On a Bailey Patent type hand plane and other similar designs, the frog is held in place by two screws that allows the frog to be slid forward and backward. How should you use this function? The answer depends on many different details. In the book “A Beginners Guide to Bench Planes” I wrote
“When setting up a plane to smooth, the width of the mouth and the distance of the chip breaker to the end of the cutting edge of the iron work in relationship to each other. Getting this set properly is more of an art than a science. A too tight of a mouth with too close of a chip breaker will cause clogging. Too much of a gap when setting the chip breaker can cause tear out in some work. Different types of wood act differently so there is not a magic formula.
Setting the frog ahead on some planes help close up the mouth. This will take some trial and error because there are many many different types of plane seats, some patented, some not. Some work better in allowing the frog to be adjusted and with some it is best to let the chip breaker work to your advantage.
The following image is an explanation of how the mouth helps the chip breaker in stopping tear out.
To go into further detail, there is a breaking point you need to find with your plane. The second image shows what happens when the mouth is to wide, but to wide is dependent on the type of wood, the thickness of the shaving, the distant of the chip breaker from the end of the iron, the angle of the tip of the chip breaker and even possibly the thickness of the chip breaker along with a laundry list you’ll read shortly..
When Leonard Bailey first produced his movable frog in the mid 1800’s the frog sat on a T shaped base and the frog “leg” hung lower. This meant when the frog was moved forward the blade still had support most of the way to the bottom of the sole. The irons were thicker as were the chip breakers.
But as the chip breaker evolved, and the irons got thinner, the need for a tight mouth became less of a requirement. It’s not that a tight mouth doesn’t help, it just doesn’t need to be quit so tight, so if the plane was manufactured with a tight mouth in the first place, a small variation wouldn’t have much of a detrimental effect.
So a quick recap of what effects all of this:
- The type of wood (along with moisture content, hardness, etc)
- How close the chip breaker is to the end of the iron
- How polished the chip breaker is
- What the angle of the chip breaker is
- The thickness of the iron
- The stiffness of the iron and chip breaker
- The seating of the iron to the frog
- The frog seat
- The tightness of the frog screws
- The thickness of the mouth behind the iron
- The thickness of the mouth in front of the iron
- How polished the face of the mouth is
Here is the seat and frog of a Stanley Bailey type 2. Note how thin the mouth is allowing the frog to sit close and be lower and closer to the bottom of the sole.
This next picture is a cut away view of a Stanley type 11. As you can see the frog now sits on a raised platform. It’s not terrible, but you can see how moving the frog forward and allowing the iron assembly to be free floating could cause some chatter in certain circumstances.
And over time different manufactures did different designs to make a better plane
This next picture is a Type 17 Stanley
This next cut-away is of a Bedrock. Look how far the cutter would be unsupported if this was adjusted ahead.
A type 2 Sargent. Note how far this would be unsupported if it did not rest on the mouth. The one thing this design had was a heavy solid frog seat. I’ve found it still works best if the iron is supported at the bottom.
Sargent made these late models like the following Dunlap. The distance from the sole to the frog makes it a requirement that the iron sit on the bottom section of the mouth. In my opinion though, this is one of the best user planes of the era this plane was made in.
Note the good heavy and solid base for the cutter on the Sargent Shaw patent #15. This make a solid base for the cutter to sit on. It also eliminate to much possibility that the plane will work correctly if the cutter doesn’t sit on it.
So in practice, it’s probably best to experiment to find the sweet spot on your plane. I would suggest starting with the frog face and the rear of the mouth set perfectly parallel. Under no circumstance should the frog be back so far that it raises the iron off the face of the frog. Then find the proper setting for the chop breaker given the distance to your mouth face. This comes from using it. If it works and works well, it’s set where you want it. Making sure the mouth face is clean and polished will help and the chip breaker should have an angle of 80 degrees and be polished at the edge. This is all part of tuning a plane.
With most reasonably well manufactured hand planes, the mouth will be small enough to be affective with this configuration. If you find it isn’t you can then try to move the frog forward. Doing a little experimenting will show you how you need to set the plane. If the mouth s to wide, and the frog doesn’t allow for the support, that’s when a thicker iron will help. I haven’t found to many planes that require this, but it’s still a possibility. The thicker iron will make the mouth smaller. A thicker iron also provides more rigidness, but that is typically not a requirement if everything else is as it should be.