Steel plate distorting when machining surface

[Men IfrParticipant @menifr84251]

I have some steel bar approx 6x20x150mm. I machine the surface off one side and it warps quite badly – I believe it's the surface tension.

The difficulty is I need it reasonable accurately flat and parallel so I have to take a lot of material off the other side and now need to go back to the 1st side and re-machine. I spent hours machining the surface off two of these yesterday, surely there has to be an easier way – I'm thinking there must be a type of steel stock I can buy that is reasonably flat to start with and will not distort if I machine one surface?

As usual thanks in advance for the replies

Ian

[Men IfrParticipant @menifr84251]

How to avoid it?

[JasonBModerator @jasonb]

Typical of bright cold drawn bar. You could start with Black hot rolled bar which has less trapped stress and usually stays flat or anneal bright bar before machining.

What is the final cross section you want? Starting with Gauge plate ( ground flat stock) may be your best bet as this has a more accurate surface.

Can you post a sketch of the part and give an idea of it's use also do you know the grade of steel you have so people can give a more informed answer

 

J

PS if it took you hours to machine the surface then you also need to look at your machining methods!

 

Edited By JasonB on 17/12/2017 09:27:16

[JOHN MOSLEY 1Participant @johnmosley1]

If you use Bright flat (BDMS) this will distort as you describe due to the cold working in the skin. For machining flat bar use hot finished, yes it has a poor finish but that what has to be used in industry. Weld on BDMS can also cause stress cracking. As an apprentice a lot time ago I fought the problem you are having, its not fun.

[Brian HParticipant @brianh50089]

Gauge plate would be one suggestion, possibly would not need to machine it. Another idea would be to stress relieve the plate by heating to red and cooling slowly.

Brian

(Jason beat me to it)

Edited By Brian Hutchings on 17/12/2017 09:27:26

[SillyOldDufferModerator @sillyoldduffer]

Posted by Men Ifr on 17/12/2017 09:06:00:

I have some steel bar approx 6x20x150mm. I machine the surface off one side and it warps quite badly – I believe it's the surface tension.

…

Don't be disheartened Ian, it's not your fault. Learning about the properties of materials is all part of the learning experience. If you have the time and money, these discoveries are 'quite interesting'. If you're working against the clock on something expensive, it's 'quite annoying'. That's 'quite annoying' as in seeing the world through a red mist and finding an urgent need to kick the cat.

I worked with Bright Mild Steel for a long time without encountering warping. I wrongly thought people were exaggerating when they mentioned it. It's caused by the way BMS is made. Basically, during manufacture, one side of the bar may become tensioned like a spring whilst the other side is compressed to keep it straight. Cutting into the bar destroys the internal balance and the bar bends.

It's possible to relieve the internal stresses in BMS by making the bar red hot and allowing it to cool very slowly. I don't have the facilities to do that.

Most of the time I risk using BMS and get away with it. When caught out I switch to Black Mild Steel and use that. In short:

• Bright Mild Steel saves a lot of machining – unless it bends.
• Black Mild Steel won't bend – but it needs a lot more machining.
• Alloys like Gauge Plate save machining time and don't bend – but are expensive.

And some of us do this stuff for fun!

Dave

[IanTParticipant @iant]

"It's possible to relieve the internal stresses in BMS by making the bar red hot and allowing it to cool very slowly. I don't have the facilities to do that."

​Where I've had this concern, I've placed the work in a garden incinerator whilst burning tree/hedge trimmings (after its been going for a short while) – and kept it stoked up for a good while longer – then left everything going overnight (with the lid on). The part (buried in the ashes) will still be very hot the next morning, so I usually leave it to cool right down. I can't say for certain that this is sufficient (and it certainly isn't controlled) but the BMS bits so treated have behaved themselves afterwards.

Something which they may have done anyway of course but it made me feel better. I've also given this treatment to some castings that had some hard spots and it also seemed to help them – but it was very subjective, so I can't be certain. Helped my confidence whilst machining though, so it worked on one level at least….

Regards,

IanT

[vintagengineerParticipant @vintagengineer]

You could try skimming the opposite side to pull it back

[Men IfrParticipant @menifr84251]

Posted by vintagengineer on 17/12/2017 12:24:31:

You could try skimming the opposite side to pull it back

Basically that is what I have done but of course that then creates another distortion so I've gone back to both sides several times to try and get them flat.

[Men IfrParticipant @menifr84251]

And of course the other side is very warped so I tried machining across the part mounting it many times in an attempt to take maybe 0.1-0.2mm off rather than doing the whole lot in one go to take 1mm off the ends and 0 in the middle…

[Men IfrParticipant @menifr84251]

Re the heating – I have a plumbers MAP torch (very useful and cheap by the way) so think I could get the piece red hot – I will try that next time first.

Also I can take the surface finish off on my belt sander it is probably worth doing that before milling as its v.quick to do both sides (and try to re-balance the stress at vintage engineer suggests) them take maybe 0.1mm off both sides on the mill..

[Men IfrParticipant @menifr84251]

• Bright Mild Steel saves a lot of machining – unless it bends.
• Black Mild Steel won't bend – but it needs a lot more machining.
• Alloys like Gauge Plate save machining time and don't bend – but are expensive.

That's a great summary I looked at gauge plate – would be perfect but would be ~5x the price for the steel rod I paid however TBH may be worth the time saved.

Also why does black mild steel need more machining?

[MuzzerParticipant @muzzer]

Because it has a black, scaly surface. Generally you don't want that on the final part so you have to machine it off. The surface of bright steel is often good enough to use as-is. Depends on the application.

Murray

[JasonBModerator @jasonb]

Better to remove the scale by pickling in acid as it can blunt HSS tools and then machine the texture off.

[SillyOldDufferModerator @sillyoldduffer]

I may have got the wrong end of the stick about the time and temperature needed to stress relieve bright mild steel. A quick Google just showed I may have confused 'Normalising' with 'Stress Relief'. Normalising is rather complicated. It involves taking steel up to about 850C, holding it at that temperature for a couple of hours and then letting it cool down slowly in the furnace. The whole process takes several hours or more. (Normalising might be the answer to the EN32B thread, what do do about hard lumps in steel, rather than Ian's bendy problem.)

For stress relief, one site simply advises heating to 650C and allowing the steel to cool in air. Much simpler.

Can anyone confirm that Ian's idea of heating to red-heat with a torch and leaving it to cool naturally is all that's needed? If so, I'll start doing it!

Dave

[Phil H1Participant @philh196021]

This might not be the most helpful post but if you can not stress relieve the bright bar – please do not think that black bar will be fine.

Perhaps we were all very unlucky but as apprentices – we only ever used black bar for all the test pieces and it can also make nice banana shapes when machined.

PhilH

[Neil WyattModerator @neilwyatt]

Posted by SillyOldDuffer on 17/12/2017 14:03:46:

Can anyone confirm that Ian's idea of heating to red-heat with a torch and leaving it to cool naturally is all that's needed? If so, I'll start doing it!

Works for me.

Neil

[Anonymous]

For cold drawn steel section I anneal before milling, rather than specifically stress relieve. I put the whole lot in the electrical furnace and set the temperature to about 830°C. Once at temperature I let it "simmer" for an hour or so, and then turn the furnace off. I let the work cool over night in the furnace.

I must have been lucky as I've never had hot rolled steel go bananas during machining, even though I don't heat treat beforehand.

Andrew

[Phil H1Participant @philh196021]

Andrew,

Yes maybe unlucky – but it would typically go if you machine one edge of the black bar. I agree that it might not be as bad but it does seem to go. Also very noticeable when you machine quite a decent chunk then try to surface grind it. Again, not massive amounts but certainly a good couple of thou'.

Phil H

[MikeParticipant @mike89748]

It seems that this warping can happen with rolled plate or strip in any metal. When I was into making fishing reels I first tried to fabricate the spools from aluminium plate. It always warped, no matter what I did to it in terms of heat treatment so had to resort to turning the spools from bar stock. The wastage of material was huge, but it was the only way to go.

[colin hawesParticipant @colinhawes85982]

The problem of long machined strips warping occurred on almost every job I had to do on press tool stripper plates. When the clamping was released after machining the bar could be guaranteed to bend so I always had to use a fly press to straighten them after machining. This is a tricky operation too although this item doesn't usually have to be machined on the reverse side. Colin

[Men IfrParticipant @menifr84251]

Posted by Muzzer on 17/12/2017 13:04:26:

Because it has a black, scaly surface. Generally you don't want that on the final part so you have to machine it off. The surface of bright steel is often good enough to use as-is. Depends on the application.

Murray

Ah OK – This is what I have and yes it bent like a banana when I took one surface off the 6mm bar

[Men IfrParticipant @menifr84251]

Will the black surface rapidly blunt a carbide mill as well?

[norman royds 2Participant @normanroyds2]

I have just read a few post trying find out what you are trying to do I think the distorting is probably due you using the wrong cutter just a guess regards norm

[ChrisHParticipant @chrish]

Re Jason's comment "Better to remove the scale by pickling in acid" – what acid?

Would citric work or does it need something much stronger?

Chris

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