The advantages of making internal threads with a singlepoint tool are that large threads of various forms can be made and that the threads are concentric to the axis of the work. The threads may not be concentric when they are tapped. Difficulties are encountered when making internal threads. The tool is often hidden from view, and tool spring must be taken into account.

The hole to be threaded is first drilled to a diameter in. less than the minor diameter. Then a boring bar is set up,and the hole is bored to the minor diameter of the thread. If the thread is to go completely through the work, no recess is necessary, but if threading is done in a blind hole, a recess must be made. The compound rest should be swiveled 29 degrees to the left of the operator for cutting right-hand  threads (Figures I-337 and I-338).

An alternative method sometimes used by machinists is to simply turn the boring bar 180 degrees so that the tool bit is opposite and the cutting edge faces downward (Figure I-339).With this method, the compound is swiveled to the right for right-hand internal threads. The advantages of this method are that the cutting operations can be more easily observed and the compound can remain in the same position as for external right-hand threads. For both methods, the threading tool is clamped in the bar in the same way and aligned by means of a center gage (Figure I-340).

The compound micrometer collar is moved to the zero index after the slack has been removed by turning the screw outward or counterclockwise. The tool is brought to the work with the cross slide handle and its collar is set on zero. Threading may now proceed in the same manner as is done with external threads. The compound is advanced outward a few thousandths of an inch, a scratch cut is made, and the thread pitch is checked with a screw pitch gage.The cross slide is backed out of the cut and reset to zero before the next pass. Cutting oil is used. The compound is advanced a few thousandths of an inch (.001 to .010 in.).The infeed on the compound is calculated in the same way as with external threads; for Unified internal threads,use the formula for the depth of cut with the compound set at 29 degrees.

Often, it is necessary to realign an internal threading tool with the thread when the tool has been moved for sharpening or when the setup has moved during the cut. The tool is realigned in the same way as is done for external threads: by engaging the half-nut and positioning the tool in a convenient place over the threads, then moving both the compound and cross slides to adjust the tool position. The exact amount of infeed depends on how rigid the boring bar and holder are and how deep the cut has progressed.Too much infeed will cause the bar to spring away and produce a bellmouth internal thread. If a slender boring bar is necessary or there is more than usual overhang, lighter cuts must be used to avoid chatter.
The bar may spring away from the cut, causing the major diameter to be less than the calculated amount or that amount fed in on the compound. If several passes are taken through the thread with the same setting on the compound, this problem can often be corrected.

The single depth of the Unified internal thread (Figure I-341) equals The minor diameter is found by subtracting the double depth of the thread from the major diameter. Thus, if the formula is

A UNC nut must be bored and threaded to fit a stud. Find the dimension of the bore. Cutting the Internal Thread
The following is the procedure for cutting right-hand internalthreads:
Step 1 After correctly setting up the boring bar and tool, touch the threading tool to the bore and set both crossfeed and compound micrometer dials to zero.
Step 2 Feed in counterclockwise .002 in. on the compound dial.
Step 3 Turn on the lathe and engage the half-nut lever.
Step 4 Take a scratch cut. Disengage the half-nut when the tool is through the workpiece.
Step 5 Check the thread pitch with a screw pitch gage.
Step 6 Apply cutting fluid to the work.
Step 7 Feed the compound in an appropriate amount.
(See Step 7 of “Cutting the Thread” in Unit 11 for an explanation of depth of cut when threading.) Slightly less depth for cutting internal threads than for similar external threads may be necessary because of the spring of the boring bar.
Step 8 When nearing the calculated depth, test the thread with a plug gage between each pass. When the plug gage turns completely into the thread without being loose, the thread is finished.
Step 9 Take several free cuts (passes without infeed) when nearing the finish depth to compensate for boring bar spring. Use the test plug between free passes.
Step 10 The inside and outside edges of the internal thread should be chamfered.

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