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 2 years ago.

Viewing 13 posts - 31 through 43 (of 43 total)
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  • #46729

    Jason
    Participant

    Yeah, posterboard is just cardstock.  It’s not hard to cut…just not happy with the performance of the drag knife overall.  The slop between the 8mm screw and the bearings is the biggest issue…that and generating proper gcode.

    That TOCK is pretty crazy.  The extra axis to rotate the knife is pretty nuts…I see he included something for generating the required gcode (for mach) but wonder how flexible it is.  And honestly overkill for anything I want to do 😀

    There was talk about something similar in the flitetest thread about the needle cutter.  There was also talk about using a similar mechanism for creating the needle motion…but in the end it just adds more complexity without any major benefits.

    Honestly the TOCK just seems like more to go wrong than a needle.  It’s basically the same thing just with a blade instead of a needle so it has to rotate.  But with a sufficiently fast needle timed to your head movement there’s no need to deal with rotations.

    My needle cutter does work on cardboard now that I added the glue to hold the needle on the bearing…it does do better a little slower than the speed I use for cutting DTFB though.   Maybe I’ll try a bit more cardboard this weekend just for fun….

     

     

    I’m just not interested in putting a lot of time into getting the drag knife working better since I already have tools I’m happy with that do everything I need.  Still fun to have built it…and I do see how it can be useful for some things.  Just nothing I do regularly.

    #46741

    Barry
    Participant

    The router tables from Esko have vibratory knives on them.  You can’t really see the blade moving, it’s like a quarter millimeter or something like that.  They’re also servo controlled to turn with the cut.  They could zip through corrugated like a hot knife through styrofoam though.  Wicked accurate too.  Could cut vinyl sticker material without cutting the paper backing.  Some of the blades they used were solid carbide.

    #46746

    James Donnelly
    Participant

    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.

     

    Yeah, posterboard is just cardstock. It’s not hard to cut…just not happy with the performance of the drag knife overall. The slop between the 8mm screw and the bearings is the biggest issue…that and generating proper gcode. That TOCK is pretty crazy.  

    Yes, chances of me making one are slim to none to be honest.  I’d rather build a massive laser.

    The router tables from Esko have vibratory knives on them. You can’t really see the blade moving, it’s like a quarter millimeter or something like that. They’re also servo controlled to turn with the cut. They could zip through corrugated like a hot knife through styrofoam though. Wicked accurate too. Could cut vinyl sticker material without cutting the paper backing. Some of the blades they used were solid carbide.

    I mentioned the sign shop near my work for free foam board.  They have a 10’x15′ bed machine and use tangential oscillating cutters.   I didn’t see it in action but it looked alot like this:   https://www.youtube.com/watch?v=q-zehi0UQUI

    I asked to see the blade / cutting tool though, and was surprisingly small – looked a bit like the blades that go in the chinese small drag knives.

    I guess they use these cutters industrially because they are very fast.  I’m not sure the TOCK will ever be faster than a needle cutter or laser, and the blade looks too long to do fine work anyway.

    #46750

    Jason
    Participant

    I’d expect the TOCK to be faster.  The needle isn’t super fast…but I’m not in a big hurry 😉

    A couple people have tried speeding up the needle to use it in production work but haven’t had much success.  I currently run mine at about 9k RPM on the needle and 15mm/s feedrate.   That lets me cut an average 20″x30″ sheet of foam in about 15 minutes.  Though some more complex designs take longer and some simpler ones take less…but on average it’s about 15 minutes a sheet.

    The problem with speeding it up is tricky.  First spinning the flywheel faster gets hairy quick, the forces ramp up and things start to fall apart faster.  Second even with the needle moving quicker it doesn’t eliminate needle drag.  In fact in sometimes makes it worse.  Since the machine movements and the needle movements aren’t synchronized the machine is moving while the needle is at all points of it’s motion.  The needle only cuts right as it hits…but the rest of it’s stroke while it’s fully in the foam it’s a potential drag.

    So if you watch it closely you can see it start to deflect and drag on the downstroke, then re-align on the upstroke.  It’s still fast enough that the individual punctures overlap and you get a clean cut – but as it’s moved faster laterally the drag on the needle goes up even if the speed of the needle increases.

    That’s why I have to slow down for cardboard – it has more drag than the foam since it’s stiffer and the problem gets worse.  A shorter needle would probably help (mine is currently 2-3mm longer than it needs to be – I left it long so I could sharpen it more times before replacing it.)  but the best solution is just slowing down.  I was running around 5-10mm/s to get reliable cuts in cardboard without stressing the needle.  At 15 which works well for foam it was just knocking the needle off constantly.

    Actually – now that I have the vac table and my cuts are looking even cleaner I should try speeding up the foam cuts a bit more…I’ve actually run it at 20mm/s but the cut quality diminished.  So now that I’ve improved cut quality with better work holding I may be able to give it a bit more speed.  I’m also curious to see how cardboard does on the vac table and if I can do any better with it now.

    #46756

    James Donnelly
    Participant

    @jason I’m with you on speed, not the top priority.  Love the vac table by the way – I voted, but you didn’t need me, you’re killing it.

    Keep us posted on the cardboard.  If you get a chance to try the double wall stuff, 5-7mm, it would be great to hear your results.  If it’s reliable I don’t care how slow.

    I have a question now about whether oscillation is as important as tangential control.  Interesting test here, which looks more accessible than the TOCK:

    Industrial example with no oscillation (claims small radii are possible, I thought maybe oscillation helped with that):

    Cutting Foamboard with an AXYZ Tangential Knife

     

    #46759

    Ryan
    Keymaster

    The nicer vinyl cutters use this but they also have some crazy expensive software to go along with it. Do you have any leads on some software that can do the tangential gcode? I am sure it would be pretty easy to get a stepper on there, but the code is the hard part (says the mechanical guy).

    #46760

    Ryan
    Keymaster

    As I watched your linked video the next one to play was tangential gcode from inkscape…dam…If I would have only waited a minute to post sorry guys.

    #46761

    Jason
    Participant

    I would guess oscillation mostly helps when the blade starts to get dull.  With a fresh sharp knife just dragging it works very well – but as it gets dull sometimes a bit of sawing motion starts to help.

    So I’d assume the oscillation is more a “backup” so you don’t have a big job go south if the blade gets dull partway through.  That and helping production shops get the most from their tooling.

    Interesting that inkscape gcodetools can do tangential…I have a spare stepper sitting here…and my wife and daughter are out of town this weekend so I can focus on my projects…hmm…..

    #46764

    James Donnelly
    Participant

    Ryan, yes I was imagining myself being willing to get coding to make a post processor translate tangential moves into the gcode – then I found that video/information that Inkscape can do it.

    Jason, I also have a spare stepper.  Interesting project, eh?  What you say about oscillation makes sense.  Are you thinking that the design we’ve used could be adapted or something else.

    Seems to me to be a good starting point, although you need to be able to set a zero offset.  I was planning to improve my initial adjustable offset mod by beefing up the material around the sliding dovetail and adding a screw.  If that’s useful I can continue, but if you have a simpler approach in mind…

    #46765

    Jason
    Participant

    I honestly don’t have any particular approach in mind…not even sure if I’ll actually pursue it or not.  I’ve kind of got a backlog of planes I’ve cut but haven’t actually built yet  and some I’ve built but haven’t finished wiring…and now that the weather is getting nicer here I’m itching to get some of them in the air.

    So…don’t wait on me to make any progress.  But one evening I may get an itch and try something 😉

    #46823

    James Donnelly
    Participant

    Don’t worry there is no chance of me waiting for anything.

    I was interested to hear if you had any ideas.  I have something sketched out now and will enjoy trying to do it myself.  Just don’t want to duplicate effort so if you do it first, great.

     

    #46825

    Barry
    Participant

    I would guess oscillation mostly helps when the blade starts to get dull. With a fresh sharp knife just dragging it works very well – but as it gets dull sometimes a bit of sawing motion starts to help. So I’d assume the oscillation is more a “backup” so you don’t have a big job go south if the blade gets dull partway through. That and helping production shops get the most from their tooling. Interesting that inkscape gcodetools can do tangential…I have a spare stepper sitting here…and my wife and daughter are out of town this weekend so I can focus on my projects…hmm…..

    Usually you’d use the oscillating cutters on thicker materials,  multi-layer corrugated, or expanded pvc foam, for example.  They also help with complex shapes and fine detailed cutouts.

    #46832

    James Donnelly
    Participant

    Usually you’d use the oscillating cutters on thicker materials, multi-layer corrugated, or expanded pvc foam, for example. They also help with complex shapes and fine detailed cutouts.

    Do you think multiple passes will compensate for oscillation for cutting double wall card?  I haven’t managed to go through it in one pass yet with the drag knife, but 2 passes seems fine.

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