Spade drills used for heavy duty hole creation are very different to normal twist drills. They comprise of two parts and deliver several advantages for in-situ machinists and for manufacturers carrying out repeat production runs.
The main advantage is that a spade drill includes a replaceable cutting blade. This is secured in a fluted holder which can be conveniently replaced when it reaches the end of its usable life. There are many other advantages too, but also a few drawbacks to be aware of.
If you use portable drilling machines and want to find out if a spade drill is right for your type of machining projects, we’ve listed the key benefits and a some of the limitations below;
Advantages of using a spade drill
- The two-part design allows the use of a wide range of inserts, suitable for drilling many different materials.
- For a longer tool life, inserts are typically made of powder high speed steel or carbide, combined with various coatings.
- Replaceable inserts eliminate the need to regrind large drills and can be replaced while the tool holder is still within the machine tool.
- Reduced tooling costs. One holder will hold several different inserts of different sizes.
- The tool holder is not carrying out the cutting, so can be made from less expensive steel, helping to keep costs down.
- Holders are available with taper & straight shanks.
- To aid the ejection of cutting chips, holders are available with either helical and straight flutes.
- Inserts include an integral chip-breaker which improves drilling stability, making it possible to drill in one-pass.
- The insert design includes ‘chip breaker grooves’, for directing the small, narrow metal chips out of the hole.
- Holders are available with the option of ‘through coolant’, delivering it to the cutting edge of the drill through the body of the tool itself.
- Although recommended cutting speeds are typically 20% lower than twist drills, feed rates can twice as fast.
- The way in which the tip of the insert is formed allows thinner ‘core drilling’, reducing axial resistance and improving self-centering.
- The spade drill’s stability makes it possible to drill holes up to 6 diameters with an unpiloted tool.
- Despite the advantage of delivering faster feed rates, the surface finish of the hole can suffer because of this.
- Spade drills are not normally available in small sizes: (Diameters of 0.75” or more).
- For ad-hoc applications requiring drilling of only one or two holes, a twist drill is likely to be a more an economical option.
- If coolant is unavailable, a twist drill might be the most reliable choice when drilling difficult holes in hard-to-machine materials.
- The twist drill’s forgiving nature also can provide added security and piece of mind for applications where a broken tool could damage an expensive part.
Spade drills work best at moderate speeds and with a heavy feed. Feeding too lightly may produce either long stringy chips or chips reduces to almost powder. Chip formation is very important because if heat absorbing ‘c-shaped chips’ aren’t produced there is a danger of smaller chips jamming and packing, which may break either the tool or the workpiece. Using a drilling machine that is powerful enough to maintain the required feed rate will help to prevent this from happening.
You can find out about the Mirage range of high torque portable drills here
George Schneider, Jr. CMfgE, 2002, Cutting Tool Applications.
Karnash Tools: http://www.karnasch.tools/en/spade-drills/