Quote from: matt sanders on Aug 25, 2014, 07:46 pm
Fixing to the Roof:
-   Does anybody have theories about how the metal Base was attached to the concrete Roof?
-   I wondered if it might just be cemented on, but note that in each of the four sides is a Weep-Hole, to allow water to escape, so cement might have fouled that drainage.
-   Perhaps there was an additional metal bracket, that isn't shown here.  But note that the top of the concrete shell has five holes in it, which are not blocked.  (Central hole for lamp cable, and four ventilation holes.)

Keeping it simple:

-     Take a 3/4" O.D. pipe (the nominal diameter of the centre hole in the lamp base bracket if my calculations are correct) of appropriate length to use as an attaching rod & conduit and thread both ends, one (the top end) threaded significantly further than the other (exact length calculable once drafted).

-     Thread a 3/4" I.D. nut on the short end and slide a bracket over the pipe's other end and down to the nut to allow attachment of the inside lamp base that connects to the interior roof/wall conduit once installed (and to distribute the load on the concrete when torquing the nuts).

-     Push this assembly up through the roof and have someone up on top drop a flange washer over the pipe (to distribute the load on the concrete on that side), thread on a 3/4" I.D. nut over the top of it and torque this assembly down.

-     Place the cast iron Beacon base with attached cross bracket down over the protruding pipe end, nestling it over a raised area that acts as a key in the roof (we know Mark 5's have these). Thread another 3/4" nut over the pipe and torque down the base.

-     Thread a reducer over the top of the pipe to bring the O.D. thread diameter down to the 1/2" standard for lamp fittings.

-     Make electrical connections to the lamp holder and thread on to the reducer, making sure to ground the whole assembly using the third wire of the cord fed through the conduit.

-     Attach the struts and lens, insert the lamp bulb, and attach the Beacon cap.

-     Done.

I don't know if this is the way it was actually done, but it seems dead simple and allows replacement of the base without having to mess with the interior fittings at all.

To make it even simpler, if everything has been designed and built to the right specs, you could potentially eliminate the washer and first top conduit nut if the Beacon base's cross bracket is designed to fit flush with the slightly raised, flat-topped (square) key in the concrete roof by just attaching the bracket directly to the roof using the threaded conduit and nut (the bracket being attached to the base by four screws, it would still allow replacement of the main part of the base independent of the cross bracket or other electrical considerations in future). It wouldn't leave a lot of room for errors in design and build, however (though you could use washers as spacers if there was a small gap).

Next time either of you are at Crich, if you have access to a small camera on a semi-flexible cable like an endoscope (hopefully with its own light source) you could attempt threading it up through one or more of the vent holes in the ceiling to try to see what the current attachment method is, as well as what type of lamp holder/fixture is being used.

Another thing you can do (after turning the power off) is unscrew the cover to the "base" fitting for the interior hanging light (it's held on by two screws per the pictures Mark took) and take a look at how that fixture's base is attached to the ceiling. You might also see any conduit that goes through the roof and how it is attached.

EDIT: In looking at those Mark 5 boxes again, the square key area under the lamp is rather low profile so there is little doubt that there was a gap between the attachment bracket and the top of the raised key that "locked in" the lamp base. If you look at Mark's photos of the bottom of the base, there is what looks to be a cleaner circular area that has less rust around the central hole in the bracket. This may be confirmation that there were flange washers used to fill in that difference to allow more secure attachment of the base without deforming the bracket or stressing the base (cast-iron is rather brittle under stress).

Since doing these diagrams for the Crich Lamp, I've visited the museum again, and made my own replica lamp (see the Build Diary section). 

But anyway, this extra visit showed me that - despite what these photos seem to suggest - the sides of the Square Base and Top Disk are in fact vertical (not slanting), and the Top Disk (around the Dome) is in fact horizontal, and not slanting.

So, the true design is slightly simpler than that shown in my diagrams.  I'll update the diagrams when I get a chance, but thought I should make a note here, in case anyone is treating them as gospel(!)

Matt, how is the Crich lamp base in comparison to the lamp base on the Trench plans? (Or the entire lamp for that matter?) If they're the same, you could just cut-and-paste the Trench lamp parts as a separate diagram. That should save you a little work. (Shouldn't it?)

Dino.

Ask and ye shall receive. I missed this when it was posted.  I was going to ask about the lip on the cap because I want to add one to my TARDIS' lamp.  The whole thing looks significantly larger than a TARDIS lamp.  Of course the roof, base, and corner posts are all more substantial (along with the bottom crossbar on the walls.)  Looks like I'm going to have to get out the ladder and the tape measure next chance I get.