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Tips & Techniques Pages
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Herein are my notes on various techniques for the milling machine. They are to date a rather disorganized collection of anecdotes found elsewhere. Over time I will organize these and test each one, discarding those that don't work so well and emphasizing those that do. For now, I am using a font convention to differentiate the tips: If I have written a tip in this font, I have personally verified it. If it is written like this, I found it on the Internet and am awaiting verification. These are the ones to take with a grain of salt.
Most people have heard of Tramming your mill, but what about squaring? Tramming generally refers to alignment along the axes that are designed to move if the head can be swiveled on the mill. Squaring involves taking the mill apart to really get things lined up. It's done once in a blue moon, such as when you first get the mill or if your mill doesn't seem to be cutting accurately even after tramming in. Shim the Column or the Base? One sure way to ignite a controversy is to bring up the topic of leveling as it relates to out of square lathes and mills. There is a school that says you level the lathe's bed and the rest is a function of the machine itself. There is another school that wants to use level as "close to correct" and then run a test bar with further adjustment of the leveling until the lathe cuts without taper. The first school sees this as adding a twist to the bed and is horrified. The second school sees it as a practical solution to a problem and wonders whether the first school realizes that. Recently the same sort of argument broke out around milling machines, specifically the Tormach. It's an interesting thread, with both sides weighing in. Philbur addresses the purest camp clearly with this remark:
I think that shimming the bed must be the last resort, not the first, for correcting a tram error. Tramming the table tells you that the spindle is not perpendicular to the table surface (assuming the surface is flat!), it doesn't tell you why. The column may not be square to the table, or the spindle may not be square to the column, or both. Twisting the bed will most probably mask one error by introducing a second error. The correct method is to identify each error individually and correct it without influencing any other alignments. OTOH, no less an authority than Tormach's Greg Jackson himself says to shim the base instead of the column:
I'm with Jackson on this one from a practical standpoint, although he has sent me correspondence claiming that all problems with out of squareness can be traced to a stand that is not level, something I don't agree with. It may be that the base is fine and the column could be shimmed, but if you can do it from the base, that seems an easier/better approach. If nothing else, try it that way first and take some measurements with your DTI to see how close you're coming. Also note that for this to work out well, you can't bolt the machine to the stand. What you're doing is using leveling feet on the base to jack one corner or another, so the base has to be able to rise and fall relative to the stand. Squaring the Column on my IH Mill Before I attempted to square my mill, I leveled the machine to the table. I measured my squareness before and after leveling and the difference was substantial. So substantial that you can probably get perfectly square just by tweaking the leveling feet of you mill (perhaps out of actual level but until your machine is square), just like with a lathe and just as Tormach's Greg Jackson says.
Before attempting to square the column, be sure to level the table! The easy way to check squareness is with a dial test indicator in the spindle, and a cylindrical square on the table. You need to measure 2 planes corresponding to X and Y, so I positioned the cylindrical square twice:
Cylindrical square is inline to measure whether the column "nods" forward or backwards from vertical. The indicator should stay put as the head jogs up and down...
I started at the top and went down 8". The need barely moved a tenth!
Now we rotate 90 degrees and we're going to check whether the column leans left or right by moving the head up and down and checking against the square... I was out about 1 thou left to right and nearly 3 thou of "nod" forward. This was easily fixed with a little shim stock. Having squared the head, I went on to tram it as well.
An alternative if you don't have a cylindrical square... Dressing the Table In The table looked like it was used as an anvil. Something had to be done to level it up. I then took a fly cutter and made a 0.005" deep pass down the table. I made sure the cutter overlap was over the tee slots and the cut pattern was a uniform series of spirals in both directions. The 0.005" cut only hit on half the table surface and it mainly just leveled the dings in the table. I then stoned the table lightly with 10 wt oil and a fine India stone. Head Squaring the head is generally what is meant by the term "tramming" in the head. Proceed to the section below to square/tram the head. QA Tests for a Mill Tormach's inspection sheet shows some excellent tests you can make on your mill to determine its squareness and accuracy.
Tram is the squareness of your mill head to the table. There is tram parallel to the x-axis, and tram parallel to the y-axis (sometimes called "nod"). Depending on your machine, you may have a swivel head that is designed to cut at angles other than square for more flexibility. For machines with adjustable heads, you need to check the tram fairly often and rest it. I try to check the tram on my mill whenever I begin a new project. That's really not often enough. Most machinists I've talked to check tram when they come in every morning, and quite a few will also check if someone else uses the machine during the day. The point is, if you need accurate cuts and the best finishes, your mill needs to be in tram. Tramming the Mill More Quickly With Your Quill DRO At some point, I developed a procedure that I find easier and faster. Here is my basic setup with the DTI on my Indicol and a couple of 1-2-3 blocks to provide clearance over the vise:
Basic tramming setup... The goal is to have the DTI have the same reading on either side, indicating the spindle is square with respect to the table. The Indicol is not the best tramming setup, BTW. A proper tramming bar would be more rigid and less "jumpy". For example:
Here's a nice tramming bar that goes in a collet... At some point, I decided to try using my quill DRO and the DTI like a sensitive height gage. I would raise the DTI off the 1-2-3 block on one side, lower the quill until I saw DTI motion, and press the zero on the quill DRO. Then I raise up off the block, flip it around to the other block, and lower down until the DTI registers. Now I can read on the quill DRO the difference between the two sides. Next I bump the head in until the Quill DRO/"Height Gage" reading is 1/2 what it started out. Repeat the procedure until you're within acceptible limits. I was able to get pretty close in 2 cycles of this:
Head is now trammed within 0.001" on about a 10-12" circle. That's pretty close!
Notes from the Web A note about gibbs. On column mills, the gibbs affect the tram of the head. If you like to tighten or lock them every time you perform an operation, you will limit the accuracy of your head tram. Better to adjust them fairly tight perhaps 4 times a year, tram after the adjustment, and then leave the gibbs alone if best accuracy is desired. Tramming your mill to a "thou or two" may not be good enough. If you move your table to a different spot to drill than where you indicated your part at you get projected error. also, try a test indicator in an indicol rather than your co-ax it will tell you if your co-ax is cheesy or not. I recommend Interapid. Use a singlepoint test indicator...the coax relies on a mechanism and precision alignments and fits therein. I am a fan of Interapid's offerings in that category. Do yourself a favor, spoil yourself and buy an interrapid incicator, I bought a .0005 and aquired a .0001 over 20 years ago, the .005 has been repaired once, otherwise they have served me faithfully ever since, their only downside most of us machinests never see is they are effected by strong magnets, so guys that only grind don't use them...turning on a 6" permag chuck will damp .0003 tir out of an interrapid...didnt belive it until I was shown it by a man with far more experience ID grinding than me. Other than that hands down they are the nicest deal going, a guy turned me onto them back in 1984, so I guess it has been 22 years...I was 19 then hehe, 41 now, still on that same indicator. Never move the knee after picking up location. Learn to tram head dead nuts on a 6" circle. When the head is not trammed dead nuts and you move the knee, the location changes. If you are out .004 on a 6" circle, and you move the knee 3" after removing your co-ax, you are now off .001 on location. In addition to the Interapid, get an Indicol that will attach to the spindle with a tool in place. This will allow you to use your indicator at any time. The easier it is to use, the more often you will use it. Also, get a couple of small mirrors to read the dial when its facing the column. Sticking your neck around back gets old very fast. I did not think about the knee changing the position of the tram, however, I was aware that the quill would throw you off, for example, if you tram the head with the quill retracted, and make your cuts with the quill extended several inches. It just magnifies your tram error, correct? The reverse also works, if you tram your mill with the quill fully extended, then the accuracy is higher when retracted. Tram your head over the largest circle you can swing (ie the width of the table). Use a dial or finger indicator and read directly on the table surface. No blocks, rings or anything else required, all will introduce error. Personally I tram the head without locking the knee, because I almost never use the knee lock at any time for the work I do. Centre you workpiece with a finger type indicator of some sort, the result will be right, whereas it sounds like the Coaxial type are not always trustworthy. You can even use a plunger-type indicator if that is all you have, and if you have enough room. The magnetic base will stick to your drill chuck and away you go. Sounds clumsy (it is) but use what you have. I don't take much notice of the suggestions about not moving the knee, sometimes you have to. Try not to, ie plan ahead. I once had a guy working for me that had a Blake Co-ax. He scrapped quite a few parts before I made him stop using it. Unless the quill is in perfect tram, you are changing the center location, if you move the knee up or down, after you indicate the center of the workpiece. Error can be even more, if there is any wear or play in the knee gibs. One more thing to try: put a cylindrical square on the table, and use an indicator to check whether the z axis is parallel to the square's axis. It could easily be that your z axis ways have excessive wear and its time to rebuild. It's VERY IMPORTANT to always lock the vertical travel of the knee before you do anything, whether it's taking a measurement or making a cut. With the knee loose, it will pull away from the column, but the magnitude and direction of the movement are unpredictable. Whenever you unlock the knee, its final position depends on the amount of weight on the table and the location of the center of gravity. But the center of gravity changes every time you move the table on either axis. You first position the Z axis as required, then lock the vertical travel, then do whatever else you need to do. On assuming your tram is goodt. We re-indicate the tram every morning, before every fresh setup, after heavy milling, after a suspicious measurement, and of course, after a crash(tho'that's never happened to me ). In mold work it is imperative as you usually doing onesy work and weld is so detrimental to the life of tool. After a week of indicating the whole shop in it is quite second nature and takes little time. As for the rig to indicate with we always called this a "tram bar". I find indicol's and those silly Co-ax indicators a waste of money. I made this bar by tracing off my mentor, he traced his off of his mentor and who knows where the pattern started. In this part of the country this type tram bar is common in many mold shops and other shops. It is a simple project and was the first tool I made when I took up mold making. It is simply CRS c&h with an ejector pin (drill rod will work) for a post, all thread for the screw, the universal sleeve is from Starrett, and then a ball swivel from Starrett or B&S. I welded an extension to the swivel. This rig will tram all the diameters and squares you will see. If needed a second snug and rod can be added to extend range. This will clamp down and bite hard and not wiggle or droop (a problem with the half baked Indicol with the nylon friction washers. Do yourself a favor and whittle out one of these and you'll never have any issue with mystery location and flyer holes. well i have to make a coment or two here. first i stone the table to remove any burrs then i use a 1" gauge block under the indicator so i dont worry about the indicator jarring or moving due to jumping over the T-slots. second in reference to the Blake coax use. these indicators are so quick to use for an initial location it amazes me anyone would not want to use one. but having said that i always go back and tweek the last few thou with at least a .0005" indicator but usually with a .0001" indicator, after all if your within .005" or less it just takes a few seconds to be dead nuts.
Once you've got the mill trammed, your next step is very likely going to be putting a vise onto the table. does the vise attach by bolts into the T slots of the table? if so, then you have a simple way to check square, you need a dial indicator, a indi-col device, and then put the indicator into the indicol's small end, and put the large end onto the spindle, and run the table's X axis. the amount of change or angle on the indicator will tell you which way the vise need to be adjusted its a long process but you will get it ----------- Mount an indicator in a drill chuck, bearing on the fixed jaw of the vise. Traverse the table (X axis) back and forth, and adjust the vise position until you get the same reading at both ends of the fixed jaw. Given that you have a swivel base, there's a shortcut to get you close: Take a reading at the center of the jaw, then at one end. Calculate the difference between the two readings. With the indicator still at the end position, rotate the vise to remove HALF of the difference. Several cautions: 1) Make sure the indicator arm is actually moving. I've seen people do this with the arm hard against the stop, and think they have the vise adjusted perfectly 2) If the jaw is mounted with two screws, take a reading directly above each mounting screw and use these to determine whether the vise is square. The pressure of the mounting screws can distort the jaw. If the jaw was ever mounted with any dirt or contamination underneath, it can be permanently distorted, even if the obstruction was later removed. 3) Make sure the fixed jaw is straight in the first place. As noted above, it can be bent by the pressure of the mounting screws. This may be a bigger challenge than squaring the vise on the mill. ----------- Quick and dirty... clamp a dowel or other piece of bar stock in the spindle. Tighten one side of the vise pretty well and leave the other just barely snug. Just make contact with dowel on the face of the fixed jaw at the tight end of the vise. Move loose end to where it is definitely a bit out of line. Crank table under spindle with dowel in contact with the fixed jaw. When you have gone the entire length of the jaw, come back and make one more pass for insurance. NOW break out your DI and tweak it if you don't like what you see. I did this today for a quick setup. When I put the DI in to check, I had less than .0005 in 4" on the vise jaws. Close enough for what I was doing by far. Same deal with a horizontal, but grab a fairly large diam slotting cutter very lightly with the vise. Tighten the bolts. Put a DI on it now. If the vise jaws need to be parallel to the arbor, you might be able to come up enough to grab the arbor the same way. If not, do it the hard way. ---------- To get the fixed jaw close, I'll use either a drill bit or a dowel pin in a drill chuck. Set a gap by eye at one end, duplicate it as best as possible at the other end. Once you get it where you can barely tell the difference by eye it's time for the indicator. Saves being way off and having the test indicator needle spinning wildly and forgetting which way is which. ----------- Some vises move only a tiny amount from loose to tight - my Kurt changes less than .010, with the table bolts pulled forward. Others move at least one time zone. With those, as Bob said, snugging the bolts and making final adjustments with a hammer is the easiest way. An 8 oz ball-pein is a good size, small enough that you won't damage the vise. If you vise is newer, or has newer jaw plates, indicate directly from the fixed jaw. If the jaws are well used & look like the surface of the moon, a tall parallel (clamped in the vise) gives the indicator a nice surface to contact. ----------- Last but not least, in Secrets of a Machine Shop, the author mentions that you can eyeball to within 0.002" by putting a piece of long stock in the vise such that you can eyeball the visual gap between the stock and the table's edge. This assumes that edge is actually parallel to the table's motion--something to be checked on an Asian import! You could also use a steel rule. One fellow starts out with the rule in the vise, eyeballing the gap. He claims his accuracy with the rule is 0.010", which is a good place to start with an indicator.
Use a Coaxial Indicator to Center on a Hole or Round Feature Accurately I got a knock-off of the Blake Coaxial Indicator from Shars and after trying it out decided, "Man, what a cool gadget!" Here it is in action as I center the spindle over a hole so I can use my boring head to make a shoulder on the hole:
Picture that long feeler as being sort of like a very exagerated finger from a dial test indicator. It goes against the inside of the hole, or there are curved feelers for the outside of a round boss. You turn on the spindle at the lowest rpm, say 100rpm, and hold the horizontal rod. This keeps the indicator facing your way while the feeler sweeps around the circumference of the feature you want to center on. As the feeler sweeps around, the needle will "kick" because the spindle axis is not centered. Now adjust the X and Y axis handwheels until the needle kicks as little as possible. That's all there is to it, you are done very quickly. What a nice gadget! Adjust the axes one at a time and don't even think much about what's needed. Simply turn the wheel and observe whether the needle kicks more or less. Turn it in the direction that kicks less. When you've minimized the kick on one axis, do the other. I circle back a second time around and got to almost no kick at all. I did not set up the Indicol and DTI to check the result as I was in a hurry, but visually, after I used the boring head to make the shoulder, it looked spot on. It tool longer to stick the Blake in the R8 collet than it did to adjust the X and Y axes, or so it seemed. Apparently they can also be used to dial in a 4-jaw chuck. I'm going to love having this little gadget around I think!
Make sure your mill is trammed in properly before attempting any accuracy drilling or boring! Couple of suggestions... -use vee-jaws for a better grip on the part (4points of contact instead of 2) -use a 120 deg spotting drill rather than the center drill...cups the drill point better IMHO and just as rigid -lock table & saddle gibs as suggested...noting any change in the DRO readings and compensating appropriately. On the center drill, if you use one, and I understand why, they are short, rigid, and common in the shop, so only use it to SPOT, then it is a 118 degree spotter, all is good, don't drill deeper than the drill point, and surely don't drill deep enough to actually start to get a center. I elarned how bad this is for the following drill in harder materials, the tips of the flutes start cutting first and get chipped, it cannot be good for the drill in softer material either is my thinking. It comes to mind your mill may have some "lost motion" somewhere, so with a good indicator you can dial in, and come back to 0 on the DRO from differant directions and see what ends up happening. Also I have never seen this, BUT your quill travel may not be square to the spindle bearings, I cannot imagine how that could happen on a good machine, but you can check for it. It sounds to me like you are getting a reasonable result for drilled holes. Use a new centre drill and a new, good quality, drill. Make sure you ease the centre drill in, using high revs, cutting oil and clear often. Same with the drill, slow down to correct speed, cutting oil, clear often. Don't use re-sharpened drills on a job like this, keep yourself a good set for jobs like this. I disagree with the idea of only dimpling with a centre drill. Make a decent centre in my experience. Maybe you need to start thinking about boring your holes if you want exact locations, but this will slow you down a lot, single point cutting is a another (harder) skill to learn! As to making a "good center" with the center drill, trying that in some 4140 HT or even harder stuff will illustrate the condition that learned me to NOT do that, actually a 120 degree spotting drill is better anyway but not as commonly avail within arms reach in the shop. if a hard material chips off the drill tips when running a 118 drill into a 60 degree center, I have to think the same process occurs with softer material, but is not as apparent because it is not as extreme. If you watch, feel, and listen to a drill you can tell it is unhappy when starting (sounds japanese eh) but it gets very happy when fully engaged and doing the job it was made to do, a 120 spotter gets it to the happy place quicker :-) I agree "spark out" of the drill with the quill against the depth stop (or a short G04 in cnc)helps get a better location, it gives any spring in the machine and setup a chance to work itself out. I never want to seem to cut down anothers experience, their method may sure add quality to MY work if I apply it, and I think the key is finding the WHY in the process that makes great parts, sometimes it is not truly a result of the intent of the guy that found it but it cannot be denied that it works. I also wanted to add that adding a "boring" step to the process with an endmill in a collet can truly make for better positions, honestly if the endmill is only taking out a little per side it may be truly as accurate as single point boring, and a following drill or reamer will follow very close to that established position. The drill generally "wants" to go straight and starting it off that way helps it do that...they can and do wander as depth increases, but if they are started off position they cannot help but walk like mad. Dealing with Runout, Especially When Reaming If you have runout, consider the following. If you have an ER collet you can tap the tool in. Measure, find the high spot and keep taping it until it gets runs true. Works with collet holders but not usually EM holders. We do it at work all the time seeing as harldy any of our machines run true. They all need new spindles as far as runout is concerned. Good tapers, bad bearings, usually. one of our machines runs dead nuts untill you clamp a tool in the spindle and put tension on the bearings... Then it is out .0015 at 6 inches from the face, which with PCD Reamers is not acceptable. Here we go again guys. From the men at Spindle Grinding Service.....Check the taper. Not only for runout, but use some Dykem 107 high spot blue on the holder and clamp it in to check contact. If the taper is worn it will cause runout in your holder. That would be my very first check. Something tells me that is the problem. Could possibly be drawbar pressure, but the taper would be my first guess. Precision Reaming Dowel Holes Whenever we get an oversized dowel pin hole we get out the ball bearings. Stick one in the offending hole and give it a tap with a hammer. Now the pin will hold securely, and as long as the hole location was correct before it should work for you now. I've had to do this more times than I care to recall. Now this is for in-house tooling. I don't think I could sell this to anyone if they saw me doing it. With the pin in it though, you can't even tell. Also, with reamers the top portion of the shank is annealed, and will "float" into the hole without boring it. If you hold it below the anneal you will end up with a very inaccurate boring bar. The ball bearing idea is a good one, but not for pallets that my customer needs to work with. In house though this is an excellent idea. Any thoughts on diameter of ball to hole? 2X? 3X? I only ask because too small a ball would put a lot of force radially outward and too large a ball would put a lot of force axially downward. I need to think about this some... We do a lot of dowel pin holes, up to 1/2". We have had good luck by spot drilling, drilling about .030" undersize, boring with a 2 flute end mill that is .015" under, & then reaming. The endmill is chamfered about .030" x 45 deg, & most of the flutes are ground off, leaving about 1/8" of full diameter at the front. We use from .006" to .012" IPR feed. Using the endmill gives a quick & dirty way to get true position & the reamer will follow the hole. This process is not as good as boring for true position but is pretty close & a lot faster. Sometimes to repair a loose dowel hole, I will tap it with the same size form tap, i.e. 1/4-20 form tap for a 1/4" dowel. The tap will raise ridges to tighten the hole and with a little loctite, I am done. We also made an alignment block, just like a tapping block, with about .002" clearance for the dowels. It makes dowel installation easy. More Boring Accuracy
Tip: Brushing on a coolant will increase the range of optimal speed. In other words, it lowers the speed where things work best as well as raising it, creating a broader "sweet spot" of speeds that is easier to hit. If you are having a hard time getting things cutting nicely, why not try a little coolant? If you are using carbide, be careful not to drench the coolant in suddenly, especially if things are running pretty hot. Sudden cooling can crack the carbide. Apply the coolant steadily and from the beginning of taking the cut. Tip: Different materials and operations respond to different coolants. I keep a set of condiment squeeze bottles (cheap at the dime store) ready to hand with the following contents: - Kerosene - Tap Magic - Heavy Sulfurized Cutting Oil Some folks also do well by spraying on WD-40, and there's always a couple cans of that floating around. Tip: Coolant can improve surface finish and tool life. Interestingly, it can also reduce tool life after you exceed certain SFM's--see Dry Machining below. One of its major roles in both improving surface finish and improving tool life is clearing away the chips to minimize chip recutting. BTW, this is one of the great advantages of climb milling as well. Getting the chips out of the way can be done with just a steady stream of air directed at the work, so by all means, try that if you have compressed air and don't want to mess with coolant. Tip: Dry Machining. Most tooling reps advocate shutting off the coolant over 400 sfm and going with dry machining. You should still use a blast of air to clear the chips, however. The consensus over on Practical Machinist is this really works well to increase tool life. Some few think you still get some surface finish advantages to use some coolant during the finish pass. ---------------------- When roughing, an air-water mist is fine. For finishing, use a cutting oil. -------------------
Cutting oil will reduce chatter. Chatter is just a harmonic resonance, so anything you do to change the "tuning" of the instrument can reduce the chatter. Speed up or slow down the spindle if you have variable speed and you can often get rid of it too. ---------------------- Rumor has it to use "A-9" aluminum cutting fluid for the finest aluminum finishes. Green can, green fluid, much better than parifin, kerosene, lard, or WD-40. ------------------- Widgitmaster's coolant of choice: Actually I bought the Synthetic Coolant, it mixes with tap water at 6oz / gallon, it has rust inhibitors in it! This coolant it water clear at first, and gets a little cloudy as the way lube gets mixed in! I have not had any rust, and this coolant dries leaving a sticky film of oil! It works really good on aluminum, stainless, cast iron, steel and plexiglas! Enco #325-6565, $36.16 per gallon of MISTIC-MIST concentrate, makes 30 gallons! -------------------- How to make a fog buster The fog buster is a coolant option many recommend, particularly those that don't want to deal with the mess of flood cooling and are afraid of the health issues associated with coolant mists (bad for the lungs). A fog buster delivers a stream of coolant embedded in an air stream without converting the coolant to an aerosol mist. The end result is the coolant winds up on the workpiece and not in the air. There is a good article on Practical Machinist about this, as well as the following additional commentary: The Fog Buster works this way with a single needle valve serving as a mixture control. Since the Coolant and air are pressurized equally, and the air is the mainline, the single needle valve acts as a mixture control to limit the amount of coolant added to a fixed amount of air. The net effect is that with a single needle valve you can dial all the way from a near flood coolant to pure air. If you search the message archives, Mike Hanz posted some great pictures and a parts list for a homemade Fog Buster back in 2003. He used a precision "panel" needle valve from McMaster, part number 48965K24. This needle valve counter-sinks into the brass block, so you get a neat, compact package without the valves hanging off the ends. Mike also turned me on to the water filter canister, part number 4422K3, for the coolant reservoir. This is the same tank the commercial Fog Buster unit uses. I think that were I seeking a coolant option somewhere between a chip brush and flood cooling, and more than just an air stream, the fog buster would be ideal. I'm not sure it is worth the trouble to make versus buy, but it isn't a very hard project either.
Milling Large Plates in the Kurt Vise
Put the jaws on the outside of the Kurt vise to hold a big plate... Accurate Z-Axis Adjustment on the Mill (aka I love it when a gadget actually works!) In working on the Turner's Cube, being able to determine Z-axis precisely is imperative. In addition, I am still having a hard time achieving the level of accuracy I routinely get on my lathe over on my mill. On the lathe, getting it accurate to a thousandth is never a problem, and even if I am not paying much attenting, things come within 3 or 4 thousandths. I haven't spent much time fiddling with tenths yet, but someday I will. On the mill, I seem to be doing good if I hit the high single digit thousandths, say accurate to 7 or 8 thousandths. I know its my technique, and its just not good enough. I mention somewhere below that one has to allocate time to experimentation, so tonight I resolved to experiment with z-axis accuracy, and along the way, dug out a purchase I had almost forgotten to test for the purpose. In the end, I tried four different methods. In each case I used my granite surface plate and digital height gage to determine how closely I had come.
Using the granite surface plate and height gage to determine z-axis positioning accuracy... To use the height gage, I zero it on the 1-2-3 block and then add 1" to the reading off the top of my aluminum cube. On the mill, I used each of 4 methods to find the top of the block, zeroed my quill DRO, dialed in a desired depth of cut with the fine quill adjustment, made the cut under power feed, and then checked how close I came to the expected result. Incidentally, the process of finding the top of the workpiece (or side on a lathe) is called a "touch off." Here are the results of each z-axis setting method: Touch off by feel: For my 1st Method, with the spindle stopped, clunk down the cutter onto the top of the workpiece. Zero the DRO and go from there. This produced a result with an error of 0.012". Not very good! The error was relatively repeatable, so one could add the fudge factor. In the end of the day, the cut was 0.012" deeper than desired. Touch off by sound: For my 2nd try, I was gently lower the spindle under power and listen for when the cutter started to cut. This method proved slightly more accuate, and resulted in 0.0085" too deep a cut. Still not very good. Touch off with paper: The traditional old school method involves holding a piece of cigarette paper (rumored to be exactly 0.001" thick) on the workpiece and gradually lowering the cutter until it starts to catch the paper. Add another 0.001" and you are at zero! Not having any cigarette papers, I used standard laser printer paper. I cut a 1/2" wide strip so I could hold onto one end from a safe distance, and waited for the cutter to grab. In my case, I got a grab at 0.010", not 0.001", but at least it was a nice round number and pretty repeatable. Z-axis Presetter: At this point, I thought I was done and would be using the good old paper trick. But I had a vague recollection that finally came back to me. Did you ever by a gadget that looked like a good idea, but before it ever arrived you started doubting it would work well, stuck it up on a shelf, and never actually tried it? I do that way too often. In this case, thing I remembered was a Z-axis Presetter I bought off eBay around 8 months ago. Search for "Z Axis Presetter" on eBay, and you'll find the ubiquitous 800watt listed Chinese machine tool gadget. They look like this:
A Z-axis Presetter from eBay seller 800watt... I have a dim recollection of some CNC guys recommending one of these to use with CNC machines. How does it work? Simple. There is a little knurled knob visible on the bottom left. It has a "test" and a "use" position. Set it to "test" and an internal standard swings into place so that if you press the anvil on top with your finger until you hit the stop, you'll have exactly 2" from top of anvil to bottom of gadget. You rotate the dial to zero in that position. Now reset the knob to "use", place it atop the workpiece, bring the cutter down until the needle registers, zero the needle, zero your DRO, and you should be exactly 2" above whatever the presetter is sitting on. So, not expecting much, I plunked the sucker down atop my aluminum cube in my Kurt vise on the mill, cranked the head until the cutter almost touched. Locked the head and cranked the quill with the fine adjust until the needle zeroed, zeroed my DRO, removed the presetter, cranked down another 2" with the fine adjust, zeroed the DRO again, added 0.010" for a modest cut, ran the cube through under power feed, and hauled the block over to the surface plate to see what I had done. The desired result was 2.396". I brought the height gage down to take a reading which was, drumroll please, 2.396"!!!! Holy uncanny accuracy, Batman! The silly presetter actually worked, and it worked well, and even though the quill travelled 2", and I expected the worst, it came out deadly accurate! Hooray for the gadget that actually worked. I guess I'll be using the silly thing more often. At $39, it was worth it! Playing With Blocks (2-4-6 Blocks, That Is) I always look carefully when an experienced machinist is showing how he has done something. Here is the Fidgiting Widgitmaster showing how he squared a 6" x 6" plate in his vise using 2-4-6 blocks to lend support (they stick up out of the vise pretty far otherwise):
Squaring a plate in the vise with the aid of some 2-4-6 blocks for support... Made me want to go buy a set of 2-4-6 blocks, although they sure aren't cheap!
Flycutters A fly cutter will often produce the best surface finish because they allow you to finish a very wide area in one pass with no overlap marks. They make a distinctive sound as their single cutting edge makes its cut on the workpiece. It's a noise that drives some machinist's crazy and can best be characterized as a loud thumping (often called "hammering") that sounds like its about to tear itself apart. You can easily convert a facemill to a flycutter by removing all but one insert, and it is interesting to experiment with the results. Reducing the number of inserts will necessitate a reduction in feedrate, but it may be worth it in terms of improved surface finish. Some people swear that the best finish for aluminum is a flycutter large enough to cut your material in a single pass equipped with a PCD insert of the largest possible radius. Like most mill cutters, your fly cutter will perform better with positive rake on the cutting tool. I haven't yet seen any positive rake brazed carbide tools (which seem like the most common thing people stick in their flycutters), so I suppose you would either need an insert like a CCMT or you'd need to grind your own tool from HSS with some positive rake. Someday I want to build an R8 fly cutter that has a built in CCMT insert. I'll use a design similar to my dovetail cutter, and I don't think it would be a hard tool to make. Here is a sample of the surface finish possible with a fly cutter and off-the-shelf brazed carbide lathe tool:
Not bad, not great finish... This was one of my first cuts with the tool, I was squaring a block for my vise stop project. I didn't run the spindle nearly as fast as I should have for a great finish, but it still came out pretty decently. The moral of the story is that fly cutters are cheap and easy to use. Fly cutters are very sensitive to tram. If you think about it, the cutter is apt to cut a concave instead of flat cut if your mill's head is not trammed properly. Watch your workpiece carefully as the leading edge moves through the center of the fly cutter. All the cutting should have been done as it was travelling the first half of the way if you head is in tram (think about it carefull, that's one geometry for one direction, so you actually have to see this behavior cutting in both directions to be sure). If it cuts more on the second half, you can be sure the tram is such that the head has that half tilted downward slightly. You can see the tramming effects in this photowhich has had the cuts accentuated via some Photoshop tricks:
The mill head is close to being in tram, but you can see that one set of marks is a little heavier than the others. If you're really out of tram, you can only see one set of marks. Also, the larger the fly cutter diameter, the more the tram effect is exaggerated.
If you are going to grind your flycutting tool, here are some useful pointers I have gleaned from my Internet travels: Fly Tool Cutter Geometry Here are photos of a flycutter cutting tool that was given to me by Widgitmaster from CNCZone. This is the geometry he uses when flycutting aluminum for his mini-routers, and the surface finish is superb:
It sits in the flycutter with a slight down angle like this. Note the large smooth radius of the cutting edge, and the considerable positive rake...
A little more oblique view of the cutting edge. You can see how it is relieved from contact from the workpiece except along the cutting radius...
From behind the cutting edge. I've got a little divot in it, unfortunately. The edge is somewhat delicate and should only be used for aluminum...
The underside of the tool... The surface finish Widgitmaster gets on his aluminum work is nothing short of spectacular with one of these!
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