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crane (8)

Original page created on 02/06/2018; updated on 12/02/2024.

Photo-etching part design

Choosing the thickness of the sheets

As already said, the sheets will be 0.2 or 0.3 mm thick depending on the objects. Obviously, the more thicknesses, the higher the price will be. But doing everything in 0.3 seemed difficult to render the foot walk appearance correctly.

Bending sheets

Note: the following has been modified to get stuck more closely to the calculation generally applied to sheet bending.

The bending takes place on lines engraved in half depth. The width of the appropriate grooves must be determined. It is generally considered that the sheet stretches on the outer face of the bend and compresses on the inner face; the neutral axis does not vary. The following diagram is obtained before / after bending at 90°:

The outer bending radius is equal to the thickness. The width of the half engraving is equal to the length of the neutral axis, whose radius is ¾ e.

The length of the neutral axis (quarter circle) is:

L =	3	e ×	2 π	= e ×	3 π	≈ e × 1,178
	4		4		8

Results (all values in millimetres)

Thickness	Calculated Width	Rounded Width	Recommended Width
0.16	0.1885	0.19	0.18
0.20	0.2356	0.24	0.22
0.30	0.3534	0.35	0.33
0.40	0.4712	0.47	0.44

Remarks:

It can be seen that the minimum engraving width (0.12 mm) is respected in all cases without any problem.
According to members of the Loco-Revue forum (C. Dufresnoy and F. Tavernier not to name them), it is often recommended a width equal to the thickness increased by 10% (in my calculation, it’s more like 18%, but it’s better to trust the experience). These are the values listed in the last column of the table.

Flattening objects

This is the most difficult part of the project. There is probably software that can do it, but…

Here is how I proceed on a simple example: an angle bar.

From the 3D model:

Cut in the angle, first horizontally (command SECTION):

Then vertically:

The corner part is eliminated. There are now two independent plates touching each other by an edge.

Turn the vertical plate to the XY plane (ROTATE command):

Move a plate by the desired groove value, for example 0.25 mm (MOVE command)

Fill the gap with a solid of lesser thickness than the parts, e.g. half the thickness (RECTANGLE then EXTRUDE commands):

Gather these three pieces into one (UNION command):

Now we must turn this “flat” volume into filled areas capable of providing an etching mask, for each of the faces.

Explode the solid, which turns it into a set of surfaces called regions, then delete all the regions (EXPLODE then ERASE commands) except the upper and lower:

Create two layers, one for each face of the mask, one coloured magenta (or other!) named sup, the other coloured cyan named inf.

Sorry, I don’t have the English version of the dialogues.

Creating layers

The remaining upper regions are placed on the sup layer and the lower ones on the inf layer.

Here, we have a small problem: when you want to print the regions, which however appear solid, only their outline is printed! The only parry I found is to hatch them.

Caution: AutoCAD offers two hatch commands. Only one allows associative hatches, i.e. hatches that will follow the changes in the object they fill. This is very important here because it will allow us to change the position or even the size of the parts without having to repeat the hatching process every time!

The command that interests us here is BHATCH which is obtained directly from the Draw toolbar.

Draw toolbar

When you run this command, a dialogue box appears, in which you first have to choose the type of hatching: we take the SOLID type to completely fill the objects. Once done, we select the objects to hatch. We must proceed for each layer (inf and sup) separately. Just frame the whole parts; no need to hatch each part individually. So it’s pretty fast.

Boîte de dialogie Hachures

Distribution of parts on the surface

We must now distribute the parts on the planned surface, as small as possible, without weakening the whole. For this, I draw a polyline around each part or group of part. Then I offset it 1 or 1.5 mm outwards: it will form the boundary delimiting the part.

Delimitation

When all these boundaries are done, they give a good overview of the area occupied by each, leaving at least 2 mm between each. The important thing is to get enough material left to properly keep all parts together. We can then look for the optimal placement on the surface. It looks a bit like the placement of devices on a circuit board, even easier…

Note: I filled the unoccupied areas with parts for another project.

Placement

Caution: when moving the parts, make sure that the two layers inf and sup are displayed, otherwise only one face will be moved, resulting in a disaster! Reminder: it is possible to go back, but it is better to save the work as frequently as possible!

Finishing the etching mask

The next step is to fill all the remaining space, to avoid exhausting the etching bath too quickly. Place a rectangle encompassing all the parts, here in A5 format, then use the BHATCH command again.

This must be done for each side of the etching mask. To keep it legible, place these “hatches” on different layers, named inf-fill and sup-fill (for example), and a colour similar to magenta and cyan, but lighter — or darker, it’s up to you…

Note 1: This view (upper side) shows the filling without the parts.

Note 2: The filling may go wrong. The most likely causes are either an unclosed contour or two contours touching one side. In this case, remove the hatching, correct the error and start again.

Filling

All we have to do now is to place tabs so that the parts won’t fall into the etching tray, only one side if we want them to be half etched, easier to eliminate. As I don’t have much experience with this (tab number and spacing), I was inspired by examples of commercial kits (view of the lower side).