Key factors for micro machining success

by

Arthur Turner, director, Rainford Precision Machines

How times change. Ten years ago, it was a struggle to speak to people wanting repeatable accuracy better than 10 μm, yet today, low-figure micron and sub-micron tolerances appear to be the normal way of tolerancing workpieces. Another major change is the physical sizes of features that are now required on the workpieces. This does not mean that the workpieces are very small, but features such as a wall thickness has a tight tolerance, drilled holes have a smaller diameter, threads are getting smaller and materials are either getting softer or much harder. So, what is industry doing today? This piece will try to describe what is achievable in a number of disciplines in micro machining, that is to say by a lathe or machining centre.

Forty years ago, I worked for a manufacturer of aircraft parts and was given the task of drilling holes 0.25 mm (0.98 in) in diameter in a cobalt chrome material. This was a monumental task given the knowledge, machines and coatings that were available for tools back then. Today, I have no worries about providing tools over ten times smaller in diameter to customers, knowing that, given the right information, these holes can be successfully drilled. My last order before Christmas was from a customer wanting 0.01, 0.015 and 0.02 mm (0.0004, 0.0006 and 0.0008 in) diameter drills, which I know they will have used correctly.

A key factor in micro machining is having tool concentricity correct. On a machining centre, tool runout is easily controlled either by buying a machining centre with nearly zero spindle runout or by using a tool holder where the tool can be independently controlled away from the shank. These days, tool holders are readily available from stock.

Another key factor is getting the feeds and speeds correct, since micro machining goes by a different set of rules compared with normal machining, especially when it comes to drilling. It is important not to run the spindle too quick as the tool needs chance to cut. Also, you do not want to create too much friction at the web, which is in constant contact with the workpiece material, unlike an end mill, where a tooth will cut and then have a short rest.

Customers are often surprised that tools as small as 0.01 mm (0.0004 in) in diameter can be produced, and yet there are a number of manufacturers globally that do this day in and day out, some are even capable of producing two fluted drills at these dimensions and giving them hole depths up to 6xD and 10xD. This is testimony to the fact that their tool grinding machines are really well produced, with attention to workholding, wheel measurement and positioning. They are well-assisted by the oil coolant suppliers and, more importantly, the raw carbide suppliers, who are developing systems to provide carbide grains small enough to be bonded together and still be machined to really small micron tolerances. Additionally, manufacturers can offer cutting tools in cubic boron nitride (CBN), which is popular because of its stability, as well as diamond, polycrystalline diamond (PCD) and, naturally, high-speed steel (HSS).

Drilling takes some of the uncertainty out of machining because, like a piece of chalk, the strength is in the drill’s longitudinal axis and not across its width. Therefore, more issues need to be considered if using small-diameter end mills. Parameters are influenced by the material, forms to be cut, surface finish and type of tool, resulting in the tool needing to be very resilient to wear, and consideration needs to be given to whether coolant is required and if so, whether it is water-based, oil mist or pure oil. Each job has its own unique set of challenges and all these different factors must be taken into account. One customer had a long machining cycle but improved the quality of the job by 75 percent as a result of changing from a water-based coolant to oil mist (for minimum quantity lubrication (MQL)).

The smaller the diameter of an end mill, the less options that are open to its geometry. In macro tooling, the use of ball nose, corner radius and square end mills is quite normal, but the smaller you get, the more difficult it becomes to grind a corner or ball nose radius and you might ask are they required? The main purpose of milling is to produce a shape or form, and the diameter of the tool can be much bigger than the shape or form required, thus eliminating one headache.

If you are talking about end mills below 0.1 mm (0.004 in) in diameter, generally they have a single flute, and their cutting edges vary in length, for example, 2xD, 4xD and 6xD are available. If you require CBN tool material, they are generally only supplied at 1xD, therefore depths of form are reduced. Again, it must be stressed that tool concentricity is very important.

However, milling takes many different forms, some of which require the use of a slitting saw or circular form cutter. Slitting saws can be manufactured from 0.1 mm (0.004 in) width and in standard series increment in 0.01 mm (0.0004 in) thicknesses. To produce a radius with a circular form cutter, normally a 0.2 mm (0.008 in) width, would be considered the starting point, but challenges are always being put to tool manufacturers.

For threads, our watchmaking friends show how micro parts really should be made, not only with the screws they manufacture on lathes but also with threaded holes in plates. A good example is an S0.30 thread whirling tool with an outside diameter of just 0.21 mm (0.0082 in). This tool only has one tooth, but if more teeth are required, why not try a multi-fluted S0.8 thread whirling tool with an outside diameter of 0.6 mm (0.024 in).

To expand a bit on the subject of threading, how about applying it to sintered tungsten carbide? “It’s not possible” I can imagine you saying, but it is. As with all machining, if you have tool concentricity and the correct feed and speed, it is easily achievable. Threads from M2 to M8 can be machined with maximum depths from 4 to 24 mm (0.158 to 0.945 in).

Firstly, a hole is needed in the tungsten carbide. I expect you are thinking electrical discharge machining (EDM), but no, I propose drilling a hole. Drills are available from 0.4 mm (0.016 in) diameter and require 20,000 rpm, but once the drill diameter increases to 0.5 mm (0.020 in), the spindle speed drops quite rapidly to 15,000 rpm, so it is in the range of most quality machining centres available today. Additionally, the surfaces of the tungsten carbide need to be milled, and tooling is available from 0.2 mm to 6 mm (0.008 to 0.236 in) in diameter, in either ball nose or corner radius design. It should be noted that the surface finish can be as good as Ra 0.01 µm (0.0004 in).

As the workpiece material is hard, it is very important to have a good tool concentricity, ensuring that each tooth takes off the same amount of material and does not differ on each rotation, as this damages the tool.

Machining micro features on a workpiece invariably raises questions regarding accuracy. Can the workpiece be positioned where the feature is required and within the agreed tolerance? Is the measuring machine accurate enough to measure to the required dimensions? Is the machining centre toleranced to position the feature within the target area? There are many machines being offered that refer to a national standard that may have been set a number of years ago. Does that standard include an allowance for reversal error? What is the tolerance for circular interpolation in both clockwise and anti-clockwise directions? There are lots more questions that can be asked, but suffice to say there will be at least one manufacturer specifying the accuracy of their machine in terms of what you are looking to achieve on your workpiece, and this is the information that you really need to know.

I always ask customers “Is the tool running true?” I have stated this several times, although it cannot be stressed enough, tool concentricity is very important. Spindle speed and feed rate can be varied for one tool on different machines, but the feature ensuring that the workpiece is correct is tool concentricity.

To ensure components are machined to the correct tolerance, it is important for you to ask the question “Is the workshop temperature consistent?” To state the coefficient of linear expansion, steel will grow (or shrink) with every degree change in temperature, and this is not just applicable to the workpiece material but also the machine and the spindle. Therefore, when trying to machine to tight positional tolerances, it is important to start with a warm workshop so that the deviation over time is quite small. For those who can afford it, a temperature-controlled workshop (to better than 2˚C if possible) is advisable, but it should be ensured that there is no direct access from it to the outside of the building as someone leaving a door or window open would mean your efforts have been wasted.

Companies are good at temperature controlling their measuring facilities so why not their workshops? Why have machines near windows or doors that can open to an uncontrolled environment or located adjacent to a heater? Ensure spindles are at operating temperature before sending the tool through a laser tool measuring system. Think about how to warm up the coolant system so that both the machine and workpiece are at the same temperature.

Many companies assume they can achieve their goal but inevitably fail because they have not addressed some of the aforementioned points. In sum, achieving the required size, tolerancing and surface finish is possible, you simply need the correct tooling for the job, the environment to be a stable temperature and the machine and cooling fluids to be warm.

Arthur Turner

Rainford Precision Machines

https://rainfordprecision.com