Servo - May 2009

for hand tapping.

Recommendation

Uncoated taps work perfectly fine in softer materials; if that is all you work with, then save some money. If there is some chance you will need to tap harder or abrasive materials, then the fairly small extra cost of oxide over nitride is worth it. Many people buy TiN coated taps purely for the bling value. If the extra cost is small, then go for it, but I don’t generally see much value.

Tap Material

Taps are made from various steel alloys; as always, higher performance costs more! If you only need to tap a few holes in softer material, then the cheapest HSS steel is all you need. For longer tool life and for tough materials, the more expensive alloys are sometimes necessary. For cheapskates like me, part two will cover sharpening taps to get the maximum life from HSS.

Carbon Steel

This is your cheapest option — too cheap in my opinion. If you need a few holes tapped in a soft material, this is the most economical option; just remember the taps will go blunt quickly and are often less accurately made.

HSS Steel

This is the best choice for most applications. Taps will last well in

aluminum and mild steel but will blunt quickly in alloy steels and are risky to use in hardened steel or titanium.

Cobalt and Vanadium Alloys

Taps with cobalt and vanadium are only required for tapping hardened steel, titanium, or very abrasive materials like fiberglass. Some brands are quite brittle and require great care to use.

Thick Threads vs. Thin Threads

The many standards for thread geometry could fill a book; however, readers of SERVO are only likely to be interested in three of them. They are UNC (Unified National Coarse), UNF (Unified National Fine), and metric. Most thread standards have different pitches. As the pitch becomes finer, the threads become smaller, shallower, and closer. Unified National threads are specified in TPI (threads per inch), while metric is specified in thread pitch (the distance between consecutive threads).

For most applications, coarse threads have these advantages:

• Easier and faster assembly, with a reliable start and less chance of cross threading.

• Nicks and burrs from handling are less likely to affect assembly.

• They are less likely to seize in extreme temperatures and in joints where corrosion will form.

• They are less likely to strip when

threaded into lower strength metals.

• More easily tapped in brittle materials and materials that crumble easily.

In some areas, fine threads are an advantage:

• Screws with fine threads are about 10% stronger that coarse threads due to their greater cross-section area. This is a win when the tapped material is hard, but is not so important in soft materials where fine threads are more likely to pull out.

• In very hard materials, fine threads are easier to tap. You are much less likely to break a fine tap in hard steel and titanium.

• Fine threaded screws can be adjusted more precisely because of their smaller helix angle.

• Where the length of engagement is limited, they provide greater strength. A fine thread is usually best when tapping thin sheet metal.

• Thinner wall thickness can be used because of their smaller thread cross section.

Recommendation

As a rule of thumb, always select a coarse thread unless there is a specific reason from the above list to use a fine thread. I stick to coarse threads where possible simply to reduce the number of taps and screws I have to buy.