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A carbide drill is a drilling tool made from carbide alloy and can be used for cutting operations on a wide range of materials, from carbon steel to heat-resistant alloys. Carbide alloy is characterized by its high hardness and the fact that its hardness decreases only slightly even at high temperatures. It is a material made by mixing tungsten carbide and cobalt, which acts as a binder, and adding titanium carbide among others to suit specific purposes. This article explains the advantages and disadvantages of carbide drills, as well as points to consider when selecting them.
Characteristics of Carbide Drills Compared to High-Speed Steel (HSS) Drills
In metalworking, the representative types of drill materials include carbide drills and high-speed steel (HSS) drills. Selection needs to be based on the requirements to be met, such as machining time and cost. The advantages and disadvantages of choosing carbide drills include the following:
Advantages of Carbide Drills
The advantages of carbide drills include excellent wear resistance and suitability for high-speed cutting.
Excellent Wear Resistance
As for the material of the drill, there are high-speed steel (HSS) drills made from steel alloyed with elements such as tungsten, chromium, and cobalt. However, carbide exhibits superior wear resistance compared to HSS. When used as a drill, a significant advantage of carbide drills over HSS drills is their longer tool life.
Suitable for High-Speed Cutting
Carbide has the characteristic of being less prone to hardness reduction at high temperatures, which means that even during cutting operations where the tool can become very hot due to high speeds, its performance is less likely to degrade. HSS can experience a reduction in hardness due to the high temperatures generated by high-speed cutting, making carbide drills more suitable for high-speed cutting applications.
Disadvantages of Carbide Drills
In contrast to the advantages of carbide drills mentioned so far, the lack of toughness and the significantly higher cost compared to HSS are disadvantages of carbide drills.
Lack of Toughness
A weakness of carbide drills is their low toughness. Low toughness means that the tool is more likely to break under bending or vibration, requiring proper management of chips which can cause these issues. On the other hand, the low toughness also means that the drill is less likely to bend during machining, allowing for high precision in the drilling process.
Expensive Compared to HSS
Carbide drills are more expensive, which is a disadvantage compared to HSS. However, due to their superior wear resistance compared to HSS, using carbide drills could potentially result in lower running costs under certain machining conditions.
Representative Types of Carbide Drills
The typical drills made from carbide material include solid drills and indexable insert drills.
Solid Drill
A solid drill has an integrated structure from the shank to the cutting edge, featuring a wide range of small diameters and capable of achieving high machining accuracy. As the cutting tool wears out and its performance decreases over time, it can be re-sharpened to restore its sharpness, which is a significant advantage in terms of long-term running costs.
Indexable Insert Drill
An indexable insert drill has a body and cutting edge that are separate, allowing the cutting edge part of the drill to be replaced. This means that when the cutting edge wears out, only the edge needs to be replaced, not the entire drill, which has advantages such as making it less likely to make mistakes in protrusion length. Since only the cutting edge is replaced, especially types that combine multiple tips are used for larger hole diameters, offering superior lineup and cost-effectiveness.
Selection of Carbide Drills
There are various types of carbide drills, and selecting one requires considering factors such as hole diameter, depth of machining, machining accuracy, and workpiece material.
Considerations for Diameter and Depth
When selecting carbide drills based on diameter and depth, the following points are important:
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Consider the Diameter
When performing drill machining, the dimension of the hole becomes slightly larger than the tool diameter due to mounting accuracy, bite, and vibrations that occur during machining. This is one point that needs attention when selecting a tool to achieve the targeted hole diameter.
Consider the Depth
For holes where the depth is more than ten times the diameter (L/D≧10), long drills are used. Long drills become more unstable the longer they are, increasing the risk of the drill breaking or the hole becoming misaligned during machining. Therefore, drills for guide hole machining are selected to improve the accuracy of hole positions.
Consider Accuracy (Tolerance)
When high precision is required, such as for fitting tolerances, finishing processes such as reaming or boring may be conducted after drilling. The diameter drilled needs to be slightly smaller than the target diameter to accommodate finishing, so care must be taken when selecting the drill diameter.
Checking Work Material
When selecting a carbide drill, it's necessary to consider the relationship with the workpiece material and whether coolant holes are required.
Select Tool Material According to Work Material
The material of the carbide drill needs to be chosen based on the workpiece material being machined. Tool manufacturers set recommendations based on the material to be cut, such as P-grade for carbon steel and N-grade for aluminum alloys, so selecting the tool material accordingly is effective.
Choose Tools with Coolant Holes
When machining materials like low carbon steel or aluminum alloys, chip breaking and evacuation can become issues. In such cases, choosing tools with coolant holes allows for machining while ejecting coolant, minimizing the impact of chips.
About Machining Conditions with Carbide Drills
When machining with carbide drills, it's necessary to set the machining conditions considering tool length and chip evacuation.
Match to Tool Length
The protrusion amount of the drill tool changes significantly depending on the depth of the hole being machined. As the protrusion amount increases, the impact of tool deflection also increases, necessitating the setting of machining conditions such as cutting speed and feed rate according to the tool length.
Consider Chip Evacuation
It's important to set machining conditions that allow for smooth evacuation of chips generated during cutting, as chips winding around the tool holder or elsewhere can have adverse effects, regardless of whether the chips are segmented.
Guidelines for Machining Conditions
The recommended cutting speeds and feed rates for carbide drills vary by the material being cut, as follows:
| Work Material | Cutting Speed | Feed per Revolution |
|---|---|---|
| Carbon Steel (S50C) | 120m/min | 0.2mm/rev |
| Stainless Steel (SUS430) | 80m/min | 0.15mm/rev |
| Titanium Alloy (Ti-6Al-4V) | 40m/min | 0.15mm/rev |
| Aluminum Alloy (A5052) | 200m/min | 0.10mm/rev |
When inputting cutting conditions such as "spindle speed" and "feed rate" into the NC program, please refer to the tables in the following catalogs for guidance:
- [Nachi] Aqua Drill EX Series Catalog (July 2022 Edition - CATALOG NO. 2292-5)https://www.nachi-fujikoshi.co.jp/web/pdf/2238-7.pdf
- [Nachi] DLC Drill & Mill Series Catalog (July 2022 Edition - CATALOG NO. 2238-7)https://www.nachi-fujikoshi.co.jp/web/pdf/2292-5.pdf
These resources can provide detailed recommendations for cutting speeds and feed rates based on the material being machined.
