|
QUICK FACT GUIDES FOR MACHINISTS
|
|
GUIDES |
Because the tapping operation follows the drilling operation, there is a tendency on the part of some drillers to overlook the importance of properly drilled holes on the subsequent tapping operation. Yet for the tapping of high quality threads, particularly at production speeds, properly drilled holes are of vital importance, and it should be borne in mind that the tap is not a corrective for poorly drilled holes. Generally considered, there are two types of holes, open or through, and blind or bottoming. Depending upon the type of material, the class of thread, and the method of tapping, the following styles of hand taps are used for open or through hole tapping. In steel and other types of malleable material, tapped by machine, the "Gun" and Plug Tap are most commonly used. For cast iron and non-malleables, the Plug Tap will give best results. Some holes may be considered blind and yet not bottoming, for the reason that it may be possible to drill the hole considerably deeper than it is necessary to tap. In such cases it is possible to proceed as for through holes, using the same style of tap as would be used if the hole were really a through one. However, if the thread must extend to the very bottom of the hole, a Bottoming Tap should be used. In all cases and in all types of holes, good clean drilling a reasonable degree of size maintenance will add considerably to the ease of tapping good threads, and will make for better size control, will often prevent loading of tap threads, and will lessen the strain on the tap. Quite frequently, taps of special design are provided with pilots, preceding the threaded section, for the purpose of maintaining alignment. Controlling the size of drilled holes for such taps is most important because holes looser than a free fit for the pilot size defeat the purpose of the pilot, while tight holes add unnecessary, if not excessive, load to the tap. "Go" and "Not Go" Plain Plug Gages for controlling the size of drilled holes area good investment where taps with front pilot are required. Such gages are also a worthwhile investment for other drilled holes to be tapped in production quantities. Of equal importance when tapping to the bottom of a hole is the matter of clearing out the drill chips, or other accumulated material before starting to tap. Tap breakage frequently results from the impact of the tap upon a bed of drill chips at the bottom of the hole. Another type of hole that frequently presents difficulties in tapping, is the one prepared for tapered threads. When tapping with a tapered thread tap such as a Taper Pipe Tap, it should be borne in mind that every tooth of the tap in the hole is cutting, whereas with a straight thread tap only the chamfered portion and the first full thread does the cutting, the remainder of the straight threads acting only as a guide or follower. Naturally, the tapered thread tap carries a much greater load which increases as the tap advances into the work piece. To lighten this load, the hole should be bored or reamed taper before tapping. If holes for taper pipe threads are reamed with standard taper pipe reamers before threading, the tap will only be required to cut away the full depth of thread. Sizes of tap drills used in Stainless Steels are very important and should be the largest size permissible. This factor is especially important when producing fine-pitch threads, in which case a few thousandths difference in hole sizes, together with the flow of metal, may produce sufficient thread depth to completely fill the tap thread root and cause difficulty. Especially when soft Stainless Steel is used, there is considerable metal flow during tapping, which can result in an undersize hole after tapping, there-by making it difficult to remove the tap without tearing the threads. The subject of drilled hole size and thread height are inseparable, but a definite thread height for all sizes, and under all conditions, is not practical Therefore, the tap user must determine for himself what is best for his particular requirements, but, by analyzing the following subject, the problem can be very easily solved. 1. Diameter and Pitch of
Tapped Hole As to the first consideration, "Diameter and Pitch of Tapped Hole," let us consider whether the tap required has a fine or coarse pitch in relation to its size, bearing in mind that the coarser the pitch the smaller the height should be. It is suggested (assuming the design of the tap is correct for the requirement at hand) that the 75% thread height be tried first, and, if this results in tap breakage, the thread height should be reduce gradually until satisfactory conditions are produced. In doing so, the question may arise, as to whether such a reduction might possibly jeopardize the strength of the thread. This can also be readily determined for we know that a nut with only 50% thread height, made to standard specifications, will break the bolt before it will strip the threads. It is also well to consider some of the basic peculiarities in our threading system to better understand why tapping problems arise from this source. the table below covering a group of tap sizes in the National Coarse and National Fine Thread Series, shows what percentage the double thread height is in relation to the basic major diameter and plainly reveals that the smaller the tap the greater the thread height proportion.
After fluting suitably for this thread height, weakness develops rapidly as the taps grow smaller; therefore, more care should be exercised when using small taps. Now as to the "Nature of Material Being Tapped," we should bear in mind that soft stringy metals such as copper, drawn aluminum, monel metal, nickel silver, etc., due to their malleable nature have a tendency to flow toward the crest of the minor diameter while being tapped; therefore, the minor diameter of tapped holes in these material will be smaller after the tapping operation, and this condition should be taken into consideration when deciding upon the tap drill size for such materials. The toughness and hardness of the material should also be considered. Some of the alloy steels and metals that are very tough or of a high hardness can be tapped more successfully with less than 75% of thread and where these characteristics exist the smallest thread height possible should be adopted. The "Depth of Tapped Hole" is also very important. When tapping holes to a depth of more than one and one half times the diameter of the tap, especially when using machine screw sizes, it will be found advantageous to use larger drills than the 75% type. Drills which will give a 50% thread will be found entirely satisfactory
|
|
Copyright © 2002 QUICK FACT GUIDES FOR MACHINISTS
|