Guidance required for small 3D project

Hello all, I was asked by a friend if I could machine a small part in hard wood or aluminium. He had asked a 3D expert to make the part on a 3D printer and the result was unusable by dint of poor adherence to the measured specifications. Here are a couple of images followed by the questions which indicate how the project will look when finished.

My questions concern how I would plan to cut the internal and external radii. I know the dimension which the internal radius must fit. With the proviso that I keep the material between the internal radius and the external radius correct for thickness, the external radius should take care of itself. The red section in the following image shows this effect. The only parameters I have is been given apart from length, hole sizes, shape and position is that the internal radius must fit a diameter of 22mm and that the maximum thickness of the material at the centre is precisely 5mm.

What I am hoping to learn are what calculations I must complete to create the internal radius. Will the external radius require similar calculations? Should I machine the internal radius first? Any help or advice will be very much appreciated. Thank you.

If you’d provide a measured drawing, I believe I could work up a BlockSCAD/OpenSCAD program which would make an STL which you could then make G-Code from using MeshCAM or pyCAM or FreeMill.

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You are an absolute star, Will. My gratitude is boundless. I will work on making a sensible drawing later this evening.

do you have the file used for 3D printing?


No, unfortunately Arjan. What a good thought that was. Thank you.

Had a lot to do today and so I had restricted myself to working the arithmetic for arc line length. Sadly, my brain cells that had once held such information had gone to sleep and I could not remember how to calculate it. I found a really neat online calculator for just calculating the arc line length and I have now drawn much of what I need. A few measurements still to be added. (the ratio I chose was 4:1 to give you a fighting chance). The lesson here for any personnel from the senior members club is this: if all else fails, cheat! :grin:

What’s the inside diameter?
Are the OD and ID concentric or as you drew earlier? (Is the wall thickness constant?)

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The inside diameter MUST fit around a 22mm cylinder and that size is absolute. The OD MUST be 5mm thicker than the ID at the centre of the item, I understand it provides some support so needs to be the full thickness. Otherwise the OD it can thin out as it would naturally, as shown in my earlier illustration. The reason for the failure of this part is that the OD is undersized by 1.4mm

I wonder if you could find an aluminum tube with a 22mm ID that would work.


A good idea. We have access to copper pipe of exactly that dimension but it is very thin walled. Probably no more than .5mm in thickness. I cannot see a method of building up ten layers other than soft soldering it. It is a thought that I will keep in mind. The slotting would likely be trivial if the workpiece could be held. Cutting metal is different in many respects from cutting wood. I have several Niagara cutters on order and they are all capable of cutting soft metal. Thanks for the suggestion Neil.

3d printing would work but the printer type used was incorrect. For something like this you would get better results having it done on an sls printer such as the ones at, not an at home fdm printer.

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Okay, let’s redraw all of that and get the specific dimensions:

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Modeling in BlockSCAD we begin with:

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Rearranging things and nesting transforms we set up:

and it’s pretty obvious that making at least one module will make this easier.

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Make a hole:

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Repeat w/ minor variations:

The file is available here:

Exported to OpenSCAD and modifying a parameter for better quality we get:

Attached: (64.2 KB)

The .stl in that archive can be loaded into any 3D CAM tool such as MeshCAM, pyCAM, or FreeMill to be cut out using appropriate tooling in the material of your choice.

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A further thought — probably the best free/opensource CAM option would be to import the .scad file into FreeCAD and work up the toolpaths based on its analysis of the geometry, which I believe would be better than the triangle tessellation of the STL.

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Holy moly! That is an amazing piece of work, Will. Thank you very much! :clap: :+1: You accurately inferred all of the geometry sizes without me specifying what they were. That Freehand software is amazing when it is driven by expert knowledge. I am very much obliged to you for your work and skill in helping a relative idiot like me to achieve useful beginner status.

I could not use pyCAM or FreeMill because I am using a Mac. I have downloaded the 15 day trial of MeshCAM v8, which looks like it will be a very useful piece of software so I guess that @robgrz will be happy. I was thinking about machining height maps particularly with reference to the cubes I want to assemble, so that will help me along in that direction.

I am also going to brush up against how to undertake and manage flip milling with this little project too. Just as I think that I have understood something, the learning curve has a laugh and gets another tad steeper. It is almost a sheer vertical wall right now. :rofl:

FWIW, FreeCAD could not load the file because the OpenSCAD format it accepts as an import to the OpenSCAD workbench is the .csg file.

Glad to be able to help out!

Isn’t pyCAM opensource and available in a form which will run on either Linux or Mac OS X?

I could have sworn that there was an option (perhaps in an OpenSCAD workbench?) to import a .scad.

Here’s the .csg: (460 Bytes)

Some useful notes and links on two (and more!) sided cutting at:

Looks like there might be such tubing available:

but unfortunately, I’m not finding a vendor which sells individual pieces in small lengths.

There’s a close Imperial size:

but a suitable chunk of alu. is less (and likely to be of an easier to machine alloy).

The only metal 35mm o.d. tubing at McMaster-Carr is stainless:

but wall thickness is just 1.5mm