Best way to machine a cylinder

[Michael CallaghanParticipant @michaelcallaghan68621]

Best method of doing the following job. I wish to make a split smokebox. I have made the cylinder out of 2.5mm brass. The design of the smokebox calls for holes to be made for the chimney, regulator arm, exhaust and cylinder pipes. So a few holes around the cylinder. Then I have the holes for the inner support for the split halfs. Then the slitting operation itself.
I propose to do the work on the milling machine. So what would be the best way to hold and support the cylinder during machining. The cylinder is 450mm o/d. Thanks

[Brian WoodParticipant @brianwood45127]

Can you cast cerrabend round it at some point that leaves access to all areas, including access to machine a flat base on the cerrabend for mounting the smokebox securely?

Brian

[JasonBModerator @jasonb]

At 450mm dia is there enough head room on your mill to do it on a rotary table mounted vertically on packing?

If so fix a disc of MDF or ply to the rotary table and then sandwich your smokebox between another disc of PLy/MDF. You can then rotate it round to do the various holes with drills and boring head and finally split it with a slitting saw which can cut through the ply/MDfF

Does it matter that it won’t be a full circle once the thing has been split? If you don’t want the saw gaps then make from two larger halves.

[Nigel Graham 2Participant @nigelgraham2]

I’ve tackled a somewhat similar task by making a mandrel laminated from old pieces of laminated chipboard on a central axis made from M20 studding; the assembly then turned to a close fit in the tube to give the metal full internal support.

I machined it on a large lathe with Tee-slotted saddle, using the lathe as a horizontal borer, but the same principle can be applied to mounting the work on a rotary-table. In that case the central bar is machined to fit the central hole of the table.

[Michael CallaghanParticipant @michaelcallaghan68621]

Thanks chaps. Some fun ahead for sure

[BazyleParticipant @bazyle]

Is 2.5mm brass going to be strong enough for a 2ft smokebox? Some of the things hanging on and off it are going to be a bit heavy. Edit – 18″ still big.

[Michael CallaghanParticipant @michaelcallaghan68621]

It’s well reinforced. Cost was a large factor in picking the material. It’s all getting very expensive these days

[Nigel Graham 2Participant @nigelgraham2]

Not only well reinforced but if this is for a locomotive, the chimney saddle is spread over a sizeable area, and the door ring and connection to the boiler barrel will add a lot of strength.

What is the engine? 450mm….   At the larger scales or, e.g, narrow-gauge practice, mild-steel is more usual for a smoke-box – and a lot cheaper than brass.

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