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| Flatness on machined (CNC milled) parts |  |  | |
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| Posted by: drgaz ® 06/18/2009, 12:55:40 Author Profile eMail author Edit |
Does anyone know the best flatness that can be achived on a milled component? I have a part (approx 150 x 120mm) 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. Cheers! |
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| Posted by: Kelly Bramble ® 06/18/2009, 17:34:47 Author Profile eMail author Edit |
Flatness depends on the material and geometry characteristics. 1) What is the material?
Modified by Kelly Bramble at Thu, Jun 18, 2009, 17:35:21 |
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| Posted by: joebeck ® 06/18/2009, 13:02:32 Author Profile eMail author Edit |
What's the flatness tolerance and how large is the part? |
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| Posted by: drgaz ® 06/24/2009, 03:32:43 Author Profile eMail author Edit |
1) What is the material?
Aluminium Al 6061-T6 (HE20) 2) Can you post a picture of the geometry?
3) What is the flatness required for? Engineering/design reason?
4) What is the size of the part?
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 part, but have just switched to a supplier in China and the flatness is terrible. 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... |
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| Posted by: drgaz ® 06/25/2009, 17:17:43 Author Profile eMail author Edit |
Anyone any thoughts at all? Any advice gratefully appreciated! |
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| Posted by: Kelly Bramble ® 06/26/2009, 07:56:00 Author Profile eMail author Edit |
Assuming that the geometry never has a really thin section near the surface requiring the flatness, manufacturing should be able to deliver .002" over five square inches. If strength is not an issue one can machine the surface close to that required then stresses relieve the material and finish the machining the concerned surface to approximately .001 per eight square inches or better. I just happen to have experience designing high performance RF (microwave - air/waveguides/etc.) component and assemblies. Generally, flatness was always over specified on flange connections and other mating surfaces fearing RF leakage. When really worried we designed and installed BAL and RF Spira Shields at the seams. Also one can machine in a labyrinth groove near the seam area to allow the material to deform under compression load at assembly. The PCB you're referring to could also have an elevated trace of silver or lead at the seam interface area that would deform and fill voids/gaps between the board and aluminum under compression. Don't forget that one can also use silver epoxy and other gasket material to RF seal the seam. |
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| Posted by: drgaz ® 06/29/2009, 11:16:21 Author Profile eMail author Edit |
Hi Kelly,
Thanks for your advice and suggestions |
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