Trimming The Flues To Length After Rolling
There are two ways to flue a boiler. One is to cut the flues to the exact length needed at the time of installation. The other method is to cut the flues to the same length and then custom trim each one to the desired length during installation. Since access to the band saw used to cut the flues was limited, all the flues were cut to length at one time.
Invariably when the flue sheets are welded into the shell there will be slight variances in how parallel the two sheets may be to each other. In addition the sheets may have a tendency to deform due to the heat of the welding process. In the case of this boiler’s construction the two flue sheets were within 1/16” of being parallel however the welding process caused both flue sheets to take on a slight concave shape making the centers of the two flue sheets 3/16” closer to each other than the sheets were where they were welded to the boiler shell. In addition the flues were cut to a length determined by the drawings instead of waiting until the boiler was welded together and thus the flues ended up being about a 1/4“ longer than necessary. All of these variations ended up requiring from 1/4“ to 7/16” to be trimmed from the ends of the flues at the top of the boiler.
In the photo is the tool that was custom made for trimming the flues. It consists of a center-guided end mill in a sleeve. Housed in the sleeve, the distance from the end of the sleeve to the end mill inside the sleeve determined the finished height of the flue. In the center-guide of the mill was placed a long, guide rod that included a length of Teflon sleeving that acted as the guide and bearing within the flue. The center-guide was placed in a flue and the trimmer started rotating using the air drill set to a very slow speed. The trimmer was pushed onto the top of the flue where the cutting action of the internally mounted end mill trimmed down the flue until the outer sleeve contacted the flue sheet. The trimmer was removed and the next flue was trimmed.
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