how to find the center of a circle?

[southpier]

want to develop a system for making pulleys from styrene stock.

I have a punch set which will net .189", .157", .128", & .096". for my most used scales of 1/25 & 1/24, these will be close enough to 5", 4", 3", & 2 ¹/₂" discs respectively. I know these are not exact but this isn't the conundrum.

to wit: after the discs are punched, i'll want to stack them on a piece of .020" brass rod for aligning & mounting.

Q: how can I get consistent centered holes in the discs?

unfortunately, my punch is a rough steel & plexiglass unit which all but prohibits me from scribing crosshairs and cutting them concentrically. it is similar to the micro-mark unit, but has guide posts on both ends of a single row of holes.

thanks for any help, jigs, or formulas.

[64SS350]

You could buy a set of 'through' punches, typically used in the steel industry. These are the same principle as your die cutter, with the exception that they have center points on the end.They should fit your cutting unit.

[Ace-Garageguy]

This is the simplest method I know of to find the center of a circle, and I use it frequently in the fab shop. All you need is a compass and a straight-edge. You don't even have to measure anything. Unfortunately however, your circles are way too small to make this practical.

http://www.mathopenref.com/constcirclecenter.html

[Foxer]

use a set of brass tubes that fit inside each other and use a sharpened rod to mark the center. Good if one tube will fit tightly around the circle you want the center of.

I asked this here and there are some good ideas in this thread ....

[Old Buckaroo]

I use one of these templates (available at art and drafting supply stores) It has "cross hairs" outside of the circle. I place the disc in the hole it fits and use tape or a scribe and straight edge to mark the center cross.

[Aaronw]

The circle template looks like it would work, and is better and easier than what I was about to suggest.

Once you have the centers marked a spring loaded center punch would probably do a good job of getting a good starting bite for a small drill. They are cheap ($15-20) and easy to use, just push down until they pop, on thin plastic you might not even need the small drill.

[southpier]

thanks for the suggestions and link to the other thread.

since I'm only making (4) sizes, I might try cobbling a "template" out of styrene similar to the above. if there's minor inaccuracy in concentric alignment, I trust there'll be other blunders to distract the viewer.

[Mike Kucaba]

I don't even know if I want to chime in here, but "transfer punches"is what I'd use. They have a point at the end and you TAP them to mark a spot on the surface of whay you want to drill a hole in. I don't know if a set would even have the sizes you require.

[southpier]

not sure of your trepidation, but a quick search let's me know my punch holes are anywhere from .002" - .008" larger than fractional sized transfer punches. I don't think tolerances need be closer than that, nor I capable of creating them.

so now the search begins for 3/32", 1/8", 5/32", & 3/16" transfer punches without having to buy a 28 piece set.

thanks.

[southpier]

well, seems HF has just what I need for a price I can afford http://www.harborfreight.com/28-piece-transfer-punch-set-3577.html

and since it's less than 2 miles from Hobby Lobby & Michael's in the other direction, I think we're going on a Field Trip this afternoon!

[southpier]

  On 1/6/2015 at 3:00 AM, 64SS350 said:

You could buy a set of 'through' punches, typically used in the steel industry. These are the same principle as your die cutter, with the exception that they have center points on the end.They should fit your cutting unit.

thanks; I just wasn't getting it. I really do need that second cup of coffee in the morning!

[Cato]

Joe,

If I understand...

May not work for you but I mark hole centers on a sheet of styrene first, along a straight line. Then use a common woodworking awl, sharpened to a good point, to punch the center holes. THEN use the circle template to mark the circles and punch / cut out.

Works on brass sheet too.

Edited January 6, 2015 by Cato

[sjordan2]

http://www.c-lineproducts.com/templates/6x9-Pie-Chart_Practice_Template_C-Line.pdf

Simple. Make a square around the circle, then draw a diagonal from each corner to the opposite corner.

Edited January 6, 2015 by sjordan2

[Ace-Garageguy]

  On 1/6/2015 at 9:13 PM, sjordan2 said:

http://www.c-lineproducts.com/templates/6x9-Pie-Chart_Practice_Template_C-Line.pdf

Simple. Make a square around the circle, then draw a diagonal from each corner to the opposite corner.

Sounds good, but to get an accurate center it has to be a square square, and it might be a little fiddly to do with a 1/8 inch disc already punched out.

His best bet, by far, is going to be to use a template similar to the drafting tool shown above, with holes just the right size for his dots to drop into. Then he can lightly draw lines continuing the lines on the template, and where they intersect, presto.

Really gotta watch line thickness on things as small as 1/8 inch, too...if you want accurate centers anyway.

[Roadrunner]

Two ways I can think of; print concentric circles on a piece of clear decal paper, use the decal on a clear thin plastic sheet, align your piece under the clear sheet, then mark center with a sharpened needle in a pin vise.

A variation of the method posted above, is draw four arcs, location is irrelevant, as long as origin is on the circle and the radius is longer than half the diameter, connect the lines at the intersections, and that's the center.

[Ace-Garageguy]

  On 1/9/2015 at 2:03 AM, Roadrunner said:

A variation of the method posted above, is draw four arcs, location is irrelevant, as long as origin is on the circle and the radius is longer than half the diameter, connect the lines at the intersections, and that's the center.

Yes, that is the way I usually do it in the shop, but I didn't find a quick video of that method, and it's still kinda difficult to do on a 1/8 inch circle.

  On 1/9/2015 at 2:03 AM, Roadrunner said:

... print concentric circles on a piece of clear decal paper, use the decal on a clear thin plastic sheet, align your piece under the clear sheet, then mark center with a sharpened needle in a pin vise.

That sounds like a good, simple approach. Maybe just draw circles on a piece of clear plastic, using a fine-line Sharpie and a circle template like the one above.

[Roadrunner]

  On 1/9/2015 at 2:12 AM, Ace-Garageguy said:

Yes, that is the way I usually do it in the shop, but I didn't find a quick video of that method, and it's still kinda difficult to do on a 1/8 inch circle.

That sounds like a good, simple approach. Maybe just draw circles on a piece of clear plastic, using a fine-line Sharpie and a circle template like the one above.

Probably almost impossible, but a useful bit of information nonetheless, for those that may be contemplating finding centers on larger circles, for whatever reason.

[southpier]

well amazon came through where the local harbor freight failed and my transfer punches arrived today. obviously I've spent the bench time tonight making pulleys to develop a system.

I cut the biggest piece of .030 styrene which would fit into my punch and shimmed it with a scrap. this way I could center & punch 4 discs of the above sizes before moving the stock again.

used the closest sized transfer punch to the hole in the die jig I have. only needed to spin it by hand to dimple .030 styrene. then followed through with the matching cutter punch. so I had a disc with a center dimple. in retrospect, I probably could do the same thing with regular jobbers drills, but I digress.

used outer two of one size and another one size down as a spacer, and strung them on a piece of .020 brass rod after drilling the holes and smoothing over with some 600 grit.

after a drop of zap acc and a few minutes wait time, chucked it into the dremel for truing & smoothing with a 400 grit stick and some klingspore sanding foam. also held the applicator from and with Tamiya green label against the pulley groove while it was spinning to seal it.

came out okay, but the real ones need a combination of .020 pulley faces and .030 spacer to be able to wrap chartpak tape around for a belt. maybe I can get to those tomorrow.

[Foxer]

Interesting. Hadn't seen transfer punches before and I see how they worked. It might make my problem of getting the center on the end of a solid rod also a bit easier .. just need a sleeve with ID same as the rod OD. And a punch that will go in.

You'll have to show your pulleys here Joe!

Edited January 9, 2015 by Foxer

[southpier]

these were from the first batch. all pieces are .030 styrene. to me they look bulky, but the steps making them will work. I think the middle piece will stay at .030, but the outsides need to be .020 or even .015. i'll probably putz around with them over the weekend.

next week's project: spark plugs.

who's got an idea how to drill a lengthwise .020 hole through a piece of .040 hex rod?

[Foxer]

Pulleys do look good!

  On 1/9/2015 at 6:13 PM, southpier said:

who's got an idea how to drill a lengthwise .020 hole through a piece of .040 hex rod?

Now we're into my problem. Most of my center finding on rods has been solved with the simple centering tool shown below. It's made from short peices of successive brass tubes that fit with close tolerances. The smallest piece is a sharpened brass rod, but I also use drill bits as the "punch" to mark the rod.

The white rod in the photo is .04" hex rod. Unfortunitly it is too small fo fit tight in the closest tube so will no twork with these tubes easily. Maybe some masking tape wrap would do it. Do look back at the link I posted as this is the same question I asked. other than it being a hex rod. a .02 hole in a .04 rod IS very small and tight tolarance. I have recently drilled some .04 hex rod but by eye. It got close enough to make a couple hex nuts when cut and didnt need to go 100% straight .. was "straight enough".

Edited January 9, 2015 by Foxer