Trimming light covers

Whats the best way to trim the headlight/spotlight covers.
The ones I've bought aren't that good a fit curvature wise either. Will the screws hold them in place ok?
How about drilling the perspex? do you have to be really careful?
Thanks
Simon
 

Jim Pearson

Lifetime Supporter
Hi Simon,

Acrylic is easy to drill but also easy to crack - especially if the holes are close to an edge.

I'd suggest you grind the drill bit to give it a zero or negative rake - ie instead of the flutes having a positive spiral and therefore a tendancy to 'screw' into the job - the flute is ground between parallel and slightly negative relative to the axis of the drill. Hope this makes sense - hard to describe, easy to sketch.

I'd also protect the area to be drilled with masking tape to avoid the spinning swarf scuffing the surface, and use a bench drill if available.

Good luck.

Jim
 

Tim Kay

Lifetime Supporter
Simon,

I use a band saw then a table belt sander. Make sure you protect the surface so not to scratch.

Also, as Jim mentioned, to keep from cracking when drilling the holes I start with a very, very small drill bit and step up gradually. I used three size bits to get to the final bit. Not sure what your material is but Lexan is a bit easier to work with and is less likely to crack than acrylic.
 
For the covers I make, I suggest to guys to first lay the cover on the car for the best curvature fit. Trace out the body flange edge with a Sharpie marker, and trim down to the line with a Dremmel tool (rotary cutter). If you cut around the piece counter-clockwise the "heat boogers" stay on the excess and the cover stays clean. Once an edge sits nicely in the flange, use it as your anchor side and continue to draw and trim until it all sits in. For drilling, do as Tim says and use a very small and very sharp bit for pilot. Press lightly and also back out of the hole as you go so the plastic won't climb the bit. When you screw it down, snug to just hold the piece on. Over tightening will dimple the plastic around the screws and soon will lead to cracking.
 

Trevor Booth

Lifetime Supporter
Supporter
Try This site www.cyro.com
After trimming and sanding you can heat the edge with a pencil flame butane torch. The edge will melt slightly and produce a smooth 'clear' finish.
 

Peter Delaney

GT40s Supporter
Simon, after trying a jig-saw, then a band-saw (both set on super-slow), I still managed to crack a side window. Peter Ransom (DRB) put me on the right track - a diamond blade on an angle grinder. It worked a treat - just 4-5 very light, quick runs & you end up with a super-clean cut.

For finishing off the edges, I used a belt sander clamped upside-down to the bench (a proper belt linisher would have been better). After each test fit, I marked the amount to be removed from the edge with a red fine-tipped pen, then ran the piece lightly over the belt until the red line had gone. Test fit, re-mark, & do it again - lightly.

For drilling the holes, I found a drill-press the best as you can control the pressure accurately. Unless you have a very low-angle drill bit, it will tend to "bite" - a hand-held drill will just run thru & probably crack the piece, whereas a drill-press will allow you to go a lttle way in, back out to let things cool, go back in a little, etc, etc.

After I had done all my holes & countersunk them, I heard that this was not a smart idea - c/s heads try to split the persex when tightened up ! Apparently the best approach is to have straight-thru holes & to use pan-head bolts/screws with thin nylon washers.

Makes sense when I think about it - I'll know how to do it properly next time !!

Kind Regards,

Peter D.
 

Pat Buckley

GT40s Supporter
On the full width windscreen I made for my Kirkham out of Lexan, I hand filed and sanded to final shape after the jig saw work was done....worked well and the edge looked good.
 

Howard Jones

Supporter
Here is everything you want to know about Lexan type materials. Shape, sand, cut, drill, hot, cold, etc. etc.

www.cyro.com

Then click techknowlogy center

or search "drill" or "drill Bit"
 

Lynn Larsen

Lynn Larsen
Very good advice in the posts here! I found a little grinding disk on a dremel to be very effective in taking a standard twist bit and flattening the cutting edge so that it doesn't bite. (These bits can be bought, but it will probably take some sluething to find them and its easier and cheaper to make your own.)

Working backwards from final -> rough:

I have also used the technique that Peter used to do the final sanding: a flat sander in a vice. Rather than a belt sander, though I used a flat inline vibratory sander which is a little less agressive. To get "close" to the line, the belt sander would be very good or I have found that a 4" drum sander chucked into my drill press works nice for the rough sanding process. (Any less diameter and you'll get bumps.) The direction you move a cutting bit in a dremel or other rotary cutter for the initial cutout is critical as described by the heat boggers (I just love that technical term, but was the best advice I had in cutting my clear bits down.) Mark is 'daman when it comes to working acrylic or polycarbonate. There are several types of carbide cutters to choose from like the spirals, the very rough spiky ones that can be found in grout removal kits, etc. and you should experiment with them and pick the one you like best. I ended up liking the spiral bit and using a slight angle with the top leaned forward in the direction of travel. This seemed to provide less wander and the bit was far less likely to develop a flutter which can rapidly chew a nasty chunk out of the work. This is another good reason to take the last 1/8" or so off with a sander; it may take a while, but the fit will be worth it. The sander also allows you to put a bevel on the edge which you may or may not need depending on the crispness of the edges of the window detent. Also don't for get to keep in mind that the paint will fill this in somewhat.

Lynn
 
Hey Pat- have you had good luck with your plastic windscreen? That was my first intention but then I became worried about scratches from the wipers and irregularities in the forming process creating wavy vision. Did you form it yourself or have a shop do it?
 

Pat Buckley

GT40s Supporter
The windshield I mentioned is on a Kirkham Cobra....it worked fine although I should add that it was fairly low....

I formed it myself - the forming was not compound.

A good product to keep any of the plastics clear of scratches is Plexus...btw.
 

Robert Logan

Defunct Manufactuer - Old RF Company
Simon,

At Roaring Forties workshop in Australia we also use a cutting disc (3mm. thick) and sanding disc but on an air driven grinder.

We have tried many ways of cutting and cracked numereous pieces and the cutting disc is by far the most consistant.

Best wishes,

Robert
 
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