Tech Talk Article
In my previous article, I mentioned that the lathe was my favoritemachine tool. That is because there is a certain elegance to theturning cut. A sharp, wedge shaped tool takes a continuous, uniformchip at the least expense of power and with satisfying smoothness. Inmy work as an Instrument Maker and also doing repair work on printingpresses, I have had to make some rather hairy lathe set-ups. I dependedon the gentleness of the turning cut and trusted that I could get atricky job done without a wreck.The only reason that I like milling is that it is easier than filing.What I don't like is that the milling cut is always an interruptedcut and the chip is always changing in thickness. In practice it meansthat the machine is going to vibrate and there is always going to befriction, heat and cutter deflection.I have a Van Norman No 38 MEA in my shop. It has a No. 50 Taper spindleand 15 horsepower to drive it. She's a Fat Girl - weighing in atsomething over 7,500 pounds. I was fortunate to buy her used from anaircraft plant so she has very little wear. THAT machine will MILL! Idon't mean just take a heavy cut, what I am getting at is that the heavymachine will take a light cut and leave a finish so fine that I have hada customer mistake it for surface grinding.The reason that I mention the big Van Norman in an article for hobbyistsis to get the point across that because of the nature of the millingcut, Nothing Beats Size and Weight! The Milling Machine must havesufficient mass of metal to dampen the vibrations of the milling cut andresist the push of the milling cutter without being deflected or thesurface finish of the cut will not be smooth.So far the best milling machine I have seen for the live steam hobbyistis, of course, the Bridgeport. The reason is this: when they designedthe Bridgeport they modeled it after the No. 2 size heavy duty millingmachines made by Brown and Sharpe, only for economy they left out allthe mechanical parts of the feed works in the knee and they gave themachine a simplified vertical milling head.They sold it as a "Tool and Die" mill and were very fussy about thequality of construction and the machine's alignments since they wereselling against large, well established machine tool manufacturers whowere just a short distance away, Brown and Sharpe in Providence and VanNorman in Springfield.In our high school shop in 1960 we had a Bridgeport Mill and all of usin the shop class just loved the fact that the hand cranks turned soeasily and that the little milling head ran so quietly that it didn'tscare us. Our other milling machines were made by Cincinnati and VanNorman and they were intimidating beasts!I am sure that this experience was shared by many others my age whichled to the fact that Bridgeport dominated the light milling machinemarket until the imported "clones" appeared 20 years later.There are some other very fine small milling machines to be found onboth the new and used market. On of the best have owned was theClausing vertical mill. This machine was very nicely made and I checkedit for alignments and it was dead-on. It had a No 2 Morse taper spindleand used No. 2 Morse taper collets.The only reason that I sold it was that at the time I was in myapprenticeship and accustomed to heavier machines and, being young andfoolish, I tended to overload the poor thing. If one can be satisfiedwith working with end mills 1/2 inch and smaller, then this is one ofthe best machines a steam hobbyist can buy.With the exception of some of the parts of the Major Beam Engine, thereis no Stuart Turner model steam engine that this machine can't handle.Both Sheldon and Hardinge made small horizontal milling machines. Themain disadvantage of this type of mill is that they have no quill ancan't be used as a drilling machine. If you are limited in shop spaceand can do accurate layout, then a small horizontal mill and a drillpress are the most economical way to go. I have a Sheldon in my shopand run small parts in production quantities on it. It has also cut itsshare of Stuart Turner's iron.I am vary of the imported so called mill-drill machines with the round,drill press type column. I ran one of them and found that they werepoorly fit together and were very prone to deflection of their partseven when taking light milling cuts. Personally I believe they can bedangerous when used as a milling machine.There are also imported machines that look similar to the Clausing millI mentioned earlier. Although I have no experience with them, I thinkthey would be all right, but like the Clausing, they should be treatedgently - no heavy milling cuts.The little table top milling machines sold in catalogs and magazines areprobably going to be a disappointment. First, they don't have the sizeand weight to withstand the reaction of the milling cut. Second, theyare so small that by the time you have a drill chuck and a drill in thespindle, there is no room for the work piece. Both the tradesmanmachinist and the hobbyist are interested in success in completing ajob. Fighting the limitations of a machine is one of the quickest waysto make a person loose interest in the trade or the hobby.Some notes on milling machine techniques. It is always best to have anexperienced machinist go through the basic points of the milling processbefore attempting a milling job on your own, Milling machines are notas forgiving as lathes. Proper set up is 90 per cent of the job and amilling machine set up simply MUST be a Safe set up.Be sure you know the difference between climb milling and conventionalmilling. The diagrams in textbooks always show the conventional and theclimb cuts in terms of a horizontal milling machine cutter. End millscan be put into a climb cut with the same unpleasant consequences.In conventional milling, the cutting edge of the cutter or end mill isgoing in the Opposite Direction of the feed of the work. In climbmilling, the cutting edge of the cutter or end mil is going in the SameDirection as the feed of the work. In a climb cut, the cutter will tendto pull the work into itself by taking up the backlash of the feedscrew. This is always exciting and usually ends up in a wreck.Heavy Industrial milling machines like our Van Norman have backlasheliminators on their table feed screws so they can climb mill, There isNO small milling machine that I know of that has such a device - Don'tclimb mill!Small vertical mills all have quills for drilling and boring. This isvery nice and it lets you do what used to be done in expensive jigborers right in your own shop. There is a very big "However" associatedwith quills, and that is the quill must be retracted all the way up whentaking milling cuts. Even more important is the responsibility of themachine operator to be SURE that the Quill Lock is set tight beforetaking a milling cut.If you go to enough shops where there are Bridgeports, you are certainto find some fool milling with the quill stuck down. This practiceruins the main head casting. Don't mill with the quill down.The helix of an end mill will exert a downward force on the machinespindle, if the quill lock is not set tight, the cutter will draw thespindle down into the work or the machine table. If you look at enoughused milling machines, you will find quite a few with mill marks intheir tables. Set the Quill Lock.All small vertical milling machines have one or more swivel jointsbetween the overarm and the milling head. This allows the head to beset at an angle and also relieves the machine manufacturer of having tohold the tolerances for squareness of the spindle to the table. Thatmeans that the operator must be able to check the spindle for squarenessto the machine table.This is done with a dial indicator held on a bent arm that is chucked inthe machine's spindle. This is best shown by an experienced machinist,the point here is if you don't know how to do that, get a machinist todo it and show you how and also, think twice before setting the head ofa vertical machine at an angle because it will always have to beindicated in afterward. By the way, if a machine has been moved, evenif it is only across the shop, the head will have to be checked forsquare. It will invariably be knocked out.Learn how to use an edge finder to locate the spindle center over theedge of a work piece. Then you can locate holes in relation to thefeatures of the work piece and measure the locations with the dials adscrews of the milling machine. This is much easier than laying a jobout and unless you are an expert layout man, it is more accurate.For those who are mathematically inclined circular hole patterns can beput into rectangular co ordinates by using high school trigonometry. Inmost cases, the locations of the holes will be more accurate than ifthey were done on a rotary table, especially the small diameter rotarytables most hobbyists and many machine shops have.A perfect example of this is the machining of steam engine frames andcylinders. Once the cylinder is bored, the head bolt holes are locatedin relation to the bore and drilled in the same set up. The same coordinates are then used on the engine frame after the crosshead guide isbored. The parts will fit together perfectly the first time. If a jigis made to hold the cylinder heads in a milling vise, the boss that fitsthe cylinder bore is indicated true to spindle center and again the sameco ordinates are used to drill the head bolt holes.While on the subject of drilling in the milling machine, there is no needfor center punch marks when locating holes by rectangular co coordinates.Center punch marks keep drills from "walking" when they contact thework. When locating a hole in a milling set up, always use a centerdrill to make a starting spot for the drill. I can't count how manytimes I have seen amateurs and people who say they are machinists expecta drill to go directly into a work piece and stay true to machine centerwithout some kind of starting spot.Never trust a milling machine vise. always check the stationary vise jawfor parallelism to one of the machine's axes using a dial indicator.Use the indicator to check to see if the flat surface of the vise at thebase of the jaws is parallel to the top of the machine table. I veryseldom use a swivel base under a milling machine vise. Yes, it looks tobe convenient, but the swivel base is a probable source of parallelismerror and it also decreases the available space between the spindle andthe table top.The milling machine table is the "operating field" of the machine. learnhow to make set ups using blocks and clamps to hold work to the table.It is all right and in fact recommended to run an oil stone or even afine file lightly over a milling machine table after it has been wipedthoroughly clean. any object that strikes the table is liable to leavean "upset" on the surface which will cause a vise or fixture to beclamped out of parallel with the table top. The stone or file will findthem so they can be taken down even with the table surface. Do thisalso to the bottom surface of vises and rotary tables, they often aredamaged while sitting around off the machine.Milling machines, especially the Bridgeport type machines see some veryrough service in various shops. This makes finding used mills quitedifficult. The CNC machines are making many manual machines available,but the price of used small mills hasn't been depressed as far as theprice of manual lathes. Buying a milling machine is going to beexpensive no matter if the machine is new or used. As in finding a niceused lathe, a used machine tool dealer is a good person to talk to. Ihave a tendency to trust a good used Bridgeport more than I would trusta slickly promoted new import.The hobbyist can do very well with an older ( late '50s - early 60's)Bridgeport with the short table length and the nine inch cross slidetravel on the knee. There is a possibility of finding an older "roundram" Bridgeport that is in really good shape. These older Bridgeportsare very fine mills only they aren't sexy anymore. remember, thehobbyist isn't looking for extended slide travels so the smaller versionsare just right and very economical to buy.I hope that I have been of some help.
Jim KizaleApril 2003