Calculating a radius for a tramel bar.

[pren]

Evening all.

I've recently been working on some projects that include a gental curve in their design. So far, I've been using a tramel bar and router to create these curves. This has all worked fine but the set-up of the tramel bar has been very much trial-and-error to find the correct radius.

One example is this coffee table top that I've been working on.

The length of the top is 1220mm and the depth of the curve is 85mm at it's deepest point.

Is there some way of calculating the radius of curves like these by only knowing the dimensions of a section of the perimeter, without having to play around with SU?

Thanks in advance

Bryn

 

[Chris Knight]

Certainly you can - try this for starters. Your 85mm refers to the segment height.

 

[wizer]

If you've got that far in SU already then working out the trammel should be a breeze. Draw out your router base and then work back from there.

 

[Chris Knight]

This might be useful when it's inconvenient to use a trammel or lack a place to put the centre point. http://chestofbooks.com/crafts/metal/Sh ... ethod.html

 

[WellsWood]

Bryn, the formula you need is here.
If you google "radius from chord height" you'll also find it expressed differently as in this one.

They amount to the same thing, and your radius comes out at 2231.3mm

 

[wizer]

FWIW, I'd apraoch that task from a different angle. Namely make a template and route around it. I'd cut the shape on the bandsaw and finesse it with spokeshaves/sandpaper.

Just in case you didn't want to muck about with trammels.

 

[WellsWood]

Yes but you still need to know the radius, otherwise how are you going to mark a line to bandsaw to? Template routing doesn't get you out of it either - you still have to make it, in exactly the same way as going directly to the table-top - it just makes repeat cuts more reliable.
I know you could use the "spring a batten" technique to form the original curve, but to get the accuracy that router and trammel provides being able to work out the radius wthout all the tedious trial and error is a real bonus.

 

[wizer]

ahem, I print my templates on the plotter...

 

[Steve Maskery]

If you don't want to make a trammel that long, and you are happy to go the template route, you can rip a piece of 6mm MDF and spring it to the curve. Plot you three points (ends and rise), hold the strip in place and get a mate to run a pencil around it.

For this kind of application it's perfectly adequate.

Cheers
Steve, who is not quite as poorly as he was this time yesterday!

 

[MikeG.]

Whilst I would always do it Steve's way (using a homemade spline.....plastic sections such as curtain tracks are my normal starting point)......

.........the answer to the above question is a radius of 2230.76mm. Given the way you lot plane, I'm sure you'll be needing the second decimal place!!! :wink:

Mike

 

[jasonB]

Another option is to use a tremmel to swing the router with say a 6mm bit and just cut 5mm deep. Then cut away the waste on the bandsaw before using a bearing guided cutter to finish the job running the bearing against the edge of the previously cut groove. I've used this method with a 5.00m radius, couldn't get much more in the workshop.

If springing a batten I tend to make a 4 or 6mm template as its easier to refine the curved edge on something this thin and then use the multitrim against this after rough bandsawing the workpiece.

Jason

 

[pren]

Ah.


Algebra.



My sworn enemy! :evil:

:lol:


Thankyou very much for all that help, folks! I very rarely use SU for designing things, so a quick 'old-school' method of figuring out trammel lengths is what I was after, ta!

Fortunately my good wife speaks maffs, so she can explain to me (in her most condescending of voices) exactly what to do. :lol:

Out of interest, Wizer, how would you go about finding the radius of the table top using SU? I did resort to randomly creating and moving circles, in the hope that one would fit the curve! This is exactly what I've been doing with the trammel arm in the W/S. (Please excuse. I no very bright! )

I have used the batten-spring method before. However, unless it's made strong enough to run the router up against and off-set for base allowance, it does mean making up a template and I'm stingy and don't want to use more material than necessary. :roll: :lol:

Thanks again!

Bryn

 

[Shultzy]

Bryn, draw the table top in SU. Highlight the arc, click on window entity info and the radius is displayed.

 

[WellsWood]

...or just find a very very big dinner plate to draw round.

:wink:

 

[MooreToolsPlease]

If you're using sketchup, if you select the dimension tool, and click on the arc, it will tell you the radius of the arc :wink:
Rather than all of that algebra to confuse your brain.
I just drew it out quickly, and it told me the radius was 2231mm,
I think thats close enough for woodwork :wink:

 

[bugbear]

This might be useful when it's inconvenient to use a trammel or lack a place to put the centre point. http://chestofbooks.com/crafts/metal/Sh ... ethod.html

Excellent reference! I've not seen that one before.

Discussed on the OLDTOOLS list back in 1999.

http://swingleydev.com/archive/get.php? ... =1#message

And also used to calculate the grinding for a scrub blade here:

http://www.geocities.com/plybench/scrub.html#blade

BugBear