Finding the center of a circle?

[bluestag]

I have made a disc, the same diameter of the disc supplied with a loco kit.   The 2nd one has a large hole in the middle.   It is a former for the boiler.   I want the firebox to have one as well, with a small hole as perfectly in the center as I can achieve, for a small screw to clamp it to the cab face.   So that I don't need to solder the boiler to the cab prematurely.  

 

I've googled it, and found a few suggestions.    All of which look manageable with wooden discs six inches or larger.   My disc is a bit over 1", and marking out on it is a challenge.

 

Does anyone have a method that is achievable on such a small disc?   16thou brass, btw.

 

Kevin

[bluestag]

To answer my own question.     I blued the disc with a blue sharpie.    I drew two chords on the face of the disc.   Using a ruler, they were four 7mm scale feet across.  Or as well as I could, and marked the half way point.   As well as I could.   Then used the end of the rule for a square, drew two lines perpendicular to the chords, at their half way points.   Where they cross is the putative center of the disc.  I have a nice small compass, with two sharp points and a screw to set the width very exactly.    I was able to feel the intersection of the two scribed lines, and measure out to the perimeter.   Then a 30thou drill (.75mm?) was felt into the point, and a small divot made.   The compass was used again to judge how true the divot was.   The divot was drifted as needed by holding the drill off perpendicular, and deepening the divot slightly.   Measured again.   Repeat about four times until the center was judged to be within 5 thou of being correct.   Drill thru.    The cab face is half etched down where it is visible, with rivets and other detail left proud.   There is a half circle defining the land for the boiler.   The disc was laid on the face of the cab, and being a bit smaller (16 thou or there abouts) than the land on the cab, it was possible to judge the placement of the disc as centered.   The drill bit was brought to bare, and a divot created.   Once it was a few thou deep, it was possible to see it thru the hole in the disc.   It was off a bit, so again the divot was drifted.   Eventually I drilled thru.

 

Then both pieces were drilled out to clear the screw that would hold them together.    A screw was soldered to the disc, and a nut run on it to tie it to the cab face.   The alignment was NOT perfect.   Not to worry.   Try though I might, I had not drilled the hole in the disc perfectly on center.   I dialed the disc around on the face of the cab, and found an alignment that was quite close.   I scribed a line in the disc to define the ideal alignment, and hey presto.   An hour's worth of work and two jewler's saw blades later, I am ready to make temporary connections between the cab and boiler.    I do hate to be rushed!

Edited October 26, 2023 by bluestag

[Camperdown]

Plenty of advice online:

https://www.wikihow.com/Find-the-Center-of-a-Circle

[GWR_Modeller]

You could have used a compass to draw a couple of concentric circles of about the right diameter on paper then aligned your disc on top, estimating its centering by eye, and fixed with a bit of glue.  Then mark from the other side through the paper which would have an obvious centre.  The trouble as you found with constructing perpendiculars is at small dimensions the thickness of the lines is a significant source of error.

[BR traction instructor]

...I would use a hole stencil as used by technical drawing creators to align the circle and allow a straight edge/fine marker across the marks provided on the stencil to locate cente...

 

BeRTIe

Edited October 26, 2023 by BR traction instructor

[D-A-T]

As a tool-a-holic I’d have used it as an excuse to buy one of these (I’ve already got one).

Faithfull CSQUARE38 38mm Finder Capacity Center Square https://amzn.eu/d/8BgJ5tP

[BluenGreyAnorak]

For a one off task, you could cut a square out of plasticard that the disc fits snugly inside and then draw lines across from the diagonals.

[peterfgf]

I made one similar to these years ago in an apprentice shop:

https://www.axminstertools.com/hand-tools/marking-measuring-levels/centre-finders

Peterfgf

[Deano]

I would set a compass to the radius of the circle you want to find the centre of. Then scribe arcs across the centre from at least two points of the edge of your disc. Where they intersect is the centre.

Edited October 26, 2023 by Deano

[bluestag]

7 hours ago, BR traction instructor said:

...I would use a hole stencil as used by technical drawing creators to align the circle and allow a straight edge/fine marker across the marks provided on the stencil to locate cente...

 

BeRTIe

The circle has to be a very exact diameter.   I soldered the former from the kit that is supposed to go in the smokebox to a bit of brass and fretted and filed until I had a copy.

[bluestag]

6 hours ago, BluenGreyAnorak said:

For a one off task, you could cut a square out of plasticard that the disc fits snugly inside and then draw lines across from the diagonals.

That would demand accuracy as much or more than the technique I used.

[bluestag]

7 hours ago, D-A-T said:

As a tool-a-holic I’d have used it as an excuse to buy one of these (I’ve already got one).

Faithfull CSQUARE38 38mm Finder Capacity Center Square https://amzn.eu/d/8BgJ5tP

That's a NICE tool!    Wish I had one.  And it's the right size.

[bluestag]

7 hours ago, GWR_Modeller said:

You could have used a compass to draw a couple of concentric circles of about the right diameter on paper then aligned your disc on top, estimating its centering by eye, and fixed with a bit of glue.  Then mark from the other side through the paper which would have an obvious centre.  The trouble as you found with constructing perpendiculars is at small dimensions the thickness of the lines is a significant source of error.

Yes, I was scribing the lines with an xacto knife.   One gets pretty fine lines.

[bluestag]

5 hours ago, Deano said:

I would set a compass to the radius of the circle you want to find the centre of. Then scribe arcs across the centre from at least two points of the edge of your disc. Where they intersect is the centre.

The disc has already been cut out.   It was never marked.   I soldered the prototype disc to some brass stock and cut and filed the stock down to a disc.   There was no place to put the pivot of a compass.

[doilum]

Place the disc on a sheet of graph paper. Construct a square round it. Join the diagonal corners. Centre located.