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How times change? Ten years ago it was a struggle to speak to people wanting repeatable accuracy better than 10 microns. Nowadays, low figure micron and sub-micron tolerances appear to be the norm for tolerancing workpieces. Another major change is the physical sizes of features that are now required on 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. What is industry doing today? As a micro-machining person, Arthur Turner takes a closer look at what is achievable.
Forty years ago, I worked at an aircraft manufacturing company and was tasked with drilling holes at 0.010” inch (0.25 mm) diameter in cobalt chrome - a monumental task given my knowledge and the machines and tool coatings of the era. Today, I have no concerns over providing tools more than ten times smaller to customers. Armed with the right information, holes can be successfully drilled. One customer recently ordered 0.010, 0.015 and 0.020mm (0.0004, 0.0006 and 0.0008 inch) diameter drills, which I know he will have used correctly.
A key factor in micro-machining is having tool concentricity correct. On a machining centre, tool run-out is easily controlled, either by buying a machining centre with nearly zero spindle run-out or using a tool holder where the tool can be independently controlled away from the shank. These holders are readily available from stock from numerous suppliers.
A second key factor is getting feeds and speeds correct. Micro machining requires a different set of rules, especially when drilling. An important fact is not to run the spindle too quickly, as the tool needs a chance to cut - and you don’t want to create too much friction at the web which is in constant contact with the workpiece, unlike an end mill where a tooth will be in continuous ‘short-term’ contact.
When discussing tooling as small as 0.010mm diameter, customers are very surprised that these tools can actually be manufactured. Yet there are numerous manufacturers that can produce drills and end mills at these dimensions that can give machining depths up to 6xD and 10xD. Similar size end mills generally have shorter cutting lengths.
This is testament to the really well produced tool grinding machines that pay remarkable attention to workholding, wheel measurement and positioning. They are well assisted by the oil coolant suppliers and importantly the raw carbide suppliers who are developing systems to provide carbide grains small enough to be bonded together, yet still be machined to micron tolerances. These tool manufacturers can also offer CBN, diamond, PCD and HSS products.
Drilling into the challenges of micro-drilling
When customers drill they are taking some of the uncertainty out of machining because like a piece of chalk, the strength is in its longitudinal axis and not across its width. Therefore using small diameter drills creates fewer issues than small end mills. Parameters are influenced by the material, the forms to be cut, surface finish and the type of tool. This results in the tool needing to be very resilient to wear. Consideration also needs to be given to whether coolant is required and if so, is it water based, oil mist or pure oil? I cannot easily supply an answer, as each job is a different challenge and due consideration has to be applied to each individual question asked. One manufacturer had a long machining cycle but improved the quality of the job by 75%, just by changing from a water based coolant to oil mist (MQL).
When the end mill diameter is small, the geometry options are reduced. In macro tooling, ball nose, corner radius and square end mills are the norm, but as we get to micro, the ability to grind a corner or ball nose forms becomes more difficult.
With polymer machining, carbide end mills generally only have a single flute when going below 0.1mm diameter, the tool cutting edges will vary in length with 2xD, 4xD and 6xD readily available. If you want a CBN tool, they are generally only supplied in 1xD length, reducing form depths - again it must be stressed that tool concentricity is critical.
A common thread in micro-machining
For threads, our watchmaking friends show how small parts really should be made; not only with the turned screws but also with threaded holes in plates. A good example is an S0.30 thread whirling tool with an OD of just 0.21mm. This tool only has one tooth but if more teeth are required why not try a multi-fluted S0.8 thread tool with an outside diameter of just 0.6mm.
Having discussed threading, how about putting threads in sintered tungsten carbide? I can hear people saying ‘not possible’, but it is. And with all machining, if you have spindle concentricity, the correct feed and speed - it’s easily achievable. Threads from M2 to M8 can be machined with depths from 4 to 24mm. Firstly, a hole in tungsten carbide is required - again I can hear people thinking about EDM, but no, let’s just drill a hole. Drills are available from 0.4mm diameter and require 20,000rpm, but once the diameter increases to 0.5mm, the spindle speed drops rapidly to 15,000rpm, so it is in the range of most high-quality machining centres.
Additionally, surfaces of tungsten carbide can be milled and tooling is available from 0.2mm to 6.0mm diameter in either ball nose or corner radius tool design that is capable of surface finishes as good as Ra0.010µm.
Answering the questions around achieving micron precision
Machining small features on a workpiece invariably raises the question of accuracy, is it 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 are manufacturers specifying the accuracy of the machine by what it will produce on your workpiece, which is the information that you really need to know.
With all customers, I ask the question “is the tool running true?” and this phrase can be read a number of times in the previous paragraphs, but its importance cannot be stressed enough. Spindle speeds and feeds can be varied for a tool on different machines, but tool concentricity is what ensures the workpiece is correct.
Each element of small feature or tight tolerance machining will raise questions as to how all the elements fit together. There are now more companies thinking they can achieve their goal, but not achieving it. This is because they haven’t addressed issues mentioned here. Surface finish, size and tolerances are all achievable, we just need to tackle the problems correctly - and not just assume that particular parameters are acceptable.