Additional problems such as cross-contamination of metric and imperial threads can further complicate this method. Specialist tooling is available for accurately and safely determining the correct thread size for a chosen application. A variety of thread gauges are available for many metric and imperial thread sizes, and can be used to identify both male and female thread types.
Ring gauges can identify male threads such as screw or bolts, and plug gauges can identify female threads, such as those found in nuts or pre-tapped holes.
How to Measure Screw Thread Pitch Measuring Metric Thread Pitch Metric screw threads are measured in millimetres, and are determined by measuring the distance between two adjacent screw threads. Thread Gauges Thread size and pitch can also be identified using a basic trial-and-error method of simply attempting to use a component of a known size with the unidentified product — if the two fit together with some but not too much resistance, they are likely the same size.
About The Author: Daniel Daniel is the Quality Manager at Accu, with a degree in History, his passion for all things mechanical burns brighter than ever. Read More. What is a Washer Used For? New larger threads are installed with a specifically sized tap.
The inserted coil has the outside diameter of the tap, but the coil inside diameter matches the original thread. Taps and dies are cut to match the desired thread, and also have a helix angle.
This is more difficult to see because the threads are not continuous around the tap or die. This helps prevent build up of cut material in the tap. It is sometimes possible to tap a damaged internal thread to a larger size, and then use the corresponding bolt or screw. This repair may not work if there is little extra material around the damaged threads. This repair is sometimes possible, but the bottom bracket should have all threads removed before tapping.
The original thread inside diameter is approximately 34mm. The bottom bracket shell inside diameter should be 35mm to correctly cut threads of 36mm. Generally, tapping a bottom bracket to the larger 36mm x 24TPI standard is a very difficult slow process. It is also very hard on the taps. Another option for some external thread repair is a thread file. Hold the die parallel to the helix angle and push the file across the damaged threads. When cutting new threads in a blank hole, the hole size must be appropriate for the tap.
This is size is called the Tap Drill Size. The tap cuts and removes a certain amount of metal, leaving the internal thread. If the hole is too small, the tap will have difficulty removing material, and a great deal of force in turning the tap will be required.
The tap will bind in the hole and begin to gall and pull up material, leaving a poor internal thread. Manuals on machining contain tables and charts to determine the correct tap drill size. If no chart is available, a simple formula for both SAE fractional and metric threads can be used. There is a simple conversion for thread per inch into pitch.
The pitch distance from crest to crest is the inverse of the threads per inch. The correct size hole in this example would be 0. Notice that the coarser the thread, greater the difference between tap drill size and tap size.
Finer threads would have less of a difference between the tap and hole size. Using a metric example, a hole is required for a thread of 5mm x 0. The correct size hole would be 5mm - 0. There is resistance to turning the bolt, as the fastener gets tighter.
Some resistance comes from friction and rubbing between the internal and external thread surfaces. Because of this, it is common to prepare the threads with lubrication. This can take for form of liquid lubrication, grease, or an anti-seize compound.
Even liquid threadlockers provide some lubrication during tightening. As a simple rule of thumb, if the thread size is small, such as a derailleur pinch bolt, a liquid lubricant is adequate. If the thread is large or the torque relatively high, such as a pedal thread or bottom bracket, use a grease or anti-seize compound.
There are situations, however, where a manufacturer may recommend no lubrication on the fastener. If is useful to lubricate the threads and under the head of the bolt, especially when the bolt head is turned during tightening. Threadlockers are special adhesives used in many industries and in many applications. These are available through Park Tool. This hardening and expansion is what gives these materials their special feature.
However, threadlockers should not be used to replace proper torque and pre-load when clamping load is important. This compound acts primarily as thread filler. If the part is removed, the compound tends to break down, so use a liquid threadlocker to supplement. Threadlockers come in different grades of strength. There are compounds that are stronger and extra procedures are often necessary when disassembling, such as heating with a heat air-gun. Most thread-locking compounds are designed for metals.
They are usually not intended for use with plastic, and may both harden and weaken the plastic. Retaining compounds are intended for press fit applications such as pressed studs.
The retaining compounds tend to have a higher viscosity than the thread-locking compounds. Many retaining compounds require special technique for removal, such as excess force and or mild heat. Retaining compounds can provide a useful repair on marginal press fits, such as a headset cup that is a poor fit to the frame. It is the habit of many mechanics to put the thread-locking compound on the external thread. This is typically not an issue, but in some cases this is not appropriate.
As the parts are threaded together, the excess compounds backs up toward the head of the thread where it spreads about. Other parts may be inadvertently contaminated. Using an I. Many thread ID calipers provide both a caliper and a seat angle gauge in one tool.
The I. Important: When matching gauge measurements to thread charts keep in mind that threads on connections that have been in-service may be worn and distorted from use, causing inexact comparison to the thread tables. For English threads, British threads and other European threads the thread pitch gauge measures the threads per inch. However, for metric threads the gauge will identify the distance between the threads. The seat angle gauge is used by placing the gauge angle on the sealing surface.
The centerline of the fitting end and the gauge should be parallel. In the English system the thread size and pitch number of threads per inch are given, along with the thread type. Measuring Threads Using the thread pitch gauge , align the gauge on the threads and make sure it is snug. Match the measurement to the a thread c hart. Then measure the thread diameter with the I. Measurin g Sealing Surface Angles Female connections are measured by inserting the ID portion of the gauge into the connecti on on the sealing surface.
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