Backlash and Your CNC - Things You Need to Know

( NOTE : This article references FANUC controls but is basically applicable to all CNC controls. )

A machine is a machine is a machine. Just because the words CNC are attached to your machine tool doesn't mean it doesn't get old or lose it's accuracy. And one of the main reasons your CNC machine losses it's accuracy is due to the ever infamous backlash.

What is backlash ?
The axis motion that makes up your machine tool is done through the use of ballscrews attached to your machining center's table and spindle housing or your lathes tool turret. The nut for the screw is usually attached to the table or turret and is connected to the ballscrew which is connected to your drive motor. As the motor turns the ballscrew, the nut moves the table or turret and your machine has motion. All ballscrew assemblies have some "slop" or backlash at assembly - the match between the screw and the nut. Basically backlash is the amount of motion the screw has to make when reversing direction before the nut and therefore the table or turret start to move.

How is backlash compensated?
Using the machine tools CNC controller, the builder can tell the controller how much motion is lost when the axis reverses direction due to the backlash. This value is stored in the machines parameters and when the particular axis goes to change direction, it looks in this parameter to know how much motion it needs to have (how many revolutions of the screw it needs to make) before the axis will physically start to move. The value of the parameter is usually in MM, although they may be in INCH settings in some instances

Why should I care ?
As the machine tool wears or as contaminants get onto the ballscrew and therefore in the nut, the original backlash settings lose their accuracy and therefore effect the accuracy of the machine tool. Positioning problems arise, straightness problems arise, as do a host of other related problems. Basically, the machine does not meet the specs like it did when it was new.

As mentioned above, sometimes contaminants can get onto the screw and then get carried into the nut. Although most nuts are protected against chips and debris, poor conditions can sometimes force the debris into the nut causing premature wearing of the screw and a pronounced backlash problem. Those contaminants can range from coolant to cutting chips. That is why it is essential to keep the machine areas clean and free from an excessive amount of chips. If chips are allowed to accumulate, they can become packed and when the machine tool moves, it forces the chips under guards and into areas where they shouldn't be. Eventually they get forced into the screws and nut areas causing un-repairable problems. Ballscrew replacement is not a cheap repair. Keep the expression: "An ounce of prevention is worth a pound or cure" in mind when planning your maintenance efforts.

What can I do about backlash ?
The normal method for adjusting the machine's backlash involves adjusting the backlash parameter values. This can be done by a qualified technician or you can give it a try. Outlined below is a brief but complete explanation of how to check for backlash and how to adjust it in FANUC controlled machine tools.

How often should you check it ? Recommended time frame would be about every 3-6 months. If you create the following sample programs in your memory and leave them there or upload and download them from a shop floor PC, you shouldn't spend much more than one hour or so keeping your machine accurate and at the same time you'll be checking for any other damaging problems. For example, if you see the backlash changing drastically, you might find a way lube problems or chip build up problem before they cause bigger problems.

How much backlash compensation is acceptable ? As mentioned above, all machines have some backlash adjustment, even when brand new and at ship time. As the machine wears, that value needs to be increased. Normal wear might have .005" - .010" adjustment in a ballscrew. If the value needs to be more than .010", it might be time to take a deeper look. Also, you need to check the backlash at various areas of the screw as it might be wearing more in one area than another. One example might be on a machining center where the set-up people always mount the vise or fixture in the middle of the table. Looks good but also causes a massive amount of wear in one confined area. the best scenario is to mount the vise or fixture all over the table, changing the location for every job - spreading the wear around evenly.

The best way to check the backlash is to first clear out the current parameter value in the control. The various parameter numbers for the variety of FANUC controls are listed further down in this page. First, write down the current values, then clear them by setting them to zero. Then make the machine move through the memory mode. We have found discrepancies in the past between the machine's handle or MPG mode and the memory mode, so we recommend you run the machine through MDI or through the machines memory mode. Below are a couple of sample programs for FANUC controls that you can use to gather your backlash data. Remember, the backlash is the amount of wasted motion when the particular axis changes direction.</p>
<p>If possible, check the backlash at different areas of the screw. On a machining center, mount the block in different areas of the table and check. On a lathe, check the backlash as various distances away from the chuck. If the values are different in the different areas, this could mean that the screw is worn in one place different than others. On a lathe, this tends to happen close to the chuck where the majority of the cutting is performed. You can's do much about to prevent it on a lathe but on a machining center, you can help yourself by mounting the chuck or fixture in various places on the table to allow for even wear. If you find big differences in the backlash in different areas, it may be too late and you may have to replace the screw.

Machining Center Backlash Adjusting Program.
If you have a Vertical or Horizontal machining center, the following program will give you an idea of how to create a program to test the backlash for each axis.

The following is a sample program for the X axis. Start the program with an indicator mounted to the spindle, touching a block mounted on the table, touching the right side of the block.

You can let the program run a couple of times to make certain that you get the same readings at the M00's in the program. The difference between Reading #1 and Reading #2 is the amount of backlash in your X axis.

You can use the same style program making changes as required to perform the same function for the other axis as well. Basically, you just want the machine to move one way then back, stop so you can and collect the indicator reading, then move the other way and back and collect that reading.

CNC Lathe Backlash Adjusting Program.
If you have a CNC lathe, the following program will give you an idea of how to create a program to test the backlash for each axis.

The following is a sample program for the Z axis. Start the program with an indicator mounted to the spindle or chuck, touching a block mounted on the turret or the tool turret itself, touching the spindle side of the block or turret.

Once you collect the value and know the backlash for your machine, you'll need to adjust the parameter values. Parameter values for FANUC controls are usually given in MM  values, without the use of decimal point. So, for example, a parameter value of 30, actually means .030 mm - the decimal point is imaginary and placed three places from the right. You can use the following conversion formula to change your backlash data to mm, then enter that value into appropriate parameter - don't forget to drop the decimal point and add any zeros as required.

MM = inch x 25.4

For reference, 1mm = .0394 in.

On a CNC lathe, the value can either be a radius or diameter value. Since there is no easy way to tell, input a radius value then re-run the test program. Adjust as necessary and make a note so next time you will know.

When you're done, you should re-run the particular axis program again to double check that you did the backlash adjustment correctly. When you re-run the program, you should see less than .0001" backlash.</p>

FANUC Backlash Parameter Numbers.

Listed below are the parameter numbers for the various FANUC control models. One note, lathe controls are T models whereas machining centers are M models.

FANUC Version 6T :

X Axis = Par # 115

Z Axis = Par # 116

FANUC Version 6M :

X Axis = Par # 115

Y Axis = Par # 116

Z Axis = Par # 117

4th Axis = Par # 118

FANUC Version 10/11/12T :

Par # 1851

Seperate line for each axis.

FANUC Version 10/11/12M :

Par # 1851

Seperate line for each axis.

FANUC Version 0T :

X Axis = Par # 535

Z Axis = Par # 536

FANUC Version 0M :

X Axis = Par # 535

Y Axis = Par # 536

Z Axis = Par # 537

4th Axis = Par # 538

FANUC Version 16/18/20T :

Par # 1851

Seperate line for each axis.

FANUC Version 16/18/20M :

Par # 1851

Seperate line for each axis.

NOTE : This 16/18/20 series of control can have a seperate backlash amount when moving at a feedrate and for moving at the rapid rate. This is an option - check with your machine tool builder. If this is the case, Parameter number 1851 is for feedrate and # 1852 is for rapid. You can use the programs above, just change from G00 to G01 and add a feedrate to test for the feedrate backlash amount.

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Your Way Lube System - Friend or Foe ?

Your CNC machine is equipped with an automatic oiler system. Great ! You won't have to think about oiling the machine and an alarm will tell you when the tank is dry. What a great device ? Right ?

Well, that is the design. Unfortunately, along with the "automatic" description of the system comes the "out of sight, out of mind" aspect of the system. Because many people know it is an automatic system, many people put it out of their minds and simply wait for the alarm to come up showing that the tank is empty and needs to be filled. But what if that alarm never comes on because the tank isn't empty ? Why wouldn't the tank be empty ?

As your machine gets older, the way lube system will require service just like any other mechanism. The main problem, which often gets overlooked, is that the "tank empty" alarm never comes on because the tank never drains and nobody ever notices it. Now your machine runs for months on end with no lubrication on the ways and when you finally notice a problem, it's too late. Here is the "Rest of the Story ..."

PROBLEM :  The machine's ways are not receiving any way lube oil.

SYMPTOMS - IN ORDER OF SEVERITY :

• Positioning / Repeatability Problems
• Axis makes noise when moving
• Axis Drive motor overload alarm coming when the axis is moving

POSSIBLE CAUSES :

• Way Lube Pump burned out.
• Way Lube Pump distribution flow set too low.
• Way Lube Pump filter CLOGGED
• Way Lube line BROKEN
• Metering Units are CLOGGED

POSSIBLE CAUSES EXPLAINED :

(1) Way Lube Pump Burned Out : If the way lube pump is burned out, obviously there will not be any lube getting into the system. These pumps are usually set using a timer system. There is basically two types of timer systems used :

• The pump is on a cam and the way it works is that the pump is always running. One gear turns another which acts like a step-down system and the second gear raises a "primer" lever. When the lever reaches the top of the stroke, the "primer" lever is released and the oil is pushed into the lines. This whole cycle can take 5-20 minutes meaning that even though the pump is always running, the lines get lube only every 5-20 minutes.
• How to Check It : Take a flashlight and look in the tank or remove the oil tank. Once looking inside, you can see the main gear that should be constantly moving. It may be at a very slow pace, but you will see it moving.

• The pump is set to an electrical timer set in the controls PC (programmable controller) or an actual physical electrical timer in the cabinet. This type of timer only supplies power to start the pump for every cycle.
• How to Check It : On some pumps there is no primer lever but a light comes on on the tank when the pump is activated. Make sure this light comes on every 5-20 minutes or some other sign comes on to show the pump is activated every 5-20 minutes.

(2) Way Lube Pump distribution flow set too low : As stated above, the way lube pump usually is set using a timer system. The flow amount that gets distributed into the lines during every cycle is usually set and adjusted at the pump with a manual setting mechanism. This type of adjusting mechanism is usually a knob that can be turned higher or lower to set more or less flow. Also, just look at the primer lever. During the mentioned 5-20 minute cycle, you should see the primer lever raise slowly and then start to drop after reaching the top of the cycle. Check the stroke of the lever - short stroke, less flow.

• How to Check It : The normal pump usage is in an 8 hour shift, you should fill the tank every 2-3 days. Also, you should see way lube flowing onto the ways. Always remember, the more flow the better. Yes, it may contaminate the coolant but that is better than ruining the ways and thus the machine just to save a couple of bucks.

The photo above shows a way lube pump unit which includes a manual flow control device. Adjusting the white knob adjusts the amount of lube being distributed per one cycle of the lube pump. When this type of pump is working correctly, you can see the white knob rising slowly then retracting, pushing the lube into the lines. The amount of rise and fall, and therefore the amount of lube distributed, is determined by the flow adjustment.

(3) Way Lube Pump filter CLOGGED : The way lube tank usually has a filter between the tank itself and the oil line that starts the distribution. This filter is usually in the tank itself at the bottom of the primer lever or in-line right after the main distribution line leaves the tank. It will get clogged over time, especially if there is no filter at the oil fill hole or if someone takes off the filter when filling the tank.

• How to Check It : Disconnect the main lube line where it exits the tank to feed the system or after the in-line filter if so equipped. When the cycle reaches the pump stage as outlined above, oil should flow through this connection. The flow should be strong at this point. If not, remove the oil tank and search out the filter or remove the in-line filter. They can often be cleaned with a cleaner but the best remedy is to replace it.

(4) Way Lube line BROKEN : Oftentimes a lube line in the system gets crimped or broken during machining or during service. These way lube systems are usually "pressurized" so to speak and if the pressure is released at one point, say at the broken line, the oil will flow all to that point, depriving all the other lines of fluid.

• How to Check It : When the pump is in the pumping stage, the primer lever should fall slowly. This is due to the fact that it is pushing the oil into the system. If a line is broken, the primer lever will fall quickly as all oil is funneled to the broken line area only. On systems without a primer lever, the pump may have a pressure gauge on the pump. During the pumping cycle, the pressure should register for a couple of seconds as the oil is pumped into the lines. If the pressure is low or does not come up at all during the pumping cycle, a line in the system may be broken.

(5) Metering Units are CLOGGED : In order to create the "pressure" of the system needed for even distribution, each oil line leads to a "metering unit" where the flow is lowered and the oil is discharged. When the pump forces oil into the lines, they all fill and flow to the metering units where the flow is stopped. Each metering unit is set to discharge the desired amount or "drops" of oil and perform their individual duties. Since some areas require more lube, the metering units can be different for each line or area. Since these metering units have actual valve type components in their very small bodies, over time these units can be become clogged or the inner workings can become stuck.

• How to Check It : This is a much harder area to check. The best remedy and prevention is to change these units every year as part of a yearly maintenance program. Because these units allow only drops to flow through, they are harder to see when troubleshooting. These metering units are usually located in "clumps" around the machine. Several lines lead to these central areas and lube lines are branched out from here to the various areas of the machine. Replacement metering valves should be obtained from the machine tool builder or dealer to insure that you are getting the correct replacement part. When changing these units, pay close attention to the flow arrow that is commonly marked on the units themselves. This arrow shows the direction of installation and flow. Check the original unit before removal and replace accordingly.

The photo above shows an example of some metering units. These individual fittings are usually located in one or two main terminal blocks that feed certain areas of the machine such as the axis and ball screws. As the system fills with pressure and lube, these fittings discharge the lube at their pre-set flow rate into their lube lines. Over time, like cholesterol in the arteries, these units become clogged and no longer allow lube to exit and thus deny vital areas of the machine the way lube they require. As part of a yearly maintenance program, metering units in the machine should be replaced as a precautionary measure.

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