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Machining-center survey: looking for Rumpelstiltskin.

Remember Rumpelstiltskin? He was that little gnome you could lock in a room overnight, and he would spin your straw into gold, but only if you were willing to pay a rather steep price.

Well, that's what manufacturing people are looking for today. They want to be able to load up thier Rumpelstiltskin machining center with parts, turn off the lights, lock up the plant and go home, and return the next day to find all their raw castings spun into profitable finished parts. But is this only a dream? No, Alice, it's been done, but the big question is, are you willing to sacrifice your firstborn to get it?

According to machining-center suppliers, their customer's number-one priority goal today is untended operation. Although few expect to get there in one giant leap, they want to build in that direction, and their new machining center, as the core tool in that grand plan, must have factory-of-the-future capabilities.

But the costs are high, and it's not just the hardware and software. Even with the best of controls and feedback devices, there's always the danger that your new $600,000 + machining system will destroy itself if serious trouble occurs with no one around to shut it down.

Thus, fear of a high-cost failure is a severe psychological barrier that also must be overcome. You must have complete trust in your system before you have the courage to turn off the lights and go home.

What's machining center?

In defining this survey article, we made certain assumptions to keep its size within practical limits. We eliminated the subject of machine controls entirely (because it extends into nearly all areas of metalworking), workhandling, and all machines with a turning emphasis. We wanted to focus on the basic million/drilling machining center for prismatic parts, and specifically on its mechanical aspects, for now.

That leaves this basic definition: standard automated centers that expand basic milling operations into multiple drilling, boring, tapping, etc; that include (as stardnard) some form of automatic tool changing; and that attack the normal size range of parts--not those megabuck machines for huge parts that are hardly commodity tools. There are about 150 US companies supplying machines in this category today. (Not all of them responded to our survey, and many will probably not have the staying power to survive the decade. Like the fallout we've already seen in robotics from a similar peak number of suppliers, the rush to supply you with machining centers indicates there's a far greater supply than demand.)

The table at the end of this article details basic machine specs from the 33 companies who responded to our survey. Although this is hardly everyone, most of the major players are represented, and their response to your inquiries should match their enthusiasm in contributing to this article.

In the genesis of machining centers, the horizontal-spindle center evolved from the milling machine, and the vertical-spindle center from the drilling machine. With the addition of automatic toolchanging, each still tends to favor its original function. Vertical spindles tend to be lower horsepower, relatively small in diameter (less friction), and higher speed to favor drilling, while horizontals are heavier and slower to favor milling.

There are approximately two verticals in the field for every horizontal. Verticals are ideal for 3-axis work on a single part face with little or no part indexing required. Long, flat parts are much easier to fixture on a vertical, and all spindle thrusts are readily absorbed by the table. A big factor is verticals are less expensive than horizontals, thus easier to justify, although it may take more machines to complete the part.

Horizontal-spindle machines work at right angeles to their tables and thus put torque into the part (more of a work-holding challenge). Bigger parts, though, benefit from their own weight, and are easier to shuttle in and out of the work area of a horizontal machine. Most horizontals have rotary-table options so that all four vertical sides of the part can be easily addressed, Horizontals also benefit from gravity's effect on chips.

New Twists

A variety of clever new head designs is blurring the distinction between vertical and horizontal. According of Cincinnati Milacon project engineer Don Ward, "Interest in 5-axis machines in smaller sizes is increasing, and not just for aerospace applications. We have recently introduced a medium- to large-size machine (40"-cube work envelope, 30-hp spindle), with a tilting head and a very competitive price. To be able to tilt the spindle and get as good tooling performance as you could with either a vertical or horizontal machine is a big benefit. In the past, these machines were very expensive."

In automatic toolchanging, the push is on to reach infinite tool capacity. One interesting development by Mori Seiki is the family tool concept: a standard size "mother" toolholder for heavy-duty milling operations is supplmented by a family of smaller insert holders for lighter-duty drills and tools that are stored in their own secondary (30- or 60-tool) tool-storage and changing system, and popped into motherholders kept empty in the main magazine.

In spindles, the emphasis today is on reaching higher speeds. The stepping-off point seems to be 6000 rpm--beyond that, you are really on your own. Users who demand higher-speed spindles may well find themselves eventually running those spindles at more normal speeds, feels Milacron's Ward. "Our research people have established 6000 rpm as the best practical speed for maximum utilization of the cutting tool. At higher speeds, you risk cutter, fixture, or part problems. Our efficiency curves all appear to peak at 6000, so that's where we tend to draw the line for our standard machines." But they, like others, will sell you higher speeds if you're confident you can master them.

High speed can mean high spindle temperatures and this is one factor behind the move toward sensing spindle-carrier temperature and compensating for it, although duty cycle is more often the cause of spindle heat buildup. Obviously, temperature compensation is no required at low cutting speeds and moderate tolerances, but if your requirements are more demanding, you may need to consider it.

Thermal shifts are definitely the next frontier to be tackled by machine makers trying to move to new levels of machine accuracy. They knwo you want the parts you cut in the afternoon to resemble parts made in the morning. Mitsubishi, for one, is promising repeatabilites to 0.000 060", based, in part, on measuring heat rise in the machine's base and column, and they will soon announce electronic spindle cooling (not just measuring spindle temperature, but actually controlling it) that will hold the rise to less than 3 C.

As temperature starts being taken more seriously, more people are spending the money to temperature-control their shops, and taking the simple precaution of warming the machine up for a half hour or so before cutting parts on the first shift.

Futures

A key battleground for the future will be finding some common ground between buyer demands and seller capabilities in the area of basic accuracies. The boring-mill people--the DeVlieges, SIPs, etc--with their 0.0001"/ft accuracies are well ahead of the medium-tolerance machining-center people who may soon be adopting the former's software-correcting and manual-scraping and finishing methods to further tune their machines and close the tolerance gap.

There has been tremendous improvement in the past few years in positioning accuarcy, machine alignments, and thermal compensation, but much more to be done by both manufacuture and user. Where are the brave souls willing to face the reality of what all those tiny isolated-accuracy claims would add up to in terms to realistic composite workpiece-position and toolpoint accuracies?.

The industry's greates unmet need is for in-process QC--the ability to measure a part while it's still in the machine. That will require major imrovements in the machine's real-would positioning accuracy and repeatability, day in, day out. Users today are willing to sacrifice a little machining time to qualify the part and know right away that the part is either okay, can be reworked by modifying the matchining cycle, or is unsalvageable and should be scrapped without wastling any more machining time.

And the other key requirement is flexibility. Top management knows that hardly any market today is fully predictable, or bankable. Therefore, you can expect to have trouble getting any new machining center approved today that's not flexible.
COPYRIGHT 1985 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1985 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Sprow, Eugene E.
Publication:Tooling & Production
Date:Oct 1, 1985
Words:1395
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