Help..can I salvage this fence?

[Johnbro]

Making a William Ng miter sled, and per his suggestion I laminated 3 pieces of 1/2" Baltic birch for the front and back fences. Once the glue dried and I jointed the top and bottom edges, I see that they are both bowed more than .030" at the center across 24" of length. 

Is this fixable? or do i toss them and get a better (ie flatter) piece of BB and make a new pair? I only really need the back fence to be flat, the front fence doesn't do anything. I thought of clamping them together to make them straight and gluing or screwing a piece of hardwood on top like a beam but on its side to hold it straight, but don't know if that will work.

Any ideas out there?
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[Wild Turkey]

Can't you face-joint the pieces?

If it's just .030" I'd pencil mark the faces heavily and get after it with a sander.

[frigator]

Same thing happened to me. I followed his advice and the plywood was not flat. I ended up making mine out of quartersawn yellow Pine that has stayed flat. You could try jointng the fence but plywood is tough on knives.

[JGrout]

What I am going to suggest is for the next time you want to glue stock together 

make the bows oppose each other when you lay out the parts 

And if you want 1 1/2" use 2 pieces of  3/4" 

As for your current quandary you could glue a leg to the back of the fence perpendicular to the bow. ( it would look like a piece of angle iron) It would reduce the cut depth of the sled but you should be able to salvage it.

Edit to add: it is possible that you could pull the bend out when you mount the fences to the sled. I am not sure how that sled goes together but a very shallow dado or rabbet to butt into or against and screws would pull out the bow as well

Joe

[Hank Knight]

I think your idea of gluing a hardwood strip on the top of the fence would work, except I would glue a stout strip on both the top and the bottom. Rip the plywood fence to accommodate the thickness if the two strips. I'd use 5/8" thick hardwood at a minimum, 3/4" would be better. Clamp the fence to something flat then glue and clamp the strips to the fence. I've added hardwood strips to the tops and bottoms of sled fences several times both for appearance and to provide a new surface for screws to correct an out-of-square alignment, but never to straighten a bowed fence. I don't see why it wouldn't work for that too.

[fixtureman]

We have fixed stuff like that with bondo and sending it through a sander

[Johnbro]

Thanks for all the replies. I didn't want to joint it because it is plywood, but now I think I will put a hardwood face on it (maybe 1/4") and joint that face flat. The back side doesn't matter, only the side that the work touches. This will be my ultimate cross cut sled (I don't want to have to make another) so I want it to be perfectly accurate. If you haven't seen the William Ng youtube video on how he gets his perfectly square I highly recommend you take 30 minutes and watch it.

[handi]

Gluing three layers of bowed plywood together would ONLY result in a bow if they were clamped up in a bow or if you used the wrong glue. 

Had they been clamped together on a truly flat surface, provided there could be no 'glue creep', they would remain flat.

I suspect you used titebond 2 or an equivalent to glue them up. Titebond 2 (and 3) do not actually dry hard. They remain very slightly flexible. This allows for 'glue creep', movement of the glue that in this case, resulted in your gap.

Glue creep is why when we used to glue up bent stringers and handrails for circular staircases, we used resin glue, not wood glue. 

Titebond 1 dries hard and brittle. It resists glue creep better than 2 and 3.

Now, to fix your issue, clamping a straight edge to your rail should allow you to force it straight, and then you can screw it to the base of your sled. Properly screwing it down should easily hold a 0.030" bow straight.

[Cooler]

Perhaps the moisture from the glue is causing the warp, and the ply is not pre-warped as assumed.

[Steve N]

Awesome sled video. I have made this sled a number of times. Some from 1/2", some from 3/4" for the fence. I can't say that 1/2" is flawed, it does take more work to get right though. Options are thickness of ply. solid stock, or a sammich. All work pretty well. I personally prefer solid wood (Hard Maple) because I don't like to screw into the edge of ply. It will wallow out over time. Plus if not perfect off the glue up, a jointer can make it so, and quickly. Jointing plywood is like swimming in a septic tank, not something I will do. If you wanna, let me know, I'll take picchurs for ya