[jeanyes]

Fairly often when creating a roughing tool path, it will not remove the top layer of material if it is thinner than the "stepdown" depth even though it is thicker than the "stock to leave" setting. This happens in both 4.3 and the latest 5.0 beta. I generally only mill MDF so I haven't bothered posting about it but it's time to figure out what I'm doing wrong. I've included a link to a 4.3 sample file below. In this file you'll see I've set my object .25" below zero and increased my material up to zero. I've set my stepdown amount to .25 and set stock to leave at .0625" - as you can see in the image below it does not create any toolpath to remove any material from the top surface. It should remove .1875" to leave the surface with the setting of .0625"



Anyone know what I am doing wrong? What's odd is that this only happens about 50% of the time. Half the time it does it just as I would expect.

http://www.jeanastie.com/share/E101_sample_r4.3dm

[wdaehn]

The root cause is the "box" or "stock model".

By default you are drawing something, then select this in Madcam and the box to cut material out of it will match the outer dimensions. So what you are essentially saying is, the top level and the side does not need to be cut as it is of perfect size already. But the box (stock model) should actually be the size of the block you are cutting it out from - or something inbetween.

Hence the standard procedure is to create the enclosing box, then enlarge the box in length and width by little bit more than the cutter radius and raise the z level of the box to the actual height of your raw material height.

If the box is higher than your work_piece_height + roughing_step_size + roughing_material_to_leave, the roughing will cut the top as well. If the difference is less it will assume that this plane requires planar finishing only.


So my standard procedure for cutting a 30x30x30 shape out of a 70x70x32 block using a 6mm drill would be to enlarge the box by 4mm and the height set to the real value: 38x38x32. For roughing the stock to leave is set by me to 0.5mm and hence as long as the step size is less than the remaining 1.5mm the roughing will cut the top material as well. You can even make the box higher than it really is, e.g. 33mm to ensure the roughing goes through it once.

[wdaehn]

Watchout for the approach method being used so that the cutter does not believe to step down in some free area although it is not as you essentially lied with specifying your box. I said it is 38x38 but actually it is 70x70.

[svenakela]

Roughing is roughing, like the name really states. If a full increment (step down) can't be done it will not be done. You should use Facing to swipe flat surfaces to correct height. If you cut MDF only you can go hard on the roughing and then let the Facing step take longer time and produce a higher finish.

[jeanyes]

wdaehn: Yeah, sometimes I do play with my box size so that it is sure to get the top surface as I want it to. However, lower surfaces will still get left with more material than I would like depending on where they are at based on my settings.

svenakela: The "Facing" under the roughing section does not do correct height either. It just rounds the corners instead of sharp points on the path. If by "Facing" you're referring to just running a planar path then yes, I could always run an planar pass after roughing but set it to leave the desired amount of material I set in roughing. It just seems like it would make sense to have an option in roughing to do this so that you can save time. Because some surfaces are already exactly as they need to be, the extra planar step would be wasted time over those areas already at the correct height. Or am I misunderstanding you? I'm not a machinist by trade so I may be missing something obvious about the normal process for this.

Thanks,
Jean

[wdaehn]

Well, I guess what you expect is not the way roughing has been implemented. Its algorithm seems to be
* Take the box height (12mm)
* Divide it my the step size (12mm/2.5mm=4.8steps)
* round up that number to an integer so that each step takes the same amount of material (round to 5, 12mm/5steps = 2.4mm real step size
* Let z be constant and cut all material that needs cutting


So in your example, you expect a roughing on height = 10mm, but actually it is at 12mm-2.4mm= 9.6mm hence no cut on the top level. If your workpiece would be 9mm high, the roughing would go through all and you would have 0.6mm left on the top surface.

While I can see that in your case that roughing strategy might not be the optimal preparation of the material prior to finishing, neither it is said that roughing is the only way, nor that you cannot do more. Just imagine you would not have a flat surface but something curved. In that case either you do what roughing does and keep the z level constant so that you can cut with high speed or you start varying the z level to follow the contours and dive into the material more often which is not good for the cutter.

You have multiple options now
* carefully pick the roughing step size to match your dimensions
* set the step size to the maximum amount of material you want to cut while finishing
* Do roughing first, then finishing with stock to leave and a final finishing
* Using z-cutting-planes break the model into multiple parts, e.g. first does cut all from 12mm to 10mm, second from 10mm to 2mm and last does the remaining 2mm to 0mm

[svenakela]

wdaehn: Yeah, sometimes I do play with my box size so that it is sure to get the top surface as I want it to. However, lower surfaces will still get left with more material than I would like depending on where they are at based on my settings.

svenakela: The "Facing" under the roughing section does not do correct height either. It just rounds the corners instead of sharp points on the path. If by "Facing" you're referring to just running a planar path then yes, I could always run an planar pass after roughing but set it to leave the desired amount of material I set in roughing. It just seems like it would make sense to have an option in roughing to do this so that you can save time. Because some surfaces are already exactly as they need to be, the extra planar step would be wasted time over those areas already at the correct height. Or am I misunderstanding you? I'm not a machinist by trade so I may be missing something obvious about the normal process for this.

Thanks,
Jean

I would say the opposite, the facing operations (AKA Planar Finishing) gives much better control and optimization possibilities, roughing doesn't benefit from an option like that. If you run soft materials it doesn't really matter, if you cut 6061 or any tougher material you need to revise your cutting strategies and start using Roughing as roughing and the other options for the fine processing.
Lets say you are going to prepare a mold in steel with sharp corners and islands in pockets. You can't do that in one swipe with a big cutter, can you? Then, the roughing is made with a big cutter, Z-level and Planar with one or several smaller tools and in the end Pencil tracing and possibly Re-machining.
If you're going to make the same thing in for example MDF, you can go directly on Z-level or even directly on a Planar if the cut is not that deep. You can set the Planar to make all surfaces by setting the angle to max. Same thing with Z-level. So the answer is that what you want to do is already there, but not in in the Roughing strategy.

[jeanyes]

Thank you guys for your information. I can see now that running the planar finishing step twice is the proper procedure for cutting harder materials. I may even start that for MDF as I sometimes have shallow crevices created (material separation instead of actual material cutting) on my surface due to too much material being taken off on my final planar cut. It will be simple to run a planar finishing step right after my roughing... before my zlevels. Then I'll follow with a final finishing and pencil tracing as usual.

Thanks again,

Jean