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My Minnie

Taking on a part finishing princess

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Nick Welburn24/05/2022 21:32:12
134 forum posts

Got started tonight. Had a look at the cylinder casting, lots of surfaces out of parallel. So I’ve set a new reference surface up and started to pull it together.

Now the bottom of the casting has been machined, but it rocks on the boiler so it’s clearly not concentric.

it’ll need a recut to put it right. I’m thinking could use my boring head to do an interrupted cut down that’s parallel ti the new reference surface.

Any thoughts?

Nigel Graham 224/05/2022 23:44:48
2287 forum posts
33 photos

I've not tried it for a cylinder saddle but others have, successfully.

Some use a between-centres boring-bar and set the work on the lathe cross-slide - assuming a slotted cross-slide.

Others use as I take it you mean, a boring-head in a milling-machine.

Either way of course needs a very rigid, accurate set-up; low speed and gentle feed, and to allow for the bar springing.

Unless you have some form of radius-measuring instrument, it would be worth making a radius-gauge from suitable, fairly thick plate (or if you have it, an offcut of the barrel material) to test the surface without disturbing anything.

JasonB25/05/2022 07:16:58
23070 forum posts
2769 photos
1 articles

between ctrs boring bar is more likely to give a parallel cut, unless you have a good rigid setup a boring head tool may deflect in the cut making it a bit smaller radius at the bottom of the cut.

Good thing with a between ctrs bar is you can easily remove it and test the boiler against the cut, unlikely to have the head room to do that on the mill.

What method did you use to end up with the rocking and are you sure there is no distortion in the boiler barrel they can move during construction.

Nick Welburn25/05/2022 08:25:49
134 forum posts

Hi Jason,

The cylinder was part machined when the project was abandoned by the previous owner.I have no idea how but the finish isn’t great.
My lathe doesn’t have a slotted cross slide. It’s an Amadeal 7*14 cj18b not a big machine. Thankfully the material machines easily, I think it’s a gun metal casting.

I’ll have a look at a between centres approach but I can’t immediately see how it could work.

Nick Welburn29/05/2022 14:34:11
134 forum posts

Right I've had play. Have I trashed the part, maybe....

So my challenge was broadly the radius under the cylinder casting was not the same radius as the boiler. In addition, the faces on the cylinder were not perpendicular to each other.

I chose a register plane, and pulled that straight. I've then machined the faces of cylinder to parallel / perpendicular.

I've also used a boring head to re-cut the curved face. This sits nicely. on the boiler now, it it isn't parallel to the top surface.

I could put a fresh cut over it, but how thin is too thin in the casting over the boiler? its sitting at around 4mm, so I could correct it, then I'd be at perhaps 2.5mm? is that too thin for a viable gun metal casting? I've then got to sort out the fact that the machined bore is likely no longer in register to the new outer faces..

Should I just spring for a fresh casting?

JasonB29/05/2022 18:26:07
23070 forum posts
2769 photos
1 articles

2,5mm is getting a bit thin and will also have a knock on effect to all the valve gear as it is likely to put the cylinder ctr line below that of the crank. The inner lifting link almost touches the boiler barrel as it is so any less and you won't get full movement of the reverser.

Nick Welburn30/05/2022 20:33:13
134 forum posts

Ordered a new one, instead of trying to sort out someone else's mistakes I can start on my own.

Went for one from GSS model supplies as it was a solid £25 cheaper than the AJ reeves one. Wonder if its just markup or if there is a difference in casting quality? Time will tell!

Nick Welburn02/06/2022 16:48:30
134 forum posts

Good progress today, started to get the water pump together, I think this is probably more complex than anything I've tried on a Stuart engine with multiple surfaces to machine and multiple interlinked passages internally.

I've got the bones of it sorted. Needed to order a few extra 40tpi taps and dies, and a reamer set. They should arrive from Tracy next week.

I hear from what I read that the water pump on Minnie tends not to be effective. I don't quite understand this as to why, are there any obvious mod's I should make? All I can imagine is that its prone to air locking? Does need additional bleed ports in the caps? say a 10BA set screw or similar?

The gland nut has a bizarre dimension on it that I don't understand. Its a 0.384", thats not obviously metric or imperial? why the random size?

Could well be linked to the next question... where is a good source of 'O' rings? I'll need them for the piston and the pump.

JasonB02/06/2022 17:22:22
23070 forum posts
2769 photos
1 articles

The heat of teh boiler makes them less effective as they warm up, you can add insulation between pump and pad or relocate elsewhere.

The 0.384" is so that the O ring is compressed enough to seal but not so tight that there is too much resistance.

I think Blackgates sell the various rings as a set.

Nick Welburn02/06/2022 20:16:56
134 forum posts

Awesome thanks Jason. I do wonder how I’d have managed to make anything without your sage advice. I’ll get some ceramic board to insulate the water pump. I’m also going to hide a manual one on the side of the tender as well. I want this thing to be useable

i was a bit nervous about getting this built, it’s starting to seem achievable. It’s just like a Stuart but with more bits and actually a chunk of the dimensions are non critical.

Nick Welburn03/07/2022 15:37:01
134 forum posts

Right then. I’ve got a new casting for the cylinder. Starting to think about how to tackle it.

I think I need to machine two reference surfaces one on the top and one perpendicular to that.
These will act as reference surfaces to allow me to set up to bore the cylinder and the mounting surface.


Paul Kemp03/07/2022 20:04:11
732 forum posts
27 photos

That is where I would start, then I would go for the third. Top, side and then carefully square to those the rear cylinder face and the bore, I assume it has a trunk guide? This needs to be square to the bore in all directions.

if you have access to Traction Talk you can see how I did the cylinder for my 6” - way bigger than yours and I have some and access too some big machines but the same principle. Top face was done on my omnimill with the vertical head then the valve chest face with the horizontal spindle on the same machine both at the same setting with the cylinder saddle appropriately packed up on the table and clamped down. Then transferred to the horizontal borer clocked up on an angle plate to the spindle the bore done, cylinder end location for trunk guide faced and the saddle cut all without moving it from the table.

one point to bear in mind if you are using a gasket between the cylinder saddle and boiler you need to machine the saddle to the diameter of the boiler plus the thickness of the gasket otherwise you will have a gap in the middle when you try to fit it.


JasonB03/07/2022 20:16:44
23070 forum posts
2769 photos
1 articles

I just did mine as Mason described in his articles/book all on the lathe getting the bore in early and then using that as the ref for all the other faces. If you do the bore and rearward cylinder end in one setting then they will be true so the rear cover and the guide bars fixed to it should also be square to the bore.

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