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Sharpening Files

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James Alford13/05/2020 07:57:18
465 forum posts
79 photos

I have a large number of old files, most of which have become blunt with use and abuse. Several of them are very old, having been inherited from my grandad in the early 80s when they were already ancient.

Does anyone know whether soaking them in acid to sharpen them is actually effective? I have seen numerous suggestions to use either white vinegar or battery acid, but nothing that really says whether it is worthwhile.


Tony Pratt 113/05/2020 08:01:23
2035 forum posts
12 photos

I did try leaving one in a strong solution of citric acid stronger than vinegar] with little effect.


Clive Foster13/05/2020 08:30:21
3173 forum posts
113 photos

Back around 1980 or thereabouts I purchased some reconditioned files for about 1/3 rd of new price. Where from I no longer recall, possibly via Brer Whistons log loving cat. They seemed to work well. Bastard and mill cut types as I recall it so rather coarse. Big ones too 12" long or so. Allegedly process gave an extra 50% of life. Which I'm not going to disagree with. On reflection I may still have a millcut one about the place.

According to my then boss these would have been done electrolytically with the file suspended opposite a flat metal electrode and effectively reverse plated. The long sides of the file tooth lost material being able to "see" the electrode whilst the undercut side was hidden and pretty much unaffected. Strong acid with some additives needed I think to ensure the material stayed hard and wasn't degraded by the process.

Research in Model Engineer (I think) uncovered the simple dip in acid process. That clearly relies on the acid in the undercut being trapped, rapidly becoming exhausted whilst that on the long sides is able to flow away and be steadily refreshed. Clearly a much slower and less controlled process. As I recall it he sweet talked some strong acid out of the chemists and gave it a go with "worked, sort of" results which was more than the chemists predicted.

Conclusion was "not worth the palaver of messing with strong acid" but it might be possible to fine tune a working process for coarse files. One for the interesting but life is too short file methinks.

Can't see a weak acid doing anything beyond possibly chemically deep cleaning the files. Process clearly needs to be fast to maintain a good differential between eating the long and short sides.

I'd hazard a guess that most home shop reported success is more cleaning tiny swarf particle build up off the tooth tips than sharpening.


DiogenesII13/05/2020 09:11:18
589 forum posts
234 photos

I have done it, with the aim of removing rust from a job-lot of ancient new and used files - they sat in a quart of vinegar for up to two weeks (checked at intervals).. ..the "new-but-rusty ones cleaned up okay and are good. Those that were new but pitted, worn, or marred in any way weren't any better.

Once the "edge" has gone, any erosion just seems to replicate the profile as is at the time of dunking, or perhaps even to make it worse - I guess it's in the very nature of the process to remove the angularities and thinnest parts first.

Some of the "new" files were a bit pitted and "raggy" here and there, and they are truly awful to use - they "pin" horribly.. they went in the scrap box (suddenly, and with great emphasis) during the last job that involved EN3b..

So no, it didn't work for me, other than to remove rust.

Sakura13/05/2020 09:17:09
46 forum posts
1 photos

I have done it occasionally using battery acid (Sulphuric acid). It works well but getting hold of battery acid now is difficult after the recent acid throwing incidents and subsequent legislation.

Andrew Tinsley13/05/2020 10:52:53
1616 forum posts

I have used reasonably concentrated, hot phosphoric acid. I simply dumped the files into the solution. The results were good. It extended the life of the files by a reasonable amount. Certainly cost effective, as the the phosphoric acid is cheap on Ebay.

Please note that the files were not "as good as new". I simply got a significant extension to their lives. In my opinion it was worth the effort. Others may not agree.


SillyOldDuffer13/05/2020 11:12:18
8912 forum posts
2000 photos

Somewhere on the forum are the results of a trial I did sharpening old files with Sulphuric Acid. Just before sales were restricted so I used Acid Drain Cleaner diluted down to about 30% (battery acid strength-ish).

It does work, but don't expect as-new condition. As I recall it did better on some files than others. Don't know if the variation was due to the degree of wear, or the type of steel. I wasn't scientific about how long files were left in the acid, but I'd guess there's an optimum time for each file at which maximum sharpness is achieved. Removed too soon, and the file will still be on the blunt side. Left in too long and the acid will start reducing the teeth so it clogs faster in use.

Clive makes an interesting point about an electrolytic process; I hadn't thought of that. File re-sharpening used to be a 'thing', and I guess that's how it was done. Electrolysis is much more controllable than dunking, and I expect they knew exactly how to get consistent results from it.

I didn't try sharpening with an Organic Acid (Citric Acid, Vinegar etc). Being food-grade weak acids they are expensive to buy and slow acting - days rather than hours or minutes. Provided you can get it and take suitable handling precautions Sulphuric Acid is much better.

Is it worth the fuss and bother? Probably not.


peak413/05/2020 11:19:22
1788 forum posts
193 photos

See also this thread on the subject on this forum.


AdrianR13/05/2020 11:20:18
597 forum posts
36 photos

Now I have read something about sharpening files using acid and electrolysis. It was written by someone who used to do it at work. I thought it was an article in MEW but it could have been a forum post too.

Anyone else remember it?


Perko713/05/2020 12:11:41
427 forum posts
33 photos

On a related subject, cleaning files rather than sharpening them. I have a set of needle files marked 'Diamond Brand Made in the Peoples Republic of China'. They are about 40 years old and have seen a lot of abuse over the years. Most of them are still good but one or two have become badly clogged with bits of soft metal such as aluminium, solder, diecast zinc and other assorted materials used in a variety of model railway and household objects. I would really like to clean them up a bit but don't want to damage whatever residual cutting edge they still have. Some have recovered through being used for filing bits of steel which has cleaned out the grooves nicely, but others have so far resisted that treatment. Any suggestions for how I might go about this which does not involve scraping out individual grooves using wedges of brass under a magnifying glass? Thanks

Neil Wyatt13/05/2020 14:29:10
19079 forum posts
736 photos
80 articles

The best method depends on precipitating a compound that protects the face of each tooth from the acid and this process was used commercially and apparently can make files almost as good as new.

But I can't remember the details.... a search for it online may throw up details of the process.

For cleaning needle files, zinc can be removed with acids and aluminium with caustic soda. Solder could be trickier. Obviously take care using these.


Ian Johnson 113/05/2020 15:11:15
376 forum posts
102 photos

Try sand blasting them!

Company I used to work for did this to old files and gave them a second life. I think it was more to do with keeping a tight hold on finances! When they found some money they bought new files.


Andrew Tinsley13/05/2020 16:10:11
1616 forum posts

Clive may well be right about etching files, just removing the junk and hence seem to have been improved! It certainly wasn't the case with the phosphoric acid treatment. I meticulously clean my files after use. So the acid treatment does resharpen the files to a useful extent.


Trevor Crossman 113/05/2020 16:38:54
152 forum posts
18 photos

I remember from my apprentice training days 60 years ago an instructor on a sheet metalwork course said that worn files were reclaimed by an electrolytic process using conc. Nitric acid. I cannot remember the details, but definitely not really something that could be done in our backyard workshops 😲


oldvelo13/05/2020 18:02:12
296 forum posts
56 photos


No need for messy dangerous acid to sharpen files just use two or three table spoons of Washing Soda per gallon of water. The ratio is not critical but an excessive ammount of washing soda will NOT speed up the process.

Suspend the files in the solution hanging verticaly in the centre of a plastic bucket. Two pieces of steel on oposite sides of the bucket wired together.

A DC power supply from a battery charger connect the + positive to the files and -- Negative to the steel plates. Hydrogen gas will be produced by the process so adequate ventilation is required

Depending on the power of the DC supply will determine the time taken to restore the files.

Do NOT be tempted to speed up the process using a DC welder as it cause the solution to boil violently.

NO stainless steel to be used in the solution. Dispose of the solution by chucking it on the lawn it will kill mosses and lichens

James Alford14/05/2020 07:25:20
465 forum posts
79 photos

Thank you for all of the suggestions and the experiences. It sounds like it will be worth a try if I get hold of some acid. I might be able to rescue one or two of the less worn files.

i may try and make some scrapers out of the others.


Edited By James Alford on 14/05/2020 07:25:49

John Paton 114/05/2020 15:57:41
324 forum posts
20 photos

Interesting stuff re 'sharpening'. Electrolytic with washing soda sounds worth a try as would be handy for controlled de-rusting of car parts too.

Re declogging files, I will mention pushing the end of a flat strip of 12x1mm steel across the file along the grain. This quickly serrates the end of the strip of steel with a tooth form which cleans out the grooves in the file. I bit of paraffin or WD40 helps too.

Sorry to all those well aware of the technique but may be useful for those who dont, especially if they have just loaded their best file with ally or even lead!

Speedy Builder514/05/2020 17:11:49
2654 forum posts
219 photos

Peak 4 Bill, that link brings me back to the current post, but I do remember a post was written about a year ago on this same subject, but I can't find it.

Michael Gilligan14/05/2020 20:32:00
20289 forum posts
1064 photos

... is this the one ?



oldvelo15/05/2020 00:45:25
296 forum posts
56 photos
Posted by John Paton 1 on 14/05/2020 15:57:41:

Interesting stuff re 'sharpening'. Electrolytic with washing soda sounds worth a try as would be handy for controlled de-rusting of car parts too.

De rusting and removing paint or declogging files. The parts need to be connecteted to -- Negative terminal on the DC power supply and the sacrificial Piece to + positive

One of my many hobbies is restoring wood an metal working machinery and have used this technique on my projects successfully. You need to scrub the black oxidised layer from the job under running water. Then remove the water from the job with CRC WD40 etc to prevent immediate rusting if left to air dry.



Edited By oldvelo on 15/05/2020 01:35:37

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