Marking a circle in three

[Doh!Nut]

I dont know if this is a tip or a hack

I needed to mark-up three holes in a circle - equi-spaced

I put a light score on the face to show the correct radius -easy enough
For the 120degree separation I assumed that the Jaws on my chuck were accurately 120degrees apart.
I found that I could put a mini-level on the cut out for the chuck jaws.
This gave me a repeatable (and I hope accurate) positioning, it was really surprising how much bubble movement there was for seeming little chuck rotation.

I then put a light score on the face using the cross slide.
The three lines intersect pretty perfectly on the centre

Nick

 

[sortafast]

you could always go buy one of those plastic protractors that you used in grade school just to double check. Assuming you don't have one of these:

 

[gadget_lover]

Interesting tip. Thanks for sharring it

Daniel

 

[MCFLYFYTER]

That sure does look funny. A modern lathe with an old school level. I'm not trashing it, it just looked odd. It looks like it worked well.

 

[StrikerDown]

:shakehead What a HACK!!!!!

Actually that's not too bad an idea, but I am now wondering what you are making?

 

[Fred S]

another trick is to use a bar of the correct length between the front way and the jaws, the bar length is not critical, but it is nice if it hits teh jaw pretty square. I used to use a mag base with a length of allthread run into one of the holes and cut square on the end.

 

[Doh!Nut]

The Level is certainly old Skool, one of my Grandfathers

I can see how Freds suggestion would work also, I might use that as a cross check if I have to do this again.

The work piece is a heat sink for a 3 p7 Dive light. The positioning of the LED needed to be accurate as I have no room to spare to fit 3 * 28mm reflectors in a 60mm body.
The three holes on the back side has the mounting points to connect it to the tail cap.

Nick

 

[PhotonFanatic]

The fact that the three lines intersect perfectly is no indication that your three angles are symmetrical. They have to intersect perfectly if the tool was at center height. :devil:

 

[StrikerDown]

The fact that the three lines intersect perfectly is no indication that your three angles are symmetrical. They have to intersect perfectly if the tool was at center height. :devil:

Agree. If the lines are above or below center they will form a triangle in the center of the disk. This does not however indicate the outside end of the lines being equidistant on the circumference of the disk.

 

[65535]

I'm gonna go out on a limb here and say if it looks good by eye, and the method used would produce decent results in the first place that for a flashlight it won't be super important that it be perfectly perfect.

 

[precisionworks]

The easiest way to do that, and also faster, is to trig out the size triangle that will fit in the inscribed circle. Set a compass to triangle leg length, mark three times, all done.

 

[PhotonFanatic]

The easiest way to do that, and also faster, is to trig out the size triangle that will fit in the inscribed circle. Set a compass to triangle leg length, mark three times, all done.

Trig out a triangle? Who needs trigonometry, or rather, who really understands trigonometry? :devil:

Here's the mechanical solution, if you have a divider:

That creates your 60° angle, and then you just continue marking off the segments around the circle. Everybody had a Starrett divider, no?

 

[precisionworks]

Here's the mechanical solution, if you have a divider:

Very neat, without having to do the calcs Your method arrives at the same end point in a different way.

who really understands trigonometry?

Only enough to lay out parts, using the trig tables in Machinery's Handbook & TI-36X sci calculator (which does polar rectangular calcs a lot faster than I can).

 

[McGizmo]

With a compass (or divider) and PhotonFanatic's method, you can easily lay out the center points for a "7-UP" LED placement. If you set the compass to the center to center dimension you seek on the LEDs you then mark your center LED's center point and then draw a circle based on it. Mark any point on this circle as the center point of one of the external LED's and then use the compass from that point to mark the next one and on around the circle you go.

The sides of a hexagon are equal in length to the radius of a circle that includes all 6 of its points. In the drawing below, is a circle of diameter 1 and radius of .5 with an included hexagon. I think you can see how a compass can generate these points and in the example, the point on the right of the circle coincident with the axis was the first point chosen with compass scribes generated from it in a counter clockwise repetition. For three points, you would use every other point as you work around the circle.