Construction of  Scribing Tool for Fuel Tanks etc

Tools 'n' Tips Article by J B Wolstenholme

 

 

Editors note...#1, #2 #3 etc refer to numbers in photos.  [1], [2], [3] etc refer to notes at bottom of article.

For years I have been using make-do methods for scribing/re-scribing fuel tanks and the like, after losing detail in the construction stage.  The idea is based on the lathe principle, but using easily obtained materials and not requiring workshop facilities[1] to produce. As can be seen in the photos, I have used a Clarke Spreader Clamp[2]
#1  2BA locking screw to locate sliding mechanism. Tapped
[3] for 2BA bolt. 
The cut thread was then 'wiped' with thin cyano to strengthen. #2  Scrap resin
[4] piece with formed channel to saddle clamp on bar (33 dia x 17mm) #3  Scrap resin[4] plate (33 dia x 4mm) drilled and fixed to #2 with panhead self taping screws. Offset one screw to allow for locking screw #1.  #4  Scrap resin[4] 'sidewalls'(4mm thick). Glued one side, then fitted second whilst feeding brass rod pivots through holes for brass tubing. Used cyano glue. Brass tubes/sleeves then  fitted left and right.  #5  Brass collet[5] (Allen key grub screw to fix scribing tool) and a length of brass tube, together with the brass pivot rods (#6) were soldered into pre-drilled holes (at 180 deg) in collet, in one operation.  #6  Brass rods, need to be a 'snug' fit in #7 to prevent sideways 'play'.  I didn't have correct rod/tube sizes, so I reduced diameter of rod to fit tube
using sandpaper.  #7  Brass tubes for pivot. #8  Scribing tool ground from 4mm silver steel rod. It is secured in and removable from brass collet/tube. Modify as necessary for commercial scriber.
#9  Removable clamp plastic pads.  Drilled at centre points to take tanks etc.  If you drill an array of circumferential holes around the centre points, it allows variations in component size/angle of scriber.
#10 Plastic (10mm) and 'pin', were removed from the inside to maximise travel of 'sliding mechanism'.

Click on images below to see larger images

  

Notes:
[1] Modeller's vice, hand drill, junior hacksaw, files/sanding block, micro torch for soldering.
[2] Available in 4 sizes; this is 300mm (UK supplier, Machine Mart). The button gives lock release and the lever, finer jaws adjustment.
[3] 'Homemade tap', produced by cutting 4 slots x 8mm (at 0, 90,180, 270 deg) in a 2BA brass bolt and very slightly tapering the end with a file, to thread the pre-drilled 4mm hole.
[4]  I keep all casting blocks from resin parts. These I think came from Aires stuff.
[5]  R/C aircraft modellers use these for wheel fixing.
J B

Photos and text © by J B Wolstenholme