JPS6224205B2 - - Google Patents

Description

【発明の詳細な説明】 この発明は、鋼板等の金属板を重ね合わせた状
態で、２枚以上の金属板に対し穿孔作業を継続し
て行なえる金属板の重ね穿孔用コアドリルに関す
るものである。DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a core drill for overlapping drilling of metal plates, which can continuously perform drilling operations on two or more metal plates, such as steel plates, in an overlapping state. .

最近、金属板に対し大口径孔を深く穿孔可能な
コアドリルとして本出願人に係る特開昭57−
96710号公報に記載のコアドリルが提案されてい
るが、このコアドリルは、第１図に示すように、
各刃片６の下端面を内側より外側下向きに傾斜さ
せた構造からなるので、単一の金属板に対する穿
孔作業には非常に優れているが、重ね合わせた金
属板に穿孔する場合、一枚目の金属板Ａの穿孔作
業が終了した際に、切抜廃板Ｂの下端周縁につば
状のバリB′が生じ、このバリB′がコアドリルの各
刃６ａ，６ｂを被つた状態でコアドリルと共に回
転するため、次の金属板Ａに対する穿孔作業が阻
害される。従つて、一旦穿孔作業を中止してコア
ドリルから切抜廃板Ｂを除去した後、再び穿孔作
業を開始しなければならず、作業上不便で作業能
率も悪いものであつた。なお、上記先願のコアド
リルは第２図に示すように、各刃片６の前面を、
穿孔用円筒体１の中心から各刃片６の前面内端へ
引いた直線ｌに一致させた構造になつている。 Recently, as a core drill that can deeply drill large diameter holes in metal plates, Japanese Patent Application Laid-Open No.
A core drill described in Publication No. 96710 has been proposed, but as shown in Figure 1, this core drill has the following features:
Since the lower end surface of each blade piece 6 is slanted downward from the inside to the outside, it is very suitable for drilling into a single metal plate, but when drilling into stacked metal plates, it is difficult to When the drilling work of the metal plate A is completed, a flange-shaped burr B' is generated on the lower edge of the cut-out waste plate B, and this burr B' covers each blade 6a, 6b of the core drill together with the core drill. Due to the rotation, the next drilling operation for the metal plate A is obstructed. Therefore, it is necessary to once stop the drilling operation and remove the cut-out waste plate B from the core drill, and then restart the drilling operation, which is inconvenient and has poor working efficiency. In addition, as shown in FIG. 2, in the core drill of the earlier application, the front surface of each blade piece 6 is
The structure is made to coincide with a straight line l drawn from the center of the cylindrical body 1 for drilling to the inner front end of each blade piece 6.

この発明は上述の点に鑑みなされたもので、特
に２枚以上の金属板の重ね合わせた状態での大口
径孔の穿孔に際し、刃先部に切屑が停溜すること
なくスムーズに排出され、各金属板に対する連続
的な穿孔作業が可能で、しかも生産性に優れ、耐
久性にも富むコアドリルを提供することを目的と
している。 This invention was made in view of the above points, and is particularly useful when drilling large-diameter holes in two or more metal plates stacked one on top of the other. The purpose of the present invention is to provide a core drill that is capable of continuous drilling work on metal plates, has excellent productivity, and is highly durable.

以下、この発明の実施例を図面に基いて説明す
る。 Embodiments of the present invention will be described below with reference to the drawings.

第３図〜第８図において、１は下端を開口した
穿孔用円筒体で、その上端にはシヤンク２接続用
のねじ孔３が貫通して穿設されている。また、円
筒体１の下部周壁１ｂの外面を、上部周壁１ａの
外面より半径方向へやや膨出させて、その厚みを
上部周壁１ａより厚く、かつその厚みが下方に向
つて途中より漸次やや増大するように下方に向け
てスカート状に拡げている。 In FIGS. 3 to 8, reference numeral 1 denotes a cylindrical body for drilling with an open lower end, and a screw hole 3 for connecting a shank 2 is penetrated through the upper end of the cylindrical body. Further, the outer surface of the lower peripheral wall 1b of the cylindrical body 1 is slightly bulged in the radial direction than the outer surface of the upper peripheral wall 1a, so that its thickness is thicker than that of the upper peripheral wall 1a, and the thickness gradually increases slightly from the middle toward the bottom. It expands downward like a skirt.

更に、下部周壁１ｂの外周に、切屑排出溝４を
等間隔に設けて下部周壁１ｂの各排出溝４間を刃
体部５に構成するが、排出溝４の深さはその溝底
が上部周壁１ａの外面と一致するようにし、ま
た、排出溝４の後壁縁を下端に向け回転方向へ前
進傾斜させて溝の巾員を下端に向けて漸次狭くな
るようにする。なお、切屑排出溝４は下部周壁１
ｂの外周に不等間隔に形成する場合がある。 Further, chip discharge grooves 4 are provided at equal intervals on the outer periphery of the lower peripheral wall 1b, and a blade part 5 is formed between each of the discharge grooves 4 of the lower peripheral wall 1b. It is made to coincide with the outer surface of the peripheral wall 1a, and the rear wall edge of the discharge groove 4 is tilted forward in the direction of rotation toward the lower end so that the width of the groove becomes gradually narrower toward the lower end. Note that the chip discharge groove 4 is located on the lower peripheral wall 1.
They may be formed at irregular intervals around the outer circumference of b.

そして、刃体部５は、その下端面を回転方向へ
下向きに突出させて、回転方向下向きに傾斜する
前縁との交叉により下端隅角を鋭角状に形成し、
超硬合金の刃片６を刃体部５の前縁に接して下端
隅角部に埋設するが、各刃片６の前面は第７図に
示すように円筒体１の中心から刃片６の前面内端
に引いた直線ｌに対して反回転方向へ外向きに傾
斜角θで傾斜させる。 The blade body portion 5 has its lower end surface protruding downward in the rotational direction, and forms an acute lower end angle by intersecting with a front edge that is inclined downward in the rotational direction.
The blade pieces 6 made of cemented carbide are embedded in the lower end corners in contact with the front edge of the blade body part 5, and the front surface of each blade piece 6 extends from the center of the cylindrical body 1 to the blade piece 6 as shown in FIG. It is tilted outward in the counter-rotational direction at an inclination angle θ with respect to a straight line l drawn at the inner end of the front surface.

また、各刃片６を交互に内刃６ａと外刃６ｂに
構成し、内刃６ａは刃片６の下端面外側寄りを内
側下向きに傾斜角αで削除し、刃片６の下端面内
側寄りにおいて内側下向きに傾斜角γで削除する
と共に、外側下向きに傾斜角δで削除して刃片６
厚の略半分に相当する刃幅Waをもつ下向山形に
削成され、外刃６ｂは刃片６の下端面外側寄りに
おいても内側下向きに内刃６ａの傾斜角αよりも
小さい傾斜角β＝γで削除して刃片６厚の略半分
に相当する刃幅Wbをもち、刃片６の内面と鈍角
で交叉する内側下向きの傾斜刃に削成されると共
に、刃片６の内側寄りを前記下向山形の内刃６ａ
が下方に突出するように水平に削除する。更に、
各刃片６前方の排出溝４の下端部をその外面より
内方下向きに傾斜させて削除することにより面取
りして各刃６ａ，６ｂ先端を排出溝４下端より下
方に突出させる。すなわち、各刃６ａ，６ｂ前方
の排出溝４下端部は、その壁厚が下向きに漸次薄
く形成され、排出溝４下端より外側上向きに傾斜
したガイド面をなすので、各刃６ａ，６ｂによつ
て切削された切屑は、それぞれ直前方の排出溝４
下端部の傾斜したガイド（面取り）面に沿つて排
出溝４上方へ導かれる。 In addition, each blade piece 6 is configured to alternately have an inner blade 6a and an outer blade 6b, and the inner blade 6a has the lower end surface of the blade piece 6 on the outside side removed inwardly downward at an inclination angle α, and the lower end face of the blade piece 6 on the inside side. At the approach, it is removed inwardly downward at an inclination angle γ, and outwardly downwardly at an inclination angle δ to form the blade piece 6.
The outer cutter 6b is cut into a downward chevron shape with a blade width Wa corresponding to approximately half of the thickness, and the outer cutter 6b is inclined downwardly inward even on the outer side of the lower end surface of the blade piece 6 at an inclination angle β smaller than the inclination angle α of the inner cutter 6a. = γ and has a blade width Wb equivalent to approximately half the thickness of the blade piece 6, is cut into an inwardly downwardly inclined blade that intersects the inner surface of the blade piece 6 at an obtuse angle, and is also cut toward the inside of the blade piece 6. The downward chevron-shaped inner blade 6a
Remove horizontally so that it protrudes downward. Furthermore,
The lower end of the discharge groove 4 in front of each blade piece 6 is beveled by slanting downward inward from its outer surface and being chamfered so that the tip of each blade 6a, 6b projects downward from the lower end of the discharge groove 4. That is, the wall thickness of the lower end of the discharge groove 4 in front of each blade 6a, 6b is formed to be gradually thinner downward, and forms a guide surface that is inclined outward and upward from the lower end of the discharge groove 4, so that the lower end of the discharge groove 4 in front of each blade 6a, 6b is The cutting chips are removed from the discharge groove 4 in front of each
It is guided upward into the discharge groove 4 along the inclined guide (chamfered) surface of the lower end.

なお、外刃６ｂおよび内刃６ａは、第５ａ図お
よび第５ｂ図に示すように、内刃６ａが外刃６ｂ
側の刃片内側寄り下端面より下方に突出すると共
に、外刃６ｂが内刃６ａ側の刃片外側寄り下端面
より下方に突出し、また内刃６ａの刃幅Waと外
刃６ｂの刃幅Wbとは等しい。 Note that the outer cutter 6b and the inner cutter 6a are such that, as shown in FIGS. 5a and 5b, the inner cutter 6a is the outer cutter 6b.
The outer cutter 6b protrudes downward from the lower end surface of the outer cutter on the inner cutter 6a side, and the outer cutter 6b protrudes downward from the lower end surface of the outer cutter on the inner cutter 6a side. Equal to Wb.

第８図は、前記穿孔用円筒体１をセンターピン
７付きシヤンク２に接続した状態を示し、第８図
において、穿孔位置決め用センターピン７の基部
が、シヤンク２の中心部に穿設した縦長孔８に挿
入され、またこの縦長孔８内にはセンターピン７
を２段階に先端方向へ附勢するための一組のスプ
リング９，１０が装填されている。 FIG. 8 shows a state in which the cylindrical body 1 for drilling is connected to a shank 2 with a center pin 7. In FIG. The center pin 7 is inserted into the hole 8, and the center pin 7 is inserted into the vertically elongated hole 8.
A pair of springs 9 and 10 are loaded to bias the tip in two stages toward the distal end.

１１は給油装置で、この装置１１によりシヤン
ク２の縦長孔８に油を供給し、センターピン７を
介して各刃片６に給油される。 Reference numeral 11 denotes an oil supply device, and this device 11 supplies oil to the longitudinal hole 8 of the shank 2, and the oil is supplied to each blade piece 6 via the center pin 7.

次に、上記実施例のコアドリルにより重ね合わ
せた金属板に穿孔する際の作用を説明する。 Next, an explanation will be given of the operation when drilling holes in stacked metal plates using the core drill of the above embodiment.

第６図および第８図において、シヤンク２を回
転電動機（図示せず）に取り付けてコアドリルを
回転させながら、センターピン７の先端を一枚目
の金属板Ａの穿孔中心位置に押当て位置決めす
る。それから、コアドリル全体を金属板Ａ側へ押
付けると、センターピン７はスプリング９，１０
に抗して円筒体１内に引込み、次に内刃６ａが金
属板Ａに切込み、引き続いて外刃６ｂが金属板Ａ
に切込み、大口径孔の穿孔作業が開始される。こ
の時、前記給油装置１１により給油も開始する。 In FIGS. 6 and 8, the shank 2 is attached to a rotating electric motor (not shown), and while rotating the core drill, the tip of the center pin 7 is pressed and positioned at the center of the hole in the first metal plate A. . Then, when the entire core drill is pressed against the metal plate A side, the center pin 7 is moved by the springs 9 and 10.
The inner cutter 6a cuts into the metal plate A, and then the outer cutter 6b cuts into the metal plate A.
A cut is made into the hole, and drilling work for a large diameter hole begins. At this time, the refueling device 11 also starts refueling.

ここで、内刃６ａと外刃６ｂによる穿孔態様を
詳しく説明すると、前記したように、内刃６ａと
外刃６ｂにより穿孔作業がなされ、内刃６ａの刃
幅Waの切屑とこの切屑とは独立した外刃６ｂの
刃幅Wbの切屑が連続して生じる。 Here, to explain in detail the drilling mode by the inner cutter 6a and the outer cutter 6b, as described above, the drilling work is performed by the inner cutter 6a and the outer cutter 6b, and the chips of the blade width Wa of the inner cutter 6a and this chip are Chips having the blade width Wb of the independent outer cutter 6b are continuously generated.

ところで、外刃６ｂの下端面は、第５ａ図に示
すように外向きに傾斜しているが、外刃６ｂの前
面が第７図に示すように外向きに傾斜させてある
ので、まず外刃６ｂの最前方位置にある最下端部
より金属板Ａに対する切込みが開始され、外刃６
ｂにより削成された切屑は上向きの力若しくは上
向きの力と外向きの力を受けて外刃６ｂ前方の排
出溝４の外面と金属板Ａの穿孔外側壁面との隙間
を上方に排出され、また、内刃６ａの下端面は第
５ｂ図に示すように下向きの山形をなし、かつ内
刃６ａの前面も第７図に示すように外向きの傾斜
させてあるので、内刃６ａにより削成された切屑
は上向きの力と外向きの力を受けて内刃６ａ前方
の排出溝４の外面と金属板Ａの穿孔内側壁面との
隙間を上方に排出される。 Incidentally, the lower end surface of the outer cutter 6b is inclined outward as shown in FIG. 5a, but since the front surface of the outer cutter 6b is inclined outward as shown in FIG. Cutting into the metal plate A starts from the lowermost end at the forwardmost position of the blade 6b, and the outer blade 6
The chips cut by b are discharged upward through the gap between the outer surface of the discharge groove 4 in front of the outer blade 6b and the outer wall surface of the perforation of the metal plate A under the influence of an upward force or an upward force and an outward force, Furthermore, the lower end surface of the inner cutter 6a has a downward mountain shape as shown in FIG. 5b, and the front surface of the inner cutter 6a is also inclined outward as shown in FIG. The formed chips are discharged upward through the gap between the outer surface of the discharge groove 4 in front of the inner cutter 6a and the inner wall surface of the perforation of the metal plate A under the influence of upward and outward forces.

なお、外刃６ｂ下端面の水平面に対する傾斜角
βおよび内刃６ａの下端面の水平面に対する傾斜
角γと外刃６ｂ前面の外刃６ｂ内端と円筒体１の
中心を結ぶ直線ｌおよび内刃６ａの前面の内刃６
ａ内端と円筒体１の中心を結ぶ直線ｌに対する傾
斜角θとは、特に限定するものではないが、傾斜
角βおよびγが傾斜角θに比べて大きくなり過ぎ
ると、外刃６ｂにより削成される切屑が内向きの
力を受けて円筒体１の内側へ入り込もうとして穿
孔作業を阻害するおそれがあるので、この点を考
慮して傾斜角βと傾斜角θを設定する必要があ
る。 In addition, the inclination angle β of the lower end surface of the outer cutter 6b with respect to the horizontal plane, the inclination angle γ of the lower end surface of the inner cutter 6a with respect to the horizontal plane, the straight line l connecting the inner end of the outer cutter 6b on the front surface of the outer cutter 6b and the center of the cylindrical body 1, and the inner cutter Inner blade 6 on the front of 6a
The inclination angle θ with respect to the straight line l connecting the inner end a and the center of the cylindrical body 1 is not particularly limited, but if the inclination angles β and γ become too large compared to the inclination angle θ, the outer cutter 6b will cut the inclination angle θ. Since there is a risk that the chips formed will receive an inward force and try to enter the inside of the cylindrical body 1 and impede the drilling operation, it is necessary to take this into consideration when setting the inclination angle β and the inclination angle θ. .

このようにして、一枚目の金属板Ａに対する穿
孔作業が終了し、第６図に示すように下端周縁に
バリがほとんどない略円柱状の廃板Ｂが切抜か
れ、この切抜廃板Ｂは円筒体１内でコアドリルと
共に回転するが、各刃６ａ，６ｂの穿孔作業が廃
板Ｂにより阻害されないので、次の金属板Ａに対
する穿孔作業が継続可能で、穿孔作業を継続する
ことにより二枚乃至それ以上の重ね合わせた金属
板Ａに対する穿孔作業が連続して行なえ、目的の
大口径孔を穿設できるものである。 In this way, the drilling work for the first metal plate A is completed, and as shown in FIG. Although it rotates together with the core drill inside the cylindrical body 1, the drilling work of each blade 6a, 6b is not obstructed by the waste plate B, so the drilling work for the next metal plate A can be continued, and by continuing the drilling work, two sheets Drilling operations can be performed continuously on one or more stacked metal plates A, and a desired large-diameter hole can be drilled.

以上説明したように、この発明の金属板重ね穿
孔用コアドリルは、例えば、鉄柱に対しＩ型鋼の
桁材を接合してボルト止めする際、両者にボルト
孔を一連に穿設する場合や、熱交換器内の複数の
じやま板に一連に貫通孔を穿設する場合に、各金
属板を重ね合わせた状態で穿孔作業を行なつて
も、本発明の内刃は第６図に示すように刃片厚の
略半分に相当する刃幅をもつ下向山形刃からなる
から、前記先行のコアドリルのように、一枚目の
金属板に対する穿孔作業の終了後に各刃先（特に
内刃）が金属板から切抜かれた廃板によつて阻害
されることがないので、コアドリルの有効深さの
範囲内で連続的に穿孔作業を継続して複数枚の金
属板に一連に連続した目的の大口径孔を穿孔で
き、これにより穿孔作業の能率向上が図れると共
に、各金属板を重ね合わせた状態で一連に穿孔で
きるため、各金属板（特に、二枚目以降）の所定
位置に正確に穿孔することができる。しかも、本
発明は外刃も内刃と同様に刃片厚の略半分に相当
する刃幅にし、各刃片前方の排出溝下端部をその
外面より内方下向きに傾斜させて削除することに
より面取りして内刃および外刃の先端部を排出溝
下端より下方に突出させ、また内刃側刃片の下端
面外側寄りを外刃が下方に突出するように内側下
向きに削除し、外刃側刃片の下端面内側寄りを内
刃が下方に突出するように水平に削除して前記先
行のコアドリルの特徴部分の基本的構成を踏襲し
たから、切屑の排出がスムーズで穿孔効率が終始
変わらず一定して仕上がりがよく、また刃先の形
成作業における刃片の削成部分が少なくて生産性
に優れ、刃部の耐久性が高い等の効果を奏する。 As explained above, the core drill for overlapping metal plate drilling of the present invention can be used, for example, when joining and bolting an I-type steel girder to a steel column, when drilling a series of bolt holes in both, or when When drilling a series of through holes in a plurality of metal plates in an exchanger, even if the drilling work is performed with the metal plates stacked one on top of the other, the inner cutter of the present invention can be used as shown in Fig. 6. Since it consists of a downward chevron-shaped blade with a blade width equivalent to approximately half of the blade thickness, each cutting edge (especially the inner blade) is Since it is not obstructed by the waste material cut out from the metal sheet, the drilling operation can be continued continuously within the effective depth of the core drill to drill the target size in series on several metal sheets. Diameter holes can be drilled, which improves the efficiency of drilling work, and since each metal plate can be drilled in series with each metal plate stacked on top of the other, each metal plate (especially after the second one) can be drilled accurately at the specified position. can do. Moreover, in the present invention, the outer cutter has a blade width equivalent to approximately half of the thickness of the blade, and the lower end of the discharge groove in front of each blade is removed by slanting it downward inward from the outer surface. The tips of the inner and outer cutters are chamfered so that they protrude downward from the lower end of the ejection groove, and the outer side of the lower end of the inner cutter side cutter is removed inward and downward so that the outer cutter protrudes downward. The inner side of the lower end surface of the side cutting piece is removed horizontally so that the inner cutting edge protrudes downward, following the basic structure of the previous core drill, so chips are discharged smoothly and drilling efficiency is improved from start to finish. The blade has a consistently good finish, and there are few parts to be ground on the blade during the cutting edge forming process, resulting in excellent productivity and high durability of the blade.

【図面の簡単な説明】[Brief explanation of the drawing]

第１図は先願のコアドリルによつて金属板に穿
孔する態様を示す概要断面図、第２図は一部を省
略した同概要底面図、第３図〜第８図はこの発明
のコアドリルの実施例を示し、第３図は正面図、
第４図は底面図、第５ａ図は外刃の断面図、第５
ｂ図は内刃の断面図、第６図は金属板に穿孔する
態様を示す概要断面図、第７図は一部を省略した
同概要底面図、第８図はシヤンクを接続した状態
を示す半断面正面図である。 １……穿孔用円筒体、１ａ……上部周壁、１ｂ
……下部周壁、２……シヤンク、３……ねじ孔、
４……切屑排出溝、５……刃体部、６……刃片、
６ａ……内刃、６ｂ……外刃、７……センターピ
ン、８……縦長孔、９，１０……スプリング、１
１……給油装置、Ａ……金属板、Ｂ……（切抜）
廃板、B′……バリ。 FIG. 1 is a schematic cross-sectional view showing the manner in which a metal plate is drilled using the core drill of the prior application, FIG. 2 is a schematic bottom view of the same with a part omitted, and FIGS. 3 to 8 show the core drill of the present invention. An example is shown, FIG. 3 is a front view,
Figure 4 is a bottom view, Figure 5a is a sectional view of the outer cutter, Figure 5
Figure b is a sectional view of the inner blade, Figure 6 is a schematic sectional view showing how to drill into a metal plate, Figure 7 is a schematic bottom view of the same with some parts omitted, and Figure 8 shows the state in which the shank is connected. It is a half-sectional front view. 1... Cylindrical body for drilling, 1a... Upper peripheral wall, 1b
... lower peripheral wall, 2 ... shank, 3 ... screw hole,
4...Chip discharge groove, 5...Blade body portion, 6...Blade piece,
6a...Inner blade, 6b...Outer blade, 7...Center pin, 8...Vertical hole, 9, 10...Spring, 1
1... Oil supply device, A... Metal plate, B... (cutting)
Scrap board, B′...burr.

Claims (1)

【特許請求の範囲】[Claims] １ 上端にシヤンクを有し、下端を開口した穿孔
用円筒体の周壁外周に、溝幅を下向けに漸次狭め
た複数の切屑排出溝を間隔をあけて設け、該排出
溝間における円筒体周壁により形成される刃体部
は、下端面を回転方向へ下向きに突出させて回転
方向下向きに傾斜する前縁との交叉により下端隅
角が鋭角状に形成され、各刃体部前縁に接して下
端隅角部に刃片を埋設し、各刃片前面を円筒体中
心から各刃片前面内端に引いた直線に対し反回転
方向へ外向きに傾斜させ、各刃片を交互に内刃と
外刃に構成し、内刃は刃片の下端面内側寄りに刃
片厚の略半分に相当する刃幅をもつ下向山形刃か
らなり、外刃は刃片の下端面外側寄りに刃片厚の
略半分に相当する刃幅をもち刃片内面と鈍角で交
叉する内側下向きの傾斜刃からなると共に、内刃
側刃片の下端面外側寄りを外刃が下方に突出する
ように内側下向きに削除し、外刃側刃片の下端面
内側寄りを内刃が下方に突出するように水平に削
除し、各刃片前方の排出溝下端部をその外面より
内方下向きに傾斜させて削除することにより面取
りして内刃および外刃の先端部を排出溝下端より
下方に突出させたことを特徴とする金属板の重ね
穿孔用コアドリル。1 A plurality of chip discharge grooves whose groove widths are gradually narrowed downward are provided at intervals on the outer periphery of the peripheral wall of a cylindrical body for drilling that has a shank at the upper end and is open at the lower end, and the peripheral wall of the cylindrical body between the discharge grooves. The blade body formed by the blade body has a lower end surface that projects downward in the rotational direction, and the lower end angle is formed into an acute angle by intersecting with the front edge that slopes downward in the rotational direction. bury the blade pieces in the lower corners, tilt the front surface of each blade outward in the counter-rotation direction with respect to a straight line drawn from the center of the cylinder to the front inner edge of each blade, and insert each blade piece inward alternately. The inner cutter consists of a downward chevron-shaped blade with a blade width equivalent to approximately half of the thickness of the blade, located closer to the inside of the lower end of the blade, and the outer blade is located closer to the outside of the lower end of the blade. It has a blade width equivalent to approximately half the thickness of the blade piece, and consists of an inwardly downwardly inclined blade that intersects the inner surface of the blade piece at an obtuse angle, and the outer blade protrudes downward from the outside of the lower end surface of the inner blade side blade piece. The inner edge of the lower end of the outer blade is removed horizontally so that the inner blade protrudes downward, and the lower end of the discharge groove in front of each blade is slanted inward and downward from its outer surface. A core drill for overlapping drilling of metal plates, characterized in that the tip portions of the inner and outer cutters are chamfered by removing the groove so that they protrude downward from the lower end of the discharge groove.