On this Page I'm going to run through the do's and Don'ts of glass drilling, in an effort to put your mind at rest regarding this decidedly dodgy (according to some) area of DIY.

Why drill in the first place I hear you ask.? Well simply put, it saves an awful lot of extra plumbing if you can go through a pane of glass rather than around it. This has several benefits in that, it  lessens the work pumps have to do, to negotiate the plumbing in question ( See Plumbing ) and the fact, that the whole set-up becomes neater and more professional in appearance, rather than having loads of pipes sprouting out of the top corners of your tank or sump.

The types of drill bit used for glass drilling are  made from a tube of hardened steel which has had the outer edge and front face coated with a material impregnated with fine industrial diamond. (see Pic 1). Typically these drills cost a small fortune compared to your average drill bit, so you will probably need to order especially through a glass dealer, online, or on ebay. You should expect to pay anywhere between £10-£40, or you can buy a set of the most common sizes for around £50.00 from B&Q. Quality does vary though so beware...Typically a good quality hole cutter should have a very thin wall. The thicker it is, the more work the bit has to do and more heat is generated.

(Pic 1)

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If you are an avid DIY'er, then you might consider getting a set for long-term use as they really are invaluable, especially when considering the cost of transporting several large tanks or sumps to a dedicated dealer over the years, and then the actual cost of drilling the holes themselves. If you are a constant tinkerer, with plans to expand your tank collection over several years then a purchase is worth while, (plus you can always hire them out if you so wish to fellow reefers). If you do decide to buy your own, then I would advise just to get the most common sizes that cover most bulkhead fittings, I.e. 27mm , 45mm  a 55mm and a 65mm. The exact sizes depend largely on what type of plumbing you prefer, be it Marley waste/overflow pipe or the dedicated metric VDL. either way, check the bulkhead drilling sizes prior to the purchase of the drills themselves. The sizes above correspond to the following VDL bulkhead fittings. 25,32,40,50mm

The Second item you will need, is a decent drill with a variable speed trigger, and preferably a 'torque' adjustment as well. The Main reason for these items is that, to get the drill Bit to bite properly into the glass, and to prevent it skidding over the surface on first contact, it is essential to have control over the speed of rotation. The adjustable Torque 'once set to minimum' prevents the drill from catching and possibly shattering the glass panel should it wedge or grab.

Now for the drilling.

The first thing to do is to decide where, and how many holes you are going to drill. IMO its a bad idea to drill too close to the edge of a panel, especially at its lower edges where water pressure is at its highest. To this end I would always advise at least 1.5x the holes diameter between the outer edge of the drilled hole and the edge of the panel itself. Likewise, to retain panel strength, you should leave at least 1.5x the hole diameter  between the outer edges of adjoining holes when drilling several. (see pic 2)

Firstly I should make it clear that you cant 'always' expect mm accuracy when doing it yourself, so I shouldn't try drilling too close to something that may restrict the outer edge of the bulkhead fitting once inserted through the hole. Your best option is to simply draw a circle where you want it, 'the same size as the hole you are drilling' using a permanent marker leaving yourself a bit of room to spare. You can make a template using a piece of thin wood or plastic sheet that you have already drilled a hole through, and tape this to the glass to act as an initial cutting guide.  Oce the bit has worked a nice grrove into the glass and is spinning steady you can remove the template and drill without the guide.. ( See pic 3 ). 

                                       (pic 2)                                                                            (Pic 3)

Once you have the drilling underway, its crucial to be patient. Don't force the drill as this will overheat it and damage it. Likewise you will cause a hot spot in the glass and crack or even worse, shatter it from heat expansion or too much pressure. Slow and steady is the order of the day. On a regular basis, say every minute or so or if it starts to sizzle, the drill should be dunked and spun in a cup of cold water to get rid of the excess glass powder that is generated and to cool the drill bit which will ware excessively and loose its diamond coating if not kept cool. At the same time you can rinse the drilling area to cool the glass, and to check on your progress through the panel. (see pic 4). If at all possible, the best way, is to transfer the tank, or panel to the garden and gently run a hose over the hole constantly whilst drilling. If indoors, adding washing up liquid to the water will make the water froth as the drill cuts. This will prevent water from dribbling everywhere and will hold the glass powder in suspension near the cutting area to make cleaning up easier. Another way is to turn your job so that you are drilling downwards and then make a ring out of plasticine so that it forms a damn and keeps water in the cutting area.

Using a gentle oscillating motion whilst drilling will help progress through a pane, 'I.e. moving the power drill in a very small circular motion'. This also 'slightly' widens the hole, and reduces the chance that the drill bit will bite and grab if the hole is too tight round the bit as it passes through the glass panel. After a period you will hear a gentle splintering sound as the leading edge of the drill breaks through the other side at some point. At this stage you should stop and rinse the drill bit again and clean any glass powder build-up in the grove that you have cut. Then gently start working your way through. 'Important note....Under no circumstances push or tap the drill bit through.....Let the drill do the work right to the end.' Some very minor splintering will nearly always occur, unless you are very lucky, or have amazingly steady hands, but once the bulkhead fitting is in place this wont matter, or be visible anyway. I find it best to gently support the drills chuck by letting it spin in my hand during this last stage (at a low speed I might add ), as it reduces the chance of the drill slipping or grabbing as it comes through the other side. Once through, you can remove the drill and slug, and gently clean up the edges of he hole with a thick cloth some water and emery paper. (See picture 5)......please please, don't run your finger round the inside of the hole afterwards..........you 'will' get a cut.....And under 'no circumstances' should glass powder be ingested, so clean your hands and that cup you were using, and Hoover up round the work area. Whilst some minor splintering is inevitable, ultimately the size and severity of this is dependant on how gentle you are. Minor splinters are insignificant, but large chunks may weaken or cause stress points in base panels, especially on large tanks if the base support isn't 100% square and flat.

                                  (Pic 4)                                                                                (Pic 5)

Hints and Tips.

1. Get a piece of scrap glass to practice on, prior to attempting it on the real thing......A £2 piece of glass is well worth it, considering your first attempt will usually be your worst.......Practice makes perfect and all that. (See pic below)

2. Take your time. and 'always' wear goggles or at least, safety glasses.

3. Drilling tanks that already have water in them is not a good idea, even if you have drained it to below the drilling point, especially considering the risks to your animals if it all goes wrong and you do crack the panel. Although it is do' able and I have done it myself. I don't recommend it..

4. When doing a sump, its worth putting a couple of spare holes at key points should you wish to feed a further piece of equipment at a later date or add another return pump. These extra holes can always be left with a bulkhead fitting attached to a closed flow tap until needed.

5. Most mistakes are through inadequate kit, or simply rushing. Use the right tool for the job and you wont go far wrong.

6. Keep the number and position of holes, sensible in the interests of structural strength, and pressure distribution. A panel that looks like a giant sieve wont be very strong.

It is highly recommended that you always leave at least 1.5x the diameter of a hole between its outer edge and the nearest edge of a panel. Equally you should leave at least 1.5 X the diameter of a hole between its edge and a neighbouring hole.

7. Keep the bit lubricated.....an overheated drill bit, will get damaged and cost a fortune to replace, not to mention the danger of cracking the overheated glass panel.

'Practice, prior to the leap of faith '

Have fun.

Simon.

My sincere thanks go to Danny, the owner of Watford Aquarium, for the loan of the drill so I could complete  this page........Thanks Danny.