geometry expert required

[Brandlin]

Hang on a minute - just rethinking.

Are you making a scabbard for the sword? Then mathematically the curve of the sword needs to be of constants radius ... ie an arc. Or you wont get the sword in the scabbard.

If thats the case then this you tube video is based on geometry and will give you an accurate arc without string or bending sticks and much more accurate than joining up dots freehand

https://www.youtube.com/watch?v=ykzQurvEqOo

 

[custard]

What's the curve for?

If it's extremely shallow (ie a very large radius), or if you're building a jig/former that has to be geometrically precise, then you complete the calculation previously detailed and then make a corresponding jig along the lines detailed by Katellwood. Although when I do it I use a router instead of a pencil and cut a master curve directly.

Actually, that's what I did up until a few years ago, nowadays I just sub it out to a CNC shop and get them to make me a master curve template.

However, it's pretty rare for those requirements to crop up in day to day furniture making. For 99% of curves I do what Andy recommended and just use a whippy bent piece of timber lath and a pencil (actually I use a fibreglass sail batten as they're compositionally more uniform). This won't give you an absolutely precise geometric curve (ie a section of a perfect circle), but visually it's close enough and it's quick and convenient.

Good luck!

 

[MikeG.]

Hang on a minute - just rethinking.

Are you making a scabbard for the sword? Then mathematically the curve of the sword needs to be of constants radius ... ie an arc.........

Piston-fit sword? I suspect that a scabbard is going to be plenty sloppy enough to cope with many a (non-arc) curve.

 

[Brandlin]

Hang on a minute - just rethinking.

Are you making a scabbard for the sword? Then mathematically the curve of the sword needs to be of constants radius ... ie an arc.........

Piston-fit sword? I suspect that a scabbard is going to be plenty sloppy enough to cope with many a (non-arc) curve.

A scabbard is usually made by laying out the two sides of the scabbard and drawing around the blade and carving out that section then gluing them together.
So whilst not 'piston-fit' as you say a good scabbard makers pride themselves in making scabbards that do not rattle or allow the blade to move within the scabbard let alone 'fall-out'.
It wont take much difference in rate of curvature along the blade to make the blade bind simply due to the geometry.

Try it out. take your stick method, draw a catenary sword blade and another scabbard and slide one over the other.

 

[MikeG.]

........Try it out. take your stick method, draw a catenary sword blade and another scabbard and slide one over the other.

.........'coz I've got loads of spare time for stuff like that.

No, you're right of course, but I suspect that at the level of accuracy we're talking about here the difference between a catenary and an arc will be negligible. In fact, an arc can be a catenary, as can a straight line.

 

[galleywood]

Hi Sunnybob

I calculate the radius to be 6.25 metres - using trigonometry calculations.

 

[MikeG.]

Hi Sunnybob

I calculate the radius to be 6.25 metres - using trigonometry calculations.

Close, but not quite:

I've since looked up and found Sagitta, but it doesnt help because i dont know the radius.

You do now. It is 6260mm (if the curve you are after is an arc [ie part of a circle]).

Calculated using DraftSight.

 

[Brandlin]

........Try it out. take your stick method, draw a catenary sword blade and another scabbard and slide one over the other.

.........'coz I've got loads of spare time for stuff like that.

No, you're right of course, but I suspect that at the level of accuracy we're talking about here the difference between a catenary and an arc will be negligible. In fact, an arc can be a catenary, as can a straight line.

Of course you've got time - you're here arent you?

I agree about the scale and shallowness of the arc in this instance.

However I can't let THIS go...! :shock: :roll: (hammer)

In fact, an arc can be a catenary, as can a straight line.

 

[galleywood]

MikeG

I guess my old school trig book has not got enough decimal places!

 

[MikeG.]

.......Of course you've got time - you're here arent you?.......

I'm working. Really, really hard.

 

[sunnybob]

Wow! what a lot of jeaniearses there here.

While I thank everybody for taking part, I have no way of checking all those calculations, and that is also not what I asked for. That is too complicated for me. the 6 metre bit of string might have worked if i had a 7 metre laying out area and a couple people to help me, but i dont on each count.

MikeG gave me the exact answer I asked for. With his "every 10 mm height" I managed to draw it out and cut the paper template onto a piece of beech, and it looks good. I was very pleasantly surprised to note that the first one I made (without the use of any type of measuring implement) was pretty damn near to spot on. i had just made the handle flat and ended up with a cutlass shape.
I also tested the only "bendy" bit of wood I had, which was a 10 mm beech dowel 6 ft long, and it also lined up right on to Mikes marks.

No scabbard folks, its just a practice wooden sword.

I have a couple thin slices of bubinga from the original sword, and i think i will glue those to the sides of the blade so that I can get laminations when I angle the front edge.
I'm running low on Custards amazing rippled ash, so this handle will just be round and whipped with thin flat leather cord.
All good fun.

 

[ColeyS1]

Sent from my SM-G900F using Tapatalk

 

[MikeG.]

Well, you'd be the winner, Coley, if I hadn't got there before you.

 

[NazNomad]

Well, you'd be the winner, Coley, if I hadn't got there before you.

So 20½ ft wasn't right?

 

[NazNomad]

... the 6 metre bit of string might have worked if i had a 7 metre laying out area and a couple people to help me, but i dont on each count.

If you had a piece of string 20½ inches long, you could've drawn an arc using that and just multiplied the measurements by 12.



Occam just phoned, he wants his Gillette Mach 3 back. :lol: :lol:

 

[MikeG.]

Well, you'd be the winner, Coley, if I hadn't got there before you.

So 20½ ft wasn't right?

No, it is 11.6mm short.

 

[MikeG.]

.........If you had a piece of string 20½ inches long, you could've drawn an arc using that and just multiplied the measurements by 12........

20.538 inches. Occam says hang on a sec.........

 

[NazNomad]

Damn you and your ruler that can measure 0.038 of an inch. :-D

 

[ColeyS1]

I showed my workings though lol.

It's one of the most useful formulas I know for the workshop. When I was an apprentice I use to make a lot of concrete formwork. This got used alot. Quite often I'd have two battens clamped together for length, then tramels stuck in my workmates bench so I could swing the arc.
It's rare I'd need to go as big as sunnybobs, but it wasn't unheard of.

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[ED65]

Just curious, does anyone here actually own a bokken, or a katana for that matter?