Diamond paste on MDF - Not for me

[Reinis Kanders]

Patrick,

Where do you find those chisels useful? Banging around the house for carpentry or are they also good for woodworking?

Thanks.

Also the HAP40 PM HSS in one of my sets of chisels. Yet another useful purchase inspired by one of Derek's reviews.

[Bruce Haugen]

I used to work at DeLaval Enterprise Engine Division where we made such beasties. Long before my woodworking days began in ernest.

We didn't make an 18 cylinder model that I recall. Did make a 20 cylinder version.

jtk

We scrapped out that Worthington and a similar sized Nordberg and replaced them with a 18 cyl Wartsila, reliably and continuously making 10 MW. I loved that engine, because it could go from cold start to full load in a little over 5 minutes. I worked in the control room and never saw it run, though.

[Patrick Chase]

Patrick,

Where do you find those chisels useful? Banging around the house for carpentry or are they also good for woodworking?

Thanks.

Being completely honest they're not my most heavily used chisels. Unlike Derek I don't often work in super difficult woods (though I'd convinced myself I was moving in that direction at the time).

With that said, Derek hit the nail on the head in his "4 chisel steels compared" review. Note that the PM-HSS chisel wasn't one of the 4 he reviewed, it's what he used to finish the job after they all failed. They're built for heavy chopping, and they keep going in circumstances that cause edge failure in every other chisel I own, including the PM-V11 ones. As a rule I do exactly what Derek did: Start with more "reasonable" chisels, and switch over to the PM-HSS ones if they don't hold up. It doesn't happen very often (at least not since I straightened some issues in my technique out) but when it does they're life savers.

The thing to be aware of is that mundane water stones stones like Shapton Pros or Besters are basically useless on these chisels. Select IIs will cut them just fine, but don't deliver as refined an edge as you can get on O1 or PM-V11. I suspect that even with PM processing these things have moderately large carbides, and you need a sharpening medium that can cut the carbides instead of just pulling them out. In practical terms that mean moncrystalline diamonds or CBN, both of which are harder than any carbide. I use a combination of diamond paste on plates and 3M diamond films and get very nice edges fairly easily. Those cheapo Amazon/Ebay lapping discs for rotary sharpeners should also achieve a decent edge, but I hesitate to use them on PM-HSS because they appear to use polycrystalline diamonds and are therefore vulnerable to sharding (wear) at relatively low pressures.

[Reinis Kanders]

Thanks for the info. I definitely have multiple means to sharpen them. They could be useful in Doug Fir type of wood that can sometimes chip edges, but I have some marples beaters for it already. Those cheap lapidary disks do not last for me, especially the way I use them for initial lapping of bad chisel backs. Also I find that at higher grits, e.g. 1200, chisel seems to want to bounce on them, but that could be my worksharp.

Being completely honest they're not my most heavily used chisels. Unlike Derek I don't often work in super difficult woods (though I'd convinced myself I was moving in that direction at the time).

With that said, Derek hit the nail on the head in his "4 chisel steels compared" review. Note that the PM-HSS chisel wasn't one of the 4 he reviewed, it's what he used to finish the job after they all failed. They're built for heavy chopping, and they keep going in circumstances that cause edge failure in every other chisel I own, including the PM-V11 ones. As a rule I do exactly what Derek did: Start with more "reasonable" chisels, and switch over to the PM-HSS ones if they don't hold up. It doesn't happen very often (at least not since I straightened some issues in my technique out) but when it does they're life savers.

The thing to be aware of is that mundane water stones stones like Shapton Pros or Besters are basically useless on these chisels. Select IIs will cut them just fine, but don't deliver as refined an edge as you can get on O1 or PM-V11. I suspect that even with PM processing these things have moderately large carbides, and you need a sharpening medium that can cut the carbides instead of just pulling them out. In practical terms that mean moncrystalline diamonds or CBN, both of which are harder than any carbide. I use a combination of diamond paste on plates and 3M diamond films and get very nice edges fairly easily. Those cheapo Amazon/Ebay lapping discs for rotary sharpeners should also achieve a decent edge, but I hesitate to use them on PM-HSS because they appear to use polycrystalline diamonds and are therefore vulnerable to sharding (wear) at relatively low pressures.

[Patrick Chase]

Thanks for the info. I definitely have multiple means to sharpen them. They could be useful in Doug Fir type of wood that can sometimes chip edges, but I have some marples beaters for it already. Those cheap lapidary disks do not last for me, especially the way I use them for initial lapping of bad chisel backs. Also I find that at higher grits, e.g. 1200, chisel seems to want to bounce on them, but that could be my worksharp.

Out of curiosity what failure mode do you see with the discs? Do they get dull like AlOxide such that they require increasing pressure to cut, or do they become finer-grit like SiC such that they keep cutting but remove less material and leave a finer scratch pattern?

All of the ones I have appear to be polycrystalline, which means that they initially fail by sharding. Big clumps of diamonds shard into smaller ones, leaving a finer/shallower/slower cutting surface. Is that consistent with what you see?

The thing that a lot of people miss is that the stated hardness of diamond is for a single crystal. That's a reasonable predictor of life for monocrystalline diamonds (what DMT and Atoma use), but it's completely meaningless for polycrystalline diamonds (cheapo plates/discs). What you care about with poly diamonds is the bond strength in the "clump", and that's usually far lower than the hardness of monocrystalline diamonds (or the hardness of monocrystalline CBN for that matter).

With that said, well-optimized polycrystalline abrasives can be absolutely amazing in some applications, for example 3M 663FC a.k.a. diamond Trizact. Those are brually expensive though - 663FC is $120 for a 1x30 belt (just the thing for your Harbor Freight special!), or 12 times as expensive per square inch as the cheapo discs we're discussing in this thread. As far as I can tell the discs are just commodity polycrystalline clumps graded by size and then electroplated onto steel.

EDIT: One thing you might try is 3M 663X diamond lapping film. It's a heavier-duty, larger-particle-size version of the 668X film that LV sells for hand-lapping. Like that film it uses monocrystalline diamonds resin-bonded to a polyester backing, though the resin layer is more durable in 663X. An 8" diameter PSA-backed sheet of 60-micron 663X runs about $40, or $30 in quantity. Quality diamond abrasives aren't cheap, unfortunately. I bought a 25-sheet "inner" (typically the MOQ for volume pricing) of 3x6 60 micron 663X a while back and it holds up extremely well for hand-lapping. I think it's beefy enough for machine lapping if you're reasonably careful not to let tips "dig in" and keep the temperatures below where the PSA starts to creep.

[Reinis Kanders]

Out of curiosity what failure mode do you see with the discs? Do they get dull like AlOxide such that they require increasing pressure to cut, or do they become finer-grit like SiC such that they keep cutting but remove less material and leave a finer scratch pattern?

Initial degradation is that they leave finer scratching pattern, but soon afterwards they just stop cutting. Yesterday I actually tried older 100 grit plate and it was baraly doing anythin, but of course it managed to make a couple of deep scratches. I am still fairly satisfied with these disks because they dissipate heat better than sandpaper or some other psa backed lapping medium.
I recently started to use .5 micron film instead of strop and like the keenness it imparts, but I am not sure if it really has any practical impact.

[Patrick Chase]

Initial degradation is that they leave finer scratching pattern, but soon afterwards they just stop cutting

Yep, that's how frangible abrasives like polycrystalline diamonds (and SiC) die. I still use them as well because at the price I can afford to treat them as a consumable, but as noted above I don't use them on anything really hard because I suspect that they'd fail so quickly as to be uneconomical.

A tightly graded 0.5 micron film like that (I assume you're using the ubiquitous green 3M 061X sheets that LV and many other outlets sell) is the equivalent of 20K-30K grit depending on whose system you use as a reference. As you say the results look really pretty, but probably don't deliver much practical benefit. That doesn't stop me from using 0.5 micron pastes/films though :-)

FWIW I don't have skipping problems on the 3K discs. I use them to quickly hone secondary bevels on tools that dull frequently but don't need to be critically sharp, like mortise chisels. I've never tried to lap a back on one though.

[Pat Barry]

George must have missed this thread. He has advocated using green compound on MDF many times. I am sure he has some insight to add to this thread. I do know in our industrial lapping processes we use diamond slurry on a relatively porous ceramic wheel and other industrial processes use diamond slurry on cast iron. With both method you need to dress the wheel frequently to keep it flat. We only use monocrystalline diamonds for our work.

[Chris Parks]

If anyone is looking for readily available and cheap cast iron for diamond honing they should consider brake rotors from cars whether new or used does not matter but the used ones would have to be machined of course, trucks use far larger rotors and would be worth having. Flywheels from manual cars are also cast iron and would give a large flat surface but also might need machining to flatten before use. One thing about using a flywheel would be it is unlikely to move in use.

[Jon McElwain]

That's not my observation. Where do you find a certified $20 granite surfacing block?

$31 here:

http://www.woodcraft.com/product/144...2-a-grade.aspx

I use the wet/dry sandpaper method - I'm not a sharpening aficionado, but the paper has worked well for me. After the paper I take it to a whetstone. Biggest issue I have with the sandpaper is that it curls after a few minutes of use.

[Patrick Chase]

George must have missed this thread. He has advocated using green compound on MDF many times.

Or he may have correctly recognized that I wasn't saying anything about that. I was careful to spell out two things: This thread is about diamond paste, and MDF with diamond paste is "not for me" (meaning that the issues I raised may or may not matter to anybody else depending on their subjective preferences).

FWIW I use green compound on both leather and MDF for carving tools etc. I think that it's a different situation than diamond in a few respects:

1. It's generally the last honing step, so you don't care as much about small flatness issues since you're not going to turn around and try to lap on another paste on a board with a different wear profile. All that matters is that the tool is flat enough to function correctly, and that's a pretty low bar for everything except chisel backs.

2. Green compound has a very different and less mobile/penetrating vehicle than either oil- or (especially) water-based diamond pastes, so the flatness issues in particular may not be as severe depending on how much of that was due to simple abrasion vs softening due to absorption. I'm pretty sure that the reason the MDF dished faster with aqueous pastes than oil-based ones was because it's water-permeable (actually hygroscopic, which even worse). The solid wax-ish carrier for the compound wouldn't have that issue.

3. Green compound and MDF are both pretty cheap. It isn't as much of a cost issue as with diamond paste if you have to recharge to get the cutting speed back up or even move to a new spot every time you want to flatten something.

4. The green compounds that most people use have a significant amount of larger calcinated Alumina particles mixed in with the CrO2. Leonard Lee spells this out pretty clearly in his book. I suspect that the fact that abrasive particles recede into soft backings (like MDF and leather) is actually a feature when used with the green compound, because it allows you to start out cutting fast (using mostly the Alumina particles) and still end up with a fine finish (on mostly CrO2 once the Alumina has receded to the same level). This may also explain why people are able to jump from fairly coarse stones to the "0.5 micron" green compound and quickly get good results - If I did that with a tightly graded 0.5 um CrO2 film (3M 061X) I'd be honing all day.

[Patrick Chase]

A granite surfacing block is near the cost range of a sheet of MDF. Is this not a good option for sharpening or flattening?

Sorry about the delayed reply, but... a 32 sqft MDF panel (64 usable sqft for honing) goes for about $30. It's about the same cost per unit area as sandpaper, so it can be treated as a consumable lapping medium.

A granite surface plate is about $30 for 0.75 sqft, so it's not really cost-effective as a lapping surface for pastes.