Bright Mild Steel vs Black Mild Steel

In another thread there's a warning that Bright Mild Steel might warp after machining. This is supposed to be due to stress relief after cold rolling and the advice seems sensible.

Thing is I often use Bright Mild Steel and have never had a problem with warping. Looking Bright up on the web I found this advice from a supplier, my bold:

Bright drawn mild steel is an improved quality material, free of scale, and has been cold worked (drawn or rolled) to size. It is produced to close dimensional tolerances. Straightness and flatness are better than black steel. It is more suitable for repetition precision machining.

Bright drawn steel has more consistent hardness, and increased tensile strength.
Bright steel can also be obtained in precision turned or ground form if desired.

Now I'm wondering. I've not had a problem and here's a vendor recommending it for precision machining. Have others had lots of bad experiences with Bright Mild Steel warping, or is machining it low risk in practice? Using Bright rather than Black saves a lot of time.

Also, complaints about poor quality steel are common on the forum; could this be down to it being of the Black variety? None of the Bright Mild Steel I've bought has been of poor quality.

Thanks,

Dave

If you mill away one side of a bright bar, it can bend like a banana, I've done it.

If you machine it symmetrically, e.g. for a shaft you rarely get issues.

Neil

The problem is with machining flat bar along its length ; There is not usually a problem with tuning round bar but there probably would be if you try to mill a flat along its length. Colin

Just what Neil says if you keep the cutting balanced it will not really move, take a big lump out of one side and you can see it move before your eyes.

As an example this crank cut from 2x1 BMS moved about 1/16" , both ends moving away from me.

This one from 25x50 Black did not budge one bit

As for quality if you just by a bit of unknown black bar you could be getting recycled old fridges, but a bit of known grade black bar and it can machine up quiet nicely. The second crank above was EN3 and machined up quite well compared to something like a bit of rebar.

I suspect that "precision" simply refers to the OD being consistent, rather than precision in terms of tenths, which is important when using collets on a repetition lathe. Hot rolled steel is generally oversize with a wide tolerance.

Many cold drawn materials warp if machined asymmetrically; I've had the problem with brass rectangular section. As a matter of course I anneal cold drawn sections before milling; and use hot rolled when I can. Anecdotally hot rolled seems less prone to rusting, although it's more difficult to get a good finish.

I've certainly bought poor quality cold drawn sections, and from so called professional suppliers. Hot rolled seems more consistent.

Andrew

In my experience black steel often includes strings of slag, and other odd patches. Perhaps this is because I get odds and ends from a local steel-frame and agricultural engineers, and their major concern is weldability, not machining. But that is one price we pay for living in the sticks. Styx?

Cheers, Tim

Does that prove metric stock is superior?

Neil

Now I know how you got to 10,204 posts Neil .

When I buy metric plastic stock it often comes a good deal oversize to the nominal, for e.g I buy 22mm Nylon 6.6, and it actually measures 22.7mm, However, the roundness of the extruded bar is often left to be desired, so I presume this is the reason why it seems a fair deal over 22.

Michael W

Are 3 posts all the same a record?

this is fairly standard practice for flat bar too, if you mill one side at full depth, you will often find you have trouble holding the size due to the inherent stresses.

But if you take both sides, half as deep, you'll get a more accurate cut. They will also be more parallel to each other.

Michael W

To answer the original question, I've never had bright mild warp on me. Maybe I've just been lucky.

You can use Bright Drawn Mild Steel for machining, but it helps if you stick it in the fireplace first to get nice and hot and let it cool slowly over night.

I too have experienced flat section BDMS moving quite dramatically when milling one side of it. The heating and slow cooling relieves some of the stresses from the cold rolling process.

Phil

Are 3 posts all the same a record?

Might have been. But if it was it is now FOUR all the same on this thread!

In a past life I used to machine large lumps of steel up to 60" diameter. We used to rough them out and leave them for a couple of days to let the stress relieve it self, then final machine to size. Some of the brake drums we mad would go egg shaped over night!

Record has got stuck in the same groove, he is just trying to catch me up

If you mean they all show the same number of posts then that increases as more posts are made so Neil's first post on the forum will also show 10205 posts

I had a problem last night that the forum wasn't responding after I posted, so I refreshed the page several times before it would load.

It obviously accepted the 'new posts' although it wouldn't send them back to me!

I also have 4,000 posts as 'Stub Mandrel' so my real total is over 14,000, of which several are quite useful

Neil

Thanks for the replies chaps. Now I have no doubt that Bright Mild Steel moves when you machine it but today's experiment has left me confused about how much it matters. I think this is another 'it depends' problem.

I cut a 7" length of 3/4" square BMS bar and checked each face for flatness. Looking for light under a steel rule I found that 3 faces were 'perfect' and one was slightly 'off'. (Yes, this is a crude test.) Then I centred the bar in my milling vice with the 'off' face down and cut a 10mm wide slot across the top, 1mm deep per cut.

1. After removing the first 1mm the bar bent up at the ends, perhaps 0.3mm. But after moving the bar to the bench to take a picture, the bar had returned to normal.
2. Replacing the bar in the vice, I increased the depth of the slot to 5mm with a series of 1mm deep cuts. There was no sign of warping after any of these cuts.
3. After the 6th cut I found the bar had again bent up at the ends by about 0.3mm. After removing the bar from the vice to check the faulty underside, I found the underside was now flat. That was temporary. Repeating the check after a few minutes I found that the bar had returned to it's starting state, ie with 3 good flat faces and one slightly off.

I suppose what I'm seeing is that my particular example of steel is slightly stressed in the area I cut. Machining it does cause distortion, but the distortion disappears after a few minutes or when the work is released from the vice.

Jason's examples are bigger than mine. Possibly rolling and drawing stresses increase with the size of the steel bar.? I mostly work with small section steel, which might explain why I haven't noticed any warping before.

Thanks for the answers: it's clear that Bright Mild Steel does warp when you machine it, but you might get lucky!

Cheers,

Dave

Despite the length of the bar it's only the length in the region of the slot that is affected. And the deeper you cut the less the distortion, as the stresses tend to be concentrated near the surface. Try milling a millimetre or two off the whole length.

Andrew

And just in case there's still any doubt here's a picture. It's difficult to see but the steel on both sides of the saw cut is warped away from the cut. This was to be a tool holder for a shaper.

Regards, John.

Edited By Jss on 06/07/2017 14:37:08