Can you harden maple by sanding too vigorously?

[Frans Waterlander]

Hi, am new to the forum and relatively new to woodturning. Had my lathe for about 18 months and made about 20 medium sized objects, mostly using hard maple.

Is it possible to sand with too much force, overheating my maple coffee grinder base and hardening the wood to the point where is was almost impossible to take any more wood off with a just-sharpened scraper? I had to apply more than one pound of force on the scraper to get it to remove more wood!

I sanded so much (and so vigorously) because I had much tear-out of the end grain and had to fold the sandpaper twice so I had 4 layers of sandpaper between the wood and my ungloved fingers to prevent burning my fingers.

 

[Jake Debski]

Hi, am new to the forum and relatively new to woodturning. Had my lathe for about 18 months and made about 20 medium sized objects, mostly using hard maple.

Is it possible to sand with too much force, overheating my maple coffee grinder base and hardening the wood to the point where is was almost impossible to take any more wood off with a just-sharpened scraper? I had to apply more than one pound of force on the scraper to get it to remove more wood!

I sanded so much (and so vigorously) because I had much tear-out of the end grain and had to fold the sandpaper twice so I had 4 layers of sandpaper between the wood and my ungloved fingers to prevent burning my fingers.

Frans, it sounds as though you created enough heat to produce case hardening of the surface. When it comes to turning wood, excessive heat heat is your enemy.

 

[MichaelMouse]

Sure can, as you discovered. That's why a lot of people have shiny surfaces with ugly scratches embedded in them when they press too hard. The way to release the "case-hardening" is to dampen the surface, raising the fibers you heated by excessive pressure, superdrying the surface. Sometimes it may take two cycles. Use the last grade of paper lightly after the surface dries and see if it cuts.

You may do good as well as evil with burnishing. Turners of old used to grab a handful of shavings and rub them hard into the spinning work instead of using micromesh or other small grit. You can do it too. After a sanding with the grain with 320 (P400) sandpaper, take a grocery bag or other brown wrapping and burnish with it. Thinned oil finishes will still stick, but water-based, for obvious reasons, are not recommended.

Here's a demo burnish on maple.

 

[Frans Waterlander]

Thanks for your replies. Given the problems case-hardening caused in my case, why would one want to burnish wood on purpose? To make it harder so it will wear less in use?

 

[Bill Boehme]

First of all, casehardening is a kiln drying surface defect and is not going to substantially have an effect on turning.

If you are sanding with paper that is not worn out, then it is also not going to have any effect on the hardness of the wood. Wide belt sanders that are used to "plane" wood sand far more "vigorously".

It appears that your tools are beyond dull. The fact that the end grain of the wood had significant tear out it good evidence of that. The fact that you are applying so much pressure is even further indication that the tools are completely dull. Basically, you should not be applying any force at all with a scraper. Did you use a burnisher to pull a hook on the edge of the scraper after sharpening?

Since your sandpaper is getting so hot, it tell me that it probably is worn out ... and as Bruce Hoover says, "Worn out 120 is not 220". Trying to sand with worn out paper will only serve to get the wood and your fingers blistering hot. With some woods that have a high resin content, getting it really hot can cook the resins near the surface and create a bit of glaze. You will know if this is happening if you see "corns" developing on the sandpaper. However, while sanding with worn out paper is nonproductive, it doesn't have a great impact on how the wood turns.

 

[MichaelMouse]

Thanks for your replies. Given the problems case-hardening caused in my case, why would one want to burnish wood on purpose? To make it harder so it will wear less in use?

Burnishing makes it shine. There are a lot of turners who spend and sand through a half-dozen grits to get a glow like the one 320 and a paper bag produced on the pictured sample. It's even more cost and time effective on turnings with lots of coves and beads, because their worst offender is normally heel burnishing from the tools used to make them, which disappear when the entire piece is burnished.

So don't press when you sand. Pressure times coefficient of friction is the equation for total friction, and thus total heat produced.

The reason it's called case-hardening, as is the similar effect caused by overdrying the outer portion of the wood in a kiln, is because its analogous to flame-hardening steel. In the kiln they normally use a final rehydration cycle with steam to release the case-hardening. Works for us, too.

 

[Barbara Gill]

Before I jumped in here I wanted to confirm what I thought I knew as fact. Casehardening has nothing to do with hardening the wood. It is a drying defect caused by drying the surface of the wood out faster than the inside resulting in drying stress. Woodworkers who do flat work have experienced the results when the casehardened board they are ripping on the table saw grabs the blade. There is a simple test for drying stress in boards. Dry wood cannot be casehardened .
We as woodturners even if we have turned green wood to completion and are sanding it are working with a much thinner product. Even if we heat the wood up during sanding it will not become casehardened.
As was said, we sometimes intentionally and sometimes unintentionally burnish the surface of the wood. This does two things: the surface becomes reflective and a penetrating finish will not penetrate as it should. Burnished wood can be removed by cutting either with a tool or more coarse sandpaper.
As a turner who mostly used a penetrating oil I would not want to burnish the wood.

 

[MichaelMouse]

Might want to turn to the FPL sources to review "casehardening." Yes, I still write it in quotes. It is indeed the condition of a dryer layer of wood over wetter. In the case under consideration, created by the heat of friction. In the kiln, by an interior lagging the loss from the surface.

Wood is never completely dry unless you have cooked it down to zero moisture content. Under those circumstances you would not be able to "caseharden" it by drying further. Such is seldom, if ever, the case. The wood is at equilibrium with the moisture content of the air at least on the surface. The interior of larger sections will lag. Overheat the surface, and the wood will contract as it loses moisture. According to the folks at Madison, it's pretty much a linear rate from the FSP to oven-dry. So the surface will contract on the interior by a percentage or two - whatever we manage to heat away. This produces the condition defined as casehardening in The Wood Handbook
"Casehardening. A condition of stress and set in dry lumber
characterized by compressive stress in the outer layers and
tensile stress in the center or core."

A few might recall the result if we overheat endgrain when sanding, those checks which form and then hopefully close as the surface readsorbs water from the atmosphere or from our damp rag. That's "casehardening" tension being relieved.

 

[Barbara Gill]

Dr. Gene Wengert,
"Casehardening, as you appreciate already, has nothing to do with hardness. It results earlier in drying when the shell tries to shrink (because the fibers are drying under 30% MC) but cannot. At the end of drying when the MC is uniform, there will still be this shell that is larger than the core. You cannot casehardening dry wood."

As a former professor and extension specialist at Virginia Tech and researcher at the US Forest Products Lab, Gene teaches over 30 practical wood processing classes and seminars a year for the wood products industry, including sawing, edging, grading, drying, machining and gluing.

 

[MichaelMouse]

Makes you wonder why they inject steam to rehydrate at the end of the kiln cycle, if he's correct in saying that the wood is equalized by then.

http://www.fpl.fs.fed.us/documnts/fpltn/fpltn-213-1958.pdf

you'll recognize the drawings from Hoadley.

 

[DOCworks]

Maybe someone already discussed this or maybe I'm off base. But using your scraper after sanding is going to be really hard on the tool. Particles left over from sanding especially if you don't wipe it down are going to dull the burr very quickly. If I need to scrape after sanding I usually wipe down the bowl with DNA, this raises the grain and dries very quickly, careful of the fumes. Most of the time when I have problems, it's because the tool is not sharp, be it skew, gouge or scraper.

 

[john lucas]

I go with the dull tool theory. Even the hardest woods that I turn will cut cleanly with light pressure with a sharp tool. You say you are using a scraper. did you raise a burr on the edge? If you can't feel a raised burr when running your finger across the edge (not down the edge, you don't want a cut finger) then it needs to be sharpened. You can do this by taking it to the grinder. If you sharpen until you see sparks come over the edge it will usually raise a burr. You can also raise a burr with a diamond hone. You polish off the old one by rubbing the hone across the top and keep it flat. then push the hone up the bevel toward the edge. Do this several times until you've raised a good burr.
Another possibility is the angle you approach the wood with the scraper. The handle should be higher than the cutting edge. You may have to raise it higher to find the good angle where it cuts.

 

[MichaelMouse]

I go with the dull tool theory. Even the hardest woods that I turn will cut cleanly with light pressure with a sharp tool. You say you are using a scraper. did you raise a burr on the edge? If you can't feel a raised burr when running your finger across the edge (not down the edge, you don't want a cut finger) then it needs to be sharpened.

Even a sharp tool will fail to cut when improperly presented. In this case, lacking other information, I think he's trying to scrape round and round on a face grained piece. Further, I think he was using the scraper broad into the wood, resting flat on the rest. Would have been so much better had he cut from the face or base progressively across the grain, but the word "push" just doesn't support it.

Burrs left from grinding are fragile, and will break off and disappear rapidly if pushed into endgrain. They might also appear not to cut because those ugly torn spots are actually below the level of the burnished surface. I much prefer a burnished and turned edge which presents itself as a microblade rather than a ragged line of leftover particles. That would work like a low angle shave. With no turned edge, just a deburred surface with maybe a back bevel ("negative rake"), you can also scrape pretty cleanly.

Not sure I buy the tool being dulled by embedded grit here. Seems, given the flat presentation, that it would, at worst regrind a finer edge. Not that there would be much grit embedded in that burnished surface. Nor that using liquid would remove any more embedded grit than a dry wipe or dust.

 

[Dean Center]

I'm not as expert as the other posters, but it seems to me I get heat build up if I'm sanding at too high a speed. Sometimes it seems like this glazes the surface over and the paper won't cut as effectively, sometimes with sticky shiny stuff (is that 'corns'?) clogging the paper. If the paper is too hot to hold, it's a reminder to me to turn the speed down.

 

[Bill Boehme]

... sticky shiny stuff (is that 'corns'?) clogging the paper...

That is correct. It comes from the resins that get cooked out of the wood when it gets really hot. These resins mix with sanding dust and clog the paper.