Advice on marking/cutting shallow curves

[cflansbaum]

one of the design elements I like to incorporate in my pieces are (usually) stretchers with shallow curves.

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In the above, the curve extends over 26" but is only 1/4" deep at it's widest part.

In the past I have used a Lee Valley Drawing Bow, but it's difficult to use with only 2 hands and impossible for such shallow curves.

Ideally I'd like to be able to mark with a pencil and then cut with router these curves (usually I cut close to the line with a bandsaw and then smooth to the line by hand).

I've looked with interest at this thread regarding cutting ellipses / ovals and am intrigued by the jigs, but not sure it would work in my situation.

Anyone have experience with similar or have advice?

thanks

Carl

 

[Dave Ronyak]

Carl,

I'm not familiar with the LV device you mentioned.
Have you ever watched Tom Silva create an arc?  He takes a thin strip of wood (of uniform cross section), places a nail at each end of the arc, places the strip on edge against those nails, then pushes the center of the strip upward the amount of curvature he wants.  But 1/4 inch height over 26 inches width might be fairly difficult to make a good template for routing smoothly.  Unless I am doing more than one, I draw the curve on the workpiece rough it out on a bandsaw and drum sander, and hand finish with files and sandpaper.  I don't see the benefit of making a template for just one piece, although it possibly enables a better (more uniform) final result.

Dave R.

 

[nickao]

I use the Bows and have two, they work well.

Woodgeek the string trick should work fine with that curve you need. But I have never tried it with such a stretched out ellipse before. The problem is you would need the string really long and the two nails so far apart it may not be feasible. That curve makes a HUGE oval.

I'll look up a method in my book and see what it has to say about it. I know I read a sure fire method to make those subtle curves I just can not remember off hand.

 

[chris mann]

For drawing shallow curves like that, these work much better Blending Curves.  Especially in one handed mode.

I tend to make my templates for curves in hardboard and use a spokeshave and/or sanding blocks to smooth them out after cutting close to the line on a bandsaw.  When using a template bit on the router, you need to be aware of grain direction, or you can/will get chipout.

And depending on your technique, shallow convex curves can also be made on the jointer. 

 

[Jesse Cloud]

Chris nailed it.  Those bending curves are sweet. 

If you want to route from a template, consider this to overcome the grain direction issue:

Make your template a couple of inches long on each side.

Once your template is fair and smooth, make a copy, using a pattern bit on a router.  Actually make a few copies in case of damage, etc.

Use double stick tape to put all the templates together in line.  On the drill press drill a hole that will fit your dowels in the extra space on each end.

Separate the templates.

Put one on the workpiece with a little doublestick.  Go back to the drill press and drill through the workpiece where you have the holes.  Place a long dowel through the template and the workpiece.  Place a second template on the bottom of the workpiece using the dowel for alignment. 

Trim the dowel if its long enough to get in your way, but leave a little stub to ease removal.

Now you have a template on both the top and bottom.  When you come to a place where the grain goes in the wrong direction, just flip it end for end and keep trucking with the reversed grain.  I usually mark where to route on each template.

Its probably a good idea to do this on scrap first.  If the piece doesn't come out smooth and square, you probably didn't drill the hole at exactly 90 degrees.

If you are making a lot of these, its probably worth making a carrier board to clamp the piece on and keep your hands far away from the sharp spinny stuff.
Hope this helps.

Jess

 

[nickao]

Or just get this type of bit and do not mess with removing the template at all:

Top/bottom bearing on one router bit

Top and bottom bearing on the same bit saves a lot of time messing with lining up templates on the "flip". It's not necessary to mess with the template with this bit.

Just watch the video on the bottom of the page. They work as advertised!

 

[nickao]

Katana are the premium MLCS bits.

I have tons and I love them.

I have never had one go dull or break since 2001.

 

[cflansbaum]

Dave -  I forgot about that trick, I may try something like it.

Chris -- I have some of those blending curves as well.  I find them good for copying/making complex curves but I haven't been happy with them for long gentle curves, but I'll play with them again.

Doing a search on Google for "drawing shallow curves" I found the following.  Interesting...  I'm busy the next few days, but I may try and make a jig out of this method.

Carl

 

[nickao]

That just a variation on the nail and string method.

 

[Dave Ronyak]

Thanks, Nick re the Katana bits and the use of a double bearing flush trim bit to simplify pattern routing of curves.  Katana's pricing is lower than the Freud bit I bought a year ago.

Dave R.