Drag knife not making sharp corners

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This topic contains 42 replies, has 8 voices, and was last updated by  James Donnelly 3 weeks, 1 day ago.

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  • #25876

    Jason
    Participant

    Was playing with my drag knife this weekend and some of the real fine details and sharp corners were pulling up. I was cutting it on a glass mirror using the roland style cutter with a brand new blade with Ryan’s knife holder.

    Could it be too much pressure? Not enough pressure? Cutting too slow, too fast?

    #25883

    Jason
    Participant

    Are you using gcode that compensates for the drag knife? Because the tip of the drag knife trails the center point of the tool you have to add extra pivots in the gcode to compensate.

    Here’s a page from a drag knife manufacturer that explains and includes an excell spreadsheet for adding the compensations to existing gcode files:

    Add Corner Actions to your Tool Path

    I believe dxf2gcode can also do the compensations, but I’ve not had any luck getting dxf2gcode to read any of my dxf files yet 🙁

    #25886

    Jason
    Participant

    Well that just added a new layer of complexity LOL. Now to see if I can figure this out. Laser cutting is much easier… I am aware of the dangers of vinyl and lasers. Ventilation is good and I do not do much vinyl nor do I staying the room while its cutting.

    #26327

    Jason
    Participant

    @vicious1 will estlcam with arcs off run the drag knife properly?

    #26408

    Mic Vee
    Participant

    I’d really like to know the answer to this also. Estlcam has a drag knife setting for Estlcam firmware. I plan to test it out soon, but I’m curious if anyone has used it already.

    #26416

    Ryan
    Keymaster

    Well you should essentially be getting .025mm (or maybe .05mm) rounded corners from the tip of the carbide being back offset to make sure it drags in the right direction. For small letters and things this makes a big difference, large stuff you hardly notice it.

    My vinyl cutting software had that setting built in, But I don’t own it anymore.

    To use CAM software you would have to overcut the corners, that might work.

    This could be a cool request for Christian from ESTLCAM, He might really like the opportunity to make his amazing software more versatile. just a tip offset setting and it could work for the small drag knifes or the large razor blade style.

    #26419

    Jason
    Participant

    Yes a proper drag knife setting and laser settings would make this a totally perfect software package.

    #26913

    Mic Vee
    Participant

    Estlcam does have a drag knife setting for the Estlcam firmware. I finally got around to trying it. The only parameter you can adjust is the blade offset, but it seems to work decently well (the origin corner is the trickiest).

    It’s important that you set the blade correctly and get your Z axis zeroed properly, so the corner actions will have the desired effect. I think I can get even better results if I spend more time tinkering.

    Picture of a square I just cut: https://www.amazon.com/photos/share/HuOccrTcwfNJIZ9W9gkq3d3nDJ3ckiJ3xQu0yNOqdAo

    I’m running Estlcam firmware on RAMPS.

    #27135

    Jason
    Participant

    Little rough but pretty close. Did you set the corner overcuts? What angles did you use?

    #27224

    Mic Vee
    Participant

    Yeah, a bit rough, but better than nothing and I think it can be improved upon. My current MPCNC setup is far from ideal for a drag knife, so someone else might do better just because their machine is set up better. My waste board is not completely flat, which is absolutely needed for corner actions to work correctly.

    I didn’t use overcuts because I wanted to see how sharp I could make the corners with the Estlcam drag knife settings. Maybe I’m missing something though. Aren’t overcuts used for rounding off outer corners?

    #43894

    Guys, where did you find the Drag Knife possibility in Estlcam? Can you post a short manual how to use this?

    #43897

    Ryan
    Keymaster

    Drag Knife, Vinyl Cutter

    How To

    1 user thanked author for this post.
    #44115

    Ryan thanks again for the links! Due to some reason even Google was not able to find them

    So I was experimenting with my Drag Knife and kind of Depron Foam (quite hard foam of 4mm height), and i havent found any solution that will be both easy and acceptable. I found that when cutting all 4mm at once, on every sharp corner I have to lift the knife and re-prime it (slowly move it into new cut). That means that in ESTLCAM you can not automatically generate such drawings, for example for rectangle you will need to create 4 separate lines with 4 separate starting points for the knife…. Which is quite sad and boring..

    Am I doing something wrong?

    I am still wondering what was that Drag Knife support Mic Vee was mentioning? I havent found a single mention of Drag Knife in ESTLCAM docs

     

    #44123

    Ryan
    Keymaster

    4mm? I would drag a endmill though it, drag knife usually are meant for much thinner material up to sandblast mask at maybe 1.5mm.

    #44127

    dkj4linux
    Participant

    Those Donek drag-knives use a utility blade and can do 1/4″ or so. They’re not cheap but do the job nicely, it appears… note the Z moves with change of direction (part of the corner compensation)

    I helped a guy visualize why mangled corners and the need for corner compensation once with a simple illustration — stab butter with tip of butter knife and twist.

    — David

     

     

    2 users thanked author for this post.
    #46458

    James Donnelly
    Participant

    I started with the little Chinese cutter as recommended and sold by Ryan, and found it fine for vinyl.  I’ve made some nice decals with that.  I’ve found fairly sharp corners no problem with no corner actions on that because the blade and offset are so small.  But I haven’t done much lettering where it would show up more.

    Having played with cardboard cutting on a laser, I’ve turned my attention to cutting out bigger shapes in cardboard with the MPCNC.  Hence I made one of these:

    https://www.thingiverse.com/thing:2494258

    But put an additional spring in the assembly.  Using that I can cut 7mm cardboard in several passes, but it fails occasionally (blade sideways, emergency stop!), so I’ve been thinking about building variable offset into the design.  Donek’s tools come with different offsets for different depth and cut radius specs.  Only trying to cut circles at the moment, but would use the Donek spreadsheet to do corner actions if needed.

    Funny that you showed up dkj4linux, I came to the conclusion on the train this morning that the answer for thick cardboard was to build your needle cutter!  I was always destined to do that, but this has given me the push.

    #46464

    Jason
    Participant
    Funny that you showed up dkj4linux, I came to the conclusion on the train this morning that the answer for thick cardboard was to build your needle cutter! I was always destined to do that, but this has given me the push.

    I was actually going to suggest trying the needle 😉  I’ve done some cardboard with mine and it can work…but is definitely trickier than foam and foam board due to the extra resistance.  The flywheel style cutter is a far better choice, I don’t think a crank style cutter would stand a chance against cardboard as the crank would probably deform from the pressure.  I did find I had to slow it down quite a bit compared to foam, and the other issue I had was my needle coming off the bearing – a drop of CA glue seems to have solved that, but a day later moebeast shared his printed needle retainer and it’s probably a better option.

    2.8w laser can also do well on some cardboards…but I’ve had mixed results.  Air assist helps a LOT, without it I tended to catch the cardboard on fire before getting a full cut.  The cases that girl scout cookies come in proved ridiculously hard to cut with a laser.   I kind of wonder if they’re treated with something to make them fire resistant…but then again they also tended to burst into flames before cutting so maybe not 😀

    #46475

    James Donnelly
    Participant

    One of those days I guess Jason.  It was your instructable I was reading on the train this morning.  You and dkj4linux were my bookmarks for the needle cutter.  Thanks both for all your great work.

    The birth of this process for me was using my new 40w laser cutter and Fusion 360 slicer (I think it used to be 123d make), and finding the a4 footprint a bit restrictive.  On that machine, I need to cut 7mm double wall cardboard with the same speed and power(15mm/s, 17mA) as 3mm ply just to go through in one pass , so it is surprisingly tough stuff.  Silly of me to assume that it would be as easy as foam on the needle cutter.

    From my experiments, 3mm or single wall card cuts just fine with the big drag knife.  The thick double wall is very strong stuff by comparison though, so I expect it will give a lot more structure to projects.

    I already had in mind the flywheel over the crank from reading you earlier – I want to be able to push the envelope I think.

    #46478

    Jason
    Participant
    One of those days I guess Jason. It was your instructable I was reading on the train this morning. You and dkj4linux were my bookmarks for the needle cutter. Thanks both for all your great work.

    Well, you’re inspiring me right back.  I love my needle and laser…but also love having as many tools as possible as options 😉  And I’ve been wanting to mess with a drag knife (I have the vinyl one which is nice for vinyl, but I want a bigger one to play with.)

    So just fired up onshape and modified the knife you shared to have a hicwic mount:

    Screen-Shot-2017-10-23-at-9.20.03-AM

    Will have to print it up tonight and give it a try.  I’m still not big on drag knifes overall due to the need for the extra motions…but figure I should give on an actual try 😀

    #46484

    James Donnelly
    Participant

    Quick work!  I’m new to CAD, having done stuff as needed in Sketchup for years.  I’m using F360, and learning.  Hope to be in a place where I can work as quickly as that soon.   Anyway your design might turn out to be useful because I recently started printing bits of hicwic.

    Let us know how it goes.  To honour the OP and thread, I promise to try out and comment on Donek’s corner actions XLS once I’ve managed to try moving the drag knife offset.

    #46492

    Jason
    Participant
    Quick work! I’m new to CAD, having done stuff as needed in Sketchup for years. I’m using F360, and learning. Hope to be in a place where I can work as quickly as that soon. Anyway your design might turn out to be useful because I recently started printing bits of hicwic. Let us know how it goes.

    Well, I already had the hicwic mount drawn up from some other projects so it was pretty simple to just add the bearing housing to it 😉  F360 is nice, but doesn’t run nearly as well on my computers as Onshape so I do most of my stuff in Onshape.  But I want to try F360 for CAM since it will run on my macbook and Estlcam won’t….but I do love estlcam and my macbook is just an air so it doesn’t run F360 very well.

    To honour the OP and thread, I promise to try out and comment on Donek’s corner actions XLS once I’ve managed to try moving the drag knife offset.

    Look forward to seeing how it goes.  Main reason I haven’t tried it is I don’t have excel…and don’t know how well it will work in google sheets.  And it’s setup for imperial measurements and I do all my CAD/CAM in metric…so not huge on dealing with conversion issues.  I keep hoping estlcam will get drag knife actions for non-estlcam firmware.

    #46565

    Jason
    Participant

    So looks like it will work:

    2017-10-23-21.39.12

    I’m not sure about the spring I used…may be a little stiff.  But I had one that was maybe a little too stiff and one that was definitely not strong enough to do anything.  Figured I can always cut down the stiff one if I need it softer.

    Now I just need to figure out the gcode.  Since the donek tool is all in imperial I’m not sure how to go about using it if I even can as I do all my gcode in metric…Even if I worked in imperial in CAM my gcode is all output in metric since that’s what Marlin expects.  So not sure what the best approach is here.  I may just look at what their code does and see if I can whip up something that does in another language.

    @James Donnelly – Any tips based on what you’ve done so far with your drag knife?

    I’d like to try this on some poster board I need to cut…but I’m not sure that’s going to be thick enough to work well, may not have enough for the blade to bite into when rotating.  I could try it on some dollar tree foam…but cutting the current foam with a blade is really sub-optimal as it tends to tear a lot…and I can seldom cut a full sheet of foam with one blade without wearing out the edge of the blade anymore.

    Still, I do want to play with this some just need to figure out the best way to adjust the gcode.

     

    #46571

    James Donnelly
    Participant

    Lookin’ good!  My spring is quite soft, and I think it does help, but I’m not certain.  I think the lower pressure on the piece leads to more stability in the blade.  Lower pressure does mean more passes though, but I’ve found pushing the speed doesn’t hurt to a point.

    Regarding the spreadsheet, I suspect it would be hard to translate to Google Sheets or Open Office as the macros do some of the work.  I’m willing to try it out in imperial and use Excel.  I think the best way forward for general use with that would be to implement it again with Python – not a small undertaking unless you’re jeffeb3.

    My two hopes for cutting thick card (no so into foam board, no DollarTree here in UK, and it’s not so cheap) are:

    Offset

    Last night I managed to increase the offset a few mm by moving the blade in the current arrangement.  It didn’t seem to help.  I might end up trying to re-model the design to allow for adjustable offset.  Would involve possibly splitting the bottom assembly, mounting the lower part in a groove and indexing with a screw.  The problem here is that with increased offset, your minimum cut radius is going to increase.

    Blade shape

    The standard blade leaves a triangular protrusion.  This means with deeper cuts there is quite a lot of blade length in the material asking to turn and snag.  When I’m next in the shop, I plan to grind down the back edge of a standard blade to see if a much steeper blade angle (like an Exacto or steeper) will help.  I’ll be wearing safety glasses when I try it out :O

    #46597

    Jason
    Participant
    Lookin’ good! My spring is quite soft, and I think it does help, but I’m not certain. I think the lower pressure on the piece leads to more stability in the blade. Lower pressure does mean more passes though, but I’ve found pushing the speed doesn’t hurt to a point.

    Well, I did have some softer springs…but they were MUCH softer.  I’ll just have to experiment and see how it does I guess.

    Regarding the spreadsheet, I suspect it would be hard to translate to Google Sheets or Open Office as the macros do some of the work. I’m willing to try it out in imperial and use Excel. I think the best way forward for general use with that would be to implement it again with Python – not a small undertaking unless you’re jeffeb3.

    Well, I’m a web developer by trade and have done some similar projects…I’m just not very experienced with excel and am not sure if they’ve got the macros encoded or protected in some way that will prevent me from figuring out what they’re doing 😉   I’ll probably give DXF2GCODE another go.  It can supposedly do drag knife corrections – but I’ve never had any luck getting it actually do…well…anything.

    When I’m next in the shop, I plan to grind down the back edge of a standard blade to see if a much steeper blade angle (like an Exacto or steeper) will help. I’ll be wearing safety glasses when I try it out :O

    I noticed in the Donek video of cutting adams foam he mentioned that the top wasn’t super clean and that switching to an exacto blade instead would probably improve the cut quality.  So you’re probably on the right track there.

    BTW – I uploaded my HicWic mount version to Thingiverse:  https://www.thingiverse.com/thing:2603958

    Kind of surprised at how many people are adding it to collections and liking it already 😀

    #46651

    James Donnelly
    Participant

    Good to see your creation on Thingiverse.   Drag knives must be cool after all!

    I’ll get around to measuring spring pressure with a scale once I start bringing things together.

    I spent some time looking for evidence of what exactly the drag knife support amounted to in dxf2gcode, and couldn’t work anything out either.  I didn’t look too hard because I’ve heard good reports from people using the Donek spreadsheet.  I haven’t looked at that in depth either but at first glance, the macros are needed to support working inline on a file you supply.  I don’t expect the calculations to be macros or protected.

    Anyway, progress with the blade mods.  I shaped the blade as discussed the grinder. First I mounted it oriented tall and steep like in this pic:

    blade

    But I found the snagging was even worse than the standard.  I was just going to give up on the blade mod approach, when it occurred to me that when the blade angle wrt the cutting material is too parallel, it’s going to be harder to cut through.  So for an extreme test, I mounted the same blade (cut down a bit more) like this:

    blade-mounted

    Well this time, it cut out the circle very nicely and reliably.  Obviously there is too much of an offset – it’s fine for circles.  But the key seems to be the shallow blade angle for thick material.

    Before I did these tests, I’d started modelling the adjustable offset version of the knife:

    dovetail_lower
    dovetail

    But now I am thinking of working in a blade rotation angle mod, so you can independently control offset and entry angle.

    #46657

    Jason
    Participant

    I found that getting dxf2gcode going on macos is a bit tricky….there’s no binary distro and the source is tough to build since it has requirements that also don’t have easily installed versions available.  So…

    I waited until I got home and tried it on one of my windows machines.  Finally got it going…and seemed to generate some drag knife friendly gcode…but when I cut with it the results were less than spectacular.

    My phone battery was dead so I wasn’t able to get a photo of the results…but they were pretty ugly.  Corners were torn and even some straight lines weren’t very straight.  It seems the 8mm bolt can shift around in the bearings a little which causes some slop.  But the bigger issue is the corner actions didn’t seem to clean up the corners very well.

    I suspect part of the problem is that even my brand new fresh from the package blade wasn’t super sharp at the tip which cost accuracy.  But the main problem seems to be it was keeping the knife down too low when doing the corner moves so it was ripping and tearing stuff as it rotated.  Not sure if it’s just me not being familiar with the settings in dxf2gcode or if it’s just dxf2gcode doesn’t do great corner actions…or if it’s just my blades.

    Will do some more experiments…but overall…not super impressed.  My laser cuts poster board great other than leaving the edges darkened, and my needle does my foam (there’s no way the drag knife would do foam anywhere near as well, the blade just can’t do the tight curves I can do with the needle.)

    It’s nice to have in my toolbox and I can see it being nice for things that don’t have sharp corners.  But overall…I’m kind of meh on it.

     

    #46663

    James Donnelly
    Participant

    Yes you said it right.  I don’t think it should depend so much on an ultra sharp blade though.

    But yes, this thread started about sharp corners, whereas you and me are also interested in thicker materials.  But unlike you I don’t need sharp corners because I’m actually trying to make a scraplight (like Graypants).

    I did say I’d look into sharp corners and I will, but I think I’m on to something about thicker material.  I measured offset of the design we’re using and in a standard set up, it’s 4mm at the tip of the blade.  This means that with 4 thick material, you’ve lost half your offset.  I think cutting 7mm as I’ve been with multiple passes, when the blade is at the bottom, the top of the blade is touching the material with barely any offset.  Having a more vertical blade reduces this effect, but makes it harder to cut through the material.

    I think the answer is to calculate offset according to the thickness and blade angle, so at the top of the material, you have a good enough offset for a follow effect, but not too much to increase minimum radius.  Then cutting in one pass I think.  I’ll carry on with the variable offset version design.

    Where I think you’re correct is that thick material + drag knife + tight radiuses / corners = fail.  You can’t have a more upright blade, and that mean the cut line at the bottom of the material is always behind the one at the top.  You can still to good stuff with the big bed cnc:

    https://www.pinterest.co.uk/explore/cardboard-furniture/?lp=true

    Might have to keep going until the giant laser gets built!

    #46664

    Jason
    Participant

    Well, I’m not even trying thick material yet.  Figured I’d start with posterboard since it should be “easy”.

    This was the first result:

    20171025_132954

    Ick.  Oh, and I forgot the worst part earlier…the gcode dxf2gcode created crashed Marlin before it finished.  I ran the code with the knife lifted up first for a dry run and didn’t notice…I just thought it didn’t return to home after the cut finished – I did think it was odd that my LCD was showing all 0’s on the coordinates though.

    Then I ran it on the actual posterboard…and when it went to do the last few inside cuts marlin rebooted.  I haven’t looked at the code yet to see just what command it sent that caused that 🙁

    Because honestly the cutting results overall at that point were so unimpressive.

    I probably will play with it more…but I’m really not in any rush since I don’t have a need for it.

    #46708

    James Donnelly
    Participant

    @jason, sorry to hear it didn’t work out great with the foam.  I’ve found a sign maker near work, and they agreed to me grabbing any scrap I want, so I’ll be trying at some point with foam board.  I don’t know what posterboard is called in the UK.  Only UK links I’ve found reference plain old card, with no foam.  Is that the stuff?

    I don’t know why that would be a challenge to cut, but I will know if I try.  I suspect something is wrong.

    The 7mm cardboard is cutting very well on a prototype of my variable offset knife now.  I didn’t bother with a fixing screw and just set the offset to +2mm of the initial design and glued things up.  Pretty clean lines and no snagging now – might actually be practical for medium to large cardboard structures without tight corners

    I plan to try running a router bit through the card as an alternative to the knife, but by all reports you get fuzzy / rough edges, and it’s very hard on bits.  Not fun if you have 50 layers to cut.

    I’ve also ordered the emax brushless motor + esc, so will be trying with a needle cutter too.  Hopefully this can work as this would be the best.  The nuclear option will be to make one of these:

    https://www.thingiverse.com/thing:1436870

    PS: I agree about the play in the bolt seated in the bearings.  I guess the answer is to get some 8mm rod and thread both ends to accept a nut, that way it’s a smooth surface travelling inside the bearing so the spring movement will be smooth.

    #46717

    Ryan
    Keymaster

    That thingiverse link is nuts. If you do end up trying it please please share the results. Seems to be well thought out and complete.

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