Dimensioning Puzzle (Alibre Drawing)

[Nigel Graham 2]

On Nigel Graham 2 Said:

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I take Nick’s point that dimensioning anything from a point out in fresh air is bad, but in this case I have no choice.

However [!] … if that point actually represents the centre of rotation of your rotary table;  what could be more logical ?

MichaelG.

[Nigel Graham 2Participant @nigelgraham2]

My thoughts entirely – and of course this is the functional as well as manufacturing axis.

The point about radii from thin air was related to being able to inspect components in a factory, needing a datum edge or surface on the piece of metal itself.

Also unless the factory is making its own products the inspector is unlikely to know what the part is, as the customer’s drawing would likely bear only some bland name like “Link”, or just an arcane “Pt. Num”. Of course, the Inspection Department might be equipped with radii-measuring instruments, but still don’t have the item’s physical reference centre to measure from.

[JasonBModerator @jasonb]

It would be interesting to know how Martin would have dimensioned this part to avoid rejecting it or any part for that matter that has a curved edge where the ctr of the arc is outside the material.

I’m not so sure that mine and Andrew’s links are a “simpler component” at least not to make. Nigel has some simple straight edges and a couple of corners to lop off, the two extra small holes are not complex items to place either. mine has 5 internal corners with a radius at the root of the rod and suspension holes, three additional curved surfaces to machine on the inner edge of the part and three external radii around the rod and suspension holes. All these items need to flow smoothly from one to the other which takes a lot of care on a rotary table to position the part correctly and not overwind the handle.

[Nigel Graham 2]

On Nigel Graham 2 Said:

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The point about radii from thin air was related to being able to inspect components in a factory, needing a datum edge or surface on the piece of metal itself.

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Entirely possible to do that with a suitable jig which includes a pin or ball as a reference point.

MichaelG.

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P.S. __ Back in the 1970s KODAK [in Stevenage] installed a pretty massive Coordinate Measuring Machine specifically to check the plastic mouldings for the body of the little 110 cameras. … The item under inspection was tiny compared with the measuring device, but sometimes you have to bite that bullet.

It’s a long time ago, but I seem to recall there being 47 critical dimensions on that body, all referenced to a ‘virtual’ datum.

Edit: _ The CMM was something in this ‘bridge’ style:

https://web.archive.org/web/20130906014000/http://www.coord3-cmm.com/3d-bridge-cmm-machine-range/

[Nigel Graham 2Participant @nigelgraham2]

The company made have had that means, but the anecdote on which I commented described a specific policy that all parts had to be measurable from points or edges on them.

Why, was not stated, apart from apparently facilitating inspection.

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for clarifying, Nigel

… I have no further comment on that anecdote !

MichaelG.

[Nigel Graham 2Participant @nigelgraham2]

It ought be possible to fit adjustable stops on a rotary table to avoid over-winding. Or a block clamped to the milling-machine table engaging an arm projecting from the rotary table. (I think I’ve even done that.)

Marking the destination angles for each arc with a felt-tip pen helps, and I think meths will clean the ink off afterwards.

With a bit of planning it ought be possible to use both the cross- and long- feeds as well as the angles to machine that link entirely on a rotary-table on the mill, straight sides as well, but this approach does need full dimensions and possibly a second drawing outlining the process.

 

[JasonBModerator @jasonb]

It is unlikely that you will be able to mill the full extent of the curved slot on your link so overrunning should not be an issue.

If you were to try and machine to the full extent plus the two radial cts at the end that would need at the most a 1/16″ dia cutter which is unlikely to have flutes long enough to cut to 3/8″ thickness and even if it did flex and breakage would become a problem. Best bet would be to mill down the ctr line with a 6mm or 1/4″ bit, offset each way to bring up the width and then finish by filing unless you have EDM to hand.

Alternative for the Wagon where it will not be seen is to redesign for a slot that can be milled with a larger cutter and doe snot need the file work at the ends.

[Nigel Graham 2Participant @nigelgraham2]

I think the ‘Maid of Kent’ design assumed such techniques as chain-drilling and filing, or (given LBSC originated it) milling by that frightening method of manually swinging the blank on a pin, against a cutter held in the lathe chuck. So squared-off ends to that slot would be almost easy compared to making the rest.

They certainly would be a lot simpler to make with round ends, the centres of the end drillings being central to the radial flats, and those holes being slightly over-size to give the block a slight over-run.

We are not told the valve-gear in the original, enclosed engine, but Stephenson’s is likely, and the links may have been as on a traction-engine for simpler manufacture and because a road vehicle doesn’t have to run ever so efficiently in reverse. The one photograph I have of any detail of the engine reveals two little projecting boxes, probably of sheet-steel, on what the publicity material tells us are “quickly detachable covers”: large inspection panels held with a lot of wing-nuts. The protuberances may have accommodated the expansion-link ends in full gear.

So I think I will revisit the drawing and modify it accordingly, with round-ended slots and possibly launch-type suspension, but to the same functional arrangement. The advantage of loco links here is in being easier to fit inside a fairly narrow casing.

 

A curious detail: each of those two little boxes is topped with what appears to be a pipe elbow, turned to face its opposite number like a pair of cartoon submarine periscopes. Quite what they were is anyone’s guess! Rather crude handles for the cover itself? A simple bent strip or rod handle would have been easier.

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The ‘Maid’ represents an L&SWR D? or L? class locomotive, with either inside or outside cylinders, and inside valve-gear of either Stephenson’s or Joy type. I have no idea how prototypical the miniature motion-work is, or which option is “right”. Still, I am pretty sure there is room inside there, under the relatively slim boiler, for rounded-slot links – and they are hidden from any but very careful inspection.

The full-size engine links with squared ends were sometimes nibbled out using a slotting machine.

It was one of these that gave me my first experience of miniature locomotive driving. Graced with the name “Maid of Athens” (I don’t know why), it was our club locomotive for many years… then mysteriously vanished. I do hope she’s still going strong somewhere!

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Meanwhile I have been pressing on to get a long-sidelined ‘Stent’ T&C grinder finished. Cor! It doesn’t half make a difference when you’ve a proven set of drawings to follow, and you can see clearly how others have modified what and how on theirs!

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Incidentally, the pioneering model-engineer Dr. Bradbury-Winter built an ingenious reciprocating-drive for his lathe spindle, for such tasks as milling arcs. I think that was treadle-driven, presumably with the milling itself courtesy of a Potts spindle – or by shaping.

[Nigel Graham 2Participant @nigelgraham2]

I have just closed Alibre from trying to re-draw it afresh – I found incorrigible mistakes in my original.

Gave up.

I am trying to adapt a published design so it might seem easy (ish) to create a copy in decimals rather than thirty-twoths and sixty-fourths.

How does anyone make successful engines from drawings so poorly dimensioned? Perhaps when it comes to valve-gear near enough is good enough!

This is a launch-type link for Stephenson’s Link Motion, so suspended from a pair of trunnions whose pins have to be centred at a definite distance from the link centre-line, and exactly in line with each other.

Each trunnion is a pins in a plate rivetted to the link and its companion via three holes dimensioned only unto themselves, not to the plate edges.

Are the rivet holes shown by dimensioned locations in the link itself? No. The drawing shows the full assembly.

The critical pin location is 3/32″ from fresh air –  the centre of a slot in a separate piece of steel – and by ” 1/2″ nom. ” from the back of the trunnion . What???

Where is the back edge of the trunnion? It is marked, ” … flush with back of link”.

Where is the back of the link? That 3/32″ + nominal half-inch from the link centre-line.

So three pieces of steel until assembled, but dimensioned from each other.

The inner face of the trunnion plate has a step in it. Dimensioned from? The pin centre.

 

I thought this is meant to be a precision machine.

So I need re-calculate a lot of back-and-forth dimensions in vulgar-fractions across two components to work out just where everything really is. Then calculate completely where to drill the three rivet holes in each of the three pieces of steel.

 

In theory one can make all five parts of the complete link individually, to better than 0.005″ of the given dimensions where those matter, assemble without having to spot through holes or drill them over-size or such like antics, and so they will work properly in the engine.

That needs using the machine-tools to their potentially most accurate but also most efficient advantage.

Hence needing a new drawing dimensioned properly, but I think that’s too hard for me in CAD.

I will have to photocopy or trace the drawing and use a calculator and pencil to re-dimension that.

 

 

[JasonBModerator @jasonb]

Photograph or scan the part of the original drawing that shows these parts and post it here. Might be able to make sense of it then and see if it can be drawn.

[David JuppParticipant @davidjupp51506]

Why not use the fractional dimensions directly in Alibre?

You can enter fractions – and switch later to decimals, or if in decimals to begin with enter in fractions and the conversion will be done ‘on the fly’.

No need to use a calculator – Alibre has one built in.

If there really is interdependence between parts – model what you can of the parts, then combine in an assembly workspace, check dimensions in assembly and if necessary ‘edit here’ one or more part.

[JasonBModerator @jasonb]

Yes fractions are easily entered but displayed as decimals on a sketch long before you get to producing a 2D drawing. Also handy if you want a half or 3rd of something just enter the full amount followed by /2 or /3 rather than trying to work it out separately. Use the CAD as your fag packet sketch to convert and also take off any missing sizes.

Copy and pasting sketches from one part to another is also a bit like spotting through as they will be in the same position on both parts provided you think about layout from the start.

[David JuppParticipant @davidjupp51506]

When using ‘edit here’ you can dimension a feature of one part from some aspect of another part in the assembly.  This creates an ‘inter design constraint’.

You can choose how dimensions are displayed in sketch and in 2D drawing – you can have fractions displayed if you wish, you can change the display format later, or even change units.  The part remains the same physical size.

[Nigel Graham 2Participant @nigelgraham2]

Thank you – I didn’t know you can enter fractional dimensions – it’s decimals I need for machining.

(I am surprised so many model designs stick to specifying fractions even for turned components.)

The problem with this link assembly is not that the original drawing is dimensioned in fractions. I can just look them up on a Tracy Tools poster-size conversion-chart kept near the computer (with another in the workshop).

The confusion is from the way the parts are dimensioned, needing a lot of awkward arithmetic to work them out. Using words like “nom” on components that have to be placed accurately does not help. Nor does the holes for their fasteners being dimensioned from each other, not a datum edge.

It is possible to work out the more obscure dimensions

To me, the only way to dimension this thing is from the radii centre, the distance from that centre to the back edge, and a secondary datum given by the intersection of the back edge and horizontal centre-line. The two trunnions then follow that second datum but are dimensioned fully from that, so although you make the three parts separately they should still all fit together, assuming sufficiently close tolerances you’d need determine for yourself.

(You could work out polar co-ordinates for the rivet holes in the link itself too, but this does complicate matters.)

The copy is here. The dimension that has fallen off the edge is 2-3/16″. It is over a fold in an oddly-folded sheet I did not want to risk damaging by creasing it even further!

 

Examining the rough GA I have for the steam-wagon engine, suggests a true Launch link – as on a traction-engine – would fit and be a lot simpler to make but would need other parts re-designing.

 

[JasonBModerator @jasonb]

The only critical dimension is that the pins on the small side plates end up 3/32″ from the centre line of the expansion link’s curved slot so the back edge and position of the rivits is not critical hence the “nominal” edge distance and no actual positioning sizes for the three rivit holes.

[David JuppParticipant @davidjupp51506]

I would model the parts using fractional inches first, as it is easier to check back for consistency with the original drawing.

In the derived 2D drawing, I would set display units format to whatever is required (decimal inches in this case).  Drawing units/format do not have to be the same as those used in the model.

In the drawing you can add any datum that you’ll be using, and place dimensions however they make sense for your machining operations – again the dimensions on drawing could be arranged completely differently from those used in the model.  So use angles/radii if that is what you’ll use when machining.

 

The 3 rivit holes may be ‘match drilled’ during assembly, or spotted through from one part to the other.  Atom3D doesn’t include ‘assembly features’ which allow for ‘match drilling’ in an Alibre assembly, but you can do a ‘virtual spotting through’ by using ‘project to sketch’ in the assembly context.

[JasonBModerator @jasonb]

What David says, enter the dimensions as fractions, you can also add offsets, etc. Here the slot’s central dia has had the slot width added to diameter which gives the correct offset

Then when you click the green tick it shows as a decimal on your sketch

All seems doable from the given drawing, only exception are the two angled edges from the vertical inner edge of the link to the ends and the radii of the corner rounding neither are critical. 3 rivit hole sand the vertical back edge were copied and used to construct the small blocks.

[Martin ConnellyParticipant @martinconnelly55370]

If you want to know why a part should not have a datum in fresh air it is because different people may be doing different operations on different features using different machines on different jigs on different days. Also you would not want to start your part off using more raw material than was necessary to produce the part. If you have a couple of jigs then making them large enough to have a datum point off in the distance would take up a lot of storage space when they are not in use and may also be large enough to require cranes to start moving them around and large machines to mount them on. Imagine making this part if the radius of the slot happened to be 1500mm. If you have two of the edges as datums then all you need are three dowels to locate the part on a small jig. You can always show the dimensions from an edge datum to the centre of rotation of a feature. If you start off making this part out of a large piece and dismount it then cut it down to size you have lost your datum if you realise you need to add a hole at some point.

Martin C

[Nick WheelerParticipant @nickwheeler]

Martin, the real complaint here is that some of the dimensions are floating in fresh air without any datum. Surely that changes it from being possibly tricky to machine/inspect in some circumstances to utterly impossible?

 

[JasonBModerator @jasonb]

I had no problem tying down the “Floaters” As I said the only exceptions were the non critical edges marked in green on my last image for which I put in the two ringed dimensions.

The three rivit holes are not particularly critical but still easy enough to place even if it says 1/2″ nominal rather than 0.500″. You can work out their positions from what is given with a bit of common sense thrown in and end up with 13/128″ as what is needed. The use of “Nominal” is only there as the back edge of the link is not critical, it is the position of the pin and it’s hole in the side brackets that matters.

[Nigel Graham 2Participant @nigelgraham2]

Thank you Jason, David –

I was attempting to avoid having to spot through anything, certainly not on the real bits of steel, because that is not easy on something as small as this, but too easy to go wrong.

I was stymied because the original K.N. Harris drawing tells me to put two separate pivot-pins in line in a very precise place then makes it very difficult to do so by not giving a physical datum like an edge to measure from. In fact he seems to suggest the edge that could be used is not critical, and you measure from the middle of a slot.

The two big corner bevels can be treated as a finishing operation by saw and file, guided by a washer or similar screwed to the pin-holes.

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Nick –

Exactly the problem with this link!

I was trying to set it up as a sub-contract engineering workshop might. That is, the parts occupy separate drawings dimensioned entirely from their own physical data surfaces which correspond to those on each other. So the parts can be made quite independently but provided they are within tolerance, still fit together as intended.

Only…. the published drawing does not do that at all, and trying to work out the proper way totally foxed me.

(I know we don’t normally see, nor need, formal tolerances on model-engineering drawings but we can still try to set our own, at least on machined components. I have seen professional drawings with both 1/64″ tolerances on fractions and 0.005 or 0.010 on decimals, u.o.s. Apply that here and the two trunnion pins can be 0.03″ (1/32″ ) out of line with each other even though the individual parts are within limits.)

[JasonBModerator @jasonb]

You can dimension the brackets taking the pin as the datum as that is the critical part not the back edge.

Here I have set it up to display duel dimensions with fractions as primary to 128ths and secondary as decimal to 3 places.

[Michael GilliganParticipant @michaelgilligan61133]

Nicely done, Jason

MichaelG.

[JasonBModerator @jasonb]

Thanks, It is not a feature I personally like as it can make a drawing look rather cluttered but worth it to show Nigel what can be done and it can have the odd use. I would rather do the drawing with fractions and then save a copy with a new name and just alter what is displayed to decimal and send out or print one of each.

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