Is any amount of bow ever acceptable?

[Tyler Kaelin]

I am working on a frame for a large freestanding mirror which I plan to assemble with bridle joints.

The mirror is 60” tall and I am working with 3/4 stock milled down from 4/4 rough sawn.

After milling I have a very slight bow to both of my stiles/longer sides. It is tough to get 5+ ft of 3/4 stock perfectly square even on my long bed jointer.

It’s tough to measure because I don’t even have a long enough reference surface but putting both concave sides towards one another to exaggerate the gap I have about a 1/4” gap over 65” which is shown in the attached pictures. So each piece has probably 1/8” of bow.

A few questions for the hive mind-

1. Is it ever acceptable to have bow in your stiles for a frame? Will the rails help pull this in to square?

2. Would it help if I inset the mirror like making a frame and panel door instead of a rabbet to try and pull the stiles square? The pieces are long and thin enough that I can easily press the bow out with two fingers? Would prefer to do as a traditional picture frame to make the glass replaceable in the future.

3. Generally, how square is square enough over longer distances with thinner stock? Even if I started with 5/4 I feel like it’s tough to truly get perfectly square stock this thin.

[andrew whicker]

How thick is the mirror + backing?

I had a door I built recently that was about 5 ft tall and 1" thick (shaker style). On some of the stiles I vacuum pressed a few together (lamination). I figured as long as my table was flat then the glue line would be strong enough to take out any bow that did exist. I was right on about that.

I also added a middle rail to help stabilize the door (per advice on this site).

So you could try something like that if you have that set up.

[Tyler Kaelin]

The mirror is only 1/8”. Not sure yet on backing. Likely just some 1/8” hardboard just a hair oversized all the way around to help protect the mirror from any compression.

A vacuum clamp is a good idea but Unfortunately not something I currently have access to.

The third rail in the middle is another good idea. While the back won’t be immediately obvious the piece won’t be wall hung so will need to have a think on a decent looking way to do that.

[Brian Runau]

I don't think clamping it in the corners would take the bow out. When you assemble the frame is the bow noticeable to the point it bothers you? On the bow side cut the rabbit for the mirror wider, then have the mirror cut to just fit in the corners so the mirror is still behind the rabbit in the middle where the bow is greatest. Brian

[Warren Lake]

your question about bow some carpenters put or some a bow in the style on the lock side of a door that way its under load when its shut and doesnt rattle.

[Thomas McCurnin]

The traditional way of handling large frame and panel construction is to add a third, or even fourth, rail (horizontal piece) mortised and tennoned and to build the frame out of 1" stock, with the center rails being less thick, about 3/4"

[Steve Demuth]

If you're confident the wood is now stable, then the question of whether the bow is acceptable is aesthetic. Unless it's a very thin mirror (width-wise), it probably won't be noticable.

But, if you wanted to get the bow out to be sure, it's a pretty simply job to true two matched stiles using a hand plane. Plane a bit (1/64 ish) off the four respective ends on the concave sides, then butt the pieces together as you've already done to visualize the bow, and mark with a pencil how far back the pieces are mating from the ends, so you know how long of cuts to take in the next round. Rinse and repeat until you've got satisfactory visuals. Then true the other sides on a table saw.

Or put in a third rail to pull the siles together as suggested by Brian.

[Lee Schierer]

Can you flip the pieces over so the center of the bow is against the wall then fasten the top and bottom to the wall? The wall will push the bow out. a 1/8" mirror is quite flexible over the length you war dealing with and once mounted to the wall it will be flat, so all will be good.

[George Yetka]

I would cut the rabbit for glass 5/8 deep and use a light 1/2" x 2" piece for hardwood to pull the styles in.

[Carl Beckett]

Philosophically, there is ALWAYS 'SOME' amount of bow. (no material is perfect, especially wood).

So it is a question of what is acceptable.

Functionally (does it hold the mirror and go together) As you note, the stiffness of the pieces are not so great so it may be the mirror holds the wood down (with some clips on the back for this purpose)

Ascetically (judgment on your part. Do edges line up the way you want, does it look shabby) Do non woodworking friends even notice? (I notice many things that others do not)

[Jim Dwight]

My jointer is an old Inca with only about a 3 foot bed. So I can't straighten edges with it very well. So my standard way to straighten longer stock is to use my track saw. I used it to straighten all the boards for my 10 foot dining table for glueup. If you have enough material and a track saw this would be one "fix". A table saw can also do this by fastening a long straight board to the one you want to straighten. Otherwise if your rabbet for the mirror is deep enough you can "life with it" or pull it out with a center rail as already discussed. I also wonder if the wood might also move in the future. But my experience is the bow mostly comes from relaxing tension in the wood when I cut a wider board into narrower pieces. I try to allow for some straightening when I rip the boards down.

[Mel Fulks]

Most of the mirrors are thin and flexible…that’s how they make FUN HOUSE mirrors . If you want your house to be a “fun house”, make
sure it’s a heavy beveled edge mirror that shows straight, and that kind won’t show the rough innards of the rabbit !

[Lee Schierer]

Most of the mirrors are thin and flexible…that’s how they make FUN HOUSE mirrors . If you want your house to be a “fun house”, make
sure it’s a heavy beveled edge mirror that shows straight, and that kind won’t show the rough innards of the rabbit !

A little trick with beveled mirrors is to paint the rabbet black. Then you won't see any defects in the bevel of the mirror.

[andrew whicker]

Most of the mirrors are thin and flexible…that’s how they make FUN HOUSE mirrors . If you want your house to be a “fun house”, make
sure it’s a heavy beveled edge mirror that shows straight, and that kind won’t show the rough innards of the rabbit !

Aren't fun house mirrors polished metal and no glass?

[Jay Knobbe]

I recently completed 2 mirrors for a client and had the same concerns. My mirrors were narrower but taller. Based on what I learned consider the following:
1.Use 1/4" HDF or a good quality MDF for the backing. It will certainly be flat and offer more stiffness than fiber board.
2. I had my 1/8" thick mirror professionally mounted at a glass shop. They used an adhesive backing that prevents the glass from breaking into shards all over the place in case of impact. Similar to your windshield.
3. That 1/8" bow will pretty much disappear once assembled. I had that much, maybe a bit more and it came out fine.
4. Beveled glass could really stiffen things up but I'm pretty sure we were told it is available in 1/4". It is very heavy and much more expensive. Plus it really can't be cut to a specific size. You make the frame based on the mirror size available.
5. Paint the inside of your rabbet either white or black or stained same as frame.
6. As someone else mentioned I did have the mirror cur 1/8" smaller in all directions so I could handle it and set it down without fear of breaking the glass and then suffering 7 years of bad luck! ;-)
Sorry, I couldn't figure out how to flip the images. They are straight on my phone and PC.