Hi, Gang: I have a piece of Plexi 1/4" thick that needs four holes drilled in it. Size yet to be determined, but under 1/4". I did this in school thirty years ago but not since and need a memory upgrade. Any tips appreciated.
IIRC, that material undergoes a tremendous expansion rate on temperature change, so suggest drill oversize if the piece is very large at all--otherwise, it will crack--or else your fastener heads will break off upon weather changes.....
I always reduce the grind angle on bits I use for drilling plastics. I picked up a set of mid quality drill bits at Harbor freight, and used them for various items, and when they needed resharpening, I just gro9und them to a lesser rake angle and put them asside for plastics. Less rake and reduced pooint angle keeps them from grabbing and chipping the exit side of the holes, and really makes a nice clean hole.....I use center cutting endmills as well in some cases, for larger holes.
============================================== Put some color in your cheeks...garden naked!
With hole diameter less than thickness, there shouldn't be a lot of problems. You will probably have some chipping as a regular drill exits. If the hole is very near the edge, you may crack the material hole-to-edge.
If you can buy a drill bit specifically made for plexiglass, most of these problems will disappear. Any plastics supplier will have the drill bits.
Really? I always used cooling fluid (water + oil). But I never realized that this might be the cause for cracks. Yes, I had some/many fine but _only_ after glueing. I always thought that it has something to do with the way I glue (UV-hardening glue).
I don't think the oil CAUSES the cracks, but, as I understand it, the machining produces microcracks. The oil gets into these and causes them to spread (propagate). Eventually the whole area around the hole or cut just about crystallizes and crumbles from all the little cracks. Tapped holes are especially prone to this. Anyway, it DOES happen.
Water does indeed seem to be the safest coolant/lubricant. ANY residual oil is asking for trouble. I have tried wax, and this does not seem to have the same degree of problem as oil. Perhaps some, but I've not decided one way or the other yet. Some micro-cracking is normal.
Like many things, the plastic can have internal strains that are released during the cutting process. This may aggravate the cracks. Sometimes the plastic can be annealed to relieve this effect. It is heated evenly to a temperature below it's melting point, and allowed to set for a while, then slowly cooled. This may reduce cracking. I've not done this, and usually get decent results without needing it.
What I do that DOES help a lot ... AFTER the cut is finished, seal any cracks(most are invisible at this point). This can be done with either heat (fire polishing)if it's an open surface, or a solvent chemical like Methylene chloride (liquid acrylic cement ... nasty stuff, use suitable gloves and good ventilation) which can be used even inside a hole. Even lacquer thinner may work fairly well (depends on the brand, as all are NOT alike). The solvent needs to melt the surface slightly, thus fusing any tiny cracks into the main body of the plex. A drop into a small hole should work fairly well. Let the plex dry and harden before trying to use the part. Some minor rework may be needed after for critical surfaces.
Sounds reasonable. So I accept your oily oppinion. :-)
Oh yes, they do! I think Plexiglas XT (extruded) is worse than the GS (or G? anyhow, the casted one)
I did this and it helps. But you can make it wrong too. 60 deg. C for several hours is OK, 70 deg. C will make new wraps. :-(
Polishing helps most. 240 grit, then 400 grit whet, then a special Plexi-polish. For glueing, polishing is not required, the 400 whet is good enough.
I had bad experiences with acetone. It introduced cracks (_very_ fine, but a lot). I think that the plexi swells a tad with the solvent, deforms and when the solvent vapors you get the cracks. Not a proven theory, just my experience. So I stay away from _any_ solvents (except for glueing, but there the solvent vapors much slower).
Take a look at a plexiglass pepper mill. They will be molded, with maybe a little machining done to hold the grinder parts. And then again, maybe not. However, it doesn't matter. They ALL develop cracks eventually. The oil from the peppercorns gets into tiny cracks, scratches, whatever, and cracks form. ERS
The worst think is that the cracks may not form for months. It depends to some degree on how much stress is put on them. I built some vacuum chamber lids out of 2 inch plexiglass that were about 28 inches in diameter. I had to polish out all scratches. The polish used contained no oils or alcohols. The lids worked fine but were prototypes. The production ones went to a different shop that was better equipped to do parts as large as these. All the lids they made failed when stressed with the vacuum. Turns out they used coolant with soluble oil and polished out scratches with abrasive powder mixed with the same coolant. All the subsequent lids they made were kept well away from any oils. ERS