Saddle machining

[andyhigham]

Over 20 years ago when I built my sweet pea I fabricated the smokebox saddle from sheet steel. I made it tapered in to about 30mm wide under the smokebox which gave a nice straight run for the steam pipes but over the years it has become apparent that it does not provide enough support, the bolts holding the smokebox to the saddle will not stay tight.
I have bought the aluminium casting and thinking about the best way to machine it. My initial thought is to clamp it to the mill "upside down" and mill the attachment to the frame surfaces and mill the bottom flat. Then bolt the bottom to an angle plate on the mill table, clock up the previously milled part and bore the radius using a boring head.
Clamping will take some serious thinking about

[builder01]

I used a boring bar turned between centers to machine the large radius on the saddle. As you mention, you must first machine the bottom of the saddle. I did the bottom and sides on a shaper. Then clamped it to the saddle on the lathe. You get automatic feed this way also! My bar is 1" diameter, here is is cutting a 5-1/8" circle (2-9/16" radius).

DSCN0986 - reduced 1 by Builder16, on Flickr

DSCN0987 - reduced 3 by Builder16, on Flickr

DSCN0985 - reduced 2 by Builder16, on Flickr

[ettingtonliam]

I was lucky with my saddle, the radius needed was 3", and I had a 6" dia side and face cutter about 1/4" thick. I clamped the saddle to an angleplate on the mill, fitted the cutter to a spigot, and just traversed it up and down with the quill feed.

[Deleted]

I machined the bottom and sides first as others have stated. I supported the radius on some round bar and levelled off before machining. IIRC I did the rad itself by hand as the outside cylinder exhaust passage spigot is part of the casting and thus would obstruct any machine cutter. Luckily the casting was very close to size to begin with, perhaps that was Don's intention, I'd have to reread his words to recall if this is the case.

Pete

[andyhigham]

Boring the saddle on the cross slide is out as it is too tall for my 330mm swing lathe
I think I will machine the surfaces that fit between the frames and as much of the bottom as I can, then reposition the clamps and machine the remainder of the bottom.
If I then mark through the frames and drill the attachment holes I can bolt some pieces of square bar to the saddle and use these to clamp to an angle plate on the milling machine. These square bars can be clocked up to give me the boiler centre. Then bore using a boring head

[stevep]

Sounds like you now have a plan, Andy.