I am working on modifying a CAD design for a 3D printed wagon, so far produced at 2mm and 4mm, for 7mm scale. My background is firmly 2mm, with 4mm "by exposure" - for 7mm, all I have is a collection of part-assembled kits donated to my local club, of varying age and completeness.
Those requesting the wagon in 7mm gave the suggestion of "make it work with Slater's" -there isn't much standardisation in 7mm axle fittings, it seems - and so I've been working with those.
Setting up the wagon such that the Slater's top hat bearings sat flush with the inside faces of the W irons (41.7mm apart), I found this leaves a surprising amount of sideways play in the axles - perhaps as much as 2mm. Although the wagon runs well on a test track, the ~2mm of sideways play feels excessive. Comparing to some other 7mm wagons, found that:
A distance of 40mm between the insides of the "rim" of Slater's top hat bearings is common
At least some kits include a moulding on the inside of the W irons that inset the top hat bearings to achieve this
On attempting to match that, the axles are a tighter fit, but the issue becomes that it's very challenging to insert the axles. The W irons need to be bent out by nearly 3mm to fit - the distance from the outside face of one wheel to the opposite end of the axle being ~43mm, fitting into a ~40mm gap. This is not an amount of bending that a 3D printed resin takes kindly to, and it didn't take me long to cause damage by repeated insertion and removal.
So, my query for 7mm modellers - what do you expect from a kit?
The Slater's top hat bearings sit flush with the inside face of the W irons. The axle has some sideways play, but it's to be expected.
The bearings need to be inset to avoid any play in the axle, and the W irons must handle being pulled out a fair distance to accommodate.
Simply don't bother with including the W irons and axleboxes in a 3D print - they'd be poor quality compared to what you're used to, and you'd prefer to source sprung W irons/axleboxes from somewhere.
