March 2009 / Volume 61 / Issue 3|
By Bill Kennedy
Making a simple-looking part sometimes requires a complex series of operations. That was the case when Falcone Precision Machine Inc. had to produce 8,000 pieces of a 0.916 "-long × 0.247 "-wide × 0.056 "-thick electrical connector made of Kovar, an iron-nickel-cobalt alloy. True position for a series of 18 holes in the connector was to within 0.002 " of the holes’ centerlines, and specifications required part edges to be sharp with no burrs. The parts had to be burr-free because the customer intended to fill some of the holes with glass and set in 0.018 "-dia. wires and gold-plate the entire connector to enhance conductivity.
Courtesy of B. Kennedy
Falcone Precision, Latrobe, Pa., created a workholding setup to maximize productivity, repeatability and efficiency. In one vise on the table of a Fadal 3016 vertical machining center, a 6 "×⅜ "× 0.l00 " Kovar blank was clamped for machining of one side of the connector. A second vise had fixtures to hold five connectors cut from the blank after machining the first side.
According to Mark Falcone, co-owner of the shop with his brother, Kevin, the part thickness dictated that the parts be cut apart for the operations in the second vise. “Otherwise, if you turned the blank over and put it in the second vise, you couldn’t keep it flat enough,” he said.
In the first operation in vise No. 1, an Atrax ¼ "-dia., square-shoulder, TiAlN-coated carbide endmill run at 6,000 rpm and a 36-ipm feed rate roughed the perimeters of five parts and facemilled the tops. Then the same endmill moved to the second fixture and roughed the second side’s perimeters of the then-separated parts. Next, an identical but fresh ¼ "-dia. endmill finished the top and perimeter of the parts in the first vise and finished the parts in the second vise.
Those parts already featured holes created on vise No. 1. There, 18 hole positions were spotted with a 0.046 "-dia. center drill run at 7,300 rpm and 3 ipm. Twelve of the spots, for through-holes, were 0.010 " deep, “If you went too far, then the drill would chamfer the hole too much,” said Kevin. The six remaining hole positions were to receive a 0.070 "-dia., 0.004 "- to 0.007 "-deep spotface with a 0.028 "-dia., 0.020 "-deep counterbore in its center. Those six locations were spot drilled to a depth of 0.020 ". “If we didn’t go to that depth, the 0.028 "-dia. endmill we used to make the counterbore would break because it was just plunging straight down,” Kevin said.
The through-holes were drilled with a 0.062 "-dia. cobalt drill run at 5,000 rpm and 2 ipm. When through-drilling was completed, an M01 command stopped the program to enable visual verification that the tool wasn’t broken. “That’s how high maintenance the part was,” Mark said. “We had to watch for broken tooling on every run.”
Finished size for the through-holes was 0.0655 " in diameter, which was achieved via circular interpolation with an Atrax 0.062 "-dia. endmill run at 7,300 rpm, plunging at a 7-ipm feed rate and then circular interpolating at 5 ipm. Kevin said: “We used the endmill to make sure we held position. That way, we didn’t have to worry about checking the location on every part. We were able to let it run and go do other things.” The circular interpolation is a prewritten program provided on the Fadal control, and programming was handled primarily through subroutines.
After milling the through-holes, the same 0.062 "-dia. endmill ran at 7,300 rpm and 2 ipm to rough the six 0.070 "-dia., 0.004 "- to 0.007 "-deep spotfaces. A fresh endmill of the same diameter then plunged and interpolated the spotfaces “just to make sure we had sharp corners in that 0.070 "-dia. spotface,” Kevin said. Then a 0.028 "-dia. endmill run at 7,300 rpm and 1 ipm plunged the six counterbores in the spotfaces to 0.020 " with a ±0.003 " depth.
When the finishing endmill moved from the first vise to the second, it encountered burrs and interrupted cuts as it passed over the through-holes made in the earlier operation. Burrs also remained on the part periphery. To remove them, the finishing endmill followed the part’s edge, stepping in 0.010 " around the perimeter. Then the tool milled the connector to its final 0.056 " thickness in a series of 0.001 "-deep passes. “If you take too heavy of a cut, it takes a bigger burr,” said Mark. Finally, a 0.065 "-dia. reamer ran through each through-hole. “We threw the burr back down into the hole and sized the hole in case the endmill didn’t machine perfectly round,” Kevin said.
The final machining operation was a ¼ "-dia. endmill pass to cut the five parts on the first vise apart. Those were moved to the second vise, a new blank was clamped in the first vise and the process began again until 8,000 parts were completed.
To guarantee that the parts were free of burrs, Falcone Precision built a series of sanding fixtures and used 600-grit sandpaper to polish the top, bottom, sides and ends of the connectors. “You look at it and it’s a simple part,” said Mark, “but when you are trying to hold all those dimensions on it and keep the edges sharp and burr-free, it’s not.”
For more information about Falcone Precision Machine Inc., call (724) 539-0560 or visit www.falconeprecisionmachine.com
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