Dimensional inaccuracies are unfortunately a common problem when having parts machined . The advantage of lower costs and fast lead times sometimes needs to be weighed against the tolerance and quality requirements. For the majority of parts and projects though, standard Chinese machine shops can deliver more than sufficient quality as long as you find the right one and requirements are clearly communicated beforehand.
First, let us look at why you might experience issues and a number of things you can do to avoid them;
– Engineering drawings – one of the main reasons dimensions might be out of spec is because they supplier didn’t know what was required in the first place. Without a detailed 2D engineering drawing calling out the critical dimensions, most shops will apply a blanket tolerance to your part if they apply one at all.
– Equipment and maintenance – as with anything CNC machines come in a massive range of colours and sizes. On the back of Chinese manufacturing’s rapid rise there was an equally rapid growth in companies making equipment to support this. Unfortunately, a lot of the equipment is very poor quality with inherent inaccuracies, add to this they run for years with minimal maintenance and that they can make parts at all is often all one can ask for.
– Tools and drill bits – often very cheap, poor quality and certainly not being changed often enough
– Machine axes and fixtures – parts that should be made on a five-axis machine being made on 3 axis machines is a great way to keep costs down, but you are going to sacrifice some accuracy, especially if the fixtures are poorly designed or built
Of these problems the one with the most direct control is to submit clear engineering drawings detailing critical tolerances with every part. But with high tolerances comes higher costs. As a designer it is easy to apply strict tolerances across the entire part, happy in the knowledge that when it arrives as long as it is to tolerance it will fit perfectly. But holding tight tolerances on all the dimensions especially on some complex parts is incredibly difficult and very expensive. If you want to reduce costs and scrap rates you should relax any tolerances possible. You need to be clear about which dimensions are actually critical, understanding everything involves balance.
The quality and maintenance of both equipment and tools is a more difficult problem to deal with. A visit to the supplier is obviously highly recommended; there you should be looking closely for maintenance and calibration records, which all ISO certified companies should have. If that is not possible you can still ask about the calibration records and for some pictures of the equipment.
Again there is little you can do to ensure drill bits and machine tools are of the required quality and are being changed often enough, though due diligence in selecting the supplier should ensure some level of safety and as long as you select a partner who values quality and long-term cooperation over short term profits you will be ok.
If your part requires 5 axis machining then you need to be explicit about this upfront, a lot of very complicated parts can be machined to a high degree of accuracy on a three axis machine using multiple setups, but each setup is going to cost you in dimensional accuracy.
The most essential tool you have to ensure compliant parts is to understand your product and the end user’s requirements as fully as possible and to communicate this to the supplier. The common theme running through my last few articles is continued here, the key to getting what you want from your supplier is clear and proper communication. As long as you are explicit about what you need and what features and dimensions you are concerned about you will have far fewer problems finding a company who delivers what you want for a reasonable cost.
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