Why has my mild steel bent

[DellParticipant @dell]

I am in the process of making a copy of Levin’s diamond watchmaker graver sharpener ( see first picture), I started by making the arbor to mount the diamond wheel to my Pultra then moved onto the base that fits onto the mounting point where the tool rest goes.

I started by milling the steel square ( or should that be oblong ), then cut a slot 4.8mm deep X 8.6mm wide then to cut the T slot I had to use a woodruff cutter because I couldn’t get a T slot cutter small enough, very slow going on my very small Warco WM12 , as I am going to nickel plate to guard against rust I needed to sand to remove tool marks but when I offered it up to the sander it only sanded the edges not the centre it’s not a problem with this part because it’s only the base but for future reference it would be nice to know if cutting the T slot was the cause.

I haven’t been able to find anyone with the tool to be able to get measurements so I am having to do it best I can.

Dell

Dell

Edited By Dell on 03/08/2023 17:16:15

[DellParticipant @dell]

[Frances IoMParticipant @francesiom58905]

hot rolled steel ? often will move if cuts unbalance the built in stresses – if you can’t take balanced cuts then I’ve seen suggestions to anneal the block and cool slowly might reduce the effect.

[Bo’sunParticipant @bosun58570]

I was given to believe that happened to Cold Rolled Steel.

[old martParticipant @oldmart]

The stresses in the surface skin are reduced by milling a little off both sides on cold rolled steel. You might start with removing about 0.005" to 0.015" each side in turn, up to 0.100"total, so starting with some spare thickness helps a lot. The tee slot will then will hopefully not affect the straightness.

Edited By old mart on 03/08/2023 18:18:57

[Ramon WilsonParticipant @ramonwilson3]

Bo'sun is correct.

Cold rolled steel (CRS) or 'Bright Mild Steel' flat bar has a lot of stress within the material that is released upon cutting any surface. Depending on the stock section and the amount of material removed the bending can be quite excessive.

Hot rolled steel, usually black in colour has far less stress locked in and usually machines quite well. The material is not always square however so a different approach in intitial sizing is required.

There are ways of eliminating the stress when machining CRS – annealing it beforehand is one, the other is to machine it down to block size (the size of the part) by taking successive cuts from each face to eliminate or reduce the bending to a minimum

Hope that helps – Tug

[JasonBModerator @jasonb]

As Bo'sun says it can happen with cold rolled (Bright) steel and other metals for that matter. The final cold rolling intriduces stresses into the metal and when you cut unevenly into it the stress will be released on one side but not the other.

As your tee slot only cuts through one side there is not much hope of trying to balance the cuts by doing the same each side.

Two options, start with hot rolled (black) material which will need a bit more work to get a clean surface or stress relieve bright by heating to red heat and allowing to cool slowly which will end up making the surface look like black bar

[DC31kParticipant @dc31k]

Posted by JasonB on 03/08/2023 18:26:34:

…heating to red heat and allowing to cool slowly which will end up making the surface look like black bar

Are there any solutions within the means of a home workshop to prevent this happening?

Would wrapping in stainless steel foil and including a bit of paper that would burn off the oxygen work?

For small stuff, could you coat in borax paste as if you were silver soldering and then dissolve off once cooled?

Would something as simple as surrounding the bar you want with another bar on each side of it tightly wrapped with wire be good enough to stop the bit in the middle being oxidised?

[JasonBModerator @jasonb]

I have tried it with flux which worked, not tried the others but may well help.

[DellParticipant @dell]

Thanks for all the replies, so next time ( if there is a next time ) I will coat in borax and heat to red then let cool.

Dell

[Anonymous]

Technically sheet and plate are cold rolled. Sections such as round/hexagon/rectangular are cold drawn through a die. Hence the description BDMS, equals bright drawn mild steel. It is the distortion caused by drawing through a die to reduce the size that causes the surface stresses.

The issue occurs on any material that is cold drawn. In the past I have had serious bending problems when machining brass sections.

Andrew

[FulmenParticipant @fulmen]

Posted by DC31k on 03/08/2023 18:46:21:

Would wrapping in stainless steel foil and including a bit of paper that would burn off the oxygen work?

For small stuff, could you coat in borax paste as if you were silver soldering and then dissolve off once cooled?

Yes and maybe. I haven't tried foil myself, but it's a well known trick among knifesmiths and toolmakers.

And while borax chould work, I've only used boric acid. It has a much lower melting point (170C) meaning you can apply it by sprinkling it over a hot part or rolling/dipping the part in dry powder. Once molten it dehydrates into a viscous glass-like coating that will dissolve in water.

PS: You can make boric acid by acidifying a borax solution with muriatic (hydrochloric).

[Peter Simpson 3Participant @petersimpson3]

I have tried to machine a little off both sides on connecting rod and numerous other narrow cold rolled steel parts.. They always bend like a banana. Black steel gives much better results when machining these type of parts. The only issue is there is a very limited size options when purchasing black steel stock. I believe all stock is now metric sizes.

[old martParticipant @oldmart]

When I was working, we used to stress relieve lots of steel before machining at 190C for a couple of hours followed by cooling naturally. This did not completely remove the stress, but must have helped because it was used extensively on aircraft parts.

[Clive HartlandParticipant @clivehartland94829]

I have always heat treated metal, both brass and steel when machining.if you dont it will bend. Especially long items.

Brass is the worst to bend when maching, also if filing. The flat steel bar needs both sides machinging.

[Ramon WilsonParticipant @ramonwilson3]

Why would you need to protect material from oxidising if you are annealing it in the first place to stress relieve it for machining all over?

Just heat treat it then machine it. A good home method of annealing steel is to plunge it at red heat deep into preserved fire ash and allow to cool slowly. The ash needs to be deep enough to allow a good insulation all round.

Incidentally this method can be used to soften hardened steel balls for drilling etc for use as governor balls.

Wrapping to prevent oxidising is only necessary if the surface actually requires preserving after machining eg hardening

Tug

[not done it yetParticipant @notdoneityet]

All materials are stressed when worked, Some more than others. Even hot rolled steel may have some stresses within the mass, but far less than cold rolled equivalent. Even think springs – they simply return to original when the strain is removed (as long as they are not over-stretched!).

Annealing (normalising) is the process to get the product to the state required (if not hardening and tempering). Remember, too, that heating, then cooling, will cause some minor change in shape. Finish machining, after any heat treatment, is likely necessary if precision is to be retained.

[JasonBModerator @jasonb]

Posted by Ramon Wilson on 03/08/2023 22:20:28:

Why would you need to protect material from oxidising if you are annealing it in the first place to stress relieve it for machining all over?

Tug

It may be you just need to cut a notch out of one side like the OP's part machine a tee slot and don't want the part to cup as the slot closes up/opens out. Same reason bright bar is not good to use for loco frames as it moves when the cut outs for the axle horns are done.

Heating larger sections will take some time and is likely to leave some scale on the surface which would need cleaning off in some way, protecting the unmachined surfaces would save having to do that

My reason for initially mentioning that the surface would be affected by heating was to make anyone doing it aware that what started bright would not be after heating so they should bear that in mind either allowing for a skim to clean things up or a bit of draw filing if minimal. Also people tend to by bright as they want something to look bright so don't get the black, as heating will make it black they may as well just start of with black material provide dit can be had in the size spec they want.

While on the subject for those who have not used it black bar and flat often has rounded edges or corners so if your part is tight on size you may want to buy larger Example some "black" 20×40 cut for crank webs

Edited By JasonB on 04/08/2023 07:07:06

[Ramon WilsonParticipant @ramonwilson3]

Hi Jason

Cutting a notch in one side is not 'machining it all over' which is the point I'm making – why bother if it's to be removed.

I very quickly learnt so many years ago that using bright mild steel surfaces as rolled as a finished surface is not ideal – indeed my first frames did exactly as you describe.( Later frames were 'black mild steel' with the oxidation removed before machining).

Machining any small component either in part or all over from stock CRS is always fraught with the likely potential of distortion so its either annealing or careful and slow removal of metal working from face to face to eliminate the stresses relieved and that is not always successful in that aim.

The surfaces of cold rolled steel vary with size and they are not always as flat one would assume so initial machining all over is often necessary.

I've been luck over later years in having a small amount of offcuts of a steel termed UHB11 to use. It is completely stress free and large cuts can be taken from one side of a component without any fear of distortion – but most are not so lucky

Hope you are well – R

[DellParticipant @dell]

As I said in the original post , I squared it off meaning I milled all four sides and both ends but it looks as though I didn’t milk enough off both sides to release the stresses.

Dell

[JasonBModerator @jasonb]

Dell, which way did it bend?

From your first post I assumed it had cupped, either opening or closing when you cut the tee slot in which case skimming all 4 sides would not have made much difference. It is not so much machining to release stresses but balancing the stresses on each side so a big slot cut in from one side will not be balanced hence cupping

If it bent along it's length on the initial clean up then flipping it over so you take small amounts from each side in turn until down to finished size can help rather than doing one side then the other.

[Nigel BennettParticipant @nigelbennett69913]

It also happens with brass.

Fortunately I was able to batter it straight again and use it, but it does illustrate the locked-in stresses at the surface of drawn material.

[DellParticipant @dell]

It wasn’t along its length it was is the side to side that dished ,as I say it’s not a problem with this part , I was just wondering for future reference when something more critical in size is needed, I have since measured and the slot I cut in first 8.6mm wide now measured 8.67, although it is less the further in towards the closed end but that would make sense as more material there. so not a great amount but if it was some critical part 0.07mm could make a difference between fit and not .

Dell

[DalboyParticipant @dalboy]

being a new metalworker someone will correct me if my thinking is wrong so here goes how low in the vice is it being held. would holding it just by the lower section where no material is being machined away stop this happening, that is to say no pressure on the top closing it up. I know that vices for milling machines are meant to hold square.

Hope that makes sense

[Ramon WilsonParticipant @ramonwilson3]

Dalboy, – unfortunately no, the vise will not prevent the stresses releasing though it will do so until the vise is undone.

Dell's deep cut of the tee slot is what was the main cause and is something I would have expected to happen if done without allowing the stress to ease by lighter machining and releasing from the vise for those stresses to ease gradually.

As Nigel says brass is particularly bad for this but can sometimes be reshaped it the part will allow it.

A similar situation to this was experienced at work. Thin, slightly distorted heat treated parts could not be held down to the magnetic chuck by magnetism to be ground initially – the magnetism would actually straighten the part which would then resume its distortion once released. Parts, sometimes many of them, would be stuck down to the chuck using double sided sticky tape and the initial cut gingerly taken to take the bow out. The sticky job of removing the tape had to be done before holding them in the correct fashion to bring them flat. A one off might similarly be held in a vise to create a flat surface before using the mag chuck.

Not quite the same as distortion caused by stress release on CRS but similar in the way the material is dealt with to create a flat surface

Please note – I am not advocating holding parts down by the sticky tape for milling !!

Tug

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