If you need to accurately locate and space a couple of holes to match those in any item the job can be done very easily.
The example is one of our underslung LPG tanks, it needs a couple of reinforcing straps made up to mount it strongly to the vehicle. The following technique works well for any item that needs two holes accurately spaced.
It is so convenient that you do not even need to measure the hole spacing!
These two straps need accurately spaced holes....
Put a bolt offcut in the lathe and turn a nice sharp point on it (you can grind it freehand but it won't be as accurate). The angle doesn't matter as long as it is 45 degrees or more.
Now screw the bolt into the job (with the pointed end presented upward) a good few threads so it doesn't shake.
The pointed bolt will be your scriber .....
When the first hole has been drilled simply fit it over another bolt (don't let the workpiece go too far down the bolt or spacing accuracy will be diminished).
Then move the workpiece in an arc over the pointed bolt. Press down a little. The pointed bolt will scribe a nice clear arc across the metal.
To finish up, find the centre of the workpiece widthwise and centre pop. Then drill the second hole and have the pleasure of seeing both holes located so accurately that the workpiece simply drops over BOTH bolts, as shown in the first picture.
.....and the workpiece will be your Trammel
The techinique never fails, both workpieces drop straight over their respective bolts without any filing or opening up of the clearance holes. Note that we have 4 pointed bolts fitted in the tank mountings. This makes marking out all 4 holes on the vehicle's floor a piece of cake. We simply jack up the tank to the required position and allow the pointed bolts to make 4 centre marks on the underside of the floor, then simply drill the holes. It works every time.
The straps drop onto their respective bolts first time.....