Rapid Traverse Speed

Rapid Traverse Speed

One of the items under the menu item Options - Settings - Machining is Rapid Rate Test.

The test is designed to find the optimum Rapid Traverse rate for WELmill which can be supported reliably by the controlling PC.

The PC software "drip feeds" data to the mill controller when requested by the controller. The PC organises a "packet" of data and sends it to the controller. The faster the axes of the mill move the shorter the time between requests for more data. There comes a point when the time between requests for data is less than the time it takes the PC to organise and send a packet. The system will, of course, fail at this point.

A PC is a brilliant general purpose machine but less than ideal for real time critical jobs like controlling a CNC mill. As well as running WELmill it will have other programs running in the background: press Ctrl-Alt Del to see the list. There may be other programs which have been minimised: look at what is to the right of the Start button. Laptop PCs, especially the lower cost ones, often have design compromises which make the operating system do work which, in a desktop machine, would be done by hardware. All these demands prevent the PC giving 100% attention to WELmill.

In addition to software and firmware demands, some PCs have hardware which can sprint while others have hardware which crawls.

Hence the Rapid Rate Test to find the optimum Rapid Traverse rate which the PC can reliably support. It is a series of tests which run automatically. Each test lasts one minute. The tests analyse the performance of the PC as it tries to keep pace with increasingly frequent requests for data, the Test Rate, by the WELmill controller.

The displayed Test Factor results are just numbers which indicate the processing time available to the PC between data requests. The higher the value of the Test Factor, the better the PC can support the WELmill controller at that Test Rate. For reliable operation at a given Test Rate, the Test Factor should be above 10. The Test Factor values should decrease as the Test Rate increases. If you get a weird result such as a low Test Factor at a low Test Rate there may be a fault with the PC or it may be that you are using an old or budget price laptop with major design compromises. In such cases, run the test several times to see if the weird result persists. If it does not, or pops up only now and then, the PC may still be usable for controlling the mill.

At the end of the test a recommended maximum Rapid Traverse Rate is displayed. This should be entered in the Settings dialogue in the Machining Parameters section. You can enter a slightly higher value and see what happens - but be prepared to smile should the system lock up after 40 minutes machining of a 41 minute job.

For best PC performance when running WELmill, close any minimised programs. Turn off background programs such as Norton Doctor and similar maintenance programs. Do not use the PC for another job, eg word processing or surfing the web, when running WELmill. Ideally, dedicate a PC to running the mill: just have a version of Windows and WELmill on the PC, nothing else other than, perhaps, code generating software which is always closed when WELmill runs.

We recommend a maximum rapid traverse rate of 500mm/min even though the mill's original specification says 600mm/min is the maximum. By all means try 600mm/min, provided the PC can keep pace. Remember that this is an open loop control system: there is no positional feedback from the mill to the PC. The faster the axes move, the greater the risk of stepper motor steps being missed and the finished part therefore being dimensionally incorrect. Experiment and find the limits of the PC and mill.

PC Specification for Running WELmill