So, I do remember high school geometry when the taught me how to find the center of a circle with two arcs. But somehow, I always end up off by just enough to be visible. SO....

What techniques do you all use to find the center of a round piece, for example a piece of dowel, a disk, or in my case a cylinder base? There's got to be something better than eyeballing it.

us Cabinet Makers do it yet another way that is proven to be flawless.

a straight line anywhere thru the circle, then bissected and using a square. then along that line you divide to find exact center.

or use this method,
http://www.makeitsolar.com/science-f...cle-center.htm

the square and straight line is simpler too, but here is the compass method,

again, explanations are here,
http://www.makeitsolar.com/science-f...cle-center.htm

Use any 90 degree object to draw a square around the edges of the circle. Lines drawn corner to corner through the circle will give you the center.

That method is a little different from what the taught me in high school (you know, bisect the dinosaur with a antelope bone)...but it looks just as troublesome.

yes it is, that's why we do the srtaight line, bissect with a square, then measure to center.
you asked for the compass method though.............takes 15 seconds to do it the CM way

look at the 4th image uploaded above and that's what i am talking about.
once you have those 2 lines, it's done.

Lay your dowel in a 'V' block on a flat surface. Use square to find center by lining up square to apex of 'V'. You can rotate the dowel in 'V' block to mark again and where the lines intersect is center of end of dowel. If using drill press then lower bit into 'V' block to center the block. Clamp block. Raise bit then lay dowel into 'V' block. Now, you are ready to cross drill hole. If you want to end drill or mark the dowel, you could also make a series of tubes that slip within each other. The dowel will fit inside and you can start to end drill using this as starting guide. I made a aircraft ply jig with different sized holes to fit different sized dowels with a small 'on-center' hole for drilling. End drilling dowles isn't very accurate- the bits try to wander!

I'll actually be drilling my resin cast cylinders and I managed one with the eyeball method. But figured there had to be a better way to do eight more. I like the idea of making up some sort of drilling guide. The challenge is that the resin cylinders (with details) can't really be "clamped" under the drill press. On the first one I drilled a small guide hole, then a larger hole and a larger hole before finally using a Foster's bit while holding the cylinder in my hands (wrapped in a cloth). It wasn't an ideal operation.

DOH!!..just realized what Don was asking........ARUP got it while the rest of us overcomplicated things

glad someone had their coffee today

You'll prolly have to make some kind of drilling fixture for something like those resin cast cylinders. I was 'reading' into your post since I know you have built using dowels as framework and such. Your Alb looks very nice BTW.

This is what I want to do, but with a better method than I used before.

Looks like you are trying to lighten them up more than anything, right? If so drill as large a hole as you dare then rattail file the rest. Can the plug be modified so that the cast part has the hole down center?

Abufletcher,

Using a regular compass, just draw a circle with the diameter of the base of your cylinder on a piece of paper or cardboard.

Cut it around and place it over the base.

The center marked by the compass on the piece of paper will be visible for you to mark thru on the cylinder.

Even after finding the center, drill a pilot hole for the forester bit to follow. I would be more inclined to drill with regular bits stepping up a size or two each pass. For me, I would not go over 1/4" so you could use a dowel for mounting. Might not be lighter but it would be solid. A bigger casting and I would want it more hollow.

After you mark the center of the base of each cylinder, you could build a wood clamp to hold the cast part while you drill it with the press drill.

That clamp will fit over the round regular surface.
A piece of leather will protect the part from scratch (or from accidental rotation when the wall of the cast part becomes thinner and thinner).

To align the hole with the part, a hollow support (like a metal can) over the table will help.
Since the material is soft, you may be able to hold the wood clamp with your hand while you drill bigger diameters in small steps.

Lots of good, practical ideas here. Thanks! I still don't know exactly how I'm going to attach the resin cylinders to a 1/16" ply base. I was thinking of just epoxying them in place but am concerned about engine vibration. I could use the dowel idea but here would only be a 1/16" bit of dowel in a hole.

The dowel can pass deeper and more glue put on from under. Have 1/4" pass below.

The thing is that the dowel CAN'T go any deeper at least for the first two cylinders since base plate would already be sitting right on top of the engine mounting flange.

The clamp in the sketch seems a good idea but I would favour supporting the other end too to ensure it does not go out of line. I would not hold anything by hand when using a Forstner bit or similar. If the bit grabs it can throw the work out for your hands and maybe into your face! I was lucky to dodge the wood and not get my fingers drawn into the bit on the only occasion I tried this! Never again.
If the parts are costly then it would be worth using such a clamp fitted on the faceplate of a lathes to bore the centre away. Find a friendly model engineer with a lathe!
To find the centre of a circle for practical purposes these gadgets are the thing.....
The first is a simple plastic give away device used by woodturners and the other a Moore & Wright No 825A for engineers. Both use the same principles, 2 lines are drawn at about right angles and a third to check for errors, they should all coincide at the centre. There is also 'jenny leg' (odd leg ) dividers for the same purpose where the non pointed leg goes on the outer surface of the cylinder to scribe the arcs.
[img][/img]

You are welcome.
What about this one?
Any bolt would need to be manipulated from outside the array of engines.