Problems modeling an accurate curved basin form for cnc carving

Please can someone help with the best method and extension to use to model an accurate hand wash basin form that can be used for CNC carving.
I have watched a number of vids. on various plugins like curviloft and soapskin as well as the stansard sketchup tool and i am sure I am the weak link in the process. The issue is that the model needs to be accurate and have pre defined falls to the water outlet which is off centre ie. not positioned centrally on eithe x or y axis.
The basin i wnt to create is a round cornered rectangle in plan 420mm x 400mm x 150mm deep at the outlet with rads of 50mm on the corners. The problem is im not sure which tool or process to use to achive an accurate smooth construction that will have an functional fall at the base.
I can model the basic 3d form, a box with no top and rads to the corners and edges but i cant work out the transition to the bottom surface with a gentle continuous smooth fall to a specific point for the outlet.


How “smooth” does it have to be? Would something like this work?

The model was too big to upload here, but it’s in the Warehouse:

1 Like

Maybe? fredo corner@ 50mm then drag down the hole on z.

because of some off geometry, tied it by breaking area up to get a more even flow.

Wow! Thanks so much for such quick responses guys.
Im guessing the Mr.Jims used curviloft to create the form, but how is the question?
And the WhiteBunny method is where i got to but i couldnt get the base formed correctly.
Both methods could work however both have an issue i am trying to overcome. The transition from the rad. at the base of the wall to the surface that forms the bottom of the basin need to not have any ‘valleys’ accross the surface coming from the corners or the transitions between planar surfaces. What i would like to achieve is a more pillow like bottom to the basin, much like what can be achieved in soapskin bubble but with a continuous smooth transition from the wall base rads. The problem is i dont know how to make this work properly.

So both methods you guys have tried would work if this smooth base can be included.
Oh! i forgot to mention that the wall should have a small flair angle toward the top so Mr.Jims already has this but the WhiteBunny😉 idea just neede to flair a little at the top which is a simple adjustment.

As a side question, how do tou record your screen moves as you have in the example you posed as this is a great way to explain an idea?

Thankyou again!

Licecap is one app and ScreentoGif is another. A search will reveal more.

thanks for the heads up ill check it out.

This is where I got to and ran in to problems and assumed I was using the wrong method. I have exaggerated the base Z move to illustrate the problem. I put a taper on the sides and then applied the rounding then i pulled the centre down with the hole in it like you did, however the rad that is applied to the corners is 90 deg. and independent to the flat centre of the base so when the centre is pulled down is there is no smooth transition from the rad. to the base, as seen in the 2nd image from the side.

Also as I previously said, I was hoping to make the base more like a soft pillow without any obvious valleys to the round outlet at the lowest point. I cant see how this can be achieved with this method of construction or am i just missing something?

I forgot to mention that I deleted the innermost outline on the bottom, so that the joining line to the plughole was more tangental to the curve at the bottom - if that makes sense. I had also subdivided the bottom to minimize the valleys that result from sketchup assigning its own geometry around the hole.tangent

Also you might find Fredo’s curviloft plugin will make a better transition from the square base of sink to the round hole.

what i finished up with.

sink.skp (461.4 KB)

can you tell me how you model this?

I used a plugin called Spirix (it’s not particularly user-friendly). To make the form, I created the rectangular outline and then grouped it. Tracing around it, I used it to create lines and arcs in sequence and then used Spirix to create an ordered group of edges (a royal PITA). The tracing started at the center of the bottom line so that the outline starts and ends there … each line segment and arc has 24 segments or 192 in all. The circle was added with 192 segments and then grouped. “Tracing” around this with two separate 96 segment arcs and using Spirix to create a second group gives a circle of ordered edges that align with the 192 segments of the first outline.

Once the groups are created (a1 and a2), it’s fairly simple to create the surface using the two groups and an “arc” morphing function that creates the shape:

The plugin can be found here:

Here’s a quick version using Bezier curves and Curviloft. You can edit the curves any way you want to get the shape you desire and add more to refine the shape etc
Personally I would use SUbD for this type of thing but that’s another whole learning curve on it’s own.


Again THANKYOU for your efforts with this, your help and ideas are greatly appreciated! And serve to demonstrate to me multiple different methods I had not considered.

Whilst the method below seems to have worked (possibly?) I think your methods may result in a more refined surface and better CNC result so I will have a go with them too…

Below is how far I have got.
Because this basin is to be placed in a restricted space there is very little room for the outlet which has to be made off centre in order to keep the basin in the available space and of a decent size.

I started with a box. Once I created an angle to the box sides and removed the top I then used round corner to soften the edges and corners. Using the resulting form without adjustment caused the back wall transition to the outlet to be a strange shape due to the proximity I think, so I removed some of the radiused corner splines on the back wall in order to try to increase the space for the curve and achieve a smoother transition to the outlet. I used Curviloft by spline to create the base of the basin and which I think may have worked to a degree…

Below you can see a problem I cant figure out.
Curviloft kept twisting the result and I tried all the tools to see if i could sort it in the extension without success. I also tried reversing the faces to the splines and then deleting the faces before the lofting but that didnt work. In the end I mirrored and inverted the splines relative to each other and it worked fine.

Can anyone tell me what is happening and is there an easy fix to this as it took a while to work out?

Thanks again

I have tried to create the form using Box’s method but it seems because I have tapered the sides of the basin to achieve non vertical walls (I pulled each side of the basic form in by 8mm at the base) I cant achieve a smooth transition where the 4 quadrants of the resulting lofted form meet.

I also attempted to make an adjustment in the start pint of the transition in to the base of the basin by drawing the curves from a plane positioned 50mm up Z from the base of the basin so as to keep the space within the basin to a max, but I ran in to the same problem with the creasing at the quadrant joints.

I have included the file and of the pic. below. The small Rads. to the LHS and the big to the RHS.
The structure in the middle is the origin of both methods.

Please can someone tell me what I am doing wrong. Is it the wrong tool to use given the alterations I have made to the original method from Box, or am I just being dumb again and missing the obvious?

image Curviloft Loft still nopt working properly.skp (1.3 MB)

WOW! thats a whole lot of work for what I thought was a fairly simple form. What do i know…
I looked at Spirix and you are correct, it looks horrible to use and difficult to learn, but quite clever in what it can do.

That’s why it’s so popular :wink:

It’s deceptively simple looking … I originally misunderstood your description (I thought it was a bowl-shape). A “smooth” transition from a square to a circle with a sloped wall and floor isn’t terribly difficult when the drain is centered, but when it’s offset toward a corner, you end up with a varying length along the radius and a varying tangent to the drain along the perimeter:

When I encountered this shape in my mold-making days, we would CNC the male form and then sand it by hand to “blend” the creases.

[added] This is somewhat difficult to see, but the bottom should be a flattened cone centered on the drain … the pic below is the result of a 2 degree angle around the drain intersected with the outline of the sink:


The shape you are trying to make is one of those awkward ones that are possible but strangely difficult to achieve in sketchup. The combination of arcs and straight lines can be problematic, causing stresses in the geometry.
If you are planning to do a lot of this type of thing I would suggest you look into learning how to use SUbD and the associated Vertex Tools and Quadface Tools By Thomthom. The first two aren’t free, but very cheap.
Here are two examples that took several minutes.


In a surface modeler, this would be created by using nine ruled surfaces:


The ruled surfaces can then be used to created a constant-radius fillet surface that tangentially joins the ruled surfaces since they are continuous by definition. In SketchUp, however, the faceted surfaces don’t align nicely. I tried using RoundCorner on this shape, but it had numerous overlaps (or maybe I’m not using it right):