A Milling Machine is usually the second major machine tool any aspiring metalworker will obtain, after a lathe. More correctly known as a vertical mill as horizontal mills do exist. Whereas a lathe mainly turns objects in a circle, the milling machine will make objects square and cut slots as well as performing tasks that can also be done on a lathe such as drilling and tapping (being more flexible for this), boring holes and sawing (using a ‘slitting saw’) and mitering tubes.
Basically it consists of a drill mounted on a column which can be moved up and down, pretty much the same as a pillar drill. The head is wound down slowly and has an accurate depth scale on the operating handle. There is also a plunge section as you would find on a pillar drill with a large handle which can be used for rapid drilling but is locked out for more subtle milling operations. The column can often be tilted from side to side to enable cutting at an angle but in practice this is not very useful and it is usually better to mount the work at an angle. The mill can be used as a drill press. Some mills are sold as mill/drills but I am not sure of the difference. Drills can be mounted in a drill chuck but other tools are best mounted in a collet chuck system which is more accurate and secure. The drill head itself can only move up and down, so the shaping of the metal is achieved by moving the table in two axes, against the milling cutters of which there are several types. Collet chucks and the like are usually attached to the mill head using a morse taper. These have a tapped hole in the top which is to further secure them in the drilling head with a threaded bar. Unfortunately a lot of potentially interchangeable lathe tools have a tang (a projecting bit of metal) which is to facilitate their ejection from the lathe tailstock. This means they will not fit into the mill head. I am mystified as to why tooling manufacturers do not make interchangeable tools.
*(Belatedly I have discovered my mill can accept tools with a tang by fully extracting the drawbar, I do not know if all mills are capable of this. I have not seen any reference to it anywhere so it may well be worth asking if you are considering a specific purchase)
Making bicycle frames doesn’t require any of these sophisticated machine tools as most parts for building are freely available but they will undoubtedly enhance your capabilities, in particular making lower cost tools and fixtures to help with the process, though the initial cost rather wipes out the longer term savings. Unfortunately as with a lathe purchase, buying a milling machine is only the start as you will undoubtedly spend as much again on tooling in order to achieve your aims as you rapidly find you require a vice (a separate vice for drilling is also a good idea as they are different), milling cutters, collet set for holding the cutters, drilling chuck and drills, reamers, a boring head, parallels, angle plates, V block, clamping set, a rotary table; not to mention the plethora of measuring tools such as bore gauges, micrometers, height gauge, surface plate you may have already had to buy to use your lathe.
Additionally you will obviously need to mount your mill on a flat surface, usually on a tray, or purchase a matching stand. You will also benefit form some form of containment shielding around the mill as the swarf from the cutting will fly everywhere and the small screens provided on the machines are not very effective and often get in the way of some tasks. It is vital to wear protective glasses / goggles whilst milling. One of the best items I subsequently bought was a workshop vacuum which is a huge time saver.
Subsequent to buying both my lathe and milling machine I have spent more time making tools and fixtures to use on the machines both to enhance the use of the machines and to use in frame building, than I have making bike frames.
The first thing I made on my lathe was a heat sink for brazing seat tubes, the first thing I made on my mill was a replacement bolt strike plate for my back door! My own mill, like my lathe, is relatively small and I have already found some limitations I have had to find ways of overcoming. It is however the largest machine I could fit in my workshop. Affordability aside the usual rule applies to get the largest machine you can, on the basis that you can do pretty much everything you can do on a small mill on a large one, and much more. However mine is a typical small manual machine and creating stuff is very satisfying but SLOW, especially for beginners. A small mistake can even ruin hours of work, but I would say this gets rapidly less likely with practice. Many things I have already made or intend to make will not be cost effective. A rear dropout I made would be a typical example. Steel is expensive, especially in small quantities and when you do not have a local supplier, due to weight, postage adds another wedge. Most small bike building parts will be turned out on CNC (computer controlled) machines which will run automatically when set up and can produce a piece I guess in a few minutes that would take me hours, so it is probably cheaper to buy the parts. Manual machines can be speeded up by fitting a Digital read out (DRO). These are also available for lathes and effectively is a screen and some sensors which monitors the table movements and enables more rapid and accurate control of the table movements, rather than relying on memory. However, they do add to the complexity of the set up but can usually be retrofitted.
A note about mitering. V blocks are fairly easily made on a mill and are a convenient way to mount tubes. They are most easily made by resting a block of material in another accurate V block (usually one you have bought) and cutting straight across as shown. However, although tube blocks can be mounted in the milling machine and cut with a hole saw, the geometry of the set up is such that the tube, usually contained in a block, has to be mounted sloping upwards and doing this requires the use of either angle plates to set the angle accurately, which is tricky, or maybe a tilting vice or tilting angle plate. All of these require extra bits of kit, whereas on a lathe only the blocks are required to secure the tubes and the machine easily adjusts to the correct angle. Therefore I will undoubtedly stick with the lathe for mitering
Nearly all machines are made in China and most seem to be made by Seig, though they are often re-badged in different colours with slightly different specifications. I decided to buy a German (Austrian) made lathe from Pro Machine Tools in Stamford who also supplied my lathe. They also supply slightly larger machines from Italy which are made not too far away from the Campagnolo factory. They appear to be the only importer of European made machines in the country. Mills are heavy, mine weighed 80 kg and I needed to reinforce my bench. This sort of weight requires three people to lift it, though I managed single handedly with a block and tackle. The equivalent Chinese models were much heavier. It is possible to get milling slides for lathes. These are in effect a small milling table mounted instead of the toolpost at right angles to the lathe chuck, the chuck being used instead of the milling head. I have no doubt a lot of milling tasks can be carried out on these but on small lathes these slides are small and I decided would not offer me the flexibility I required. A large slide on a full sized lathe I am sure would be more useful. Many lathes also now can be purchased with add on milling attachments, so effectively you get two machines in one. I do not think these are terribly popular but may appeal to some. My objection to these machines is principally the working height. I can get away with using a lathe at a standard bench height despite a certain amount of stooping, but a mill I needed mounting higher up for comfort.
Using these machine tools is greatly enhanced, in my opinion, by learning how to use them from a professional and I went on a short course at Axminster Tools which I reviewed separately. There are very many helpful videos on YouTube and many relatively simple books available, mostly in the realms of model engineering. I often forget that bicycle frame builders, especially amateurs, often have an engineering background and are often already familiar with these tools. Also many will have used them in school. Unfortuately I had rather academic schooling and only managed 1 year of woodwork.