Press fit question

[uuu]

I need to know how big to make Jessie's front and rear smokebox rings.

The smokebox tube is steel, 5 7/8" OD, 5 5/8" ID, which I've turned from a larger lump (not all the turning was done in the Myford). The front and rear rings are gunmetal castings and will be turned to a shoulder, for insertion into the steel tube by about 1/4". The drawing says "5 5/8 inch press fit".

No other fixings are shown. Potentially the front ring could be a lighter fit, to be removeable; but the rear ring is there for good, and holds up the front of the boiler.

So the question is - how much bigger than the steel tube should I make the gunmetal rings?

Wilf


P.S. Other threads have already noted lack of tolerances on most of our drawings, so I'm not trying to re-open that debate.

[jma1009]

hi wilf,

surprised no one has replied yet!

i dont think a press fit per se is required as loctite can be added. however aim for a press fit. this isnt always easy with a smokebox as the slightest distortion or 'out of round' could easily result in a sloppy fit. in my experience a what might at first appear to be a very tight press fit quickly becomes otherwise - holding up to a good light with the front ring partly pressed in will show up bits of daylight where the tube isnt quite round.

do the front ring first. try on both ends of the tube.

be very careful with the rear ring as overtightening of the 4-jaw jaws will distort it.

taking into account small distortion of the tube, i would machine the front ring 5 thou over size to start with, and very small reductions may be required afterwards plus the use of a leather hide mallet.

cheers,
julian

[jma1009]

i should perhaps have added that in my case i dont make the smokebox till after the boiler is made - making the rear ring fit both the smokebox and bored out first for a good fit on the front of the boiler barrel. i wouldnt make the smokebox till after making boiler.
cheers,
julian

[springcrocus]

Mar 15, 2015 23:34:52 GMT jma1009 said:

i should perhaps have added that in my case i dont make the smokebox till after the boiler is made - making the rear ring fit both the smokebox and bored out first for a good fit on the front of the boiler barrel. i wouldnt make the smokebox till after making boiler.
cheers,
julian

Useful advice, thanks for that, Julian. I have yet to make my smokebox and boiler and shall remember your comments.

Steve

[uuu]

Thanks, Julian, that's useful. Since i have turned the somkebox tube, I'm hoping it's closer to round thsn a rolled one. In independent discusussions I've been advised to try and increase the length inserted, as far as the castings allow, and to try and add a few rivets.

I'll let you kniw how I get on, though it may be a while, at my slow pace.

Wilf

PS I have bought a boiler. Cheating.