Anecdotes 2

This topic has 34 replies, 21 voices, and was last updated 3 February 2013 at 11:45 by Brian Wood.

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18 January 2013 at 16:10 #109128
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
There is always discussion of limits , fits and accuracy on this site so a I thought that I'd tell you about one or two oddball examples that have appeared correctly on real parts drawings .

(1) On a modest size and quite simple steel component :

Radius of 6000 inches +/- 200 inches

What do you think the part was and why such an odd dimension and tolerance ???
Advert

18 January 2013 at 16:10 #22420
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
18 January 2013 at 16:42 #109131
Ed Duffner
Participant
@edduffner79357
Peco Setrack 1:1 scale?

or part of the Hadron Collider?
18 January 2013 at 16:58 #109136
JasonB
Moderator
@jasonb
I sould think the decimal points were missing maybe it should be 6.000" +- 0.002"
18 January 2013 at 17:08 #109137
Durhambuilder
Participant
@durhambuilder
Hmm, doesn't necessarily need to be a complete circle, it could be a short arc?
18 January 2013 at 17:31 #109140
Martin W
Participant
@martinw
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.
18 January 2013 at 17:33 #109141
Sub Mandrel
Participant
@submandrel
Something like a point frog? 500 feet +/- quite a lot sounds like the spec for full size rail radius.

Neil
18 January 2013 at 22:49 #109171
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
Its a dimension and tolerance which appears on a drawing of an aeroengine component .

Less sophisticated examples of the same component can be found in more commonplace engines .

???
19 January 2013 at 00:40 #109182
jason udall
Participant
@jasonudall57142
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&quot..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.
19 January 2013 at 00:50 #109184
_Paul_
Participant
@_paul_
Modest size, simple & part of an aero engine….. could it be the curvature of a rocker arm? or perhaps the prop boss?
19 January 2013 at 09:11 #109190
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
Hi Jason ,

Close enough !

The pins and journals on some Bristol radials were crowned to reduce local stresses .

6000 inches +/- 200 inches was a typical value .

Don't know what the good component yield percentage was in manufacture but they would certainly have been inspected and approved/rejected individually .

Regards ,

Michael Williams

Edited By MICHAEL WILLIAMS on 19/01/2013 09:19:32
19 January 2013 at 09:19 #109191
Stovepipe
Participant
@stovepipe
Looking at Argos website yesterday – a "Dremel" accessory set, weight 450 KG. Blimey, some accessory !

Dennis
19 January 2013 at 13:52 #109236
Ed Duffner
Participant
@edduffner79357
Posted by MICHAEL WILLIAMS on 19/01/2013 09:11:47:

Hi Jason ,

Close enough !

The pins and journals on some Bristol radials were crowned to reduce local stresses .

6000 inches +/- 200 inches was a typical value .

Don't know what the good component yield percentage was in manufacture but they would certainly have been inspected and approved/rejected individually .

Regards ,

Michael Williams

Edited By MICHAEL WILLIAMS on 19/01/2013 09:19:32

But how would you go about checking this tolerance? Isn't it more sensible to reduce the figures to workable/measurable values?
20 January 2013 at 19:14 #109426
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
Hi Ed ,

The crowning has to be of very large radius compared to the component – really its just a very slight reduction of diameter of pin (or whatever) at outer ends .

The 6000 inch +/- 200 inch figures appeared on engine drawings which were done in the 1930 to 1950 period .

Crowning is still very important on many engineering components but the means of specifying on drawings and means of manufacture have moved on considerably .

Regards ,

Michael Williams .
20 January 2013 at 19:19 #109428
Phil P
Participant
@philp
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
21 January 2013 at 00:19 #109465
jason udall
Participant
@jasonudall57142
Posted by MICHAEL WILLIAMS on 20/01/2013 19:14:58:

Hi Ed ,

The crowning has to be of very large radius compared to the component – really its just a very slight reduction of diameter of pin (or whatever) at outer ends .

The 6000 inch +/- 200 inch figures appeared on engine drawings which were done in the 1930 to 1950 period .

Crowning is still very important on many engineering components but the means of specifying on drawings and means of manufacture have moved on considerably .

Regards ,

Michael Williams .

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)……
21 January 2013 at 21:13 #109579
Sub Mandrel
Participant
@submandrel
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
22 January 2013 at 00:56 #109608
jason udall
Participant
@jasonudall57142
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…
22 January 2013 at 08:06 #109615
Michael Gilligan
Participant
@michaelgilligan61133
Posted by MICHAEL WILLIAMS on 19/01/2013 09:11:47:

Hi Jason ,

Close enough !

The pins and journals on some Bristol radials were crowned to reduce local stresses .

6000 inches +/- 200 inches was a typical value .

Don't know what the good component yield percentage was in manufacture but they would certainly have been inspected and approved/rejected individually .

Regards ,

Michael Williams

Edited By MICHAEL WILLIAMS on 19/01/2013 09:19:32

.

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
22 January 2013 at 10:20 #109619
jason udall
Participant
@jasonudall57142
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…
22 January 2013 at 11:50 #109632
Anonymous
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
22 January 2013 at 12:47 #109639
jason udall
Participant
@jasonudall57142
Posted by Andrew Johnston on 22/01/2013 11:50:06:

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
22 January 2013 at 13:08 #109641
Joseph Ramon
Participant
@josephramon28170
Perhaps just hand held emery to relieve the centre would do the job.

Joey
22 January 2013 at 23:27 #109736
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
Rollers in roller bearings are crowned in volume production and these are of even flatter curvature . More sophisticated methods are available now but for many years the crowning was done by tumbling .

Very flat radii on components are easy to measure optically – again there are new sophisticated methods but for many years checking was done by the fringe counting method .

Need to make a distinction between the diameter of the component as such and the crowning effect when considering measurement methods .

The original Bristol components were hand lapped . May sound hard work but it was a doddle compared to making some of the other engine parts . It may well have been that the actual dimension was just a ' guide in principle ' and that a slight fairing off of the outer ends of pin was usually in the right ball park dimensionally after a skilled worker had done a few by trial and error .

Quick and often used ' workshop ' test for crowning is to spin component on glass (barrel wise ) . If it spins freely crowning is present and probably adequate .

Other things are sometimes crowned – one example is of gear teeth for demanding applications .

Michael Williams .

Edited By MICHAEL WILLIAMS on 22/01/2013 23:32:52
22 January 2013 at 23:55 #109740
Michael Gilligan
Participant
@michaelgilligan61133
Posted by MICHAEL WILLIAMS on 22/01/2013 23:27:59:

Quick and often used ' workshop ' test for crowning is to spin component on glass (barrel wise ) . If it spins freely crowning is present and probably adequate .

Michael Williams .

Thanks … I will remember that one!

Bound to come in useful sometime.

MichaelG.
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