question about BMS

[Garry Coles]

Hi everyone
Can anyone tell me if BMS flat in various sizes should be heated up to red hot, so as to stop the distortion after cutting out for loco con rods or anything else for that matter. I have read a couple of articles about this problem, but not sure if it applies to BMS or not.
Cheers
Garry in sunny Jersey !!!

[jackrae]

If it's bright then it's almost guaranteed to be stressed from the drawing process so needs normalising before you make any longitudinal cuts or machining. The only exception is when cutting a length from stock since the stresses usually act across the width and thickness so won't produce distortion in cut off lengths. However angle iron can bend when you cut off a length since the stresses in the flanges may not be equal.

Normalising takes time to be effective - dependant upon bulk of the material, not simply heating to red hot and then cooling. The bulkier the section the longer the "soak" time required. I've used the old technique of lighting a wood fire big enough to heat the metal to red heat then letting the fire burn itself out and removing the metal once the ashes are cold. The ash permits the metal to cool slowly, and the wood fire doesn't burn hot enough to "burn" the metal.

If you want a demo of the effect of stress relieving take a piece of your stock and make a longitudinal cut down its length. The two parts will separate like a pair of bananas as the cut progresses.

good luck
jack

[waggy]

Garry,

Take care heat treating free cutting mild steel, if you don't support the bar I've found it sags and defeats the object somewhat! Put your bar on a thin bed of sand prior to heating, this worked for me when I heated the steel for motion parts I was making. I spread the sand on an old steel tray, about 1/4" thick, bedded the bar and applied the blowtorch, no distortion.

Jackrae is right when he says the metal distorts, it will even move when held in a vice for machining. Bl**dy annoying at times.

Waggy.

[cardiffpat]

It would be very handy if commercialy stress relieved BMS were available, does anyone know were to obtain it?.
Is the hot rolled black steel as machinable as BMS, if so then what is the advantage of BMS?
regards Pat.

[chris vine]

It is sometimes possible to get hot rolled black steel which is the free cutting formula.

Otherwise the black stuff can be rather difficult to machine well.

Chris.

[SteveW]

Pat,

Re: advantage of BMS over Hot Rolled.

BMS arrives nice and shiney with nice flat surfaces and square corners but the rolling process that got it like this caused a lot of trapped stress. It's quite near to declared sizes so is good for using as-is. If you have to go inside the stock size for the shape you're looking for either normalize it or work in from all sides a bit at a time to release the trapped stress slowly.

Hot Rolled (HR) usually arrives with a black crust of oxide from the hot rolling process. This can be quite hard to get through but needs to go before you start anything. The dimensions are less accurate and the edges/sides are usually rounded.

So far I've found using HR is a whole less messing around and if you take care any non-free cutting issues aren't a problem.

[JohnP]

I had a black steel plate that I wanted a bit cleaner so I left it in the pickle - citric acid - for about a week(!), checking fairly frequently. The black faded to grey and I'm pretty sure I was down to the native steel.

JohnP

[havoc]

Only today I realised I missed something in following this thread.

I planned on making the frames of my next loc from BMS. But since this will involve a lot of cutting in the bar I start fearing this isn't the way to go. So I have a few questions regarding alternatives for BMS:
- if you heat treat it, does it become softer? If not, then that would be the easiest if it doesn't twist to much.
- what is the easiest to get the black skin of HR? This would be the next alternative. But it would involve a lot more work.
- what about using sheet metal? How safe is that? I look more related to BMS, but has it the same stress problem?

[jackrae]

The normalizing will leave the cold drawn steel "softer" since the stresses provide some degree of hardness/toughness to the material. This is also why you need to avoid over-heating in the normalizing process, hence my suggestion of the wood fire.

Trying to normalize a wide thin strip will almost inevitably result in some buckling of the material but this can be "bashed out" by judicious use of a soft hammer

If you don't wish to use hot rolled black for your frame sides then a piece guillotined from sheet really is the way to go. Although this is also hot rolled rather than drawn but is then "cleaned up" following the rolling process to produce a fine finish.

If you really want something to be proud of you could go for ground flat plate but that's spending money for the sake of it.

When ordering your sheared plate allow an 1/8th oversize since the shearing process invariably leaves a mark at the cut edge which won't look too good in the finished model

jack

[burnettsj]

Or the even easier method - get them laser cut !!

[jgb7573]

For cleaning Black MS, use phosphoric acid (I believe this is the active ingredient in rust removal stuff like Jenolite). I use this at about 1 part acid to 10 of water. When you take the metal out of the pickle, it'll be a grey colour. Rinse in water and immediately coat with oil (WD40 or something) as otherwise it will turn red with rust before your eyes.