Scraping planes & cap irons/chip breakers

[ian maybury]

This may not be news to anybody - but while trialling a Veritas scraping plane last night after talking cap iron set up for planes earlier (the big one with the tilting blade holder - got it accidentally set much too coarse at first, and was surprised to see it peel quite a thick shaving off) the thought surfaced that the geometry of the cutting edge probably ends up quite similar to that of a plane running a very close set highish angle chip breaker - as described in the paper and web pages mentioned in the earlier threads. The ultimate in a close set gapless cap iron you could even say.

The final burnishing of a 45 deg bevel at around 15 deg seems likely (if the blade is vertical) and making allowance for some springback to deliver a rake/pitch of something like maybe 30 deg (like a low angle plane), with a back clearance of something over the 15 deg angle of the burnisher. Tilting the blade forward using the adjuster will probably increase the pitch to something more like standard, and increase the clearance - it leaves room for some fine tuning of these angles in use.

It's probably not by accident that scraping planes handle tear out well, and ditto in the case of planes with close set cap irons….

Given the range of geometries we may accidentally produce when setting up the blade for one of these planes it's hard not to wonder just how sensitive the various angles are. It'd be interesting to see the work on cap iron placement extended to take a look at the effects of pitch and clearance - although no doubt there's those that know the story exactly from experience...

[bridger berdel]

the biggest stumbling block to a burnished edge being able to replace a smoother with a cap iron is probably the softness required to run the bur- the edge just doesn't last all that long.

[ian maybury]

For sure a scraper isn't necessarily a replacement for a smoothing plane Bridger. I wasn't so much thinking about that as just suggesting that all three tools (the scraping plane, the smoother and the bevel up smoother) despite being seemingly very different are actually all using the same principle to avoid tear out on difficult woods.

i.e. the geometry of a turned scraper plane edge is actually not very different to that of a bevel down plane blade with a very closely set cap iron. Experience suggests that a simple steeply angled bevel (as in the case of a bevel up plane) delivers very similar results too.

Which suggests that all three systems are actually chasing the same objective in terms of chip handling. i.e. having peeled a chip off using a sharp edge comprised of two polished faces to quickly turn it up through a tight radius by bouncing it off a steeply angled surface - which due to friction imparts enough of a reaction force to prevent splitting of the wood ahead of the cut.


ian