10 replies to this topic

[Hellenic tifosi]

How much time does it take to manufacture a new carvon-fibre chassis from scratch, after the initial drawings have been competed?

[Wellington]

Hi

The first prototypes may prove a real puzzle to manufacture. Once you have the design, the most difficult lays still ahead!

In fact, the challenge is to arrange the layers of prepreg in order to make them fulfill their mechanical role. One of the advantages of the use of composites is that you can build a structure with the least material in the most mechanicaly competent quantity. To achieve this, you "just" have to have a look at the stress beared by your structure, and with this data you place your material in order to counter it (by arranging the direction of the fiber). But if you can have a theoretical solution for this, it does not mean that it is easy to realise inside the mould: there may be weaknesses in other parts (like the junction between two parts for which the stress is different). Thus, identifying the stresses and translating them into correct stacks of prepreg is no little job.

Then, there is still the fear that the shape of the chassis itself makes things difficult when moulding. Prepregs are tacky but often not enough, thus may not stay where put (desperatly discouraging when you are fighting with your material...). And on the other hand, each and every problem relative to the use of composites may occur, like exothermal damage or the ousting of gases imprisonned in the material.

There are often big surprises when you are to take the parts off the mould. You may see that internal stresses due to the cure bend some parts or even that the fragile junctions such as those mentioned above break under the slightest stress. You need to re-arrange the stacked layers of prepreg, to try and minimise thickness at some places, etc...

Do you have such project in mind or in completion? I would be very interested.

Regards

[Ben]

The only hard data I've got is from Autosport (27495) from a feature on McLaren's efforts to redo the MP4/10's tub for Mansell.

Ergonomic/packaging study 3 days
Production mock-up 3 days
Driver feedback 1 day
CAD definition of surface geometry 3 days
Modify chassis pattern 10 days
Paint chassis pattern 5 days
Produce new moulds 4 days
Component manufacture 10 days
Assembly prior to machining 2 days
5-axis machining 6 days
Final monocoque assembly 5 days
Initial assembly of chassis 2 days
FIA crash testing 2 days
Painting 3 days
Car build and set-up 6 days

This is a total of 65 days and they actually got the job done in 33 days.

Ben

[Hellenic tifosi]

So that means that if a team wants to have the new car ready at the January 1st for example, then they must be finished with the tub design by mid November and then start manufacturing it, so that they can assemble all different parts(enfgine, gearbox, etc) on time for the launch. Is this correct? By the way, both posts have been great and informative:up:

[Wolf]

Tifosi- there are few simple shareware/freeware programs for calculation of properties of multidirection, multilayer orthotropic materials. I can send them to You or U/L them somewhere if You're interested in D/Ling them. Or anybody else, for that matter.

[MclarenF1]

I was wondering if the first CF monoque

[desmo]

A bit from Peter Wright's "Formula One Technology":

"Specialised Mouldings was the first company to use carbon fibers to stiffen and reinforce the glass fiber body panels it manufactured for the race car manufacturers such as Lola, McLaren, and Chevron...Initially, CF was available only as individual tows- 3mm bundles of fibers- and these were laid up as a 50mm square grid on the back of the flass fiber panels. As soon as the textile companies such as Courtaulds began to weave CF, the material was used to make whole panels, replacing glass fiber in the lay-up of composite body panels.

"The first CF monocoques from McLaren and Lotus appeared within a wek of each other in early 1981. John Barnard's MP4/1 used a monocoque molded from pre-impregnated CFRP/Al honeycomb by Hercules Corporation in the US. Hercules...used the most advanced techniques available to mold the monocoques around an internal Al mandrel, and cured it under pressure in an autoclave.

"The in-house approach of Lotus was very different for the T88 monocoque. A large sheet of CF/Nomex honeycomb sandwich was hand laid-up on a flat table (actually a sheet of plate glass). Then the cured the inner skin was grooved and the sheet was cut out and folded into the monocoque around a jig. The whole approach was similar to making a card model- 'score here; cut here; fold along A-A; glue B to C.' When the joints were reinforced and machined Al bulkheads were bolted in, it formed a stiff and lightweight structure."

[Timm]

I doubt the MP4 Fat-Arse edition would be the best example. I mean - 'all' they had to do was widen it. The important bit is defining the structure at load bearing points and regulated/aerodynamically critical areas.

I distictly remember Patrick Head saying that how long a tub took to build depended on 'how hard we kick our blokes'

Once the chassis packaging and the tub shape has been defined, the structural engineers will define thicknesses and lay-ups. following that, the pattern is made, prepared and moulding is made from this. Then lay-up can begin. Actual lay-up could feasibly be the swiftest part, depending on the complexity of the lay-up, there could be delays while it's consolidated in the oven (This squeezes the earlier layers onto the mould and allows lay-up to continue). Honeycomb can be tricky to include also. Furthermore, if there is more than one piece in the tub, each part may have to be done first - and then bonded together. Modern F1 tubs are one piece with a seat-back bulkhead & dash. Earlier tubs were two piece and had 3 or more bulkheads that had to be bolted/bonded in.

We had a cool tool at GKN when we were laying up composite nacelles for Airbus' engines. Eack laminate was cut to shape and a kit of these laminate parts was given to us. The position and orientation of each bit was critical and a laser mounted in the roof of the factory projected the outline of the laminate piece onto the correct location on the mould. In addition the the outline, there were datum points to assist us.

I recommend you try and get hold of some techincal articles written in the early 90's by Gary Savage and/or Brian O'Rourke. Respectively the structural blokes for McLaren & Williams, these articles are superb. Brian O'Rourkes was in the IMechE journals and Savage's were in 'Metals & Materials' magazine.

[Ben]

Good points. I think a fair bit of redifinition of the lay-up was needed for the MP4-10b though.

Which GKN site do/did you work at?

Ben

[jetsetjim]

This is how we built our CFRP chassis:

Week 1-2: Pattern Manufacture. The top and bottom half of the chassis patterns are CNC machined from large blocks of Tooling block. CNC cutter paths are generated from IGES files taken from the CAD files. Once the patterns are finished, they are coated with a specific paint which seals the pattern. Once the paint has cured and hardened, the patterns are then coated with mould release agent. (Frekote is the usual chemical)

Week 3: Mould Manufacture. Once the patterns are correctly prepared, then it is possible to begin the lay-up of the moulds. In the case of our chassis, it was a 3 piece mould. The bottom split in half, and the top was a complete structure. The moulds have to be manufactured from specific CFRP material, as it has to undergo 2 "cooks". The tooling CFRP was supplied in two thicknesses. 200gsm material was used first to define all the edges on the chassis, and is basically the first ply of the mould lay-up. This material also cures to a better surface finish, which ultimately gives the final chassis a better finish. The pattern is then put into a sealed bag and a vacuum applied to push the ply hard into the pattern, again for edge definition. The next 8 plys are then layed up using some 640gsm material, which is a nightmare to use!! After about 4 plys, the patterns is vacc'd down again. The final ply (ply 10) is another layer of 200gsm material. Once these plies have been laid, the pattern is vacc'd down once more, and then cured in the autoclave. The cure time for this procedure is a long slow cook, with the pressure in the 'clave being held at about 90psi. Once the pattern comes out, the mould is removed, trimmed up, and then put back into the 'clave for a second cure. This post cure raised the Tg of the material, which prevents any distortion of the mould when cooking the final chassis. This whole process takes a week, and has to be done quickly, as the tooling CFRP has an outlife of only a few days. I.e: You have to have the mould layed up and cured within about 3 days, or the Carbon and Resin system go off.

Week 4-9: Chassis lay-up. The lay-up of the chassis varies from team to team, but in our case, we used a mixture of T800 cloth, and T800 UD. It is more common these days to just use T1000 cloth, with M46J or M55J UD materials. The ply lay-up, and material orientation will have been defined by the Composite designers, the Stress Calculation department and also as a result of FEA analysis.
The laminators are given a lay-up book, which they have to follow. In our case, the outer skin consisted of 17 plies, mostly UD material. Once these plies are laid, the mould goes into the 'clave for it's first cure. This was at 120°C, 90psi. The next procedure is to fit the honeycomb core. Again, the thickness of the core at different places is defined both by regulations and by the design department. Once all the core has been laid, it goes back into the oven to cure the glue film that has been applied beneath the core. The cure this time is typically 120°C, 40psi. Again the mould comes out, and the next procedure is to fit all the solid carbon inserts in the areas required. There are as many as 100 inserts in a chassis. After the inserts are fitted, the inner skin plies are then laid up. In our case this was 11 plies. Once all these are laid, the moulds go back into the 'clave for their 3rd cook. Again at 120°C, 40psi. Once they come out, the chassis halves are trimmed to the edge of the moulds, and then the two haves are bolted together, to allow the splice lay-up to be done. This splice lay-up is what joins the two halves of the chassis together to make a one-peice structure. The final cure is then carried out on the chassis complete.

Week 10-11: Chassis trimming and machining. The chassis complete is removed from the mould and first sent to the trim shop. All excess carbon edges are removed, and the final shape emerges. The next procedure is to take the chassis to the machine shop, where all the holes are drilled to allow all the components to be fitted to the chassis. Keencerts are fitted where necessary, and then the chassis is returned to the trim shop.

Week 12: Bulkhead fitting and body fit. This is the final part of the chassis manufacture. Here all the relevant bulkheads and other misc. panels are bonded to the chassis. As a final job before release, the trim shop do a body fit on the car, to ensure good panel fits with the undertray, sidepods, nose, and engine cover. After this, the chassis can then be released for painting.

This is typical of a new chassis build, and the procedure can probably be reduced by a week and a half these days with 24 hour shifts. The overall time for chassis builds though from start to release for build is still around 7 weeks.

[Ben]

Great post Jim.

Who's the 'we' by the way?

Ben