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PixelCNC: Fast/Easy CAM for Signs, Engravings, Art, and More!

Generates a variety of 3-axis CNC router/mill toolpaths and G-code from images and models! · By Deftware Industries

Toolpath Options

A topic by GF357 created 86 days ago Views: 53 Replies: 4
Viewing posts 1 to 3

How feasible would it be to add options to toolpath control... specifically in relation to feed rate. 

Here's scenario A: you're on the last pass of toolpath (profiling, pocketing, any operation really) and you'd like to automatically have the feed rate change to certain percentage of the initial rate. This would be either a last pass option, or better, an option based on thickness of the stock (percentage or actual dimension).  

Scenario B: feed rate is modified based on zones. Picture an archery target with it's concentric circles, and each annulus would potentially modify the feed rate. 

Developer

Hi GF357, thank you for the suggestion!

The only related feature that has been under consideration thus far has been with regard to dynamically controlling feed/speed rates (spindle RPM for machines that support it) to maintain a constant spindle load in spite of varying engagement between the cutter and material. I'm curious what exactly the purpose of the feature you propose would be because you did explain the effect clearly enough but I'm at a loss as to what this would be useful for. I apologize if I come off as presumptuous, and by no means intend to seem insulting in any way, but what it sounds like you're talking about is a bit like an alternate means of producing a sort of finishing cut pass? If that's not the case then please elaborate because I'm always curious as to what users envision a need for :)

For purposes of ensuring a clean final surface it is possible in most cases to simply create a finishing pass operation that cleans up previous operations' cuts at different feed/speed rates. The "Leave Stock" parameter enables operations to be treated as roughing passes, leaving a thickness of stock material which allows for a finishing pass to then come in and clean up the rest of the workpiece at different feed/speed rates - and even entirely different cutters that are better suited for the task than what roughed out the piece to begin with. Multiple finishing passes can be performed too, with each one leaving successively less stock material behind to ensure a flawless final surface. I mean no insult if you're well-rehearsed in the arts of CNC and if none of this is new to you and you still think there's a need for a variable feed feature then consider my interest piqued!

The leave-stock parameter, however, is only applicable with the operations which generate toolpaths from the project's actual surface contours and form, such as for relief/emboss type carvings. This excludes the Medial-Axis Carve, Profiling, and Pocketing operations due to the fact that these operations generate toolpaths directly from the project input image itself, with no regard for any 3D geometry to 'leave stock' of any thickness on. They simply generate a toolpath based on a 2D image with no intention of producing or conforming to any  3D form because their depth and offset parameters allow for cuts that are completely independent of the generated 3D project mesh.

At any rate, as far as the 2.5D type operations like pocketing/profiling/horizontal are concerned - which feed through cuts horizontally at incremental depths - it wouldn't be much of a hassle at all to implement a means by which users could impart a fractional decrease in the feed rate or spindle RPM of successive cuts by depth or concentricity. Specifically for the pocketing operation though it's not difficult to produce the equivalent of a finishing pass by using another pocketing operation with the cut depth and max depth both set to the exact final desired depth, and a finishing feed/speed rate set. This would only take care of the bottom of the pockets which is where a profiling operation would come in right after to clean up the 'walls' of the pockets at the finishing speed/feed rates. If your flutes are long enough to reach the depth of your pocketing max-depth then you could clean up the walls with the finishing pocketing operation itself instead, because the profiling operation is only necessary if your cut depth is less than the max depth of the pockets themselves and you couldn't just combine finishing the bottom of the pockets with the walls in one shot.

Again, I am in no way trying to be insulting or presumptive as to what your skill level or existing CNC knowledge may be! I'm only trying to provide some pointers in the event that they may be useful because many PixelCNC users are (surprisingly) actually complete newcomers to CNC. But if what you're suggesting has another reason that such a feature would be of use then consider me all ears! Perhaps it could be a means of providing an opportunity for something of a pseudo-finishing pass built right into an operation - without having to explicitly define one (or more) to get the job done, and that doesn't seem like a bad idea at all. Let me know!

Thanks again,

Charlie

I don't take any part of your reply to be insulting, no worries there. I'll start off with something to think about - not every CNC machine cuts.

So, for scenario B with the zone modified feed rate, the tool in this case being a pen (or a laser with fixed output level, or an air brush with a fixed nozzle size and flow rate) would leave less ink (or the laser would burn not so deep/dark, air brush would leave less paint, etc). This absolutely could be accomplished with multiple overlapping tool paths creating the different levels of the gradient, but that also increases the run time of all the tool paths and goes over the same ground multiple times. But if a zone feed rate (zone gradient effect?) were implemented similar to your spiral operation (with the center being user defined, along with # of rings, eccentric VS concentric) the toolpath would go faster the farther from center the tool got. This could have an interesting effect on a crosshatching texture, too.  See the attached picture of a test piece I did. It's an attempt at Bender on canvas using Sharpies. Sharpies and canvas don't mix well, even if the pen holder is spring loaded - the canvas chews up the tip rather quickly. The gradient effect would work quite well on the stripes, but could also be useful to force focus in a particular spot. I realize after writing all of this that it's a request for a niche function few people might use.

As for dynamically controlling spindle speed - that sounds like a good complimentary feature for trochoidal milling.

The scenario A function that I described is essentially a finishing cut pass. In a discussion about another piece of software used for cabinets, objects that were below a certain size had the option of having the last pass slow to a specific feed rate in an attempt to not damage or eject the piece.  I imagine the use of either spiral upcut or downcut bits combined with not babysitting the machine because it's not cost effective in a production shop, having the machine slowdown on the last pass cuts down on tear out or projectile ejection. For the interior of pockets, spiraling the tool all the way to desired depth inside the pocket profile at a slow steady descent  usually works as long as the piece in the center doesn't cause tear out when milled away. While cutting a piece of aluminum for a Jeep switch face plate, one of the slugs from a switch hole got caught on the bit just before it was completely cut free and it ended up pulling the whole piece up and into the bit (of course it was the part that was farthest from any of the 6 bolts holding the stock down). Had I either manually slowed the feed rate down, or the toolpath generator auto-magically applied the size less than X @ last pass rule on the switch holes, the slug probably would have been cut free without damaging the main piece.

Developer

Ah, I knew I had to be missing something! It's an interesting idea for sure. Have you tried doing it with the image itself? I don't know what exactly you're doing to produce toolpaths that are almost exclusively level, other than perhaps setting project Z depth to a very tiny size and pretending that's your cut depth for the Sharpie to traverse. If you took your image for the stripes and added a radial gradient to them and toolpathed off that using the contour-carving type operations (i.e. parallel, spiral, etc) then you could get the Sharpie to be lifted off more, or have less pressure, at either end of them. That likely won't be as good as changing the feed rate though.

I'll have to think about that because there's a lot of different ways to go about something like that. For instance, would you want to set a percentage of feed rate change per inch of depth/step-over? Or would you just want to say "at this point we feed at this rate" and then pick a distance point "at this distance and beyond we're at this feed rate". That would allow at least a feed rate gradient based on radial proximity to the original center point.There's really a huge array of possibilities, you could also have a set of points all over the project that are blendpoints for different feed rates, or have square/diamond/star shaped concentric zones of either blending between feeds or sharply stepping between them.

Then would you just want a linear transition from one feed to another or something more like a polynomial transition that gives a different dynamic? You could have plain old linear, but maybe something like an ease-in or ease-out cubic transition would be better. Maybe the user should just have control over the curve's start/end derivatives to change the feed rate blend dynamic.

There could even be a new marker/pen tool that just doodles on the project material surface. That might be neato too.

Well I'm not going to make any promises just yet, but I'll definitely keep a location-based variable feed rate idea cooking in the back of my mind, because there is definitely some value there as a feature for manipulating the final appearance of something. I think it could be one and the same with the variable feed system for maintaining a relatively constant spindle load - spawning visible points across the project for an operation which affect the feed rate as the tool approaches them. We will see! This is something that I probably won't be able to get around to until at least 6 months from now, or more, depending on how things go with the features currently lined up for me to knock out. In the meantime feel free to keep sharing your work here for other users to see. It's a pretty cool use case for PixelCNC!

Thanks,

Charlie

I treat the Sharpie's similar to a drag knife . Disable the spindle, set Z to top of material, define depth of cut for the pen at .05", and set profile or pocket toolpaths to .05". There's a 2.5D function in VCarve called fluting that will perform a ramped cut over the length of a profile, and it has options  for:  

  • start depth and final depth of cut
  • tool type
  • to make the flute have a linear ramp, to ramp at start,  to ramp at start and end (think bowl/concave shaped), to ramp for a fixed length
  • to define the ramp as a linear motion or as a smooth motion (similar to cutting a pocket with a radius end mill VS flat end mill)

Because I was focused on just the flat drawing at the time, I hadn't thought about applying this to the Z axis (or even an A or B axis for rotary jobs), but it makes sense - merging the two scenarios that I originally wrote about as it boils down to modifying the feed rate (or spindle rate) based on possible parameters:

  • distance to/from Z coordinate
  • distance to/from X-Y coordinates
  • distance to/from A-B coordinate (tricky if you allow your rotary to spin more than 360 degrees)
  • spindle speed target = ST; spindle speed actual = SA; adjust feed rate by F%, adjust spindle rate by R%

As for implementing the gradient function or better yet, the radial modifier, I think that a simple linear percentage as you move between zones would give the best performance. I imagine that having more than one modifying layer could be tricky, do you default to the lowest value, the highest value, or a value between the two?

I know you can't or shouldn't make promises - this is only a suggestion based on an idea I had from a recent project.  I can probably achieve a similar effect from having multiple overlapping toolpaths at different feed rates... I'll end up making a tool holder/mount that holds a modified fountain brush with an air line that keeps the reservoir  pressurized.  The trick will be to either hand edit the G-code to pulse a solenoid for semi-constant air pressure, or to find a way to have a post processor insert an M-code based on the distance traveled when the tool/brush is in use. Or just sit and watch it and make sure the ink doesn't run out or spit all over the project. It would probably be easier to just to paint the damn thing by hand... if I could paint.