Wednesday, 12 August 2020
MECHANICAL INSTRUMENTS

Types of Thread Gauges

The thread gauges are essential used for measuring screw threads.The types of screw threads are. 

  1. Caliper gauge
  2. Ring screw gauge
  3. Plug screw gauge

Caliper gauge

The caliper gauges are similar to gap gauges with threads provided on anvils. The front anvils with full thread form acts as GO gauge, to check the threads below the size limit.

Caliper gauge

The screw with exact effective diameter  and pitch errors will not passing through GO anvils, Whereas eliminating pitch error by reduction ineffective diameter enables the screw to pass through GO as well as NO GO anvils. This leads to rejection of screw threads.

Ring screw gauge

The ring screw gauges are used for gauging bolts during production. It is provided with a system of limits to check the dimensions of the part. A thread ring gauge may be used to check the pitch diameter, major diameter, etc.

Ring gauge

By using ring gauges, the lead errors of thread can be easily determined. A special precaution must be taken to avoid excessive pressure on gauge, while checking five or more threads.

Plug Screw Gauges

The plug gauges are used for gauging internal threads. It is made to the minimum dimensions of internal threads, to ensure accurate inspection. The different elements of threads such as major diameter, minor diameter, effective diameter, etc., being checked, define the form of thread, angle and pitch.

Thread plug gauge

Generally, NO GO gauge is used is for gauging major, minor and effective diameter. For gauging parallel internal screw threads, different gauges are used.

Go gauge to check the tolerance on minor diameter and minimum effective diameter, whereas the NO GO gauge to check the tolerance and maximum effective diameter, these gauges are generally made up of special steel followed by hardening.

Types of pitch measurement methods

The types of pitch measurement methods are base on the

  • Internal screw threads
  • External screw threads

Pitch measurement of internal screw threads.

  1. Pitch measuring machine
  2. Tool maker’s microscope

 Pitch measurement of External screw threads.

  1. Screw measuring machine
  2. Microscope method
  3. Zeiss pitch or lead measuring instrument
  4. Matrix pitch measuring machine

Pitch measuring machine

The pitch of an internal screw thread is measuring by adapter. This adapter is consists of a bar which can be placed into the ring and end of the bar, stylus is fitted with the help of a thread.

Measuring Machine
                                           Measuring Machine

The ring gauge is held on a chuck or face plate of the machine. Which is incorporates rings up to several sizes.

In this machine a special surface plate is required with a slip gauges and dial indicator, for very large rings. And, micrometer is used to measure the movement of the stylus.

Tool maker’s microscope

Tool maker’s microscope is consists of a base with a vertical supporting column. The optical head is mounted on the column with a clamping screw, by which it can move up and down.

Tool maker’s microscope
Tool maker’s microscope

The screw whose elements are to be measured is placed on the table and its position is adjusted, till the intersection of cross lines seen through the eyepiece coincides with apex of angle. The reticule is rotated to bring one cross line parallel to one edge of the angle and reading is noted by viewing scale through the eyepiece.

The reticule is then again rotated to bring the same cross line to the other edge and the scale reading is taken in this position. The different between, these two readings is either the angle turned through or 360 degrees.

Screw measuring machine

Using the screw measuring machine, pitch of external screw threads can be easily measured. It consists of series of threads with non- uniform pitch. The pitch is the one which aligns perfectly with thread being tested. Therefore, the accuracy of measurement is based on sighting method.