In this write-up, I am going to show you how I attach and use a drag knife on my systems.

I am not a drag knife expert, and these are just some of the methods I have used with success.

Please note that what I describe here will work on both my CNCCS system and the KRMx02 with a quick change adapter.

This is a Roland style drag knife blade. I have seen these in 30, 45, and 60 degree versions.

These are the blades used in most of the smaller drag knives meant for cutting thin stock.

This is a Roland blade holder meant for a Silhouette cutter/plotter.

This makes for a very inexpensive drag knife system as it comes with the holder and 5 of each of the blades mentioned above.

The blades are held in place with a small magnet so they are very easy to change.

This blade holder also uses the above mentioned blades. It is meant to be installed into a 1/4” router collet, making it very easy to adapt to an existing CNC system.

This holder allows you to adjust the tension on the blade, but is more difficult tune to your cutting mat.

This is a much larger drag knife meant to hold a box cutter blade.

While it is not meant for fine detail cutting, it can be used to cut thicker materials.

These are also meant to be inserted into the router collet on your router.

You can not change the angle of the blade on this design so you have to purchase a knife holder more specific to your application.

This is a DIY drag knife that I built a while back. It like the Donek knife, is designed to hold a box cutter blade.

This particular knife is based on a set of plans sold by Grunblau.

I have found that having the ability to hold your cutting stock firmly to table is one of the most important factors when using a drag knife. For this reason I use a fixture to hold an adhesive cutting mat.

I own a Silhouette Cameo plotter/cutter and it uses these 12” x 12” tacky cutting mats. This system works very well and while there are other ways to hold your cutting stock, such as vacuum system. I will be using these adhesive cutting mats.

You can get them here:

Strong Grip Adhesive Cutting Mat

Standard Grip Adhesive Cutting Mat

To make the cutting mat fixture I start with a piece of 1/2” MDF 14” wide by 15” tall.

I add a 1/4” rabbit 1/4” deep to all four sides, as shown here.

The rabbit is added to allow the clamps to hold the fixture in-place without getting in the way when surfacing the fixture.

I then attach the fixture to my CNC table.

I cut a pocket in the center of the fixture 13.35” wide by 14.35” tall and 1/32” deep. This ensures I have an extremely flat cutting surface.

I then apply spray adhesive to the top of the fixture.

You can get the adhesive I used here:

Elmer’s Craftbond Spray Adhesive

You don’t want a permanent bond, so let it dry for about 5 minutes before attaching the cutting mat.

Once the adhesive has dried (it will still be tacky) attach the cutting mat.

I used a scrap piece of 3/4” aluminum angle to allow me to align the mat perpendicular to the edge.

Here are two completed cutting mat fixtures. I like to keep both a standard grip and strong grip fixture ready for use.

Note that the cutting mats are consumables. After each use the mat will loose some of its adhesive strength. IE, after a time the strong grip will drop to standard grip strength. The standard grip mat will drop to light grip.

How long this will take will depend on the material you are holding, and how diligent you are at replacing the mat cover when not in use.

I decided to use a simple Roland style blade with a blade holder meant for a silhouette cutter/plotter.

You can pick one up here:

Drag Knife Blade and Holder kit

The kit comes with 5 30 degree blades, 5 45 degree blades, and 5 60 degree blades.

To mount the blade on my machines, I created a custom quick change Z-plate and tool holder that holds the blade securely.

This Z-plate and tool holder will fit any machine equipped to accept a quick change Z-plate.

See how to add one here:

CNCCS Quick Change Z-plate

It is simply a Quick Change Plate (described above) with two holes 2-1/2” apart 1/2” from the bottom of the plate to hold the tool holder.

The tool holder does not have to be this shape as long as the main blade holder hole is in-place.

Note the three red holes. These are pilot holes. They should be made the entire thickness of the stock. They should also be cut first.

These holes add little dimples on the end of the stock that act as a guide when drilling your mounting holes.

I had to change the Holder to a clamp style as the original would not properly secure the knife holder.

The new design is the same as the older one except the screw clamp holes were added. Note the slit added to allow the clamping action.

This new design is included in the drawing files as “Drag Knife Holder 3”

When testing on thin materials, I like using some 5” x 8” card stock. Its inexpensive and is a good medium for testing your cut depth.

For card stock, you want to make sure you are not using the strong grip cutting mats.

This particular card stock is .008” thick so I will be using that to setup my cutting paths.

I am not going to use any special knife cutting software. I am just going to setup the paths as though it is a very small end mill using Vcarve.

I have set my cutting depth to .009. We should be able to cut cleanly through the material with minimal penetration into our cutting mat.

I saved the toolpaths and loaded them into Mach3.

The next step is to adjust the knife blade so that it is just touching the card stock.

Be sure to re-tighten the lock nut once you have adjusted the blade.

Once the blade has been set, zero your Z-axis.

As you can see here, the brute force approach when cutting with a drag knife can yield less than optimal results.

The tiny tears you see here, because the knife blade was not oriented properly prior to the cut.

Since the blade is slightly offset, it will self correct its orientation. The problem is that as it turns it will cause tearing. The thicker the material and the more offset the knife blade has the more tearing that will occur.

In this test I am going to use the same drawing and settings, but I will add a 1/4” ramp to each cut. This should allow the blade to turn without too much tearing.

As you can see, the results were much better. I could also lengthen the ramps to better improve the results.

Just know that with some shapes you wont be able to get the length of ramps you may need to keep from tearing.

This test is almost identical to the first test, but instead I will be cutting squares.

As you can see it suffered from the same tearing issues as the circle test.

I added 1/4” ramp and most of the tearing has been eliminated.

If you look close at the corners, you can see they are not square and seem to be misshaped.

This is due to the offset of the point from the center of the blade.

The more the blade offset, the more pronounced this error will be. In some cases this may prevent a complete cut on and object.

Unfortunately, there is no way to compensate for this without some special provisions in the CAM software, which Vcarve does not have.

I have been a longtime user of SheetCam CAM software. In my opinion it is some of the best software for plasma cutting I have ever used.

The price is also very nice, I think it is currently less than $150.

To do drag knife operations, you will need to purchase the drag knife plugin for about $35, and enter the licence into the plugin form, as shown here.

Here I imported my small squares DXF as a new part and have added a new drag knife tool profile to match the geometry of the the drag knife blade I am using.

Notice the offset and swivel depth settings. These will effect the accuracy of your cuts.

The swivel depth is the depth the blade will go to make swivel moves. As a default I often choose half the cut depth as a starting point.

Here I moved all the cuts into a new layer called square cuts using the Edit Contours function.

I right click in the operations panel and add a new drag knife operation.

I set the following fields for the operation:

• Layer=square cuts

• Tool=T203 (The drag knife tool I created)

• CutDepth=.009”

• Depth per Pass=.03”

• Feed Rate=50 IPM

Once the operation is generated, you will notice some small paths added to each of the corners. These tell your CNC when to start turning in order to compensate for the knife blase offset.

For the blade I am using these would be set to .01, but I increased the offset to .03 so that you can see them better.

At this point I can generate my G-code. I do this by hitting the P (PostProcess) function.

The saved SheetCam toolpaths and loaded into Mach3.

The results were less than overwhelming. You can see the edges still are not square.

I was relying on the settings given with the knives which gave an offset of .25mm or .098”.

I decided to do several cuts decreasing and increasing the offset.

What I came up with was a .027” offset.

I also changed the swivel depth to .03”

The results were the best I have seen so far.

Offset setting is too small.

Offset is too large

In closing I want to talk a little about the the strong grip cutting mat.

I would not use any kind of paper backed contact paper or card stock with the strong grip cutting mat.

The backing to the contact paper or card stock can stick to the mat. While you can remove it, it will ruin your cut parts.