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Sometimes a solution can be a problem. Or so is the story of backlash in CNC machining or machining in general.
Backlash is the space between adjoining gear teeth.
This space exists to allow the gear mesh without jamming.
And to allow lubrication.
So, the backlash is the solution to keep the machines from jamming.
The more the amount of backlash, the less likely the machine is to jam.
And, unfortunately, also the less likely the machine is to be error-free.
This space also causes the machine to be inaccurate and unresponsive, increasing wear as loose-fitting moving parts collide together.
When the gears are pushing to one side, there is a small clearance in the opposite direction.
When you push the gear to the other side, they have to travel this tiny space before they come in contact with touching the other side.
While machining, a lot of gears are working together to perform the functions.
So, while moving the machine one way would be fine.
Getting it to move in the other direction would be a bit delayed.
And thus, backlash causes inaccuracy and inefficiency.
Backlash is also present in screws, nut bolts, or any mechanical system where the driving member does not connect to the load.
What is Backlash in CNC Machining?
Some sort of backlash also exists in CNC machines, especially in converted CNC machines.
Backlash in CNC is especially alarming.
Even the most closed-loop system can’t detect if the axis hasn’t moved even though the motor has.
Generally speaking, backlash causes even CNC machines to perform poorly and inaccurately, especially the converted ones.
Besides the ‘clearance’ between gears, backlash can also be caused by loosening or deforming mechanical parts.
Most often than not, you can reduce backlash in CNC-converted machines by replacing lead screws with ball screws.
The fundamental difference between lead screws and ball screws is that a ball screw uses a ball bearing to eliminate friction, and lead screws do not.
Generally, ball screws are better suited for cnc operations. Ball screws allow smooth motion, efficiency, accuracy, precision, and prolonged continuous high-speed movement.
Lead screws, on the other hand, are made for more straightforward transfer applications that doesn’t require speed, accuracy, precision, efficiency, and rigidity.
All in all, ball screws are better suited for CNC machining as they are more reliable and accurate. (more on this later)
Compensating Backlash
Most manual machinists tend to compensate for backlash by covering up the ‘clearance’ on the handwheels.
All you need to do is make sure to feed enough in the direction you will be cutting to take up all the backlash.
Some machine control software such as Mach 3 performs the same routine.
However, a machine can only guess and is, therefore, less efficient in compensating for backlash.
Most software relies on the operator to tell them how much backlash is present.
And with a CNC machine, it is pretty difficult to gauge precisely how much backlash is present as it can vary as your lead screws wear or your machine needs repairing.
Compensating for backlash can be helpful with lathes as they have limited movement.
Some g-codes force the motion to compensate for the backlash if you are concerned about accuracy.
Finding the Source of Backlash
As we already mentioned, most likely, the majority of backlash is coming from the lead screws.
Lead screws have weaker nuts that tend to wear off faster than the screws.
Moreover, if you tighten a nut to eliminate backlash, it can cause too much friction.
There are other causes for backlash too. But lead screws are the most common.
Therefore, you must first consider replacing the lead screws in your machine to reduce backlash.
One of the most common alternatives for lead screws is ball screws.
The nut in the ball screw uses recirculating ball bearing, which helps in reducing friction and increasing tightening tolerance.
There are two types of ball screws: ground and rolled, with the latter being more accurate and backlash-free.
Some other sources of backlash can include:
- Motor drive mechanism
- Slideways and or Gibs
- Machine Flexure and Rigidity
Reducing Backlash
There are two ways you can reduce backlash in a CNC machine: software compensation and hardware compensation.
On the software side, you can use your machine’s control unit to adjust the amount of backlash compensation for the three axes.
On the hardware side, you can either install an anti-backlash nut assembly on the lead screws or replace the lead screws altogether with ball screws.
If you identify the backlash from other sources such as Gibs and Machine Flexure, try lubricating the parts to avoid unnecessary friction.
In fact, lubrication should be an inherent part of machining.
If possible, invest in an automatic oiler to lubricate all the parts of the machine in one go.
If the problem persists, you might have to have your machine checked out by a specialist.
Adjusting your gibs is a daunting task that is better left to experts. If not done properly, it can deal some irrevocable damage to your machine.
Same with machine flexure and rigidity issues.
Typically 0.005” to 0.007” backlash per axis is acceptable on a CNC machine.
You can use software and hardware compensation to reduce the backlash to zero. However, you should avoid being obsessed with completely eliminating it from your machine.
As we mentioned at the beginning of this article, a little backlash is essential to allow thermal expansion and contraction, debris extraction, and lubrication.
Be sure to keep checking your machine every 3-4 months to verify the anti-backlash system you installed is working correctly.
If you find any minor backlash increment, you can reduce it using the CNC control software.
Conclusion
CNC Conversion Plus has been designing and automating machines for the last 40 years. We have the expertise in converting manual machines into CNC as well as preparing modified CNC machines on demand.
When we design your CNC machines, we make sure to consider every little aspect of machining. So you get high-performance, smooth CNC machines that can serve you for years.
We make sure all our CNC machines are backlash-free and accurate, so you don’t have to go around making any sort of adjustments.
We prepare our machines with quality electronic and hardware components, giving you the optimum CNC machining experience.
If you are looking for CNC machines with optimized backlash and performance, think of CNC Conversion Plus.