Kerfing Plywood for bending - guidelines?

[Jim Underwood]

I have CNC and programming capabilities, and would like to create my own kerfed plywood parts for bending. (I am aware of bendable products on the market which I'll only pursue if the kerfing does not work.)

What I need is a tutorial or guidelines for kerfing depth, width and spacing on 3/4 ply.

I've been searching the internet but not much luck...

Anyone aware of such resources?

Thanks,

[Jeffrey Makiel]

Jim, I cannot give you a direct answer. Perhaps some other folks will join in. But I have a couple of thoughts from my own experiences.

1. Cut some narrow scraps of plywood and try cutting various kerf spaces in them and see how much you can bend it. This can be time consuming if you're looking for a wide range of curves. The depth of stock is not important and only a function of clamping strength, not breakage.

2. The kerfs are usually pretty deep such that the veneer and a very small amount of the adjacent ply are left. I would estimate that the material that is remaining is less than 1/16" thick.

3. Even after you kerf bend, you still may see kerf lines telegraphing thru to the veneer side. Sanding is the only option I know of to remove these sublte lines, but the veneer on plywood is so thin these days, it's a challenging task.

-Jeff

[Neil Lamens]

Hi Jim:

What is the extent to the bending you are trying to do. Is it a wave, a quarter round, what's the final shape your looking to do?

Neil

[Jim Underwood]

I'm wanting the guidelines so that I can program the object to do whatever it should do depending on radius, thickness, length etc.

In other words this is a project that takes into account the variables and changes the dado length, width, spacing, and depth if need be.

[Stephen Clem]

It would be one helluva geometry exercise but I could probably create an excel spreadsheet that would take inputs of length, curve radius, etc and spit out the required spacing and depth.:cool:

[Earl Kelly]

I would think that for the time and effort you put into this, you could buy the products designed to do this, 3 times over. If you're determined to do this, I would only use MDF. Ply is not going to be as consistent because of the ply orientation and the varied depths of cut necessary to achieve tighter or looser curves.

Earl

[glenn bradley]

You probably found this one but, with it and some tests I think you'll get the pattern:

http://www.valpo.edu/mathcs/ResearchPapers/kerfcuts.pdf

[Per Swenson]

Ok,

I am not talking furniture here nor CNC.

We have done a lot of raised floors, Bedroom islands, band space,

interior carpentry stuff. Circular stairway molding.

I just set the Miter saw to a 1/2 deep cut on 3/4 stock and

cut it up at 1/4 to 3/8 spacing. Just chop chop as the saying goes.

Bends good. Here is a inside and outside, show ya what I mean.

Per

[Jim Underwood]

Thanks all for the responses.


I would think that for the time and effort you put into this, you could buy the products designed to do this, 3 times over.

That's probably true Earl. But I'm probably going to do this many more times than three. This is an object that is going into a design program for use by several people... to be used over and over and over...

So I think it's worth the effort and time.

And as the photos above prove, it can be done, and done pretty well with plywood. I saw some photos on another forum that showed some nice bent ply work also.

This will have application in MDF as well though, as soon as I get the object programmed. Actually, I have the object mostly finished, but have a couple of changes and additions to make, so that I have control over a couple more things with the click of a couple buttons....

Thank you for the analysis on kerf bending Per. I'll take a better look at it tomorrow. It looks as if it may be of some help.

If anyone else has some input I do appreciate it.

[Earl Kelly]

I'm curious, Jim.

What's your application, cabinets, millwork or furniture? Paint or Stain?

Earl

[Larry Rasmussen]

Not sure if it will help much for your application but I happen to have a couple of 2'x4' pieces in the back room waiting for me to turn them into a Plasma TV stand. I believe I got them from Rockwell but you would have to check. They are MDF, 1/4" thick. The cuts go 3/16 deep with a mere 1/16 of solid MDF. The grooves are 1/16 wide and the solid counterpart is 3/16 wide. Or you could look at it like there is 3/16 between each groove. I am suprised at those numbers, it looks smooth on the face when bent, no little lines.
Happy bending,
Larry

[Jim Underwood]

I thought I'd posted something yesterday evening.. but it seems to have disappeared...:confused:


I'm curious, Jim.

What's your application, cabinets, millwork or furniture? Paint or Stain?

Earl

The application is mostly cabinets Earl. I do some furniture pieces, but they are mostly fancy standalone cabinets (like islands and entertainment centers).

And the finish can run the gamut. Paint, Stain, Natural, Glazed etc. It's all Lacquer though.

Sometimes I run millwork around the arches, but I won't be kerfing those. I just run that through a W&H shaper/moulder.

[Jim Underwood]

Ok.. just a follow up for anyone who might possibly, on the remote chance, be interested.

The radius of this particular table is 19 inches. The skirt is 4 inches wide.

I cut 1/4 kerfs, 3/8 apart, and left 1/8 in the bottom of the kerfs. It bent fairly well til it broke....:D

This morning I'll cut another one and leave 3/32 in the bottom of the kerf - so we'll see what happens in a little bit.

[Earl Kelly]

Jim,

I didn't intend to come across as confrontational. I hope you have sucess with your project. Last project I did with saw kerfs was many yrs ago.

If your interested here's a couple of links to give you an idea of how I build curved projects.

http://earlkelly.com/CurvedVeneerPanel.htm

This is what I used the curves on.

http://www.earlkelly.com/xoddics.htm

Earl

[Jim Underwood]

No problem Earl.

Very nice furniture by the way.

I cut another piece on the router this morning, and made the kerf just 1/32 deeper.

Now the part bends like there's no tomorrow....:D


You can the previous kerfed part that is broken. The bent one is just sitting there pretty as you please. All it took was cutting through the longitudinal lamination and scoring the cross lamination next to the veneer.

[Jim Underwood]

And here is a peek at the project and what it's wrapping around. We may have a slight problem with the flats telegraphing through, but we'll see:

[Earl Kelly]

Yes, it's a fine balance to cut enough to get flexibility but not too much. That's why I don't like the ply for kerfing, the grain structure can mess you up. MDF has none. If you have a piece of MDF core material you might want to see if that works any better.

If you intend to do a lot of curved work a vacuum press can open all new doors for you. Basically the size and shape are endless, from 3" radius panels, panel doors of any shape, to circular stair railings.

Thank you. I try to stay away from cabinets and built ins. There's always someone that will do it faster and cheaper. So, it's hard to compete when your a one man shop.