A tip for filing a plane mouth - surface texture
One of the best reasons to teach, is that it forces you to really think about how you actually do things. This seems obvious, but there have been several times where I have been caught saying one thing, but actually doing something very different when I demonstrate it. I try to remind myself of this as often as I can, and the other day, I realized that a previous blog post was missing a pretty important element - surface texture. Several years ago (I think), I wrote about filing the mouth on a plane but omitted this aspect. So here is the updated version which more accurately reflects what I am actually doing:)
I recently finished lapping a K18 and the next step was to file the mouth. I had fully tuned the blade - flattened the back (no back bevel), and honed the bevel. When I make a plane, the mouth opening is too small for the blade to pass, so I have to file the mouth open to allow the plane to take a shaving. When you are trying to achieve a mouth opening in the 0.006" range, you need a fully tuned blade in order to work accurately. I also know full well that the perfect edge will be damaged during the process, but such is life... and on the plus side - you get pretty good at sharpening!
The shot above shows the set-up. Pretty simple - the plane inverted in a machinists vise with leather lined jaws. There is also a leather pad underneath it to keep the front of the plane from being damaged.
Here I am scribing a line to file to. In truth, it is not a hard line to file to, but more as a point of reference for filing. It always takes a few strokes to get the muscle memory back for this type of fine work. I use an 8", single cut mill bastard to start with.
I file at 90 degrees to the sole.
If you click on the photo above, you can see the effects of a single file stroke. The texture and therefore the color of the filed area has changed. This gives me a fantastic visual reference for where I have already removed steel. I file across the entire mouth opening, keeping an eye on the scribe line and watching the texture change.
Once I have filed to the scribe line, I will use a No.0 file to draw file - sliding the file side to side instead of a forward push stroke. This once again changes the surface texture and color so you can see where you have filed and where you still need to work. I am only draw filing the 90 degree area I have just filed. A No.0 file is so fine that there is little risk of taking off significant material and affecting the size of the mouth opening. In the photo above, you can see that I had draw filed about half of the leading edge of the mouth opening - the shiny area as opposed to the dull grey area.
This next photo (above) shows the entire mouth edge after draw filing.
Once the draw filing of the mouth edge is done, it is time to file a bevel to reduce the size of the perpendicular surface (the shiny part). I return to the 8" single cut file for more aggressive filing. The photo above shows the file held at an angle, and the photo below shows the change in texture and color of about three quarters of the bevel. Click on the image for a larger view where you can see the shoulder where the filing stopped.
I am careful not to remove the entire shiny, draw filed surface. I want to keep an area perpendicular to the sole that is about 0.015" wide.
After the bevel has been filed, I go back to the No.0 and draw file the bevel to make the texture and color consistent again.
At this point, I re-install the blade - keeping it back from the leading edge of the mouth, and lap the sole on my surface plate. I gently lower the blade to see if it passes through the mouth opening or if I have more filing to do. If there is more filing, I return to the first stage and repeat the process.
There is usually a stage where the blade is really close to passing through, or one side passes through but not across the width. Or sometimes passes through, but is too tight to be useful (a 0.001" mouth for example). If the blade passes but is too fine, I will not proceed with the 8" single cut file, but will use a 1" wide, No.0 file so I am removing very little steel with each stroke. I will still draw file with the narrow No.0 and go through all the stages, but I will not use something as aggressive as the 8" single cut file.
I use surface texture as a point of reference for lots of other shop tasks. When honing bevels on a plane iron for example. The change in texture and color tells me how far the camber goes towards the center of the blade. I also use it when shaping wood and metal - introducing deliberate texture in a single direction so the next file or rasp will alter the texture telling me where I have worked. This is especially useful when shaping round or curved work. Sometimes you don’t even need to change the file or rasp - simply maintain a single direction of cut over an area and then change direction to see where you are. These textures can become quite complicated, but with some practice, can be used as a road map as you work and eliminate a lot of guess work, and ultimately, reduce the chance of disaster.