OVERSIZE TAPPING
Heli-Coil STI tapped holes may become oversize for any one, or combination of the same factors that cause standard tapped holes to be oversize. Heli-Coil provides two insert systems to correct oversize STI tapped holes.
(The Heli-Coil Insert, itself, is the first choice method of correcting standard holes that are oversize.)
OVERSIZE INSERTS
A tapped hole is considered oversize if the Hl (No Go) gage enters more than 3 turns (2 turns for Metric
threads). If this Hl gage enters more than 3 turns, the amount of oversize is not known except by an educated guess based on the degree of wobble of the gage.
This oversize condition of an STI tapped hole can be corrected by an Oversize Insert if the actual pitch diameter of the hole is within Heli-Coil oversize limits, or if it can be retapped with an Oversize STI Tap to within Heli-Coil oversize limits.
For critical applications, Class 2B or 3B Oversize Gages can be ordered for gaging the oversize tapped hole.
This enables determination of the actual class of fit. These are listed in Table 7.
When it has been determined that a tapped hole is oversize, it must next be determined if retapping with the Oversize Tap is necessary. If the Oversize GO Gage will pass through the full length of the hole, it is already large enough to accommodate the oversize insert. The Hl gage can be used to determine if it is a Class 3B or a Class 2B hole.
If the Oversize GO Gage does not enter, it is necessary to retap with the Oversize STI Tap. All proper tapping procedures should be followed to minimize the possibility of the Oversize Tap itself cutting oversize.
The Oversize Tap may remove, approximately, from one ten thousandth of material up to several thousandths.
In removing very small amounts of material, a tap may simply burnish and not cut. In order to minimize this
problem, Heli-Coil Oversize Taps have been made to a slightly larger pitch diameter than if they were cutting an untapped hole. As a result, a Class 3B hole may not be attainable. Also, a Class 2B hole is not always
absolutely assured, depending on the condition of the original oversize hole.
If it is determined that the oversize tapped hole is so large that it is beyond the correction range of Oversize Inserts, a Twinsert assembly can be used
