A Different Kind of Sharpening Thread

[Trevanion]

Before I start, I don't want to get into what rubbing steel against an abrasive surface method is best like every other thread seems to, in fact, it's not about hand tools at all so if that's your thing you might want to drop out of the thread now.

Something I've been wondering for a while now, Is there any gain to be had from sharpening planer/moulder knives to shaving razor-sharp? I've seen people advocate it a few times now, have their knives ground and then go through extra steps to get a mirror polish on the knives just like on hand tools to get a "better" cut finish. But I've been wondering whilst you may have a better cut surface initially, does that fade away and the knives get duller far quicker due to their very fine edge? I don't really see the point in it because planed surfaces should be sanded or hand-planed to remove the radial cut marks/washboard effect so having absolutely razor-sharp knives seems a bit pointless then. I've also seen people advocate razor-sharp knives for cutting curly and interlocked timbers but standard practice to mostly eliminate tear out with difficult timbers is actually a back-bevel up to 15-degrees on the knives, which produces an excellent surface on figured timbers.

You read in old books of "jointing" knives which essentially comprised of holding a carborundum stone on the rear bed of a surface planer or screw feeding a stone across thicknesser blades to effectively dull the knives until they all cut in the same cut-circle, which created a better finish with multiple slightly dull knives than a single sharp knife doing all the cutting.

Has anyone got any thoughts? I'm not sure whether there's any real benefit to it or not. I've definitely honed knives out of the packet to remove burrs and even out the grinding a bit but I think polishing to razor-sharp might be a pointless extra step.

 

[Deadeye]

Weeellll.... I think "sharpness" is a conceptual problem, and that issue is a prime cause of some of the controversy around "sharpening".
An early indication of trouble is the definitions being circular - "Sharpness - the quality of being sharp".
When you try to pin down "sharpness" - for example, as S.I. units, it's evasive.
Look closer and there's a hotch-potch of edge radius, hardness and brittleness. Look closer still and edge irregularity comes to the fore: the tomato-cutting example, which sees a parallel in woodworking in shearing cuts of planes or helical (shearing) cutters on planers.
To take a guess at a direct answer to your question:
I suspect that the reduced friction from a more polished surface and the reduced radius at the leading edge are marginal gains for the metal of that hardness, flexibility and brittleness in fine edges, and that much greater gains will be had by changing the attack angle or, particularly, by including cross-ways motion with the cut.

One final thought (well, you did ask!). Green wood cuts easier than seasoned - presumably because of the moisture content. So there is somehting going on at a cellular level in the substrate that changes it's response to an edge of given "sharpness".
I think that might be a very interesting area of research; I can find nothing published.

There, you've brought the research scientist out in me, even if no viruses to be seen!

 

[ED65]

Something I've been wondering for a while now, Is there any gain to be had from sharpening planer/moulder knives to shaving razor-sharp?

I believe the answer is a conclusive yes, sharper is sharper with all that implies.

No need to believe anyone else on the subject – seeing the improvement in surface with your own eyes is enough to convince on its own, or it's not. If you do find the results convincing whether it's worth it to do it every time is going to be down to you. It's clear from other contexts that for some the (10?) minutes this takes for conventional planer knives won't be worth the saving in time elsewhere in the process; not as a routine thing at least, maybe now and then for special timber?

In case you haven't seen it I'll link to this video from William Ng, Fast and Easy Way to Sharpen Jointer and Planer Knives.

I've seen people advocate it a few times now, have their knives ground and then go through extra steps to get a mirror polish on the knives just like on hand tools to get a "better" cut finish. But I've been wondering whilst you may have a better cut surface initially, does that fade away and the knives get duller far quicker due to their very fine edge?

Unfortunately I have to refer to one of the threads on sharpening hand tools. In one D_W just posted the results of a long and extensive test that I think shows conclusively that you do get a much longer-lasting edge the further you take it. This tallies with a lot of other tests on edge longevity BTW, although not all though. But not all edge-retention tests can be taken as directly applicable to cutting wood due to differences in material and cutting method (slicing action instead of push cuts).

If you do the whole process yourself there's the obvious advantage of saving money and time each and every time the knives need it, plus the likely extended interval between sharpening will save further. So even without any improvement in results there is gain there.

Beyond this, as you mention yourself, sharpening (or honing) your own knives gives you the opportunity to tailor the edges in the same way that some hand plane irons are tailored, by e.g. adding a back bevel to improve performance on hardwoods and especially on highly figured hardwoods. You're never going to get a finish-ready surface straight from the planer but the reduction in tearout can be really significant from the pics I've seen.

 

[MikeG.]

........– seeing the improvement in surface with your own eyes is enough to convince on its own.......

I think the point of the OP is that the quality of the surface finish is unimportant anyway, as it is always followed up by sanding or hand-planing and/ or scraping. If the only point of polished knives is to produce a surface you don't need, then there is no point to the extra work.

However, you then went on to show that polished edges last longer, and that is certainly a valid reason to at least consider doing the extra work. Personally, the biggest faff and taker-of-time in the whole process is re-setting the knives into position, so a few extra minutes working on the edge would be neither here nor there.

 

[Lons]

I never polished my planer blades and just treat the machine as a way to accurately dimension stock reasonably close enough for finishing, if using woods prone to tear out I just make it a few mm bigger than needed and do the rest by hand so must re evaluate now as it makes sense to polish the blades.

I have one of those sharpeners on a handle, a bit like this but mine is one sided and I find it enough to keep the blades sharp until nicked or at the grinding stage.

 

[Woody2Shoes]

I wonder if there is a burnishing effect - as the cutting edge dulls and becomes a less effective cutter it becomes a more effective burnisher which tends to compensate. I know that my hand tools do both at the same time.

 

[Hornbeam]

When you sharpen hand tools you are aiming to get the bevel and the back meeting at a perfect angle, however the sharper the tool, the more fragile the cutting edge produced. I suspect for planer and moulding knives any benefit from that initial "super sharpness" will be lost in the first few minutes of use.
Most turning tools are used straight off the grindstone as I suspect any benefit from fine honing would be lost after the initial cut
Lots of people have done examinations of how sharp an edge they can produce but I dont know if the edge has been checked after different periods of use to show how it wears and holds up
Ian

 

[Trevanion]

Unfortunately I have to refer to one of the threads on sharpening hand tools. In one D_W just posted the results of a long and extensive test that I think shows conclusively that you do get a much longer-lasting edge the further you take it. This tallies with a lot of other tests on edge longevity BTW, although not all though. But not all edge-retention tests can be taken as directly applicable to cutting wood due to differences in material and cutting method (slicing action instead of push cuts).

I would never have thought that a far finer edge exceding 1200G would last longer than an edge at say 240G. But as you say, does that also apply to radial cutting where the knives are cleaving off material at high speed rather than a gentle slicing push cut? Perhaps this isn't a totally fair comparison because I can't say 100% what the material composition is, but I use HSS Tersa knives in a machine which are polished on the cutting edges and are razor-sharp, they seem to dull far quicker than HSS "standard" knives in a different block in the same machine, on the flip-side the Tersas are very quick to swap out.

If the thought of the finer you go on the knives the longer the edges last is true, it may be worth the extra half an hour to gain an extra bit of time between having to swap out. It's just in my head a super fine edge is more liable to chip and burr up when hitting knots and such compared to an already slightly rough edge.

 

[D_W]

I tested plane blades, but have no clue if that would hold true on a machine planer that could run into dirt, etc. There's a pretty clear reward for hand plane blades of reasonable quality, as long as someone doesn't negate it by making a sharpening process take eons.

 

[ED65]

........– seeing the improvement in surface with your own eyes is enough to convince on its own, or it's not.

I think the point of the OP is that the quality of the surface finish is unimportant anyway, as it is always followed up by sanding or hand-planing and/ or scraping.

Granted further work will always be needed. But the brightness or clarify of the wood isn't the only surface-quality issue as I think you're thinking? If it helps reduce tearout that alone could make it worth doing, to reduce the amount needing to be taken off by the next process.

When you sharpen hand tools you are aiming to get the bevel and the back meeting at a perfect angle, however the sharper the tool, the more fragile the cutting edge produced. I suspect for planer and moulding knives any benefit from that initial "super sharpness" will be lost in the first few minutes of use.

This doesn't appear to be the case in practice. FWIW: Get Longer Life and Better Performance from Planer and Jointer Knives

The quality of the ground surfaces that you start with is obviously going to be a factor here, at least as much as how far one subsequently takes them. Although the greatest gains seem likely going from coarse-ish to something a lot finer I suspect in 99% of cases there's benefit to doing something to commercially sharpened knives.

I would never have thought that a far finer edge exceding 1200G would last longer than an edge at say 240G. But as you say, does that also apply to radial cutting where the knives are cleaving off material at high speed rather than a gentle slicing push cut? Perhaps this isn't a totally fair comparison because I can't say 100% what the material composition is, but I use HSS Tersa knives in a machine which are polished on the cutting edges and are razor-sharp, they seem to dull far quicker than HSS "standard" knives in a different block in the same machine, on the flip-side the Tersas are very quick to swap out.

Yeah the alloy could be different, and worse some cheap HSS isn't actually what it claims to be! Plus there are a host of other possible variables. Same bevel angle on both knives BTW?

The only way to definitively find out is a direct like-for-like comparison, take out the unknown variables.

It's just in my head a super fine edge is more liable to chip and burr up when hitting knots and such compared to an already slightly rough edge.

Totally get that.

 

[D_W]

Unfortunately I have to refer to one of the threads on sharpening hand tools. In one D_W just posted the results of a long and extensive test that I think shows conclusively that you do get a much longer-lasting edge the further you take it. This tallies with a lot of other tests on edge longevity BTW, although not all though. But not all edge-retention tests can be taken as directly applicable to cutting wood due to differences in material and cutting method (slicing action instead of push cuts).

I would never have thought that a far finer edge exceding 1200G would last longer than an edge at say 240G. But as you say, does that also apply to radial cutting where the knives are cleaving off material at high speed rather than a gentle slicing push cut? Perhaps this isn't a totally fair comparison because I can't say 100% what the material composition is, but I use HSS Tersa knives in a machine which are polished on the cutting edges and are razor-sharp, they seem to dull far quicker than HSS "standard" knives in a different block in the same machine, on the flip-side the Tersas are very quick to swap out.

If the thought of the finer you go on the knives the longer the edges last is true, it may be worth the extra half an hour to gain an extra bit of time between having to swap out. It's just in my head a super fine edge is more liable to chip and burr up when hitting knots and such compared to an already slightly rough edge.

I don't machine plane much, but I'll admit I don't take very good care of my machine planing setup, either. It's just a portable dewalt planer, and the blades don't stand up to much, so they're usually nicked almost immediately.

However, refinement of an edge isn't what leads to edge damage, but rather edge strength. If a steel is too soft or too hard, the initial edge that seems really sharp and clean will be a transient one, even if it lasts a little longer. In cleaner material, it'll still be transient, but it'll last somewhat longer.

In hand planes, it's more clear, because the edges do mostly slicing rather than beating. Even if an initial edge seems transient (which it is), the edge is sharper at every step along the way, which leads to longer life and less effort on the user.

Being sympathetic to that and not the machines, I'll admit that as long as my lunch box planer will beat its way through wood, I'll plane off the nicks. When it starts to suffer performance issues, I'll change the knives - if it works hard between those two things, I'm less sympathetic to machines and electrons. If that causes the machine to break, I'll just buy another one, but given my habits, nothing particularly nice.

Sharpness (as a function of cutting ease) is a matter of two things - angle (which will determine how well a blade avoids or tolerates some damage) and finish. I wouldn't be surprised on a machine if the proper angle was more important than absolute sharpness - especially on blades that are specified to be fairly soft and tough rather than hard, keen, and unforgiving to less than perfect conditions.