Corkscrew Milling Guide-Pin Holes

New technique opens holes four times faster than spade drilling, and saves one shop $35,000 per year.

Sinking large-diameter blind holes into tough hot-work steels isn’t easy, but diemakers often have no choice. They need to create guide-pin holes in mating halves of a toolset, and unless those holes are perfectly round and smooth-sided the sweat fit with the guide pin won’t hold. Plus, if the mating holes don’t align exactly, the die set can seize or be damaged when put to use, endangering the machine and anyone nearby.

The problem is perhaps most extreme in big forging dies, where guide-pin holes run big enough to hide a bagel—two to four inches in diameter by more than four inches deep. Standard index-able mills haven’t worked well on guide-pin holes because they cut out-of-round or score the bore with zigzag tool marks, leading to loose sweat fits with the guide pins

For many tool-and-die shops, prevailing practice is to do it the old fashioned way. Open the hole with a spade drill, then finish-bore for final size, roundness, straightness, and finish on one half. Repeat for the other holes until all four are done. The problem is it’s a very slow process. It can take a full day to complete just one tool set this way, even when all goes well.

That’s the bad news.

A new method
The good news is that several forging shops have switched to a new method that cuts machining time for guide-pin holes by four to one, and eliminates a lot of error sources, too. They corkscrew-mill the holes with a new Ingersoll Hi-Pos+ index-able end mill. It completes a whole die set in 90 minutes, and produces a hole that looks like it’s been bored. Dimensions are accurate within 0.0005 in., and the bottom is a true flat, 90° square from the wall. And, there’s no need to move to a second machine just for the holemaking.

“The horsepower required to spade-drill such large holes can easily stall most modern CNC machines used in fine cavity work,” says Ingersoll product manager Konrad Forman, who has helped spearhead the new practice. “For that reason we’ve seen many diemakers moving the toolset to a jig borer or heavy-duty drill press just for the guide pin holes. Moving to a second machine slows things down, and also introduces another source of error. With corkscrew milling, there’s no stalling problem — and no shuttling between machines.”

Beyond orbital milling
Corkscrew milling involves simultaneously feeding on all three axes: advance on the Z axis while interpolating on the X and Y axes to enlarge the hole. It’s a step beyond orbital milling, where the Z-feed is done separately from the X-Y interpolation. The cutter’s centerline follows a helix. It’s a programming step up from orbital milling, where you plunge to depth on the Z-axis and then interpolate.

Since there is only a small contact area between tool and workpiece at any instant, cutting forces are much lower than in spade drilling. And, completely eliminated is the friction between drill flutes, chips, and sidewall of the hole.

Absolutely straight sidewalls
The new Ingersoll index-able mill works here because the edge of the inserts are manufactured to trace a true helix with respect to the cutter’s centerline, leading to absolutely straight sidewalls and a 90° bottom. Insert geometry also promotes very free cutting with uniformly low cutting forces. The result is the kind of finish you’d get with a solid carbide end mill. By contrast, kinematically, conventional square inserts following a helical path simply cannot create a square corner at the bottom of a blind hole, and can’t avoid leaving lap lines in the sidewalls.

Customary practice with the new method is to corkscrew-mill both holes to identical diameter and location, then enlarge holes in the top die by 0.005 inch for clearance. Next, the guide pin is turned to 0.003-0.005 inch larger than the bottom die hole, to create a tight sweat fit. Finally the bottom die is heated to expand the hole enough to accept the guide pin. When everything cools and contracts, the pin is locked firmly in place.

Case in point
One forging-tool shop using the new method standardizes on 3-inch guide pins and high-nickel 4130-type steel, Rc 38, for big dies. A typical die measures 20324312 inches. Diemakers mill the holes with a 1.5-inch Ingersoll Hi-Pos+ index-able end mill at 3000 SFPM and 40 IPM feed. In about 90 minutes per die set, they complete all four holes and produce and “sweat in” two guide pins.

Based on current volume of two die set per week on average, the new method saves about $35,000 a year in machining time.

Ingersoll’s Forman has helped a couple of forging tool makers take it a step further, correcting for differential thermal expansion that develops over time between the mating halves of a forging die in use. “By noontime in many forge shops,” he explains, “ the bottom die runs 200 degrees hotter than the top die because it has spent more time in contact with hot billets. So its guide-pin holes are 0.005-0.010 farther apart than those in the top die. This, two, can cause seizing and tool damage.”

Accordingly, Forman advises to mill the hole in the top die as a very short slot rather than a true round. The few thousandths of slot length are enough to accommodate the spread of the guide pins as the hot die expands. “While you can’t readily create a slot with a spade drill or boring tool, it’s easy with an index-able mill,” Forman says.

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