Bench plane mouth opening / frog adjustment size

[James Spangler]

I'm a new contributor to SMC forums and this is my first posting, so please excuse any goofs here...

I've been retired for three years now and I enjoy restoring old rusty/crusty bench planes as a hobby - for me it's rewarding to bring back to life an old tool. I've kept a couple, given away a few as gifts, and have even sold a few. I'm not a frequent woodworker, but am gratified when I get feedback that a plane I've restored is being used successfully by someone who is.

I watched many on-line videos on restoring and tuning-up planes and I'm always learning something new about that process.
For example, I was surprised when I first learned that the chip breaker was much more than just a gizmo to stiffen the iron.
Since then I pay much attention to the leading edge of the chip breaker when tuning-up and testing a plane that I'm trying to restore.

But what I haven't seen is a good explanation of best practices of when to adjust the frog forward or back. I understand that you'd want to have a mouth opening large enough to permit shavings to sail upwards without hitting parts of the mouth, but is there more to it than that? For example, if I just set the frog back leaving a larger opening in front of the cutting edge is that somehow suboptimal in how the iron cuts through the wood in certain situations? I *think* I've seen advice that you should move the frog forward if you're trying to get fine wispy shavings. Is that true, and if so why?

[Jim Koepke]

Howdy James and welcome to the Creek.

Here is a post of mine from over a decade ago about rehabilitating old planes > https://sawmillcreek.org/showthread.php?114373

It is archived in the Neanderthal wisdom/FAQs > https://sawmillcreek.org/showthread.php?103805 < along with many other useful posts.

The mouth setting doesn't have as much to do with preventing tear out as a properly working chip breaker. Most of my planes have the frog set so the back of the mouth is
co-planer with the face of the frog. This helps to support the blade to avoid blade resonance or chatter.

Many years ago some of my planes were set with a tighter mouth. This was before learning about how to use the chip breaker.

Now days my only mouth adjustment is with bevel up planes when working on convex faces. This helps to eliminate the blade digging into the work.

jtk

[Jerome Andrieux]

But what I haven't seen is a good explanation of best practices of when to adjust the frog forward or back.

The frog is adjusted to obtain the desired mouth opening, taking the blade thickness into consideration.

A plane is usually set for one job and is adjusted rarely, to manage special timber, wear, etc. Scrub, fore planes, jointers, jacks ...

Some planes are polyvalent or more refined, and may adjusted more frequently. Smoothers, jacks again.

A wide mouth opening is good for heavy cuts and shaving but it doesn’t provide any pressure on wood fiber just in front of the blade, nor any support so you are at risk of "tipping" at the end of a narrow piece and of course of tearing out reverse grain.

A fine mouth opening is something like twice the thickness of your shaving. This provides support and pressure very close to the edge, which was the preferred way of preventing tear with single iron planes. Infills, Japanese planes, coffin smoothers were set like that. Such a narrow mouth opening makes the plane to clog easily on less than good wood though.

With double iron planes, with the chip breaker set close to the edge (less than half a mm), a mouth opening of "less than a mm" is good enough to bring good results, no tear out, no clogging, no tipping.

[James Spangler]

Thank you Jim and Jerome!

Thank you Jim for the links to the other articles - very interesting. You've reinforced my focus on getting a good edge and mating surface on the chip breaker.

And Thanks Jerome for the reply. I had not considered the aspect of the pressure on the wood provided by the area just in front of the mouth.
I assume when you mention double iron planes, you are referring to an iron with a chip breaker attached?

[Tom Trees]

If tearout is of concern...
Beware of taking the advice on closing the mouth up, with a regular Bailey style plane!
The area in front of the mouth has nothing to do with anything regarding holding fibres down, the cap iron does that all day long without needing that malarkey.

If you haven't a mad camber or rounded corners on the cutter, then you will be able to set your cap iron to, or just under 1/32" for most tough timbers.. (further away and the cap stops working concerning tearout)
Hone the leading edge of the cap iron, to around or just over 50 degrees, as many folks like David Weaver suggests (David W on youtube) or Derek Cohen might suggest.
It has effect on the distance needed to work correctly, can be less forgiving to adjust the plane in use, if honed steeper.
If honed less than 50 degrees, then the cap iron needs to be closer, equalling less camber...

Mr Weaver's name is darkened, as he has made a lot of publications on the subject, as no-one seems to listen to fellow creeker Warren Mickley, who got him to actually try using the cap iron to it's full potential, so I have him to thank, as I learned from David's publications.

To this day!!! many folks are still unclear about this, it is unquestionably better performing, look for anyone who honestly "tried it and went back to using scrapers"

Going back to setting the cap iron
Two iron's needed, one with a cap set no further than 1/32", with less camber/distance given for a tough batch,
and another iron that can be set half that distance again, an un-noticable camber until paired with the cap iron, so not honed to suit any setting in particular.
i.e.. If you can set the cap iron to 1/64" away from the edge, then you will have a cutter profile that can be closer if needed.

If you try closing the mouth of your plane on the former setting, then it will be really hard to push for no reason at all.
If you try closing the mouth of another plane with the latter setting, then the plane will refuse to work, full stop.

Many folks just need that wee extra push to stop thinking the area in front of the mouth is of any benefit, if the cap iron is set correctly.
Keep the frog in line with the plane as Jim says.
A bit of wax should be on the bench, as the plane will hold itself better into the timber than you will have noticed before!

Have fun making straight shavings and treating every timber the same as if it were just some agreeable species.
No skills or tricks needed around knots or reversing grain, just sail through from one direction the same as with clear stock.

All the best
Tom

[Jim Koepke]

Ask Dr. Google < setting the cap iron < and it should lead to a post on wood central by David Weaver.

SMC Terms Of Service doesn't allow posting links to other forums.

jtk

[Jerome Andrieux]

Thank you Jim and Jerome!

Thank you Jim for the links to the other articles - very interesting. You've reinforced my focus on getting a good edge and mating surface on the chip breaker.

And Thanks Jerome for the reply. I had not considered the aspect of the pressure on the wood provided by the area just in front of the mouth.
I assume when you mention double iron planes, you are referring to an iron with a chip breaker attached?

Yes, "double iron" means blade and chipbreaker. It may be an antiquated term nowadays, as most bevel down planes are "double iron". There is no need to go for a very narrow mouth opening on those. Just narrow enough to make it confortable on narrow pieces.

"Single iron" planes are mostly bevel up planes, but also some bevel down Japanese and infill planes, maybe some high angle smoothers. They usually feature a thick blade to limit chatter. Some Japanese blades are almost 10mm thick, compared to the usual 2mm Stanley They can feel great in use on adequate wood.
Some will prefer using a low angle plane, with an high angle (think over 60 degrees, sometimes approaching 90) bevel up blade and a tight mouth on difficult very hard wood grain, because an high angle is tougher.

There is no silver bullet, just many options and preferences.

There is nothing like experimenting for yourself

[Tom Trees]

For me, a close set cap iron is indeed the closest thing to a silver bullet, I've ever encountered in woodworking.

Never fails, and works well past what one would describe as a sharp cutter.
If not quite close enough, tearout will be evident quite soon when the edge is not as keen as it was,
Many think that is as good as it gets, when in fact, it suggests something's amiss.

Easier to push compared to having a tight mouth, and less tendency to nose dive effect off the ends, so very handy for short pieces.
Grain orientation is of little concern,
No discounting really gnarly timbers from the stock
and you don't have to spend any money on super steel or hone for said super steel,

Undoubtedly the silver bullet in my workshop.
Tom

[Scott Winners]

Welcome James. Where possible, I like the bedding surface of the frog and the bedding surface of the mouth to be coplanar, lying in the same plane. This gives the plane iron, the blade, maximum support; I end up with the smallest amount of chatter where the blade jumps up and down in relation to the wood surface I am trying to make flat or smooth.

I mean no disrespect to other excellent plane makers. In my shop I use Stanley Bailey planes only for bench planes, but am buying from Lie-Neilsen or Veritas for specialty planes like router planes and shoulder planes and so on. Rabbet. Skew. Plow.

What I find, with the frog and mouth coplanar, is if I need a bigger cut I just reach for a bigger plane, something that is a whatever 'and a half' - with one exception. Factory plane irons, up here, are subject to a LOT of pitting. They can't be sharpened because of the rust pits. When I am looking at a plane that is otherwise a user, if the price is right, I buy the plane, and a new (thicker) replacement iron from Lee Valley or Hock, and know I am going to have to open the mouth of the body a little bit to work with the replacement (thicker) iron. I am just not in a position to go looking for vintage irons to keep my planes running.

100, 200 years from now Baileys with Stanley irons and factory mouth widths are gong to command a price premium among collectors, but users are going to have openened mouths, replacement irons and probably have been well oiled by the artisans using them. I would rather leave my children, grandchildren and future woodworkers user Baileys with with no rust, openened mouths, some furniture I built, and replaced irons rather than otherwise pristine planes with pitted factory blades.

FWIW my core planes run from 4 up to 5 1/2 inclusive. If/when I need a bigger bite I reach for a wannabe (not Bailey) #4 with a heavily radiused blade and a wide open mouth.

It is possible to move the frog back a bit on any Bailey and take a bigger bite. I don't find it neccessary. When the frog and mouth are coplanar I can use the blade depth adjustment knob to modify my depth of cut. For deeper cuts I just twist the knob and go. You might internet search "Chris Schwarz stock prep by hand", it is only one of 16 million and 27 similar videos on youtube of what a properly tuned plane can do. If I was making jewelry boxes my core planes might be 1, 2 and 3. My next tool will likely be a electronic planer for when I need to take off an eighth or more.

[Jerome Andrieux]

For me, a close set cap iron is indeed the closest thing to a silver bullet, I've ever encountered in woodworking.
Tom

Hey Tom.

This is also my preference as I only work with domestic European wood, mostly making furniture.

Some people, using more difficult woods, or doing different jobs, such as music instruments, tools, etc, would prefer the advantages of a steeper edge. Indeed a 60° bevel (up) is just tougher than a 34° (Down) for the same surface result.

[Tom M King]

It should not be assumed that the mouth size is of no importance. Depending on what wood you are planing, you can lift out small chunks that you don't want to.

The really close cap iron is only needed on finishing planes, sometimes including the jointer, and then dependent on the wood you're planing.

On other planes, I prefer to just get things out of the way, for hogging off wood, including shavings that may only be 6 thou thick with a try plane.

As far as straight shavings meaning anything, I find planing speed has more effect on that than anything else. I can take the same plane that will produce straight shavings when planing slowly, and make the shavings curl up tightly, and fly off out of the way by planing fast. I don't want to be pulling every shaving off my forearm with every stroke.

[James Spangler]

Thanks again to everyone for the great info. I love hearing the details, getting down into the weeds of this topic!
And thanks to Jim for pointing to the David Weaver video, which lead me to his "Setting a Cap Iron" article in Wood Central.
His Figure 1 ("Plane Anatomy") in that article nicely depicts the relationship of cap iron to iron within the plane with recommended measurement settings.

[Tom Trees]

Hey Tom.

This is also my preference as I only work with domestic European wood, mostly making furniture.

Some people, using more difficult woods, or doing different jobs, such as music instruments, tools, etc, would prefer the advantages of a steeper edge. Indeed a 60° bevel (up) is just tougher than a 34° (Down) for the same surface result.

But you need to keep it sharp, which is not so much the case with a correctly set cap iron and traditionally honed angle iron.
Fair enough, apparently you can buff the edge as Weaver and others are now doing, for a substantially longer lasting edge,
but the problem for one doing it Neanderthal style is if you're dimensioning tropical species by hand, you cannot take a decent thick shaving from any plane with a tight mouth.

The influence of the cap iron isn't a one trick pony, as some make it out to be.
A 1/32" camber can take quite a scoop if you want,

I have a cheap no.4, which has a lot more camber and that gets used sometimes, but I rarely have to use it, not possible on some examples though, as
the interlocked and reversing grain of a tough example of iroko will just annihilate the edge.
That can be a rather dusty frustrating experience, tearing out chunks, as most tropicals are rather irritant.
Hard work means sweat and dust sticks.
A close set cap iron makes things as clean as if you were planing clear pine, so is a much less troublesome if you happen to be reactive to certain timbers.
The only dust you will have gets caught in the plane.

As anyone working specifically with tropicals or highly figured timbers, who actually uses the cap iron to its full potential will tell you,
they haven't used a scraper for dimensioning/smoothing work on the flat since.
Look Warren up, I believe he hasn't done so since the 70's.

Tom

[Rob Lee]

Hi -

Just another perspective - a moveable frog is a manufacturing solution too. It is by far the most cost effective way to produce cast iron planes with a fine mouth. (second would be a moveable toe for an adjustable throat). The moveable toe solution is a better choice for planes where it is intended for the user to adjust the throat.

Just a guess - but I'd bet 99% of Bailey purchasers never touched the frog.

Cheers -

Rob

[Jim Koepke]

Just a guess - but I'd bet 99% of Bailey purchasers never touched the frog.

My guess is you'd win that bet.

jtk