4.8 Spindle index pulse
When threading, it is essential for Mach3 to know the exact angular position of the spindle.
This can be sensed in one of two ways which are similar to each other.
Figure 4.11 shows a spindle with a slotted disc on it. The slots are detected by a sensor consisting of an infra-red LED and a photo-transistor mounted so that the disc turns between them. They is on the aluminium block shown below the disc. Mach3 has an input for one or more pulses generated each revolution of the spindle.
It is also possible to use a reflective opto-sensor "looking at" white stripes on a blackened pulley or gear.
This requires more adjustment when setting it up to get reliable results but may be easier to arrange.
The disc shown in figure 4.11 has four slots in it. The one at the eight-o'clock position is wider than the others by at least 50%. The other slots are approximately evenly spaced around the disc. Precision is not required here as Mach3 calibrates itself and even discovers
the number of slots (or stripes for a reflective sensor) that are in use.
However, spindles with a reasonable amount of inertia work fine with just one slot. Our advice is initially to use one slot or mark unless you already have a disc with more.
For reliable running Mach3 needs a pulse of at least 200 microseconds from the slots. You can calculate your slot angle or width as follows:
Suppose the maximum speed of your spindle is N (rpm).
The slot angle required alpha (degrees) is given by
alpha = 0.0012 x N
If the diameter of the disc is D (inches or mm) then the slot width W (in same units) will be
approximately
W = 0.0088 x alpha x D
Hardware issues and connecting your machine tool
Rev 1.84-A2 4-13 Using Mach3Turn
So for example if maximum spindle speed is 3000 RPM and disc diameter is 110 mm then:
Alpha = 0.0012 x 3000 = 3.6o
Slot width = 0.0088 x 3.6 x 110 = 3.48 mm
Big slot needs to be 50% wider = 3.48 x 1.5 = 5.22 mm
The example in figure 4.11 has three slots at about 4 mm and one about 8 mm wide.
You will see that you cannot use a disc with many slots or holes in it or use a conventional encoder. The pulse would be too fast. The original index sensing arrangements can usually be simplified when retrofitting an old lathe . These often had multiple slots for speed
sensing together with a single index slot.
As well as for timing threading operations, Mach3 also uses the index pulse to display the actual speed of the spindle and can implement a feed-per-revolution, rather than feed-perminute, based on it.