I’ve seen a chart that is wrong, so I’m posting this here. These are based on Nicholson files, one of the more common brands of decent files in the USA. Getting a close match makes high spots and low spots less of a concern. Half round files aren’t semi-circles, one side is round and one side is flat. Half-round files are actually about a 120 degree arc, or a third of a circle. These are for American pattern files:
8″ is 1″ diameter – For mitering the top tube and seatstays and downtube (bb notch) to a 28.6 seat tube, and stays to some hooded dropouts.
10″ is 1-1/4″ diameter – For mitering main tubes to 1″ (31.8mm) or 1-1/8″ (36mm) headtubes, the top tube, seatstays and downtube (bb notch) to an oversize 31.8mm seat tube, and stays to some hooded dropouts.
12″ is 1-1/2″ diameter – For mitering main tubes to 44mm and some tapered headtubes, seat tube, downtube and chainstays to BSA bottom brackets and some hooded dropouts.
14″ is 2″ diameter – For seat tube, downtube and chainstays to 2″ OD T47 bottom bracket shells (unconfirmed)
Summary: You need an 8″ file if you have a 28.6mm seat tube. A 10″ file helps mostly with straight steerer headtubes. A 12″ file helps with BBs. You can still miter with just an 8″ file. You will need a large round file for mitering bridges.
Keep in mind, you can use a file one size up at an angle to effectively cut a deeper crotch. This can be useful for working down the ears since the right size file will cut a little undersize and there may be some rocking if you don’t keep the file level, but the surface will not be flat to the other tube, so it should be touched up with the right size file or a size smaller. This is less important with lugs or welding. Lugs have a higher tolerance for poor fit, as the lug provides alignment and joint reinforcement, and welding can get better penetration with a little bit of beveling, or even a slight gap, though it becomes harder to tack and fuse without the base metal melting away from the root.
Round File Diameters
Round files are mostly for cleaning up fillets. It helps to have a round file with a diameter at least as large as the radius of the fillet. This is a decent compromise between being able to form a round fillet and being able to target specific areas to remove metal. Using a file that matches the radius exactly limits visibility and harder to control because both toes of the fillet must be controlled at the same time to prevent undercut. It also limits the ability to go a little deeper to remove a divot. Using too small of a round file will tend to leave flat spots or gouges and require lots of targeted shaping, then smoothing, and so on. You can shape a fillet with just a 1/4″ chainsaw file though.
A 6″ round file is not recommended. Chainsaw sharpening files are 8″ long, in various diameters up to 1/4″, smooth cut, and much cheaper. You can often get several chainsaw files for the price of one round file.
| Length (American) | Inch | mm |
| 6” | 7/32 | 5.6 |
| 8” | 9/32 | 7.1 |
| 10” | 3/8 | 9.5 |
| 12” | 1/2 | 12.7 |
| 14″ | 5/8 | 15.9 |
TPI
The following tooth count is from Bahco half-round files, but they match up with their more limited selection of round files, converted from TPC to TPI. They seem to match up decently with the limited selection of files from Snap-On who reports TPI. Flat mill files tend to be finer than round files.
| Length (American) | Bastard (Coarse) | 2nd Cut (Medium) | Smooth (Fine) |
| 4″ | 38 | 53 | 74 |
| 6″ | 30 | 43 | 61 |
| 8″ | 25 | 38 | 53 |
| 10″ | 23 | 30 | 43 |
| 12″ | 18 | 25 | 38 |
| 14″ | 15 |
I would not use coarser than 25 TPI for shaping fillets, as it tends to leave deep gouges. If you use it near the toe, you may find there are gouges that can not be removed. It is okay for getting rid of lumps and rough shaping. Coarser files are better for rough shaping because they tend to follow the existing fillet curvature less. Fine files require more pressure per material removed, and they tend to skate and dwell in groves and make grooves deeper. They should mostly be for finishing the surface in preparation for sanding. A chainsaw file works fine for this with a diagonal motion from one to to the other. An inexpensive one can be used to to the initial shaping to deal with residual flux that may dull better files. Chainsaw files are smooth cut.
Kuromori 