Hi all.
What are the basic rules of thumb when designing a chassis in CAD and what sort of level of detail do you work to?
If you were to CAD a chassis made up of box section would you CAD it as if it were individual sections of steel then add fillets of weld etc, or would
you design the whole thing as a solid and then hollow to the correct wall thickness?
I'm starting to read up on chassis building with books currently on order. If you were to hollow out then your CAD would actually be different to
reality where I'm guessing you wouldn't cut into a main length ways member to add in a diagonal brace, you'd be more inclined to but
weld it on to the surface?
Feel free to point me to other posts or books. I will search through the forum soon!
I started this process some years ago following the Ron Champion book and using Solid Edge (at work). I modeled each part as a section of hollow
section then created an assembly of parts for the chassis. Maybe not the correct way to do it, but it worked for me.
That said I ended up buying an MK Indy as a kit!
A wire model? Centre line for each tube seems a logical way to draw. Then (if possible) increase the thickness of the line to view. Bit of a guess!
depends on what you want the CAD model for -
if it is for construction only then solid bars i.e. external surface of structure is fine - keep it as simple as possible to reduce time/effort
if it is for FE modelling fro calculating torsional stiffness and suspension forces then you will need to model the hollow tubes
if it is just to look pretty then a pen and paper will be fine.
Million ways to do it and none of them are wrong, its whatever you feel comfortable doing and designing. Plus depends on intended use.
When I did a Uni project designing a Formula Student Chassis, I created it as a wire frame using points and lines. This meant I could move points
through the coordinate system and modify the design with ease. This was done in a single geometrical set as the chassis would be one piece rather than
sub-frames. I then applied an outer surface bars/tubes to the lines which I would then apply a thickness too later.
Once I had this, I created a BOM group set of individual Part Bodies that would be the uncut bars and make ordering material much easier. Bars and
tubes overlapped but that didn't matter, it could be hidden. This was where I added a wall thickness to the surface.
The 2nd group was the final product ready for manufacture (i.e. you knew what angles and cuts needed to be made to the stock bars). This had all the
tubes cut and connected, again as individual part bodies per bar.
I then created a 3rd group for pretty pictures where I assembled the part bodies together adding in the fillets to act as welds. This looked very nice
and could give a much better representation of total weight and design for the posters.
To do any FEA on the chassis is a whole new task all together and depends heavily on what system you are using. Some will need joins connecting as per
group 2 and applying the constraint values i.e. welded, others as group 3 and measured as single solid items.
Thats all my way of doing it working happily in Catia V5 in GSD (surfaces) However I know some people who will much prefer the use of Part Design
(Solid) and will pad/hollow out the tubes. Again, no way is right or wrong, its how you feel comfortable getting the outcome you desire
quote:
Originally posted by Ugg10
depends on what you want the CAD model for -
if it is for construction only then solid bars i.e. external surface of structure is fine - keep it as simple as possible to reduce time/effort
if it is for FE modelling fro calculating torsional stiffness and suspension forces then you will need to model the hollow tubes
if it is just to look pretty then a pen and paper will be fine.
Thanks, plenty to consider there. Even to get a specific geometry there are a number of ways to do something with CAD, so I'm not surprised the
options are wide.
For the main part CAD is overkill for what I'm doing as so long as the inspector is happy about how secure the seat is and the seat belt anchor
points and I don't get wet from road spray when my doors are on then all are happy! :-D
I'm getting a feel for how I would take things if it went further so the habits I learn now are useful if I undertake a larger project later on
rather than working in a way I'd have to unlearn later.
I'd admit my way of approaching the modeling made use of the CAD skills I already had.
quote:
Originally posted by nick205
I'd admit my way of approaching the modeling made use of the CAD skills I already had.
I would say both 
The best way is to have a skeleton model to "Hang" your parts off.
1. make an empty assembly
2. Make a skeleton part which allows you to play with and visualise the form. Include points or cso at ends to ease the assembly function. Assemble
this as the first part in the assembly.
3. Make individual parts and hang them on the skeleton in the assembly.
this works really well and is flexible, if your good with cad or want to learn the techniques you can make the skeleton drive the other bits.
Pete
quote:
Originally posted by watsonpj
The best way is to have a skeleton model to "Hang" your parts off.
yes its a great technique
I've used it on small designs with just a few parts to large machines with thousands of parts.
You can really get the skeleton to do a lot if you want like articulate doors, suspension etc and the parts follow it.
build in some parameters and tweak them and see the bits jump to position on regen.
Pete
I believe solidworks has a tube and weldment feature so you can draw a 3D sketch of the frame and then it will work out what cuts are needed where to
complete the frame. Because it knows how the parts are related it can then manage fea
Have a play with the tube/pipe tool.