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Description
M8 DIN 376 C machine tap – with straight flutes
Technical data and dimensions to DIN 376
| Technical data | |
|---|---|
| Thread | M8 x1.25 (metric coarse thread, right-hand) |
| Standard | DIN 376, reduced shank |
| Flute | straight |
| Chamfer | form C, 2–3 threads |
| Thread tolerance | 6H |
| Material | HSSE (HSS-Co5) |
| Overall length | 90 mm |
| Threaded section length | 22 mm |
| Shank diameter | 6 mm |
| Square drive | 4.9 mm |
| Tap drill | 6.8 mm |
| Manufacturer | Narex |
M8 machine tap The Narex straight flute tap cuts threads in cast iron, brass, bronze, and steel. Chips removed by the cutting edges remain in the flutes and leave the hole with the tap. A straight flute moves chips neither upward nor downward.
The form C chamfer ends 2–3 threads from the tip, allowing the tap to work in through holes and blind holes. Cobalt-alloyed HSSE steel and a 6H tolerance produce threads for M8 screws.
MaterialHSSE, 5% cobalt
Flutestraight
ChamferC, 2–3 threads
Tolerance6H
Where chips remain in a straight-flute tap
The flute holds the chips until the tap is reversed out
A straight flute runs parallel to the tap axis. It has no helix to carry chips upward. The chamfer cutting edges are not ground at an angle, so they do not push chips downward. Short chips break into fragments and remain in the flute next to the cutting edges that removed them. They leave the hole only when the tap is backed out.
1
Straight flute – chips remain in the flute, next to the cutting edges that produced them.
2
Form C chamfer: 2–3 incomplete threads on the tapered lead.
3
The tap rotates clockwise to cut a right-hand thread.
4
Individual fragments exit at the top, onto the workpiece surface.
5
A few fragments fall below the tip and exit through the bottom of the hole.
6
Feed 1.25 mm/rev, equal to the pitch of the M8 thread.
Straight flute – thick core and chip space
Straight flutes are shallower than spiral flutes, so the tap core is thicker. A thicker core withstands torque better, making the tap less likely to break during machine tapping. Each flute acts as a small chip pocket. It works best with short chips that break into fragments on their own, as in gray cast iron and brass.
Form C chamfer – full thread 2–3 threads from the tip
The chamfer is the tapered lead of the tap, which removes material gradually. Form C has 2–3 tapered threads, equivalent to 2.5–3.75 mm, since each thread corresponds to a pitch of 1.25 mm. In a blind hole, the full thread therefore ends close to the bottom. In a through hole, the chamfer only needs to extend approximately 4 mm below the workpiece.
DIN 376 shank and machine mounting
Reduced shank 6 mm with a 4.9 mm square drive
DIN 376 specifies machine taps with a reduced shank, meaning a shank smaller in diameter than the thread. For M8, the shank diameter is 6 mm and remains constant from the threaded section to the square, with no neck. The minimum hole diameter for an M8 6H thread is 6.647 mm, so the 6 mm shank fits inside the thread. The tap can pass through a hole longer than its 22 mm threaded section. The 4.9 mm square at the end of the shank transmits torque from the holder to the tap.
Mount the tap in a tapping chuck with a 4.9 mm square socket or in a tap collet. On a drill press or machine tool without rigid tapping, use a chuck with axial compensation. It compensates for the difference between the machine feed and the thread pitch, preventing the threads from being stretched or compressed.
▲
Clear the flutes when tapping deep threads
Chips remain in the flutes until you back the tap out. In a deep hole, tap halfway, back the tool out, remove the chips, and finish the thread. An overfilled flute jams the tap and can break it.
Cutting data and tap drill hole for M8
Cutting speed, spindle speed, and feed for HSSE steel
These values apply to HSSE taps. Spindle speed and feed rate were calculated for a diameter of 8 mm. Feed per revolution always equals the thread pitch, which is 1.25 mm. In a deep blind hole, reduce the speed by 20–30%.
| Workpiece material | Cutting speed [m/min] | Spindle speed [rpm] | Feed [mm/min] |
|---|---|---|---|
| PMedium-carbon steel 500–700 MPa, e.g., C45 | 8–12 | 320–480 | 400–600 |
| PAlloy steel 700–1000 MPa | 6–10 | 240–400 | 300–500 |
| KGray cast iron, e.g., EN-GJL-250 | 10–18 | 400–720 | 500–900 |
| KDuctile cast iron, e.g., EN-GJS-500 | 8–15 | 320–600 | 400–750 |
| NFree-machining brass and bronze | 20–30 | 800–1190 | 1000–1490 |
Spindle speed = 1000 × 12 m/min ÷ (3.14 × 8 mm) ≈ 477 rpm; feed = 477 rpm × 1.25 mm ≈ 597 mm/min
12 m/min is a cutting speed in the lower half of the range for gray cast iron. 8 mm is the tap diameter, and 1000 converts meters to millimeters. 1.25 mm is the pitch of the M8 thread – the distance the tap advances into the hole per revolution. Set approximately 480 rpm and a feed rate of approximately 600 mm/min.
M8 thread in a blind hole step by step
1Spot drilling
90° center drill
2Drilling
6.8 mm drill bit to a depth of 22 mm
3Tapping
feed 1.25 mm/rev, chips exit in the flutes
✓Inspection
blowing out the hole and checking with an M8 6H gauge
Depth 22 mm = full thread 16 mm + chamfer 3 threads × 1.25 mm + allowance 2 threads × 1.25 mm for the tip and chip fragments.
Which materials to choose straight flutes for
Cast iron, brass, bronze, and steel
HSSE is high-speed steel with approximately 5% cobalt. Cobalt helps the cutting edge retain its hardness at high cutting temperatures. Choose straight flutes where chips are short and break on their own: in gray and ductile cast iron, and in free-machining brass and bronze. In C45 steel and alloy steels, it taps through holes and shallow blind holes.
Gray cast iron can be tapped dry or with emulsion. In brass, bronze, and steel, lubricate generously with emulsion at a concentration of 5–10% or with tapping oil.
▲
Long chips clog straight flutes
Stainless steels 304 and 316, wrought aluminum, and soft low-carbon steel produce long, ductile chips. These wrap around in the flutes and jam the tap. For these materials, choose a spiral flute tap for blind holes or a tap with a form B through-hole chamfer for through holes.
M8 machine tap The Narex DIN 376 C tap retains chips in its straight flutes and removes them from the hole when backed out. The form C chamfer leaves the full thread 2–3 threads from the tip, while the thick core withstands torque. The 6H tolerance produces a thread that fits M8 screws with a 6g tolerance. The 6.8 mm drill bit and feed of 1.25 mm/rev remain constant for every material.
Questions about the M8 straight flute tap
Drill bit, chamfer, and hole type
A 6.8 mm drill bit. This is the thread diameter of 8 mm minus the pitch of 1.25 mm. A hole with a 6H tolerance can range from 6.647 to 6.912 mm, so a 6.8 mm drill bit falls in the middle of this tolerance zone.
C is the chamfer form, meaning the tapered lead. The form C chamfer spans 2–3 threads, equivalent to 2.5–3.75 mm, and allows tapping close to the bottom of a blind hole. The longer form B chamfer, spanning 3.5–5.5 threads, is used in through holes.
Straight flutes retain chips until the tap leaves the hole. Spiral flutes carry chips upward along the helix, while the chip breaker on a form B chamfer pushes them down the hole ahead of the tip. Straight flutes provide a thicker core than spiral flutes.
Yes. Chips remain in the flutes, while individual fragments fall to the bottom. Drill the hole deeper to accommodate the chamfer and an allowance: approximately 22 mm deep for a 16 mm thread. Blow out the hole after tapping.
DIN 376 has a shank smaller than the thread diameter — for M8, the shank is 6 mm with a 4.9 mm square. This shank passes through the tapped hole. DIN 371 has a reinforced shank: for M8, the shank is 8 mm, the same as the thread diameter, with a 6.2 mm square.
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