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I have a project which involving several mating parts and also shaft guided through bearings, my issue here is that I don't have much experience with tolerances and manufacturing processes and I cannot determine the perfect balance between cost and accuracy. I am using Aluminium 6061 for raw material. For example I have some guidance "shafts" which are "made" by removing a large material from the part, I made some research and I saw that a +- 0.025 mm is a typical tolerance for a decent CNC machine. My question is, could I go down with that value, in terms of cost or is already a tight tolerance ?

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It's going to be an uphill battle from that point on. While you can get every part of the machine and the process to tighter tolerances, their errors may compound to the sort of tolerance of 0.025; you may manufacture relatively simple parts with lower error but whether you succeed or not is more up to luck than your efforts - whether the tolerances of the tool, the table drive, the head drives, the frame rigidity, the sliders rigidity, and the material itself add up and ruin the accuracy or cancel each other resulting in near-perfect workpiece.

One nasty property to consider is the internal stresses of the material, resulting from the manufacturing process. If you remove large parts of it, the stresses that were canceled out by say, two outer surfaces pulling with equal strength against the central bulk pushing, will result in the workpiece ending as soon as you unlatch it. I had worked with 7mm brass sheets, and upon removing most of material from one side, I't observe good 2cm of deflection on a 30cm bar.

Regardless, if you aim at sub-0.01mm accuracy in all domains you can control (tool shape, material alignment, table flatness and machine being level), and the machine is good quality, you can expect better than 0.03mm accuracy in the result. If you want better, you'll need to get into the "guts" of the process - drives of the head and table, bearings, vibration dampening etc.

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  • $\begingroup$ Thank you very much for the answer. I am the designer and I am about to give the project to production. You just make my work harder. I have a gap in my knowledge and imagination, for example if I will make an aluminium shaft smaller with 0.01 mm always that the hole (using true position tolerance, for orientation and form) the assembly will always work ? $\endgroup$ Feb 11 '19 at 12:16
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    $\begingroup$ @StefanMerfu: You can always declare negative tolerances. Note how that goes for nuts and bolts tolerances: say, both nut (inner) and bolt (outer) diameter are declared as 10mm, but they will always fit, because the bolt's tolerance is (...making up the numbers, I don't recall the exact value) [-0.05 ; -0.15] meaning a bolt of 10mm exactly would be out of specs, exceeding the bracket. Just provide a tolerance top bracket of shaft that is lower than the bottom end of the hole bracket, and it guarantees they will fit. (add some extra margin for oxidation, contaminants, lubricant etc too...) $\endgroup$
    – SF.
    Feb 11 '19 at 12:21
  • $\begingroup$ Could you provide me a safe value for extra margin ? $\endgroup$ Feb 11 '19 at 12:28
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    $\begingroup$ @StefanMerfu: Sorry, a little too long since I've been into that stuff. I'd suggest looking into standards; the aforementioned nuts and bolts (ISO 965-1) are one safe place where you'll find generally accepted tolerances for various diameters of shafts. It's a bit grueling work finding the right numbers (never mind free copies of the standards) though, so I'm not going to do it for you. $\endgroup$
    – SF.
    Feb 11 '19 at 12:33
  • $\begingroup$ No problem, you just help me a lot, thanks for all the good stuff from your experience. $\endgroup$ Feb 11 '19 at 12:34

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