Stuart 10V Steam Ports - How to drill 'em...

[mwilkes]

Hi folks - new to this forum and building a Stuart 10V (my second engine - first one was a little stuart oscillator - http://markwilkes.blogspot.ie/2012/02/stuart-steam-oscillator-part-iii.html).

My question is this: why do they ask for diagonal steam ports in the cylinder on the plans? Surely it's much simpler to go straight down from the port face, then straight in at right angles from either end of the cylinder?

That way you don't have to compensate if you bored the cylinder slightly larger than specified. I had to bore out to 20.5mm because the casting was so badly cored....i'm terrified of drilling at x degrees and missing the steam port :hDe:

thx

 

[kvom]

Assuming that's a slide valve, I agree that it's a bit nerve-wracking. However there's not really much choice. The passages need to open at the end of the cylinder bore and be sealed by the heads. There's really no good way to have a passage that's parallel to the bore.

I've found that once the cylinder is in the vise at the correct angle, it's very possible to position the drill off to the side and verify visually. You can make the steam ports deeper if needed. Since you are drilling at the lip of the bore, start by milling a flat.

 

[Philjoe5]

If you drill straight down from the port face and then straight down from the end of the cylinder, you still risk not having the drilled holes meet.

Drilling holes at an angle is really not difficult. Start the hole with a centercutting end mill of appropriate size to ensure the drill doesn't wander.

I do know your concerns though. This is one of the steps where you're really trusting the designer to have made the right calculations but double check the angle on the plan with a protractor and also calculate the distance for drill breakthrough to the port so you'll be ready to ease up on the quill feed.



Good luck.

Phil

 

[gbritnell]

You can definitely drill straight down. The only thing that will be critical is the depth of the port coming in from the valve face. I would advise to drill the ports from the top and bottom of the cylinder first then when you're coming in from the valve face you will feel the drill break into the existing hole so you'll have a better idea of when to stop.
Just for your knowledge it's not really that hard to figure the angle needed. In trigonometry terms it's called the tangent of the angle. You can calculate how far from center your vertical end port will be. You can measure how deep the valve face port is. Subtract the 2 dimensions and you have one side of the triangle, it's called the opposite side. Now calculate the dimension from the top of the cylinder to the centerline of the closest end port on the valve face. This is called the adjacent side. Now divide the opposite side (small number) by the adjacent side (big number) and this will give you the tangent angle of the triangle. You either need a trigonometry chart or almost everyone has a scientific calculator these days.
Let's say the measurement from the position of the vertical port to the depth of the horizontal port is .125. Now the measurement from the head face, or the depth of the spotfaced pocket to the centerline of the port is .438. Divide .125 by .438 and you get .2854. This number will be the tangent of the angle which is 15.9 degrees.
Simple trigonometry is quite easy and is sure helpful for the modeler.
gbritnell

 

[mwilkes]

Thanks folks - great responses. I'll do some maths, bite the bullet tomorrow & letcha know

 

[Brian Rupnow]

MWilkes---In this age of 3d cad, I sometimes forget that not everyone has 3D cad software available at their fingertips. However, we all still have access to pencil, paper, straight edges and protractors. Your cylinder should be small enough and your drawings clear enough to draw it out full scale, or even double full scale to satisfy yourself about where exactly things are going to end up in the area of your steam ports. Good luck.----Brian

 

[mwilkes]

@Brian, funnily enough, that's exactly what I'm doing now!

 

[mwilkes]

Did the maths, drilled the holes. Warm feeling of pleasure and relief seeing the drill coming through in the right place!

Centre-drilled straight in just a teensy bit further than the milled slot:

Used my protractor contraption to set the angle - this is a bit precarious...

...and lo and behold, light at the end of the tunnel.

Thanks chaps.

 

[Brian Rupnow]

I knew ya could do it!!!---Its only the first time thats a bit scary.---Like some other things I won't mention!!!---Brian

 

[Philjoe5]

Nice work. Looks like you did your homework. Congrats Thm:

Phil