The format of a tapping cycle is very similar to G81:
Here are two related formulas for calculating tapping feedrate F with known spindle S and either pitch Por number of threads per inch TPI.
G98 G84 X.. Y.. R.. 2.. F..
Threads made by tapping can create problems just like threads machined on a lathe. The problems can be quite numerous, so it is always important to categorize them and narrow down the cause. Generally, tapping prob lems can be filed in four categories:
A hole that was drilled earlier can accept another op- eration, such as boring, reaming or tapping. Let’s look at tapping first. Tapping is a combination of forming and cutting. The tap has to be selected strictly for its size and the hole has to be drilled in such a way that there is enough material left for the tap. The drill selected for tapping is commonly known as the tap drill.
This formula is always used for metric threads, but it can also be used for imperial threads, which requires cal culation of the pitch first:
G99 G84 X.. Y., R.. 2.. F.. G74 has the same format.
There are several features in a typical tapping program that the CNC operator should be aware of:
Stripped thread I Dimensional problems
Surface finish problems
P= 1 / TPI
I G84 is used for right-hand taps with M03 spindle
rotation – right-hand tap must be used
This is an extra step, which is not necessary. A better way is to use the TPI directly
Programmers will use a tap drill most suitable for the job. There are numerous tap drill charts available, sup plied through vendors, manufacturers and other sources. You can also find a typical chart in the Reference section of this handbook, on page 271.
A modified example from the drilling section can be used as a sample of what to look in the program for a tap.
I G74 is used for left-hand taps with M04 spindle
rotation – left-hand tap must be used
Spindle speed is selected for the machining conditions
Feedrate is always calculated (see below)
Tap will most likely break, if the tap itself is too large or the pre-drilled hole is too small. Dull taps are more likely to break than new taps. In the program, check if the XY coordinate positions of the tap drill and the tap match. Check if the spindle speed and feedrate relation ship is correct. Sometimes a different type of lubricant may solve the problem.
For blind holes check the depth of the drilled hole. Most drawings allow certain flexibility for the hole depth, as long as it does not break through the material. Tapped depth in blind holes should be checked periodi cally, so the full diameter tap depth is maintained. Re moving chips prior to tapping may be another way to eliminate tap breakage.
, R-level has been increased to allow extra space
for acceleration with high feedrates
– 1.4 –
In the example, the feedrate was calculated using the last formula:
F = 600 / 16 = 37.5 NOTE – It is not unusual to change spindle speed at the machine to create optimal conditions. Changing spindle speed for tapping is no exception, as long as the feedrate is changed accordingly
For example, you may find that the programmed spin- dle speed 5600 can be changed to $700. It is not enough to change the address S only, but the feedrate F has to change as well: S600 F = 600 / 16 = F37.5 $700 F = 700 / 16 = F43.75 (F43.7)
. Z-depth should always guarantee full diameter thread
• Taps vary in geometry – select the most suitable one
In the less likely event, the reverse of the above state- ment is also thru – if you change the feedrate, you must also adjust the spindle speed. For the number of threads per inch TPI use this formula:
Since tapping is a cutting operation combined with forming, the shape of the tap has to be formed for the thread to be accurate. That means maintaining the thread pitch. The great majority of taps have only a single start, so the pitch and lead of the thread are the same. Remem ber, that feedrate for tapping (or threading) always con siders the lead of the thread, not the pitch. This is not an issue in tapping, but it is very important for threads with more than one start (multi start threads). To review:
• PITCH is the distance between corresponding point
of two adjacent threads
S = F x TPI
Finding causes for a stripped thread may not always be easy, but trying the remedies suggested here should always be the first step. Dull threads, wrong alignment, hard material, poor setup – they all may contribute to the problem.
One solution is to underfeed. Take 2-5 percent off the calculated feedrate and try again. An even more innova tive solution, particularly for very fine threads, may be to underfeed on the way into the material and overfeed on the way out. The problem with this solution is that tapping cycle uses only one feedrate for both motions. Writing a simple macro is the best solution. Since mac ros belong to manual programming on a much higher level, the following macro will be provided with mini mum explanation.
It is written for a feedrate of 80% in and 120% out. This percentage will most likely require some degree of experimentation, but has been proven to work well.
For pitch P, use this formula:
The program shows a tool T03-388-16 TAP. The tap drill program is not listed, as it does not bring any new infor- mation. The typical (most common) size of the tap drill is Ø5/16 (0.3125), based on standard charts.
(T03 – 3/8-16 TAP) N22 T03
(TOOL CHANGE) N23 G90 G54 Goo x0.9 71.1 5600 M03 (HOLE A) N24 G43 21.0 +03 M08
N25 G99 G84 RO.3 Z-0.7 F37.5
N27 X1.4 Y0.5
N28 G80 21.0 MO9
(RETRACT) N29 G28 21.0 M05 (RETURN TO MACHINE ZERO) N30 M30
(END OF PROGRAM)
is the axial distance measured over one revolution
For example if you want to change the feedrate of F37.5 to F35.0 the spindle speed will change to: S = 35.0 x 16 = S560 which is the same as S= 35.0 / 0.0625 = 5560
Tapping feedrate is a relationship between the pro grammed spindle speed and thread lead (pitch for nor mal tapping). Metric threads are always defined by pitch, for example, M10x1.5 is a 10 mm thread with a pitch of 1.5 mm. Standard V-shape imperial threads have the same form but are defined with the number of threads per inch (TPI).
CNC Control Setup for Milling and Turning
CNC Control Setup for Milling and Tuming MACHINING HOLES
Macro call example for imperial units, where the argu- ment T is the number of threads per inch (TPI):
G65 P8020 R0.4 20.25 $750 T36
G99 –++–*– Spindle CW
Boring – or more accurately – single point boring is ba sically a sizing operation. It adds roundness and cylin dricity to the hole, improves its surface finish, and with a micro attachment on the boring bar, hole size can be adjusted very accurately.
All remaining cycles are defined as boring cycles, starting with G85. Their differences are often subtle. A short list below illustrates the order of basic boring mo tions, programming format and brief motion details.
Surface Finish Problems
Rough surface finish is generally the result of the tap pushing itself into the material rather than cutting it smoothly. Changing tap geometry often eliminates the problem of rough threads. Also, check the tapping head – some designs allow torque adjustments. Small tap drill may also contribute to poor surface finish, If the CNC machine has rigid tapping capability, use rigid tapping rather than the traditional tension/compression type tap ping heads. Rigid tapping always surpasses regular tap ping, but the machine tool has to support this feature. Underfeeding a couple of percent may also help.
08020 (SPECIAL TAPPING MACRO – IMPERIAL) (R = #18 = FEEDPLANE) (Z = #26 = TAP DEPTH) (S = #19 = SPINDLE SPEED – RPM) (T = #20 = NUMBER OF THREADS PER INCH == #3003 = 1 GOO Z [ABS [#18]] S#19 M03 #3004 = 7 G01 2-[ABS (#26]] F[#19/#20*0.8] M05 Z#18 F[#19/#20*1.2] M04 #3004 = 0
#3003 = 0
685 Feed to depth > Feed out
G98 G85 x.. Y., R.. z.. F..
G99 G85 x.. Y.. R.. 2.. F.. Feeding out with a regular boring bar may actually damage the surface due to the release of tool pressure.
This is the preferred fixed cycle for REAMING
Both G84 and G74 tapping cycles can be represented graphically, similar to other cycles described in this sec tion. Watch for spindle rotation and correct tap selection.
G87 Cycle G87 Backboring (** G98 MODE ONLY **)
G98 G87 X.. Y..R.. Q..z.. F.. Apart from the requirement of a special tool, this cycle starts from the bottom of the hole and feeds up. It is used mainly when the backbore diameter is only marginally larger that the existing hole. G98 mode only.
Macro call example for metric units, where the argu ment Tis the thread pitch:
START IN MO3 MODE G84
and R/H tap
G65 P8020 R3.0 26.5 5750 T0,75
08020 (SPECIAL TAPPING MACRO – METRIC) (R = #18 = FEEDPLANE)
= #26 = TAP DEPTH) (S = #19 = SPINDLE SPEED – RPM) (T = #20 = THREAD PITCH ==== #3003 = 1 GOO Z [ABS [#18]] S#19 M03 #3004 = 7 G01 2-[ABS (#26]] F[#19**20*0.8] M05 Z#18 F[#19*#20*1.2] M04 #3004 – 0
#3003 = 0
Z-depth – spindle CCW
START IN M04 MODE G74
and L/H tap
Note the OSS abbreviation – OSS stands for oriented spin dle stop. Also note the Q-address – this is amount of the bar shift. Oriented spindle stop and the shift and their ef fect on tool setup is described in detail for G76 fixed cycle on page 124.
The 3000-series variables disable or enable single block and feedrate overrides. Take these examples for what they are – only examples that can be adapted.
G86 Cycle G86 Feed to depth > Stop > Rapid out
G98 G86 x.. Y.. R.. 2.. F..
G99 G86 x.. Y., R.. 2.. F.. A very useful fixed cycle for rough or utility type boring. When the tool retracts at a rapid rate, the spindle does not rotate, and the relief of tool pressure will leave a straight vertical line on the hole surface. In fact, G81 cycle that is closely associated with drilling can also be used for rough or utility type boring. The difference is that it will leave a helix on the hole surface, since the spindle is always rotating. G86 or even G81 cy cles are often used when the hole has to be round, but surface finish is not critical.
Z-depth – spindle CW
G88 Cycle G88 Feed to depth > Stop > Manual operation … For all practical purposes, ignore the fixed cycle G88. It is used for very special purposes only and appears in part programs extremely rarely (if ever).
Adjustment in tapping speed (don’t forget to change feedrate, too) is one possible approach. Using coated taps will prevent metal sticking to the thread, and so will a suitable lubricant. Changing the tap drill size slightly will also change the percentage of the thread depth. Us ing high helix tap geometry minimizes pressure. These are all possible solutions to consider.
Riaid Tanning Rigid Tapping
Rigid tapping does not much change in the program, but the tap is installed is a solid holder (rigid holder). Spindle synchronizes the tapping. There are many bene fits, but the CNC machine has to support this feature.
There is no need for illustration,