I always tell budding machinists that the initial layout work is crucial to achieving the results required for precision machine work to take place in the machine shop.
Initially before the machine work could begin on this piece of billet aluminum that is part of a project that I'm working on, layout work took place on the layout table with a vernier height gauge being used to precisely scribe the lines in place as to where the holes were to be drilled.
Once the material had been secured in place on the angle-plate mounted on the table of my milling machine, the next step was to center the scribed lines where the holes were to be drilled in precise alignment with the center of the milling head.
To accomplish this I used the device you see here that I have owned for 20 years, long before there were laser alignment devices that are utilized today in machine shops.
This device has a built in light powered by two AA batteries that projects a beam of light through a ground glass which projects a bright dot on to the surface of the piece requiring precision positioning before the machine work can begin.
If you click on the photo and look close at the face of the aluminum material just back of the large 1" hole, you can make out the two scribed intersecting lines with the pinpoint of light centered precisely over them.
Once the light beam has been focused and is projecting a small sharp pinpoint of light, its as simple as using the X and Y axis movements of the milling machine table to center the work precisely under the center of the milling head.
Once this is done, the alignment tool is removed and replaced by the cutting tool required to do the work.
Here is where the initial layout work took place.....