WINTER 2014

108 THE DISTRIBUTOR’S

108 THE DISTRIBUTOR’S LINK Greenslade & Company Inc. 2234 Wenneca Avenue, Fort Worth, TX 76102 Tel: 817-870-8888 Fax: 817-870-9199 Email: sales1@greensladeandcompany.com www.greensladeandcompany.com INSPECTING SCREW AND BOLT STRAIGHTNESS by Larry Borowski President When a screw or bolt’s length exceeds eight times its body diameter (example: 1/2-13 X 4”) straightness becomes a concern for many end users. If a long screw or bolt is not sufficiently straight it can create significant assembly problems for the end user, which can then result in customer complaints or rejects for the fastener supplier. When measuring bolt straightness, any area containing swell under the head should be avoided. ASME B18.5 states; The length of any permissible swell on the body under the head or neck of bolts should be excluded from checks for shank straightness. A good rule of thumb is to hang about 1 times the shank diameter outside of the jaws when performing this check. Straightness gages in ASME standards Several of the American Society of Mechanical Engineers (ASME) B18 standards appendices contain an attribute type of gage for determining part straightness. The pictured gage consists of a base plate with one stationary side rail and one adjustable side rail. The gap between the rails is set with a pair of micrometer heads. This style of gage is quite versatile and cost effective, as it will accommodate a range of diameters and lengths. Most of the ASME standards are consistent in their straightness specifications. Screws and bolts up to and including 12 inches in length can be bowed .006 inches per inch of length. Parts exceeding 12 inches can be bowed .008 inches per inch of length. If a user’s application requires a part straighter than these limits, they must specify their required straightness limits on their print and/or purchase order. In the case of our 1/2-13 x 4” long example, the maximum allowable bow would be .024” (4” length x .006” per linear inch). When using this attribute type gage, you would add the allowable bow (.024”) to the basic diameter (.500”) of the bolt, and set the gap in the jaws at .524”. You would then rotate the fastener 360º in between the jaws. If the part binds, you have exceeded your maximum bow or camber. If it rotates freely, you are within your allowable bow or camber. The camber gage illustrated in the ASME standards and shown in this article is an attribute gage. It will only indicate to the user whether or not the part being inspected is within the specified limits for straightness, but will not indicate the exact magnitude of the camber or bow in the part. Indicating gage for measuring the amount of screw or bolt camber To determine the extent of the bow in a part used for either gathering statistical process control (SPC) data or for final inspection documentation, an indicating, variable type of gage such as the CamberChek® is needed. The CamberChek® gage is similar to the rail-type gage illustrated in the ASME standards except the non-stationary side rail slides on two precision bearings instead of a fixed position. The gage is designed so that gravity provides a constant closing pressure against the sliding rail. A digital indicator contacts the outside of the sliding rail for indicating the exact amount of bow in the parts being inspected by the gage. please turn to page 171