Compass Cutter Really Sucks!

[cdavenport]

Originally Posted by Getter 1

Medved,

I have a compass cutter very similar to the one in your pic there and I too had similar troubles with it at 1st. Eric covered all the tips I could've given especially the hold the cutter stationary and rotated the part :P

I usually cut all small circles by hand so the cutter hardly ever gets use for anything other than circle bulkhead parts.

However I do toss this question up for any who can answer.

I have bought my 1st Airship model of the USS Macon and of course it has circle bulkheads. They are fairly large and so I want to use the circle cutter on them. Now most models I've come across till now have had the centers marked, this one doesn't. I was wondering if anyone had a good tip on finding circle centers.

G1

Somebody skipped geometry class. You will need a dictionary to define the terms because I am a teacher and now it's time to pay the piper!

1. Draw a chord on said circle, perferably one that has an even measure.
2. Bisect the chord with a ruler divided in 1/64ths (the smaller the scale, the greater the accuracy).
3. Locate the midpoint of the chord with the ruler.
4. Draw a perpendicular to the chord at that point that bisects both arcs of the circle.
4. Measure the midpoint of the perpendicular.
5. That is your center.

[cdavenport]

Oops! Forgot to mention; airships are not perfectly circular in their cross section. Because they were constructed from duraluminum frames covered with fabric, the fabric stretched and sagged between consecutive longerons.

Actually, the look of a dirigible cross section is the same as modern computer algorithms that represent circles in games and CAD drawings; circles are constructed as a series of straight lines arranged equidistant around a central axis.

[Getter 1]

Cdavenport,

Well I did take Geometry and did well in it but the formulas didn't stick in the noggin as it was over 15 years ago :P I completly forgot that method. Man i really need to find a good Geometry book for reference. Funny saying I "want" a school book :P

Thanks everyone for pointing out and even reminding me how it's done :D

Hee hee I do a lot of my design work not in CAD but in MS Paint using geometry but I only needed a few basic formulas. Now wher I have trouble in this method is figuring out how to make cones using old school methods. CAD does all that stuff for you but I want to know how to do it the old way before CAD.

G1

[Getter 1]

Medved,

Sorry for getting this off topic :P

G1

[John Bowden]

On cuttin small(er) circles........ -JimG posted a picture a while back showing the build of his design of the DH Beaver....... he had a pair of finger nail clippers on his workbench........ and I had a Homer Simpson DUH! moment.

The curved shape lends itself to shaping small circular disk from paper.

Hey give them a try......... what do you have to lose at about a dollar (US) a pop.

[Getter 1]

That's an idea that's bizzare enough to work.

G1

[rman]

I use nail clippers all the time on small parts. I have straight, concave, and convex ones

[medved]

Thanks for all the feed-back everyone. I think I have some more good ideas to start working with now.

On circle centers - I always use the way I was shown in Tech Drawing at school -
Using bow compass
Select a circle; :p
Set bow compass to a guess-timate of slightly more than the radius;
Set point on outer circle edge (i.e. the actual line) draw an arc bisecting the center;
Set point on a spot roughly opposite & draw another arc.

The center of the X is the center of the circle.
One can do more opposing arcs to increase the accuracy.

Using perpendiculars takes slightly longer but these can also be constructed using the above method but starting on a straight line.

Quick & easy to recall. Will have to test this method against SCEtoAUX's tool.

@Getter1 - no problem - & similarly off-topic, u just reminded me of how much I liked Spokane & the area of Mt. Rainier etc (& all those trees waiting to be turned into models... LOL) Briefly lived down by Puget Sound. Had a great view of the water & the Cascades that I often miss.

@The Hermit - I have been thinking that might ultimately be the 'only' way to really do it, not to mention that the disciplined focus pays off in other ways. (Case in point - my home repairs have improved since I started learning this art )
A lot of Old School stuff has way more longterm & broad-based benefits than one imagines when being forced to do it @school (learning multiplication tables vs using a calculator in exams for example).
However as Lala points out when faced with 000's of circles, it would be nice to find someone else to do it for you properly

Thanks Don now I can search for any tips & examples by those members.

To any designers out there - It might be nice if you guys put a list of approximate diameters with models instructions, eg:- part 001 = 4mm, to assist in selection of punches etc. (But maybe some are & I just haven't noticed it yet).

I decided to email Rotring DE to see if they have the rotary cutting heads easily available locally - I really liked the principle & @ approx AUD$8.00 they are reasonable enough to try I think.

Will post back any findings.

[medved]

I just had a look at Johnny Svenssons' cutting tips. He says about clock-wise & anti-clockwise cuts for circles - I infer this to mean he is pushing the cut away from himself, in contrast to my feeling that a sweeping drawing towards oneself would be the motion req'd.

Am I missing something?

Can't argue with his results tho!

[TheWebdude]

I am in the same camp as the Hermits' Mom . I cut them freehand, covering roughly 30° of the circle per cut, from about 1 o'clock to 3 o'clock per cut. Practice is the key.