How do i match a radius?

[David Ruhland]

I am trying to figure out how to match the radius of the truck box so the board slides down inside and fits tightly to the bed. Thoughts/Suggestions please

Thanks...

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[Gary Ragatz]

If you have access to one of those devices that woodturners use to copy a profile (a contour gauge), I think that would work.

You could also get some Play Doh and mold it into the radius, flatten the sides so you can lay it down on your work piece, and trace the radius.

[Grant Wilkinson]

I have one of these

http://www.leevalley.com/us/wood/page.aspx?p=32523&cat=1,42936,42958

[Prashun Patel]

45 degrees wouldn’t for for you?

[Nick Decker]

Or this:

https://www.amazon.com/MLCS-9327-Woodworking-36-Inch-Flexible/dp/B001S2RAUW/ref=pd_bxgy_469_3/143-6814657-6840248?_encoding=UTF8&pd_rd_i=B001S2RAUW&pd_rd_r=2e1a7ef4-471e-11e9-a09e-af4805a7e305&pd_rd_w=cNStZ&pd_rd_wg=3gTUw&pf_rd_p=a2006322-0bc0-4db9-a08e-d168c18ce6f0&pf_rd_r=ZT49362AB8K4W267NWAV&psc=1&refRID=ZT49362AB8K4W267NWAV

[Marshall Harrison]

Easiest way is to use a contour gauge.

https://www.amazon.com/s?k=contour+gauge+duplicator+tool&crid=R4YAQEG1DDOF&sprefix=contour+g%2Caps%2C154&ref=nb_sb_ss_i_3_9

Push it against the radius of the truck bed then transfer that to your 2x4.

[Charles Taylor]

I have one of these

http://www.leevalley.com/us/wood/page.aspx?p=32523&cat=1,42936,42958


I have a practically identical model from some other source. Ideal for the job, as long as the fingers have sufficient reach, or at least enough of the fingers that you can sketch in the rest.

[David Ruhland]

not sure... haven't attempted it yet

[David Ruhland]

Easiest way is to use a contour gauge.
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Thats what they are called! Thanks.... That is what i was thinking, but called it " a thingy with nail like wires" lol

[Marshall Harrison]

Thats what they are called! Thanks.... That is what i was thinking, but called it " a thingy with nail like wires" lol

yeah, it had me baffled for a moment too. My first Amazon search was for "radius gauge" but that is a different animal. I have one in my workshop but I had to think about what it was called.:)

[Steve Eure]

You can get a flexible curve ruler like the one Nick posted at Michael's and Hobby Lobby for about the same price. Beats waiting for shipping if you have one of them close by. I've actually used a piece of #10 copper wire to replicate a curve once or twice. Not the best tool for the job, but was what I had on hand and it did the job.

https://www.michaels.com/staedtler-mars-18in-flexible-curve/10192326.html
https://www.hobbylobby.com/search/?text=slexible+curve

[Jim Becker]

My method...a piece of cardboard and some scissors. :D Really...that's exactly what I'd do for this particular application. Once I had a template that was really close, it would be easy to get that on paper and figure out the numeric radius in the measuring system of choice so that it's repeatable. Or you could just use the template as a template with a pencil and then cut.

[Al Launier]

Same as Jim; make a cardboard template & you're good to go.

[Brian Tymchak]

Same as Jim; make a cardboard template & you're good to go.

that's the first thought that came to mind for me. Also, you could scribe the curve with a compass such as is done to fit cabinetry to uneven walls.

[Tim Bueler]

My method...a piece of cardboard and some scissors. :D

Ditto.

I would make the first radius to fit and transfer that to a second piece of cardboard. Both pieces long enough to overlap. Set in place and tape together. You now have a template with not only the correct radii it is also the correct length.

[Nick Decker]

Full disclosure, I bought one of those flexible curve rulers 5 or 6 years ago and it worked fine for the project I bought it for. Haven't seen it since, but I know it's out there somewhere.

[Lee Schierer]

Simply scribe the line on a piece of cardboard with a pencil compass.
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[Ken Fitzgerald]

Simply scribe the line on a piece of cardboard with a pencil compass.
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The only problem to remember with this method is you must keep the compass perpendicular to the surface being traced at all times as you trace it to get an accurate representation. Other than that, it is a great way to trace a surface to make a template.

[David Ruhland]

thanks for the pic! Great idea

[Jacob Reverb]

Scribe it with a compass?

Doh -- Lee beat me to it.

[Marshall Harrison]

The only problem to remember with this method is you must keep the compass perpendicular to the surface being traced at all times as you trace it to get an accurate representation. Other than that, it is a great way to trace a surface to make a template.

There is an even bigger problem with this method; no new tools are involved. everyone knows that half the fun of woodworking is in acquiring new tools. :D

Can I get an amen?

[Ken Fitzgerald]

There is an even bigger problem with this method; no new tools are involved. everyone knows that half the fun of woodworking is in acquiring new tools. :D

Can I get an amen?

I apologize for my mistake!:o

[Lee Schierer]

There is an even bigger problem with this method; no new tools are involved. everyone knows that half the fun of woodworking is in acquiring new tools. :D

Can I get an amen?

He might not own a pencil equipped compass. :D

[Bill Orbine]

As it has been said before....45 degree cut - done! Not good enuff? OK... Find come soup, coffee, paint or whatever can that might come close to the radius on the truck bed to trace a radius. But really...... how far is the edge of vertical 2x4 from the side of the box bed in the picture you provided? That distance would be about the radius!

[David Kreuzberg]

Look around the house for anything circular: plates, saucers, pan lids, coffee can lids, jar lids....

[al heitz]

You can get a flexible curve ruler like the one Nick posted at Michael's and Hobby Lobby for about the same price. Beats waiting for shipping if you have one of them close by. I've actually used a piece of #10 copper wire to replicate a curve once or twice. Not the best tool for the job, but was what I had on hand and it did the job.

OR a metal coat hanger bent to shape. I bet there's one in your house waiting for you.

[Keith Westfall]

Cut a 45* and then cut those 2 points again. You'll be getting pretty close - nothing a disc sander couldn't fix...

[andy bessette]

Elementary.

Use a pencil compass to scribe the curve onto a scrap of cardboard, which you can then cutout with scissors to check and refine the fit before transferring it to the workpiece.

[Myk Rian]

, jar lids....
^^^^This^^^^

[Martin Wasner]

Look around the house for anything circular: plates, saucers, pan lids, coffee can lids, jar lids....

That'd be my solution.

Scribing it in would be my second choice

[Jason Roehl]

Cut a 45* and then cut those 2 points again. You'll be getting pretty close - nothing a disc sander couldn't fix...

Exactly what I did on mine, minus the disc sander--the board is under a tonneau cover.

[Jon Barnett]

Measure the approximate radius with a tape measure, cut that radius in a scrap piece of wood and test fit. You should be able to sneak up on a perfect fit with a couple of tries.

[Steve Demuth]

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Practically speaking, any of the methods suggested work. Personally, I'd go with Jim's cardboard and scissors.

But, for the measurement-minded, you can determine the radius directly. Given the length of any chord (line between two points on a circle) and the distance of the chord's midpoint from the circle, there is a simple formula to get the radius of the circle. Call one half of the chord length a, and the distance from the center of the chord to circle, b. Then the radius of the circle, r is:

r = (a^2 + b^2) / 2b

So, if you've got a part of a circle and want to know the radius, you can:

Make a straight, square stick to a known length
mark the centerpoint on the stick
place the stick across the inside of the arc you have to work with
use a combination square to measure the square distance from that center point to the arc.

That gives you a and b. r follows from the formula.

The big challenge with this method is that if b is very small compared to a, then any error in b is magnified by the formula. For example, if you have an 8" stick (a = 4) and measure a .25" distance to the circle for b, the radius of the circle is 30.13" - 30 1/8" to a woodworker. But if b should have been .24" inches, the circle is really 33.45" radius. A 1/100", .4% error in measurement becomes a 3", 10% miss on the radius.

So, it works best if you've got a quarter circle or so to start with.

[Phillip Mitchell]

Wow, you guys really know how to be anal woodworkers :D

Just clip the offending corners off at a 45 and call it a day. This is some 2x framing lumber going into the back of a truck, not a piece of furniture.

You could scribe it with a compass or figure it out with geometry equations, but why bother if you don't need to for the application??