Stuart S50

[Allen Smithee]

Today I had to hand over wads of cash to order a selection of inferial reamers and BA/ME taps & dies rather than rework the drawings to use the metric sizes I'm tooled up with. So it looks like the rest of the build will be more or less to the drawings, although I'm tempted to make the cylinder caps from brass rather than steel as it would look cute.

Having said that I'm still tempted to make the crank web from a bit of 1/4" steel plate rather than the 3/4" length of 1-1/2" bar provided:



I'm struggling to see the reason for all that scrap steel!  My other thought was to dig out my keats fixture and machine the web with an integral crank pin. I probably won't, but we'll see how I feel when I get to it!!

AS

[Jasonb]

I'm struggling to see the reason for all that scrap steel! 

Can't have it both ways. In an earlier post you were asking for more material to hold now you are saying it's waste

Why not saw off a 1/4" slice and machine from that and save the "waste" for something else, the EN1A round stock will be nicer to machine than a bit of unknown plate

[Allen Smithee]

The crank web drawing is (for me) remarkably unhelpful because while the dimensions might describe the part they don't give me anything I can actually machine to  for the two angled faces. No doubt Jason will tell me that they are perfectly adequate because a real machinist will turn the round features and then mark out the angled faces on the back of an apprentice for hand filing or shaping with a stone club or something, but I have a lathe and a (sort of) mill and that's what I want to use.

So the first task was to redraw it in Fusion 360:



This allowed me to extract meaningful dimensions:



I even extracted this useful dimension which would mean I could mount the part in a rotary table at the defined distance and use the angle directly for the two cuts:



...although that's a hard way to do it so I think I'll just centre it in the rotary table and use that 425thou dimension on both sides (and I can drill/tap the crank pin hole at the same time without changing the setup). But this exercise does how that CAD can be useful as a measuring tool!

I need to do some pondering about this, because this built-up crank inherently means milling and turning on large offsets from a thin 1/4" shaft - probably need more attention to shallow cuts at correct feeds and speeds than I normally bother with...

AS

[propforward]

Modelling the part is a good approach - it can give you a sort of electronic dry run to help think through the machining sequence. For what it's worth, I think you're doing a great job on this engine and I am enjoying your build.

[Jasonb]

School boy geometry really.

Scribe lines to the large and small dimensions, and join where they intersect with the outside of the stock, no need to know the angle. hacksaw just clear of the line and then set line level in the mill vice and mill to the line. And to think someone said I was making things complicated

If you are playing with CAD work out what you need to space a pin in the crank pin hole off the vice jaw by and use that to set the angle

[propforward]

The crank web drawing is (for me) remarkably unhelpful because while the dimensions might describe the part they don't give me anything I can actually machine to  for the two angled faces. No doubt Jason will tell me that they are perfectly adequate because a real machinist will turn the round features and then mark out the angled faces on the back of an apprentice for hand filing or shaping with a stone club or something, but I have a lathe and a (sort of) mill and that's what I want to use.

Best to just do it your own way and have the satisfaction of the end result. If someone chooses to openly laugh at you rather than offer positive encouragement they are best ignored. In doing it your way, you may decide as you go that there was an easier way, or a quicker way, or whatever, but that really doesn't matter when your own approach works. You may afterwards decide that in future you'll use another method, but that doesn't matter either. I am looking forward to your successful outcome.

[Allen Smithee]

Ah, I see.

I don't want to mark lines and mill to them by eye - I have machine tools with calibrated dials so that I can machine to dimensions. What's the point in having a machine that will hold to less than a thou if you just use it to visually line up with a scribbled scratch?

The model took less than 10 mins to do and it produced data that allows me to just mount a rotary table to do two accurate cuts without disturbing the setup. In only a little more time I could have produced the same data using a pencil and "schoolboy geometry".  The point is that data only needed to be developed once - if the drawing had been done properly those dimensions would already be there. The scabby drawing with non-useful dimensions just makes work for every builder.

I've been looking at S50 build logs and videos, and it seems this issue has arisen several times. It also seems that the cut-outs aren't really there to form a counterweight (although that's useful). Their main reason for being there is so that the crankshaft can be fitted over the edge of the base casting to run in the enclosed trough. So the dimension is actually very simply defined:



The desired clearance over the edge of the casting (A) will need an easily measurable dimension for the tangential distance between the flat and the centre (B). Then for a given counterbalance (or just visual) effect there will be a required angle between the flats (σ). To machine the part the builder needs just A and σ to be able to rattle of both flats datumed to the shaft centre. Add one further dimension for the crank throw and the true position of the crank pin hole can be drilled at the same time without moving it from the fixture.

I used to read riot acts to draughtsmen (and a few draughtswomen) about dimensioning shop drawings USEFULLY so that the drawings contained the information needed to make the part. To do this the draughtsperson had to think about how the part would be made, and if this involved getting off their stool and heading down to the shop floor to ask how it would be made then that's just another school day in their education which might ultimately lead to fewer un-manufacturable drawings being sent down.

Apols for the rant, but it's a hobby-horse of mine. The purpose of a drawing is to communicate a design intention, not to document a designer's thought process. A drawing which amounts to an exam question (requiring further working and a slide-rule or calculator) before the part can be made is like a broken pencil - utterly pointless.

Not that I feel strongly about it or anything...

 

AS

[propforward]

I have that same conversation with my engineers routinely. They have a habit of just slapping any old dims on a drawing to get the job out to the floor. Hoping to work them through that a bit. Getting them to talk to the shop is the key - and they are getting the hang of it, happily. Why is that so hard? My favourite part of the day is going to the shop and talking to the machinists and welders.

[ettingtonliam]

Personally, I'd have used marking blue, marked out the shape with scriber lines and light punch dots, sawn to shape, filed to line, finishing with draw filing, and 'breaking' the edges just enough to remove any trace of the punch dots. Estimated time for a component this size, 10 -15 mins beginning to end. Each to his own

[Allen Smithee]

I'm not in that environment any more, but 30ish years ago I was a process engineer in a company making electronic connectors and associated goodies (I was introducing Statistical Process Control approaches). I would expect my guys & gals to walk the floor at least twice a day, and on the last working day before Xmas I would give them all ties (purchased from charity shops) that they had to put on before heading down to run the gambit of the assembly women. Anyone who came back with an intact tie (not cut off, stolen, shredded, dipped in bonding agents or stapled to shirts) clearly didn't have a close enough working relationship with the staff, and got re-educated (one of my guys usually came back de-bagged and covered in lipstick, but he was one of nature's babe-magnets).

But I encouraged the women on the shop floor to come up to the engineers who created the drawings and seek a personal explanation of what they meant. It was always educational for one or both of them. When it came to the toolroom I encouraged engineers to invite toolmakers into the discussions on both the designs and the manufacturing techniques because I was fed up with seeing un-manufacturable designs leave the office. One of my favourites from an even earlier job in the underwater business was a sonar pressure fitting which had a pin to release a ball-valve. It was a phosphor-bronze part with a phosphor-bronze pin and three O-rings that looked something like this in cross-section:



The pin was 2.5mm dia in a 2.8mm bore. Quite how those undercuts for the O-ring cavities were to be made doesn't seem to have occurred to the designer, let alone how the specified 5x3x2 O-rings were going to get in there afterwards...

AS

[Jasonb]

As I said earlier the drawings are old and were done for makers who had little more than a basic lathe, no mill, no micrometers not Rotary table and certainly no way to measure angles to much accuracy. So would have been marked out and made as per the above post and what I said earlier. They are of their time, fair enough if you want to move on and have better equipment but there are still beginners who will just have basic machines and tools which are who the model is aimed at.

[propforward]

The pin was 2.5mm dia in a 2.8mm bore. Quite how those undercuts for the O-ring cavities were to be made doesn't seem to have occurred to the designer, let alone how the specified 5x3x2 O-rings were going to get in there afterwards...

AS

That's a corker!

I'd like to pretend I'm immune, but just a couple of weeks ago I was reviewing one of my own designs when I realized it wouldn't go together. Glad I caught that before it went out anyway.

Yesterday I had a really fun conversation with the programmers figuring out how we are going to fixture a huge steel weldment I designed, as well as getting into the GD and T on it. I love that kind of collaboration. Actually climbing up on the mill table to look at it when it gets there will be even more fun.

Makes me feel a bit silly when I'm twiddling the handles on my little mill. 

But this is why, as you do, I try and machine by the numbers. Just trying to emulate what I see at work, in my own small way.

[ettingtonliam]

Apart from summer jobs in small sub contract machine shops in the West Midlands when I was a student, I haven't worked in production engineering, but did spend a lifetime in the construction industry. You probably wouldn't believe how complicated the reinforcing can be in a bridge deck, especially at the ends where the bearings are. My first boss in the design office used to make us bend up scaled down links and bars from big paperclips just to check that it would all fit together on site.  Later, on site, which is where I spent most of my career, I've lost count of the times I've had to have reinforcement redesigned 'on the hoof' and rebent in order to make it possible to fit all the links and bars together.

I used to get really angry if sent an updated drawing which just carried the note 'Generally revised', but didn't give any indication of what exactly had been changed.
I used to try to insist that any change was 'bubbled' (cloud shape drawn round it), but it didn't always happen.
I'd been well trained in technical drawing at school by a teacher who had spent 30 years in railway drawing offices before changing career to become a teacher of technical drawing and lower school maths.

I had a friend who did a lot of 1 off prototype work, and he would sometimes build the prototype from a series of back of envelope sketches, and only do the 'proper' drawings once he'd made sure the thing could be built, and would work.

[Charles Lamont]

Two practices I learned early and carried through my career: when going on to any shop floor, my first habitual action, as a safety precaution, was to tuck my tie inside my shirt; and the largest words on any drawing were "IF IN DOUBT, ASK".

[ettingtonliam]

Plus 'DO NOT SCALE'!