Rigidity, backlash and runout
Rigidity is how much the machine flexes under cutting force. Backlash is lost motion when an axis reverses. Runout is how much the bit wobbles off true. All three show up as poor finish and wrong dimensions, and all three are measurable in your own shop.
Rigidity
Every machine flexes under load. The question is how much, and whether it flexes enough to show in the work. Cutting force pushes the tool sideways; the frame, the gantry, the Z assembly, the router mount and the bit all give a little, and the sum is the error in your part.
| Where it flexes | Typical worst offender |
|---|---|
| The frame | Light extrusion with few or poorly torqued fasteners |
| The gantry | A long unsupported span, especially on wide machines |
| The Z assembly | Often the weakest link on hobby machines |
| The router mount | A clamp gripping a round router body |
| The bit | Excessive stickout, which is free to fix |
| The workpiece | Thin or poorly supported stock flexing away from the cut |
Symptoms of insufficient rigidity: walls that taper, dimensions that vary with cut direction, chatter, and a finish that is good on light cuts and poor on heavy ones. The response is to reduce depth of cut and engagement, not to slow the feed.
Backlash
Lost motion when an axis reverses direction. The motor turns, but the tool does not move until the slack is taken up. Causes: loose belts, worn leadscrew nuts, a loose coupler on a motor shaft, a pinion with too much play, or a gantry that is not properly preloaded.
Measuring it
- Mount a dial indicator so it reads against a fixed point on the moving axis.
- Jog in one direction well past the indicator's range, then come back to touch it and zero it.
- Jog further in the same direction by a known amount, then reverse by the same amount.
- The difference between the indicator reading and zero is your backlash.
Without an indicator, a cut test works: cut a large square, measure it, then cut the same square approaching from the opposite direction, and compare. Different results mean backlash.
Why it matters more than the number suggests
- Circles come out with flats where the axis reverses.
- Lettering and fine detail look wobbly, because they reverse direction constantly.
- Climb cutting becomes risky, because the tool can pull into the slack.
- Dimensions depend on which direction the tool approached from.
Software backlash compensation exists on some controllers. It is a patch, not a fix: it helps with steady state error and does nothing for the sloppy feel or the climb cutting risk. Fix the mechanical cause where you can.
Runout
How much the bit wobbles off the true axis of rotation, measured as total indicated runout with a dial indicator against the shank.
| Source | Notes |
|---|---|
| Collet | The most common cause. Worn, dirty or damaged collets are cheap to replace. See collets. |
| The bit itself | Cheap bits can be ground off centre |
| Spindle bearings | Real but less common. If everything else is clean and it still wobbles, this is next. |
| Debris in the collet | A single chip is enough. Clean every tool change. |
| A dropped or bent bit | Carbide does not bend, it chips, but the shank can be damaged |
Effects: the cut is wider than the bit diameter, one flute does more work than the other so it dulls faster, the surface finish suffers, and small bits break more often.
Drive types
| Drive | Backlash | Speed | Notes |
|---|---|---|---|
| Belt | Low if properly tensioned, and it stretches over time | Fast | Common on hobby machines. Check tension periodically. |
| Lead screw | Depends entirely on the nut. Anti-backlash nuts help. | Slower | Simple and cheap. Nuts wear. |
| Ball screw | Very low | Fast | The best of the common options, and the most expensive |
| Rack and pinion | Depends on preload | Fast, and it scales to long axes | Common on large machines where screws would be impractical |
The routine maintenance that keeps it
- Check and retighten frame fasteners periodically. They loosen with vibration.
- Check belt tension, and look for wear at the teeth.
- Check for play in V-wheels and adjust eccentric nuts to remove it without binding.
- Keep rails and screws clean. Dust packed into a lead screw is abrasive.
- Check motor couplers for loose grub screws. A slipping coupler looks exactly like lost steps.