Machining Times (Body)

[Andy K]

I've started setting up CNC programs to machine the plug for our body. I'm getting disturbingly long programs (to the tune of 7-10 hours per part... and I have twelve total due to size limitations of our CNC). I just want to get an idea of how long you guys have spent machining your plugs.

Thanks

Andris Kanins
McGill Racing Team - Body Design

[James Waltman]

What material are you using?
Is there a lot of surfacing or mostly geometric shapes?
Are you doing the machining yourself or have you found someone to do it for you?
What kind of machine and tooling are you using?
You say that you are setting up the programs – does that mean tool paths, G&M code, or just a solid model.
What CNC experience do you have?
How have you selected feed rates and spindle speed?

James Waltman
waltmaj@cc.wwu.edu
http://dot.etec.wwu.edu/fsae/
Formula SAE
Vehicle Research Institute at
Western Washington University

[Andy K]

To answer your questions:

1- We are using a styrofoam type material
2- There are many sweeping curves, no perfect geometric shapes
3- Machining it ourselves in our shop
4- Machine doesn't matter, what matters is that it only has a range of 17"x38" in the y and x-directions respectively. We were planning on using an 1" ball end mill w/8"cutting depth... Both of these factors have led to my decision to split the plug into nearly 12 sections of various sizes.
5- Toolpaths... I have the design on CAD software and have created toolpaths for it. There just seem to be none that provide me with a good 'shape' with a decent machining time.
6- CNC experience: enough
7- Feed and spindle speed have been maxed out. This has helped reduce machine times further, but not enough to warrant the exorbitant price in the Cost Report, IMHO. I could spend the time carving it by hand resulting in a good, if not better, finish than using the machine...

Andris Kanins
McGill Racing Team - Body Design

[James Waltman]

4.3.4.1 General Requirements
The reported cost of the prototype vehicle will exclude R & D, tooling (e.g. jigs, moulds, patterns and dies), and capital expenditures (e.g. plant, machinery and tools).

So I don't think that you need to worry about machine times for the cost report. I thought that you found someone to do the work for you and you were worried about tying up the machine for two weeks. I guess if you have to sit in front of your own machine the times are still too long.

Does "Styrofoam type"� mean home insulation? I am not sure how easy Styrofoam is to machine. It seems to me that it will deform at high feed rates and not come off in chips, then it may spring back some after the tool has passed. It seems like it might melt if you are using high spindle speeds. But I haven't machined any so these are just guesses, maybe you have tried some parts already. The foam we have used here is dense and the chips (mostly dust) are very similar to sand. It is hell on the machines because it is very abrasive. To get your run times down I would try to get the foam to a rough shape fast (jig saw?) then run your surfacing in the mill.

Steps for some smaller molds that have been made here:
Machine foam to desired shape
Cover all surfaces with Bondo (or similar)
Machine Bondo
Finish sand
Prime/Paint and lay up parts

Very little handwork is needed and the parts are especially accurate. If you just machine the Styrofoam you will have to spend a lot of time getting a surface ready to lay up on (Bondo, sanding, Bondo, sanding and on and on). The goal of using a machine is to get rid of those steps.

I would think that extra caution would be necessary if you want to vacuum bag a Styrofoam mold (or it may collapse). Be careful with the acetone around your mold so it doesn't melt...

James Waltman
waltmaj@cc.wwu.edu
http://dot.etec.wwu.edu/fsae/
Formula SAE
Vehicle Research Institute at
Western Washington University

[Andy K]

I am going to try to machine a few pieces and see how long it really takes...

Thanks, you've given me some additional ideas...

Andris Kanins
McGill Racing Team - Body Design

[matt giles]

Be careful what type of foam you are bondoing over. Most foams will melt slightly under the bondo. This adds hours upon hours of extra bondoing and sanding. I suggest laying up fiberglass over the foam to protect it, or I think urethane foam won't melt but it is really expensive.

Matt Giles
Kettering University FSAE
Composites Team Leader

[James Waltman]

Matt,
Do you mean to use a layer of fiberglass inside the mold on the surfaces that you are going to lay up on or on the other surfaces as general protection? If you put fiberglass on the inside you need to account for it's thickness in your machining and it will need to be prepared for layup (Bondo and sanding). Lots of hand work again. The ultimate goal should be to have a part that comes out of the machine requiring very little additional work.

Check out Aircraft Spruce's catalog:
http://www.aircraftspruce.com/catmain.php
From the main catalog select composites and then foam. I think that we have been using stuff like Last-A-Foam or Poly-Urethane it is tough to tell from the pictures.

Let us know what you decide and how it comes out.

James Waltman
waltmaj@cc.wwu.edu
http://dot.etec.wwu.edu/fsae/
Formula SAE
Vehicle Research Institute at
Western Washington University

[Travis Garrison]

Correct me if I'm wrong here, but isn't it the polysester resin in bondo that cuases the problems with styrofoam? So wouldn't fiberglassing the mold be worse than simply using bondo?

If it were me, I'd stay away from the styrofoam altogether, just not worth the headache...



Travis

[Brent Howard]

Yeah, you don't pay for jigs, patterns, coding time, etc. I think that the reason for this is that as you produce more units the cost of these moulds approaches zero. This kinda keeps with the same way the rules used to be where you have to cost the car absed on 1000 units production.

Brent

www.ucalgary.ca/fsae (http://www.ucalgary.ca/fsae)

[Andy K]

OK, so cost isn't a problem - I guess time is not on our side... I 'think' we have decided to use the std. hotwire method.

We had originaly planned on laying up a relatively thin layer of fibreglass over the styrofoam (after machining) only to realize that the curing of the resin is an exothermic process and may melt the foam... Also, we are hopefully avoiding the use of Bondo by using plaster and a good layer of primer over that.

I now know what to avoid for next year...

Thanks

Andris Kanins
McGill Racing Team - Body Design

[Scott Wordley]

Epoxy resin is fine to use on polystyrene foam, but don't use polyester.

Last year we had our body plug machined from polystyrene and then had a urethane coating sprayed over the surface and then finished it by hand with lots of sanding and bondo.

Finishing the plug by hand ended up being a huge amount of work so this year we're looking at machining the plug from MDF or something similar.

Regards,

Scott Wordley & Roan Lyddy Meaney
Monash FSAE Wingmen
http://www-personal.monash.edu.au/~fsae

[Michael Jones]

Cost - yep, pretty well everything around plug manufacturing and finishing is not included. Thank God, or the body would cost more than the rest of the car.

CNC - It would be nice, I supopse, but given the size of the plugs, the materials you generally work with (MDF or wax, perhaps - can't see machining foam, but perhaps) and, at least in our case, the general inaccuracy of the models, I think it's overkill.

Bondo - does not eat foam appreciably. At worst, it makes parts kind of softish, and you might lose a millimeter or so. When you realize that you're bound to be 1/4" if you're good, a mm or two is irrelevant.

Epoxy resin -- and glass is good for finishing the mold and giving yourself a more stable surface to work on for detailed plug sanding, which should of course be minimal since no one's going to see it anyway.

We spend as much or more time on finishing sanding than plug preparation. That's what you see after all. But the better the former steps, the better (and lighter) the finished product.

And above all else, I think it's important to consider body design and manufacture more an art than a science. OUr 2001 body was overengineered and overrationalized, and was off the frame by 6" in some places. The 2002 body was off by a 1/2" at worst, and the process was much more intuitive than formal. Basic common sense and iterative checking should suffice. Make it look cool otherwise.

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Cornell Racing
http://fsae.mae.cornell.edu

[Andy K]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Michael Jones:
And above all else, I think it's important to consider body design and manufacture more an art than a science.<HR></BLOCKQUOTE>After joining the team I was convinced that it was a science... I had originally thought that it was more about feel and checking dimensions once in a while. I may have proved to the other guys that this really is the case.

Andris Kanins
McGill Racing Team - Body Design

[Michael Jones]

Some valid arguments either way...but there's some benefit to sketchy solutions, even if they do take a bit longer to hash out at times. Certainly if this was a mass production effort (hell, even if you had to make two or more) CNC'ing body plugs makes perfect sense and is worth the effort. Good luck in it all...

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Cornell Racing
http://fsae.mae.cornell.edu