edge to edge joining plywood to hardwood

[sepra13]

Hi all,
I am making a table top of plywood within a frame of hardwood. I would like to make them flush, but I am concerned about them staying flush over time.

I've heard of using a "scratch line" instead of trying to get them flush, but I have no idea what a "scratch line" is.

I'd like to hear any tips, tricks, experiences with flush or scratch line before I get a very expensive lesson!

Thanks!

[LCHIEN]

i think maybe they are referring to routing a very small, narrow grove - rectangular or V, right over the junction of the two kinds of wood. THis will hide any minor discontinuity in height and add a decorative touch. The key is that a height mismatch of a couple of thou can be easily felt by sliding a fingertip over the joint... but with the groove its not that apparent. A V-groove will be much easier to keep clean.
Use a trim router with an edge guide to do this.

[JimD]

You can actually put a small rabbet on the solid wood prior to glueing it on to do this, if you want. I would probably just glue it, possibly using biscuits, and not worry about it. I have edged plywood with solid wood multiple times, sometimes using the rabbett at the edge and sometimes not. Getting the solid wood flush without sanding through the plywood face veneer has been an issue but not a height difference between the solid wood and the plywood. All I can say about this is BE CAREFUL. Best is probably to use a block plane with masking tape on the plywood and then sand carefully. I have a router jig I use I think I saw in Wood magazine that works pretty well and takes less effort than the block plane. Biggest reason I see for the rabbett or v groove is so you do not have to flush up the pieces so you do not risk damaging the plywood. If you finish the solid wood and plywood with something that seals the wood, neither should move enough to make a noticable height change.

Jim

[cabinetman]

A "scratch line" as you put it can also be called a "reveal". Like Loring said it creates a purposeful differential between the hardwood and the plywood.

Your choice of where it's referenced may affect how it looks. It could be in three different areas.
1. In only the plywood.
2. Between the plywood and the hardwood.
3. Only in the hardwood.
It is simply machined using the outer edge of the hardwood as a guide, and using some variant of a detail bit. Any place it is machined will be a dust and dirt collector. Or it can be done to the individual pieces before glue up as Jim noted.

Making the hardwood flush with the plywood can work out very nicely. If the plywood and the hardwood have good mating edges and each are at 90 degrees, gluing and clamping can bring them together with a fine hairline showing if any. That line could just about disappear using the same species of hardwood and plywood, where the grains would run parallel and visually that joint line would not be that visible.

For different species of hardwood and plywood, especially with drastic differences in color, like Maple and Walnut, the joint line is an expected line.
The final leveling of the hardwood to the plywood can be done first with a trim bit with a bearing, and then carefully scraped and/or sanded flush. Done properly, finishes off beautifully. Movement issues for the hardwood aren't usually a consideration as the edge doesn't carry a very wide (crossgrain) section of wood.
.

[poolhound]

Basically I agree with Jim - he's always right

Carefull joints and biscuits, plus the fine tuning already mentioned should take care of your needs. The only other thing I would add is DO spend the extra on some quality plywood from a good lumber supplier - Dont try and make it from BORG ply.

I would also plan your work so that you mill your edge stock first. Let it aclimate for a day or so then double check it for straight and sq and fine tune if necessary. Then cut your ply and add the edging all in one session. Dont be tempted to cut the ply first and then leave it lying around while you prepare the HW. This is just introducing an opportunity for the ply to warp. the better the quality of ply the less likely it is to happen but its always best to get the edging on ASAP.

[Tom Miller]

How would they not stay flush over time -- IOW, what are you concerned will happen. Unless the glue joint fails, I don't see an issue. Unless I'm missing something (happens all the time).

Getting them flush to begin with may be another story, but you could get plenty of advice for that.

Regards,
Tom

[Wood_workur]

http://eagleamerica.com/face-frame-v...er/p/117-1002/ is a good bit to make a decorative bevel, but it will not work at the corners.

I really don't see there being any sagging of the edge if you use biscuits to join it (when you cut them into the plywood, put 2 sheets of paper under the jointer so they are cut just the slightest bit higher.)

[sepra13]

How would they not stay flush over time -- IOW, what are you concerned will happen. Unless the glue joint fails, I don't see an issue. Unless I'm missing something (happens all the time).

Getting them flush to begin with may be another story, but you could get plenty of advice for that.

Regards,
Tom

I am concerned about movement of the two different materials over time - maybe not an issue if I seal them properly. I'm also concerned about getting them flush without sanding through the veneer, as this is my first try at this kind of joinery. Also, since the ply is a 3/8" thick, I was going to rabbet the hardwood and glue in the ply - no biscuits (ply not thick enough?). The hardwood frame will be 3" wide, so I don't think using a trim bit in the router will work?

I'd prefer to make them flush, but just don't want it to look bad because my skills are not up to the task yet.

thanks for all the advice

[drumpriest]

Another option is a tongue and groove joint, cut a dado in the hardwood on the TS and a tongue on the end of the ply (router table or nibble away with TS.. Will take a bunch of tests to get it setup right, but then your ply won't be able to twist in relation to the hardwood, and you can get it nice and flush across the whole span. Without doing something like that or biscuits it's hard to keep the ply aligned over the whole span. With a rabbet it would work if you glue it up right. Plenty of pressure downward on the ply to ensure it's seated well in the rabbet across the span.

Lately I've been leaving the hardwood about 1/32" proud, and planing it down with a block plane and hand scrapers, you definitely need to be careful to not destroy the veneer.