$20 Drag Knife

[billt]

I have come up with two drag knife designs that can be made for about $20.  Both designs allow mounting in a 1/2" router collet.  The first design is based on the Roland Blade Holder, and is spring-loaded to allow for slight surface variations - this one would be used for thin materials like Vinyl.  The second design is based on an X-Acto #1 knife with external bearings and is not spring loaded - this would be used for thicker materials like cardboard or wood veneer.

Some initial tests included a hobby drag-knife, which did not cut very well due to the sloppy fit of the knife blade, and did not rotate well when I tried to add a spring.  To work well a drag knife should definitely use roller bearings of some sort.

The "Roland Blade Holder" can be purchased on E-bay for less than $15 including blades and shipping.  Modify by removing all of the components and then first add in a short section of tubing (1/2" x 3/16"OD or 5mm OD) as a spacer, add the magnet/bearing, cut the spring in half and add, then add the set screw.  On the top add a short section of 1/2" plastic tubing which acts as a bushing to fit into the 1/2" collet.  Finally add the blade which snaps into fit the magnet/bearing.  Spring pressure can be adjusted up or down by tightening the set screw.  Both pieces of tubing you should be able to get at a home store or hardware store for less than $1 per foot.

The X-Acto knife design uses just the front portion of the knife - unscrew the handle and set it aside.  The bearings are 5/16"x1/2" (I ordered qty 10 of R1810ZZ from VXB.com for $30 including shipping, but you could order a minimum of two at $5.77 each if you can find a suitable spacer).  File the knurling off of the blade sleeve so that the bearings can be added, and epoxy into place.  A 10-24 nut can be used to tighten the blade.  There are many blade options, the scoring blade #16 came with my blade set ($5 for knife plus blade assortment).  There are many others that you can try, for example #10, #19, or #23 look promising for drag-knife cutting.

I posted a drag knife Python script in a previous post.  It works, but I am working on some refinements for selecting the start points and making the rotations all CCW.  It can be found at:
http://www.cambam.co.uk/forum/index.php?topic=3628.0

Cheers - BillT

[Dragonfly]

I haven't posted in the drag-knife script thread recently because I had to solve some mechanical problems. Some of them - self inflicted
The problem was that with the corrected tool trajectories there were no visual result when trying on foil. I knew that I slightly overdid the knife holder hole on the carrier I made and there was some play. So I decided to make a new carrier. Several days went drawing, making some parts, then again drawing and changing the design. Finally a new carrier (to held in the spindle chuck) came to being and I was very pleased with it. Now I can change the spring and adjust its tension and the moving part slides on two pins with no play.
But the result on foil was again a failure.
So I took the Roland knife holder and had a look inside. It appeared that trying to have the heel cap on as much thread as possible I've adjusted the knife too deep and its shaft (2.5 mm OD) got out of the ball bearing leaving there the thin neck ( ~1 mm) and the knife was rocking unsupported radially.
While putting and taking out things something got stuck, I applied some brute force without much thinking and there came out a piece of the brass capsule where I knew the magnet was inserted, the magnet itself and in addition - three tiny steel balls and a tiny steel cup.
Left it for the night for my frustration to pass and in the morning, after some pondering and head scratching I understood why there was no second bearing in there. The cup and the balls formed a kind of axial bearing and the conical shaft end lying in the center of the thee balls tripod.
But I needed a new capsule and decided to make one. Had no suitable brass piece nor tools for it. So I went for Acetal (Delrin). And I made it. I am really starting to love my machine which I built in home garage conditions.
Here's what the rear end support looks like. The red part is the magnet.

[Dragonfly]

Now that the knife holder was repaired and adjusted, together with the new carrier there are the first tests with the expected results.

[lloydsp]

Very nice!

Lloyd