Cross hole drilling

It is not difficult to drill cross holes on some parts. The structure of these parts is usually simple (such as aluminum valve body), the holes are not too deep, and they are easy to center, and the customer can tolerate some small intersections between the two holes. glitch.

But for other parts, such as P-2 tool steel, injection molds for complex medical devices, drilling dense, high-precision cross-holes is far from easy, even for experienced mechanics. I also have a deep headache.

Even the simple cross-holes are difficult to drill, including fast tool wear, poor chip removal, difficulty in removing burrs, and tool deflection that can cause the most robust bit to break. However, there are also some solutions that make the task of drilling cross holes easier.

Dan Habben, an application engineer at Sumitomo Electric Hardmetals, believes that drilling cross holes is always a problem. He smiled. "Maybe the best advice I can offer is: don't drill cross holes!" He is working with auto parts suppliers to find drills on various diesel engine parts (from gearbox to hydraulic) The best way to cut the cross hole. “Our customers need to cut a large number of aluminum alloy and gray cast iron die castings, and burrs are one of the main problems we encountered, especially when machining aluminum die castings. In hydraulic systems, the cleanliness of the holes is critical. Any chips or burrs on the workpiece may enter the hydraulic system and damage the hydraulic or hydraulic pump."

One possible solution is to perform an effective cutting edge preparation for the drill bit and adjust the feed rate and cutting speed as appropriate. Habben said, “In this case, we usually recommend trimming the drill bit by grinding a 45° chamfer on the outer edge of the drill bit, with a slight edge preparation, for example, at the cutting edge. A small T-shaped rib is prepared on it, or a passivation of about 0.08-0.10 mm is performed. It is especially important to use sharp tools when drilling cross holes."

Matt Sippel, senior product applications engineer at Melin Tooling, agrees: “To optimize cross-hole drilling, we have been doing an important job of using a special blade on our drills with coolant. The tip is usually slightly flat (such as a drill tip angle of about 140°), and the peripheral edge is slightly chamfered, and sometimes the edge is prepared. In the aerospace manufacturing industry, this method is tried and tested, it can be drilled in the cross hole. The drill tip protects the drill tip."

But Habben says this is only part of optimizing cross-hole drilling. “When the drill tip is drilled through the cross hole, a rule of thumb should be followed: reduce the feed rate by approximately 50%. This helps reduce the number of burrs and also reduces the deflection of the drill bit. The current back hole is eccentric to each other. This is especially true."

In addition, the drilling sequence must also be considered. Habben said, “Many valves are machined with orifice tools to machine the seat and then drill the small cross holes in the hydraulic system, which are often easily eccentric. In this case, it is important to plan the machining sequence correctly. For example, you should first drill a small diameter hole in the hydraulic system and then machine the seat with an orifice tool, as larger diameter tools are usually shorter in length and provide better control of flexural deformation."

However, if the drill itself has poor performance, finer edge preparation will not help. When the cross hole is drilled, great cutting force may be generated, and the flexural deformation of the drill bit may cause a big problem. Al Zaitoon, sales and marketing manager at YG-1 Tool Company, explains that indexable inserts, which are widely used in most machine shops, are not suitable for drilling cross holes.

According to Zaitoon, “When the cross hole is drilled, some kind of support needs to be provided to keep the drill bit stable. Unlike the other drill bits, the blade drill has no blade behind the cutting edge to keep the drill bit stable.”

Zaitoon describes the typical condition of cross-hole drilling: “For example, you drilled a 9.5mm cross hole with a 12.7mm solid carbide drill and then drilled another hole into the hole. There is a length of 50mm or 75mm, so the drill can be stabilized when drilling from the previous hole into the latter hole. This is difficult to achieve with flat or blade drills because they are easy to swing. ."

Subie Electric's Habben also believes that blade drills are not suitable for cross hole drilling. Solid carbide drills and replaceable drill bits are recommended. “Blade drills do not perform well in cross-hole drilling because they have only one effective cutting edge and are easily deflected. For solid cross-bores with a diameter of less than 12.7 mm, the solid carbide drill is the only option,” he said. "But he also pointed out that for larger diameter cross holes, the cost of the tool needs to be considered. The cost of the drillable drill bit is only about half that of the solid carbide drill, and the machining accuracy is also good. “Many cross holes are deep holes, and the hole depth may reach 12-20 times the hole diameter. When the hole size ranges from 3.2-25.4 mm or more, such a long solid carbide drill bit may be quite expensive, large diameter drill bit In particular, each bit can cost between $500 and $800."

According to Habben, the geometry of the replaceable drill bit is essentially the same as that of a solid carbide drill. It has two integrated cutting edges. The drill tip is screwed to the serrated joint of the shank. This connection has a high positioning accuracy and transmission strength.

The use of advanced tool coatings and large amounts of coolant also helps to optimize machining due to the large amount of cutting heat that can be generated when drilling cross holes. Habben said, “Coating is very beneficial because in cross-drilling, there is a lot of time to reduce the feed rate, which means poor chipping performance.” He pointed out that Sumitomo Electric offers a thickness of only a few microns. TiAlCr coating. This coating is prepared using nano-coating technology, which means that the coating is very thin and very smooth, although its cost is much lower than that of the diamond coating, but the tool life is comparable to that of the diamond coating. ”

Habben believes that chip removal is critical. “In order to flush away the chips, high pressure coolants with a pressure greater than 600 psi (pounds per square inch) should be used. When you drill cross holes with large diameters (eg 9.5 mm or more), you must use a coolant flow that increases the flow rate. Cooling pump. When drilling a hole with a diameter of 12.7mm, the coolant flow should reach 5-10gpm (gallons per minute); when drilling a hole with a larger diameter, the coolant flow rate should be 15-20gpm. Provides the flow required to compensate for coolant loss when drilling through the cross hole."
Habben finally pointed out that mechanics tend to ignore one thing - the drill diameter error (TIR) ​​should be less than 0.025mm, preferably better than 0.013mm. Reducing the run-off error is a key element in improving tool life and drilling accuracy. For many processing plants, this sounds like a demanding one, but Habben believes that this can be easily achieved with high-quality hydraulic tool chucks or hot-mounted tool chucks.

Rob Brown, product manager at United Mechanical Engineering (AMEC), agrees with the above recommendations. He said, “The conclusion that the blade drill is not suitable for machining the cross hole is absolutely correct. There is no blade on the tool and there is only one effective cutting edge. However, the flat drill still has a tape with a length equal to the size of the blade itself. It provides limited support, but flat drills are still much more stable than blade drills.” Brown does not recommend a flat drill with a guiding structure on the drill body if the machining requirements are not fully met. To improve the stability of the drill bit when it breaks through the cross hole.

Brown said that, to a large extent, flat drills are only suitable for drilling holes with a diameter of 19 mm and larger. “It depends on the processing conditions, but we usually also consider using flat drills, because in cross hole drilling, Most of the holes are deep. Flat drills are more cost effective than indexable inserts or solid carbide drills. We can supply high speed steel and carbide flat drill bits. Need to withstand a lot of impact, or the workpiece material is inconsistent, we recommend the use of high-speed steel cutter head, because it absorbs the impact better than the carbide cutter head. However, if the processing equipment in the workshop is very rigid and production If the efficiency requirements are high, the carbide tip should be used."

Like its competitors, Brown also believes that cutting edge preparation is very important. “We grind a chamfer on the outer edge of the belt, called SK2 trimming. This will help improve the stability of the drill while extending tool life and reducing burr formation. For cross hole drilling, success The secret is to keep the tool stable."

Tool manufacturers do not have any controversy about feed rates and cutting speeds. Brown said, “When drilling a cross hole, you must use the recommended feed rate and cutting speed. If the hole is deep, you need to reduce the feed rate and speed. We tell the user to reduce the feed when drilling the cross hole. Rate is a rule that must be followed. The problem is that doing so can affect the formation of chips, but it helps the drill to smoothly transition to the other end of the cross hole. Once the drill bit is fully drilled into the back hole, the normal feed rate can be restored."

The intermittent nature of cross-hole drilling presents another challenge. Nika Alex, a product expert at Mitsubishi Materials USA, said, “From a programming point of view, cross-hole drilling is quite difficult. Because of the need to drill and drill the workpiece, you must increase and decrease the cutting speed accordingly. Also, due to the large Some of the cross holes are deeper, so the drill used is also very long, which usually means first drilling a front start hole with a shorter drill bit, and then replacing a long drill bit to drill the back hole. This means The drill bit may cause vibration and deflection before it is fully drilled into the rear hole - if this happens, the cutting speed should be reduced immediately."

Like other peers, Alex also recommends reducing the feed rate by at least 1/2 when drilling through the cross holes. “Once the crossover is passed, the feed rate can be increased until it is machined to the far end of the cross hole. I recommend that the feed rate be reduced at the intersection to prevent the bit from shifting. Then, once the drill angle is drilled When you drill into the rear hole completely, you must increase the feed rate immediately to avoid chip control problems."

Hans Liechti, manager of the tool division at Mikron, has a different opinion. Although he also acknowledged that cross-hole drilling is quite complicated, he believes that there should be no problem in calculating the correct tool path using CAM software. “This was a big problem in the past, but now it’s just a piece of cake.” His department specializes in the production of hole-machining tools with a diameter of less than 6 mm, including various solid carbide drills, internal cooling drills, and custom step drills and reamers. And orifice machining tools.

If asked Liichti which tool is best for drilling cross holes, his answer may be "crazy": "Our standard tool line is called Crazy Drill. We have developed a special drill for a user. When he came to the cutting demonstration, the drill's machining performance exceeded its expectations by twice. He said that these tools cut like crazy." Although the name is a bit weird, the Crazy drill has the need for cross-hole drilling. All the necessary conditions - 140 ° drill angle, double-edged sipe, internal cooling channels and solid carbide body. “When using these tools, you can use large cutting parameters with a feed rate up to 5 times the normal value. Then reduce the feed rate when drilling through the cross holes.”

Liechti believes that the biggest problem with cross-hole drilling is that the front and rear holes are severely eccentric. “All of our drills are made of hard alloy and therefore have excellent rigidity, but if the two holes are eccentric with each other, the tool has a risk of breaking. In this case, we recommend almost Close to the larger drill angle of the end mill."

Although tool manufacturers compete with each other, there is still a lot of consensus on how to optimize cross-hole drilling, and most tool manufacturers agree with the following recommendations:

1. High quality solid carbide drills should be purchased and used, followed by interchangeable drill or flat drills. However, blade drills should not be used.

2. To help chip removal, reduce cutting heat and lubricate the tool, use high pressure, high flow coolant.

3. Understand the two-step machining programming method for the cross hole, adjust the feed rate and cutting speed to maintain the straightness of the hole and avoid damage to the tool.

4. Passivate the cutting edge on the outer edge of the drill bit to reduce burr formation and extend tool life.

5. Adopt high-rigidity processing equipment and processing sequence to minimize the deflection of the tool.

Drilling cross holes is a challenging job, and these machining techniques can guide you in the right direction.

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