Core drill bits

Credit: JMbestglass.cn

Using core drill bits needs a drill press. It keeps the drill bit steady and avoids breaking the core which plugs the hollow part of the bit.

Oscillating a core diamond drill bit is not the correct procedure. Oscillating the bit creates two undesirable things.

• It breaks off the core that  is being drilled out, plugging the drill bit, and blocking the cooling water being pumped to the drill bit.  This means the bit heats up and loses some of the diamonds. Additionally, it can heat up the glass so much that it breaks. If you are not using a flushing head with your drill, you will need to raise the bit a little from time to time, allowing water to the grinding surface. 
• Starting at an angle or oscillating with a core bit wears out the sides of the drill bit more quickly than necessary. Core drill bits need to be applied directly and vertically. This is why core bits do best in a drill press. It holds the bit in a vertical position without breaking the core being drilled out, or prematurely using the diamonds higher up the bit.

Credit:  Lawson-HIS

There are generic drill presses available for holding Dremel-type craft motors and hand-held drills. They are inexpensive and make the drilling process so much more certain to regulate the pressure. It also makes an easier start without skipping over the glass. They are so inexpensive that a few holes without skipping will pay it.

Credit: Bhole ST1542 Pico Dril

Drill speeds should be varied according to the size of the hole being drilled. This is important with the high speed Dremel-type motors.  Larger holes need a slower speed than smaller ones. The rim speed of a small bit is nearer the rpm of the drill than a larger one, because the larger one travels a greater distance per revolution than a small one. A listing of recommended speeds is given in this blog.

Hollow core diamond bits are of two types:

•     One, where a heating process attaches the diamond, is called sintered in Europe and other countries.
•     The second, where the diamond is bound with resins, is called bonded in Europe.

They seem to have different designations in North America.

Bits of the first type are longer lasting, and more expensive. These can be “sharpened” with an aluminium oxide dressing stick to expose new diamonds and maintain their effectiveness.

Credit: W W Grainger.com

Bits of the second type wear quickly and should not be “sharpened” with a dressing stone. The normal wearing away of the bonding material exposes the new diamonds.  Dressing them wears away the diamonds that could be used in drilling.

Another advantage to core bits, is that a core drill grinds out much less glass from the hole than a solid drill bit, so it takes less time to drill a hole.

One disadvantage, especially on core drills of 5mm and less, is that the core needs frequent cleaning out of the cores that get stuck inside the drill bit. To maintain efficient and effective drilling, the core needs to be poked out from the bit from the base toward the drilling surface.  This applies whether water is being pumped through the core or not.  Without clearing the core, more pressure must be used to continue drilling, resulting in larger break outs as the hole is completed, and more breaks of the complete piece.

Firing cremains to avoid bubbles

Firing with cremation remains is very similar to firing with any organic material encapsulated into glass.

Design

There are several possible design approaches.

Drilling holes is one method to avoid bubbles.  You can drill the base, put the remains on top and then cap.  Place the whole assembly on 1mm fibre paper to allow the air to migrate out through the hole and fibre paper under the glass.

Alternatively, you fire upside down and then fire polish the top.  Place the eventual top down onto the kiln washed shelf or Thinfire. Place the remains on the glass and cap with the glass that has the hole drilled.  Fire, then clean, turn over and fire polish the final top surface.

Design the piece and placing so there is a gap at the edge. 

This gives a route for air to escape.  If there is any gap left after fusing, it can be filled with a bit of super glue or other clear glue. 

Another method is to place pieces of frit or stringer at the very edge of the base glass to allow air out from under the centre of the piece.

If you do not need to concentrate the cremains in one area, you can disperse the material evenly across the piece to reduce the possibility of large bubbles.  The air and gasses can migrate to the edge through the particles, just as happens with powder sprinkled between layers of glass.

You can combine some of these methods as they are not mutually exclusive.

Firing

Fusing these pieces is, in principle, the same as encapsulating any organic material within the glass.  Slow advances are required with a 3 to 4-hour soak at around 600°C to burn out any residual organic material just as you might for thick vegetable matter.  You can add another bubble squeeze soak of an hour or so at around 650°C to gradually push any remaining air out from between the particles.  Then advance to the fusing temperature and anneal as usual.

Centering Holes for Drilling

When using larger core drills, it is not possible to see the centre point for drilling. So a different arrangement for marking the place to be drilled is required.  This example is to locate the hole for a clock spindle accurately between the points marked by the stringers.


Find the centre point and extend lines at right angles to each other across the centre point.



Then measure the radius on each arm and make a perpendicular mark on each of the radial arms. Depending on how long those lines were, you have something approaching a box.

Approximately center the drill over the center point. In this case it is a portable drill, but the principle is the same for a pillar drill.

Lower the drill bit over the hole. The radius marks allow the drill operator to see the edge of the hole and use any three of the marks to centre the drill bit within the hole.


This procedure ensure the accurate positioning of the hole.  This is especially important when fitting to existing fixing points rather than making new ones.

Other posts on drilling are here.

Reducing a Bubble

A query about reducing a bubble appeared on the internet recently.  The bubble was from between the shelf and the single layer glass.  It was a relatively shallow dome that did not seem to have thinned the glass much.

There is quite a bit of information on reducing the incidence of bubbles. Among them are my blog posts on the subject.

Eliminating bubbles 
Preventing Bubbles  

Avoidance   

My view is that large thin bubbles cannot be repaired successfully.  As the bubble forms and grows, it pushes a proportion of glass to the side.  This thickens the glass at the edge of the bubble.  Bursting the bubble and filling it with something (e.g., a piece of glass, or frit) leaves marks at the thickened edge of the bubble, so it remains a mark in the finished piece.

I recommend two ways to deal with shallow bubbles.

Method 1

Glass with a low uprising between the shelf and the glass can be successfully repaired, if the uprising is low and the glass has not thinned. In the case mentioned, the risk in simply re-firing right side up is that the bubble will increase in size. The weight of the glass may not be sufficient to pull it down except at higher temperatures – which is where the risk of increasing the size of the bubble occurs.

Instead, flip the piece over. Allow the weight of the glass to flatten the uprising. You can use a much lower temperature to flatten the glass by taking advantage of the weight of the whole piece.  This lower temperature means that you will not mark the surface so much as at higher temperatures. Don’t worry if the uprising is not central, you do not have to balance the glass on the point of the bubble for this process to work.

Take the piece to 650°C for as long as it takes to flatten. The rate of advance should be slow – not more than 100°C per hour.  This steady, slow input of temperature will allow the glass to relax at lower temperatures than rapid increases. 

You should use the smoothest separator surface that you can – Thinfire or Papyros, or a smoothed kiln wash.  This together with a low temperature will give minimum markings. 

You must observe the process from about 600°C to be able to stop the slump when the piece is flat and advance to the annealing segment of the firing.

Method 2

This post gives a further alternative. Use two shelves to compress the uprising flat. Although the post is talking about thinning a pot melt, the principle is the same.  Place fibre paper around the edge equal to the thickness of the glass piece and place another prepared kiln shelf on top. You do not need to invert the surface of the piece to do this.  It may be that you will later need a fire polish to remove any marks on top.

A plea

Do not drill holes. Especially not in the case of a shallow bubble.  The glass has not significantly thinned and so can be rescued.  Drilling a hole will only leave an unwanted mark.

Diamonds and Water Use

When drilling glass with diamonds, water has three uses.

• It cools the glass.  The action of grinding away the glass surface creates heat.  If this is not dissipated, the glass will break from the heat differentials caused by the drilling.
• Water helps to lubricate and clear the grinding dust from between the diamonds on the drill bit.
• Water keeps the glass dust that would otherwise be circulated in the air contained and easy to clean. 

Ground glass does not cause silicosis.  This is from a leading industrial safety expert:

"It is important to understand the difference between glass and crystalline silica because exposure outcomes are extremely different!  Glass is a silicate containing various other ingredients which have been melted and upon cooling form an amorphous, or non-crystalline structure.  While silica (SiO2) is a primary ingredient in the manufacturing of glass, when glass is formed under heat, the crystalline structure is changed to an amorphous structure and is no longer considered crystalline.  Ground glass is rarely respirable because the particle is too big.

Always use wet methods when grinding glass! Water captures the dust."

Source: http://www.gregorieglass.com/Health_Safety_Chemical.html

Clearing Small Core Drill Bits

Core drill bits are very useful, as only a thin wall of glass is removed, leaving the main part of the hole as a single piece. This means that it is quicker than a solid drill bit through not having to remove so much glass. However, small diameter core drills often jam up with waste glass. This is evident when the drill ceases to drill through the glass, leaving a broad circular mark as it moves around on the glass.

Inevitably, at one time or another, the glass core will get stuck inside the drill bit. This needs to be cleared before any further drilling can be successfully done. It seems to be more common with the smaller diameter drills – from 5mm down.

The broken off core needs to be cleared from the back of the bit, not the diamond coated end. So you need to take the bit out of the drill and use a thin nail or piece of wire into the hole to push the glass out.

My collection of core drills.  At the front you can see copper wire and a thin punch for clearing blocked drills

Core drills need a more accurate means of steadying the bit than your hands. Imagine that you are trying to get that core out of the glass in tact. Any wobbling as you drill will break that core. When the core breaks, it is inclined to jam the cylinder. So a drill press is almost essential when using a core drill.

They also need water like any other drilling of glass. Normally this is supplied through the hollow core of the bit, which is beyond hand held drill motors.

For something as smaller than 3mm, a solid drill that removes all the glass rather than a core is more likely to be most successful. You can get diamonds bonded to piano wire that will give you holes from a fraction of a mm up to 2 or 3 mm and these can be used in dremel-like tools that take small diameter bits.

It is absolutely NOT recommended to use a core drill at an acute angle to start the hole. The wall of a 3mm core drill is very thin and easily damaged. It is ok to start a solid drill that way, but not a core drill.

There are a number of related posts beginning with this one which give advice on the many ways to drill holes in glass.

Re-Firing Holes

Occasionally you need to re-fire a piece that already has holes drilled into it. The smaller holes tend to close up or reduce in diameter when they are larger. There is a method to resist this and still have a neat smooth hole.

To keep the holes open during a re-firing, cut a strip of Thinfire a little thinner than the thickness of the glass to be fired, roll it up tightly and put it into the hole to be kept open. Starting the wrapping around a pencil or pen makes the start easy and the roll can be tightened by holding the centre and pulling the end.

Put the roll into the hole and allow it to expand to fill the hole. It does not have to be solid. If the roll is as high or higher than the surrounding glass there is a tendency to get spikes.

This works fine on 6mm and thicker glass, but I have never tried it on 3mm glass. No reason why it shouldn't work though in my estimation if you can cut and manipulate 2mm strips of fibre paper.

Vertical Kiln Formed Holes

For vertical holes in frit-cast reliefs you can fill a drinking straw with plaster, cut while still wet and build the frit or cullet around it.

Already fused pieces can have the holes made much neater and smoother by using the above method in pre drilled holes. Another method is to wrap a thin strip of fibre paper around a pencil or end of a paint brush. Then push this circle of fibre paper into the hole. If this is the same height or a little less than the glass, it provides a clean fire polished hole, if the glass is taken to the high end of fire polishing temperatures.

Drilling Glass, 2

Using a Drill Press

It is best to have a drill press if you are doing a lot of drilling. It provides a stable drilling action and the pressure on the bit can be controlled.  It is important to ensure the bit is running true without wobble. The drill press should have instructions to help correct any untrue running of the chuck.  Make sure the drill bit is secured firmly.  Core drill bits are easier to keep true, as they normally have a threaded fixing.

With a drill press, you can drill continually until the hole is completed, or until a white paste or dust begins to appear. This indicates the drilling is being done dry and will in a few moments heat up the glass too much. When the white paste appears, back out of the bottom of the hole a little to allow water to flush the glass out. Then continue.

Keep a firm grip on the glass being drilled. Maintain its position, especially if you are intending to back off intermittently to allow water to the bottom of the hole. This enables you to get back into the hole without scratches.

If possible, submerge the piece. But if that isn’t possible, just squeeze a little puddle of water on the surface and watch it swirl around. You can see if it is pulling ground glass out of the hole by watching the circulation. Placing a plasticine or clay dam around the drill area will keep the water confined.

Don't push down any harder than you comfortably can with the tips of your fingers. Keep it steady. Listen for the sound of diamond grinding glass

If the core gets stuck in the bit, knock it out with some stiff wire or a nail. Always remove each core right after drilling. They are very difficult to remove if there is more than one in there.

Every diameter drill bit has an optimum drill speed. The smaller they are the faster the speed required. Based on what the manufacturers recommend, a Dremel running at top speed is way too fast. When using a Dremel for drilling glass, slow it down with the speed control. Drill presses do tend to be on the slow side for glass drilling so it takes a bit longer, but there are big advantages in other respects.

For other tips on glass drilling see:

Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling speeds for diamond bits in glass

Drilling Glass 1

Keeping Things Wet

It is important always to keep the drill bit and glass wet, otherwise the glass gets too hot and will break. There are a variety of things you can do to achieve this.

• Drill under water in a container
• Drill in a ring of clay, plasticine, etc., holding water. To do this, you need to make a ring about 50mm / two inches in diameter and press it around the drill site. Fill the ring with water and a little diamond coolant. This will cool the drill site and glass. Diamond coolant is not necessary, but extends the life of the bits.
• Use a recirculating water pump such as those made for indoor water features. Direct the small flow of water to the drilling site and catch the water in the bucket in which the pump is submerged.
• Use a glass drill with hollow core bits and an internal water feed. This is the most expensive but it is the best equipment with which to drill large holes.

For other tips on glass drilling see:
Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling speeds for diamond bits in glass

Drilling Glass, 3

Drilling with a Flushing Head

A flushing head with a re-circulating pump will deliver water to the drill site through the core of the drill. These are supplied complete or as a fitting for an existing drill press. This is suitable for holes of 4mm and larger. Smaller core drills are impractical both because the glass is easily trapped in the drill and the wall thickness of the drill makes them almost solid anyway.

An additional requirement is to have a means to direct the water back to the reservoir.

Every diameter drill bit has an optimum drill speed. The smaller they are the faster the speed. Drill presses do tend to be on the slow side for glass drilling, but often have ways of altering the speed. So it takes a bit longer, but there are big advantages in other respects.

Don't push down any harder than you comfortably can with the tips of your fingers. Keep it steady. Listen for the sound of diamond grinding glass

If the core gets stuck in the bit, knock it out with some stiff wire or a nail. Be careful not to damage the edges of the drill bit. Always remove each core right after drilling. They are difficult to remove if there is more than one in the core of the bit.

For other tips on glass drilling see:


Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling speeds for diamond bits in glass

Drilling Glass, 4

Avoiding Chipping

There are a number of methods to avoid chipping out the back of the glass when drilling:

• Placing a piece of scrap glass under your good glass will help avoid chipping on the backside. By pressing firmly but gently on the glass (not the bit) the bit will go through the upper piece of glass without chipping the back. This can be a difficult process to keep stable when both the pieces of glass are wet.
• Another method is to put duct tape under the glass to help minimise chip out.
  Drill from both sides to avoid chip out as the bit breaks through. Go slowly toward the bottom of the hole. When the hole is almost through, turn the glass over and drill back to front.
• Sometimes the glass is curved and drilling from the back is not easy. This is when the drill press mechanism to stop the bit sliding comes into its own. Before switching the drill on, lower it to the surface supporting the glass. You can adjust the mechanism to stop the press just as it reaches the support surface. Then place the glass under the press and the turn the drill on to begin the drilling.
• Don't push hard as you come to the end. Don't push down any harder on the drill press levers than you comfortably can with the tips of your fingers throughout the process. Keep it steady. Listen for the sound of diamond grinding glass.

All these things will help to avoid chipping out the glass at the bottom of the hole.


Keeping things wet

Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling speeds for diamond bits

Drilling Glass, 5

Drilling holes with copper tube and grit

You can drill holes by using loose grit and a copper tube of the correct diameter. It can take quite a while. You will need to have a chuck big enough to take the tube, or have a means to reduce the tube diameter to the chuck size. Alternatively, use core drills that have had the diamonds worn away.

Prepare the glass as for a drill press without a flushing head, so the water and grit are confined. The dam can be putty, plasticine, clay, or other mouldable material put around the area to be drilled.

The grit can be sandblast grit or other abrasive of about 100 to 200 grit.

Drill as normal.

If the core gets stuck in the bit, knock it out with some stiff wire or a nail. Always remove each core right after drilling. They are very difficult to remove if there is more than one in the core of the bit.

For other tips on glass drilling see:
Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling speeds for diamond bits in glass

Drilling Glass 6

There are a variety of tools that can be used to power glass drills.

Dremmel and similar craft motorsThese are light duty high-speed drills. Those with variable speed controls are especially useful. They work best for small diameter holes. They must have the speed turned down for drilling, especially for larger holes.

These can be combined with a flexible drive shaft for lighter weight.

Drill pressHowever, the most important thing to have when drilling glass is a drill press. Doing it by hand is very difficult and wears out diamond bits very fast. Dremmel and others make drill presses for their tools.

Drilling machinesPurpose made glass-drilling machines are important for larger holes and production work. Flushing head adapters are available from suppliers that will convert a standard drill press into a glass-drilling machine. These tend to be much slower than the Dremmel style motors, but are very steady. The important thing about these is that they use hollow core drill bits, allowing the water to be fed through the drill bit directly to the glass-drilling site.

Drill bits
The other tool needed is drill bits. The recommended type depends on the size of hole to be drilled.

Small diameter holes, up to and including 3mm require solid bits. These can be spade (unusual) or solid diamond-tipped bits. A number of manufacturers make solid drill bits from 2-6mm and some (especially lapidary suppliers) make the very small diameter bits from less than 1mm to 2mm.

Larger diameter holes are best drilled with hollow core bits, as less glass needs to be removed to achieve the hole. These can be used with a flushing head or simply by directing water to the drill bit, with a dam to hold the water around the site.

The bits will last longer if you use a drill press. The press keeps the bit wobble to a minimum and maintains the vertical, both helping to reduce the wear on the bit.

Hollow core bits
Hollow core diamond bits are of two types:

One -where a heating process attaches the diamond - is called sintered in Europe and a number of other countries.

The second – where the diamond is bound to the metal with resins and other chemical attractions – is called bonded in Europe.

Bits of the first type are longer lasting and more expensive. These can be dressed with an aluminium oxide dressing stick to maintain their effectiveness.

Bits of the second type wear quickly and should not be dressed.

In general a diamond core drill breaks out much less glass at the bottom of the hole than a solid drill bit.

Water pumpA further tool that is useful to have is a re-circulating pump. This can be a small fountain pump with a flexible spout to aim the water on the drilling site. A foot switch can control the water flow. A large tub is required to act as the catch basin for the water that comes off the drill and as the reservoir for the pump.


For other tips on glass drilling see:


Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling Speeds for diamond bits

Drilling Glass, 7

Drilling glass without a drill press

It is best to have a drill press for drilling holes in glass, but there are ways of doing it with a hand drill.

Make a ring of modeling clay, plasticine, putty or other mouldable material about 5cm/2" in diameter and press it around the drill site. Fill the ring with water to cool the drill site and surrounding glass as well lubricate the drill bit.  Buying sinered (more expensive) bits is worthwhile as they work much better than the cheaper resin bonded ones.

Use a paint pen to mark the spot where the hole is to be. Without a drill press, starting at an angle with a slow drill speed will stop the bit from sliding around as you establish the drilling point. As the glass surface is roughened, bring the drill to vertical. Move the drill up and down a little as you drill to allow the water into the hole. If you are using a solid or spade drill, a little oscillation keeps the bit from jamming in the hole. Do not do this with a core drill.

A Dremel running at top speed is way too fast. Slow it down with the speed control.
Every diameter drill bit has an optimum drill speed. The smaller the bit is, the faster the speed required.


For other tips on glass drilling see:


Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling speeds for diamond bits in glass

Drilling glass, 8

Hole Placement

The general rule on drilling holes in glass is that the edge of the hole should be further away from the edge than the thickness of the glass. This means that the edge of the hole on a 6mm thick piece of glass must be more than 6mm from the edge of the glass.

The calculations are simple arithmetic. 

• calculate the centre point of the hole by adding the radius of the hole to the thickness of the glass plus at least 1mm. 
• For example, to drill a 10mm hole in 6mm glass, you add 5mm (radius of hole) to 6mm (thickness of the glass) plus 1mm = 12mm as the minimum distance from the edge of the glass to the centre of the hole.

Remember this is the minimum distance. For safety and durability in architectural or heavy circumstances, an additional margin must be added.


For other tips on glass drilling see:
Centering the drill bit
Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling speeds for diamond bits in glass

Drilling Glass, 9

Drilling speeds for diamond bits in glass

Diameter 3-4mm
Speed 6000 rpm

Diameter 5-8mm
Speed 4500 rpm

Diameter 9-12mm
Speed 3000 rpm

Diameter 13-16mm
Speed 2500 rpm

Diameter 17-25mm
Speed 2000 rpm

Diameter 26-28mm
Speed 1800 rpm

Diameter 29-44mm
Speed 1500 rpm

Diameter 45-64mm
Speed 1200 rpm

Diameter 65-89mm
Speed 900 rpm

Diameter 90-120mm
Speed 800 rpm

[Based on CR Lawrence and Amazing Glazing recommendations]


For other tips on glass drilling see:

Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling speeds for diamond bits in glass

Dams for Drilling

For those who find putty or plasticene too insecure, it is possible to use a plastic bottle with the bottom and top cut off, and secured with hot glue. This will provide a non-slip dam for the water while drilling.