Customising a Dreadnought File

[Mike DonnerstagParticipant @mikedonnerstag]

I have several new dreadnought files (I think also named 'milled tooth' files) and would like to customise it to give me an isosceles trapezium, roughly square in section but with angles adjacent to the cutting edge of 75deg.

I'm wondering the best way to do this. My options, as I understand, are:

1) Cut the file to rough shape using a cutting disc in an angle grinder and attempt to grind the angle using a linisher (my belt sander, held upside down), all trying to preserve the temper of the file

2) Cut the file to rough shape using a cutting disc in an angle grinder, then anneal the file and mill the angle using a milling machine, before re-hardening. (Will fire-scale be a problem?)

3) Abandon the dreadnought file and make my own milled-tooth file (it doesn't need many teeth) from silver steel/gauge plate

Any suggestions very gratefully received, as always.

[Mike DonnerstagParticipant @mikedonnerstag]

[Martin Johnson 1Participant @martinjohnson1]

I would grind the excess of the sides, the advantage being that by working the whole edge you avoid overheating. A cutting disc might be quicker , but all the heating is in one place.

Other tricks include wrapping damp rag round the file, using damp sand to hold the file. Above all, take your time by alternating with another job – grind a bit, do somehting else while it cools, grind a bit, repeat.

Martin

[JasonBModerator @jasonb]

I'd be tempted to try a carbide cutter to mill the edges and not bother with initial ginding. Finish the edges with a diamond stone.

[DC31kParticipant @dc31k]

Posted by JasonB on 10/08/2023 13:06:26:

I'd be tempted to try a carbide cutter to mill the edges…

…and consider climb milling onto the face you are creating rather than off it. That way any burrs will not affect the bottom (or top), cutting edge(s) of the file.

[JasonBModerator @jasonb]

Better still use the side of the cutter not the end then if there were any burr it would just be at one end of the long cut.

[SillyOldDufferModerator @sillyoldduffer]

As a few Dreadnought's are available, I'd experiment with one. Might be easy to soften and re-harden with heat. Depends on the steel.

Older files were mostly made from plain Carbon Tool steel. Workable when the metal annealed, and going up to glass hard when quenched, usually in water or brine, not oil. Quenching in Brine makes steel extra hard, but the result may be too brittle. All that's needed is a decent fire, or big torch, and a drum full of salt water. Fire-scale is avoided by keeping the item in a non-oxidising part of the fire.

Try heating a file cherry-red throughout, hold for about one minute per cubic inch of metal, and then allow it to cool slowly. Should soften the metal making it easier to mill, file, grind or linish to shape. Then reheat to cherry-red, make sure the metal is all the same colour, and plunge and stir into the brine drum. (Lots of brine needed – at least a bucketful.) Although the process is simple, there's a fair bit of skill in it – judging cherry-red for example.

With luck the steel will re-harden. Won't if the file is made of HSS, because several precise and long-winded steps are needed to harden it properly. A modern file might well be made of HSS because it's tougher than tool-steel and less likely to snap.

My experience suggests Silver Steel is best for amateur hardening. Tolerant of misjudgements, though not completely idiot-proof! Yes, I have failed to harden Silver Steel.

Dave

[Mike DonnerstagParticipant @mikedonnerstag]

Many thanks for all of your answers.

In the end, I found part of a dreadnought file that I had already cut out of a larger file into a square section. Although I had drawn the temper in a spot when I cut it out, I didn't think this should be too much of a problem.

I spent several hours on an upturned belt sander to slowly and carefully grind on the 15deg angle on each side, quenching in water as I went and being careful not to sand my finger-tips! I went through three blue zircon belts, which are probably still good enough for sanding wood.

Not great photos, but hopefully they show what I achieved.

[Author]

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