SO......YOU JUST BOUGHT YOUR FIRST MINIATURE MILLING MACHINE
The next logical step after you have had a small lathe for a while and saved all your spare change is to consider buying a small milling machine to further expand your machining capabilities. Before I get into the aspects of the small milling machines most used by modelers, let me describe some of the basic features of such a machine.
Although the lathe can indeed be used for many milling operations, because of the comparatively small travels of most cross and vertical slides, only small workpieces can be milled in one continuous movement. Longer pieces would have to be shifted to complete the machining on a given surface with probable shifts in alignment or much extra time wasted attempting to re-align the work. The carriages of even the smallest milling machine like the SHERLINE, will easily provide at least 8" of longitudinal travel ( X ) and 2-3" of cross travel ( Y ). Various size vices can be installed to the machine's carriage with the normal " T " bolt or clamping systems used by the big machines. The head stock is situated vertically and it slides via a screw feed mechanism along a "V" shaped column. A lathe's head stock is normally stationary with the carriage traveling via a screw feed. In a milling machine the head stock is the one that travels along the vertical axis ( Z ) by turning the screw via a graduated crank above the head. Power is administered by an on board motor either housed inside the head or riding outside, connected to the spindle with a drive belt and pulley system. The motors can be single speed inductions motor, in which case, speed can only be altered by changing the position of the drive belt on the pulley. This can be a nuisance in some machines and easy in others depending on the design. If I had my way, I would only install variable speed universal motors on these machines so with a simple turn of a knob I could fine tune the milling speed to instantly suit the material being machined. Variable speed motors are universal type motors with brushes. They are a bit louder that their induction counterpart. The advantage is that for a given size, universal motors produce more power and torque so relatively small motors can be used to keep the weight down. Old fashioned rheostats would indeed regulate the speed but at the expense of low end torque. Today we have electronic speed controllers that are able to preserve power at the lower speeds.
After the machine has been removed from the shipping box, it must be cleaned of any packing grease. The instruction book must be thoroughly read before you attempt to turn the machine on. Choose a suitable location for it and preferably bolt it to a rectangular piece of 3/4" plywood for easy transport. The SHERLINE weighs less that 25 pounds so is not a problem to move from place to place. Ideally, it should be given its own location on the work bench. The milling machine provides three basic axial movements via dovetailed slides: longitudinal advance - left to right direction ( X ), cross advance - toward and away from you at right angles to the long bed ( Y ) and vertical movement to raise and lower the cutters or drill bits ( Z ). These component or slides are fitted onto " V " shaped dove tails which provide the perfect surfaces for smooth wobble free movement. Like any moving part exposed to friction, these surfaces will eventually begin to exhibit wear. All high quality slides will have a built in provision to adjust for normal wear. The slide rides along the dovetail on bronze plates called jibs. Behind these plates are set screws that can bring the plate into closer contact to either tighten the feel of the slide or to make up for any wear created by the constant movement of these slides. The jibs on your new milling machine may or may not be adjusted for optimum feel as that is a very objective and personal kind of feeling only the individual can decide on. To properly adjust a jib, you first advance the slide so the carriage or simply put, the part that slides along the dovetailed bed is situated along the middle of its travel limits to make sure that part of the jibs are not hanging over the dovetail bed. Loosen the lock nuts and all the screws and carefully tighten the center pretty tight then loosen it and re-tighten it lightly. Check to see if the slide moves easily but without any wobble. The reason you first tighten the middle screw tight is to indent the jib with the end of the screw to prevent it from sliding out of position later. If the slide feels very loose, tighten the screw a bit more. If it slides but it feels tight, just loosen the screw slightly. This a trial and error procedure. Lock the screw with the lock nut and proceed to adjust the outer screws in the same manner. The ends of the jibs must be adjusted equally by just tightening the screws until you feel it touching bottom and you would have to apply a relatively higher amount of torque to further tighten it. After locking the end screws, test the slide along its complete range of travel and proceed to adjust the next slide in the same manner. The cross slide and main longitudinal bed of the SHERLINE in double " T " slotted to accept a multitude of purchased or shop built holding devices, just as long as the their are compatible with the mounting slots of the machine. If the vice you presently own will not bolt directly to the mill bed, you may have to build a table with mounting holes to match the mill bed slots and then mount the vise to the plate. In fact a sort of universal mounting table could easily be designed and built along those lines.
Just about any milling machine will be able to machine surfaces that are true, square, and parallel, but achieve this, you first have to bring your vice into perfect alignment relative to the cross slide and carriage. Loosely clamp the vice either along the long or short axis and align it using a machinist square. for most rough work this is more than enough, but I normally go one step further by attaching a small side measuring dial test indicator to the mill's quill or directly to the spindle chuck and orient it so the contact is touching the clamping inner surface of the non moving jaw of the vise. With the slide advances move the vice in a direction which is parallel to the surface of the non moving jaw while looking at the indicator needle. It will more than likely indicate that the vice is not as perfectly aligned as you might have thought as the needle will probably move several marks along the total travel. Very gently tap the sides of the vice while constantly checking with the indicator until you get the needle to remain perfectly still. When you manage to achieve that elusive condition, proceed, without moving the vice, to very carefully tighten the hold down screws or clamps to secure the vice. Check the vice again and pray that it did not shift as you tightened it. Anything that you now clamp in the jaws of the vise will be held parallel to the bed so that any milling done along the edges of such a workpiece will also be milled parallel to the jaws.
The simplest form of milling is squaring up the surfaces, and it is used to bring some piece of rough stock to the proper dimensions. I am not talking about intricate shapes like "Vs" or "Ls", I am simply talking about taking a rough piece of metal bar stock and squaring up the ends and if needed, the sides, to create an accurate rectangular or square block. Begin the squaring up procedure by placing the stock between the vice jaws so it is laying on its long axis and so one of the sawn ends protrudes about 1/4" beyond the vice jaws. Assuming the that the sides of the bar stock are relatively parallel to each other, you should be able to mill the rough end and it will be squared to the sides. With an end mill secured in a milling bit arbor, lower the head until the tip of the mill is slightly below and beyond the workpiece bottom edge. With the "X" advance, bring the side edge of the cutter to almost touch the work. Move the cross slide toward you as you look at the milling machine to clear the work and turn on the motor. Adjust the speed to a mid range and advance the tool so the movement into the work is against the rotation of the cutter. So if you are cutting along the right edge of the workpiece you must advance the tool from you toward the rear column. The opposite must also apply, so if you are machining along the left edge you would feed the tool from the rear toward you. If the tool is not cutting at all it just means it is too far away from the workpiece, you need to slightly advance the work closer to the tool with the "X" advance and repeat the cut. Do not attempt to make cuts deeper than around .020" as these small machines are not built to hog out huge quantities of metal. These tool are small high precision tools and they should be used accordingly. Besides, heavy cuts and high feed rates will produce the worse quality cuts no matter what kind of machine you use. Once the first end has been machined true and clean, unclamp the work and rotate it end for end, keeping the same surface on the bottom of the vice and repeat the same procedure. This type or work will efficiently clean and square the ends of stock but it can also be used to machine a piece to a specific length. As you begin to mill the second end, you can start measuring the length with calipers without disturbing the workpiece from the vice. If you find that the work is only about .015" or so too long, do not make the mistake of advancing the tool over by that amount. Do it in several small increments and when you are only .001" away from the perfect length, do not advance the tool but instead just take a second pass as most cutters will still cut some material off on the second pass due to many factors as tool flex, work flex and component deflection. Sound scary? Is not really that bad. This is a completely normal occurrence in machining.
Lets suppose that you have a piece that now is the proper length and also has nice clean, square ends and measures 1" square in cross section but the plans for that little steam engine you are so feverishly building says it is supposed to measure 1" x 3/4". You are now confronted with the problem of removing that excess 1/4" of material from one of the sides. Well guess what? This is exactly the kind of work a milling machine exceeds at so you have nothing to worry about. Before we star milling metal away we have to determine if the top surface of the workpiece still on the vice is sitting above the surface of the vice jaws. If it is, you can proceed from there without further ado. If it is seating bellow the jaws, you will have to lay some spacers of the same thickness on top of the bed of the vice to raise the work so it clears the top of the jaws by the amount of material you need to remove plus about 1/16" extra to make sure the bit clears the jaws. Good parallels can of course be bought at a hefty cost, but you can simply substitute two blank high speed steel un-ground lathe blanks of the proper size. measure the blanks for their actual width on both surfaces as they may be slightly rectangular so you will have to now which two sides are the same width and set them down side by side on the vice bed. Lay the work on top of the spacers and tighten the jaws. The side surfaces should always be machined on opposite sides to prevent warping toward the unmachined surface due to sudden differential stress relief. The internal composition of CRS is not homogeneous throughout its cross section. The outer skin is compressed during the rolling process and slightly harder so when it is machined away, it no longer impart strength to the surface and will give toward the opposite still un-machined surface by literally bending. This is especially true with so called cold rolled metals. Hot rolled steel will not react this way but should still be machined on adjacent surfaces just to be safe. With the milling bit situated above the surface to be milled so the end of the bit is facing the surface, begin to machine it starting at the rear right corner and only with the front half portion of the end mill. proceed from right to left until the whole edge is milled. Return the cutter back to the starting position and move the bit toward the front 1/2 of its diameter and proceed to mill the next section. Continue until the surface is completely milled. Immediately flip the workpiece over to bring the opposite face up and the machined surface facing down and on top of the spacers. Proceed to machine this face and continue to mill until the thickness has been reduced to 3/4" as required by your engine plans. By allowing a part of the stock to hang past the vice jaws, you will be able to bring in a caliper to properly measure your progress. If you also needed to machine the opposing faces, you would simply flip the work 1/4 turn so the vice jaws are griping it by the freshly machined faces. Soft jaws would be required so as not to mar the fresh surfaces or you could simple sandwich the workpiece between two thin strips of aluminum sheet of the appropriate dimensions and clamp it between the vice jaws. The aluminum will probably end up slightly dented, but not your workpiece. Once again, mill the two adjacent surfaces and you will end up with a piece of dimensioned metal with square faces and ends ready to be further machined as needed.
Before I get into the more advance milling operations I will take a few minutes to talk about the mill as a drill press. There probably is not a better and more accurate drill press made than the milling machine. Milling machines have as a common place situation very accurate, heavy duty spindles, much better than those found on the common drill press. The bearings on a drilling spindle can only handle a thrust force that is in line with the spindle or in the direction of the drill bit. Milling spindle bearings can take thrust stress in the directions of the spindle but also from the side and this makes it a very steady no wobble type spindle that in very accurately adjusted at the factory to run as concentric as possible. Secondly, you can run your bits on collets which by themselves are inherently extremely accurate so the bit will run with almost no run out so the resulting hole can as close to the actual diameter of the bit. A situation that is almost non-existent with a normal drill press. Because the drill point can be bought down to just kiss the workpiece, the depth of the hole can be measured exactly from that point by keeping tract of the advance dial for the head stock. I will further describe the many types of drilling and boring operations that can be done on the mill to include locating edges of workpieces and locating hole positions by the " X & Y "coordinate system.
Lest go back to milling operations and talk about milling angled corners, chamfering edges, slotting, and keyway cutting, dovetailing slots, and milling cavities.
Joinery is not just for the woodworking aficionado. It applies also to metalwork when two member are to be joined together, is best to slot the accepting piece for a snug slide fit of the other with the two being secured with screws. To set up a workpiece for slotting, it is best to orient the work in such a way the slots is cut along the axis of the vice jaws so the work can be allowed to sit on the flat vice bed. Sometimes this will not be possible because of the size of the workpiece so it will have to be jacked up high enough on spacers so the bottom of the slot to machined clears the top of the vice jaws. Milling slots is identical to previous operation, the only difference being that the cut is deepened with each pass until the desired depth is achieved. Ideally, you should use an end mill of the same diameter as the slot you require, but if the slot has to of an exact width, you should begin with a narrower bit and after the correct depth has been achieved, the slot is gradually widened by side milling always against the rotation of the bit. You create many intricate passage ways and slots by simply by using the " X & Y " movements of the mill bed. Workpieces can be clamped at angles other than square so angled cut can be made on ends, sides. surfaces and edges of the work. To achieve accurate positioning of the work you use machining triangles and angle or bevel gauges. The cuts are performed as before, only the orientation of the work has changed. The edges can be beveled with either a " V " cutter or a dovetail cutter. The angled portion of the cutter has to be placed at the edge to be beveled and the cut is made in the same manner as when the ends where being squared. A keyway is normally cut with a special bit that sort of looks like a shaft with a small thick saw blade cutter at the end. The thickness of the cutter will determine the width of the slot created. The stock is clamped horizontally and the cutter is passed along the side to be slotted and the cut is deepened with each pass until the proper depth is achieved. A dovetail slot is cut by fir cutting a square slot and then the dovetail cutter is run through the slot to cut the angled sides. After the dovetail slot is finished, the bit has to be slid out of the slot or the slot will be permanently damaged. Boring cuts can be round, square or rectangular. Round bores are started by drilling and enlarged with an adjustable boring head. A boring head uses a regular boring bar in a sliding holder that can be indexed via a micrometer screw to slowly create a swing of increasing diameters to gradually enlarge the hole. The workpiece is clamped and the boring head is attached to the mill spindle and as it rotates, it is sent through the hole after advancing the tool out a few thousands after each pass. A square or rectangular opening is made by end milling with the X&Y movements and deepening the cuts after each complete pass. The smoothness of the milled surface will differ depending on the rate of feed of the work past the cutting tool, the cutting tool itself and the type of metal you are machining. To produce the same degree of finish when milling with bit with two cutting edges as opposed with one with four cutters, you would have to feed the work at half the speed as the four fluted cutter. The later takes four cuts per revolution as opposed to the two flute bit which only takes two cuts per revolution. This is pretty obvious so we would either increase the speed of the bit itself to twice what it would be for the four flute tool. Two flute cutter also tend not to clog up as easy as four and six flute bit will when machining aluminum. In fact, many people will recommend only two flute cutter for machining aluminum. Aluminum has a tendency to produce edge weld where the chuck actually melt during cutting if the cutter edges are only marginally sharp. There is a special cutting fluid specifically formulated for aluminum that really seems to help produce fine cuts and it can be used for any type of cutting like drilling, reaming and taping. For machining deep cavities, you can use roughing out mills which have cutting edges with many cutting facets like a file. The surfaces produced are rough but the mills cut faster than smooth fluted bits. The final finishing cuts should of course be made with the regular end mills.
Locating hole positions accurately will determine whether two components to be screwed together will fit in perfect alignment or not. The manual method has been used for decades with perfectly good results. It does require care and time under your belt to be able to do it consistently enough. As a example to begin with, lets imagine that a base plate for an engine needs several mounting holes for various components that will be bolted to it. If you look at a good machine drawing, you will notice that any locations for hole are indicated by two measurements from two adjacent edges. These can be the left and top or bottom edge. I myself, prefer the left and bottom edges, from which all other locations are calculated. Assuming that The base plate is squared and you need two holes that are situated .500" in from the left edge and the first one .750" up from the bottom edge with the second one 1.750" along the same axis, you will end up with holes that are 1/2" in from the left vertical edge and starting from 3/4" from the bottom edge, with the second hole 1" up and away from the first one. Instead of measuring 1" up from the first hole and possibly introducing some slight error that will sneak up on you. If there are many holes to spaced along that same vertical axis and you measure each subsequent hole from the last one, by the time you get to the last hole, you will end up with many small errors that will accumulate from hole to hole, placing the last one at the wrong position. To minimize the possibility of accumulative errors you should locate each individual hole location by always measuring from a single edge. In our case the two holes are 1" apart but 3/4" from the bottom edge and 1/2" from the left side edge. Layout blue should be applied to the surface of the plate and with the use of a small tri -square set at 3/4"., measure from the bottom left edge and make a horizontal scribe from the edge for about 3/4". The second location is marked by setting the square at 1-3/4" and making and identical mark 1-1/4" from the bottom edge. Set your square to a 1/2" and from the left side edge place two vertical cross marks to intersect the two previous horizontal ones. At the point where the two marks cross is where the you center punch to locate the drilling point for the holes. If the edges of these bottom plates are square to each other, it will not matter if the actual width and length dimensions are slightly off since the measurements are always taken from the same two reference edges.
Curved milling is done by attaching the workpiece to a rotary table which revolves by turning a crank located on the side. A perfect circle can cut or any fraction of it and slots or other curved contours can easily be milled in this manner.
When building the connecting rod of steam or gasoline powered engines, you will have to cut a centered slot and cross drill the piston and valve rods so the flat ends of the connecting rods can slide in them. Slots are cut with a slitting saw blade of the final width needed. The work is held and oriented so the slot to be cut will run with the horizontal axis of the mill. If the slot is to be centered, as they mostly are, the blade is brought down so it just touches the upper surface of the workpiece being slotted. Moving the saw blade away to clear the work, lower the blade 1/2 the total sum of the blade thickness and the work's thickness or diameter. This will place the center line of the blade along the center line of the workpiece. Slots are cut at a medium low speed by making repeated passes of .010" to .015" in depth until the slot is completed. Slotting blades can be obtained in a multitude of thicknesses, tooth design and number of teeth and diameters. They are held on a blade arbor that is in turn held in the spindle either by screwing or with a draw bar. A cross hole drilled after the horizontal hole is cut will always be perpendicular to the slot if the workpiece is not disturbed between the two machining steps. Cross holes that will take crank pins as in the construction of engines will more likely be reamed to a perfect diameter. To do this just drill the cross hole with a slightly smaller bit and follow with a reamer of the proper diameter at a low speed, a slow and steady feed, and plenty of cutting fluid if you machining steel or aluminum. Brass is considered to be free machining so it does not require any cutting oil.
Learning the art of milling is something that will take years of practice to fully master. Do not feel bad if at first your efforts are not as good as those you see in the machining magazines. This will come to you when you least expect it.
So far I have been talking about very general milling techniques on a small mill like the SHERLINE. There are other units that are more expensive and heftier. The EMCO/MAIER company makes a small lathe to which a small milling machine can be added. The advantage here is being able to mill work that is still held in the lathe chuck or held between centers. With an indexing plate on the lathe spindle, a workpiece can be milled in precise increments. A round workpiece can have a square or hexagonal end milled on it so it can be turned will a wrench. Specialized bolts and other components can be made this way. The PRAZI milling machine is also one of the finest small mills out there and can be purchased as a dedicated single unit or as an add on to their great small lathe. Just imagine being able mill a workpiece with automatic feed by clamping the work to the cross slide and auto advancing it past the rotating milling bit. Aside from these lathe/mill type machines, there are some very neat medium size lathe/mill/drill combination units that are referred to as multi machines. The SMITHY machine is probably one of the best in its class and although they are all made in China, the SMITHY company is American and have their own resident design and quality control folks at the Chinese factory where they are made. Service, replacement parts, and technical support is excellent. Other similar units are sold through Harbor Freight tool in Camarillo California. One is a fully manual unit, meaning that all the feeds are by hand and it sells in the $600. while a bigger unit with auto feed and thread cutting capabilities is available for just over $1000. with free freight to your the front of your driveway. It's your un-enviable job to the get it from the driveway to your shop area. If it's in the garage, consider yourself very lucky as most times, it must transported somehow to the basement. You must have guessed by now that these machines must be very heavy. They will average around the 400 to 600 pound mark. This makes for very sturdy and vibration free operation. The individual components are hefty, large and are able to withstand some pretty strong forces that would cause a smaller machine to vibrate itself to death is the process. The last option would be a medium to full size milling machine. Many Asian milling machines are now being imported by several American companies like Grizzly and Harbor Freight Tools and of course, any of the major industrial tool and supply companies. These milling machines are either floor units, extremely heavy and expensive and in my mind only to be considered for large professional production machine shops. The others fall into the bench top class, although I tend to not think of 500 pound machines as bench top tools. I have not actually worked with one of these machines but I know others who have and they like them very much. The only difference between a 500 pound monster and a 25 pound " midget " is the work and tooling capacity it can handle and the type and level of work it can perform. What I mean is this. Some of the most intricate miniature engineering I have ever witnessed was done on a small 25 to 50 pound machine. You just take several lighter than usual cuts instead of the heavy ones that the big machine can easily take. Yes, it will take you longer to machine a part but we are not production or assembly line workers, we have the luxury that this is our hobby and the object of relaxation and stress relief. If you do wish a larger machine, you will still be able to build the same size projects as with the smaller machine. Is just that when the urge hits you to build that 1" scale hit and miss engine with an 8" flywheel, you will not have to scale down the drawings so your little machine will be able to handle it or beg some machinist friend to rent you time on his big mill. Decide how much money you can afford to put into a machine tool purchase and also look at you shop space to see whether you have the room for such a machine. If you do, just make sure you put where you're going to want it for a long time. Remember that once you set that 500 pound baby down it isn't going to be easy to move around like the SHERLINE. If for some reason think that you must own a 500 pound plus machine in order to be able to perform accurate work, just take a minute to browse through some of you Home Shop Machinist issues and check out some of the work created under the talented hands of Rudy Kouhoupt. This man does not use any other machine than his very small Unimat ( smaller than the TAIG ) and a very old equally as small PERRIS lathe. For over head milling work he uses the Unimat head stock and the vertical column set up in the milling configuration. To enable him to perform that many different types of cuts and machining operations, he has designed and created a multitude of tools and attachments right on the same lathe they will be ultimately used in. When I first was exposed to this man's work I was greatly humbled as I continued to study his work, here I was with the attitude of bigger is better and buying every tool known to mankind. Instead, I should have been using the machine I did have to its fullest. If you have not seen Mr. Kouphoupt's work, consider purchasing HSM's book featuring his work, you will not be disappointed.
Some of the more specialized milling techniques will be discussed in their own individual articles or project descriptions where they are utilized. This is in no way saying that these procedures are difficult, but instead they are somewhat specific in their use so it is better to describe them as they are used during a given project.