Stuart Twin Oscillator build

Sarah · Jul 9, 2024

Whilst I am literally waiting for the paint to dry on my Stuart Progress I thought I would make a start on a Stuart Twin Oscillator, along with a friend we are building the single and twin versions.

As I didn't want to get too far ahead I thought I would make the two steam unions first. Part way through the first one and I didn't understand how the drilling was supposed to go on the left hand end which is threaded 3/16" x 40. Picture attached. The drawing calls for 1/8" dia x 1/4" deep, should that be more like 1/16" instead?

Yes, I was dumb enough to try and put a 3/32" drill in it!

Jasonb · Jul 9, 2024

I would say 1/16" dia x 1/4 deep eg to meet the 1/8" hole from the other side in the middle.

Then counterbore 3/32" x 1/16" deep

Sarah · Jul 9, 2024

Jasonb said:
I would say 1/16" dia x 1/4 deep eg to meet the 1/8" hole from the other side in the middle.

Then counterbore 3/32" x 1/16" deep

Thank you Jason

The one I've made may be okay but may be a bit thin around the under cut. I'll order some more brass .

Mark_1984 · Jul 10, 2024

I’d agree with Jason, but I’d also check the drawings to see what the unions are connecting to and make sure the counterbores match whatever goes into the union.

Jasonb · Jul 10, 2024

I was thinking about that yesterday, as the counterbore is not a common type of union in the UK for steam models. I wonder as this tends to be a beginners engine that it was felt it would be easier for them to just make and solder a simple ring onto the ends of the pipe rather than make the more common nipple/ferrule

I have a feeling the small end screws into the main casting so can't really see the need for any form of counterbore of 60deg conical bore.

bruedney · Jul 10, 2024

I reckon it is a 1/8 thru hole (1/8 x 1/4 x 2 = 1/2) and then either side is counterbored to the sizes indicated.

Bruce

Sarah · Jul 10, 2024

Jasonb said:
I was thinking about that yesterday, as the counterbore is not a common type of union in the UK for steam models. I wonder as this tends to be a beginners engine that it was felt it would be easier for them to just make and solder a simple ring onto the ends of the pipe rather than make the more common nipple/ferrule

I have a feeling the small end screws into the main casting so can't really see the need for any form of counterbore of 60deg conical bore.

Sarah · Jul 10, 2024

You are correct Jason, the small end does go into the main standard in a tapped hole. I don't think there is any need for the counterbore.

Sarah · Jul 10, 2024

bruedney said:
I reckon it is a 1/8 thru hole (1/8 x 1/4 x 2 = 1/2) and then either side is counterbored to the sizes indicated.

Bruce

Hi Bruce, if it was a 1/8" through hole then the undercut on the thread end could go completely through.

Sarah

Sarah · Jul 20, 2024

The Single Oscillator standard was machined without any problem, then we moved onto the Twin Oscillator standard. I used my 50mm face cutter to mill the base and all the other flat surfaces. I reall love the finish the 50mm face cutter leaves, as can be seen in the attched photographs. Now I've machined the rear of the standard I will drill all the holes for the control valve before I move it again.

Richard Hed · Jul 21, 2024

Sarah said:
The Single Oscillator standard was machined without any problem, then we moved onto the Twin Oscillator standard. I used my 50mm face cutter to mill the base and all the other flat surfaces. I reall love the finish the 50mm face cutter leaves, as can be seen in the attched photographs. Now I've machined the rear of the standard I will drill all the holes for the control valve before I move it again.

That is the first time I have seen that twin. It looks rather large ( I mean comparatively), so I was wondering what the cylinder diameter and the stroke are.

Sarah · Jul 21, 2024

HELP!

Whilst drilling the back side of the Twin Oscillator standard, I've followed what i now think is an incorrect dimension on the plan. On the photograph below the hole in question, indicated by the pen, shows on the plan to be 5/32" diameter. While I was drilling it I had a doubting feeling it wasn't right. On reflection I think it should be smaller. There is no indication in the plan what the hole is for, I wonder if its for a stop for the reversing valve.

I would be grateful for any one else's thoughts about the hole and if it should be smaller the best way of sorting it out.

Many thanks, Sarah

Sarah · Jul 21, 2024

Richard Hed said:
That is the first time I have seen that twin. It looks rather large ( I mean comparatively), so I was wondering what the cylinder diameter and the stroke are.

Hi Richard,

The bore and stroke are 7/16". The standard is gunmetal which seems to machine very nicely, the cylinders are made from a brass extrusion.

Richard Hed · Jul 21, 2024

The castings of Stuart are generally really great, but the drawings have a lot to be desired. I realize that when the drawings were created, they were in an age where hobbyists only needed the most important dimensions--the critical ones, but I for one, want it ALL spelled out as that is what I am used to in industry. It would not be a big cost for Stuart to update those drawings with CAD, they should have someone "in-house" who can do it. I can do one of those drawings in a few hours.

Jasonb · Jul 21, 2024

I would say it should be dimensioned 3/32" but it is not critical if you have already drilled it. It is indeed to take a peg to act as a stop for the two lugs on the valve to limit it's movement.

You could just use a 5/32 peg or take a bit of 5/32" material and turn the end down to 3/32" loctite the big end into your hole but have the right diameter sticking out.

Although it may not take too long to redraw a set of plans there is little point in doing that without actually checking things work so really whoever does the revised drawing should first check the dimensions of the castings to see if the parts can actually be got out of them and then secondly build the engine to prove their revised sizes. In an ideal world a second builder to prove them is best as the designer may assume things that they knew they drew even if they forgot to put it on the drawings. That is going to take a lot more than a few hours, I should know as I have done it in the past.

Sarah · Jul 22, 2024

Jasonb said:
I would say it should be dimensioned 3/32" but it is not critical if you have already drilled it. It is indeed to take a peg to act as a stop for the two lugs on the valve to limit it's movement.

You could just use a 5/32 peg or take a bit of 5/32" material and turn the end down to 3/32" loctite the big end into your hole but have the right diameter sticking out.

Although it may not take too long to redraw a set of plans there is little point in doing that without actually checking things work so really whoever does the revised drawing should first check the dimensions of the castings to see if the parts can actually be got out of them and then secondly build the engine to prove their revised sizes. In an ideal world a second builder to prove them is best as the designer may assume things that they knew they drew even if they forgot to put it on the drawings. That is going to take a lot more than a few hours, I should know as I have done it in the past.

Thank you for your reply Jason, the photograph is very useful.

As it is not critical I might make a peg to fit and turn the end down to 3/32".

After a surgery I was immobile for a while and I drew up a set of drawings for the Stuart Progress. I used them when I made my Stuart Progress and they were useful, but I had quite a few changes to make to them. I would be happy to make them freely available, but only after someone else has tried to build a kit from them and verified the drawings were fit to use. It's not a quick process to change drawings in reality.

Sarah · Aug 10, 2024

I did think the Stuart Twin Oscillator would be a simple engine to machine, I was wrong! Ensuring everything is level, measuring, centering, drilling all the steam passages was quite a task. I also had to do some maths (Sin and Cos) to place some of the holes.

I had a bit of a problem finding the centre for the crank shaft as the casting was a little uneven at that point. All the other holes were placed with out any problems. I did break a drill bit in one of the angled branches, I'm not sure why it to me as it was only in a short way. I was fortunate in that I was able to fix the problem with a 3mm carbide end mill. Though the end mill was toast afterwards.

Although I took care drilling all the ports and steam passages, I was pleasantly surprised when I realised the ports went through to the steam passages.

Next the cylinders

Mark_1984 · Aug 10, 2024

Looks good Sarah. A lot of work for a small part!

Sarah · Aug 11, 2024

We have machined the cylinders for our single and twin oscillators. Still have the steam passages and the mounting holes to do for the cladding

Sarah · Aug 22, 2024

I've made the top and bottom cylinder ends, being careful to make sure the drawing sizes appers sensible.

However when I try to attach the top and bottom ends to the cylinders it looks like the PCD that the three holes is on, seems a bit too small.

I've checked the drawing over, that seems okay. The three holes in the top and bottom covers are all in the right place. The covers nearly fit, but not quite.

Has anyone come across this problem before?

Stuart Twin Oscillator build

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