The 1"-thick oak tread on my spiral staircase has cracked as you can see in the picture. I can pull the two pieces flush by screwing (and gluing?) a board (or a piece of plywood?) from the bottom. But is there any sense in squirting glue into the crack?
That shouldn't be hard to fix. If it were me, I'd get a flat bladed screwdriver, insert into the crack to widen it just a bit. Using a glue like TiteBond, smear it over the crack trying to force as much as possible into the crack. remove the screwdriver, wipe it down then using bar clamps tighten the crack. Once more, you'll have to wipe down any squeeze out. Be sure to do a thorough job on the wipe down because any residue can interfere with later refinishing. All it takes is a wet rag if using TiteBond.
Yes, it is worth glueing the crack. Your plan is one way.
You could also consider drilling from one side with sufficient depth to go past the crack to glue in dowels which would also work. Need long drills and need to be accurate as coming out the face is not a desired result.
Another similar option is to drill pilot holes and insert big long screws from the side with glue in the crack.
Or you could rout slots - thinner in the middle like butterflies on the underside which will act similar to the dowels. A technique shown by those making tables with cracked or weird wood and resin.
- Scabbing a piece of plywood or even another piece of oak on the bottom is one way of fixing this, but it's not a pretty way of doing it unless you cover the entire bottom of the tread, but then you'll always see one tread that's thicker than the rest.
- Running a screw through the thickness of the tread (from the front or back) would be much better, but it can be very difficult to keep a long, thin drill bit directly on track down the center of the relatively thin tread and not have it burst out the top or bottom, making another thing to have to repair.
- You might be able to drill in from the outside edge of the tread at an angle and run your screw through the crack from there. This still requires a steady hand and a good clamp to keep the tread from moving while you drill.
- Simply gluing and clamping the crack has a high probability of lasting a long time.
Cracks in panels (like this) are usually repaired with a butterfly (or bowtie) patch. The patch will take some extra effort to make, but it can be installed top or bottom (or both, since both sides of the tread will be visible), and it can be made out of a different color wood as a decorative addition to the stair tread or more oak to try to hide it in the wood.
Whatever you use to patch it, it is essential that you get glue into the joint to reinforce it. There are syringes made specifically for injecting glue into cracks like this. You can find them at woodworking specialty stores or online (or even the specialty store's online locations). They're well worth the investment as they help make certain you actually get glue into the joint.
You'd have to understand the mechanics that caused it to split in the first place (weak joint, weak growth ring, load transfer, bad glue) to make a proper, lasting repair.
Assuming as it appears to be two boards making one step (as most circular stairs I've seen are), my recommendation would be to to remove the step, split it completely, clean the joint, add some joinery biscuits and glue and clamp back together.
In the alternative,
to widen the split, glue and clamp, though this is not likely to hold long-term unless the tread is fully supported underneath. Still better to remove the tread, pry open (if not splitting) the seam, glue and clamp, then replace.
Or you could try some English craftsmanship as seen in these double dovetail, a.k.a. "butterfly joints", at Leeds Caste in Kent, UK.
Image from BuilderBill website.
In case anyone is unclear what my answer means (?) I m suggesting an alternative method to squirting in glue into a crack to hold it together. The method is a double dovetail.