Plexiglass drilling-tips please

You can use regular twist drills but you gotta keep 'em COOL or the Plexi will melt and leave a raised ridge around the holes, or worse, melt so much that the hole becomes oversized.

Keep the drill speed low and don't try and "punch" the drill through with too much pressure.

If the Plexi is horizontal you can build a low dam of modeling clay around the spot you're drilling and put a little water in it to help keep the drill cool.

If you've got some extra Plexi, you might want to PRACTICE a little until you know you can drill the workpiece properly.

HTH,

Jeff

Roger Hull wrote in news: snipped-for-privacy@nntp.velocitus.net:

Make sure that both sides are masked: if the original covering paper has been removed use masking tape to insure that both sides are masked where the holes are to go.

Masking will reduce chipping as the bit enters and leaves the material.

Use a drill press and go slow. Excess pressure will make it crack. Cracking was my biggest problem with plexi.

Stev

One more: Clamp the piece down. If drill passes through it will pull the piece up and ... crack!

Nick

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.....

What exactly is the application ???

Another, place a pirce of wood under the plexiglas before clamping to aviod the drill breakout.

The material is very prone to cracking. I have good luck using a step drill (Unibit).

(top posted for your convenience) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net

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.

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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.

Tim.

Along with all the other tips don't use any kind of oil on it. If you do it will crack. Maybe not right away, but it will for sure crack. No doubt about it. Water is really best for a coolant. ERS

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).

astonished, Nick

Use brad and spurr bit at slow speed with wood under it. Do it all the time!

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.

Dan Mitchell ============

ACK

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).

Thanks for the insight, Nick

Here's a good web site on working with acrylic sheet. I found the info on thermoforming especially interesting. Here's the drilling section:

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