Internal profile modelling

[David Harrod]

I've been trying to model some frames to create the radio rack in the rear of a spitfire fuselage. The method I've tried to use to determine the dimensions is something like this: 

 

1. Mark a piece of tape at 5mm intervals and stick it where the rib will sit

2. Use a caliper to determine the actual distance between the reference edge and the marks (usually less than half a mm out).

3. Using a straight edge across the fuselage Half's use a caliper to measure the depth at each marked point. 

4. Use these measurements to determine the points in cad and strike a curve through them, mirror it, extrude to get a part I can print. 

 

I managed to get something that looks about right but isn't the correct profile when printed, I was expecting some tweaking but not that much. It's printing to the measured dimensions so it's. It a scale issue. Any ideas how I can improve the method or something I haven't thought of? 

 

 

[Circloy]

Why not use a profile gauge

https://www.ebay.co.uk/itm/Yato-Professional-Magnetic-Contour-Profile-Gauge-Shape-44x125mm-1-7x5-YT-3735/222022434368?hash=item33b1917e40:g:mnQAAOSw4A5YqOwu

this can be used to determine the depth at the corner of each slide with a greater resolution. (dont forget to swap from top corner to bottom corner when the profile changes direction - i.e. from going in to coming out)

 

Transfer the details to CAD & do a 2D paper print to check for accuracy before commiting to 3D - Should get you closer than what you've illustrated.

 

Link show is example only not a recommendation of actual guage or seller othe styles & sellers are available.

[wellsprop]

It depends how the plastic part is made.

 

I would determine the outside dimensions of the fuselage then carry out an offset of that profile (to get you the inside profile).

[Circloy]

47 minutes ago, wellsprop said:

determine the outside dimensions of the fuselage then carry out an offset of that profile

That relies on the part being a consistent thickness through the profile & there's no guarantee that's the case, even if the kit's been designed usng CAD. There's a good possibility the thickness varies to take into account the material flow &  fill chacteristics when moulding.

[David Harrod]

On 11/10/2020 at 20:11, Circloy said:

Why not use a profile gauge

https://www.ebay.co.uk/itm/Yato-Professional-Magnetic-Contour-Profile-Gauge-Shape-44x125mm-1-7x5-YT-3735/222022434368?hash=item33b1917e40:g:mnQAAOSw4A5YqOwu

this can be used to determine the depth at the corner of each slide with a greater resolution. (dont forget to swap from top corner to bottom corner when the profile changes direction - i.e. from going in to coming out)

 

Transfer the details to CAD & do a 2D paper print to check for accuracy before commiting to 3D - Should get you closer than what you've illustrated.

 

Link show is example only not a recommendation of actual guage or seller othe styles & sellers are available.

What method do you use to determine the reference, in this case the halfway point horizontally of the fuselage? Do you mark it or just rely on the outermost contacting teeth of the guage? 

[ICMF]

 

 

You can also do something similar with a (roughly cut to size) styrene bulkhead and CA - put some tape inside the part where you want to measure; hold your rough-but-close bulkhead in place; squeeze in some thick CA to fill the gap between the tape and bulkhead; spray on some accelerator; peel the whole thing, tape and all, off the plastic and sand/file away any excess CA.