Polishing Mild Steel

[Jason]

Hi

This will sound like a really daft question to the experienced engineers on this board, but can someone sell me what the correct sequence it is to polish a piece of steel after turning or filing, for example a buffer, how do you get a real good finish afterwords to the steel. Is it just a case of using steel wool or using a polishing mop or what?

Thanks

Jason

[baggo]

Hi Jason,

Obviously it helps to get as good a finish as possible from the initial turning or filing as this will cut down the amount of work afterwards.

The way I do it, and others probably do as well, is to start off with something like 120 grit carborundum paper and use progressively finer grades until the desired finish is achieved. The starting grade of carborundum paper will depend on the initial finish but it's no good trying to remove tool marks and deep scratches with 1000 grade - you'll be there until next Xmas! If the turned finish is very poor I usually use small files to remove any deep marks. Take care though using files in a lathe in case they catch on the chuck (ALWAYS use files with proper handles fitted!). To give the work a final polish use the finest paper with a bit of oil or a cloth with metal polish.

Edges of frames etc can be finished by drawfiling using plenty of chalk on the file to stop bits sticking in the teeth and causing more scratches.

Personally I don't get too carried away with the finish on bare metal parts as you don't see many full size locos with polished motion work, in fact often the finish is pretty rough! I'm happy with a nice matt finish.

John

[Jason]

Hi John

Thank you for taken the time to reply to my query. The advice you gave is extremely useful, I know this probably seems very basic, but as a beginner its nice to know that there is people on here I can turn to.

Kind Regards

Jason

[chris vine]

Hi Jason,

I find that when you have got all the bad machining marks out, then the maroon coloured Scotchbrite gives a very satisfactory finish. It is not a polish but leaves a nice grain on mild steel.

Have Fun,

Chris.

[ron]

Hi Jason
I'd pretty much second the previous replies, coarse emery first [if needed] then fine emery, you can get a very high polish with worn smooth emery. If it's on a lathe, top speed and smooth worn emery will produce almost a mirror finish. Draw filing with emery stretched on the file gives a nice finish, although I get a guilty concience when draw filing, it was a hanging offence when I was an apprentice.
The only thing you have to watch when polishing flat steel is not to slightly curve it, it looks better if you can keep it completely flat.
I've an old bench grinder which I converted to a polisher for other purposes, it is really only suitable for restoring something that previously had a good finish, it will not remove anything other than very minor scratches.
Ron

[Lurkio]

Hi Jason,

Ensure you make allowances when turning if you're going to polish up afterwards where finished size is important. It's easy to quickly remove a few thou. especially when using coarser grades of abrasive.
Although a mirror finish can be obtained if needed, I concur with baggo and chris - you'll find more often than not that a satin finish such as that provided by Scothbrite is the most satisfactory - faults and blemishes don't show up so much!

Lurkio.

[waggy]

Hi Jason,

Try "Wet and Dry" paper, using half / half mix of washing up liquid and water. Wash off and oil as soon as you finish, though. Gives a nice finish.

Waggy.

[Noddy]

Just a thought; what is the part that you are polishing and why?

The reason I'm asking is, if it is a bearing surface or there will be sliding contact, you may be better off going for a finish that leaves some sctatches in, and (this is contentious) some would reckon that you should not use hard abraisive (silicon carbide, fused alumina etc) on a bearing surface as some may embed itself to cause wear later. None embedding abraisives such as Diamantine (precipetated aluminium oxide)

supposedly the ideal compromise between roughness and pockets for oil to contact surface is about 40% to 60% contact. This even seems to apply where no liquid lube is used, so for example target rifle barrels are lapped with a lead lap charged with 80 or 120 grit. The lapped surface is reckoned to pick up less lead or gliding metal fouling.

For mirror like finishes, the horology text books go into excruciating detail.

I know I mentioned guns, but the majority of "gunsmithing" books are hearsay instructions on how to spoil a good gun and are therefore best avoided (especially if "written" by Harold Hoffman).

[baggo]

Other items I find very useful are the little sanding drums on expanding mandrels for use in Mini Drills such as the Dremel. I use them for finishing off coupling rods, connecting rods etc and they leave a very nice satin finish. Quick to use and, being round, fit into the curved bits! The only problem is that they only seem to come in one grit size (?) and they can be a bit agressive when new so you have to be careful and not press on too hard! Looking through the new Arc Euro catalogue I see that you can also get little flap wheels, sponge sanding drums, and tapered cartridge rolls that all look very useful.

Another dodge is to wrap a strip of carborundum paper round a scrap of rod with a slot in it to hold the end and use that in a drill or the lathe.

Ron - I'm curious as to why draw filing was taboo ? I've always thought it to be a recognised way of finishing metal edges. I was taught it in school metalwork classes 40 years ago!

John

[Jason]

Thanks to all for the advice you have given me. This should hopefully give me a head start in getting a good finish.

Just one quick question, I hear the saying "Draw Filing" a lot. Please can some one tell what the true definition of this is.

Jason

[ron]

John
I think it was taboo because it was an easy way of doing it therefore apprentices being apprentices would take the easy road and wouldn't learn to do it 'properly', in retrospect a load of nonsense but the instructors we had were the old school where everything was done by the book [or else!]
One of the test pieces we had to file was a perfect 3/4" cube of MS to be a perfect sliding fit all ways in a 3/4" hole filed in the centre of a 2" square piece of 1/4" MS plate.
Nowadays that's why I avoid filing if I can possibly manage it, phsychologically damaged for life
Ron

[chris vine]

Hi, Baggp's little sanding drums are great but you can get them in almost any grit size you like. Sometimes known as spirabands. Try looking up an industrial abrasive supplier, they will either have them or know someone who does.

Still on the subject of these little drums and finishing curves on things like coupling rods. You can put them in the vertical drill, just sticking down through the centre hole. Then you can work the item against them, using the table of the drill to keep all nice and square. Wood workers have a similar scheme with the drum coming up through a hole, rotating and oscillating up and down. Works a treat.....

Chris.

[baggo]

Jason - have a look at:

www.btinternet.com/~hognosesam/targets/targets1/drawfiling.htm

for a visual demo.

Thanks Ron, I can see why you have been put off filing! My excuse is I'm not very good at it

John

[Jason]

Thanks Baggo,

That has made draw filing very clear. There is nothing better than a visual demonstration.

Jason

[Shawki Shlemon]

If it is going to be painted ,then a good finish will suffice . If it will remain metal finish I use the following depending what it is -
1) smooth milling file, this has teeth in one direction only .
2) linisher , is one of the most used machines in my workshop.
3)I was given a polishing wheel which I mounted on a mandrel and run it in the lathe and it does a great job but takes a lot of time.I have no information on its details.
For what it is worth " we all try to make the model better than the original " is it wright ? I don't know the answer .

[Boadicea]

For what it is worth " we all try to make the model better than the original " is it wright ? I don't know the answer

Shawki - I think we do, and I think we have to. Models are always scrutinised more than the original. Not many would want a Brit in its running condition in the mid 60s though. I think we often lose track of the fact that it IS a model and there is a limit to what can be achieved in scaling down.
Like many classic cars - what we have is better than the condition on day leaving the factory.
It is always difficult to settle down on completion and enjoy the result, rather than criticising it to ourselves (and others), that is because of our own high standards. A man from Mars would say "Relax - you've achieved a lot". Not many men from Mars around though! ;D
Regards, Bo.

[Myford Matt]

I often see chalk recommended for draw filing - what kind and from where? (Blackboard?)

Cheers

Matt

[steammadman]

Myford- matt.
Any good quality chalk will do,
i was taught to use "french chalk" for finnish filing
it's finer.
Of coarse that was 55 years ago at DONCASTER plant works
" the place the BEST loco's were made.

[spurley]

Chalk is used to prevent 'pinning'; this is where the file picks up and traps a piece of swarf in it's 'teeth' and this then scratches the surface you are filing. As apprentices, we were taught to use chalk during the finishing process to help to avoid this problem and were given ordinary blackboard chalk for it.

Hope that helps?

Cheers

Brian

[John Lee]

ron said:

Nowadays that's why I avoid filing if I can possibly manage it, phsychologically damaged for life
Ron

As other post Ron, so am I... b**terds. ;D Ohh I will get over it...

I think draw filing was prescribed because it damaged the file, cutting in one area.

To be frank this is perfectionist engineer gone mad..if this is the reason. Draw filing is a method described by the late GH Thomas, whose books are both an inspiration to perfection, and source of jealousy for his skill..to me at least
John