Motorsport Composite Fundamentals: Cutting Cured Parts

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Cutting Cured Parts

05.57

00:00	While our parts are looking pretty good straight out of the mould, they clearly need some finishing touches before they're ready to use.
00:06	In this module we'll be trimming and drilling holes in our CRX fenders.
00:10	While this will be the final touches for these specific parts, we'll continue in the next module to show some void filling and surface finishing touches on some other parts.
00:19	Something to keep in mind during this module is that we're working with fibreglass and this does come with some different considerations compared to cutting other composites like carbon fibre, kevlar and flax.
00:31	We'll discuss these different considerations as we go and we'll also be adding new worked examples using these specific composites to the library in the near future.
00:41	With that out of the way, let's get into our first job of trimming and shaping the edges of our parts to match the original fenders that we pulled the moulds off.
00:50	There's a fairly clear visible line on the surface of the parts from the mould surface to indicate where this is.
00:56	But some cross-referencing with the original parts and then the use of a marker to highlight the areas is a good way of avoiding any mistakes.
01:04	The next step is making sure we have appropriate PPE as we discussed in the mould making section of the course when trimming the flanges of the mould.
01:13	This includes eye and hearing protection as well as a respirator, coveralls and latex or nitrile gloves.
01:19	The dust from cutting fibreglass is extremely irritating and very messy so we recommend doing it outside if possible.
01:27	With the part on a sturdy surface, we can use a cut-off disc for cutting metal parts on a die grinder.
01:33	In this case, we're specifically using an Inox Aluminium Oxide abrasive grit cut-off disc for cutting metal parts as this will cut through the cured fibreglass laminate very easily.
01:44	The same could be used for cutting flax composite parts although these generally cut somewhat like a mixture between fibreglass and wood.
01:51	Using a coarser grit cut-off wheel and a slower cutting speed can help to avoid burning up the edges.
01:58	Carbon fibre on the other hand is extremely abrasive and will wear down a typical cut-off disc very quickly.
02:04	For these, it's best to use tungsten carbide abrasive grit cut-off discs.
02:09	The same could be said for cutting Kevlar.
02:11	However, as Kevlar is very durable, it's also very difficult to cut and causes frayed edges.
02:17	A diamond cut-off disc is the best choice when working with this material.
02:21	Let's get back on track.
02:23	These parts are now trimmed close to the marker line, but not all the way up to the final edge.
02:28	Next, we can use a carbide burr tip and a die grinder to get right into the detailed corners.
02:33	Then a fibre sanding disc to work up to the edges and shape them with control.
02:38	We then follow this by hand sanding with a sanding block to take off any sharp edges and remove the flashing from the mould split lines.
02:46	The final step is to drill the holes for mounting the fender to the car.
02:49	Before we make some irreversible changes, it's important to line the fender up with the car and mark out these holes that we need to drill.
02:57	Again, we have some clear witness marks for the holes from the mould surface, transferred from the original pattern, so it's easy to spot them and find the centres.
03:06	Body panels usually have a lot of clearance in the holes to allow the panels to be aligned on the car and achieve consistent panel gaps.
03:13	The hardware is M6 in this case.
03:15	We'll simply drill out to 6.5mm for now.
03:18	And then when the rest of the bodywork comes together on the car, these holes can always be opened up if needed.
03:25	When it comes to the drilling process, there are a few things that we need to be aware of.
03:29	First, fibreglass often splinters and chips on the backside of the hole, like wood does when drilled.
03:35	We'll use a simple twist or jobber drill bit for this job, as this is what most people will have on hand and the chipping will be minimal with such a thin laminate like this body panel.
03:44	However, for thicker laminates, we'd ideally use a tapered drill reamer, straight flute spade or dagger drill bit.
03:51	These are the best for preventing chipping and delamination, while the straight flutes also cut a clean and accurate hole size.
03:58	We will, however, clamp a piece of sacrificial wood in place as a backing material to reduce the amount of chipping.
04:04	Our regular tall steel drill bits will cut through fibreglass easily.
04:08	However, for more abrasive composites like carbon fibre, it's best to use a drill bit with a coating like titanium nitride, or even better, a solid carbide bit if the budget allows.
04:19	A quick rub of sandpaper after drilling helps to deburr around the hole and lowers the risk of splinters from handling the fenders.
04:27	At this stage, our fibreglass fenders are essentially complete.
04:30	Again, as the rest of the bodywork comes together, they'll be sanded, primed and then painted or wrapped with whatever livery ends up on this car.
04:38	In the next module, we'll look at surface finishing a carbon fibre part, where the work is a bit more rewarding than our CSM reinforced fenders.
04:45	So for now, let's summarise our final work on these fibreglass fenders.
04:49	Straight off the mould, our parts have an area of excess untidy laminate around the perimeter that needs to be removed to find the final tidy edge of the part.
04:58	We first marked out the areas we needed to remove to avoid costly mistakes.
05:03	Then, with the proper PPE and inner space, we could get messy.
05:06	We used a cut-off disc on a die grinder to make the first rough cuts and remove the bulk of the unwanted material.
05:13	We then come back in with the same die grinder and a burr tip to get into the finer details, then a fibre sanding disc to carefully shape up to the desired edge, with a final pass of sandpaper to finish off and remove any flashing from the mould split lines.
05:28	We also marked out the holes for mounting the fender to the car and drilled them using a typical twist drill bit and some wood backing material to help reduce the chipping on the backside of the hole.
05:38	While cutting fibreglass parts like this is fairly easy with conventional tools we'd use in the workshop, there are some considerations around cutting the abrasive carbon fibre and difficult -to-cut Kevlar composites that we'll show in the worked examples using these specific materials.