I get a mo I’ll try to measure it more accurately.

 

Sorry I can’t be more definite!

 

John.

[John HaineParticipant @johnhaine32865]

OK, tried again scribing a slightly smaller circle, this time 15/16 ” dia.  Looks slightly better, and 15/16 = 0.9375 which is very close to the other measurement, so with Myford’s propensity to use whole fractions (as Meyrick says) then I think we can assume this is the actual value!

 

John.

[mgjParticipant @mgj]

john –

 

His problem is that hes on a 3 hole PCD, so while certainly you can go about it empirically, on a 3 holer as it were, you have to work from a radius, and then you can measure it directly

 

I assume from the question that Niloch has the dividing head to check fits, but doesn’t have drawings.

 

The only trick is to make sure the edge of the screw/bolt you measure up to is firm in the hole. Or you can take a max and min and then average.

 

You can equally work off an ID and add the distance.

 

But yes, the equilateral trianglulation method is another good way, but you start getting involved in chords of a circle, which is less user friendly.

 

Cats & skins.

Edited By meyrick griffith-jones on 22/11/2009 17:11:33

[NilochParticipant @niloch]

Meyrick & John,

Thank you for your erudite and helpful responses.  What do you think of the following suggestion?

(1) Placing a Myford division plate (I have plates 1 & 2, plate 4 on order from Ebay) on top of my home made plate; the hole in the middle of both is identical;

(2) use a piece of snug fitting silver steel to locate/align the two accurately;

(3) clamp together with toolmakers’ or similar;

(4) use Myford plate as a template by drilling thru’ existing holes into my home-made plate.

It  sounds too easy a solution to me, can you foresee any problems?

Thanks again.

 

Edited By Niloch on 22/11/2009 17:46:44

Just correcting typing errors!

Edited By Niloch on 22/11/2009 17:55:01

[mgjParticipant @mgj]

Personally I wouldn’t do it that way, because its awkward to hold, and you will need to make an accurately sized spotting punch to ensure you pick up centre.

I hate all this spotting through stuff, because while it seems so simple, unless I am very careful, I always manage to add errors. The only time I spot though is when I have to transfer previously marked out holes in undrilled metal. Then I use a  centre drill followed by a 1/16 pilot through both, (and then only on assemblies which are not coming apart again . Bear in mind, you wil be drilling a clearance hole of what +.004″, so your maximun cumulative radial error can only be .002″, and you have a central spigot and 3 holes to get in concert.

 

The way I did do it on the plates for my GHT head, was to offset the right amount, and then drill with a centre drill and then a clearance drill, off the 12 hole direct indexing circle on the pinion. If i didn’t have a direct indexing circle, then I’d line up properly and start cranking.

 

Others may well be better at spotting through than I, and that being so, good luck to them. However, if you do it properly you can be certain that the three locking countersunk screws will keep all  central, and you won’t get the plunger pin jamming on  one side of the circle .

 

So that would be my suggestion. 

 

 

[John Stevenson 1Participant @johnstevenson1]

23.83mm This is from a genuine Myford drawing.

 

John S.

[CirclipParticipant @circlip]

This should probably also go into the hints and tips section, but just to give a heads up to the unknowing, the sizes worked out by John H., and qualified by Sir John are PROBABLY not from a designer having a bad hair day but rather a “You’ve got to come back to us for spares” ploy.

 

  “Standard” diameters (Company Std., that was) I had to work to many years ago was to ensure that a competitors product wasn’t able to be fitted as a “Temporary” measure.

 

   Textile industry always was cut throat.

 

  Regards  Ian

[chris stephensParticipant @chrisstephens63393]

Hi John, 

Could that have been because the draughtsman’s battery was going flat in his calculator when he was told to do it in Metric, when he just wanted to draw it as 15/16″.

chriStephens

[Author]

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