Importance of polishing the backs of blades.

[HarveyWildes]

I want to run some technique questions by the hand tool experts, but need to explain how I sharpen and why first.

Generally I start with sandpaper to flatten the backs of my blades.  I test the flatness by starting with 2000 grit sandpaper, and if I see high or low spots, I go to lower grits (400-600, and even lower if there are bad problems) to flatten faster, and then go back up to 2000 grit until the back is flat and uniformly finished.  The sandpaper is fixed on a granite plate.

Once the back is flat, I use a Veritas MKII jig to set the initial bevel (either 25 or 30 degrees, depending).  I use 400-600 for this, although if the bevel is significantly off I start with lower grits.  Once the primary bevel is where I want it, I might do a pass with 2000 or I might not.

At this point, the blade has the right shape, but is not polished.  I'm happy with the results I get using this technique to this point in the process.

The next steps are to polish the back and hone the edge, in that order.  I am assuming that the back and edge both need to be polished to the same grit.  I've known people who just hone a micro-bevel at this point, but in that case the deeper grooves on the back still intersect the edge and leave it rough, which means it is not as sharp and dulls more quickly.  I haven't done any head-to-head tests to verify this, but it seems both intuitive and anecdotally true.

The trouble is that when I polish with higher grit stones, I'm not getting the results I expect.  I've been working up through waterstone grits from 4000 to 15000 (regular Shapton), and in some cases the polish almost seems worse when I'm done with the 15000 grit stone than after the 2000 grit sandpaper.  Recently I tried an 8000 grit Shapton glass stone and got really good results on the first chisel I tried when I polished the back and honed the edge by hand, but I was in a hurry to do a quick test and honed the edge by hand rather than using the MKII jig.

I think it's important to polish the backs of blades, so I tend to go back and forth between stone grits as I'm sharpening, first working the backs and then the bevels.

When I am flattening and polishing, I make sure to rinse and wipe the blades between grits, and also to rinse and wipe down the roller on the MKII jig if I am using it.

So here are the questions, all related to the fact that I can't figure out why I'm getting inferior results with my older stones.

* Do stones get cross contaminated with grit from other stones?  If so, how do you avoid that?

* Is it possible that the MKII jig is somehow to blame?  For example, do stone particles from the lower grit stones embed in the brass roller of the MKII jig and then end up embedded in the higher grit stones?  Should I be doing more than just rinsing and wiping down the roller?

* I use a very rough (but flat) lapping stone to flatten my stones.  Could that be the source of the problems?  What do people recommend for lapping stones?  Veritas sells a system that uses silicon carbide powder on glass for flattening stones, but I'm afraid to use that because I'm afraid that the particles will embed in the stones.  What about diamond lapping plates?  Do Shapton glass stones require the Shapton diamond lapping plate (which is pricey, to say the least).  If diamond lapping is required, what about using diamond lapping film on a flat plate?  It's much less expensive.

So far I haven't used the 8000 grit Shapton stone with any other stones, so no contamination would have happened to that stone.  But before I integrate it into my overall sharpening routine, I want to make sure that I'm not going to ruin it with a bad sharpening process that would contaminate it with larger grit particles.

Any thoughts or experiences would be appreciated.

 

[mouppe]

Others will chime in, but here's my brief answer.

The back needs to be polished to the same level as the bevel. Normally, however, you just prepare (i.e. flatten and polish) the back when you first get the blade, after which you only need to use the finest stone on the back. This is because when you sharpen the bevel, you are also removing material from the very edge of the back side too. So get the back of the blade polished and flattened to a high level and then don't worry about it other than a couple of swipes on highest grit to remove the burr created by polishing the bevel.

I use a Veritas honing guide and have not found grit transferral from one stone to the other to be a problem. Sandpaper may be different. I do rinse each stone before using it, and wipe down the roller too.

I use a 220 diamond plate to flatten my stones and also to remove a lot of material from the bevel if needed. From there I go to a 1000 stone, and then I finish with a 10,000 stone and perhaps a leather strop.

Hope this helps.

 

[Lemwise]

Flattening the entire back of a chisel is complete bullocks. You only have to worry about the first and last 2-2.5 cm. The only exception is when the back is convex. In general most chisel backs are not enough out of flat to make any difference. Anyone who tells you otherwise doesn't know how to use a chisel. I don't know who started the trend of flattening the entire back but he or she should be ashamed.

 

[HarveyWildes]

I once flattened the backs of some Sorby and Record chisels that were seriously convex, and it took -way- too long.  Now I just buy chisels that have flat backs to begin with - Veritas PMV11, Lie Nielsen, and Stanley Sweetheart chisels all met my expectations out of the box.  I almost wanted to work on the Sweethearts, but they were good enough.  I try not to polish more than the last inch of the back, but sometimes I get carried away.

Whether the effort that you put into sharpening is worth it depends a lot on the steel, the bevel angle, and the wood.  Sorby chisels (circa 15 years ago, rosewood handled) sharpened at 25 degrees aren't worth the effort to hone past 4000 grit because the edges turn up so quickly.  At 30 degrees, they hold an edge for a while, but at least they are easy to sharpen.  PMV11 chisels at 25 degrees can be sharpened to 8000 grit and will pare thin slices off cherry end grain for an hour.  At 30 degrees, they will pare thin slices off cherry end grain all day.  Granted cherry is not hard on edges, but if you're using ipe or jatoba, you're asking for the extra work to keep your blades sharp.  My chisels don't have to be -that- good (although it's nice), but I really want my plane blades to be that good.  It's a pain to take the blade out of the plane body, sharpen it, reinstall it, and get everything adjusted just right again.  Even if it's a Lie Nielsen or Veritas plane.  I'm gradually switching out plane blades to PMV11 and finding that I can get an edge that will plane fine shavings way longer than the older blades that came with the planes originally - the edge retention is outstanding.  And I just don't use good planes or chisels on ipe or jatoba - I use carbide planer blades and sand it,

I'm not a fan of strops because mine curves the edges over.  My edges seem to get worse after using a strop.  I'm open to a tutorial on strop technique because I know guys that get good results from them, but I haven't figured it out yet.

All that said, I'm still searching for a perfect solution, or baring that, at least a better one.  I appreciate all of the discussion.

 

[derekcohen]

My thought is that you are not flattening and polishing the back of the blade immediately behind the bevel.

The second likelihood is that you are not polishing the full face of the bevel, itself.

Frankly, sharpening should be a simple matter and take a few seconds. Efficiency is central. When you hone full bevels on a honing guide, you are being the least efficient of all. Honing guides are designed to create a secondary bevel, and are not intended to work full bevel faces. If you do the latter, it is unlikely that you will replicate the previous bevel angle, and as a result you will need to form a new face.

My method is different - I am not suggesting it, but to clarify. I hollow grind the face at the angle I want (usually 30 degrees), and then freehand directly on the hollow. The hollow acts as a guide. Importantly, there is very little steel to hone, and it is this that makes for efficiency. A secondary bevel (using a guide) does the same thing.

In my sharpening system, the initial grind is the key to a quick sharp edge. In a honing guide system, the guide must create a secondary micro bevel. When this is done correctly, fewer stones/grits may be used.

My system:http://www.inthewoodshop.com/WoodworkTechniques/UltimateGrindingSharpeningSetUp.html

With regard stropping, use green compound on hardwood - not leather - to avoid dubbing.

Regards from Perth

Derek

 

[RobNJ]

Just a theory...
What chisels are you using?  I have noticed that when sharpening Narex chisels on Shapton Glass (1000 / 4000 / 8000) I wasn't able to get a mirror polish like I was getting on my plane blades.  Apparently the CrMn content of the chisels produces a lot of black, almost gummy swarf on the stones.  As an experiment, I wiped down the stone with a wet microfiber literally after every sharpening stroke and was able to get  a mirror finish in probably 15-20 strokes with the higher grit stone!  I'm no expert, but it felt like it wasn't cross contamination of grits, but actually the swarf preventing the finer grits from cutting properly.
(as a side note, I don't need to wipe the stones after every strong with my PMV-11 plan blades - only the Narex chisels)

 

[grbmds]

[member=61231]HarveyWildes[/member] Might be a minor thing and/or I misunderstood but I both flatten and polish the back to the final grit (in my case with an 8000 Shapton Glass Stone) before I move on to any other steps.

The comment from another poster is correct. Only the end of the blade near the edge needs to be flattened. Flattening the entire back would take a very long time on almost any chisel and/or plane blade and isn't necessary. The purpose of flattening the back is to ensure that the bevel and the back meet at the thinnest "line" possible without any scratches to blemish the edge and this is accomplished by doing only the back near the edge.

Sorry if  missed something in your initial post.

I doubt you are getting stray grit from a lower grit on the MarkII rollers if you clean it off before moving on, but you could ask Lee Valley about this possibility. Stones can be contaminated with coarser grits, but the only source for that would be from either your honing guide or not cleaning the tool off before moving to the next grit (didn't see whether you cleaned the tool off but am assuming you do).

For a lapping plate I use the DMT Dia-Flat (original) which is coarse and don't have problems so I doubt that's it, especially since you polish the back with higher grit stones.

As for the micro-bevel (or secondary bevel) I create that after I've gotten the back polished and the primary bevel created and taken to 1000 grit (or 1200 diamond plate in my case).

Then you can create the secondary bevel with the 1200 stone or with the next finer grit (in my case 4000 stone) and then polish that with the 8000 stone. Finally remove the burr from the back with the 8000 stone. If you are polishing the back with an 8000 stone and finishing the secondary bevel with the same 8000 stone, then the burr which remains on the back after finishing the bevel needs to be removed with the 8000 stone. Not removing the burr or removing it with a lower grit stone will defeat the purpose of using the 8000 stone to begin with, creating scratches form the lower grit stone. The result will not meet your expectations.

I wrote this kind of quickly so hope I didn't create any confusion. Basically your process seems to be correct except I'd add in that you must make sure to remove the burr from the back with the final stone after finishing the front with that stone.

 

[Lemwise]

Just a theory...
What chisels are you using?  I have noticed that when sharpening Narex chisels on Shapton Glass (1000 / 4000 / 8000) I wasn't able to get a mirror polish like I was getting on my plane blades.

A mirror polish doesn't say a thing. Take my Sigma Select II 6000 for example, it uses silicone carbide as the abrasive but because that stuff is so incredibly hard it hardly breaks down, which in turn causes the blade to never reach a mirror polish. It is however a 6000 grit finish, just a hazy one. Don't get hung up on stuff like that.