Advice for setting screws in dense wood

Question

The Janka Hardness Test provides a scale for the hardness of woods. Toward the higher end, wood can be particularly difficult to drive screws into and even a pilot hole may not be adequate. Is there a good method for firmly setting screws into a work-piece without snapping the head?

Approximately where on the scale would these extra measures become necessary?

Answer 1

Your milage will vary. I think it will depend on a lot of factors beyond the wood - type of screw used, screw head, method of driving them, temperature, etc.

Some ways to make it easier to drive and avoid snapping the screw are:

1. Larger pilot hole
2. Larger screw, if possible (with a larger pilot hole)
3. Lubricate the screws - a bar of soap works well
4. Select a screw that does not cam out (slip out and strip the head) easily - robertson, torx, pozidriv
5. Choose a screw made from a harder material - avoid brass
6. Use an impact driver to drive the screw
7. Use a screw with a higher number of threads per inch

Answer 2

I cannot attest for at which point the woods hardness come into play but the type 17 (The type is describing the point) screw which was designed for quick penetration and can effectively remove the need for pilot holes. They are typically found in decking. I use them when I get my hard oak pallets as I have lost bits, nails and screws in that wood.

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If you take a listen to a gentleman on YouTube he would suggest you use a bar of hand soap to lubricate the screw. The man in the video only talks about lubrication but in theory that should help prevent overheating which makes the screw more malleable and susceptible to breaking.

Answer 3

I would echo the advice of the other two answers, but would like to provide specific advice on screws.

I've heard good things about SPAX screws, and they have a guide to pre-drilling sizes here http://www.spax.com/uk/technical-terms/pre-drilling which may be useful.

Personally I've always used Reisser R2 wood screws which come pre-lubricated and I've VERY rarely had problems with the screws breaking or being hard to drive in, even without pre-drilling. I work at a joinery and we make thousands of hardwood windows each year. We've tried other screws as there's always somebody trying to sell their fancy new wood screws with Torx drive and cutting tips, self-countersinking heads etc. but nothing is a patch on the Reisser screws.

If you try these screws and still have trouble, then try pre-drilling. If you still have problems after that then something else we've tried when driving in long (160mm) decking screws is applying some silicone sealant to the screw thread, by dipping the screw into the end of a tube). This lessens the friction and also makes it easier to remove the screws later, if necessary.