O.B. Bolton Marine Triple Expansion Steam Engine using castings by AJ Reeves, drawings by John Bertinat

[NMBROOK]

Bolton part eighteen

When I was reading the DRO manual for my mill, I thought I would use the pitch circle diameter function to drill my cylinder covers/block. Seems that whilst I have the buttons for these additional functions, they are not included on my particular model

A bit like when you bought a base model car and there were blanking plugs where electric window switches would be, yes I am going back a few years now

So back to school it felt like for me, I include pictures of my "spider scrawl" just to illustrate Pythagoras had a lot to do with my hole drilling

Using the centre of each bore as my datum I calculated x and y coordinates for my holes. At least I only had to do calculations for one quarter as the rest are mirror images. I could have set the covers up on my rotary table but 14 divisions does not easily divide up 360 degrees, so I knew I would end up with some sort of cumulative error, plus I would still have to drill the block.

I referenced the inside of one bore at a time using my edge finder and fitted the respective cover with some dabs of medium CA on the flange.

My holes were then drilled tapping size through cover and block to the required depth. The holes were opened up to clearance size in the cover alone using the depth gauge on the quill. The CA was then broken by warming the cover with a blowlamp and the cover removed. The holes in the block were then tapped 7BA.

It was tapping the holes for the IP cylinder that disaster/ inevitable happened. My tap broke. Fortunately it snapped just above the hole and on the way out so I carefully managed to unscrew the remaining piece of tap. The tap had done a number of holes and 7BA is only 2.5mm diameter ! I have no doubt I will break some more in the process of tapping the myriad of 7BA holes on this engine.

So this stopped play. Another tap has been ordered and the holes with the black circles around them are just a reminder these are still to tap.

One detail I have added is three " jacking" screws to each cover. On the real engine these are just threaded through the cover and tightening these allowed the mechanic to pop the seal in the gasket. These have been tapped slightly smaller at 8BA which is around 2mm diameter. I will have the challenge later of machining some square head bolts for these holes.

 

[rtwpsom2]

This is one area that even the most basic CAD will shine. You can do circular patterns and then measure their location. Though remembering your old sohcahtoa calculations is impressive in and of itself.

 

[Namabiiru]

If you had to order one tap, I hope you ordered a small handful. I agree with you that at that size they are prone to quitting.

 

[Roger Pellett]

I thought that I was the only one who broke taps. It appears that I’m in good company!

Roger

 

[NMBROOK]

If you had to order one tap, I hope you ordered a small handful. I agree with you that at that size they are prone to quitting.

Only ordered one as I had to order some other supplies and that stockist only does sets of three, 1st 2nd and plug. They are only high carbon steel, I intend to find some high speed steel ones but rather not buy sets just plug taps as most holes on this engine are blind tapped and hss taps are more expensive

 

[NMBROOK]

Bolton part nineteen

With the broken tap replaced, I was able to complete the drilling of the covers and tapping of the block for all three cylinders



The last job on the covers before painting them was to add the Fracture grooves. As I mentioned earlier, these are present on the prototype incase of over pressurising in a cylinder. They are to encourage the Cylinder cap to breakup rather than damage the block.

To turn these on the caps, I first made a mandrel from stock steel bar. This ensures the caps are concentric and also perpendicular to the cylinders centreline.The Mandrel had a thread added to allow the cap to screw on using the thread for the pressure relief valve.The rotation of the lathe means tool force will tighten, not undo the thread so no additional fixing is needed.



The grouves were then turned in each cap using a modified/reground threading tool.



Final job was to use the covers as templates to make the relevant gaskets out of 1/64" gasket material.

The completed covers ready for paint and their respective gaskets. The little arrow indicates the small flat I added for paint clearance between the two larger covers.