View Full Version : drilling in fiberglass reinforced part

07-21-2004, 08:07 AM
I need to drill some holes in 3/8" thick by 2" wide fiberglass reinforced epoxy "bar". The holes need to be 5/16" dia. with a countersink, the countersink needs to start about 1/16 below the surface so the screw head is recessed with no chance of being at the surface of the bar.

My problem is the fiberglass around the outside edge of the hole pulls up and leaves a fuzzy edge - - ugly hole. I have tried regular drill bits at a variety of speeds - - no luck. Wood bits with the point on the sides (not sure what they are called) work for about 3 holes - - then they get dull and the fiberglass starts fuzzing up. Fosner bits (I only have a cheap set) also work for about 3 holes before the edges get fuzzy. I have tried putting a piece of wood on top and clamping it down with no luck - - haven't tried metal.

I don't mind drilling these holes in two steps - - drill a hole the dia. of the countersink about 1/16 deep and then come back and drill the 5/16 hole and countersink but I have to drill a lot of holes and can't have my bits only lasting about 3 holes - - besides I don't know how to sharpen Fosner bits or the pointed wood bits anyway!

Any suggestion? type of drill, speed, etc. would be appreciated. I know that this can be done efficiently because I have seen pieces with a bunch of holes drilled and countersunk that looked great and it couldn't be too difficult because the parts weren't too expensive - - unfortunately the pieces for sale don't have the holes in the correct places for me.


07-21-2004, 08:15 AM
Carbide drill and carbide countersink. As you have found out fiberglass is very abrasive. It will even wear out carbide. You might want to try a carbide tipped masonsry drill first before you invest in a 5/16 drill. You'll have to sharpen the bit first though.
There was an article in HSM awhile back about making spot facers and countersinks that use carbide inserts. Might be wort looking into.

Mike Graham
07-21-2004, 08:24 AM
<font face="Verdana, Arial" size="2">Originally posted by kruszert:
My problem is the fiberglass around the outside edge of the hole pulls up and leaves a fuzzy edge - - ugly hole.

This is a nasty problem. As I was reading your message, my first inclination was to suggest spur-point bits (the wood bits) but as you say they dull too fast. I don't think that any bit that doesn't have a spur of some sort to cut the OD is going to give you a clean hole. I suppose if I had to do this I'd probably make a solid carbide spade bit (only because I've got access to heaps of carbide)but a regular carbide bit will give you the same lack-luster results as a freshly-sharpened HSS bit of the same pattern. The only benefit to the carbide bits is that they stay sharper longer.
You don't mention how many holes you have to make... if it's only a couple of dozen or so then you can sharpen those spur bits... just take a stone or diamond file or something like that to the inside faces of the spur. That should be all you need to do until you've sharpened the spur away, but by then you should be done a few dozen holes.

One odd-ball idea that might work is using an end-mill, as long as these things are being drilled in a mill or press. If an HSS end-mill gives you a good hole, then a carbide end-mill of the same pattern should give you a bunch of good holes.

Mike Graham
Caledon, ON

Bruce Griffing
07-21-2004, 09:03 AM
I have experience drilling small holes in G10 epoxy fiberglass circuit boards. The key to making nice holes in that material - similar to yours - is very high speed with carbide. I have used both carbide drill bits and carbide end mills. It needs to be done in something ridgid like a milling machine. The small wire holes are drilled in printed circuit boards with spindles that turn 60k rpm and above. This is of course because the holes are small. I don't have a sfpm number, but I would use carbide and go as fast as possible.

Mark Jones
07-21-2004, 10:39 AM
the gel does not have reinforcing in it.
this makes the stuff very brittle especially when its old ..more than two years and it looses its elasticity.
think what you need to be thinking of doing is lubricating with petroleun jelly as you drill.
dont think it matters what drill you use as long as you have the lube and you do not wobble the drill off the 90 degrees plane.
All the best....mark

Dave Opincarne
07-21-2004, 11:59 AM
Where does he mention a gel/face coat?

Having worked as a toolmaker for an aerospace company I know what you're talking about. My first questions are: Are you sure this is fiberglass and not aramid fiber? Is this a comercialy made part or did you lay it up yourself? While the fraying you're describing does occur in glass it is much more pronounced with aramid (as well as tool wear) since the fibers never become fully encapsulated and therefor fray. Resin starvation in glass will have a similar effect and for the same reson, the fibers are dry and therefor fray.

Assuming neither one of these cases is true here's some things you might try. The backer block is a good idea as is the use of the brad point drill. There is a drill bit used for composite tooling, but IIRC it's a propriatary design so is only available from the manufacturer. I've looked on the web but cannot find a link. I've only seen the catalog, but as I recall it did have a vauge resemblence to the brad point drill. Cutting action looked to be preformed at the lip with a high shear angle. I remember a lot of hook being formed by the flutes. I don't think there were any spurs though. Sorry if this isn't a very good description, it's been a few years. Our production dept used a carbide drill/reamer for small holes. This was a four flute reamer with a long taper on the front ending in a two flute drill point similar to a spade drill.

A couple of simple ideas that may help: Try drilling the hole back to front and then cleaning up the fraying with the countersink operation. Make a jig to hold two drill bushings in alignment on the front and back. Drill the hole through half way from each side.

OK, it just occured to me how to simulate the bits I was telling you about. Look at your spur bit. Now imagine the lip releif angle increased but still square to the drill axis and without the spurs. What you're trying to do is get the outer tip of the drill to shear the fibers first. I don't know that it will stay sharp much longer in HSS but improved cutting action might allow you to get more accptable holes with a relativly duller bit. It's also a fairly easy shape to touch up. If it works and you have enough it might be worth investing in a micrograin carbide bit and having it reground.


07-21-2004, 12:14 PM
Seems to me this might be a job for grinding, at least for the countersink. Either buy an appropriate stone or dress one to size. Should leave a clean edge.

Dave Opincarne
07-21-2004, 12:33 PM

Sorry, just re-read your post, I was focusing on the fuzzy problem which now sounds secondary to your initial problem. The fuzzing usualy happens on the back of the hole. If I now understand you corectly your having problems with delamination as the drill enters the workpiece correct?

If so the drill point I mentioned will help since it detaches the fibers first before the flute lifts the waste up. Think about a drill grabbing as it breaks through a thin sheet. You just have a lot of sheets stacked on top of each other.

Try drilling a hole through a metal block. Face the block so there is a sharp edge at the hole. The edge of the hole should be as sharp as possible and as close to size as you can make it. Clamp the block FIRMLY in place against your part with a hardwood backer block on the other side. Drill your hole. If you make your block out of steel you can dispence with the pilot point on the drill bit. Just grind the bit at 0* with a lot of relief angle and let the block locate the drill bit.

Hope this is of some help


07-21-2004, 04:52 PM
Carbide drills, and tightly sandwich the material for drilling if that's possible. I go with Evan's idea of dressing a small mounted stone for countersinking. Or I wonder if there's a carbide or diamond burr made that's already suitable. Maybe select or dress a mounted stone to debur the lip at the surface.

07-21-2004, 04:54 PM
Lots of good suggestions and things to try - - as usual this group is very helpful.

Carbide will definitely be the way to go once I find something that works. HSS will do for testing. I have several hundred holes to drill so struggling with 3 or 5 holes per drill bit would be a problem.

The problem material is fiberglass, the surface of the part is actually Teflon/polyester attached to the fiberglass rod. The Teflon/polyester layer is thin and cuts easily but when you start cutting into the top layer of the rod the fiberglass
fuzzes right up. After you get down in the material say 1/8" it cuts easily. No gel coat and no aramid. When the drill bit comes out the bottom the edges are just fine with no fuzz.

I think that I am going to need some type of spur on the edge to cut the glass. Using an end mill perhaps with some center grinding to leave the edges sharp may work. Increasing my speed may help also - - I used around 1000rpm max when drilling - - I'll try to up the speed. I have about 4000rpm max. A two step operation with the first step using a special tool to cut the initial glass may be another option.

The metal block idea also sounds interesting. Basically cutting into the surface and also holding the fiberglass in place.

Not sure about grinding this type of material.

I'll give these a try over the next couple of days.



Dave Opincarne
07-21-2004, 05:02 PM
It would have to be a very course stone to keep from loading up with resin and burning. Depending on the composite type the resin may burn away leaving a bad fringe. composites do not like to be sanded on the end fibers. Abrasive drills do exsist but the hole quality tends to be poor.

Just wondering, is this a phenolic resin? These seem to be the worst for the type of damage I think you're describing.

Dave Opincarne
07-21-2004, 05:08 PM
Sorry, Simulpost. I'll try and do a quick grind job and take a photo to illustrate the drill bit. You do want to cut at the lip first but I don't think the spurs are the best option. Check back and I'll try to post a photo


Dave Opincarne
07-21-2004, 06:37 PM
OK, here's a bad photo of a bad grind job done on a beat up wheel done in all of a few minutes, but hopefuly it's enough to illustrate the geomitry I'm trying to describe. Note the hook formed by the flute.



07-21-2004, 06:50 PM
Don't grind it, cut it. You want as little particulate matter in the air as possible. I should have brought this up before. Very fine fiberglass particles are very bad for your lungs. Use a mask.
Another idea. What about a sharpened hollow hole punch in the drill press. Slice through it.

07-21-2004, 07:57 PM
You can buy cabride tipped brad points at www.leevalley.com (http://www.leevalley.com) I use them for woodworking and they cut very cleanly. Might be worth a try