Creative fixturing puts parts in the zone

Jan. 5, 2009
But be warned, some machines limit how much of the workzone is actually usable.
A Makino a51 machining center provided Parkn Manufacturing full machine workzone use to improve the shop’s crossbow part production.

Everyday, many shops perform a balancing act in their quest to build the perfect fixture.

That highly sought after perfect fixture puts as many parts as possible into a machine tool at one time without disturbing the toolpath and it doesn’t exceed the limitations of the machine that’s being used.

It goes without saying that the more parts a shop can get on a machine the better. That is the easiest way to reduce cost per part when machine time is concerned. However, not all machines are created equal. Some offer limited actual usable workzone space.

In the case of horizontal machining centers, Bill Scandlon and his brother Bob, co-owners of Parkn Manufacturing LLC in Litchfield, Ohio, have found the best machine is one that accurately cuts precise complex part features high up in its Y axis. That machine also offers the ability to use its full Z-axis force at the top of its workzone without tipping its pallet.

“When you have to account for the flaws of the machine when designing your fixture, it can severely limit your productivity,” Bob said.

“If the machine won’t let me cut accurately at the top of the Y axis, or the pallet isn’t held firmly and the Z-axis thrust causes us to scrap a part, we have to hold the process back, and that’s not acceptable in a job shop environment,” he added.

Parkn Manufacturing specializes in highaccuracy parts for the sporting good, defense, and automotive industries. The shop typically runs jobs of 10,000 parts to 30,000 parts annually, and it has the capability to run thousands more.

When the brothers opened the shop for business, they purchased a commodity vertical machining center to get jobs going. Unfortunately, they realized that machine couldn’t handle the type of work they hoped to gain, and it didn’t allow them to use multiple part fixturing fully.

Parkn Manufacturing then got the opportunity to produce parts for a crossbow manufacturer. The job required the brothers to machine multiple parts accurately on one fixture, but the shop’s vertical machining center couldn’t fill the bill, even with an indexer.

The crossbow job prompted the Scandlons to purchase an a51 horizontal machining center from Makino. The machine accurately machines to the top of its Y axis, and its cone-pallet holding system allows for full Z-axis thrust at the top of the Y axis. The machine’s design let Parkn Manufacturing use the entire workzone fully and to optimize the number of parts that could be loaded onto each side of the fixture.

For full workzone use, machines must accurately cut precise complex part features high up in their Y axes and allow for full Z-axis force at the top of their workzones without tipping pallets.

For the shop’s existing jobs, the a51 reduced production times, often as much as 80 percent compared with the shop’s original vertical machining center. The brothers attribute these reductions to the Makino machine’s increased speed as well as to the ability to fixture horizontally on the machine.

“What used to take five minutes on our vertical machining center takes one minute on the Makino,” Bill said.

“One of our competitors machines a specific part in 17 minutes. Using the Makino, we machined the same part in three minutes, and it came out perfect,” he added.

Crossbow risers, made from 6061 extruded aluminum, were the parts that prompted the purchase of the a51. Parkn Manufacturing handles the part from start to finish.

The first operation requires two stations and 16 tool changes to drill, tap and cut a radius. Equipped with a special ball cutter and through-spindle coolant to keep chips out of the cut, the a51 profiles at 300 ipm using a 0.500-in.-diameter end mill buried 1.000-in. deep. Bill said it never slows down, and the finish is completed by a 2.500-in. fly cutter at 850 ipm.

In the first setup, two sides of the part – the flat back and the inside – are cut. In the second setup, the bottom of the piece is done. The job’s fixture holds eight parts per setup per side.

After machining a difficult 45-degree radius up one wall of the parts, the a51 radiuses the part corners and deburrs them in the final step of the second setup.

In a third setup, the machine cuts part sides that have pockets, drills and counterbores five holes, puts in countersinks, and deburrs that side.

Most holes on the crossbow risers are +/- 0.0005 in., but some are +/- 0.002 in., drilled to size. The entire part is kept within 0.002 in., and the fixture must be accurate to within 0.0005 in.

Parkn Manufacturing runs the part in three total operations, and setups are timed so that the machine is always cutting. At the end of the three setups, the shop has eight finished parts.

Bob pointed out that all the relationships of the part have to be maintained throughout machining, so the shop can machine all the critical parts in the same seutp. To ensure that, the shop cuts critical areas in the first setup to reduce the potential for stack-up errors and positioning problems.

Being able to machine accurately high in the Y axis becomes especially important on these parts because 32 of them are fixtured and several of the parts are positioned far above the centerline of the workzone. That position typically is the point at which many horizontal machine tools begin to lose accuracy.

“We needed to count on the fact that the parts on the top of the fixture would turn out exactly as the parts closest to the pallet, a position where most machines are capable of keeping good accuracy. All the parts have to be identical, even if they’re on the very top of the fixture and we’re cutting the most critical features,” Bill said.

Because at least one of each setup is on each side of the four-sided fixture, a batch of parts comes out finished each time the pallet cycles out.

“The key to this part is quantity without sacrificing quality. We have to be able to take finished parts off every time the pallet comes out of the machine to keep production flowing, but we can’t spend a great deal of time tweaking to make sure the parts are perfect,” Bob said.

“The machine has to do that for us, while running very quickly. Our fixture, combined with the accuracy and speed of the machine, allows us to do that. In fact, the machines are so fast, our guys barely have time to load up a new pallet before the one being machined finishes,” he added.

Previously, the crossbow manufacturer used four shops to produce the riser, but there were problems with consistency and on-time delivery. The combination of the Makino a51 and Parkn Manufacturing’s ability to creatively fixture 24 parts in one batch allows it to quickly crank out parts to meet demands.

In addition to improving part accuracy and production levels, the a51’s through-spindle coolant boosts tool life at Parkn Manufacturing. The shop’s tools lasted for only three or four parts per tool on other machines. With the Makino machines, tool life has jumped to 75 parts.

“We actually went out and found new tools that would maximize our speed and keep up with the machine. It gave us the opportunity to get more efficient and more accurate tooling, and saved us a significant amount overall in the cost of tooling,” Bill said.

Latest from Cutting Tools