The art of micro-machining11 March 2020
The Queen’s head has been beautifully micro-machined onto pieces of brass by engineers at the AMRC using cutting tools so small they are barely visible to the naked eye.
One of the machined portraits of Her Majesty, measuring just 1.4mm, is so small that the intricate detail can only be seen clearly through a powerful microscope. It is a major achievement for University of Sheffield Advanced Manufacturing Research Centre (AMRC) engineers Emma Parkin and Joe Thickett who wanted to create the tiny artworks to show off the AMRC Machining Group’s capabilities with the goal of bolstering industry knowledge on micro-machining to help UK firms win business.
The faces were created using a seven-axis Starrag Bumotec S-191 mill-turn centre, an industry leading high-accuracy machine tool platform. Emma and Joe’s deep understanding of the machine’s behaviour allowed them to produce four remarkably detailed Queen’s heads – with diameters of 11.2mm, 7mm, 2.8mm and 1.4mm - on a single circular piece of brass measuring 25mm.
They used micro-end mill cutting tools supplied by Sandvik Coromant to achieve the basic shape of the design before switching to tiny but incredibly precise ball-nose cutters - the smallest of which measures just 0.2mm - to carry out the detailed contouring of the Queen’s face.
“We chose the Queen as a portrait because we wanted to do something similar to a coin as it is relatable, people generally know the size of a coin; so to be able to scale it down in size yet still keep the Queen’s face on there is the ‘wow’ factor.
“We wanted to show our partners and wider industry what is achievable; that we can work to an accuracy of 0.001 mm on work pieces as small as 1.5 mm and make use of these tiny cutters whilst still maintaining detail.”
Emma drew on the vast expertise from across the AMRC to create the micro-machined portraits.
“To get Her Majesty’s face, we scanned a real coin using a 3D microscope that’s normally reserved for detecting surface roughness of texture on a material. It works by taking a series of pictures which are then layered up to create a 3D image. We then gave that digital 3D image to an engineer in the Design and Prototyping Group - Valdis Krumins, who was able to turn it into a file format I could work with called STL that is used in stereolithography CAD software.
“We were able to take that STL file and upload it into Siemens NX 12, which is a CAD design software package. In this environment we were able to make a programme for the tool paths; roughing, semi-roughing; semi-finishing; and finishing which is standard practice for any high precision machining.
“We require a high degree of accuracy and repeatability from our tool paths to avoid tool failure. The software is important because the difficulty with micro-machining is that you can’t hear the cutter’s health so you have to get the feeds and speeds right the first time.
“We started at 4mm to remove the bulk material and then went down in size. When we got to the finishing operations with the 0.2mm ball-nose we were doing micron step overs which is literally moving a micron every time, back and forth.”
It is this high precision and ability to achieve accuracies to one micron that is increasingly important for modern micro, ultra-precision and high-value manufacturing; across a broad range of applications from automotive, aerospace and medical to watchmaking, electronics, cryogenics and space. That makes micro-machining and micro-engineering big business, with latest reports (by IndustryARC) suggesting a global market value of $12.5billion last year, concentrated mainly in the US and Europe.
Emma said: “In the UK it is mostly SMEs that use micro-machining. It’s a bit of a forgotten art over here. It is expensive to do because of the machinery you need to purchase in order to achieve the accuracies desired. You may be producing tiny little parts but there is so much precision that goes into it, it ends up becoming quite expensive.
“The majority of micro-machining is produced in Europe with Switzerland being the most well-known, and because of this a lot of work gets sent there - incurring a further decline in knowledge in the UK.
“There are a lot of companies that use micro parts but there are only a very few companies in the UK now that actually specialise in micro-machining; mostly people just send work out of the country.
“We have a few places in the UK that specialise in high precision but the numbers are few. It’s more machine tool companies and cutting tool companies who tend to sell you their machines and pass on some knowledge but to me that is not the best way, which is where we come in, and what we’re trying to do - have tech that we can give out to companies to bridge that gap.
“At a retail price upwards of £600k, the Bumotec machine is not affordable for every SME, but what we can do is get those best practices out there around machine tool down selection, machine tool inspection, environmental considerations, cutting strategies, tooling, tool run out and component metrology.
“The gain is that by adopting this best practice, companies could potentially increase productivity and manufacture components that would normally be deemed as too difficult, in house.”
Emma is hoping the ‘coin’ demonstrator will lead to further micro-machining R&D and commercial projects for the AMRC, which is part of the High Value Manufacturing (HVM) Catapult network. “I just want people to come and have a look at what we’re doing. At the moment when people come to the AMRC, and here to Factory of the Future, what they see is aerospace, with massive aerospace components on display and huge machines.
“I want people to think bigger than that, by thinking smaller and walk onto the shop floor, down to the Bumotec or the Kern, and go ‘wow’ the AMRC can make some really small parts. Let’s work with them.”