road and workshop
Page 41
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by Handyman
Benchwise: lathe sense (34)
ALTHOUGH external thread-cutting has been my concern so far in this series, the fact that each male thread needs a mated part for it in one form or another, either a nut, screwed bush, bracket, torsion bar, etc, must be borne in mind. Therefore I will now turn to internal screwcutting.
With V-form threads and nuts, quite a large amount of vehicle work will be concerned with hexagon nuts, and the ordinary tap will suffice. But in the larger range of threads. i.e. screwed blocks or bushes, the ordinary tap may not necessarily do. This can be caused by reasons of length, or thread being square, acme or buttress. But, provided that the item can be mounted and revolved in the lathe, any pattern of internal thread can be cut.
Although most of the screwcutting rules already discussed will apply equally to internal work, additional points have to be learned in order to ensure success. Probably the first problem will be the choice of tool and holder; for normal V-form threads the boring tool holder with a loose cutter set at a right angle is the answer, as this permits the cutting tool to be removed for sharpening without the need to disturb the lathe settings. For square or internal acme threads this same tool holder will serve best, particularly if the actual tool is round, permitting clearance adjustment.
The diameter The next problem. especially where a bush or threaded bracket, etc, has been reclaimed by welding or is being made from new solid material, is to determine the size of the hole to be bored first. This hole must of course, be less than the diameter of the mating screw by double the thread depth, which can be established from the thread gauge or by measuring the bottom of the V with a Vernier scale, using the knife-edge section of the gauge.
Therefore the first task is to bore out at screw diameter less a double depth of thread. For example, if the thread diameter is +in., quite a common size on road vehicles, and the tpi is 16, it will be found that this thread depth is 0.040in. so the hole to be bored from solid will need to be 0.080in. less than tin., i.e. 0.670in. To find the bore-hole size for the acme thread which has an accepted clearance of 0.010in. at top and bottom of thread, make the following allowance: screw diameter, less pitch. then less an extra 0.010in. The square thread, however, is slightly less complex and, as the pitch is always equal to double the depth of thread, it is simply screw diameter less pitch.
A useful tip for the trainee dealing with internal threads for the first time is to recess the nut or female thread for a short distance inwards at the correct overall diameter of the screw; this need only be for the distance of one thread inwards, but if this is done before screw-cutting begins it will indicate the exact depth of thread throughout the operation. As the final thread cut is made, the tool tip should just be in contact with this register.
However, there are one or two points to remember when starting to cut an internal thread. Here again, it is most important to take light Cuts at the slowest speed—in fact use the back gear as much as possible, and make several successive runs at the same
setting when nearing the finishing cut. I made this point when writing about internal boring as the reason is very much the same, i.e. despite the fairly stiff tool holder and the short cutting tool, any cutting of an internal nature has a tendency to push the tool off the work, and may appear even to "ride" rather than cut full depth as set.
This fault, however, is overcome as it is with straight boring by letting the bar lose its "spring" over two or more runs. It will be noted that, although no feed has been put on, the tool will still cut lightly for the first two runs, but let this take place before attempting to set in deeper, as this action also gives a correct shape to the cut. This is also a good practice to follow after the final cut as there is also a polishing action.
Stiff threads When the internal thread has been taken to its supposed finishing cut it may be found that on offering the screw to the new thread it is stiff, or goes solid. Two or three runs at the most under no additional load other than the fme amount of spring in the tool holder, will be enough to allow the threads to mate smoothly. While this method applies with V-form nuts, even the greatest care in setting and cutting may still find the paired threads too stiff for working.
One or two old tricks may be needed. If this is a normal nut and no further thread cutting is desirable, remove it from the lathe, set it on the screw at least a full nut and, odd as it may appear, one or two good -whacks" with a hide mallet will often do the trick. The second method, should the internal thread be in some odd-shaped item, is to take a marker blue reading, and if this shows no more than high spots in the female thread a slight smear of fine grinding paste will clear the trouble—but take care to wash clean afterwards.
A more difficult problem can be met when fitting nuts to square or acme threads, when for no obvious reason the nut may refuse even to start. In this case three checks can be Made: (1) see that the pitch of both threads is identical, as even the best turners can be caught out; (2) ensure that the internal thread is deep enough; (3) if all is correct so far, leave all the lathe settings as they are and draw the tool backwards towards the tailstock with the top slide to an amount not exceeding 0.001in. and take another cut through the nut. This will cut on the back edge and will widen the groove, and is the answer to the fault if the two initial checks show all is well; this process can be repeated until the threads will engage and work normally.