Finding, marking, and drilling the perfect center of a circle

[ChuckD]

In a corollary to my question about motors a couple weeks ago, I am now wondering how to find and drill the perfect center of a very small circle.  I need to drill the back of the prop hub on my Avenger kit so I can glue it to the shaft of the electric motor.  If I'm off even slightly, the prop will wobble noticeably when the motor is turning.

 

I have a drill press, so I'm reasonably certain I can drill a straight hole... If I can only find the center.  The surface I'm working with on the back of the prop hub is a circle all of about 3mm in diameter.  

 

Anyone have any good suggestions on this?  I'd love to hear them.

 

Thank you!

[LSP_Kevin]

8 minutes ago, ChuckD said:

Anyone have any good suggestions on this?  I'd love to hear them.

 

Me too!

 

Kev

[Subodai]

I know how to use a pair of compasses to find the centre of a circle, but you'd struggle with only a 3mm diameter with that technique.. 

Any way to centre the drill head on something on it's drilling surface, mount the actual drill on the platform, then mount the prob hub in the drill itself? Would it naturally centre in the chuck (if it would fit)

Saying that, if the centre piece in question is precisely 3mm, you could mark it using Calipers by setting them to 1.500 mm and drawing them across the surface at varying angles, where they cross over should be the centre right? It's a similar technique to the compasses one, but you would be using a known radius, so you should find centre quicker.

Edited September 17, 2020 by Subodai

[dodgem37]

Found here:

https://jetsinc.com/measuring-and-testing/gauges/gauge-multi-use-rule-and-gage-general-tool-16me-measure-angles-center-finder/

 

Found here, and elsewhere:

https://jetsinc.com/measuring-and-testing/gauges/center-square-1-1-2-machinist-centering-hand-tool-center-finder-gauge/?gclid=EAIaIQobChMIiLmA2uPv6wIVRr7ACh2pwQ6SEAkYByABEgKQFfD_BwE

 

Other places:

https://www.google.com/search?q=circle+center+tool&tbm=isch&ved=2ahUKEwjzkfLZ4-_rAhUFEd8KHUZmACkQ2-cCegQIABAA&oq=circle+center+tool&gs_lcp=CgNpbWcQDDIGCAAQCBAeMgYIABAIEB4yBggAEAgQHjIECAAQGFDChwlYwocJYNSWCWgAcAB4AIABQogBQpIBATGYAQCgAQGqAQtnd3Mtd2l6LWltZ8ABAQ&sclient=img&ei=Oh9jX_OMB4Wi_AbGzIHIAg&bih=750&biw=1536

 

Sincerely,

Mark

[Greg W]

There is a shade tree mechanic way to do this.

 

1)  Use a punch and die set to make a disc slightly larger than needed. Drill the hole as near to the center as you can. 

 

2)  Slide the plastic disc down the drill bit onto the shaft and glue with super glue.

 

3)  Turn the disc at slow speed and use a sanding stick or small file to reduce the diameter. Frequently check with a circle template or calipers.

 

4)  When properly sized, apply de-bonder to the part and slide it off the bit.

 

It will take a couple tries but after practicing you will get a good result. 

 

Greg

Edited September 17, 2020 by GDW

[dodgem37]

Another way occurred to me.  You can use a lathe if you have one.  Chuck the hub into the spindle, then press a dimple into the part face with the tailstock quill.

 

Sincerely,

Mark

[Woody V]

Take the prop to a machinist and explain what you want to do. Have him make a drill guide out of round stock with two holes, one on either end, one hole large enough to snugly fit over the prop boss and another hole on the other end the size of the drill bit you plan to use to drill the hole. Slip the guide over the prop boss and drill.

 

Hope that makes sense. 

[ChuckD]

This is why I love this place.  An oddball question with a lot of really great suggestions.  Thanks, guys.

 

I don't have access to a lathe, nor a machinist, so those would be tougher, though I certainly appreciate the suggestions.  I may snag the tools linked by Dodgem37 and see how they work, but I had another idea first.  If I can determine the exact diameter of the full circle of the prop disc, I could potentially use the intersecting arcs method to make a paper template that would match up to the center.  It's a bit iffy, but it could work.  I may give that a go and see how things pan out then look at snagging the tools mentioned above.

[D.B. Andrus]

On 9/16/2020 at 7:45 PM, LSP_Kevin said:

 

Me too!

 

Kev

 

+1

[Bob MDC]

Just a random thought "vintage Port no sorry Madeira tonight" looking a jewellers tools :

 

A 3mm tube to fit over the part then a tube that fits the inside the tube a shank drill then spacers to fit the drill into the tube PVA to hold it together if you drill slowly it should work.

 

Bob

[phasephantomphixer]

Since the tip of a drill bit is center, use a drill bit same size as part (3mm) sleeved

with metal tubing that drill bit fits into as guide and turn bit enough to mark center.

[Aerobat]

I tried making my own center finder decades ago, but found this to be a quick and cheap tool. I picked it up at Princess Auto (Canada's Harbor Freight tools).

 

Center Finder

[chrish]

 

5 hours ago, Aerobat said:

I tried making my own center finder decades ago, but found this to be a quick and cheap tool. I picked it up at Princess Auto (Canada's Harbor Freight tools).

 

Center Finder

Thanks for posting, that looks really handy!

[Greg W]

A important consideration of Chucks question is, how to have a perfectly centered hole in a small plastic disc, 3mm in diameter. Most of the suggestions here are excellent in general but not for a thin, tiny styrene circle. I fear the small disc may be to small to be used with some of the tools suggested. Using a Dremel or variable speed drill as an ersatz lathe (turning at low speed) will allow such a small disc to be shaped to the desired size and have the hole centered. 

 

Question for Chuck:

 

Are you trying to center a 3mm diameter hole in the center of larger disc? Or is the disc itself 3mm across? Post the outside diameter of the disc and the size of the hole needed at the center (I am going on the assumption that the disc will be a plug or backing plate at the back of the spinner).

Once I know that, I will make one and post images/process description here, so you can see how it's done. You can do this on your drill press too but I will demonstrate it with a Dremel tool, since more modelers have access to that vs a drill press.   

 

I have to say, the telescoping tube technique looks to be a great hack (I mean that in a good way!) but it seems that the modeler would be restricted by the fixed diameters of the tubing. If you don't have the correctly sized tubes, your out of luck possibly. With the method I am recommending, simply turn down the part to whatever outside diameter is needed. 

 

Aerobat, that center finder looks great. Thanks for passing that along, going to pick one up.

 

Greg

 

 

Edited September 22, 2020 by GDW

[Rick Griewski]

Trap the part in a square that has true 90 degree edges or close.  LEGOS come to mind.  Draw diagonals corner to corner.  Rotate the part then draw again.  A very good center will present itself. 
HTH

 

Rick