File Renovation

I had quite a few VERY rusty files that I inherited from my father. The rust was so thick, you could barely see the individual teeth! I was about to bin them, when I remembered that I had a 5 litre can of conc phosphoric acid, off Ebay for a very low price. I had used this to de-rust a lot of items.

So I thought why not try the files? I prepared a solution of approximately 20 to 1 boiling water to conc phosphoric acid. In went the files and I was rewarded with much bubbling. Eventually this slowed right down and looking at a file it appeared to be covered with a jet black substance. I assumed that the solution had become too weak and so I then repeated the process. On the third treatment, the files came out just slightly black and I assumed that the rust had gone.

On drying, all looked well and when I tried rubbing my fingers over the file, I found it to have incredibly sharp teeth! More so than a new file. So it was a very worthwhile exercise! I have no idea how the files will fare long term, but filing a piece of mild steel, the removal rate was very high indeed.

Andrew.

Great tip, i'm going to try it!

Michael W

You can sharpen files chemically, although the rust/derust method is probably not the best way .

Neil

Hello Neil,

You may well be correct. however there was so much thick rust on the files that de-rusting was a first step! I have heard that some old timers used sulphuric acid to sharpen files (maybe nitric or hydrochloric acid, I am not quite sure!).

I doubt that any other chemical method would give better results, these old files are now incredibly sharp, one stroke on flesh would produce much blood! However I would caution that I have no idea if the files will last well in service. I have yet to find that out. I will let people know the long term results. It could be, that it only works with very rusty files. I will try to find my bluntest file and give that the treatment.

Andrew.

Dilute Sulphuric Acid was recommended for home sharpening files in the good old days. I've used Drain Unblocker diluted unscientifically with 5 parts water to 1 part acid. Results were mixed, ranging from a slight improvement to wow, good as new. I don't know what makes the difference: possibly it depends on the type of steel the file is made of.

It's well worth trying, but the acid isn't good for concrete floors. Don't ask me how I know.

Dave

I came across a methodology that was quitre complex. I think you first precipitated something on the upright file to protect the face of the teeth, then copper sulphate to put a layer on the back of the teeth, then clean off and etch away the face of the teeth with concentrated acid.

Or something completely different...

Neil

Hello Dave,

You may well be right about the type of steel making a big difference. I treated 9 files and they all came out good, but they were all very old files from the 1950s I would guess, maybe there is something about them that gave me such good results.

Andrew

Time was you could buy reconditioned files, presumably chemically done if my admittedly third hand information is correct. Bought some, under 1/4 the price of new I think, and they really were pretty good. I imagine Neils' methodology refers to the commercial method which would need to be more reliable and faster than simply dunking in acid. Extra process steps being easily handle in a production environment and well worth it if a faster, more aggressive, method can be used for the actual sharpening.

Standard explanation for why sharpening happens is as given by Carl. Many years ago I asked one of the hot shot metallurgical chemists at work why the teeth got sharper rather than having either a fairly even etch all round or preferential erosion of the exposed point making them blunter which really isn't the idea. I got the "Yes, thats pretty much what happens. Must keep them vertical and don't stir the acid. Probably better if the temperature stays constant too." accompanied by the expression that meant "its all terribly complicated and you really don't wan't to know more. Do you.". Never figured out if he really did know or was winging it on a bluff.

I guess that if circulation of the acid under the short, raked back under, edges of the teeth were rather worse than over the long, freely exposed, ones the acid under the teeth would be more exhausted than that on top so would etch less. Still think the point should go fasted tho'.

Clive.

That's how I think it works. If I can find an old file to experiment on I'll take before and after microphotographs.

When I first tried the experiment I expected the acid to dissolve the ridges leaving a flat bar. Not so, it really does make sharp edges. I doubt the original tooth shape has been properly restored by the acid though. A chemically treated file may not really be as good as new. Just better than it was before.

Dave

Edited By Carl Wilson 4 on 03/06/2017 18:25:05

books.google.co.uk/books?id=seMDAAAAMBAJ&lpg=PA64&ots=Bsg7DY-S2E&dq=chemical%20file%20sharpening&pg=PA64#v=onepage&q=chemical%20file%20sharpening&f=false

Edited By Neil Wyatt on 03/06/2017 19:00:34

Hello again,

I tried the etching again, this time with one of my blunt files, of recent vintage. This time I didn't need to change the phosphoric acid. Previous files were so rusty that I believe I had exhausted the acid, because etching stopped even though the mix was quite warm. There was also a huge amount of black sludge on the bottom of the plastic container, plus a very black file after the first two etches.

With the non rusty file, the etching continued until the mixture became cold. Adding more boiling water got the reaction going again. I then realised the reaction rate was very temperature dependent (forgotten my 10 degree chemistry reaction rate rule!). So I used my large ultrasonic bath, which is stainless steel plus a heater. I set the temperature at 80 degrees centigrade. After 2 hours, I fished out the file and it too, was very sharp. There was a very thin layer of black particles at the bottom of the bath.

The file was laid horizontally and no effort was made to lift the file clear of the bottom. I now have 10 very sharp files for the cost of the phosphoric acid used, which is probably around the 50 pence mark. It has worked for me, so I suspect it will work for others. Again, I do not know if the effect will last as long as a new file. But considering the cost of the operation is peanuts, I think it well worth doing, even if the files don't last as long as new ones!

Andrew.

OK, here they are, filthy adult only pictures.

My test file is an old favourite. I bought it government surplus the first time I needed a file, so it's at least 40 years old. Made in India, it contradicts conventional wisdom that Far Eastern tools are all rubbish. It's been abused - only since retiring have I learned to respect files. Most recently the file was used to take burrs off lathe turned edges.

First, the file before treatment. It's dirty and blunt. You can see brass, aluminium and steel chips.

I made a 20% by volume solution of Sulphuric Acid Drain Unblocker. This appears to be mostly concentrated sulphuric acid with additives. Although the acid isn't quite strong enough to char paper, don't splash it around. Always dilute by adding the acid to a large surplus of cold water, never the other way round. (The water gets hot; overdo the acid and the mixture will boil, spit or even go pop. A face full of hot drain cleaner will ruin your day.)

Next the same file after a good brush with a file card. You can see that the edges are dinged and that the ridge tops have a shiny polish.

This is what an almost new file looks like. The edges are in much better condition, and nothing shines!

This is the file after 30 minutes in the acid. Despite vigorous bubbling, the improvement isn't all that obvious. Out of the acid the file is covered in a black deposit (Carbon?) and has a strong unpleasant 'metal attacked by acid' smell. The photo was taken after rinsing the file under a tap and going over it with a tooth brush.

After 14 hours in the acid it's black, with small white crystals, and lots of dark grey particles. Yuk!

After another rinse clean. Now the ridges are a bit sharper. The shiny polish on the ridge tops has gone. Not perfect by a long way, but certainly much better than Photo 2. It's easier to feel the difference than to see it. Try rubbing the file with a finger before and after.

Top down photos make the file look flatter than it actually is. Next is the best I could do to capture the tooth profile. Bit rough, but it has lots of sharp edges.

I've tried acid sharpening with number of old files. This example is neither good nor bad. Some treated files last quite well after sharpening, others don't. Looking at the pictures above you can see signs of pitting. I guess this would tend to reduce the local strength of the teeth, perhaps causing them to wear more quickly than a new file would.

Andrew mentioned in his Original Post that he used Phosphoric Acid. This has a different reaction to dilute Sulphuric Acid. Sulphuric reacts with both Iron and Iron Oxide. After removing the rust it dissolves metal as well. Phosphoric Acid reacts with the rust to form an corrosion resistant layer on top of the steel. I don't think the phosphate layer is hard, so using the file probably removes it quickly. I'd guess that Phosphoric Acid is best for recovering a sharp file that's gone rusty, and Sulphuric is best for recovering a blunt file.

Of course we may be seeing another effect as well. Simply giving a clogged file a good clean sometimes works wonders.

Dave

Oh I forgot to say that my experience of, even dilute phosphoric acid, on concrete, is exactly the same as Dave's!!! The slabs on my patio now have some very clean areas! I just hope my wife doesn't notice. Some hope!

Andrew.

P.S. It is very odd that provided the solution is not very hot, I fished out the files by hand, no real sensation of acid on the hands, but do swill them with clean water afterwards.

Very interesting, Andrew

I am intrigued by your results; mainly because I think of Phosphoric Acid as a "rust converter" rather than a "rust remover" [it being the main active ingredient in products such as Jenolite].

Perhaps your usage at elevated temperature is what made the difference.

MichaelG.

.

https://en.m.wikipedia.org/wiki/Phosphoric_acid#Rust_removal

Edited By Michael Gilligan on 04/06/2017 12:29:25

I have tried another blunt file in dilute phosphoric acid at ambient temperature. It is still reacting very slowly with a little gas release to show that things are happening. I left it outside overnight and the reaction rate had almost ceased, as the day warmed up, then the reaction speeded up. I will let you know the results later.

One WARNING, when the reaction occurs at high temperature, there are copious obnoxious white fumes evolved in great profusion! DO NOT breath them in, ask why I know this!?

Looking at the micrographs that Dave has provided I suspect that the sharpness of the files is due to the very rough structure shown in the last photo. Even so, the files do cut at a rapid rate, but for how long is any ones guess. When I have some need of lots of filing, I will give some feedback on how long the sharpness lasts.

Andrew.

Did some research on my observation 'Out of the acid the file is covered in a black deposit (Carbon?)'

Not an inspired guess: it's proper name is 'smut'. Although there's some free Carbon in it, smut is mostly Ferric Carbide which is better known to steel makers as Cementite.

Respectable married men beware! Searching for smut on the Internet will be punished by She Who Must be Obeyed.

Dave

40 years ago when I started teaching Technology we used to send files for sharpening commercially every couple of years. We used to send them about 3 times. They were never as good as a new file, but a fraction of the cost. When cuts in funding took place in the 1980s I did them myself using dilute sulphuric acid. 10 mol. The same strength as used for pickling beaten copper work. Worked quite well but some I left too long ,so took to much off.

The acid attacks all the metal equally so the rounded edge which makes the file blunt is reduced. Application of electricity would spread matters up as in electro chemical deburring or electro chemical machining.

I think that files are now so cheap that it is not worth sharpening them. At school I am ordering new files for little more than I was in 1980.... But the quality is of course very poor these days!!