Which edge of the plane's mouth should I file if I want to open it up?

Question

I have replaced the cutting iron on a Stanley #4 hand plane (edit: because the old one was kaputt, not by vanity). The replacement blade I got from LV is thicker, at 3.8mm vs the 2.0mm of the old one. The mouth is now too small to allow the new blade to protrude. I measured the current mouth opening in the sole to be 5mm. I am reusing the original chipbreaker, screw, and lever cap.

To enlarge the mouth, it looks like I can file either the front edge, or the rear edge. Is there a right or wrong edge to file?

My plane has a moveable frog (only via the two top screws in the frog, this model has no rear adjuster screw), but it offers only a very narrow range of useful positions. When it's backed out by any more than 1mm from its most forward position, the base casting starts interfering with the mating of the blade against the flat surface of the frog.

That gives this frog a total range of front-back adjustment of about 1mm, if I understand correctly. When I set the frog as far back as allowed (ensuring that the blade mates well with the frog), I can't lower the cutting iron past the mouth -- the cutting edge of the blade bumps against the fore edge of the mouth.

^{Frog backed up completely. I don't understand how this position would be useful in any way and I question the design. There is about 2.5mm of material I could file before the black rib between the two legs of the frog would start interfering with the blade.}

^{Frog pushed in completely. The frog barely clears the base casting of the mouth on the rear edge. If I remove the washers under the screws, I can nudge it by another 1mm forward.}

^{The blade hits the sole before clearing the mouth. In this image, the frog is set as far back as possible, without the base casting interfering with the mating of the blade against the frog.}

References:

Rob Cosman, in "Filing the Mouth of a Plane" shows how to file the fore edge of the mouth without mentioning the back edge.
Christopher Schwarz, in the book Handplane Essentials (in chapter Metal Bodied Jack Planes), writes that you want to be able to configure a mouth opening to be 1/16" (~1.6mm) for general work, or smaller (for finer work). He describes also that the black rib between the legs of the frog can be filed if necessary, but doesn't describe a strategy for filing the edges of the mouth.

Two additional personal points I want to make, 1, that replacement iron may not be (polite for, isn't likely to be) an actual upgrade to the plane. 2, be very careful about listening to Cosman. Quite apart from his habit of nakedly shilling of things which are of both very expensive and of questionable utility to viewers many woodworkers with far more experience than he has have caught him making misleading statements and of presenting personal opinions and half-truths as hard facts (and accepted as gospel by his ardent fans, another thing to watch out for). — Graphus, Dec 04 '19 at 16:01
thank you. in this case, i tried restoring the original cutting iron, but its steel was cracking with little hairline fractures. i purchased the LV replacement blade in the interest of time (vs getting a second hand compatible one) — ww_init_js, Dec 04 '19 at 18:46
i also do agree with taking cosman’s advice with a grain of salt — this one seemed like a reasonable restoration idea however. on a separate forum, a user hinted at a video series from Schwarz, “super tuning a handplane”, which supposedly describes a similar procedure. I don’t have access to that full video to confirm. For once, Cosman doesn’t recommend buying something, so I thought maybe there’s some legitimacy to it. — ww_init_js, Dec 04 '19 at 18:54
"in this case, i tried restoring the original cutting iron, but its steel was cracking with little hairline fractures." Ah, fair enough! That might still be salvageable with a lot of grinder work but there's no guarantee so yeah, a replacement makes total sense. Out of curiosity is the LV iron carbon steel or did you go with one of the alloys? — Graphus, Dec 04 '19 at 21:11
I could post photos of the old blade and ask a few things on a separate question. I did my best to tap it back into shape because it had a big belly in the center and was taking forever to lap. I tried a Paul Sellers trick (i.e. mallet taps) to save grinding time, but probably ended up destroying the blade in the process. :/
Re: new blade, I am trying the PMV-11 alloy option. It seemed like a good tradeoff between O1 and A2, based on LV's "white paper" on steel hardnesses. But I have no experience with it -- it won't fit (yet)! — ww_init_js, Dec 04 '19 at 23:46
And I know it's not necessarily Paul Sellers' trick per se. I attributed it to him because I saw him first describe it on his plane restoration youtube video, at 00:47:06. I may have overdone it, and now here I am, learning about how to file plane soles. — ww_init_js, Dec 05 '19 at 00:07
Yeah unfortunately bashing an iron flatter, while a great trick to have in one's bag of tricks, sometimes bites you in the ass. I've never cracked an iron myself doing it (only had to use it a couple of times) but a friend cracked a really nice old iron recently doing it. Luckily it's only one crack in the centre and it doesn't seem to flex open when the iron is under tension but it's not like you can come back from that! I should point out though that he's using that cracked iron, but it's in a 5 and not a 4 [contd] — Graphus, Dec 05 '19 at 07:54
so it's never going to be the tool that last touches the workpiece. However, if you normally scrape and/or sand after you use your smoothing plane then neither is your 4 so..... — Graphus, Dec 05 '19 at 07:56

Graphus · Answer 1 · 2019-12-04T21:25:48.507

The front exclusively.

The rear surface of the mouth represents part of a reference/registration surface that works in conjunction with the frog's face to support the iron assembly, and if it's in good condition and doesn't need remedial work it shouldn't be touched¹.

In addition the metal is generally thinner at the front than at the back (not even accounting for any frog-alignment lugs) so the filing will go faster.

^{Note the angle the plane is held in the vice in the left image. Having this surface you're filing angle forwards is not always necessary, but can be, so it's best not to file it square to the sole just in case.}

How carefully do you have to file?
Depending on what sources you believe, who you listen to or trust, the mouth's front surface plays a vital role, a significant role or is of no real consequence to the performance of the plane. So, uh, somewhat?

Obviously you wouldn't deliberately file it out of square but your best effort is really all that's needed in a lot of cases (as shown by planes that don't have perfectly square or parallel mouths which work well for their users).

The way to end up with the mouth straight and square is to have a line to file to and arguably the best way to do this is to colour the area with a marker or layout fluid and then, using a square held against the plane's cheek, lightly scribe with a scriber, the point of a knife or just a sewing needle. This will show as a bright silver line against the marker ink or layout fluid which is relatively easy to see in different lighting and can't be erased when you brush or wipe away filings.

Christopher Schwarz, in the book Handplane Essentials (in chapter Metal Bodied Jack Planes), writes that you want to be able to configure a mouth opening to be 1/16" (~1.6mm) for general work, or smaller (for finer work). He describes also that the black rib between the legs of the frog can be filed if necessary, but doesn't describe a strategy for filing the edges of the mouth.

Worth mentioning here that Schwarz has gone back and forth over the years on the question of using mouth size as an important or necessary aspect of plane use — in concert with or instead of using the cap iron/chipbreaker — so his views at the time of writing this book may not jibe with what he believed, and wrote about, a few years previously..... and won't necessarily reflect what he currently believes to be the case, and now writes about :-\

I want to note that a great many much more seasoned woodworkers never adjust the frog on their planes². This includes numerous full-time professionals. Their frogs are generally set back to align their top surface with the angle of the back of the mouth, providing maximum support for the cutting iron³. So the mouths on their planes remain fixed, which lends support to the idea that the front edge of the mouth doesn't necessarily play much part in the performance of a plane. See more on where mouth size does and doesn't matter in this previous Answer.

¹ The one exception to this is possibly to slightly round over or chamfer its leading corner if it's very sharp, to help prevent damage over time as the plane's mouth runs over knots, particles of grit or grains of silica in the wood. This sharp edge can be found in planes straight from the factory as well as after a user has lapped the sole — aggressive lapping frequently leads to the front edge of the rear of the mouth ending up a virtual knife edge, which must be softened for safety.

² Despite an adjustable frog being considered a vital design element in a plane that we're told everywhere to look out for!

³ Every adjustment of the frog to bring it forward of this position essentially means the tip of the iron assembly is hanging in space somewhat which can allow for some vibration, which could lead to chatter.

in your experience, is a file with safe edges necessary? or will being careful cut it. — ww_init_js, Dec 04 '19 at 19:23
No, IME a safe-edge file isn't vital. One good tip to using a non-safe-edge file for a job of this sort is when you're working at each end of the slot to file inwards only. So you're always filing away from the side wall, making accidentally marring it much less likely. One thing you might want to do for added safety though is put tape one the opposite face of the file, so that if/when you accidentally bump the back edge it doesn't matter. — Graphus, Dec 04 '19 at 21:04
would you mind sharing a citation for the images you included (unless they were created for this answer)? If they're from an external article, I would like to read it. — ww_init_js, Dec 06 '19 at 23:12
As a rule I don't add citations to a mixed image like this, unless there's something of special relevance. Even if I didn't normally these two came from sources with conflicting and/or misleading info which I felt would just muddy the waters for you and future readers. But if you want to read the source for the one on the left it's on the Canadian Woodworking site. — Graphus, Dec 07 '19 at 08:27

Kevin Kruse · Answer 2 · 2019-12-04T16:50:21.600

Intuitively I would choose to file the fore edge of the mouth, for two reasons:

The angle is not critical. The rear edge matches the bedding angle of the frog, whereas the fore edge is just perpendicular (edit: as Graphus points out in this answer, the fore edge angle may also matter).
Filing the fore edge means the frog adjustment doesn't need to change.

Bear in mind I'm no expert in modifying hand planes and I haven't ever tried modifying a plane to fit a thicker blade before.

Which edge of the plane's mouth should I file if I want to open it up?

2 Answers2