Drag knife.

Such as Laser cutters (Must use the phrase 'sharks with frickin lazors' once per thread)
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Drag knife.

Postby RegB » Fri Jan 29, 2016 7:03 am

I want to cut some vinyl and want to NOT lazer burn it.
I found a few threads in other places that deal with building (or buying) the drag knife itself, but so far I haven't found a general solution to the drag offset issue.
Many suggestions say to just avoid radii less than some limit, or to over cut, raise re-position and start a new cut in the new direction.
They all seem unsatisfactory and don't account for undercutting on the outside of inside curves or overcutting on the outside of outside curves - - what tractor trailers do, in the US on right turns it can mess up impatient/careless bicycle riders and/or mail boxes.

So I am looking for 2D software that can DO this - i.e. handle the drag offset.

I'm supposed to say Sharks with fricki'n lazers - lets make that razors :D

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Re: Drag knife.

Postby bubbasnow » Fri Jan 29, 2016 11:30 am

if you are worried about drag offsets with software, im sure you can manually drag the lines past the intersection of issue and do a seperate lead in/out for each line segment.

doesnt seem like this guy is having any need or issues with.


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Re: Drag knife.

Postby RegB » Fri Jan 29, 2016 12:00 pm

bubbasnow wrote:if you are worried about drag offsets with software, im sure you can manually drag the lines past the intersection of issue and do a seperate lead in/out for each line segment.

doesnt seem like this guy is having any need or issues with.



Yes, that can work with straight lines, the problem is with curves.
Just like on a vehicle the rear wheels never QUITE follow the front wheels on curves, they always "short track". The tighter the curve the more pronounced the effect.
With long trailers you drive the trailer more than you drive the tractor :D

I am looking for software that imitates the path that a GOOD tractor trailer driver would take with a 53 ft trailer except the tractor would be airborne :D

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Re: Drag knife.

Postby fletcher » Fri Jan 29, 2016 12:25 pm

If you are only looking to cut vinyl, maybe something like this could work for you?

https://www.rockcliffmachine.com/produc ... bit-0-250/

The blade in these is only offset by .25mm or so. I have one for my cnc router - works very well for sign vinyl.

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Re: Drag knife.

Postby bubbasnow » Fri Jan 29, 2016 2:16 pm

RegB wrote:
Yes, that can work with straight lines, the problem is with curves.
Just like on a vehicle the rear wheels never QUITE follow the front wheels on curves, they always "short track". The tighter the curve the more pronounced the effect.
With long trailers you drive the trailer more than you drive the tractor :D

I am looking for software that imitates the path that a GOOD tractor trailer driver would take with a 53 ft trailer except the tractor would be airborne :D


Of course like you say the longer the trailer the less it tracks, but the opposite is true with a well designed drag knife. its offset from center is so minor that its "short track" is not noticeable.

in the video i linked and the video fletcher linked, you can see that no special software is required.

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Re: Drag knife.

Postby RegB » Fri Jan 29, 2016 2:51 pm

Thanks, I had found the Rockcliff site before but didn't find (&still haven't found) the 0.25mm offset spec.
That may well be close enough to zero that it approximates zero.
Of course as it approaches zero the blade will "caster" less effectively.

On the cost issue; I had hoped for something that uses standard utility knife (Stanley knife) blades and costs twenty bucks :D
Oh well, you get what you pay for (at best).

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Re: Drag knife.

Postby bubbasnow » Fri Jan 29, 2016 4:54 pm


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Re: Drag knife.

Postby teoman » Fri Jan 29, 2016 6:51 pm

Why drag it?

A cheapo servo could rotate it.
When on mobile I am brief and may be perceived as an arsl.

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Re: Drag knife.

Postby bubbasnow » Fri Jan 29, 2016 10:01 pm

teoman wrote:Why drag it?

A cheapo servo could rotate it.


that would be called "over engineering"

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Re: Drag knife.

Postby teoman » Sat Jan 30, 2016 10:53 pm

I have been knon to ponder on problems that do not really need a solution.
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Re: Drag knife.

Postby Xenocrates » Sat Jan 30, 2016 11:14 pm

Problems which don't need solved? Are you perchance a Preacher, Politician, Psychiatrist, Chiropractor, or consultant? Because most problems which don't need solved are not what I would call problems.
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Re: Drag knife.

Postby teoman » Sun Jan 31, 2016 7:08 am

Now you are insulting me. :x

Bsc computer engineering, msc electronics and phd mechatronics. (Which mean nothing in the real world)

Was programming industrial robots and creating solutions, now in the mobile device world and assistant prof.

I may not have expressed myself properly (english is not my native language) but it was mentioned that the drag knives had a minor following problem and a controlled cutting head would solve that.

Kind of like using an exacto knife or a scalpel where you determine the angle and the position kind of follows that vs a deburring knife where you manually provide the position (or force) and the knife follows a similar position but not quite the same.
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Re: Drag knife.

Postby Xenocrates » Sun Jan 31, 2016 9:40 am

I was attempting to make a joke Teo. I apologize. I'm currently working on an associates in mechatronics. I'm not precisely great at getting tone right in writing. Perhaps I should have added some form of smiley or sarcasm tag.

As far as drag knifes having following problems go, I think a 1/16 radius is enough for most things, and beyond that, it might make more sense to design around something like a laser or waterknife for cutting. Figuring out how to program the servo, while doable, would not be easy, and is likely more complicated than it's worth, especially if you want accuracy on curves. Either that, or do what was suggested earlier, and have leads in and out (which only doesn't work if you have a need to pack things really closely)
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Re: Drag knife.

Postby Eaglezsoar » Sun Jan 31, 2016 1:46 pm

Teo, I really appreciate your posts and it is great to know that you have a well rounded and extensive education but you need to chill out a little.
Xenocrates was just joking with you and everybody got it except you. We all are well rounded members and I consider us all friends as well.
Please don't take things so seriously and please keep posting on this forum. Your posts have helped a lot of members sir, and we do not want you
to be offended at any time. Peace! :) :)

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Re: Drag knife.

Postby teoman » Sun Jan 31, 2016 2:00 pm

Oh don't worry. I was not offended at all :D

I thought the angry emoticon made that clear, who would use an emoticon when they are truly angry?
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Re: Drag knife.

Postby Eaglezsoar » Sun Jan 31, 2016 2:04 pm

teoman wrote:Oh don't worry. I was not offended at all :D

I thought the angry emoticon made that clear, who would use an emoticon when they are truly angry?


Thanks, I am happy that you were not truly angry. I have made it part of my job not to allow users to be angry
about anything because down deep we are all friends sharing information. Thank you for all your great posts and
I know we will see many more. You are a key member of this forum and it is appreciated.

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Re: Drag knife.

Postby Xenocrates » Sun Jan 31, 2016 6:05 pm

I'm was worried Teo, since you've had some great posts, and we recently had Buckeye decide he didn't want to stick around(which is a damned shame), especially considering that I know I have an issue with tone in writing.
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Re: Drag knife.

Postby RegB » Mon Feb 01, 2016 7:55 am

teoman wrote:Why drag it?

A cheapo servo could rotate it.


I agree, if I were striving for "perfection" a little bit of path look-ahead could indeed aim a knife in the intended direction of cut.
It is quite possible that this is matured knowledge at some high end machines.

I am being selfish, for MY wants/needs it isn't worth the research and/or development to "get there from here" and I don't have the motivation to do it just for the satisfaction of doing it.
Without doing the math I feel that 0.25 mm of drag offset will probably produce "good enough" accuracy for my purpose.

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Re: Drag knife.

Postby enggmaug » Mon Feb 01, 2016 9:15 am

If one wants no offset at all, controlling the blade is necessary.

A servo is a good idea, and would work in most cases. But a servo also has a rotation limit. It cannot rotate more than XX°, XX often being a bit more than 360°.

That means you would not be able to cut a snail shape.... unless you pause cutting, rotate your blade 360° backwards, and proceed, for every loop.

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Re: Drag knife.

Postby RegB » Mon Feb 01, 2016 12:09 pm

enggmaug wrote:If one wants no offset at all, controlling the blade is necessary.

A servo is a good idea, and would work in most cases. But a servo also has a rotation limit. It cannot rotate more than XX°, XX often being a bit more than 360°.

That means you would not be able to cut a snail shape.... unless you pause cutting, rotate your blade 360° backwards, and proceed, for every loop.


It might not NEED a servo, i.e. a stepper might do.
Just before every cutting run make a straight line cut with rotational stepper OFF in the X+ direction on the waste area to establish X+
If there is wander, do it again after every long cut.

Anyway, right now I am absorbed with the details of unwrapping surfaces to get my patterns from :D

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Re: Drag knife.

Postby DeltaCon » Mon Feb 01, 2016 12:30 pm

in the video i linked and the video fletcher linked, you can see that no special software is required.

If dragging is not an issue that right. real Vinyl cutters do have an offset parameter though, and the drag, even with the small special vinylcutterknife is really someting that you can see at edgy corners of a cut.

I am looking for software that imitates the path that a GOOD tractor trailer driver would take with a 53 ft trailer except the tractor would be airborne :D

Usually that is done through firmware, much like the firmware in our delta's compensate for eg the delta-radius. I don't know of any 3d printing software that compensates for for deltaradius on it's own, much like I don't know of any vinylcutting (or signmaking) software that does compensate for knife offset in it's own.

A cheapo servo could rotate it.

that would be called "over engineering"


Signmakers call that a tangential cutters ;-) And they are considered top of the bill! They are really wonderful for cutting thicker materials like rubber mask for sandblasting purposes.
I never thought I would be telling things like this in a 3d print forum... ;-)
I am DeltaCon, I have a delta, my name is Con, I am definitely PRO delta! ;-)
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PS.: Sorry for the avatar, that's my other hobby!

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Re: Drag knife.

Postby fletcher » Mon Feb 01, 2016 5:04 pm

This is a little Nema14 Stepper with a DIY Tangential Knife attachment. We ran this with Mach3 for cnc routers and it worked well. Mach does tangential cutting though - I'm not too sure about Repetier.
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Tangential Knife.JPG

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Re: Drag knife.

Postby RegB » Wed Feb 03, 2016 12:07 pm

Thanks Fletcher.
I like that nema14 solution.
It would probably work better in my little cnc router than the Rostock effecter plate :D
The cnc router runs grbl, so I'll check into that.


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