Manual Control Guide for Handwheel CNC

• 1
  How a handwheel CNC control turns rotation into axis motion
• 2
  Choosing ×1, ×10 or ×100 before the tool gets close
• 3
  Pulse rate, detent feel and following error
• 4
  Jobs where handwheel control beats a full automatic cycle
• 5
  Where manual control stops being the right tool
• 6
  Automatic cycle vs handwheel jog: which fits the task

A handwheel, often called an MPG or manual pulse generator, is an encoder with a detented dial. Inside are two optical or magnetic tracks in quadrature, plus a small detent wheel that makes each click. The control reads the direction from which track leads, counts the edges, and converts counts into a commanded move on one selected axis.

The conversion factor is the scale factor, usually ×1, ×10 and ×100. At ×1 the axis moves one least input increment per pulse, which on a metric control is typically 0.001 mm. At ×10 and ×100 the same click becomes 0.01 mm and 0.1 mm. That is the whole trick: the encoder resolution is fixed, the software multiplier decides the real step.

The drive then has to execute the move. Most controls apply an acceleration and deceleration ramp even in jog mode, so a fast spin at ×100 does not slam the axis. The control also tracks following error during the move. If you spin faster than the axis can follow, the lag grows and the control can raise a following error alarm before any tool touches metal.

So the handwheel is not a mechanical link to the ballscrew. It is a request queue. Every click is a position increment the servo loop has to close, and the loop needs time. That single fact explains most handwheel problems on the shop floor.

Use ×100 while the tool is still far from the part. At 0.1 mm per click you can cover 50 mm in 500 clicks, which takes seconds. Expect to overshoot the target and then correct. This is a travel setting, not a setting for touching off.

Drop to ×10 once you are inside about 5 mm of the surface. The step is small enough to control approach speed but still fast enough to close a short gap. Most operators do their edge finding and stock checks at ×10.

Switch to ×1 for the last few tenths. At 0.001 mm per pulse, ten clicks are 0.01 mm. This is the range where a handwheel CNC operator can feel the load on a dial indicator or hear a cutter start to bite. It is also the range where a worn encoder can lose counts, so test it.

Some controls let you change the multiplier while the axis is moving. Others force a stop first. Know which one you have. Changing scale mid-move on a control that allows it is a common way to overshoot a delicate feature.

• 1
  ×100 for travel0.1 mm per pulse, use it between features, not near them.
• 2
  ×10 for approach0.01 mm per pulse, the working range for edge finding.
• 3
  ×1 for touch-off0.001 mm per pulse, final tenths and indicator work.
• 4
  Never change scale under loadSome controls allow it. That does not make it safe.

Pulse rate is the number of clicks per second the control can accept. A typical MPG generates 100 pulses per revolution of the dial, with 100 detents. Spin it at two revolutions per second and you are asking for 200 pulses per second. That is fine on a modern control and marginal on an older one.

When the pulse rate exceeds what the loop can close, the axis lags. The control sees a growing difference between commanded and actual position. Depending on the parameter set, it either slows the commanded stream or trips a following error alarm. Either way the axis does not go where you think it went.

Detent feel matters more than people admit. A dial with 100 detents per revolution gives 3.6 degrees of rotation per click, which is easy to count by hand. A smooth dial without detents is harder to count and easier to overspin. For touch-off work, detents are worth having.

If your machine throws following error alarms during handwheel jog, lower the multiplier before you touch a parameter. Going from ×100 to ×10 cuts the pulse demand by ten and usually clears the alarm with no other change.

Setup is the obvious case. Touching off a tool on a rough casting, finding the center of a bore, or picking up the edge of a plate with a 3D taster is faster by hand than by writing a probing cycle and verifying it. A few seconds of manual control often prevents a scrapped part.

First-article checks are the second case. After a program runs, an operator may need to jog to a specific feature and measure it without breaking the setup. Handwheel jog lets you move the axis a known increment and read the result on the machine position display.

Recovery is the third. When a tool breaks or a chip packs the flutes, you need to back the axis off along a controlled path. Rapid moves in that situation risk dragging a broken tool across a finished surface. Jogging at ×10 gives you a traceable retreat.

The fourth case is delicate geometry. Thin walls, deep ribs and small bores do not always behave the way the simulation predicts. An operator who can slow the feed by hand, or stop the axis mid-cut to listen, catches chatter before it becomes a scrapped feature.

Handwheel jog is not a production method. Any feature that repeats across hundreds of parts belongs in the program, with the offsets set once. Manual motion adds cycle time and operator fatigue, and fatigue is where mistakes come from.

Accuracy claims need care too. A handwheel can place an axis to one least input increment, but that is not the same as part accuracy. Backlash, thermal growth, tool deflection and fixture repeatability still apply. Jog accuracy is a positioning tool, not a tolerance guarantee.

On a control with absolute encoders, handwheel jog is safe after a power cycle because the control knows where the axis is. On an incremental system, jogging before a re-home is how you lose the reference. Check the reference state before you turn the dial.

Finally, do not jog an axis that is clamped, or a rotary table with the brake engaged. The servo will either fault or slip. Release the clamp, confirm the axis is free, then move it.