Poor milling precision - Results out of tolerance

 cnc-milling, tips-tricks, ts_evo_usage, ts_fab_usage, ts_pro_usage

▶  Possible origins

The cut part's measurement are outside the tolerance of your machine, or shapes get deformed (square become trapezoidal, circle become oval, etc...). 

Note: Here we will consider that this problem is occasional. If it happened right from the assembly of the machine, the issue might come from a mounting or part error. Please contact our support service if your machine can never reach its indicated precision rate.

▶  Description of the problem

There are a few possible origins, the main ones are:

  • CAM preparation (operation selection and parameters)
    • Machine setup mistakes
      • Low tools or accessories quality

        ▶  Solution

        Here are a few goods practice rules which should enhance the quality of your millings. 

        1. CAM Preparation

        • Use a "Roughing + Finishing Strategy" to split your operations in 2: First rough out most of your stock leaving a few tenth of millimeter, then do a finishing pass to get a crisps and precise milling.
          • Be realistic with your feeds, speeds and depth of cut. Go check our CNC milling material library and our article about feeds and speed to get a base and adapt it to your situation (end mill type, material, machine...)

            2. Machine Setup

            • Try to get a better workholding: make sure the stock is firmly secured in place and has no way to move or vibrate (this would induce a loss in precision). Check our video about workholding to learn more
              • Remember to Square your gantry after doing the Home (using the Square button on the keypad) to make sur it is perfectly prependicular to the frame (otherwise you won't obtain perpendicular lines in the milled parts either).  
                • Keep your machine in a good working state by doing some maintenance regularly. Dust will acumulate in the moving parts and this will make your machine less precise. Some parts also need to be grease to function properly. Everything is detailed in our video on the maintenance of a CNC milling machine.

                  3. Low tools or accessories quality

                  • Measure your tool's diameter and compare it to the indications of the supplier. Some of the cheaper tools can have some non-negligible discrepencies. If there is a difference, modify your tool in your software to indicate it's actual cutting diameter.
                    • Check the spindle's nut and the collet installation. Make sure they are clean and the collet is properly clipped in the nut.
                      • Don't save money on those parts. A cheap nut or collet might be slightly out of center, and this will make the end mill's rotation conical, not straight. This can create huge losses in accuracy (the longer the tool, the wider the cone will be at the end of the tool).
                        • To test the collet and nut: install the longest end mill you have and make it stick our of the collet as much as possible while still being secured. Make a 1mm deep hole in a waste board by going straight down in it. Then measure the diameter of the hole and compare it to the tool's diameter. There can be a bit of a difference due to parts tolerances, but it shouldn't be significative compared to the machine's precision rate. (Above 0,2mm is definitely a problem and means there is a deffective part).
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