The options are limitless when it comes to finding what will work best for you.
By LaTisha Shipman
Down-the-hole bits are so versatile, especially when working with a flexible drill bit manufacturer. You can truly customize your bits specific to your formation and other drilling conditions, which makes this method somewhat unique as opposed to others.
The first thing to consider when selecting your bit is the formation. Formation dictates what type of bit face and carbide we will need.
The DTH drilling method itself is best suited for medium to hard rock drilling, and not all hard rock is the same. Some formations are harder than others and some formations are more abrasive than others due to the silica content.
These are the types of conditions we’re going to look at here as we can customize the bits to get the most bit life and increase performance.
Choosing a Face Design
Let’s consider the face design first. There are several different types of face designs we can choose from: Flat Face, Concave, Convex, Convex-Concave, and Drop Center.
Flat Face bits perform well in all rock types, but especially so in hard formations. You can, though, experience some hole deviation in softer formations with this face type. Overall, the penetration rate is good.
This design might be difficult to find in stock. You may need to custom order it, which typically means you will need to place a minimum order quantity and wait for delivery. Plan ahead with this one.
Concave is probably the most common face type and the most readily available in stock with almost all manufacturers. This design is good in medium to hard formations and allows for a nice, straight hole and the penetration rate is fair.
This is what we consider to be the standard in bit design as it works nicely in almost all formations with a few tweaks for harder formations, such as adding wear pads or premium buttons.
Convex bits are best suited for hard and abrasive formations. The reason being is this design incorporates two different gage row angles leading up to the center of the bit which protrudes and is the first part of the bit to impact the rock. The second gage row angle helps to soften the blow on the outer gage, protecting it from excessive wear.
This is an excellent design to use if you are experiencing steel wash on the gage. The CV face design works best in medium to hard formations with average hole straightness but excellent penetration rates. This bit also might need to be custom ordered.
The newest face design is a Convex-Concave which incorporates the best of both worlds, utilizing the strengths of both the convex and concave faces. The deeper dish in the center of the bit, also called a CV-CC, allows for better hole straightness than what the convex face alone provides.
This design also incorporates three distinctly different gage row and face angles. The CV-CC works well in medium to hard formations with improved hole straightness and excellent penetration rates.
This design is becoming increasingly popular and many of the top manufacturers will keep this type of bit on the shelf although they might have their own name for this design.
The Drop Center face is hard to find in stock, is becoming somewhat outdated, but still has some good properties to consider. The deeper dish in the center provides for good hole straightness, and the bit performs well in sloping or broken formations with an average penetration rate.
It will most likely need to be custom ordered, so you might have to wait longer for it and then also would need to meet a minimum order quantity which differs between manufacturers. Not all manufacturers offer this design.
Air and Bit Hardness
Next, let’s consider the flushing holes. One of the most important aspects of bit design is the ability to flush the hole clean. Of course, this is only possible when you have an adequate air supply.
If you are on the higher end of the air supply, consider a three-hole flushing, as this can help get those cuttings moving quickly from the bottom of the hole.
I like to see flushing holes that are deep and wide, especially in high air pressure applications. If the cuttings are not quickly and efficiently removed, the drill bit will regrind cuttings, which leads to premature wear or failure as well as slower penetration rates.
Another common bit enhancement we can make is to adjust the hardness of the bit itself. There are several different types of heat treatment options available, which may vary between manufacturers, but some of them include induction hardening and carburization. These different types of heat treatment will harden the bit at different depths, thus providing longer life of the drill bit steel.
Carbide shape, grade, and size are other options we have in DTH drill bit design that makes this drilling method unique. The options are truly limitless.
There are several different shapes and hardness available on the market. The most common shapes are dome/spherical and parabolic.
Dome buttons are generally the most common and suitable for hard and abrasive formations. Parabolic inserts are good for soft-medium formations and not abrasive. There are multi-point, chisel tooth, conical, and ballistic buttons available with some manufacturers. Carbide also comes available in different grades of hardness.
There is a standard grade, which may differ between each manufacturer, but each manufacturer will usually always have an upgrade available to a harder or premium type button which should perform better in harder and more abrasive formations. Then there are diamond inserts for the hardest of formations, typically used in granite.
The premium buttons also come at a premium price. You can expect to pay approximately 10% more for your drill bit with premium buttons and even more for diamond.
However, you can often mix and match the harder buttons with the softer buttons such as inserting a harder gage row if you are experiencing rapid gage wear, while utilizing the standard or softer buttons on the face of the bit.
You wouldn’t want to use this combination in the convex face, though, as the center of the bit is what impacts the rock first. In this instance, you will want to request a harder face button and softer gage row button.
Being able to choose where you need to put the premium buttons when not needed on the whole bit face helps to keep the costs down. It is also important to note that not all premium buttons can be sharpened, so you’ll need to check with the manufacturer about that if you do sharpen your bits.
You have the option to mix shapes as well, such as using dome buttons on the gage row, and parabolic on the face.
We should also consider button size when determining the best bit for your application. In some applications it can be beneficial to use a larger gage row button. However, on smaller bits, this can cause an increase in torque. Discuss the pros and cons of this with your distributor or bit manufacturer.
There are several bit enhancements which can also improve drilling performance.
If you are in a very hard, very abrasive formation, consider premium carbide inserts on the gage, as well as gage protection or wear pads. The wear pads help protect and reinforce the gage row and can help the gage last longer.
Wear pads can be a flat button, or they can also be a regular dome button. Wear pads are inserted on the wing of the bit, directly above the gage row.
If you are drilling in overburden, I recommend using a casing advancement system, but that’s for another day!
You can also request backreaming or backout buttons if using a standard DTH bit. These buttons are inserted on the back side of the head and aid in drilling back out of the hole in instances where the hole might be falling in on you.
A Case Study
I recently had a customer in a difficult formation who had alternating layers of hard shale and a softer sandstone. This makes drilling with DTH difficult, but it’s necessary because of the hard shale they were in.
As the driller was transitioning between the two formations, the gage was encountering the hard shale while the face of the bit was still in the softer sandstone, which resulted in most of the impact and torque transferring solely to the gage row, rather than sharing the load with the entire face. As a result, he was wiping out the gage row buttons when coming out of the soft formations and moving into the hard formation.
When designing a custom bit, you can expect some trial and error—and a lot of questions! This takes time, effort, and teamwork on the part of the driller, the distributor, and the manufacturer.
I had a good idea of what would work for this customer. We first tried adding wear pads and premium buttons. Next, I went over his drilling parameters with him and noticed the rotation speed was a little slower than what is recommended. I had him slightly increase to 20 RPM. There was some improvement, but I felt like we could do better.
We worked together with the manufacturer to change the angle on the gage row as well as increase the gage width. We added a larger button on the gage, going with ¾-inch buttons on the gage and ⅝-inch on the face—and the customer had really good success with this bit.
The driller was able to get five times the bit life from the custom design as opposed to the standard bit they were buying off the shelf. I appreciated his patience and willingness to try new designs and recommendations. He now has a custom bit design for this one particularly difficult area he often works in.
LaTisha Shipman is the Texas branch manager for Drilling Equipment Resources. She has more than 20 years of experience in the drilling industry, with most of that time spent working in manufacturing with DTH hammers and bits. She can be reached at firstname.lastname@example.org.