Drilling and Tapping

Examples of Drill Bits and Taps

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Description

Tapping is the process of creating threads on the inner surface of a drilled hole. A variety of taps are available in the shop to match almost any screw type available, including metric and standard measurements. Information required to select the drill bit includes thread count, diameter, thread pitch, and coarse/fine threads.

Good Practices

Using Tap Guides

The tap guides, located in the same drawer as the taps, are crucial to creating a straight and usable tap. When tapping on a machine such as the Mill or Lathe, the tap is automatically centered and straight. Be wary of tap alignment when doing it manually, as the human eye isn't as accurate as a perfectly 90 degree tap guide.

Using Oil

Oil is a necessity while drilling and tapping. It prevents overheating, helps clean out the chips created by the bits, assists the cutting action, and prevents the bits from squealing.

Center Drills

Creating a drilled and tapped hole first requires using a center drill. This gives the larger bits a center to align to, since most drill bits aren't perfectly centered when mounted in a drill chuck. This ensures the larger bits will drill in exactly the right place and won't walk across the part.

Pecking

While either drilling or tapping, pecking helps ensure the bits won't overheat or break. Pecking is the process of drilling into the part a way, then backing out to remove the chips and allow the part to cool. Common practice is to rotate the handle being used about a full turn, then back a half turn. Every time the bit/tap is pulled out, as many chips as possible should be removed and oil should be added to the surface and the bit/tap.

Tapping Procedure

Steps to creating a tapped hole:

Example	Description
Tap/Drill Chart located behind the Mills in the shop	1. Select drill size from chart. This is the first place to look when selecting a tap size. Usually the desired screw size is known, so the drill bit can be selected directly from this chart.
Cutting Speeds Chart located outside of Russ' office	2. Check cutting speeds for drill bit. For center drills, a cutting speed of 1000 RPM is usually selected. However, under certain circumstances, other speeds may be necessary. For the normal drilling, speeds are selected based on material and diameter. (Note that the CNC machines have maximum speeds that may not reach the recommended values.)
Center Drill	3. Make a center hole. Set up the center drill bit in the drill chuck. Using oil on the tip of the bit and on the location to be drilled, slowly lower the bit and cut between 1/16th inch and 1/8th inch, enough to seat a larger bit's tapered end.
Drill Bits	4. Drill hole. Load the required drill bit in the chuck. The part should not be moved during this step, so the bit will already be centered. Apply oil to the bit and surface. Lower the bit until it comes in contact with the part and begin peck drilling, adding oil at every pass. Drill to the desired depth. If the hole is a through hole, be aware of the depth of your part. If it is a blind hole, use a previously defined zero as a reference.
Chamfers and Countersinks	5. Add chamfer. Additional features sometimes desired for screws are chamfers and countersinks. Spindle speed for best results should be between 150 and 250 rpm.
Metric and English Tap Guides	6. Get a tap guide. The final step is to manually tap the hole. This is done using the taps and guide blocks available in the tool chest near the manual mills. The guide blocks have several holes for different sized taps. Select the one closest to the size tap being used, and place it over the drilled hole.
Hand Tap Wrench	7. Tap the block. Using the tap wrenches, peck tap by applying a gentle pressure while turning the wrench a full turn in, then a half turn out, and repeating. Once the tap is sufficiently seated, disconnect the wrench from the tap, remove the guide from the tap, replace the wrench, and continue peck tapping to depth.
Tapped Block	8. Complete the tap. Don't keep applying pressure if the tap refuses to go any further; it has likely reached the bottom of the hole. Any pressure applied at this point is likely to break the tap. The smaller the tap, the more likely it is to break while in use, so exercise caution in speed and pressure applied.

Alternative Tapping Methods

There are alternative ways to tapping other than by hand. It's possible to tap with the aid of the mill, lathe, and CNC. All the steps before tapping the hole are repeated before these procedures:

Using the Mill:

Example	Description
Spring Loaded Center	6. Use spring loaded center. Place spring loaded center in the chuck. This will center the tap and apply the downward force.
Hand Tap Wrench	7. Secure tap in hand tapping wrench.
Mill Tapping	8. Put the hand tapping wrench between the part and the spring loaded center.
Tapped Block	9. Tap the block. Use a rotational force, no downward force. Use the tap pecking method and plenty of oil. Once the tap is resisting and is about at the bottom of the hole, stop. The tap is complete!

Using the Lathe:

Example	Description
Spring Loaded Center	6. Use spring loaded center. Place spring loaded center in the tailstock. This will center the tap and apply the axial force.
Hand Tap Wrench	7. Secure desired tap in hand tapping wrench.
Lathe Tapping	8. Put the hand tapping wrench between the part and the spring loaded center.
Tapped Block	9. Tap Lathe piece. Use a rotational force, no axial force. Use the tap pecking method and plenty of oil. Once the tap is resisting and is about at the bottom of the hole, stop. The tap is complete!

Using the CNC:

Example	Description
Tapping the Hole	Tapping on the CNC mill is fairly simple. The procedure is the same as above, but instead of tapping manually after the hole is drilled, the tap is loaded into one of the CNC mill's chucks. The program the CNC uses, MastercamX7, will take the spindle speed input by the user and calculate the exact speed it needs to lower the spindle in order to tap the part.