Drawing A Straigt Line Over A Length Of Bar Stock

[grepper]

This is not Tormek related and I wrote it to post on a machinist forum, but it's a sort of a nifty little trick you might find useful sometime.

I am machining a new handle for the compound slide on my lathe.  I hate the tiny little handle that comes with it.  Every time I turn it another layer of knuckle epidermis is removed, and the handle part is fixed to the cross piece that bolts onto the screw.  So I'm making a new, longer one with thrust washers in the handle for smooth and knuckle bashing free operation.

I need to drill two holes in the cross bar, (1) for the bolt for the handle, and (2) for the hole for the bolt to attach it to the screw.  I don't have a X/Y table for my drill press, so I need to draw a line straight down the side of the cross bar so the holes are perfectly aligned. 



Seems simple, but it proved more difficult that I thought.  How to hold a ruler against the surface and know it goes perfectly from center to center down the length of a bar?  If it's off just a little...  I know, I could put a protractor on each end... But one end is rounded, making that method difficult.  I could try to carefully measure it – problematic.

It's night here, and I was sitting on the couch next to my wife while she watched some used episode of Criminal Minds.  The living room was mostly dark, save for a light on a table at the end of the couch.  So I was sitting there holding the part pictured here, scratching my head and puzzling over how I could accurately draw the line, when I noticed something. 

Did you see it too?

So, I shined a flashlight down the length from one end.  Bet you see it now.



The orientation of the light does not matter.  As the light is moved, it just moves the reflection around the circumference of the bar. Of course the reflection is still perfectly straight down the length of the bar.

Anyway, it's a super quick way to get a perfectly straight line down the side of reflective bar stock.  I thought it was cool.

[Ken S]

Clever, Grepper!

Congratulations, you get the Popular Mechanics Home Workshop Man of the Year Award.

Ken

[Elden]

   That is a great idea, Mark. I will have to put that in file A. Probably will forget where I put file A and only be able to find B and C when it is needed.

[Herman Trivilino]

Note that that will work provided the surfaces are concentric and have zero curvature. For example, each of your sections is the surface of a concentric cylinder or right circular cone. If they were instead concave or convex along the direction of the axis, the reflection of the light rays would converge (if concave) or diverge (if convex) and then the light line would be curved. Then I don't think this trick would work so well. But I'm not sure. Some experimentation is needed!