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  • Flatness on machined (CNC milled) parts

    Discussion in 'Machining' started by GarethW, Jul 30, 2009.

    1. GarethW

      GarethW Chief Clicker Staff Member

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      Does anyone know the best flatness that can be achived on a milled component? I have a part that needs excellent flatness, but it must be by CNC milling. It would be too costly in production to use a secondary process (e.g. lapping)

      Hope someone can help me.

      More info below:

      1) What is the material?
      Aluminium Al 6061-T6 (HE20)

      2) Can you post a picture of the geometry?
      No, sorry.

      3) What is the flatness required for? Engineering/design reason?
      RF performance. This is part of a high-frequency radio module.

      4) What is the size of the part?
      It's a 95mm x 190mm part that is 12.7mm thick

      The 95 x 190mm face has lots of pockets machined into it. A high-frequency RF PCB is screwed onto that face, then a similar plate gets screwed onto the back of that PCB, thus sandwiching the PCB between the two aluminium plates. Therefore the plates MUST have excellent flatness or there will be RF leakage (this causes all sorts of problems).

      We have achieved excellent flatness (with CNC milling) no problem in the past, but have just switched to a supplier in China and the flatness is not so good. My aim is to put a very tight (but very realistic) flatness tolerance on the engineering drawing, but I'm not sure what the best flatness that can be achieved through CNC milling (without having to use a secondary process such as lapping).

      Can anyone please advise? I was hoping to find a straightforward table on the web or in a book that shows process capabilities for different flatness - but I can't find anything. Any help in this area would be gratefully appreciated...

      Cheers!
      Gareth
       
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    3. maniacal_engineer

      maniacal_engineer Well-Known Member

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      The T6 temper means solution heat treated and artificially aged. Solution heat treat calls for rapid cooling, which will set up internal stresses. Aging is usually too cool to relieve them. In china the material that they start with isn't 12.7mm (1/2 in ) so they are slicing a significant amount off one face and removing one side of the formerly symmetrical internal stress. The stress in the other side then warps the part.

      It can be fixed by taking multiple passes; for instance after taking off one surface, clamp it from the SIDES, face off the other side just enough to be flat, the clamp it to that side and face the other side again, again just enough to flatten. Or you can stress relieve at elevated temps, while clamping in position - the key is to cool slowly, however this will lower mechanical properities. You might also be able to get there by cutting the material down to 12.7 by taking some from both sides to even out the distortion.

      I would think that a flatness of .1mm would be very feasible, with .02/10 mm/mm

      On the other hand, if you bolt the two together with big bolts you don't need flatness - they will just bend! :eek: :( :shock: (ok enough emoticons already - sorry they are too much fun)

      But seriously, whatever you do, one or both of the plates will have to conform to the other, so maybe the solution is to make them softer, and then the need for precision goes away. I know you are thinking that it is taboo to bend a pcb, and this is true, but it is not a dynamic bending - you clamp it once and it sits there. So you relax the flatness to .25mm make the plates softer,make the screws bigger, and go with that.
       
    4. GarethW

      GarethW Chief Clicker Staff Member

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      Thanks for the help maniacal_engineer and a warm welcome to the forum!

      I have been getting somewhere with the flatness problem myself the last few days...

      It turns out that in the UK we used 1.2" toolplate "C250" aluminium alloy http://www.metalfast.co.uk/C250.htm
      As it turns out this has no internall stresses so does not deform when machined. As it is the correct thickness it requires less machining anyway - the flatness is fantastic on these parts.

      The Chinese 6061-T6 is from extruded stock, and as you suggested due to the inherent internal stresses, the stuff warps when faced/machined.

      Our Chinese supplier has managed to improve the flatness to within 50microns (0.05mm) which is pretty good.

      The problem I now seem to be facing is that the plating process may be the root cause of the flatness issue - the suppler tells me that they can only achieve flatness of 0.2mm after plating - 200microns! That is not so good (I mean we are only specifying 13microns of nickel/tin plate).

      Anyway I have samples of plated & unplated parts and I'm now getting the guys to so some RF testing so that we can determine if unplated samples don't cause a problem. Supplier is also looking into improving the plating....
       
    5. maniacal_engineer

      maniacal_engineer Well-Known Member

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      That plating thing is unexpexted from .05 to .20 just from plating?

      Make sure you post the reason for that to the forum. I am very curious as to what tke mechanism is for that.
       
    6. GarethW

      GarethW Chief Clicker Staff Member

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      I have spoken to UK platers and they say that this shouldn't be an issue at all, but apparently uneven plating could happen if there's not enough "agitation" during the plating process.

      Anyway, hoping to get to the bottom of this one very soon, and will let you know the outcome!
       
    7. GarethW

      GarethW Chief Clicker Staff Member

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      After loads of aggro and one trip to China I've finally made some progress...

      They used Copper/Tin plating instead of Nickel/Tin plating. I'm not sure why because we had Nickel/Tin on the drawing. I think this was perhaps a manufacturing error.

      Making some progress on the machining too. Basically, if they skim back the surface using a diamond cutter then flatness is spot-on (0.025mm). This combined with the correct plating produces an excellent flatness result (0.025mm after plating!)
       
    8. wangyangzhong

      wangyangzhong Member

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      What's your supplier's CNC RPM ?
      I think if we use a high speed CNC (RPM 30,000) and finish milling of a flat surface,
      we can achieve the flatness within 0.02 mm,
      however, we also need to finish mill the opposite side to get an accurate datum surface.
      I have made a lot of such parts for robotics in china

      Stephen
       
    9. bbuckley

      bbuckley New Member

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      Hello Gareth,
      In regards to your flatness problem, I have held .0001 flatness on a 6061-t6 plate that was 22 x 10 x .5 with .030 thickness walls on the flatness callout surface(note all dimensions are in inches), with proper fixturing, tooling and process on a 1996 Mori- Seki 3 axis mill so it can be done. Even with tooling plate other wide know as "jig plate" your will want to skim the two surface prior to starting your rough and finish machine process'. This is key no matter what material you are working with when there is a critical flatness callout. You also need to carefully look at the pressures and forces of your work holding as too much or too little has a dramatic effect on the flatness outcome,also you want to make sure that the work hodling pressures are repeatable and consistent this way you can adjust pressures to see which gives you the best flatness.
      Cutting tool geometry is another aspect you need to look at, nose radius, rake angles, tool materials & coatings are all key as well as heat generated by the cutting process ( the diamond tool is not the solution for aluminum cutting it is a very expensive bandaid which is just masking a process problem and if you started some SPC you would find this to be the case). Cutting fluids are another item of critical importance as water soluable oils can put micro thermo shock into both the tool and the work piece and are difficult to control the viscosity of, oil based cutting fluids are stable improve tool life and surface finish (which is key to tight flatness callouts).
      The other thing I started doing about 15 years ago, especially for thin walled parts, was skimming the surfaces and then sending the material out to get cryrognically treated. This process stabilizes the material by closing the gap in the micro structure and has saved me much time, scrap and aggravation over the years with flatness callouts on weldments, plated parts and heat treated parts.

      I could go on for pages with problems and solutions for flatness issues but I will stop here and hope that some of this information may help.

      Best,
      Bill
       

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