Lapping chisel and plane blade backs - a few tips

[rsser]

Crumbs, 13kg for the granite plate.

Wonder what the postage would be?!

Rated at one ten thou. That's a pretty small gnat.

[Mr Brush]

Definitely a pickup from the store job, but quite reasonably priced if it is as flat as they claim. If you use the mylar film as a sacrificial surface for abrasives, there's no reason why it won''t stay flat.

The granite plate is also a handy size, so it won't take up as much of the bench as my huge (and not very flat) glass plate

If they ever get them in stock, I'll buy one..............

[rsser]

Yes.

Seems V. may still make their glass sheet and I've emailed them about tolerances and whether it's anything more than garden variety float glass.

[ian]

Ern
I think you'll find that "garden variety float glass" is more than flat enough for lapping plane blades.
and if a piece is not flat enough a simple visual test will allow you to chose another piece.

[Clinton1]

Ern,

Get along to a monumental masons or stonemasons and take a look at polished granite offcuts. You'll need to work out the tolerance yourself, but I've always gotten 'beyond my ability to measure' flatness out of the polished granite. Can't knock back a scratch of the head and "dunno mate, $30?" for a meter by 100mm-ish granite.

[rsser]

Thanks for the tips guys.

V. responded re their glass lapping plate: it's std float glass and within .01" of a surface plate.

[rsser]

Have been pursuing another option too.

Elsewhere I posted the fantastic result I got with 2 Muji blades on the side of the Tormek wheel.

And the very disappointing result with the 3rd after having used the grading stone to try to clean the wheel side. Seemed to knock the cutting power dead.

I now have the Jet side grind bracket and with a bit of fiddling the T. truing tool might be made to work. If so, that will true and clean the wheel side and give me back the orig cutting power.

More later.

[jenkinsr]

I've been following this thread (and others) with interest; there's a wealth of knowledge here and I know I'm going to have trouble keeping my experience up with all the theory I've learnt.

I'm lapping the back of the blade of an original Stanley Nº4 plane circa 1962. I have a multi-tool on an Abbot & Ashby grinder, but I'm unhappy with the 400 grit belts I can get locally; the join in the belt is about 80 grit high, so when I tested it everything bounces over the platen

I also have a Viking wet grinder with a wobbly stone that I have tried very hard to dress flat and even. Holding the dressing tool on the side of the wheel, wedged into the side of the cast metal bath gives a fairly consistent and level surface, but it's not good enough.

When I move from the wet grinder to the abrasive film on float glass (aka Scary Sharp) I can see that the corners of my Nº4 Stanley plane blade are on a completely different planet. Poor technique or I've dressed the stone wrong, but it's a lesson learned. Not Tormek flat, obviously.

3 days, half a can of glue and a dozen sheets later I'm at last somewhere near level out to the corners and about 40mm up the blade, with some hollowing back from the edge. I've worked from 100 Grit through 240, 400, 800, ~1000/15 micron and ~2500/5 micron.

At this level (past 800) I can see that I'm not getting a mirror finish all the way to the edge: about a millimetre of the cutting edge and down both sides is relatively dull, so I'm a bit puzzled by this. I'm using a fairly strong magnet (from an old hard disk drive) but I do try to re-position it often to even out any cupping effect that it might have. I checked my progress and technique often by cross-hatching with Sharpie and making a pass or two, and I think I'm on the right track. I have tried to be consistent with my strokes and keep the pressure even, and spread the effort over the abrasive to even the wear...

Any clues as to what might cause this? Is this an inherent problem with Scary Sharp, or just my technique?

Another thing that I don't quite understand comes from a video from Fine Woodworking, where they show how to grind out a chipped chisel blade and re-establish a primary bevel. In the video, he works his way through several (3?) grades of grit, wiping out the scratch pattern of the previous one before moving to the next, then he goes back to the first (coarsest) and starts over. I sort of get it, but I'm not convinced it's necessary. Any one have any insight on this?

I've got a Hock blade and Cap due here about Monday, and Ron Hock's book sometime in August, amazon willing, but that's my budget blown. DMT and Shapton are going to have to wait.

Thanks,
®

ps. One thing about using a magnet on dry abrasive; all the filings stick to the sides and end of the blade rather than clog the grit.

[rsser]

Welcome to the marathon club ;-}

What you describe with the Stanley blade seems to be common enough with Scary Sharp.

Another holding option as described by Derek I think is a magnetic dial gauge stand. This may work better than a smaller magnet by spreading the pressure more widely.

Have you been alternating the directions of strokes from one grit to the next? That's another way of telling whether you're getting full coverage.

Depending on your facility with a bench grinder and intended use for the blade you might at this point just joint the edge to a mm back and/or camber the edge or round the corners.

Well all this is good practice for the Hock which will need lapping as well

[Blue-deviled]

Ouch, sounds frighteningly slow rather than scary sharp! Must be quite a toughened blade.

[jenkinsr]

Thanks for the feedback. I have been altering directions and using the scratch pattern to indicate when to change grits. By concentrating on the back of the cutting edge and selectively applying pressure I've got most of the hump out of the blade. It's not perfect yet but it's good enough to use - and it's time to use it.

As Ern suggested, I'll round off the corners a little. I've got a bit of Jarrah that's giving me some trouble so I might try a 5º back bevel to bring it up to a York pitch.

As far as magnets go, Derek's (we are not worthy etc.) mag base might be a bit tall and prone to rocking in my inexperienced hands. The magnet I'm using is quite low, ~5mm, enough to get purchase on but not much more. I've got a few to choose from so I might try with one of the less powerful ones next time, and see what happens...

I have a theory about why the edges get rounded using the Scary Sharp method

Ahem. My Theory: Although I'm using quite good quality abrasive papers there is give in the medium that allows the abrasive particles to move slightly under pressure, either in a cock's comb effect as the edge passes over in tension (pull stroke), or a standing wave under compression (push stroke). On the finer grades the 3M micro-finishing film doesn't move as much as it's on a firmer substrate, and that's when the effect becomes noticeable; the scratch patterns don't change right at the edge once I move to that media.

Based on a sample size of one it's not exactly watertight though. It's entirely possible that despite my best efforts the centre part of the abrasive did wear more than the edges (as pointed out by Jim Davey and many others) and that's all there is to it.

Well all this is good practice for the Hock which will need lapping as well

Yeah, I know. One of the reasons I chose the O1 over the A2!

Blue-Deviled: It's not especially tough (I've got some Global knives that feel twice as hard) but I'm not especially fast either. As I'm learning it takes me longer to set up and get going, and swapping out the abrasive on a couple of bits of glass, and cleaning them up, re-gluing without bubbles, waiting for the glue to dry... It all takes time. Shed Time.

[rsser]

Your theory about the inherent problem with SS sounds as plausible as any I've read, maybe even more so.

And it may be the case that there's more flex in Stanley blades than quality blades such that drop-offs are more likely to occur with one.

And if the best float glass can achieve is .01 tolerance then ....

sometimes we might get a great result and other times a carp one due to accumulation or cancellation of errors.

Re unequal exposure to the abrasive, I got a result on a Stanley #6 sole that was about 2 thou convex and I was happy with that. I've speculated that if you keep the abrasive about as wide as the sole and a good deal longer than it, differential abrasion should be reduced.

All good grist for the mental mill not to mention the biceps ;-}

I'm waiting on some tempered glass Worksharp discs and PSA abrasives, 150mm diam., and will have a go at mounting one of these in the drill press or lathe arbour for a lapping experiment. Reviews of the WS 3000 have indicated the risk of a higher abrasion rate on the leading edge side so some clever counter measures may have to be thought up.

Nothing comes without a price.

[jenkinsr]

I expect there will be a difference on the leading edge; to digress slightly, in a former life I had a lot to do with rally cars, and one of the problems we regularly battled with was tapered wear on disc brake pads. The leading edge of the pad always wore twice as fast as the rear, leading to a lot of travel in the brake pedal and a few soiled driving suits.

If you approach the spinning disk tangentially, with your cutting edge in line with the disc rotation you will probably see higher leading edge abrasion. If you approach it radially then the inside edge will be cutting slower than the outside edge. Depending on the width of the blade and the circumference of the disc this will be a minor problem or a major one.

Depending on the width of the blade and the bevel being cut, you might be able to even things out by angling the blade so the slower cutting inside edge is the leading edge, and the faster cutting outside edge is the trailing edge. I can see this being frought with inconsistencies though; lots of variables.

Off the top of my head I can think of two solutions:

Alternating approach: With the tool in a tangential aspect, alternate the direction of the disk regularly, without changing the aspect of the tool. Five turns forward, five reverse, rinse and repeat. That way each side gets to be the leading edge. (I don't know how the middle is going to like that, but I guess that depends on the scale of the problem).

The Venn diagram approach: move the tip of the tool in an arc from the outside edge, in toward the centre, and out to the other side. This will expose both edges to the same speed at the extremes, although one side will be in tension, the other in compression (and I don't know how significant that is either). If you couple this with alternating the rotation of the disk then it should all work out even, if it turns out to matter.

Hmmm. As you said Ern, clever countermeasures required.

Re unequal exposure to the abrasive, ... I've speculated that if you keep the abrasive about as wide as the sole and a good deal longer than it, differential abrasion should be reduced.

You're spot on there, IMHO. I did that with the 400 and 800 grit and know I know why. (facepalm) Thanks!
Oh, and thanks for reminding me I still have the sole of this beast to finish...

[Blue-deviled]

Glad ito hear the iron's not as hard as I feared! I think some study should be done on 'Shed Time' - it doesn't occur as often as one would wish, and time passes much faster than it should when inside...

On the lapping front, I'll dig up the details on some diamond/stainless steel plate (they're about a 1/4" inch thick, so stable) that a friend brought back from Japan. I've been borrowing hers for problem irons before hitting the waterstone, and it is fiendishly fast Well priced (about $50 from memory) too.

[rsser]

Good thinking Jenkinsr. I can reverse the lapping disc on the lathe (solution 1); would solution 2 have the edge end up pointing at the oncoming abrasive rather than trailing? Might be tricky.