Interapid Hole and Groove Checking Comparator

(Dial Caliper Gage)

The Swiss-made Interapid Hole and Groove Checking Comparator Gage model IRA2 has the advantage of an extended range, up to six inches, so that one gage will perform the function of many. It is a superior tool and comes at a premium price. The gage is complex. Be sure to read all of the information on this page so that you understand the tool's limitations. The hardened steel measuring anvils are exchangeable and optional accessories allow for 3-point contact measurement. The dial scale reads in .0005" or 0,01mm and the accuracy is ±1 graduation.

The manufacturer provides the following statement:

"The IRA2 gauge is an instrument with many application possibilities. Thanks to its design as well as a variety of standard accessories, the IRA2 is suited for the measuring of blind and through bores, grooves and undercuts as well as form errors such as conicity, ovality and parallelism. Due to its light weight and form, the IRA2 is easy to handle. Because of the angular adjustment of its measuring arms it has a large measuring range, going from 10 to 150 mm (0.4 to 6 inches)"

Interapid IRA2 groove checking gage made in Switzerland

The complete Interapid IRA2 set includes:

  • comparator gage (.0005" or 0,01mm)

  • three measuring inserts (hardened steel) with 2.2 mm length and 3 mm diameter (metric part 105667; inch part 105697)

  • three measuring inserts (hardened steel) with 2.5 mm length and 1 mm diameter (metric part 105668; inch part 105698)

  • three measuring inserts (hardened steel) with 5 mm length and 1.4 mm diameter (metric part 105669; inch part 105699)

  • two adjustable flat bottom contact arms for measuring diameters larger than 6 mm (The installed tip can be removed - note the black screws in the photo above - and replaced with this pair of contact arms. The flat bottom allows you to go to the bottom of a blind hole.)

  • bridge attachment (small probe) for 3-point measurement

  • bridge attachment (large probe) for 3-point measurement

  • probe holder

  • custom probe wrench

  • centering device for diameters 15 - 30 mm (.6 - 1.2 inches)

  • centering device for diameters 30 - 150 mm (1.2 - 6 inches)

  • instructions for inspection of a cylindrical bore, 3-point inspection of a groove, and control of the cylindricity of a bore.

  • plastic holding case

  • Replacement tips — contact points

Technical data

  • Application range: 10 to 150 mm (0.4 to 6 inches)
  • Minimum groove width: 1 mm using measuring points 79.105668 which are included with the complete metric set.
  • Maximum Measuring Error: after zero setting of the apparatus in a ring gage (measured diameter of 48 mm) the error will not be more than ±0.01 mm (1 division) at both ends of the indicator scale.
  • Repeatability: maximum permissible error at different arm openings is as follows (since this is a comparator gage, the instrument must always be calibrated in the same range in which measurements will be taken)
  • 10 to 18 mm: 5µm
  • 18 to 30 mm: 6µm
  • 30 to 50 mm: 7µm
  • 50 to 80 mm: 8µm
  • 80 to 120mm: 10µm
  • 120 to 150mm: 12µm

Error of linearity

Example using the above data:
  • Zero setting: 100 mm
  • Measured value: 99.80 mm
  • Real value: 99.80 - 0.0068 = 99.7932 mm
  • Clearance between measuring arms: 10 mm (0.4 inches)
  • Minimum hole diameter: larger than 9 mm
  • Measuring depth: 2.5" (62.5mm) maximum in a 1/2" (13mm) diameter hole.
  • Measuring force: 3.5 N
  • Scale division: 0.01 mm (.0005 inches)
  • Scale range: ±0.2 mm (±0.008 inches)
  • Weight: 210g
Repairs and alternatives

Repairs are possible on all Interapid gages. With over 50 years of experience in Interapid repairs we are well qualified to perform repairs of these tools. Additionally, we can provide spare parts to qualified repair shops and dealers.

This style of measuring gage is available in every conceivable cost range from cheap throw-aways out of China, to flimsy Japanese models, to sturdy and recently improved German Intertest, all the way up to these high end Interapid gages.

Special notes about instrument use
  • The instrument will not repeat correctly if it is allowed to swivel.
  • Measurement must be taken with the dial in a vertical position, not in a horizontal position.
  • The instrument may malfunction if the arms are spread more than 6"
  • The instrument does not require lubrication.
  • This is a precision instrument and must be treated with respect and handled with delicacy.
  • In case of malfunction, the user is urged not to attempt any disassembly. The entire gage should be returned to an authorized repair service such as Long Island Indicator Service.
  • Make every effort to avoid dusty environments and to stay away from grinding machines.
  • Complete usage instructions are included with each new instrument. Study them carefully.
Setting the tension of the adjustment knob

On the back of the gage, just below the adjustment knob, you'll find a black screw which can be turned in or out to set the amount of tension on the adjustment knob. If you don't like the factory preset, or if someone's fiddled with this screw, you can easily make changes.

Problems with readings and calibration

First and foremost, remember that this is a comparator gage. You will not be taking direct readings beyond the range of the dial (±.008 inches). From the charts above, you will also notice that a linearity error exists as you go above or below 1.9 inches (48 mm). Furthermore, the repeatability lessens as you approach the upper range of 6 inches. At that point, one graduation of repeatability is expected.

If you're having problems try these steps:

  • Make sure that after the dial is set to zero, you tighten the small set screw on the front of the indicator (upper right).
  • Do not use the gage in a horizontal position.
  • Contact points should not show signs of wear. You may be using heavily and unevenly worn points. Use rounded points on flat surfaces. Don't use round on round.
  • Use a micrometer to set the gage. If you use a .0001" micrometer you should be fine. If you want, set the mike to a gage block of the right dimension. The parallel faces of the micrometer will give you better readings than the curves of a ring gage.
  • Do not rock the gage left or right. Only up and down.
  • Set the ends of the contact arms as near to parallel with the measuring surface as possible. Do not let them be at angles. The tips of the arms are adjustable for this purpose.
How to calibrate the IRA2

Use 48 mm as a standard setting (1.9"). A micrometer which has been set to 48 mm will give you best results. Repeatability must be .0003" (about half a graduation on the inch models).

Keeping the dial locked in place, use a micrometer of .0001" accuracy to check the movement of the hand on the dial. As you close the micrometer .008" and then .016" you will see the dial hand move correspondingly. At that range, there is no error of linearity and your dial hand should move the full .016".

If you repeat this procedure at other settings (it would be a good idea to do it at or near the settings you normally use) you will have to take repeatability and linearity errors into consideration. See the charts above.


Interapid IRA2 complete set
Contact us for availability

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Two things to consider:

  • Amazon product photos sometimes don't match the product description. It's the description that you want to pay attention to.
  • There are imitation gages on the market (specifically Mitutoyo) and you can make sure you are getting the real item by looking for the words "sold by Amazon". If it is sold by a third party, you might want to call Mitutoyo to find out if they are legitimate distributors.

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