Multi Tap Spiral Flute & Point, NE2/NT2/GLT-1

The Multi-Tap is Emuge’s general-purpose machine tap that is engineered, ground and finished to cut a credible thread across nearly all materials, rather than being optimised for one.

A NE2 spiral flute or NT2 spiral point Multi-Tap will produce a serviceable thread in steel, stainless, cast iron, aluminium and non-ferrous materials which makes it the default workshop tap where a single tap must cover whatever the day brings.

The trade-off here is refinement: a Multi-Tap gives up some of the last increment of tap life and speed to gain that breadth. On a production line running one material at high volume a tap ground and coated specifically for that material will outperform it.

For a jobbing shop, a maintenance bench and any application where one tap covers mixed work, the Multi-Tap is the practical, economical choice and its price-to-performance is one of the reasons the range sells so well.

The designations identify the geometry and the surface treatment of the tap. NE2 is a spiral flute Multi-Tap, oxidised (steam-tempered) to resist cold welding (the phenomenon where chips galling onto the cutting edge tear the thread and break the tap). NT2 is the spiral point Multi-Tap, also oxidised for the same protection. The "2" denotes the oxidised finish within Emuge’s naming.

So NE2 is the blind-hole geometry (spiral flute lifts chips up and out) and NT2 is the through-hole geometry (spiral point pushes chips forward ahead of the tap), both with the oxide finish that protects against built-up edge in steel and stainless.

Choosing between them is a question of the hole type (blind or through) and the finish is matched to the material.

GLT-1 is Emuge’s proprietary coated finish, it is a hard, low-friction layer applied over the tap that adds surface hardness, reduces friction and resists galling more strongly than an oxide finish alone. It suits the harder and more demanding materials (stainless and the alloy steels especially) where an oxidised tap would struggle, and lets you run a higher cutting speed.

A steam-tempered (oxidised) tap is the finish for free-cutting and general steels, cast iron and the easier work, where the oxide is enough to prevent cold-welding and the tap performs well at a lower cost.

Choose GLT-1 where the material is harder or more galling-prone or where the extra tap life and speed earn back the higher price but choose steam-tempered where the application is general and the coating is not strictly needed.

Steam tempering (also called steam oxidation or black oxide) is a surface treatment in which the tap is heated in a controlled steam atmosphere, producing a thin, black iron-oxide layer on the finished tool. The film is porous enough to hold cutting fluid at the cutting edge, which lubricates the cut and it provides a barrier that prevents the workpiece material galling onto the tap flanks.

The practical effect is a drop in built-up edge (the chips do not weld to the edge and tear the thread) and longer, cleaner tap life in steels. It is the standard finish for general-purpose taps and the baseline Emuge treatment. It is not a hard coating like GLT-1 (it adds no real hardness) so for harder and more galling-prone materials you step up to the coated finishes.

Hole type decides the geometry first.

For a blind hole, choose a spiral flute tap because the helical flutes lift the chips up and out of the hole so they do not pack at the bottom and break the tap. For a through hole, a spiral point tap is faster and cleaner — the point angle drives the chips forward out of the far end of the hole. A roll (cold-forming) tap is the choice where the material is ductile and you want a strong, chip-free thread.

Within Emuge’s naming, NE2 covers the spiral flute blind-hole Multi-Tap and NT2 the spiral point through-hole Multi-Tap, both in steam-tempered finish. The same geometries are available in GLT-1 coated form for harder and stainless work.

Match the geometry to the hole first, the finish to the material and the thread form to the standard the drawing calls up.

The figure describes the maximum usable thread depth the tap is designed to cut efficiently, expressed as a multiple of the tap’s nominal diameter (d1).

A "max. 2.5 x d1" rating means the tap is rated to cut a threaded depth up to two-and-a-half times its diameter, for example an M8 tap (d1 = 8 mm) is rated to around 20 mm of thread depth, an M10 to around 25 mm and so on.

This is not a hard limit (the tap will cut deeper at a cost in chip clearing and tap life) but it is the design depth at which the tap performs at its best. For deeper threads, choose a tap with a longer chamfer and a higher depth rating, peck-tap to clear chips and allow the tap more clearance.

Threads deeper than the rated figure are the common cause of broken taps in production, so staying within the rated depth is the prudent path.

Stainless is one of the harder tapping materials because it work-hardens under the cutting edge and galls onto the tap.

The Emuge choice here is a GLT-1 coated spiral flute tap for blind holes or a GLT-1 coated spiral point for through holes where the coating resists the galling and lets you run at a usable speed and the spiral flute is essential in a blind hole to lift the tough, stringy stainless chips clear before they pack.

Run the tap conservatively (lower speed than for mild steel, plenty of cutting fluid high in EP additives) and do not persist with a worn tap, because a dull tip work-hardens the stainless and starts a downward spiral of thread quality and tap life.

The appropriate GLT-1 coated tap, run right, will give a gauge-true thread in stainless at a respectable tool life.

Aluminium and non-ferrous materials produce soft, sticky chips that tend to build up on a cutting edge and the material itself is sensitive to a tap that is too aggressive.

An Emuge steam-tempered, polished-flute spiral flute or spiral point tap (the bright, low-friction finish on the flutes) produces a clean thread with minimal built-up edge.

Avoid a heavy hard coating on aluminium (a bright or lightly-coated tap with polished flutes resists the built-up edge better than a hard AlTiN-style coating, which can encourage adhesion).

Use a spiral point in a through hole to eject the soft chips, a spiral flute in a blind hole to lift them.

For a roll tap, an Emuge forming tap in aluminium produces a very strong, smooth thread at high speed (aluminium forms readily and is an excellent forming material).

Cast iron machines to a short, crumbling chip, so chip clearing is not the challenge, the tap does not need a spiral flute to lift chips and a spiral point or straight flute tap performs well.

Emuge steam-tempered spiral point taps are a strong choice for cast iron because the forward-ejecting geometry clears the crumbly chips ahead of the tap and the oxide finish is adequate for the material.

Cast iron is abrasive, so tap life is governed by edge wear rather than chip handling and the tap should be changed before the edge dulls and tears the thread. A coated tap will give some added tool life but the standard steam-tempered spiral point tap is the value choice and handles the material well.

Avoid a roll tap in cast iron because the material is not ductile enough to form and a forming tap will fracture the thread rather than flow it.

The thread form is dictated by the drawing, you must match it exactly;

• Metric coarse (MC) covers the common metric threads (M6x1, M8x1.25, M10x1.5)

• Metric fine (MF) the fine-pitch metric series

• Unified coarse (UNC) and fine (UNF) cover the imperial UN threads specified in threads-per-inch

• Whitworth (G) covers the British fastener and pipe threads.

Each Emuge tap is ground to one thread form, diameter and pitch so the filter tree (thread form, then diameter, then pitch or TPI) delivers the exact tap.

A metric tap will not produce a UN thread and vice versa and a coarse tap will not produce the fine thread, so be precise.

The diameter and pitch filters on this category pair the thread form to the size, which is the simplest way to land on the correct Emuge tap for the job.

The tapping drill size is the pre-drilled hole diameter below the thread major diameter, leaving material for the tap to cut the thread. The correct size is a published figure for every thread (M8x1.25 takes 6.8 mm, M10x1.5 takes 8.5 mm, and so on) and is the same for an Emuge tap as for any tap of that thread form. The diameter and pitch filters on the range identify the correct tap, and the standard tapping drill table identifies the matching drill.

For an Emuge roll (cold-forming) tap the hole is larger and the forming tap displaces metal into the thread form rather than cutting it, so the drill leaves less material to displace. An M8x1.25 forming tap takes around 7.3 mm rather than 6.8 mm.

Drilling the wrong tapping-drill size is the leading cause of broken taps and weak threads, so set the drill to the table for the thread and the tap type (cut or roll) every time.

Tapping runs at a lower surface speed than drilling or turning because the tap is fully engaged and the chip must clear within the hole. For an Emuge steam-tempered tap in free-cutting steel, a surface speed around 10-18 m/min is typical; harder and alloy steels and stainless run slower, cast iron a little higher, aluminium higher still. A GLT-1 coated tap can run noticeably faster than an oxidised tap in the same material because the coating resists heat and galling.

The feed per revolution is the thread pitch, which the machine synchronises on rigid tapping and a tension-compression holder absorbs on a non-rigid set-up.

Start at the maker’s recommended speed for the material and finish, run a part, and adjust (too fast and the tap glazes and breaks, too slow and it rubs and work-hardens the material into a torn thread).

For a roll tap, speeds are higher than for the equivalent cutting tap because the forming process runs cleaner.

A blind hole is the demanding tapping application and the Emuge spiral flute (NE2 or GLT-1 spiral flute) is the correct tool. The helical flutes lift the chips up and out of the hole, preventing the chip packing that breaks taps. Run the tap with cutting fluid directed into the hole to help the chips clear, stop the tap a thread or two short of the bottom to leave chip clearance and peck-tap on a manual machine (a turn forward, a half turn back to break the chip, repeat) to clear chips incrementally.

On a CNC, a synchronised peck-tap cycle does the same logic automatically. Do not drive the full thread depth hard into the bottom of a shallow blind hole because the chamfer and flute form are designed for a usable depth up to the rated maximum and exceeding it is where spiral flute taps break.

A GLT-1 coated spiral flute in stainless, run right, will give a gauge-true thread at the bottom of a blind hole reliably.

Tap life comes down to four things, in order:

• The tapping drill size

• The cutting speed

• The cutting fluid

• Changing the tap before it is worn.

Drill the tapping hole to the correct table size (too small and the tap overworks and breaks early).

Run at the maker’s recommended speed for the finish and material because too fast glazes the edge, too slow rubs and work-hardens. Use a dedicated tapping fluid high in EP additives, directed at the cutting zone for both lubrication and chip flushing. And change the tap at the first sign of a torn or poor thread because a tap run past its edge life work-hardens the material and ruins both the thread and the next tap.

Hold the tap true in a quality collet or rigid tap holder because an out-of-true tap cuts a thread no bolt will run into and breaks early.

With those four handled an Emuge steam-tempered Multi-Tap will give its full rated life and a GLT-1 coated tap several times that.

If you tell us the thread, material and machine, our in-house engineers will specify the right Emuge tap, finish and holder for the life the application warrants.