Drag Knife

Discussion in 'CNC machining projects' started by Al2O3, Feb 18, 2024.

  1. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I broke down and bought a drag knife. Would have been easy enough to make one but I also needed a vacuum hold down table so decided to focus my time on that part.

    I machined the hold down table out of mdf and besides the grid on top, machined a plenum into the backsides between the two sheets and glued them together to keep it low profile to live (most of the time) on my CNC router table. Then sealed it all up with a couple coats of varnish.

    1 Making Vacuum Table.JPG

    Ya-know I had to get a casting into the works some how, so made this adapter for my shop vac.

    2 Vacuum Adapater.JPG

    Here it is installed on my router table. All the YouTube vids said you could just use MDF as a spoil board because it was so porus. Though it was porus, I didn't think it held very well at all through 1/4" thick piece of MDF. The peg board worked much better and was cheap. I can get two spoil boards out of a single $8 piece.

    3 Vacuum Table.jpg

    Here it is in action. There was a drag knife plug in for CamBam. I bought the SST drag knife. It's a pretty nice piece and value at $100. There was nuance learning stuff for settings and what not but after 3-4 sample cuts I was cutting good gaskets.

    These gaskets were 1/16" thick and being cut in two depth increments just to make sure the machine had enough grunt and avoid step loss. With the cycle time under a minute anyway, it's kind of a shoulder shrug. After a few minutes, gaskets were cheap.

    4 Gaskets.JPG 5 Gaskets.JPG

    I previously was clamping gasket stock in shop-made MDF jigs/fixtures, but now the vacuum table is my universal fixture and I can cut and nest any size and multiples that will fit on the vacuum table without having to make or store fixtures and set up is just the time to cut a chunk of gasket off the roll, load the program, and home the machine......only a couple minutes. -Cool beans!

    Petee716, Tops and Tobho Mott like this.
  2. Petee716

    Petee716 Gold Banner Member

    This is a very timely thread for me. More to come...
  3. Tops

    Tops Silver Banner Member

    This is a technology I am interested in. When looking before, between the Donek knife and the bespoke software, it was more cost effective for me to get a small UV laser. It's not the same 'cut' for sure, would rather have the knife for some materials, but I have fun with it especially with burning wood. The SST knife is attractive cost-wise. Looking forward to see where Kelly and Pete take this.
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    As usual, there's more to the story, and there are a lot of nuansce things I'm learning along the way.

    The first was, there's a lower limit to the size holes a drag knife can cut. What's small? Well, most of my gaskets will use >1/4" hardware so say ~.280"D. This knife will do so but is kind of on the ragged edge of what in can do, and how well it does so can be dependent upon the material and thickness. The manual says the lower limit of radius of curvature approaches the Knife Tip Offset (KTO) setting. I have not found that not to be entirely so.

    I made some quick test cuts in paper and cardboard and then quickly switched to gasket stock. When I cut .280" holes in 1/32" gasket, I set the knife offset at .055" to allow some overcut, tolerance for table flatness, and clearance above the stock so the knife body doesn't drag on the stock. When I entered all the corresonding data into the drag knife software, it initially cut .280" holes considerably undersized and I needed to input higher than actual KTO to correct this. Interstingly, this correction doesn't seem to translate at all to dimensional errors in the larger features. The software I am using is a free CamBam plugin written by a user so this may be software related.

    Comparing the Donek (~$250) and SST (~$100) knives, the SST seems to be more verstile and much better value. The SST knife has adjustable offset and a dial for setting depth/offset with very fine adjustment and the fit and finish is exceptional. Donek has a series of knives but they must be fixed geometry with respect to KTO and offset and they have totally exposed blades. Both use a standard utility knife blade.

    The only downside I've found to the SST.....since offset and extension are 1:1, you need to select more offset than material thickness so you can insure you cut through the stock typically by .010", allow some tolerance for table flatness, which can be a bigger deal for large gaskets. More offset means smaller radius of curvature cut capability so that is an aggravation to small features.

    Material can affect knife behavior and quality of cut. My first cuts were with the 1/32 green gasket stock which aramid fiber reinforced Buna rubber compond. Soft but strong. First cut looked good but holes were undersized. After iterating the KTO, I was getting dimensionally accurate nice looking gaskets. When I switched to 1/16"t stock, I was getting a lot of tear out and ugly .280 holes in the relatively soft material. Thickness also reduces the possible radius because of the amount of (straight edged) knife engagement.

    After some forum correspondence, I decided I may need a smaller knife (more on that in a minute).

    But, when I switched to the tan FelPro gasket stock, which is much harder, it cut beautifully. The 1/32 was cut in one pass and the 1/16 in two depth increments.

    I was going to make my own holder grind my own smaller knives, when a friend suggested I look at vinyl cutter offering. For under $20 delivered I got a knife holder and 14 cemented carbide knives. If you search Amazon for "Roland Vinyl Cutting" or Roland Vinyl Knife) you'll get many hits.

    the one I bought to suit Roland plotters.PNG

    Amazon.com: CAREGY Deep Cut Blade and Housing for Cricut Maker Replacement Deep Point Blade Compatible with Maker 3/Explore 3/Air 2/Air/Maker/One Cutting Machines (4 Deep Cut Blades and 1 Housing Included) : Arts, Crafts & Sewing

    Amazon.com: 10PCS Replacement Deep Point Cutting Blades Compatible with Cricut Explore Air2/Air 3/, Cutting Blades Compatible with Cricut Maker/Maker 3 Machines, Cut Thicker Materials (Deep Point Blade) : Arts, Crafts & Sewing

    This is a small knife holder for use in vinyl cutting machines remeniscant of pen plotters. I ordered "deep cut" knives because they can supposedly cut up to 1.5mm thick stock. The knives are tiny at .074" diameter and .037 KTO, and about aninch long. I chucked the holder up in my lathe and cut shoulder off so I could hold it in a collet.

    Small Knife and Holder in Extension Collet.JPG

    The inital cuts were encouraging but not always great because this quick and dirty method relies on the router bearings for the the knife to pivot properly and with that miniscule KTO, there's too much bearing drag in the router bearings to keep the knife oriented and causes it to occassionaly drag with the blade sideways. So I am going to make a holder with very low drag bearings and give it another go and see how small of a hole I can cut.

    Tops likes this.
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I looked at a laser which is how my higher use/volume gaskets are made (not by me). I'm pretty much limited to thin mineral paper because it cuts fast and easily but is just so-so gasket stock. The (hobby level) lasers doesn't cut the better gasket materials very well or at all, stink to high heaven, and leave friable charred material at the cut.....not good for automotive stuff. It can be scraped off for the most part but what a bother, and all of this is probably why my gasket supplier will only offer mineral paper gaskets.....thus the drag knife.

    Tops likes this.
  6. Petee716

    Petee716 Gold Banner Member


    Sorry I thought the photo was clearer. The one on the left takes something like a #2 exacto blade. It’s one with a 1/4” tang, not one of the skinny ones. It has a pair of internal roller bearings so it pivots very smoothly. If the pic was clearer you could read the vendors website. I’ll follow up with it tomorrow. The one on the right has one of the vinyl cutting blades Kelly showed above. That little bugger has bearings in it to so the blade pivots nicely. Neither of these cutters handles stickout very well so they’re not suited for very deep cutting. The vinyl knife is ok for thin stuff under .015 and not really great for through cutting. It works great for kiss-cutting pressure sensitive materials like vinyl (what a coincidence,lol) where you’re touching but not cutting through the backing liner. It’s also great for fridge magnets (.012-.030”) where you only have to cut to a majority depth of the material and peel it out. The one on the left works better for through cutting up to about 1/4” or so, but the knife offset can get a bit tricky as Kelly mentioned. My Vectric V-carvePro has a drag knife utility, and sometimes I utilize it, but I sometimes I just tell the software that I’m running an endmill. I generally use it for cutting shapes out of 4mm corrugated plastic or 3mm PVC sheet for signage. Nothing with much of a tight radius though.
    Neither one of them care much for paper or fibrous material.

    PS- I said the thread was timely because I’ve actually been brainstorming about a vacuum table and possibly taking a run at v-casting. I just picked up a couple of 5hp air pumps that I have yet to test.
    Tobho Mott and Tops like this.
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I did a quick and dirty test with and exacto blade but in gasket stock, they were so fragile the point on the tip breaks almost instantly before it can pivot. I may give it another go after I make a small low friction holder.

    Mine feels like it has bearings and for that reason, I thought it would work better when the body was mounted in a collet depending on its own internal bearings instead of the router bearings to pivot, but, if it has internal bearings, they must not be able to handle the load of .010-.015" depth cuts in gasket stock. The cemented carbide blade also broke after some use but that may have been because it wasn't pivoting and was being dragged sideways. At less than $1 each they're still worth a try.


    I'm with you on this and the biggest reason for (me doing) this is being able to use a spiral leadin move on the cut. In other words, I can call for a ramp into the depth of cut of any angle and that should help align the blade as depth of cut increases to the profile setting. I dont have the option of lead in moves with the Engrave MOP in CamBam, but this is easily solved by just telling the program the end mill in a profile MOP is .0001" diameter.

    When I build a small low friction knife holder, I will try this both ways with the cheap vinyl knives, with both an ordinary milling profile and a drag knife corrected profile. If the vinyl knives can't cut it (sorry :) ) I'll just grind my own from 1/8" D A2 tool steel. I really only need it for small holes. The SST knife otherwise satisfies my needs.

    Type of material and thickness are really the drivers for me. The Felpro gasket material is way harder and tougher to cut than cardboard of the same thicknes. Even with the SST knife and new blade, it will skip steps if I try to cut .062" in a single cut.

    It's already a big win for me, just trying to get it to cover the fringes of my needs now.

    Tops likes this.
  8. Petee716

    Petee716 Gold Banner Member

  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Price disparity seems similar in all commercial production versus hobby duty tooling. Most of the time you get what you pay for. I saw similarly priced/quality knives in my previous searches but could grind my own in 15 minutes or less. Same goes with the larger knives but with the low cost of utility knife blades and how many gaskets they can make, it's fractional pennies per gasket with them and really a matter of what's good enough.

    On the small diameter stuff, for me, it's a matter of what can be made to work and at what cost... and that's still an open matter for me......If the Chinese cheapies could make a dozen or two gaskets, that'd be fine but even so I'm not thrilled about a tool change to make smaller holes. No tool changer on my router......may as well mill them with a carbide spiral bit. Milled small holes are perfect each and every time.

    I'd rather push go and see a whole sheet of complete gaskets made a few minutes later.

    Perfection is the mortal enemy of good enough!

  10. Al2O3

    Al2O3 Administrator Staff Member Banner Member

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