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Marking out boiler tube.

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Chris09/08/2010 19:14:52
87 forum posts
13 photos
I have just started maring out the boiler for Northumbrian. I need two lines the length of the tube exactly diametrically opposite. What is the best way to go about this task? I have this feeing that the answer will be so obvious that I will feel rather foolish but not half as foolish as ending up with the dome and safety valve at a jaunty 5 degrees to the vertical.
wheeltapper09/08/2010 19:28:34
421 forum posts
98 photos
It could be as simple as clamping the tube to a board, then marking  down each side with a scriber set to the correct height.
but what do I know, Ive never built a boiler
JasonB09/08/2010 19:31:27
18930 forum posts
2083 photos
1 articles
I would stand it on a pair of vee blocks and use a surface gauge or better still a height guage to lightly mark either side at approx centre height. Rotate the tube 180 degrees and pick up one of those marks with the surface gauge than move to the other mark. Its unlikely it will be on the mark so repeat the process adjusting the gauge up or down until they meet.
You will then have the scriber on teh surface gauge at centre height and its a simple matter to run a line down each side. While you are at it turn the tube 90degrees and align your two marks vertically with a tri square and then scribe another two lines down the side of the boiler. You now have horizontal centre lines as well as top and bottom ctr lines
You could probably bodge up something if you don't have vee blocks and a surface gauge is not hard to make and is a very useful tool for marking out.
I should have said that all this should be done of a true flat surface, a thick piece of glass, porcelain tile or a granite chopping board from the likes of Aldi or Lidl will suffice.
KWIL09/08/2010 19:35:24
3309 forum posts
63 photos
Would it not be better to wait until you have built the boiler and before adding the tubes, mount the boiler under your mill and locate the top centre line?
Jeff Dayman09/08/2010 20:17:21
1896 forum posts
45 photos
Hi Chris
Justin case you have no marking out kit, you can still mark the boiler sides at 180 or 90 degrees easily.
Take a strip of heavy paper and wrap it around the boiler. mark a point where the strips overlap, with a fine width pen or pencil. remove the strip, and for 180 degree marks fold it half with the end of the strip and the pen mark lined up. Mark the fold line also. If you now wrap this around the boiler, you have marks at exactly 180 deg apart, so you can centre punch or scribe short line marks on the tube. To scribe lines along these marks all along the tube, lay a piece of known straight angle stock any size on the tube open side of the vee down, that is two edges of the legs of the angle touching the tube. Line up the edge to your mark and scribe away.
For 90 degree marks (ie chimney line vs boiler side lines) fold the strip twice in half and make 4 marks instead of two.
If you are careful and clever about the strip folding you can use this method for any number of divisions theoretically.
With care this method can allow markups accurate to +/- about .010" in my experince depending on the stretchiness and tension of the paper.
Hope it helps, JD
Chris09/08/2010 22:16:41
87 forum posts
13 photos
Thanks to you all. Jason, it was the fine tuning bit that I just knew would be simple.
Kwil, I see your point but being as this is my first boiler i need as many data points as possible. I'll be following Jasons idea of the 90 degree lines as well. Belt and braces and a length of string if it is available.
Jeff, that is wonderful. It sounds a bit Blue Peter but if it works to within 10 thou I'm up for it.
One thing is for certain, this is the third time since starting Northumbrian that V blocks would have been useful. Where has she hidden my debit card???
KWIL10/08/2010 08:59:37
3309 forum posts
63 photos
No V blocks??
You can use parallels bolted down, you can take two pieces of angle, clamp together and drill the common leg ("T") and then space with studding and nuts. NB angle stock is rarely at right angles or straight, even the material supplied to MEs, check it first.
Look at photos for examples.

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