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I came on this:

The teeth on Auriou rasps are slightly angled to favor right-handed users. If you are left-handed, please contact us as we can order you left-handed rasp (although we do keep some in stock). The reason has to do with the proper use of a rasp.

A rasp should be grasped by the handle in one hand. This is where the power behind the rasp comes from. The other hand should grasp the tip of the rasp in thumb and forefinger, to balance the pressure on the stroke and to give control. The teeth should be perpendicular to the direction of the stoke. Auriou rasps are handed, so you hold them at an angle to the work but push them perpendicularly to the work.

I understand what they're saying. My question is, how much does it matter? If you're using such a rasp 'off-handed', is it noticeably less effective?

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  • Quite significantly, since if the teeth are angled in the 'wrong' direction they won't bite into the wood the same way (generally they're remove wood more slowly). You can check this effect using any normal rasp, by deliberately filing at an angle to the axis of the rasp's body. Now that aside are you looking at getting some higher-end rasps like the Aurious? – Graphus Dec 7 '19 at 8:43
  • @GraphussupportsMonica - I'm not really a 'woodworker', per se. More, carpentry. But, there have been times where an ironing rasp would have been very useful. The LH/RH thing matters b/c (a) I'm left-handed, but (b) sometimes just by the situation, I'd have used it RH'd (if I'd had one). Plus, other ppl seem to be mostly RH'd. – George Dec 7 '19 at 15:30
  • If you were in the market for 1 or 2 rasps for occasional uses (especially as a left-hander) I was thinking I'd have quite a few alternative suggestions for you but unfortunately an ironing rasp is so specialist there aren't many, and my go-to recommendations don't cover this specific tool configuration. However, oddly for something as specialist as this we have a previous Q&A which might be of some use, woodworking.stackexchange.com/questions/2535 – Graphus Dec 8 '19 at 7:04
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I understand what they're saying. My question is, how much does it matter? If you're using such a rasp 'off-handed', is it noticeably less effective?

Yes, conventional rasp teeth (in both hand-cut and machine-cut rasps) are like little pyramids of steel and cut most effectively when the flat front face is pushed straight into the workpiece. Working at an angle a certain amount of the blunt side of the tooth is presented to the wood, and the trailing cutting edge can't shear as effectively.

There's no need to buy an Auriou rasp to see this effect in action. With any conventional toothed rasp, including machine-made ones1, just gauge the rate of material removal using the rasp in line with your strokes and when the rasp is at an angle to the stroke, say 15-25° (to the left or the right).

The rasp should be noticeably slower when worked at an angle, but whether this speed difference is actually of any importance will be down to the individual user.

One other thing worth noting is that you may also notice that the above leaves a smoother surface. As a result with coarser machine-cut rasps especially you might deliberately use the rasp at an angle, to reduce the known tendency of such rasps to plough deep furrows in the work. So within reason2 there's not just a single angle a rasp must be used at.

1 Machine-cut rasps have teeth formed in a hydraulic press working at high speed and the teeth are both very uniform in size and in placement.

2 You wouldn't want to use a rasp at ≥45° to the stroke. Try it and you'll see why.

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