Drilling of a composite monocoque

[dlat]

I would like to know how the other teams drill their monocoque chassis. I know we can use a drill jig or a cnc machine, but i would like to know if there is other methods. For now we are thinking to use a drill jig but we don't know if it's the best way to do it. So if you could show us pictures of your jigs it would be appreciated.

Thank you

[dlat]

I would like to know how the other teams drill their monocoque chassis. I know we can use a drill jig or a cnc machine, but i would like to know if there is other methods. For now we are thinking to use a drill jig but we don't know if it's the best way to do it. So if you could show us pictures of your jigs it would be appreciated.

Thank you

[Carrington]

DeWalt 18volt cordless is my favorite.

[Travis Garrison]

Are you looking for advice on how to place holes (jigs) or on how to make holes (which drill bits)? Both can be tricky...

[dlat]

I'd like to know how to place the holes so they are positionned the as exactly as possible

[Xeilos]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by dlat:
I'd like to know how to place the holes so they are positionned the as exactly as possible </div></BLOCKQUOTE>

Don't worry about it. It'll be fine....

You should use the rule of thumb. If you can cover you mistake with your thumb, then it never happened.

Would probably be easier if it was a rule of wrist.

[Swingtops]

I am reminded of a south park episode....

Step 1: Make monocoque

Step 2: .............

Step 3: Win Detroit!

But seriously a brad point drill bit or a straight flute carbide endmill should do the trick

[woodsy96]

Last year we tried drilling the moulds and using dowells to locate the hardpoints. Caused a couple of dramas. Before that we used to mark the points with a robotic arm and use a hand drill (with two spotters for angular accuracy)

[Tim.Wright]

There are quite a few ways to do this. Location of composite plies are typically not so accurate, and they can move as the part cures, so make sure your hard points are nice and big.

1. The best way is of course to make a metallic drill jig. Here you will typically have datum surfaces and mounting holes located off the edge of the part from which you can attach the jig. Then you drill your hole through the jig and the datum surfaces are then cut off as the part is cut to final size.

2. Next possibility is to put scribe lines onto the mould tool which will transfer onto the surface of the final part to give you a reference to which you can aim your drill. In this case, you only need to make sure the drill is nice and perpendicular to the drilling surface.

As for the drilling process - look into the type of tooling required for composites (diamond tipped drill bits I believe) because normal drill bits will blunt very quickly and will tear fibres away as it exits the part.

And finally, always, always put a sacrificial block of wood hard up against the back side of the panel you want to drill through to stop the fibres peeling as the drill comes through the part.

Tim

[Thrainer]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by woodsy96:
Last year we tried drilling the moulds and using dowells to locate the hardpoints. Caused a couple of dramas. ... </div></BLOCKQUOTE>

What kind of problems did you have?

We had the same procedure for the last three years: Put dowels into the positive mould to have holes in the negative mould. Then use dowels in the negative moulds to locate the hardpoints. This must have been pretty accurate, as our wishbones wouldn't have fit otherwise.
There are some pictures of this in our 2009 gallery.

Tim, I think using a CNC mill is more accurate than using a drill jig, as you're avoiding several sources of inaccuracies.

Regards
Thomas
AMZ - ETH Zurich

[cvargas]

I'd suggest not drilling and try molding the holes into your composite instead. Composites work best when their fiber lengths are as long as possible. By drilling you are cutting those fibers and creating stress concentrations at that point.

You are creating cracks in the matrix when you drill that will begin to propagate once loading is applied.

You can drill a composite laminate but I'd try to avoid it.

[Tim.Wright]

There are several reasons why I would not recommend moulding holes into your part.

1. Ive never seen it done sucessfully before. Composite structures are drilled all the time, from aerospace parts to race car parts, so you certainly can't call it bad design.

2. Your composite should be thicker (by building up the number of plies) in the area it is drilled, so you wont loose any overall strength.

3. Small radius details such a hole will turn out scrappy 9 times out of 10 unless the processing is absolutely perfect. Im talking about exposed fibres and out of round holes. At best you could mould pilot holes into the part, but if you want clean round holes, you have to drill/machine

4. What is to say your mould tool is accurate?

5. The composite moves as it cures and shrinks as it cools down, so its not so easy to predict the final shape of the part.

Finally, yes, a CNC mill would work perfect, but how many people have access to a mill big enough to fit a whole tub inside? Also, the amount of tooling required to position the tub in the mill would be much more complicated than making a drill jig.

Tim

[kapps]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Timo:

Composite structures are drilled all the time, from aerospace parts to race car parts, so you certainly can't call it bad design.

Tim </div></BLOCKQUOTE>


Just because it's done all the time doesn't mean it's a good design. Most times, it's done because they're still living in a sheetmetal world and don't understand how to design for composites. Yes, even aerospace companies. Trust me, I know. When you do have to drill composites, I'd recommend drilling larger and then bonding in a 2-part spacer and top hat that is bonded to the composite and not only spreads the load but also eliminates the possibility of compressing the composite when you tighten the fastener. Composites like tension but do not like being compressed and tightening a bolt through it can cause problems.

[Tim.Wright]

Of course, blindly copying people is not good design, but ignoring the experience of hundreds of other applications is plain ignorant.

For sure placing a spacer will reduce the compressive load on the composite, but was it ever a problem? Yes composite are weaker in compression than extension but it doesn't mean its jelly.

Making a spacer also increases the part count and assembly time as well as passibly making the joint more bulky, so you really need to ask yourself if its really necessary.

Tim

[Moke]

You are much better to drill a hole than to mould around a hole. Yes drilling cuts the fibres, which means there are fibres to take compression loads on their ends (1470MPa for T700s fibre). If you mould the fibres around the hole there is nothing to take any compression and a bolt can just "split" down between the fibres. I have seen it happen and done it myself thinking it would be better.

You don't get much crack propagation in composites due to the fibres, unlike a metal. So when making a metal part you should mould in the holes for sure.