Thanx for help on toggle switches - here's a compiled list

Two methods come to mind. Fund and use a drill bit for plastic. The tip angle is very wide -- something like 135 degrees. This keeps the chips small and if you're drilling by hand doesn't pull the bit through the work piece like a bit for steel will.

Another method is to drill a pilot hole any then use a sharp reamer to slowly enlarge the hole to the proper diameter, wprking evenly foem both sides of the workpiece.

Bob

Drilling plexi is best done in a drill press - so you can control the feed rate and not have the bit pull into the material faster than you want AND clamping the plexi firmly so that the bit cannot pull the material into the bit. If you do both of these and feed the bit slowly

- but not too slow since you want to cut and not melt - you should have perfect holes with no shattering. Practice on a scrap of material to get a feel for how fast to feed the bit.

Dale Gloer

V> Thanx to everyone for the great pointers to electronics supply houses. I

If youj are using a drill press with a quill lock it may help to put a bit of drag on the feed with the lock. Roger Auitman

Dale Gloer wrote:

I've had good luck using a step drill. Like this:

I can't add much to what Bob has posted. I use both methods.

A drill for Plex needs to be 'backed off' in the same way one does for drilling brass. The tip of the cutting edges are stoned back to produce a near ZERO 'rake' angle, The drill then scrapes the surface instead of digging in. This greatly reduces breakage and cracking.

Reamers are FAR less likely to crack the plastic, and leave a better, rounder, hole than a drill. Drill undersize, and then ream out to dimension.

Acrylic plastics (including Plexiglas, a brand name) vary widely in properties. Some are quite soft and drill well, others are quite hard and crack with little provocation. Older stock tends to be harder than fresh. The harder grades, expecially, are prone to stress cracking AFTER the hole is drilled. Such cracks start as invisible microcracks, and later spread to wreck the whole area. Oil in any form makes this microcracking FAR worse. *NEVER* use oil for a cutting lubricant on acryilc. If you need any lube at all, use water.

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

Agreed! The act somehting like a cross between a drill and reamer. Turn them slowly (compared to a drill). They often leave a good hole in plastic. They can also be use with a large pin vise, and turned by hand with considerable succcess.

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

"Plastics, m'boy, plastics."

Plexiglass doesn't shatter that easily - the major problem you're going to have is melting. Use a sharp bit, go slow, and stop to let it cool every now and again. If the plexiglass gets too hot, you'll get ugly white patches around the hole.

Masking tape is a good idea mostly for marking purposes - center punching is a big no-no, and most pencil or ink marks will rub off plexiglass quickly. I'm not sure what the point of drilling through wood is for, though I'm sure someone will tell us exactly why I'm stupid for not knowing that. *

I fabricate a lot of stuff from acrylic plastic. I saw, drill, ream, mill, turn, and tap the material frequently.

Acrylic plastic (or "Plexiglas" ... ONE "s") does indeed shatter readily. Some examples are softer and more resilient than other samples. Older stock is more brittle. A sudden impact, or slow continued stress, will almost always crack it. Any existing cracks (even microcracks) in the plastic serve as "stress risers", and make the breakage far easier.

OTHER plastics, like polycarbonate ("Lexan"), are more crack resistant, but are softer and scratch more easily. However, even polycarbonate cracks *VIOLENTLY* if bent QUICKLY!

The purpose of the wood OVER the plastic is to prevent the plastic from 'climbing' the drill bit. It will try to do this as it breaks through the back side, or if it 'digs in' from too aggressive a cut (or a wrongly sharpened drill). Either process causes a sudden shock to the work, and can shatter the surrounding plastic. Drilling also produced microcracks (or worse) in the surrounding plastic, making the shatter more likely.

Keep all OIL **AWAY** from acrylic when machining it. Oil GREATLY aggravates the stress microcracking, leading to eventual failure of the part. If melting is a problem, use WATER to cool the plastic, and as a lubricant.

Acrylic CAN be drilled with considerable success, but it requires either the correct tools and techniques, or just plain luck.

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

On Mon, 03 Oct 2005 14:35:49 -0400, "Daniel A. Mitchell" purred

I've used acrylic a reasonable amount and never had much trouble drilling it once I learned to be slow and careful (and to use a block of wood to drill through). the one thing I still have problems with is cutting the acrylic. The usual "score and crack" often leads to disaster and saws seem to fare even worse. Is there a good RELIABLE way of cutting various thicknesses of sheet acrylic? I would really like to be able to cut a sheet without the constant fear of shattering or edge cracking.

cat

Figure out how to keep it cool while you cut. I have a table saw with a Kool-Mist sprayer that I use when I cut Plex. Use a saw depth that will just permit the blade to break through the top. Works for me.

When drilling or tapping use peanut oil for lubrication instead of petroleum oils or solvents. Petroleum oils will be absorbed into the material and will cause crazing, cracking and deterioration at a later time. Peanut oil can be washed off with mild hand soap, and does not have any harmful reactions with the acrylic.

---Spray units & coolant:

---MSDS:

I used to center punch plexiglass, then I heard you couldn't center punch it. I kept on center punching it, and still do when I need to drill a hole. What ever the problem with center punching is, I seem to have missed it.

Most of the stuff I've used comes with a sheet of paper stuck to each side. Mark on the paper, cut, drill, etc., then take the paper off when everything else is finished.

Thin sheet can be cut by the score and crack method. About 1/8" is the thickest I've been able to relaibly cut this way, and it does leave a lousy edge.

Sawing is often better, expecially for 1/8" and thicker sheet. Use a good, SHARP, thin, carbide tipped circular saw with MANY teeth (60-80) for best results. The edges may still exhibit some cracking and spalling, but a SHARP blade minimizes this. It is also important to hold the plastic down tight on the saw table (any of the 'roller' hold-downs works well for this). If the plastic bounces up and down during the cut it will chip. Keep the saw set relatively low ... it should only stick up through the plactic about a half inch. This results in a long curved slicing cut rather than a vertical chopping cut ... this reduced chipping.

With any method, if edge finish or exact dimensions are important, I cut oversized, and finish using light cuts on a woodworking 'Jointer' to clean up the edges. I then finish by sanding. Lacking a jointer, you can use a SHARP hand plane to improve the edges. Hold the plane at an angle so it takes a slicing rather than a plowing cut. You can do it all by sanding, but it's more work.

Progress down through several grades of the black Si-Carbide sanding sheets, starting with about 100 grit, and finishing with 600 grit. Place the sandpaper grit-up on a flat surface (a saw table works well), hold the plastic vertically, and rub the plastic (mostly) lengthwise against the abrasive. It can take a while if the edge chipping is deep.

If a clear edge is desired, it can be polished with solvent, fire, or yet finer grades of abrasive.

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

I have to admit, that I've only used sheets of the stuff, the same thickness that's often used for replacing window glass. I've made melting marks, but I've never shattered it when drilling holes. For the most part, it's been pretty forgiving.

Aha! That makes perfect sense. I usually clamp down the piece with wood below, go slow, and use a bit with the least agressive cutting angle I can find. I would be concerned with getting the hole in the right place if I was drilling through wood. It would also be harder to see whether the plastic is melting.

Interesting! Why does oil do that? It gets hotter than water without turning to vapor? *

I've had good luck using a dremel or a scroll saw with a fine blade. I have to agree about the snapping thing - unless you score it about 50 times, it never works, and even then the result isn't very nice. *

You have a more gentle hand with the punch than I do I guess. Generally, it will crack or star the surface. If you're expanding the hole that might not be a big problem.

The paper is good for avoiding scratches too. Window plexiglass will scratch if you look at it funny. *

On Tue, 04 Oct 2005 04:34:27 GMT, Froggy @ thepond..com purred

Be VERY VERY careful when using peanut products in aerosol form. for a goodly number of people those vapors are fatal. Peanuts can throw someone sensitive to them into anaphylactic shock and they will die in minutes if not properly treated. The incident with peanut oil traces that shut down production on CSI last year made it clear that peanuts are a potential risk one is wisest to avoid. You won't find any of that anywhere near my shop.

cat

I think this is NOT well understood. I've heard it has something to do with an effective 'surface tension' in the plastic, and several other theories. But it DOES happen! I've not heard that the oil CAUSES the cracks (they are inevitable, anyway), just makes them *MUCH* worse, especially over time.

All cast acrylic plastic (and most other things too) has internal strains. Anything that releases these strains causes warping or cracking. Machining operations (including drilling) release such strains, as well as make new ones. The tiny microfractures that result just keep spreading, much like a crack in glass. Oil gets into these cracks and increases the rate of propagation.

And I've seen it happen, *BIG* time. The worst I've encountered are tapped holes that were cut using oil. The whole area around the hole just disintegrates over an area several times larger than the hole. Another of the joys caused by engineering students ... :-((

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