Sequence of machining - Stuart Twin Oscillator

One of the problems faced by a novice (like me) is to decide the *SEQUENCE* of machining a piece. I have the tools (drill press,lathe and vertical milling attachment), I can read drawings, and I can visualise the end result.

But where to start? Help would be very much appreciated.

This piece for example. It is the main casting for the Stuart Twin Oscillator.

You may find it useful to get hold of the MEs that covered the build of this engine starting with 4396

Usually the most useful first thing is to establish a datum surface or surfaces. The two that immediately spring to mind are the flat base and the 5/32 bore for the crankshaft. Pretty much everything else seems to be referenced to these.

I am going to jump in here with both feet and probably receive a lot of flack.

The first thing I would do is to re-draw the drawing. The more I look at it the more horrible it becomes - The lack of section lines, the lack of defined positions of the long holes, what projection (although this is obvious). Going beyond the typical model engineering drawing, the stack-up of dimensions and what is really required is defined datums. OK, all these can be worked out but re-drawing gives one a good, full understanding of the part.

The casting looks nice.

JA

Edited By JA on 15/04/2016 12:32:26

Using a web search, I can find 4396, but not 4397.... to whatever

Part 1 4396

Part 2 3498

part 3 4399

part 4 4400

Part 5 4401

It's quite a detailed description for a small engine but is aimed at the beginner and shows plenty of setups. What better excuse to take out a digital subscription and get access to all the back issues from 2002 onwards

Good idea, and I will do that anyway.
Edit: Done!

But the link at the top of this page to just results in a "page not found" error on those issues.

Cheers, Rod

Edited By Rod Neep on 15/04/2016 13:50:45

Thanks Michael.
The cast boss where that 5/32" hole is to be drilled is 1/4" diameter.
What is the advantage in drilling a pilot from both ends?

To get the best measurement on the centre of this boss I feel that it might be pertinent to mill the bottom of the base and the top of the casting first so that there is something "square" to grip the casting for drilling. Yes?

Rod, you should soon get a 10 digit subs number if you have not got that already. Once you have the number go to "settings" along the green bar at the top of the page and enter the number their in the ME box. Once that is done you should then be able to click "magazines" along that same green bar and then select ME, this will take you to several pages that show the front covers of the mags, just click on the ones I listed above.

Article shows making the base flat as the first op.

J

Edited By JasonB on 15/04/2016 14:13:50

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Rod,

It's a fairly long hole [for its diameter] and any angular misalignement can become very obvious if you only work from one end.

Like any tunelling operation: hallf the length = half the displacement [and both ends are right]

MichaelG.

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P.S. ... Apparently the articles say different, but: Me, personally; I would get this pilot hole done before anything else, and use it as my primary datum.

Edited By Michael Gilligan on 15/04/2016 15:07:13

I suppose the flat base gives you something to make sure your two drilled holes are parallel to, if drilled first you may not be able to mount the casting in the same plane to drill from either side.

.

... or, you could pass a rod through the bore and rest that on the vice jaws [or whatever].

[my 1/8" pilot bore would be reamed, after whatever drilling/tunneling operation]

MichaelG.

Edited By Michael Gilligan on 15/04/2016 16:46:27

You are ahead of me Michael and already have the holes drilled from either side. What I was querying was that if you drill half way from one side and then flip the part over how do you ensure that the second hole will be parallel with the firstas you have no fixed face to check you have the casting in the same plane.

Or are you thinking of the trick of ctr punching either end of the cast boss, then with a drill in the lathe chuck using a tailstock ctr to line the two punch marks up as you drill with the lathe, which is the best way to drill from both sides and get things to meet in the middle.

.

To be honest, Jason ... Given that the bosses are only 1/4" diameter, I would probably hope to centre each pilot hole by eye.

I'm not claiming it's the right way ... just how I would approach it.

MichaelG.

Yes without having the casting in your hand its going to have several ways to tacle it.

One thing I would suggest Rod is that before you reach for any machines give the casting a fettle to remove and casting lines etc. Then blue up all the faces to be machined and set the casting up on a flat surface, run a rule over it and if you have a simple home made scribing block or better still a height gauge use that to check where the various holes will fall. You wouldn't be the first person to put a hole in the ctr of one feature and then find that the rest of the holes/faces don't lay where you want them, a bit of averaging out is often needed. Also you can find that the two halves of the casting can be out of line so positions may need to be a bit high on one side and a bit low on the other.

J

Splendid idea.
Many thanks.

Rod

Rod, I made a similar engine for one of my model boating friends, and I would proceed as follows. The base is the natural datum for this engine, so I would set the casting up in the vice with the base uppermost, and skim the base. While the base is uppermost I would drill the mounting holes. Then I would make a sacrificial base plate out of a scrap of anything handy, making sure the sides are square with each other and overhang the casting base by 1/2" all round, then mount the casting square on the base, then use the sacrificial base plate to hold the casting and as a setting aid as you proceed. As the mounting holes are small, use additional supports and clamps to take the machining loads. I would not bother trying to drill the shaft hole from both sides, just make sure the face of the boss is flat, skim first, and start with a small centre drill just to dimple the face, the drill will snatch in bronze if you go any deeper, then use a new quality drill for the hole, this should not wander over such a short distance.

Chris Gunn

Thanks for your help everyone

This is a big step for the beginner.... and I have never done milling before. I have a vertical slide attachment for my Warco WM-240, and this is my first attempt at milling. (And also setting up the vertical slide and the work piece using an indicator).

First I milled the base of this standard for the Stuart V-Twin Oscillator, then skimmed the four edges of the base, and then the top surface so that it can be parallel with the base. Now I just completed milling the first side for the steam connections and valve surfaces.

OK... I did take it very steadily with fine cuts, as one of the problems for a rank beginner is figuring out how much of a cut to make at a time. But I did it! Next step will be to come out 5/32" and mill the end of the crankshaft bearing, then flip it over and do the other side.

Rod