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EN3 bowing after machining

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Peter Caswell12/11/2019 19:54:02
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9 forum posts

Hi Peoples

Apologies if this under the wrong topic but it seemed the most appropriate. still learning and always will be and need some help and guidance regards machining EN3.

I wanted to make some Tee nuts, I had a 300mm length of EN3 20x20mm, I machined this to a Tee profile to chop up into 21mm lengths to make the tee nuts. It was rigidly clamped on the milling table and machined, reducing the height to 14mm, the width of the foot to 19mm with a 7mm step with the remainder to a width of 12mm.

Now the problem, when I unclamped it from the mill table it had a bow in it, the foot of the Tee being concave there being a gap just off centre of 1mm, I guess it was due to stresses being released (or created) during the milling,

As I am going to cut it up in 21mm lengths I can probably still use it, but what did I do wrong and how can I prevent repeating this.

Have put the project to one side now awaiting your knowledgeable replies.

Thanks

Peter

Clive Brown 112/11/2019 19:59:40
284 forum posts
7 photos

Is it bright drawn material, 'cos that will be worse than hot rolled? You could have reduced the thickness to14mm by taking 3mm from each side of the bar.

Clive

Edited By Clive Brown 1 on 12/11/2019 20:00:12

Clive Foster12/11/2019 20:26:38
1886 forum posts
59 photos

Yup. Stresses released during machining.

By the very nature of things all matrials have surface stresses induced during the production process. Solid equivalent of surface tension in a liquid. So any asymmetrical machining, such as on one side but not the other will leave stresses trying to bend the material. If the residual stresses from production are small they probably won't have any effect but if large things can bend in a pretty spectacular manner.

Hot processed material is bought to size and left too cool down so pretty much all the surface stress sorts itself out as it cools. Hot rolled steel in particular has a rather poor finish which is usually machined off to leave something nicer so the (lightly) stressed part is taken off anyway further reducing the small effects so bending is extremely unlikely.

Cold processed materials, like bright steels, are effectively squashed to shape cold under high pressures. The closer to the edge the more the squash. So there is a lot of stress locked up close to the surface of the material. As made it usually stays straight as stresses are pretty much the same all round. Taking a decent cut off one side removes the stressed layer that side so the other one pulls, or pushes, it bent.

The exact depth of the stressed area is material and process dependant in monumentally complicated ways. In our sort of sizes you probably need to take off something over 0.04" / 1 mm or so to reduce any residual stresses in bright materials to safe, non distorting levels. Which may need some workholding creativity to stop it bowing half way through.

Clive

Edited By Clive Foster on 12/11/2019 20:28:10

Peter Caswell12/11/2019 20:42:02
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9 forum posts

Many thanks for that explanation Clive F, as Clive B1 suggested I should have reduced the thickness equally from both sides but I guess from your explanation I would have to have it on its side, so to speak, to equally machine the top and bottom with very light cuts, as you said that would mean some creative work holding. Perhaps I should try some hot rolled!.

Nigel Bennett12/11/2019 20:50:17
306 forum posts
11 photos

You can also heat up the bar to a nice red heat, hold it there for a couple of minutes and allow to cool slowly. This redistributes all the locked-in stresses created during the drawing process and distortion should be reduced.

Ian P12/11/2019 21:02:51
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2246 forum posts
90 photos

If you have not drilled the holes through it then getting it straight (enough) whilst it in a long length should not be too difficult. Worst case you could re-machine the two faces that sit under the slot ears.

Ian P

Andrew Johnston12/11/2019 22:56:45
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4935 forum posts
560 photos

I've milled a lot of hot rolled steel and never had a problem with distortion. Although there was a thread recently where someone had experienced distortion after milling hot rolled steel. If I recall it was quite a thin section. I'll see what happens when I mill the small hot rolled section to make the beading for my traction engine tender.

Cold drawn is a different animal, and not just steel. Cold drawn brass will distort as well. If I need to mill cold drawn steel I normally heat it to 800°C and let it soak for 30 minutes or so per inch of thickness. I then let it cool slowly. Of course I have the luxury of an electric furnace so soaking and slow cooling are fairly simple. I've never had a distortion problem after heat treatment.

Andrew

Tony Pratt 113/11/2019 06:49:46
929 forum posts
3 photos

Just made some 95 mm long tee nuts from bright mild steel I had in stock, to avoid distortion I put the raw material in my first open hearth fire of this season, it came out horribly scaled but all was machined away with no apparent distortion when the part was finished.

Tony

Martin Kyte13/11/2019 08:51:26
1511 forum posts
24 photos

Unless it was bowed to start with.

Martin

Nigel McBurney 113/11/2019 09:02:31
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622 forum posts
3 photos

I have a long length of key steel (en 9) 1/2 by 3/4 which I use for stationary engine gib head keys,if I take a short length say 4 inches and machine down one side it will curl up by an 1/16 or more, so I usually get it red hot with the propane torch,for a few minutes and let it cool slowly on the fire brick hearth,this relieves the stress and machines ok in a straight line. To get over the T nut with a slight curve,i would cut the nuts to length, then set one at a time in the machine vice,with narrow bit of the t upwards just as they sit in the t slot in the machine,then take a light cut either side so that the part of the nut which comes in contact with the underside of the tee slot is truly flat. It should be flat so that it will spread the load on the underside of the t slot,it does not matter if the other surfaces of the t nut are not true.

Peter Caswell13/11/2019 09:44:20
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9 forum posts

Many thanks for all your replies to my post.

I now know what to do in the future and have a good solution from Nigel McBurney to true up the parts, while in the vice I can drill and tap for the stud and all will be square.

old mart16/11/2019 18:45:40
780 forum posts
76 photos

We used to stress relieve steel at 190C for a couple of hours and then cool naturally. This is in domestic oven territory. It might work on cold rolled mild steel. We just made 11 10mm threaded  tee nuts for 1/2" wide slots from a 300mm length of key steel and didn't notice any bowing.

Edited By old mart on 16/11/2019 18:46:35

Edited By old mart on 16/11/2019 18:48:05

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