Cutting a non linear spiral thread

Hello Everyone,

I have puzzled on and off for years on how the guidance form was machined 70 years ago without the benefit of today's CNC methods

These components were made in ten's of thousands in the USA. It it superbly made and finished, the thread form has I think been ground after milling. The barrel is 1/2 inch diameter and the groove is 29 degree ACME form.

My question is how would you go aboutdoing this using mechanical methods alone to form the logarithmic shape, there must be amongst the forum members plenty of experience. I have my own ideas, but it would be interesting to see what the rest of you think. There should be 3 photos if I've pushed all the right buttons.

Brian

Edited By Wolfie on 17/02/2013 13:32:02

Edited By Wolfie on 17/02/2013 13:33:15

I don't know about then but now, not having a CNC setup, I would couple the mill table to a universal dividing head using a spiral pulley and a wire instead of gears. Od course that means a spiral has to be made thus creating a new problem.

Is it specifically a logarithmic relationship of just not linear?

Brian,

See if this makes sense ...

It is possible to vary the effective pitch of a leadscrew by running it at an angle to the slide.

[My Gaertner Toolmaker's Microscope has this adjustment, for calibration]

... Note that the Nut is allowed to float sideways.

However; instead of angling the screw, the nut can be made to follow a profile.

I see that you already have a Taper Turning attachment ... There are similarities.

MichaelG.

P.S. ... I presume that the thread would be milled [can you see any evidence?]

Edited By Wolfie on 17/02/2013 13:34:21

Yup US National Bureau of Standards developed an attachment for 9" SouthBend lathes under contract to the Navy Aeronautics Bureau. Drawings can be found at **LINK** along with a lot of other SouthBend related information. Presumably to expand production of such components to meet wartime needs by exploiting the large number of SouthBend and similar machines in schools and training establishments.

Specialist machines do it differently. One way is by chasing using a non linear master. Another is to fit a matching feed screw to the slide of a standard precision bench lathe driving in the usual manner through a shaft from a gear train. Far as I know the mould makers lathes with sliding lead screws can only do linear pitch changes .

Clive

Edited By Wolfie on 17/02/2013 13:35:21

Excellent Link, Clive

Many thanks!

MichaelG.

Edited By Wolfie on 17/02/2013 13:35:48

There are a lot of ways of achieving the non linear motion needed to generate the screw thread. In the Workshop Technology Series No 440 - Mechanical Movements - there are numerous examples of non linear gear systems using eccentric gears, elliptical gears, "snail" gears etc. Also a simple crank mechanism will give nonlinear motion, as will a whole range of cams. It would be fairly straightforward to disconnect the lead screw driving the table and then to substitute a mechanism based on one of the above, geared to the spindle that is rotating the screw. By changing cam profiles or gear shapes you could generate just about any non linearity that you want, and if the machine becomes a special, just doing this - so what , you're going to be making hundreds or thousands of them.

Keith

Edited By Keith Long on 16/02/2013 18:54:52

Edited By Wolfie on 17/02/2013 13:36:16

Hello Michael and Bazyle,

I finally managed to get the photos to load, it seemed to want to handle them one at a time, Part of my trouble is that I don't have a 5 year old in the house to show Grandad what an old fool he is! The result is I'm afraid both messy and bitty.

I don't know which non-linear form the thread is, it might well be trigonometric. Would it actually make much difference to the process of producing it? I rather doubt it.

I'll see what develops. Thanks for your interest

Brian

Gentlemen,

The spiral form is very clearly milled, starting with a plunge cut at the closed end. The finish is too good to be left at that, it has also been ground so that motion to the bronze carrier is still possible from the run out end of the thread where side forces have more effect. Under a good lens I can see the witness marks of the milling at the start, and moreover, the ground acme form seems to been achieved by angulation of the stone; the bottom of the groove finishes in an inverted Vee

Thank you Clive and Keith for your observations, I had no idea such could be achieved on a lathe.

Brian

Edited By Wolfie on 17/02/2013 13:36:45

Bombsights were amazing analogue computers. I guess (and it is a guess) that your spiral is to compensate for the apparent accelaration of the target (really the change in angular displacement) as you fly up to it. There would have been separate adjustments for altitude and speed.

Neil

Edited By Wolfie on 17/02/2013 13:37:13

Neil,

My father-in-law found it after the war and dismantled much of it. It was packed with anaeroid bellows in pairs on balanced beams, pipes all over the place, lots of gearing and shafts, all cross pinned and running in bushed diecast frames and so on.

Truly a box or wonders. I really have little idea what compensated for what, but airspeed, altitude, estimated ground speed and cross windsheer would all have played a part in accurate bomb release . The piece I've described struck me as instrument precision workmanship, a tricky thing to get right anyway and all made in vast numbers at a time of enormous pressure.

Brian

Edited By Wolfie on 17/02/2013 13:37:42

Merged. I've had to do it post by post though so they have arranged themselves chronologically, hope they are in the right order!

Thank you Wolfie, it looks all the better for it too.

Brian

Thanks Wolfie ... Much Appreciated

MichaelG.

Well done Wolfie; have a gold star

Neil

Thank you one and all for your interest and contributions, both to rectify my inept posting of pictures and for suggesting ways and means.

I think Michael Williams spoke with authority based surely on experience, which was after all what I had hoped to promote. Clive Foster also mentioned following a master.

The process has to have been simple and repeatable, these things were made by the thousand. It had simply not occured to me to use a master pattern and copy repeatedly, I was more into one off mode instead. A very useful method for other situations too.

One request for Keith Long which might also be of interest to others, is there a publication in the Workshop Technology series 440 he can refer me too? I tried Googling without a lot of meaningful results other than language workshops and trying Mechanical movements yielded a Victorian publication for 507 ideas by Brown available from Amazon, that didn't seem right either but it was more relevant.

Clive Foster's link for Southbend drawings and information was also interesting, I shall study it in detail.for my education at least.

Brian

Edited By Brian Wood on 18/02/2013 17:02:27

Edited By Brian Wood on 18/02/2013 17:04:11

Brian

The Workshop Technology Series is produced by Tee Publishing. Just looked on their on-line list and coundn't see it. My copy ,I think, was a download as a pdf, but I can't remember from where. It might even have been picked up at a model engineering exhibition. It doesn't have an ISBN number on it anywhere so no help there either I'm afraid. Another source that might be worth googling for is "Ingenious Mechanisms for Engineers and Designers". I believe it very much on the same lines, wierd gear and cam devices and multi-element linkages.

Keith

.

It's a four volume set, and absolutely superb

Here is a handy hyperlink, to give you a taster.

MichaelG.

Hello Keith and Michael,

Thank you both for the information, none of that would have readily emerged without my original posting. It never fails to surprise me what can tumble out in response, some of it can be quite obscure.

Brian