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In the world of cutting plotters, cutting technology has progressed significantly. The most commonly used technology is the CNC drag/swivel knife (Do not get confused with swivel knives for leathercraft).
Although there are a couple of knives used in CNC machines, it is essential to select the right knife blade for a CNC project in hand. Now, this can be a challenging task initially for those who are doing so for the first time.
However, it is quite simple if you know the material to be cut and its properties such as thin or thick. If you know which knife handles which type of material, choosing the right knife for getting a superior cut is an easier task than what you may feel.
So, if you wish to use a CNC drag knife, it is not only vital to know what it is and how it works but also to find out the materials for which it is the most suitable choice. So, let’s explore this knife in detail.
Introduction to Drag Knife
A drag knife is a simple tool that fits into a CNC router spindle to cut. It can cut just as a utility knife but at the speed and accuracy of a router.
It is the only knife or cutting technology preferred for very thin materials such as thin paper, thin plastic, magnet, mylar, adhesive-backed vinyl, and corrugated cardboard. It is useful for cutting thin surfaces such as films and membranes in two dimensions.
When used with the right software, the knife plays a big role in making vehicle graphics, banners, signs of professional quality, magnets, or portions of thin plastics. Due to the low shear strength of cardboards, wax papers, standard bond papers, chipboards, this knife can cut these cellulose/lignin organic composites quite easily.
The knife is widely used in single-ply cutters for cutting small circles and detailed shapes. In short, a sharp drag knife can cut a variety of semi-rigid and flexible material sheets that have high elasticity, low toughness, and high tensile strength. If you choose an oscillating blade, it shall end up pulling the elastic out of place.
Even a rotary blade will not work satisfactorily, as it will stretch the sheet without cutting it. The low level of toughness indicates that a sharp knife crack shall spread quickly irrespective of the tensile strength but a duller blade will rather end up catching and stretching.
A drag knife will not work smoothly with tough, very thick, or abrasive material sheets. In the case of very thick material, it will contact the sides of the knife’s blade to generate more friction. This, in turn, will stress the blade and will push the sheet out of place.
This knife is highly vulnerable to dulling through abrasion. The dull blade shall undergo more stress when you drag it due to which it can break. This is why this knife is cheap and more easily replaceable than re-sharpening it.
2020’s 3 Best CNC Drag Knives
Donek tools and Dog River Tools are the two popular brands that offer a variety of drag knives. Make sure you choose the right tool for the material.
Dog River Tools CNC Drag Knife Cutter
Designed by Dog River Tools, this drag knife is the most versatile one. It not only can cut a variety of thin and flexible materials but can also fit into any CNC router devices or spindles featuring a 1/4-inch collet.
You can even set it to cut 1/4-inch and 1/16-inch depth. This is unlike the knives of brands such as Donek that gives two versions to achieve both these depths. The knife can cut sharp corners and curves on paper, plastic, sign vinyl, wood veneer, textile designs, stickers, and cardboard planes with the speed and precision of a CNC router.
You can use this versatile drag knife for making engravings aluminum trophies, name tags, and brass. Apart from the CNC routers, captivatingly, you can use it with any engraving tool featuring a 1/4-inch collet, as the knife has a precision-grounded diameter of the same size. This ensures that you get uniform engraving output on uneven surfaces.
With this knife, working on harder sheets is now easier than before. Thus, this knife is ideal for those who work on a diverse number of projects.
The knife is made using a heavy-duty CNC-milled body of aluminum. On the top, a black anodized finish gives a professional look. It fits the blades of any normal utility knife.
- Sleek look
- Compatible with any CNC router or spindle
- Versatile to cut a variety of materials
- Not for professional use
Donek Tools D4 Drag Knife
Donek Tools is an esteemed name in the domain of drag knives. This Donek CNC drag knife is versatile enough to cut almost anything that a utility knife can, fit into any CNC router or spindle collet, and fulfill the requirements of a professional and a hobbyist alike. You can expect the accuracy and speed of a CNC router or mill.
The overall design is ideal for the furniture industry. It features a maximum cutting depth of 1/4 inches and a shank of the same size due to which it can cut a myriad of materials.
The knife is handy to cut ski base and snowboard wood materials, cardboards for custom packing, leather, wood veneer for artwork, carbon fiber laminates for aerospace, and motorized components.
Whether you want to do some delicate inlay work using a veneer or cut cardboards with a CNC router, this knife can help you with diverse project tasks.
The knife’s tip features an advanced coating technology due to which it is anti-corrosive. The set head on it helps in boosting your productivity. With this knife, making intricate designs on wood is easier than before.
- Rugged look
- Versatile enough to cut a variety of materials
- Anti-corrosion coating
- Less cost of CNC routing blades
- A bit learning curve
Donek Tools D3 CNC Drag Knife
This CNC drag knife is ideal for anyone regardless of the skill set. Whether you are a professional CNC router user or a hobbyist learning to craft, this knife has the ability to carve as per your requirements. You can even use it for a variety of tasks at your workshop due to its supportive cutting depth of 1/16 inches and 1/4-inch shank.
Apart from the thin materials, this knife works well for engraving brass, metallic trophies, aluminum surfaces, and name tags. Unlike other drag knives, you can use it for engraving a glass surface.
As the knife does not rotate, there is no need to run the spindle. This is useful while working on surfaces to carve intricate designs. It is mainly designed for CNC routers.
There is also no need to program a CNC router in most cases. The support for a utility blade is a smart feature, which prevents you from buying consumable blades. The knife’s construction is durable due to precision-ground stainless steel and aluminum body.
- Rugged look
- Ideal for both hardwood carvings and delicate designing
- Versatile enough to cut a variety of materials
- Less cost of CNC routing blades
Working of a CNC Drag Knife
It is simple to use a drag knife. Its blade rotates freely as well as automatically in the direction of drag. This is unlike a non-rotating blade that can cut only in one direction. It swivels as the CNC router drags it around the sheet or design. In the case of vinyl, it easily cuts it without cutting its wax paper backing.
Thus, this knife is typically dragged through its substrate. There are external elements to control their movement. It is mostly the media movement and the cutting head movement, which are responsible for its smooth working.
Most of these knives are equipped with a cutting head of a high speed, an optical positioning system, and a spring-loaded glider foot. It is possible to compare its working with the tractor plow.
Both these devices have a few factors in common. First, both are dragged through their material or surface. This is Earth or dirt for the plow and thin material for a drag knife. Second, both are regulated by external elements.
The tractor controls the plow, which triggers a movement by pulling the plow to position itself in the right direction. In the case of a drag knife, it is the blend of two movements mentioned above. Both movements allow the knife to face in the correct direction, as the knife can rotate freely in its holder.
So now, a question may arise as to how this knife can help in making sharp corners. Well, surprisingly, this knife cannot make sharp corners. The knife makes some movements in each corner without lifting from the media to create a sharp corner.
As these additional movements occur at a minute level, it is impossible to view it with human eyes. In its way, the knife can just do the trick, as per the type of material and cutting design.
It is commonly chosen for making intricate cutting designs even on billboards, media windows, and lightboxes. It typically depends on the material’s thickness the knife is working on to make such designs with noticeable distortions.
The visible distortions are more in the corners with more thickness of the media. At times, bit rounded corners or media is pushed as per the rigidity or softness of the media. Many users believe that the amount of pressure to be used while using one such knife is limited.
Well, there is a logical justification for this fact. High pressures are usually utilized for harder and thicker materials. For these rigid materials, the drag knife technology is just not suitable. Despite the limitations, it is still a robust and widely used technology.
When used with a mill’s spindle, a drag knife attaches itself as a tool to it and traces the path along which it is moving. Distinctively, the knife can self-orient itself to be tangential while cutting, as the blade is at an angle to make an offset from the spindle’s mid-area.
This angle is technically called the caster angle and it can be positive, neutral, or negative. If the blade is at a bit negative caster angle due to which it drags behind the shaft’s or spindle’s mid-area. This offset results in a bit of torque applied on the blade whenever the spindle alters its direction. This keeps the blade pointed at a tangent to the moving direction.
Here is a video you should watch for a better understanding of this tool.
Drag versus Tangential Cutting
A drag cutting technology is not the same as the tangential cutting one. In response to the restrictions of the drag cutting technology and specific requirements, the tangential knife technology has been introduced.
Rather than rotating freely, a tangential knife is under the control of an outside motor. This motor positions the knife at the intended angle. This cutting technology ensures a superior performance over that of a drag or swivel knife technology.
The tangential knife’s direction is regulated electronically due to which you get precise cuts with very high pressures. This cutting makes weeding efficient, which saves a lot of time.
The tangential technology raises the knife from the media, rotate it, and then puts it back into the media. On the other hand, a drag knife is in the media throughout the cutting time.
For cutting the corners, a drag tool rotates spontaneously in the knife holder. To direct the knife course, a drag movement is triggered supposing that the knife tip will also follow that way.
A drag knife cuts and stops before stepping into a corner and its blade is rotated via a drag movement to an intended angle without leaving the media. The machine continues cutting when the drag movement is over.
On the other hand, the blade of a tangential knife is raised for a few tough corners and rotated by a motor. Once the intended angle is accomplished, the blade is brought down, and cutting continues.
While the dragging technology is easier to use and ensures quicker cutting, the tangential technology gives more detail, is meant for rigid and thicker substrates, and ensures more precise depth adjustments.
See this tangential cutting CNC machine in action
Drag knives enable you to cut anything you would cut with a utility knife, but with the speed and precision of a CNC router. Donek and Dog River tools dominate the market of drag knives, though the former would cost you twice as much as the latter.