Anecdotes 2

Peco Setrack 1:1 scale?

or part of the Hadron Collider?

I sould think the decimal points were missing maybe it should be 6.000" +- 0.002"

Hmm, doesn't necessarily need to be a complete circle, it could be a short arc?

How about something like plates for a large fuel storage tank or a piece of railway track. Where they will finalise the diameter/radius on site by pulling into final shape.

Something like a point frog? 500 feet +/- quite a lot sounds like the spec for full size rail radius.

Neil

Moderate sized....?say 12"long .so approx 2 milli radians 0.1 degree or 6 minutes......or somthing like 3 thou on 6 "(3 thou each end of 12"..difference of straight line and arc segment....

I have programmed such radii to compensate for "barrel"shapes in journals...

6000+/-200...seems a realistic Toll...in percentage terms...

I have had (automotive production) drawings such that a drawn feature may disapear if other dimensions are at the extreem of their tolerence...(think three lengths ....over all. and two lengths in from opposing ends)....

I sometimes wonder if anyone reads these before issue.

Modest size, simple & part of an aero engine..... could it be the curvature of a rocker arm? or perhaps the prop boss?

Looking at Argos website yesterday - a "Dremel" accessory set, weight 450 KG. Blimey, some accessory !

Dennis

But how would you go about checking this tolerance? Isn't it more sensible to reduce the figures to workable/measurable values?

I once had a mate who got sacked for being a thou out on one job.

He worked in a bank at the time.

Phil

It would be , "even" today be a near impossible dimention to check with linear 1D instruments..the "radius" would be Calculated from a cloud of 2D or 3D points....

many features today are more difficult to measure/verify than they are to generate..(both in the DO and in the factory)......

Jason - how would you actually machine such a radius - some complex geometrical too to generate the radius without having a 6000 inch bar? or would you simply lap the journal concave?

Neil

How would I?

Cnc if not ground.( or if less that 1000)

Centerless grinder from ("parallel" ) with profiled wheels if ground finish..(or more than 1000 pcs)

Diamond dresser cam / former CNC generated and verified as below...

The above deals with making it..checking/inspection of 6000" radius would be the real chalenge..I would use our 2D "height" gauge and rely on mathematics to derive radius from three points.....serveral times

Just how I work in the day job...

.

It would be interesting to know a little more about these components

... Length, Diameter, Surface Finish ...

Whilst I have no historical evidence to support this; I suspect that 6000" radius +/- 200" was intended to prescribe "tramlines" within which the actual shape [including its Straightness and Surface Finish] could be whatever suited the manufacturing process.

Jason's expert description shows how he would make it now; I remain intrigued by how they would have done it in the 1930s

MichaelG.

.

P.S.   Whilst browsing, I just found this eBook about the Packard Diesel Radial.

Edited By Michael Gilligan on 22/01/2013 08:32:40

in 30's... either centerless or between center grinding .

As to measuring it....well in production I would expect 3 "snap" mikes or dial indicators ( I have 1/10,000 " clocks in my collection...) ...mounted in fixture controlling/fixing the postion along the part..... but I would be guessing...

Hmmmm, if I've got my calculations right, and assuming the journal is 2" wide, I make that a difference of 0.000083" on the radius. In other words less than a 10th of a thou. The tolerance variation would be rather smaller.

Under wartime pressures to get engines off the production line I suspect it may have been quietly ignored, plus a certain amount of muttering by machinists about draughtsmen who came up with the tolerance in the first place.

Regards,

Andrew

Quite... inspection would be challenging

Perhaps just hand held emery to relieve the centre would do the job.

Joey

Thanks ... I will remember that one!

Bound to come in useful sometime.

MichaelG.

Back when god was a boy I was shown a method of checking very large diameters with a suitable steel bar with points turned at either end. One end was held on the bottom ID while the point on the other end was arched across until it touched one side of the ID and a chalk mark made, it was then arched to touch the opposite side of the ID and another chalk mark made. The diameter could then be calculated to a few thousandths of inch by triangulation.