mack@mails.imed.com wrote: > > There was a thread here a while back on what drill bits are best for > chassis work, etc. > Are there any machinists here? Is this something that we can be walked > through? Or do I need to find a machinist to "elmer" me through this?-- I guess I'm the 'resident machinist', so I'll give it a shot. I see that there's already several responses saying 'well, no machinist has answered yet so...' Hey, it's only been 18 hrs since I checked the list! Whew! You guys are quick! Note: Some vendors will be listed at the end. OK, first and foremost....get good bits to begin with!!! I can't emphasize this enough. NEVER skimp on tools and tooling. It just ain't worth it. Are you going to do BA work for a while? Then spring for $100 and get a first-class drill index (metal holder with 113 cobalt HSS bits). You'll get all the fractionals, plus all the number series and letter series bits. That'll give you all the exact right sizes for doing tapped holes also, and it's important to have the right size if you're going to tap a hole. A few thousandths too big and your threads are junk. A few mils too small and you'll break off a tap in a second...semi-permanently embedded in your precious BA.... -Good- tooling is made from High Speed Steel (HSS), and the best grades of HSS include several percent Cobalt, thus 'Cobalt HSS'. Various surface coatings are also available. Titanium Nitride (TiN) is a popular coating, and for good reason. It adds an extremely hard, and also lubricitive, surface to the bit. This lengthens bit life and reduces sticking and galling in gummy alloys like wrought aluminum for example. Watch out for junk bits that look nice cause they have a TiN coating (gold colored). They're still junk bits! Second, only use sharp bits. I stick the dull ones in a drawer and use them for 'locating' pins, or any project where I need a piece of precision ground stock of that diameter. I do -not- sharpen 50-cent bits. The drill sharpening fixtures you get at a hardware store are junk. They are so sloppy and so non-rigid that you'll just ruin the bit. Good drill sharpening fixtures run several hundred dollars. A key to proper cutting is -balance- in the tool. If one side is ground just a thousandth of an inch different than the other side, you'll get unequal chip-loads on each flute, the drill will try to suck into the work, cut out of round, and generally make a mess of your BA. By the way, this is another difference between good tooling and junk tooling. Some of the cheap bits are so bad right from the factory that you can easily see the imbalance with the naked eye. Also, the vast majority of home shop pedestal grinders do not have the right wheels for this work, nor do they have good enough bearings to cut smoothly in this kind of work. New bits are -cheap-. A typical cobalt HSS #29 (for 6-32 screw) bit is only 60 cents or so, for a brand-new top-notch bit. Why screw around? Use 'screw machine' bits wherever possible. They are shorter than regular 'jobber' length bits, thus they're much more rigid and cut better holes. The drill index you'll buy will almost certainly be jobber length, and that's fine 'cause you'll have the longer length when you need it. But then I get 10-paks of stub-drills (screw-machine length) for both clearance and tap hole sizes of commonly drilled holes, and I use them all the time. This saves the long bit for when I really need it, and need it in good condition! One of the responses mentioned negative rake angles. While it is true that there is an optimum geometry for each different material, it is not neccesary to buy those tools for cutting plastics. The vast majority of plastics can be machined just fine with normal metal-cutting tooling. The most critical factor in successful machining of any material is to have a -rigid- machine and fixturing, and a sharp cutting tool. Another respondent mentioned the Roper-Whitney hand punches. God I love those little suckers! I have three of 'em, each loaded with a commonly used size. I ALWAYS use them instead of drilling if it's possible. They only have a couple inches throat depth, and 3/8" or so of thickness clearance, so you can't use them for every job. But where you can, they're the absolute best solution. PS: They -will- shatter cheap grades of lucite/plexiglass/acrylic. PPS: Lexan is NOT acrylic. It looks the same, nice and clear, and is used for similar applications, but it's tough as hell (and costs 3 times what acrylic costs). Lexan is polycarbonate and is what they make bulletproof 'glass' and safety glass lenses from. That's tough! And it's easily punched. Note that it is still a plastic though, and scratches easily. The punches come with a set of 8 fractional size punch/dies, up to 1/4". You can punch up to 3/16" or so in an 1/8" aluminum rack panel. Pretty good for a hand tool! As I recall, the 'kit' is about $75. I bought 10 of them at $48 or something. We used to sell them. You can get any size you want made for you at about $15/set (set of punch and die). You will need to do this if you want to make tap-size holes, as all the common tap sizes need 'number size' drills, not fractionals. Another respondent mentioned using light pressure, to avoid overheating the tool. Actually, you can get in trouble that way. Most jobs are best accomplished with a solid, but steady, feed pressure. You do NOT want the tool simply rubbing the surface. In fact, some materials, like stainless, work-harden almost instantly. I had one production job, where if the operator paused while cranking the mill table, then started again, the bit would snap right off. For critical jobs, a drill press with a leadscrew downfeed is far superior to the typical 'spider handle'. The 'mill/drills' you see for a grand or so are an excellent value for general shop use. They're junk compared to a Bridgeport mill, but vastly superior to the usual drill-press. You know, I'm -sure I had a plastics catalog around here that had machining hints for all kinds of different polymers, but for the life of me I can't find it now. But all the producers publish application notes and other lit to guide you. Call and ask for it. Here are some sources that I've found helpful...call and get their useful and interesting catalogs. MSC (tooling) 800-645-7270 Every drill bit known to man (plus some known only to alien bankers from new york city....) Roper-Whitney 815-962-3011 punches, shears, etc.. Seelye Plastics 612-881-2658 (distributor) Commercial Plastics 718-849-8100 (corp hdqtrs, branches all over US) McMaster-Carr, no number handy, but in Chicago I think. Incredible breadth of stock in tools, materials, fasteners, etc.. Tough to get a catalog, but worth asking for. Note: All major producers like Dupont etc. have web sites where you can request that data sheets and machining hints be mailed to you. Some further notes: 1) All common -wrought- aluminum alloys are pretty gummy. The extrusion alloys, 6053 etc, are horrible. The 'aircraft' alloys like 2024 and 7075 (copper based) are much better, but they're double the per-pound cost, and are subject to stress corrosion and cracking. 6061-T6 is your best all around choice. 5052-H32 is a good sheet metal choice. 2) -Cast- aluminum actually machines pretty nicely, like brass. 3) Cold Rolled Steel machines fine. If you're having trouble, you probably have hot-rolled steel, or possibly a high-carbon CRS. Even the higher carbon alloys (1045 vs 1018 mild) machine fine if you have good tooling and a rigid machine. 4) Stainless is a very 'tough' material, however it also cuts fine if using proper feedrates, spindle speeds, etc. 5) Speaking of which, there is a proper speed and feed for every combination of tool and material. As another respondent suggested, get a copy of Machinery's Handbook. About $50 from any tool distributor like MSC, Travers, etc.. An excellent, excellent reference work that you will use for 20 years. PS: The 'proper' spindle speed for drilling an 1/8" hole in aluminum is somwhere around 10-15,000 rpm!! The vast majority of drill presses are limited to 2,500 rpm or so. Thus, you need to reduce the 'feed' rate so that the chip-loading stays the same. 6) Chip-loading is an important concept. It is the size of the bite that each cutting edge takes on each pass through the material (two cutting edges on a 2-flute drill). A particular tool likes a particular range of chip-load, typically half to one-and-a-half mils. There is also a preferred surface-speed of cutting edge in relation to workpiece. All recommended speeds and feeds are derived from these two parameters. 6) There are many styles of drill point and many 'angles' for those points. Machining is truly an art form as well as a science, and there's no way I can get into the details of drill points here. Suffice it to say that if you get a -good- quality drill index, those drills will work fine for 99% of what you need to do. Tool geometry -is- a fascinating subject, and I recommend you get a book on it if you plan to do a lot of metal work. Then you -can- select the best tooling from the MSC catalog. 7) PS: Drill bits are NOT precision cutting tools. They typically create a hole several thousandths larger than the bit size, and almost always a little out of round. If you need an accurate and round hole, you must use a 'reamer'. You drill about 5-10 mils undersize and finish with the reamer (typically a $3-8 tool, per size, in the range 0 - 3/8"). Whew!!! Wow, I didn't realize I had gone on so long! Please, if you have any interest in this stuff, archive this msg!! Don't make me to do it all again next month!! Karl Motrecht wrote two wonderful books on machining. They cover drilling, milling, turning, shaping, grinding, etc.. From the basics up to fairly sophisticated operations, and they include some feed/speed info, but mostly on metallics. Published by Industrial Press (same as Machinery's Handbook) and about $50 for the set if I remember right. Good luck, and have fun 'making chips'! Machining is a gas, and one of the best skills you can ever add to your 'toolbox' if you're into electronics construction and repair. Richard Richard Hager + Ah-ha! Design Group, Inc. - + Precision CNC Technology, since 1991 - + 612-641-1797, Fax: 612-641-8681 - + "I just like to make things" So... - + please call Ah-ha! directly for CNC info - + http://www.gdic.com/ahha email: ahha@gdic.com -