Imperial sets are required, 9/64 and 5/32 being the most common. Cobalt bits are the strongest, followed by Coated High Speed Steel bits, and lastly High Speed Steel. Any will do, but better quality can save you a lot of headache.
Countersinks and reverse countersinks will greatly improve the quality of your finish. Invest in a good countersink as they will be used on stainless steel. Reverse countersinks are primarily used on the inside of prosthetic sockets to recess rivet heads.
Forstner bits are used to bore larger holes. The lab will usually have these bits available for use. You can buy some for yourself if you do not want to wait on others.
Twist Drill Bit
Twist drill bits are secured in hand drills or drill presses to create holes for attachment points. They are found in imperial, metric, or letter and number sizes. In Prosthetics and Orthotic applications, an imperial set is required, namely the 9/64 for speedy rivets and 5/32 for copper rivets.
Drill bits come in different compositions and coatings. Generally speaking Cobalt bits are the strongest, able to cut at high speeds and hold their edges longer. Titanium coated High Speed Steel (HSS) will reduce cutting temperatures, increasing the life of the bit but can lose this coating quicker if the hole drilled is not true. HSS is the most affordable and can actually be quite good if bought from a quality source.
Note: Any bit can be broken or dulled as quickly as any other due to inexperience. Drilling through stainless steel provides the greatest challenge, only use fresh bits if using HSS and only apply enough pressure to allow the bit to do the cutting. Avoid stopping half way as this can work harden the steel.
1. The shank is the portion which attaches into the chuck of the drill.
2. The body is the section of the drill bit that contains the flutes.
3. The point is the section that does the cutting.
4. Drill diameter is the size of the drill bit.
5. The flute is the channels that allow for passage and evacuation of chips and waste.
6. The raised area between the flutes is the Land.
7. Web is the center part of the body that joins the two lands.
8. Chisel edge is the line across the point of the drill bit.
a. Point angle – 118° 135° 100° 60° – the angle of intersection between the inclined cutting edges of the point. Should be larger for hard steels, and smaller for successively softer materials. Ie 118 for mild steel or average hardness, 100 for soft metals like aluminium or copper and 60 for hard rubbers or fibres.
b. Lip relief angle is The angle measured from behind the cutting lip to a line perpendicular to the axis of the drill at the heel of the face of the land. The lip relief angle measures the clearance behind the cutting lip. It determines the amount of support provided to the cutting edge. It is either 8/10, 12, and greater. Smaller lip relief angles are better for hard materials such as stainless steel. Average hardness materials would use 12. Angles up to 35 have been used for plastics, however greater angles can weaken the cutting edge.
c. Chisel angle is usually either 120 or 130 degrees, the smaller the angle the harder material. This angle can be found by placing the cutting lips horizontally, the chisel edge should be pointing towards the 1 and 7 o’clock positions
Slower speeds are required for materials that are harder, and the opposite is true for softer ones. Under normal conditions, the recommended values in feet per minute for the speed of a point are :
i. Aluminium 250 fpm
ii. Mild steel 110 fpm
iii. Stainless 30 fpm
To convert fpm to rpm use the following formula.
rpm = (4 x fpm)/ drill diameter
i.e. ¼ inch drill bit drilling into mild steel (110fpm)
To avoid chafing and catching, it is good practice in Prosthetics and Orthotics to countersink screw and rivet heads. A reverse countersink can be purchased from specialty Prosthetic and Orthotic stores to easily countersink the interior of a socket. Alternatively, a makeshift one can be fashioned from a copper rivet .
These countersinks leaves the best finish on materials that has fibres embedded or can melt. It will fray the fiberglass in laminations, however.
These work best on soft metals like Aluminium and Copper as the single large flute will not clog.
This type of countersink reduces chatter and works best on hard materials like Stainless Steel.
Quality fluted countersink cutters sometimes have the flutes (or at least one flute) at an irregular pitch. This variation in pitching reduces the chance of the cutting edges from setting a harmonic action that leaves an undulated surface. This surface ripple is also dependent on the surface speed of the cutting edges, material type, and applied pressure (or feed rate); once started it is hard to remove. Use hard axial force, and slow spindle speed of drill to avoid chattering.
Specialized twist drill that has a modified point for drilling in wood and other materials with fibres (lamination). The spur centers the bit, and the two outside points cut circumferentially leaving a clean hole.
Unitbit/Step Drill Bit
Conical in shape and has a stepped profile, the diameter of each step becoming smaller toward the point. Due to the design, one bit can be use for cutting various sizes of holes. Best used in soft materials, typically used on thin materials due to the stepped profile.
Spade Bit & Forstner Bit
Drill bits used to drill wide diameter holes. Spade or flat bits, excel at high speed boring. Can cause splintering of the fibres around the entry and exit of a hole. A single sharp spur at the centre of the bit bites into the material, guiding the bit.
Forstner bit is identifiable by its sharp cutting lips that shear away the wood from the centre of the hole leaving a flat bottomed recess with smooth sides. Used when accuracy and finish are imperative. Due to their design, Forstner bits are best used on fixed machines such as drill presses and lathes.
Note: due to the excessively large diameters available in both spade and forstner bits, rpm’s must be reduced to achieve best results.
Similar to countersinks in that they allow for screws and bolts to sit flush in the material. It enlarges the portion of the hole in which the head of the bolt sits in by making a straight sided recess. The straight sided hole reduces the chance material will split when applying a lot of torque due to hoop stress. Rarely used in Prosthetics and Orthotics.
Center punching is a technique used to dimple the surface of a material to avoid drifting when drilling a hole. Any tool with a sharp point can be used for this. Larger drill diameters have larger webs, requiring a larger center punch. Center-drills are a nice option in these situations. The material is center punched first, then center-drilled to widen the dent for the drill.