Shop made jigs warp. Is that common?

[James Baker SD]

I have made several crosscut sled for my tablesaw to improve my accuracy. I always use baltic birch ply (typically 3/4") and give the jig several coats of shellac to seal the surfaces. I live in a dry climate, but eventually (within a couple years) every jig warps or twists enough to destroy its usefulness as an accurate cutting aid. Am I doing something wrong or is this common? I am about ready to make some cabinet doors out of baltic birch and don't want them warping in a couple of years. How to the pros keep their doors flat? Our kitchen has plywood doors on the cabinets, no frames, just big sheets of ply with walnut laminate and they are still flat after 30 years.

James

[George Gyulatyan]

How are you storing your sled when not in use? If it warps it could be due to humidity differential between the top and bottom.

[pat warner]

" to destroy its usefulness as an accurate cutting aid. Am I doing something wrong or is this common?"
Royally common; you're doing nothing wrong but following convention.
All woody products, (MDF, ply, wood, masonite etc), are subject to deflection and dimensional surprises whence they soak up water molecules. Moreover, toggles, cam and other clamping devices exert unbelieveable forces on their substrates and bend them all up during use!
I use nothing but 1/2" ground aluminum (http://patwarner.com/images/router-X-cut-jig.jpg). Easily handles a 1500 -2000lbs/ft without deflection. Can be stored underwater without changing shape.

[glenn bradley]

I have three sleds with 3/4" BB ply bases and full length, adjustable front fences that have been giving years of reliable service. I added rubber feet on the edges where they stand on the cement in a mostly un-insulated shop. I do live at the foothills of a desert basin area which is quite dry (according to visitors). Temps range from the 40's to the low 100's. Just lucky? Like you, Pat is close to the water by comparison to my 'about' 60 miles from the coast. Much wetter than here.

[Ray Newman]

Wondering if the problem could be in the quality of the plywood?

At the Big Box Stores and a few local lumber yards, I have seen imported from “who-knows-where” plywood that is labeled as Baltic Birch. From talking to other woodworkers who worked with this alleged Baltic Birch, it is not a very stable product and has internal voids.

For shop made wood jigs, an application of spray on shellac with paste wax over that helps a great deal as well as storing jigs flat or hanging them so as they are not “disturbed“.

[Fred Voorhees]

I always try to make my jigs out of MDF because it is so stable. I have been using the same table saw cross cut jig for more years than I know at this point and it is as straight and true as the day I made it.

[Rod Sheridan]

Wood changes dimensions with changes in humidity.

Wood jigs can be inexpensive to make, yet they aren't dimensionally stable under all conditions.

The moisture content of the wood used at the time of contruction is important, the wood should be well aclimatised to your shop, not just brought home from the supplier.

You may or may not have great luck with wood jigs. I've found they're OK, not excellent.

There's a reason commercial accessories are made out metal............Regards, Rod.

[Lex Boegen]

" to destroy its usefulness as an accurate cutting aid. Am I doing something wrong or is this common?"
Royally common; you're doing nothing wrong but following convention.
All woody products, (MDF, ply, wood, masonite etc), are subject to deflection and dimensional surprises whence they soak up water molecules. Moreover, toggles, cam and other clamping devices exert unbelieveable forces on their substrates and bend them all up during use!
I use nothing but 1/2" ground aluminum (http://patwarner.com/images/router-X-cut-jig.jpg). Easily handles a 1500 -2000lbs/ft without deflection. Can be stored underwater without changing shape.

Great flaming gobs of <censored>! That is one impressive looking jig! Where do you get that material?