Clean Cutting Curves in Plywood

[Frank Snyder]

I have to help a buddy out with his x-mas gift tomorrow and this project he wants to make has some curves cut in 3/4" plywood. Both sides of the plywood will show, so I'd like the tearout to be nonexistent. The plywood edge will either get edgebanding or a solid edge treatment.

My thought was to cut out the curves a bit oversized with a jigsaw, then clean up the edge to the actual line using a spindle sander. I know many have had great success using an upcut sprial bit, but can this potentially lift the veneer on the top side?

Any ideas would be greatly appreciated.

Many thanks!

Frank :)

[Andy Hoyt]

Yup. spindle sander.

[Frank Snyder]

Thanks, Andy. I wasn't sure if there was a better method out there.

[Craig Coney]

You could make a template out of 1/4" MDF or hardboard for the curves, cut proud of the line 1/8", then use a piloted bit 1" long to trim it to the pattern. A regular straight or shear will work.

[Peter Stahl]

I used to score the line with a razor blade then cut off the line a hair. Also like the template idea, I know it works well with particle board and solid wood, never used it for plywood.

[Frank Snyder]

Craig, Peter - Thanks for the additional tips. I've got access to a spindle sander so I might just go that route instead.

[Lee Schierer]

One way that I cut plywood with zero tearout is to apply regular masking tape (not the painters tape) to the cut line. Remove the tape by peeling it toward the cut not away from the cut when it is time to remove it. Applying it to the cureves might take a bit more tape, but should work just as well.

[Frank Snyder]

Thanks, Lee. I've used masking tape before for straight cuts, but I wasn't so crazy about having to lay out curves with it.

[Jeff Patrick]

I cannot imagine how anyone could sand to a line with a spindle sander and keep it accurate enough to apply edge banding. Little gaps are bound to occur. I would use a router and template made of 1/4"sheet goods. You have to work above and below the template alternately using top bearing and bottom bearing bits. Always have the bit cutting with the grain and there will be no tearout.

[Frank Snyder]

Jeff - I guess we'll find out tomorrow (about not being able to sand straight). You gotta be a smoooooooth operator. Since these are curves that I'm dealing with, I'll be going every which way in regards to the grain direction. I think tearout would occur at some point with a router bit, perhaps due to a void in the plywood or a weak spot in the veneer. Sanding IMO is the safest way to avoid such a mishap. We'll find out how well I can cut and sand tomorrow ;).

[Bob Michaels]

Jeff - Isn't a template a catch 22 - how do you first make the template without the same small variations that you are concerned with.

[Scott Rongey]

Bob,

There are two tips for making the template:

(1) Make it out of hardboard because it is firm enough to guide the router bearing but soft enough to be easily shaped by sanding.

(2) For sanding to the final shape, glue a strip of sandpaper to a thin strip of wood and sand in-line with the edge you are shaping. The strip is flexible enough to conform to the desired curves but long enough and stiff enough to ensure smooth transitions (e.g. it can't dig in a small area like a spindle can).

Scott

[Scott Rongey]

Frank,

Although I've never tried them there are compression router bits designed for edging that have both up-cut and down-cut spiral portions to cut both sides more cleanly.

There is a discussion of different spiral bits, including the compression type, in this article:
http://www.woodcraft.com/articleprint.aspx?ArticleID=445

Scott

[Frank Snyder]

Scott - Thanks for the article link. It looks like a compression bit would be the ideal router bit to use for a clean top and bottom. I'll have to experiment with one for my next project. For now, I'm just going to use the spindle sander as I've already borrowed it for the next two days. Thanks again for the info.

[Jeff Patrick]

Scott's reply to Bob's question on how to make the template is right on. I also often use a block plane to fair the line smooth. The mdf is a little hard on the iron's edge, but that's what sharpening stones are for.

[Paul Greathouse]

Frank

If you have a good quality jig saw with a blade that runs fairly true, you could make a temporary zero clearance base out of plexiglas or some other see-through material. Also use as fine tooth of a blade as you can without slowing you down too much.

[Bill White]

Most CNC nesting/cutting machines use the compression bit 'cause ya don't have the tear out top and bottom.
Bill