How to Select an
Economical Vertical Machining Center for Machining Molds
|
|||||||||
|
For shops that produce molds and dies, surface finishes and cutting feed rates are of primary concern. The goal is to reduce machining time per mold, and assure that the surface finish achieved by the machining center is optimum and doesn't require hand finishing on the bench. Market pressures in the contract moldmaking business dictate that limited capital expenditure resources provide the maximum "bang for the buck." Certain factors help to determine how to select an economical vertical machining center (VMC) that delivers the needed productivity. Fast Feed Rates Figuring the right blend of spindle power, torque, and speed is the equation to be solved, and you don't necessarily need the maximum spindle speed capabilities to gain the ideal feed rates. Optimizing your metal removal rate, whether it be for roughing or finish operations, is a function of both spindle speed and the machine control's ability to stay ahead of the data-flow needs of the machining program. A high spindle speed alone is not enough. In a typical steel mold finishing application, you're probably often cutting at a rate between 800 to 1,200 surface feet per minute (SFPM), with an end mill that is 3/8" diameter. 10,000 RPMs is all you need, 15,000 RPM maximum. Spindle cost goes up dramatically above this level, with only small incremental gains in productivity. 20,000 RPM
spindles and higher can be worthwhile for very fine detail operations with a
very small OD tool, but they are overkill otherwise. In most situations, the
balance between torque and spindle speed is determined by the material being
machined and the amount of it to be removed in a rough cut. The 10,000 to 15,000
RPM range is suitable for 70 to 80% of moldmaking jobs. |
|
||||||||
|
The more complex the tool path and the higher the feed rates, the more need for fast processing speeds from the control. If the distance that the tool has to move during a given program "event" takes less time than it takes to process the block of commands, your machine hits the data-flow limit and data starvation impedes machining efficiency. The "look ahead" feature available on newer controls allows the machinist to optimize cutting speed, ensuring intelligent acceleration and deceleration of the feed rate along the tool path. It determines how to schedule the feeding of program data to keep the tool running smoothly, planning for any sharp moves that have to be made. Unless your machine is using a very high speed spindle at over 900 IPM, you don't need a "look ahead" buffer that's more than 500 to 1,000 blocks since today's processor speeds are quite good. Depending on your programming process, control software packages designed specifically for moldmaking are also helpful. A side note: in the majority of applications, integral motorized spindles are not needed to deliver higher spindle speeds. Given that they can be five times the cost of a geared or belt-driven spindle, integral motorized spindles are a luxury. Rigidity and
Accuracy The key advantage of a box way system is vibration dampening. When you make a heavy cut, the machine experiences vibration as a result of the significant forces developed. The large contact area between the box ways and the table, combined with overall rigidity of the machine, will significantly reduce vibration by increasing dynamic stiffness. As a machine tool heats up, thermal growth becomes the enemy of accuracy and threatens your confidence in holding tight part tolerances. When the machine is designed with symmetrical structures, it helps, because when heat-related growth occurs the tool center point does not move and tolerances are not affected. Higher speed spindles require air/oil lubrication that coats the spindle bearings and reduces friction, so the spindle runs cooler (and lasts longer). Ceramic bearings in steel races also help - they have a heat expansion rate of about one-third that of steel, which prevents heat build-up at higher speeds; this type of spindle also maintains bearing preload better. A closed-loop, self-monitoring refrigeration system for the spindle is the best way to ensure consistent positioning accuracy. The spindle cartridge and machine headstock are cooled by recirculating refrigerant. Ball screws are the other problematic heat-generating source. Friction can be reduced by separating the balls as they flow through the nut, so that they don't rub against each other, thus reducing friction. One way to maintain precision despite thermal changes is to use glass scales on the way units, giving you the closed-loop feedback to ensure "you got there" and that positioning repeatability is consistent. This enables a customer to cold start the machine and produce quality parts right away, but it adds significantly to the cost of the machine. Strategies where the machine builder or user tries to predict how the machine will deflect when it is hot, and that then devise predetermined compensation, do not work very well for this class of vertical machining centers. Extra Axes Don't Forget
Maintenance and Support Sizing up your current process requirements and your current machining capabilities, while factoring in future work that you can predict or will seek, is the key to investing in the right VMC. For more information contact: MAG Fadal 20701 Plummer Street Chatsworth, CA 91311 818-407-1400 Fax: 818-407-0020
Southwest TX Dave Weber Hillary, Inc. 1825 Summit #207 Plano, TX 75074 972-578-1515 Fax: 972-578-1519 E-mail: daveweber@hillaryinc.com
OK Brent Eagleburger Maruka, U.S.A. Inc. 1210 NE Douglas Lee's Summit, MO 64086 800-220-2361 Fax: 816-524-5444
AR, LA Steve Wherry Cardinal Machinery 7535 Appling Center Dr. Memphis, TN 38184 901-377-3107 Fax: 901-377-4855 E-mail: s.wherry@cardinalmachinery.com
NM Triad Machine Tool 7811 W. 48th St. Wheat Ridge, CO 80033 303-424-0268
Southeast FL Bill Koster Koster Machinery Co. 537 US Highway One North Palm Beach, FL 33408 561-627-6968 Fax: 561-845-1285
NC, SC, GA, N.FL Steven Flint Flint Machine Tools, Inc. 3710 Hewatt Court Snellville, GA 30039 770-985-2626 Fax: 770-985-2706 E-mail: postmaster@flintmachine.com
TN, AL, MS Steve Wherry Cardinal Machinery 7535 Appling Center Dr. Memphis, TN 38184 901-377-3107 Fax: 901-377-4855 E-mail: s.wherry@cardinalmachinery.com
VA Robert Browning Pinnacle CNC, Inc. 805 B Barkwood Court Linthicum, MD 21090 410-609-0805 Fax: 410-609-0804 E-mail: sales@pinnaclecnc.com
Northeast CT, NH, VT, ME, MA, RI David Shabey Compumachine Inc. 6 Electronics Blvd. Danvers, MA 01923 978-777-8440 Fax: 978-777-8490
NJ, E. PA Robert Morrison Mid-Atlantic CNC, Inc. 260 Evans Way Branchburg, NJ 08876 908-809-1100 Fax: 908-809-1199 E-mail: rmorrison@midatlanticcnc.com
NY Dan Shortino Mid Atlantic CNC 85 High Tech Ave. Rush, NY 14543 585-334-2420
W. PA & N. WV Maurice Auer 2SQ Industrial Supply, Inc. 1002 Corporate Lane Export, PA 15632 800-346-3188 E-mail: mauer@2sqpa.com
DE, DC, MD, WV Robert Browning Pinnacle CNC, Inc. 805 B Barkwood Court Linthicum, MD 21090 410-609-0805 Fax: 410-609-0804 E-mail: sales@pinnaclecnc.com
Midwest KS, W. MO Brent Eagleburger Maruka, U.S.A. Inc. 1210 NE Douglas Lee's Summit, MO 64086 800-220-2361 Fax: 816-524-5444
IL, E. MO Tim Goedeker Goedeker Machinery, LLC 2275 Cassens Drive, Ste. 126 Fenton, MO 63026 636-680-1841 Fax: 636-349-2397 E-mail: info@goedekermachinery.com
N. IL, WI Jeff Zemen Integra Machine Tool 2620 South 162nd Street New Berlin, WI 53150 262-789-1600 E-mail: jzeman@integramt.com
IN Butch Corn Machine Tool Solutions 9112 W. Jackson St. Muncie, IN 47304 765-759-9044 Fax: 765-759-9075
IA, NE Ken Erickson Erickson Machinery 409 Market St. Story City, IA 50248 800-247-3905 E-mail: ken@ericksonmachine.com
MN, WI, SD, ND Craig Conlon Concept Machine Tool 15625 Medina Rd. Minneapolis, MN 55447 763-559-1975 Fax: 763-553-7704 E-mail: tinaj@conceptmachine.com
S. OH, N. KY Jones Machinery 11118 Adwood Drive Cincinnati, OH 45240 513-825-5000 Fax: 513-825-1225 E-mail: sales@jonesmachinery.com
N. OH Dave Grega HM Technology 12955 York Delta Dr Unit C North Royalton, OH 44133 440-237-1491 Fax: 440-237-3267 E-mail: dmgtek@sbcglobal.net
MI Jennifer Giannetti VMC Technologies Inc. 1788 Northwood Drive Troy, MI 48084 248-786-3000 Fax: 248-786-3001 E-mail: jjg@vmctech.com
S. KY Steve Wherry Cardinal Machinery 7535 Appling Center Dr. Memphis, TN 38184 901-377-3107 Fax: 901-377-4855 E-mail: s.wherry@cardinalmachinery.com
|
|||||||||
