Files, Filing, Filling, and Finishing
by Bob Smalser
This article first appeared as a thread on theWoodNet woodworking forum. It was compiled and reproduced here for easier public consumption. All Text and Images are the property of Bob Smalser.
This article deals with restoring metal surfaces. It's applicable to just about any time you have occasion to pick up a file, whether to fit or repair old bronze boat hardware, antique tools or automotive parts. My training aid is a badly abused but classic hunting rifle from 1936 I recently restored.as these objects are the subjects of close scrutiny by discriminating clients and require a degree of precision useful for demonstration.
File Pattern and Type
Files come in far too many patterns and cuts to treat in detail here, but are generally either Machinists or Engineers Files for shaping, Saw or Sharpening Files for sharpening, Aluminum Files for that soft material, Rasps and smaller Rifflers with coarse teeth for wood, Needle Files for small work or large Blacksmith Files. There are also innumerable specialty files like metal checkering files, lathe files, curved tooth files, cantsaw files and even triangular dovetail files with two "safe" or smooth faces.
Either American or Swiss pattern Files are common here, the difference only being the pattern standard used to manufacture the file and how the fineness or coarseness of its teeth is measured. The shapes themselves are similar, but Swiss files have longer, thinner tapers, are slightly slimmer, are made to closer tolerances and come in finer grades of cut. They also cost more. Any edition of Machinery's Handbook by Industrial Press will provide additional detail.
Type of Cut
Single cut files have a series of parallel teeth running diagonally across the width of the file surface and are generally used for sharpening or fine finishing. Double cut files have two series of parallel teeth running diagonally across the width of the file surface with one series crossing the other. These are best suited for rapid removal of material.
Coarseness of Cut
Bastard Cut in American Pattern files is the standard for shaping and dressing steels and castings. The Swiss Pattern equivalents is #00 Very Coarse. Second Cut American is for lighter removal and for hard metals. The Swiss equivalent is #0 Coarse, with a finer grade of #1 Medium Coarse above it. Smooth Cut American is for finishing and hard metals. The Swiss equivalent is #2 Medium with #3 Medium Fine and #4 Fine above it.
Once filing skills are mastered on both metal and wood, these finer grades of file will often pay for themselves in the cost of abrasive paper saved. Flea market machinist files too worn for steel still work just fine on wood, and will save lots of sanding and paper.
Using the file in its normal direction of cutting perpendicular to the axis of the file. Files cut in one direction only and when they get dull, they are relegated to soft materials like wood or discarded.if you develop the habit of lifting the file on the return stroke, you will literally double the life of your files.
Draw filing is used to produce very smooth and true surfaces. To draw file, hold the file at right angles to the direction of the strokes, with your hands close together to prevent breaking the file. Pressure should not be great and can remain the same for the back stroke, as the teeth cut in both directions when using the file in this manner.
Break In and Care
Files are of hard and brittle steel and new files have a slight wire edge on their cutting teeth. Using a new file fresh from its wrapper to cut hard materials is a mistake, as this wire edge can break and take part of the cutting edge with it. A better practice is to break the file in.removing that wire edge.by filing lightly on brass or bronze before proceeding to steel. Files should never be used with force, as too much pressure causes the teeth to clog quickly and causes the file to rock and round the corners of the work piece. Also take note of the fresh file brush and blackboard chalk that live with the files. Keeping the work surface wiped clean of shavings and the teeth of the file clean and unclogged will prevent those disastrous deep scratches that are the bane of finishing work. Chalking the file makes it easier to clean and lessens its tendency to clog with shavings.
Files should not be stored loose in drawer or toolbox where they touch each other.I prefer to store them either hanging by their handles or upright with their tangs in a holed block affixed to the back of the bench. Yeah, I know.the files in my picture are dirty and rusty.I [I] should[/I] clean them before storage and then again before and as I use them. But what's important is clean teeth touching the work piece, and out of laziness I skip a step and clean them thoroughly with WD-40 and the file brush and a dental pick if necessary before use and with chalk and the file brush and during use.
Removing the scope mount on this first-year Remington 141 restoration was an unpleasant surprise..small, 6-48 holes are filled and the repair hidden relatively easily, but these larger, off-center 8-40 holes will be difficult to repair without the repair showing. What a rat's nest!
And the aftermarket sling swivel mounted off center will also be removed and the original finish restored, and I'll do that job first. These are threaded studs and are screwed in and sometimes also soft soldered as there is very little thread bearing surface in the thin band it is mounted to. This one is soldered. I can either unscrew it using 450 degrees of heat and pliers then fill the hole or I can simply grind and file it flush, which has some disadvantages you will see below.
I grind off the stud on the buffer grinder and crossfile with a single cut mill file until I am close to the original surface..
.then I draw file the stud flush with the surface.
.. followed by aluminum oxide wet-or-dry paper lubed with WD-40 and a sanding block in 180 through 600 grits without skipping a grit to polish. I could wrap the paper around my file instead of using a wood block.and I do that for flat surfaces.but for curved surfaces I prefer the softer wood as backing. A sanding block is important, as freehanding will result in an untrue surface.. especially in the joint filled by softer solder that is easily dished.
I then test the quality of my surface using a little cold bluing for color.I'm using Brownell's Oxpho Blue phosphate bluing solution, buffing in multiple coats after degreasing with 0000 steel wool..
..and darken the solder with a touch of Solder Black.and the result is marginal. The relatively loose threaded joint shows a rather wide band of solder impossible to completely hide. I'll redo this later using another filling technique shown below.
Moving on to the rat's nest of holes in the receiver, I chase the threads with taps to expose fresh metal.
I could thread some #6 and #8 screw blanks and solder them in as was done with the sling swivel above..and if strength is important, this is the preferred technique. On this particular rifle firing cartridges of relatively low pressure, strength isn't as important and I'll concentrate on cosmetics instead.
I cut some screw blanks to serve as unthreaded plugs and put a sharp taper on them using my "lathe"..I have access to commercial metal lathes but spinning them chucked in the electric drill against the disk sander is fast and works just as well.
Then I simply dip them in solder flux and drive them into the holes for a tight fit, crushing the female threads in the process. This will provide as small a joint line as can be made.
Chalk is applied to catch any finish-destroying flux and solder runs, and I heat the receiver slowly to 475 degrees and apply a mere drop of soft, high-strength solder, working the solder and flux around my joints using a dental pick.
Will this be strong enough? The solder I'm using contains 4 pct silver and 96 pct tin that offers 28,000 psi of strength and is resistant to all finishing processes normally applied to firearms. I could gain more strength.38,000 psi. by using a higher silver content solder that requires 650 degrees of heat, or 40,000+ psi using 1100 degree silver braze. I also could fill these holes with nickel steel rod by gas welding, or take it to a friend skilled with a TIG welder..all at higher temperatures.
My problem with higher temperatures is that I have only a poor idea of how that critical receiver steel was hardened and tempered to withstand the 35,000-psi its cartridge generates in the barrel chamber..the pressure area held together by the critical barrel-receiver threads immediately below one of my holes. The steel seems soft, or at best case or surface hardened, but I [I]do[/I] know that much over 600 degrees of heat will ruin whatever temper the steel has and I choose not to risk more heat.
I cut the plugs just proud of flush with a cutoff disk..
.crossfile the plugs even closer to flush and begin drawfiling to smooth the finish and contour the plug ends to the curved surface of the receiver.
As I drawfile, the blued steel surface becomes my index for uniformity, as missing a spot stands out like a neon sign.if this steel were bright, I would blue it first with layout fluid or cold blue just for that purpose.
When drawfiling is complete, I begin with my 180 through 600 grit abrasive paper lubed with WD-40 and my sanding block..
..and then buff the work surface [I]across[/I] the direction of drawfiling using light Knifemaker's Green Rouge and the stitched muslin wheel. Large commercial shops often use the buffer with hard felt wheels and graduated grits of abrasives in place of my abrasive paper. I've done a lot of this that way, but don't prefer it. To prevent rounded corners, 90-dollar hard felt wheels are required.one for each grit, along with matching muslin wheels.and a high order of skill is required to prevent dishing the softer, soldered surfaces. All in all, for anything less than large-scale production work, I can do the job better and more pleasantly the old-fashioned way with abrasive paper faster than I can change all those wheels.
I test my now-finished surface with my finishing solutions and discover another problem..my polished work surface is now brighter and doesn't match the rest of the receiver.and my repairs are more evident than I prefer. I could polish out the remainder of the metal surfaces to match, but this is a restoration job and I want to preserve those original surfaces to the degree possible..or I can tone down the one surface already spoiled with necessary repairs.
So I proceed to the bead blaster, mask the original surfaces, and blast the work surface with fine silica at low, 20 psi pressure. The I apply Oxpho Blue followed by Solder Black..
.and only in bright sunlight do you see evidence of the repairs.in shade and interior light you have to look closely to discern the #8 holes, the #6 holes have disappeared entirely. It's not perfect, but this is as good as I'm going to get it, so it's time to stop.
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