Cutting it Close
One of the most important, but least discussed, responsibilities at a wire harness shop is that of matchmaker. This is the person responsible for pairing the right blade with each wire so it is properly stripped or cut.
In small shops, this duty often falls on the machine operator or even the owner. Large harness manufacturers, in contrast, might have machine-setup specialists whose job includes selecting, monitoring and replacing blades. Regardless of facility size and production level, it’s imperative that the proper blade is chosen for each wire.
“Using under-sized blades may result in feathering, where strands of insulation stick out, or they may not strip the wire at all,” notes Dave Eubanks, president of Eubanks Engineering Co. “If the strip blade penetrates too deeply, it will create nicks in the conductor , and sometimes even break the blade.”
“Cutting and stripping machines always dictate the type of blade you should use,” adds Jeff Barth, CEO and president of JWB Manufacturing LLC, a supplier of replacement blades for wire cutting and stripping equipment. “Factors include how the blade is held and moved up and down, and how much clearance is provided around a wire.”
For decades, manufacturers have relied on three types of blades to strip and cut wire: V, rotary and die. All blades are made of hardened tool steel, but each type is unique in design, function and capability.
V for Very Popular
Two factors account for the popularity of V blades: their versatility and variation. Standard blades feature a 90-degree V configuration that provides good quality stripping and cutting of a wide range of wire sizes (26 to 10 AWG) and insulation types (PVC, Teflon, fiberglass).
V blades are used in sets (top and bottom) that bypass each other to cut wire. Stripping V blades penetrate the insulation at four points that, when connected, form a diamond.
“They don’t cut the insulation perfectly,” notes Erich Moeri, applications engineering manager for the Komax Group. “But, 80 percent of the time, it’s good enough.”
Usually, one set of V blades are used on a benchtop stripping machine or a low-volume stripping and cutting machine. High-volume machines and multistation systems require separate blade sets (one or more) for stripping and cutting.
V blades come in several nonstandard configurations for stripping, including the Uni-V (from Lakes Precision Inc.), full radius and tangent radius V. The Uni-V features a secondary angle with a sharp edge that provides greater insulation penetration for a cleaner strip. Full radius blades feature a half-circle (below the V) with a radius that approximates a specific AWG wire size. When closed, this blade presents a true circle profile. It is very effective for stripping thin-wall, cross-linked PVC and most rubber insulations (thin and thick wall).
U.S. and European automakers are testing the effectiveness of Komax’s proprietary V radius blade (V-Form) to cut aluminum wire. The blade has a Triton coating to resist abrasion.
“After a short while, say 1,000 wires, fine shavings of aluminum build up on traditional V blades,” says Moeri. “This buildup prevents bypassing and causes machines to stop. With our blade, shavings fall through the hole so machines keep running.”
Tangent radius V blades, when closed, produce a diamond-shaped profile. The blade features a sharp edge ground to an arc that approximates a specific AWG wire size.
“These blades allow you to process adjacent wire sizes by simply adjusting the machine’s cutter-head shut height,” says Terry Curtis, president of Wire Process Specialties, a distributor of wire processing equipment in Uxbridge, Ontario. “They also can compensate for off-center wire extrusions.”
Eubanks points out that V blades used for stripping are sharper than those used for cutting to ensure that the insulation is cut cleanly. Like all of his company’s wire strippers, the Model 8000 AutoStrip machine features separate strip and cut blades.
The benchtop unit strips wire from 32 to 4 AWG, and cuts and strips flat and multiconductor cable up to 0.5625 inch in diameter. In addition, the unit comes with a cassette that lets operators quickly insert and remove blades.
Another type of V blade is the multi-V, which strips flat wire or multiple pieces of the same wire simultaneously. Custom-blade maker JWB Manufacturing supplied 25-wide multi-V blades to a harness maker for a unique aerospace application involving ribbon cable.
“Five sections of five V blades were required to strip this cable and expose its 25 conductors,” explains Barth. “The custom blade is 3.5-inches wide, 1-inch high and 0.0625-inch thick.”
Round and Round They Go
Rotary blades, interestingly, are square, not round. Their name stems from the fact that the blades rotate around the insulation to strip it. These blades are not designed to cut wire.
On the stripping machine, one, two or four rotary blades are centered around an opening where the wire is pushed through. A wire guide is sized to the wire OD to ensure that the wire does not rotate in an oblong fashion and cause damage to the inner conductors. Then the blades are adjusted to match the wire ID for the same reason.
To perform stripping, the center edge of each blade penetrates the insulation, and the blade fixture is rotated 360 degrees for several milliseconds. Insulation material and thickness determine how many revolutions are required to separate insulation from wire.
Rotary blades always produce a very clean shoulder, which is the cut edge of the insulation, according to Curtis. They also twist single-conductor stranded wire tighter, enabling it to be inserted into a terminal block or circuit board hole prior to wave soldering.
“Traditionally, the electronics industry has preferred rotary blades for stripping thick insulation and shielding off coaxial cable,” says Rich Javenkoski, vice president of Lakes Precision Inc., a supplier of replacement blades and other tooling for wire processing equipment. “Because they can be better controlled, the blades provide an advantage when stripping small wire or thin insulation.”
Curtis also point outs that the blades’ rotating action may cause problems with nonconcentric wire. These include nicked strands or jagged cutting of the insulation.
To Die For
Each die blade is custom made to strip or cut a specific AWG wire. The most common die blade for stripping is the collinear radius (CL-R) type. It features a ground half circle at the center of its straight shear edge.
When a set of duplicate CL-R blades are closed around the wire, the circle’s radius matches the wire conductor exactly. This perfect fit produces precise stripping and clean shoulders without any conductor damage.
Javenkoski says the CL-R blade is ideal for stripping standard thin-wall, cross-link PVC, as well as wire with multilayered insulation—such as fiber over plastic and plastic over shielding. This blade may also be used in off-center-wire applications, provided the blade diameter is sized properly for the specific wire.
A variation of this blade is the precision CL-R, which consists of two pieces that fit together exactly around one-, two-, three- or oval-conductor wire. The blade’s fixed shut height makes it ideal for stripping thin or oval-shaped insulation from solid- and multiconductor wire.
Die blades for cutting feature a sharp edge that is ground to a flat collinear angle. The edges of two blades butt against each other after cutting through the wire. Javenkoski says die blades for cutting are thicker, heavier and have a larger cutting angle than those for stripping.
Over the past few years, JWB Manufacturing has supplied durable die blades to a wire processing company in the Netherlands to cut stainless steel rod. Barth says he had to customize the cutting geometry of the blades to 45 degrees to get the proper cut. These blades fit into Schleuniger and Artos machines.
Die blades are often required for stripping tougher insulations, claims Eubanks. The company’s standard die blades strip 4 to 30 gauge wire, ground-edge blades strip 8 to 26 gauge wire and precision blades with guide strip 28 to 36 gauge wire.
Another category of cutting blades that may or not be custom made are guillotine blades. These blades are used to cut insulated or bare wire, or tubing. A set of guillotine blades includes either two flat blades or a top angled blade and a bottom flat blade. The two flat blades bypass each other to cut material, whereas the angled and flat blades do the same to shear through material.
Wear and Maintenance
Generally speaking, cutting blades wear quicker than stripping blades because they’re cutting through insulation and wire. But, not all insulation materials are equally harsh on stripping blades.
PVC, a relatively soft plastic, is the least abrasive. Tougher to strip are Tefzel and Teflon, which are more abrasive, and silicone, which is relatively rubbery.
“Silicone isn’t just rubbery, it’s also very thick,” notes Moeri. “Whereas a standard PVC insulation might be 0.04 inch, silicone is nearly 0.12 inch. It can make an 18 gauge wire look like it’s 8 gauge. A thinner-edge blade—1 millimeter rather than 2—is recommended to cleanly strip this insulation because it pushes the material less during stripping.”
Based on his experience, Javenkoski says that, all things being equal, a blade that can strip 500,000 PVC-insulated wires before replacement may be able to only strip 15,000 silicone-insulated wires. To improve stripping of rubbery insulation, Curtis recommends manufacturers increase blade dwell time so the material relaxes a bit and is less stretched.
The most abrasive materials are fiberglass, Kevlar braiding, and the sheathing used to insulate fiber optics. White goods manufacturers prefer wire with fiberglass insulation because it withstands high temperatures. This wire is used extensively in wire stove elements, temperature probes and baseboard heaters.
Coatings are a great way for manufacturers to toughen the blade surface and extend wear life, says Barth. Several suppliers coat their blades with titanium nitrate.
Lakes Precision uses two types of vessels to apply physical vapor deposition (PVD) coatings to blades. PVD coatings consist of a metal vapor deposited electronically on conductive material. The standard coating (titanium nitrate) can be applied to any blade for any application. A nanocoating is recommended for applications involving abrasive insulations such as fiberglass.
As for resharpening worn-down blades, there is disagreement. Barth says he will resharpen some styles of guillotine and rotary blades, but not V blades. The amount of grinding is based on the degree of blade wear.
“We’ve never seen any customer be 100 percent satisfied when using resharpened blades,” counters Javenkoski. “Sharpening changes the blade diameters and requires the machine to be reset. More importantly, using resharpened blades with new ones can cause quality problems, and the savings are minimal. It’s just not worth it.”