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  • Distorting steel plate

    Discussion in 'The main mechanical design forum' started by COLINH34, Jun 9, 2011.

    1. COLINH34

      COLINH34 Member

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      Hi

      Can anybody offer any advice on the following :-

      We manufacture steel heater plates for the print industry. The plate is made from a "plough" ground /stress relieved mild steel and is 895mm x 695mm x 16mm thick. The plate is "honeycombed" with 11 dia holes at about 18mm pitch. On one side only of the plate, there has machined, a 3.5 wide x 4mm deep x 6000mm long recess "weaving" between the 11 dia holes. We are finding that ,even though the steel has been stress relieved, on some plates it distorts by up to 0.7mm. This is probably due to the machined recess on one side of the plate.

      It looks as though I need to find an alternative spec. steel that can be heavily machined and will not distort. Any ideas would be most appreciated.

      Many thanks

      Colin
       
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    3. srdfmc

      srdfmc Well-Known Member

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      Hi,

      How are you making those holes :? Punching or laser cutting ? What's the material used (be more precise than steel please).

      Any draft ?

      regards
       
    4. COLINH34

      COLINH34 Member

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      Hi

      We are presently using Mild Steel plate - EN1A - BS970 230M07 - stress relieved.

      The holes and the recess are machined using a CNC machining centre.

      Regards

      Colin
       
    5. srdfmc

      srdfmc Well-Known Member

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      Hi Collin,

      here is my best guess :
      - use steel with higher yield stress (around 600N/mm²) and lower the speed
      - use thicker plate (20mm) and end the process by machining the part on the side of the recess
      - Machine the recess first and then from the other face the holes progressively slowing down when your reach the recess (to create a strain gradient ).
      To resume the above machining the recess first and playing with varying speed during the milling sound more in the good direction (but you might hve alrdy done this :roll: )

      Sry for my poor English


      regards
       
    6. COLINH34

      COLINH34 Member

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      Hi

      Many thanks for your ideas.

      We would ideally just like to change the specification of the steel and not incur more costs with additional machining. But starting off with thicker material may have to be considered if a more stable steel cannot be found.
      Maybe as you suggest, going to a higher yield stress material such as EN8, would be less prone to distortion after machining than EN1A.

      Many thanks

      Colin

      PS - Your English is very good
       
    7. AndrewNew

      AndrewNew Well-Known Member

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      Interesting problem! What sort of distortion are you seeing? Is it bowing of the whole plate, with the convex side of the plate the side that you have machined? If so, I would guess that your plate has not been sufficiently stress-relieved. I'd be surprised if local effects caused by different strain fields induced by small changes in the machining processes would account for so much distortion, but I have been surprised before. I assume the holes go all the way through the plate - do you see any distortions if you just drill the holes?

      Are you able to source ground annealed plate? As I understand it annealed plate is subject to higher temperatures during heat treatment, which allows for more of the residual stress caused by plate rolling to relax out, which might reduce your distortion when you subsequently machine it.

      Is the geometry of your recess critical? If not, you might be able to change the shape to control the distortion.

      How much distortion is acceptable?

      Cheers

      Andrew
       
    8. COLINH34

      COLINH34 Member

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      Hi

      Usually the plate is bowed to some degree with the convex on the machined recess side. I must admit that most of the plates that we receive from our machinist usually have about a 0.3mm bow which seems to run midway in one direction only through the plate. The plate we have received this time has about 0.7mm bow running through it. This may indeed be a bit of a "rogue" plate. An annealed steel plate might be the answer.
      The machined recess is to enable a heater element to evenly heat the plate up for hot foiling. So it follows that the recess route on the underneath side of the plate is as evenly distributed as possible.
      I leave 0.3mm top and bottom for final grinding as the plate has to be flat to within 0.1mm. ( I am in trouble with this last plate as I will have to grind off more than I envisaged - but I might just be able to get away with it.)

      I was hoping that someone would be able to suggest an alternative grade of "non-distorting" steel which would eliminate the need for me to worry what degree of bow we are going to end up with.

      Many thanks for your replies

      Colin
       
    9. AdamW

      AdamW Member

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      Hi Colin,

      How are you specifying the 'stress relief?' All anneals are not the same. A stress-relief anneal must be below 725C, and the longer you do it the less stress you get.
       
    10. COLINH34

      COLINH34 Member

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      Hi

      Our manufacturer of the plate receives it from a local company who supplies it "stress relieved and ground". I must admit I do not have a full heat treatment specification on the drawing. This is maybe a lack of control on my part which requires rectifying. I did read somewhere that stress relieving takes a maximum of about 90% stress from the material. There will always be some residual stresses left in rolled plate afterwards. It's these stresses I will have to try and keep to a minimum by a more detailed spec. on the drawing.

      Many thanks

      Colin
       

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