Flat backs, new and old blades, and I'm confused.

[Aaron Rosenthal]

In the last 2 months back-orders have eased, "seconds" sales occurred, and I ended up with 3 new planes. I sold a few, too, but that's not my question.

Because it's winter here and uncomfortable in my garage workshop, I bring my hand tools inside and overhaul/oil/sharpen etc.
I decided to flatten all my Stanleys' irons backs; The 6 was fine, the 8 (series 4) was grossly not flat and took hours to even get 80% flat and shiny, and the 3 was OK.
I did the cap irons as well a la David Charlesworth (https://www.youtube.com/@DavidCharlesworth).

Why confused? My new planes.
I expected to have a lot of work on the Stanleys, but not on the new irons.
The planes: LV Custom #4, Custom #5 and Skew rabbet (rebate) planes. 2 bevel down, one bevel up (Skew plane).

The sheet that comes with the planes says that the backs are lapped and that only the secondary bevel be needed.
I have 3 DMT diamond plates I use for sharpening, which I assume to e dead flat, unless my square's flatness is suspect. Besides, when doing the primary and secondary bevel, I don't see any lack of flatness.
But each back I try to flatten shows the left and right sides going shiny well before I can get the middle to the same degree of polish.
I'm really not sure if it makes that much difference, so I'd like to rely on opinions here.
"Everyone" seems to opine that flattening the back of older irons is a good thing; modern manufacturing techniques get the irons much flatter than ever before, and I do understand that even these level of instruments are a starting points.
So my question is, with modern,"local" quality tooling, is the need to flatten the back still as necessary?

[Tom M King]

Sounds to me like that diamond plate might not be as flat as you're assuming it is. I would bet on a LV iron back before I would bet on a diamond plate, especially if it had been used any.

[Aaron Rosenthal]

But Tom, when I do the primary and secondary bevel, there's no arc; and it's over 3 plates.

[Michael Bulatowicz]

I have some thoughts on what might be happening, but rather than go into detail immediately let’s start with some questions that might help figure out what’s going on:
1. How much pressure are you applying?
2. Are you using the little rubber feet that came with the DMT plates?
3. Are you sliding the irons with cutting edge trailing/leading or side to side (or perhaps on a diagonal)? Along or across the DMT plates? Using the whole plate or just the center? When you sharpen the bevel, what portion of the plate are you using and how are you using it?
4. Where on the iron are you applying the pressure?

As Tom said, the LV plane irons come flat to a very high level; I very much doubt the irons are at fault. While they don’t look shiny out of the box, the backs of the irons have also been abraded to a higher grit/smaller scratches than any DMT extra-fine plate I’ve ever used.

[James Pallas]

If your diamond plate is your precision measuring instrument it’s like using a hardware store free yardstick against a Starrett metal precision rule. The reason the edge seems good it’s because it is.
Jim

[Aaron Rosenthal]

To answer your questions: I swipe length wise along the long edge of the plate. Pressure I can’t quantify, but I use 3 fingers on the top holding down the iron. I have tried to see if focusing the pressure on the centre part of the iron makes any difference but I can’t see any difference. I lubricate the interaction with water and a little laundry detergent from a spray bottle.
The rubber feet are on.
I start at the edge of the DMT plate and work a little towards the centre, and turn the plate around and work the other side.
The DMT plates were bought at different times, I use “fine”, “extra fine “ and “extra extra fine “.
I flattened the backs of my chisels, and didn’t have any issues (Narex Richter) when I got them.
When I use them same plates to sharpen the cutting bevels, primary and secondary, the irons are perfect.
Let me be clear; the planes perform stunningly. No complaints with the shavings, ease of pushing with my limited use so far. All my new planes have PMV-11 blades, and they are great. I just can’t figure out why I’m getting this pattern. And it’s only that I’m not actually making anything that my curiosity is aroused.

[Michael Bulatowicz]

To answer your questions: I swipe length wise along the long edge of the plate. Pressure I can’t quantify, but I use 3 fingers on the top holding down the iron. I have tried to see if focusing the pressure on the centre part of the iron makes any difference but I can’t see any difference. I lubricate the interaction with water and a little laundry detergent from a spray bottle.
The rubber feet are on.
I start at the edge of the DMT plate and work a little towards the centre, and turn the plate around and work the other side.
The DMT plates were bought at different times, I use “fine”, “extra fine “ and “extra extra fine “.
I flattened the backs of my chisels, and didn’t have any issues (Narex Richter) when I got them.
When I use them same plates to sharpen the cutting bevels, primary and secondary, the irons are perfect.
Let me be clear; the planes perform stunningly. No complaints with the shavings, ease of pushing with my limited use so far. All my new planes have PMV-11 blades, and they are great. I just can’t figure out why I’m getting this pattern. And it’s only that I’m not actually making anything that my curiosity is aroused.

With the rubber feet in place at the corners, pressure is going to slightly bow the DMT plates such that they'll be effectively hollow in the center; support through the whole underside of the plate can dramatically reduce this effect. The bowing from pressure is, however, not likely the primary issue unless you're really bearing down--if you're not bearing down, the bowing will be small and inconsequential.

It seems significantly more likely that it's an issue of abrasion favoring the leading edge and/or a small amount of inadvertent rocking (very difficult to avoid when working on flattening the back of an iron). What I mean by abrasion favoring the leading edge is that if you're applying fairly uniform pressure and swiping the blade to the right, the right-hand edge is going to see the most abrasion; likewise, swiping to the left, the left-hand edge will see the most abrasion. Once the back of the blade is slightly curved due to this effect, a slight rocking back and forth while swiping side-to-side is almost guaranteed.

If it turns out that the backs of the irons become too far out of flat and you need to remove material from the center of the iron, you can overhang the edge of the DMT plate (or other sharpening stone) and focus your pressure on the center of the iron; this same technique can also allow one to reduce the camber on a blade in the event that the cutting edge is more cambered than desired.

All that said, a flat back to the iron is not actually required--instead, it's important that the back be "polished" (abraded to a high grit, regardless of whether it's cloudy or shiny) right at the cutting edge and that the iron and cap iron (chipbreaker) mate well to each other. A reasonable level of flatness certainly helps ease this, but there's no need for it to be flat within a thousandth of an inch; pressure from the lever cap will help force the two together and close up minor gaps/out of flatness as long as the curve is smooth and shallow. As a side note, a thinner iron and a reasonable level of spring to the cap iron actually makes this easier--thinner irons don't need to be as flat as thick ones because they can flex more under the pressure from the lever cap, as can the cap iron. If you can set the cap iron's leading edge just shy of meeting the iron's cutting edge and yet still don't get any bits of shavings or dust between the iron and cap iron during use, the two are well-mated and the back of the iron is flat enough (as is the leading edge of the underside of the cap iron).

Regardless of how flat the back of the iron is when new, it will tend to get slightly "bellied" over time during removal of the burr when sharpening. This is, again, inconsequential as long as the iron and cap iron can mate well to each other and the iron is sharp.

On the subject of the cap iron's leading edge, the factory angle from LV (or LN) is too shallow to enable the cap iron to mitigate tearout. This isn't a problem per se as long as you're willing to make an adjustment to get it set up properly. If you ask around, you may see recommendations from 50-80 degrees (or perhaps even a wider range). I have no reason to believe that 50 degrees wouldn't work, but I can say I've had good success with a roughly 80 degree leading edge and smooth curved transition to the shallower factory bevel; if I remember correctly, it was Warren Mickley who recommended this configuration, though I don't believe it was specifically in the context of a LV or LN cap iron.

I hope this helps.

[Reed Gray]

There are diamond plates, and there are diamond lapping plates. The difference is that the lapping plates are pretty much dead flat. The older DMT stones with the holes in them are not dead flat. I am pretty new to planes, but figure they all need to be flattened on the bottoms. The table saw table, if you have a good one, or a jointer bed are pretty much dead flat.

robo hippy

[Cameron Wood]

"modern manufacturing techniques get the irons much flatter than ever before..."


Heh heh.


Only necessary if you want the plane to work well.

[Jim Koepke]

My advice on flattening plane soles or plane blades is to only do it if there is a problem that can be seen as being caused by the blade or sole not being proper.

It is much easier to mess either blade or sole up by trying to correct a condition that doesn't exist.

jtk

[Jack Dover]

took hours to even get 80% flat and shiny

Sounds like you need a faster abrasive.


But each back I try to flatten shows the left and right sides going shiny well before I can get the middle to the same degree of polish.

This can happen if pressure is not evenly distributed across the iron. Usually people tend to put more pressure on a leading part, that's the part that points in a stroke's direction (hope this makes sense). In some cases there's so much pressure that two sides of an iron aren't parallel after a while or if it's a cutting edge - an iron goes out of square. This also tends to happen on a slow abrasive, because we're trying to compensate for slower removal rate with pressure.

If you observe this on the bevel side, grinding might be the reason. It's totally possible to remove a bit more material in the middle, so outside parts get abraded first, but this usually goes away after a few sharpening sessions. There are other reasons for outsides to abrade quicker than middle parts, these reasons related to free abrasive and since you're using diamonds it's safe to assume it's not happening here.


is the need to flatten the back still as necessary?

It depends. LV, LN, Hock irons usually come sufficiently flat and. But you still have to polish the back though. Then it depends whether your abrasive is flatter than the back. Usually it takes 10 mins at most to bring it to a diamond plate level, if I had grind one of these irons as much as a vintage iron - I would probably reach out to the customer support.