Milling machines — part three
PART THREE OF OUR MILLING SERIES: REMOVING MATERIAL — WHICH CUTTERS AND WHAT MATERIAL?
Removing material — which cutters and what material?
The reason we bought a milling machine was to make, repair, or modify components and to do this we have to remove material. Hopefully from the right places. We have a variety of cutters available to us to accomplish this including:
• end mills
• slot drills
• sitting saws
— to name a few.
Once your component is securely clamped (see Part Two of this series, Showing Restraint, in Issue No. 83 of The Shed), ensure that the cutting tool is held correctly in its chuck or collet holder, keeping tool overhang or stickout to a minimum. Being rigid and stable applies as much to tooling as it does to the workpiece. Drill chucks are not designed to take side loads induced by milling and are exclusively for drills. Milling cutters should be held in a collet chuck or other suitable holder.
There are three kinds of material that the most common cutting tools are made from for the home workshop: high-carbon steel, high-speed steel, and tungsten carbide.
High-carbon steel (HCS)
As the name suggests, cutting tools made of this steel have a high carbon content, typically between a minimum of 0.8 per cent and a maximum of 1.2 per cent. This amount of carbon allows heat treatment to give a sufficient degree of hardness, which will produce a good cutting edge. It means that cutting tools can be produced from gauge plate or silver steel and then hardened and suitably tempered. Typically cheaper drill sets are made with HCS.
Note that overheating of these tools will cause further tempering of the tool and a loss of hardness.
High-speed steel (HSS)
This is so-called because cutters made from this material could be run much faster than the carbon steel cutters prevailing before the development of HSS. For general use in the home workshop, HSS is the most useful and cost-effective cutting-tool material. The main alloy added to HSS is tungsten. But HSS also has an amount of chromium, with smaller amounts of molybdenum, vanadium, and cobalt. The addition of tungsten alone does not provide the hightemperature characteristics. A balance of all the ingredients allows for very high tool temperatures without the cutting edge losing any hardness. Typically HSS drill sets are more expensive and end mills and slot drills will be made of HSS. As with all things in life, you generally get what you pay for. HSS cutting tools come in many grades and established brand names will be better quality than an unbranded cutter.
Tungsten carbide cutting tools (also known generally just as carbide cutting tools) are made of solid tungsten carbide or, in larger cutters, replaceable tungsten carbide inserts with other trace elements, and sometimes a wear-resistant coating on the surface.
They are produced by a process called ‘sintering’, where the tungsten carbide is used as a powder that is moulded under high pressure and temperature. Cobalt is included in the mix to help the particles ‘stick’ together.
These very hard cutters are able to deal with very hard materials and extremely high temperatures, although they are
more likely to chip if not held rigidly.
Tungsten carbide comes in many different grades to suit materials to be machined so the cost of keeping all the different grades makes this very expensive for the garage engineer.
All cutting tools, no matter their shape, will have clearance angles and a cutting angle called ‘rake’ or ‘helix’ to shear (cut) the material being machined. This angle varies depending on the material being machined. In production machining, cutting tools specific for the material will be used. For garage engineers, generalpurpose cutters and drills will get us by. If you are making hard work of it next time you are drilling a hole, have a good look at the cutting edges and check that you have a clearance angle behind the cutting edge.
The rake angle of the cutting tool is often determined by the material being machined or the material the cutting tool is made from.
Positive rake is the most common of the rakes you will find. Most HCS and HSS drills will be positive rake, as will HSS and solid carbide end mills and slot drills.
Some materials machine well with neutral rake — the most common in the home workshop is brass.
The widespread use of carbide tooling and the strength of carbide in compression make negative-rake tooling within industry and its high material-removal rates very desirable. Rigid machinery, tool holding, and high spindle power are required to get the best out of carbide tooling. A milling cutter with general-purpose carbide inserts can still be used in the garage workshop to take on hard or hardened materials. Just be aware that it will be working your spindle bearings hard.
Negative-rake carbide tooling is very common in industry but its use in home works is limited because of the higher power needed to drive negative-rake tooling and the rigidity required in the milling machine.
Industrial cutting tools can also be made from Stellite, ceramics, and industrial diamonds, among other materials.
Now a little bit of maths comes in when calculating the spindle speed for the drill or cutter. The spindle speed is dependent on both the material being machined and the cutting-tool material. This is quoted in many engineering handbooks as surface speed in metres per minute (also written as metres/min or m/min), or in older publications as feet per minute. We will concentrate on metres per minute. Academics do not like the use of the word ‘per’, but it is very common in the everyday language used here.
Cutting speed is calculated as the distance the cutting edge of your
Fly cutter at speed
Milling a slot with a three-flute end mill. Workpiece is held rigid within vice jaws
Note that cutter on fly cutter juts out from the body
Examples of cutters. (Top row) Second from left: fly cutter; third and fourth from left: ball-nose cutters; third and fourth from right: reamers; the circular cutters are side-and-face cutter (top) and slitting saw (below). (Bottom row) Far left: long-series end mill; third from left: ripper end mill for high-volume materials removal; fifth from right: reflex, or corner radius, cutter.
Cutters (left to right): four-flute centrecutting end mill, four-flute end mill, three-flute centre-cutting end mill, and twoflute centre-cutting slot drill (slot drills by definition are centre cutting)
A centre-cutting end mill plunges into an aluminium block
Clogged end mill
A centre-cutting mill produces a clean cut hole in aluminium block (left), while an ordinary end mill tears and deforms material (right)