We are often asked "How do you calibrate the Almen gage?" EI's Almen Gage Calibration Service is well worth the expense of having us do it. I'll try to explain the procedure we follow. You should be aware that if you are audited there is no industry standard or specification that covers this procedure. In cases like this the industry accepted practice is to defer to the manufacturers recommendations. Electronics Inc (EI) refers to SAE J442 for gage dimensions uses the following steps to provide the "Certificate of Conformity" for the Almen gage. (This is only a summary, not our actual procedure.)

1. Visual inspection of gage for function and wear. Replace indicator tip if needed. If the balls have flat spots then replace the gage head. Ball wear can cause significant errors.
2. Be sure indicator tip moves freely (do not oil). Replace batteries if needed.
3. Use a certified template to verify ball and post position and digital indicator placement .
4. Use the certified step blocks to verify that the digital indicator is accurate (+/- .0002")
5. Apply decal for date calibration performed.
   
   Note: all of the equipment used in this procedure is available in the
   Almen gage calibration kit from Electronics Inc. so you can perform
   your own calibations . You can use our factory calibration procedure.

1. Visual inspection of gage for function and wear. Replace worn indicator tip and 4-balls if needed.
2. Be sure indicator tip moves freely (do not oil). Replace battery.
3. Remove the digital indicator from the base.
4. Use a high-accuracy Coordinate Measuring Machine (CMM) to locate the 4-ball placement per SAE J-442 or use a certified template for go-no-go indication. to verify ball and post position and indicator placement.
5. Calibrate the digital indicator using the Federal Calibrator within +/- 0.0002"
6. Replace the digital indicator onto the base and set it at 0.000" true spindle position.(Use a certified flat block when setting the true spindle position.)
7. Apply decal for "Calibration performed on date" and "Calibration due date" (usually one year after calibration or from first day put into service.)

EI Almen gages are shipped with a curved "Check Block". The check block has not been measured with NIST traceable accuracy and is intended only as a weekly check of gage functionality. It is not calibrated and it is not certified.

Note: Almen gages should be re-certified on an annual basis. New gages are certified to meet the requirements of J442. The date of manufacture and original calibration date are shown on a special label. The field "calibration due date" is filled in at time of shipping and the date used is the shipping date plus one calendar year.

To obtain a copy of SAE J442 "Test Strip, Holder and Gage for Shot Peening" go to SAE.org

revised 8 Feb 2005

Tip moves freely
Push the indicator tip in (retracts) and release it to verify that it extends smoothly and easily with no hesitation caused by friction or a bent tip. Failure to have the tip extend properly can result in an inaccurate reading since the tip may not be in contact with the surface of the Almen strip. Excessive friction, caused by dirt or dust, may result in bending the strip as it is placed onto the gage. A tip force of 50 grams will cause an error by bending the "N" strip 0.0001". A tip force of 350 grams will bend the "A" strip 0.0001". Digital indicators used by Electronics Inc. are manufactured to meet this especially low requirement. The standard extension spring used in most digital indicators may have tip forces over 300 grams and should not be used. If you must repair or replace the extension spring be sure to measure the tip force. back to top

Batteries
The batteries should last approximately one year with normal use. The battery type is CR2450 and should be available at most camera or computer stores. Two are required. back to top

Certified step blocks
A quick and easy way to verify the calibration of the digital indicator is to use a series of calibrated step blocks that fit conveniently on the Almen gage. These special gage blocks are supplied as a complete set with steps of .005"/.010"/.015"/.020"/.025".(Note: step blocks are also available in metric .2/.3/.4/.5/.6mm) Each block is placed onto the gage (one at a time) to allow the indicator tip to extend to a specific range. The blocks have a unique feature allowing a quick "zero" setting by pushing the block to the left side of the gage. Pushing the block to the right side of the gage allows the step value to be displayed. This technique provides simple and hightly accurate gage calibration. These certified step gage blocks are available from Electronics Inc. and are covered by U. S. Patent No. 5,780,714. back to top

Check block
The check block supplied by Electronics Inc with the TSP-3 Almen gage is manufactured to an approximate value of 0.024" ±.002". This block is not to be used for calibration. It is recommended that this block be used weekly to verify the consistancy of the gage perfromance. By placing the check block onto the gage and recording its value each week the operator can detect any changes that may affect gage performance. Any time error is more than ±.0005" the gage should be inspected and/or repaired. A check block is NOT a calibration block. It has NOT been measured with high accuracy and is NOT traceable to NIST.back to top

Templates
A simple template is available from Electronics Inc which can be used to confirm the dimensional accuracy of the Almen gage components, namely the position of the balls, posts and indicator tip. The template has been manufactured to meet the construction requirements of SAE J442 and is certified by NIST traceable laboratory. The template is placed onto the gage and is used as a go/no-go conformity check. If all of the balls and posts are aligned properly, as shown by the template, then the gage is in compliance with SAE J442. back to top

If you are audited
Do not let the inspector use the check block as a qualifier for the #2 Almen gage. First, the check block is not described in J442 and and it is not traceable to NIST. Second, there are no (known) calibration procedures in effect that describe placing a curved block onto the gage as a go/no-go test of the gage. At the present time the only means of "certifying" or "calibrating" a #2 Almen gage is to refer to the drawings in SAE J-442 and verify ball placement and related dimensions. Electronics Inc has developed a procedure for calibration. back to top