Q for the guys with drag knives

[motoguy]

I'm considering adding a drag knife to my table. I'd like to pre-cut masks for some of my items that are time-intensive for hand masking. I'm weighing the options of adding a drag knife to my table, or just buying a vinyl plotter to cut disposable vinyl masks.

I'm assuming I'll have to build a box (probably from MDF) to put on the plasma table, for vinyl cutting. Easy enough to add some holes and a port, and use a vacuum to hold the material down (or whatever method is common). How flexible are these cutters with Z axis variance? My slats are no longer flat, due to all the plasma cutting. Notches, dross, small welded parts of previous cuts, etc. I'm curious what's involved in leveling my "cutting box" (for vinyl, though I assume routing as well). Do you just shim it until it's measures level, then clamp it down somehow? Is that "close enough", or do we need a machinists level here?

The small plotter-knives look to be a quick and easy way to add a drag knife. Put a holder on my Z axis, and I'm good to go. They're also spring loaded, which is good (depending on the answer to the question above). However, I'd also like the option of cutting magnetic material. I've found .030" magnetic material, which vehicle door signs, fridge magnets, etc are made of. Will these plotter knives cut material like this? Or am I going to be limited to vinyl/paper/etc only? Magnetic material wouldn't be a problem for something like this Donek knife (http://donektools.com/), but it's $250. For about 2x that, I could buy a dedicated vinyl plotter (http://www.uscutter.com/Value-Cutters)

Looking for input from those of you who have already been there. Thanks!

[acourtjester]

I have built and used a drag knife on my table and it is not that hard to use. I buy the blade holder from eBay for about $15 with blades. I made a free sliding mount for it so it uses the weight of the mount/holder for the down force. It cuts very well I copied the one Sean P made and if you go back and search here you will find his info and video. I just tape the corners down no need for a vacuum setup. Now the magnetic material I have not used of have any input about that. Also there is a guy selling plans for routing aluminum to make a direct copy of the one you talked about. That one uses a utility knife blade and may work with the magnetic stuff, you may need to find a stainless steel blade (non-magnetic).

[sawdust1]

Depending on how much you plan to do you could find an old Cricut Personal cutter. Should be less than $50. Has all the parts you need for a pressure adjustable cutter. You can dial up or down the pressure, the solenoid will become your z axis if you wish. Toss the x and y axis with the casing. The blade holders are excellent and the blades relatively inexpensive, used will likely come with both.

David

[SeanP]

I just place a piece of mdf or plastic sheet on top of the slats and tape on the vinyl sheet around the edges, just 4 corners quite often is enough.
If you use a linear bearing type design you can get a reasonable amount of floating travel, I just have 2 magnets to mount it on the z axis plate.
I think the secret is having just the right weight on the cutter, I think it's 110grams.
This is the link to the setup and other info:
http://www.plasmaspider.com/viewtopic.p ... 7a8#p89542

[SONIC]

I have a drag knife on my CNC router, and I have a vinyl cutter (USCutter)
I do not use the drag knife.
The vinyl cutter is cheap and easy and works way better.

Without a vacuum table you can't reliably cut anything with a drag knife, as it just drags it around. So you'd need to add the expense of a shop vac and material to build a vacuum table. Then add the PITA factor of moving it on and off the plasma and accounting for unlevel surfaces.

Get a vinyl plotter and be done with it.

[SeanP]

Without a vacuum table you can't reliably cut anything with a drag knife, as it just drags it around. So you'd need to add the expense of a shop vac and material to build a vacuum table. Then add the PITA factor of moving it on and off the plasma and accounting for unlevel surfaces.

Get a vinyl plotter and be done with it.

I'm sorry sonic but I don't agree with that, the cutter I made is no problem over a uneven surface.
Maybe cut quality on the corners is not quite as good, but if it's just a few bits here and there it works a treat.

Did this job the other week no probs.

https://goo.gl/photos/dFnaFbbYc31fzSEy6

[SONIC]

Fair enough!
You're doing better than I ever could.

How much fiddling does it take to make those cuts? How do you adjust cut pressure?

[acourtjester]

Here are some images of drag knifes I made (copy of Sean's design) what I found was how much the cutter expends below the drag knife has a great deal on how well it works. To much and you cut the paper backing which can cause problems.

DSCN1677 A.jpg

DSCN1681.JPG

DSCN1005.JPG

DSCN0998.JPG

[SeanP]

Fair enough!
You're doing better than I ever could.

How much fiddling does it take to make those cuts? How do you adjust cut pressure?

To be honest it's way easier than cutting metal!
The cut pressure comes from having the correct weight pressing down on the cutter which is floating rising up and down the linear bearing.
Then just having the right amount of blade protruding from the holder so as not to cut through the backing paper, same as a vinyl cutter I guess.

There is a drag knife module for sheetcam I see which would probably give even better corners, but its not to bad without really.
Although I just noticed I didn't have loop sharp corner tuned on in sheetcam for that last job, that would have helped,,,,,always something