Preventing Plywood Splits with CS Holes - Woodworking Talk - Woodworkers Forum
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post #1 of 6 Old 10-29-2015, 01:07 AM Thread Starter
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Preventing Plywood Splits with CS Holes

I am (still) building my shelving units for my home office. They are a very simple design with 3/4" oak plywood, oak strip edging in front, and a piece of plywood paneling for the back. No fancy joinery, I just plan to use three countersunk, brass colored screws to fasten the two sides to the top and bottom. And the back will be nailed into a 1/4" x 3/8" dado cut with a router after the top, bottom, and sides are assembled. I made some wall hung shelf units with some nice wood grained particleboard shelving using this basic design and I am trying to roughly match them. I can not get that particleboard shelving any longer so I was forced to move up to the oak.

I have the plywood cut into 12" (+/-) boards and the front edging strips glued on. I have cut the top, bottom, and sides for the first unit. My question is about drilling the holes for the screws. I got away with using a combination screw-form drill with the particleboard, but using it with the plywood worries me. I am afraid that those countersunk holes at 3/8" from the cut edges of the sides will tear the top layer of oak veneer, leaving a jagged edge.

After giving it some thought, I came up with the idea of using a Forstner bit to cut the outer edge of the countersink, perhaps only 1/64" deep or through the outer layer of veneer. Then using the combination bit to form the screw hole.

What do you think of that plan? Is there a better way?

Should I stain and varnish the pieces first to allow the varnish to help hold the veneer together? Should I use some masking tape over the hole locations? Any other suggestions?

If I do stain and varnish first, I know I will probably have to touch up the holes after drilling. Do you see any other problems?

And any suggestions for controlling the depth of the Forstner bit? This is too big to use the drill press so I am assuming I will be using a hand held drill, probably battery powered.
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post #2 of 6 Old 10-29-2015, 02:32 AM
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Id give the countersink bit a try on a scrap piece of the ply before anything. You never know, it could cut just fine. If your existing countersink fails however, id recommend finding a wheldon style countersink, or failing that a regular single-flute countersink. Either should leave a smoother cut than the normal multi-flute style on most self-countersinking bits.

Honestly, the forestner bit idea seems way to complicated to me. Trying to drill an extremely shallow hole with the forestner, then drill a pilor hole centered on that, then trying to get a good countersink on top of that, changing bits all the while. Seems like a logistical nightmare

I need cheaper hobby
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post #3 of 6 Old 10-29-2015, 08:23 AM
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Not wild about the exposed screws.
How about dowels? Loose tenons? Pocket screws under top, bottom and shelves?
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post #4 of 6 Old 10-29-2015, 06:33 PM Thread Starter
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Well, for fine furniture I am not wild about them either, but this was supposed to be a quickie project. That's why I started with the particle board shelves that I presently have on two walls in my home office. Cut to size, screw together, and mount them on the wall: quick and dirty. The only varnishing was on some trim strips. They weren't a work of art either, but look OK there.

Now I can no longer get that particle board but these shelves will be in that same room so I am trying to match the existing shelves. And I am trying to get this done as I must clear the project out of my garage so I can complete the work of constructing my shop there.

Another consideration is I am 71 years old and have some problems, like a bad knee. I have surgery scheduled tomorrow. But I will have to assemble these shelves in the garage for proper fit, then take them apart as I can not carry them from there to the office. I will reassemble them in the den, just outside of the office and slide them in on the carpet. Then erect them and walk them into position. I have carefully checked all clearances and hope I have not missed anything. So, for better or worse, I am probably stuck with screws.

They don't look too bad for an office. Especially mine.

This project has taken far, far too long and I just want to get it done.

Originally Posted by Pirate View Post
Not wild about the exposed screws.
How about dowels? Loose tenons? Pocket screws under top, bottom and shelves?
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post #5 of 6 Old 11-17-2015, 12:10 AM Thread Starter
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I am recovering from my knee surgery and getting back to this. I see there were no further suggestions on this so I guess I am on my own.

I have several drills so I would not have to constantly change bits. So I am going to try the Forstner bit thing after staining and varnishing. I am going to put some blue masking tape on the plywood and then sandwich it with a scrap that has the three holes pre-drilled. I will stop drilling with the Forstner bit when it starts cutting the blue tape. That should effectively limit the penetration of that bit. I have a couple of scraps to try it with. Wish me luck.
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post #6 of 6 Old 11-17-2015, 07:09 AM
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another idea ....

You could use a hole punch to pre-cut the veneer the same size as the countersink:

There are different types of countersink bits. Some have 2 flutes, some have 3, others have as many as 8. Some have the pilot bit built in, others do not:

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 11-17-2015 at 07:12 AM.
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