Checking Drawbar force

It is critical that your PM (Preventive Maintenance) Program includes a regularly scheduled (at minimum every 90 days) inspection and check of the drawbar pulling force in your spindle. All machining centers use a drawbar (item #1) that has spring packs that pull the tool holder (item #2) back into the spindle. Only mechanical force is used to push the drawbar “out” to release a tool shank during tool changes. This pullback force created by the spring packs will wear or fatigue over time and the proper pullback force will weaken. This loss of pressure allows the shank of your tool holder to move “out” of the spindle socket with radial load. There is a min/max force point for every brand of machine and type of spindle.

Why a Drawbar Force Is Measured

Drawbar force gauges allow early detection of problems with the spindles’s drawbar spring package (solid springs or Belleville washer packs). Verification of the performance of the drawbar clamping system will help prevent damage to the spindle taper, other critical machine parts to maintain machine accuracy, tool holders, and ultimately the safety of the operator.

Drawbar force measurement has been increasing in importance over the past years with the introduction of different machine spindle tapers, and higher spindle RPM capability. The machines are are used in metal working applications, woodworking applications, plastic applications, and carbon fiber materials.High speed machining is considered when spindle speed exceed 10,00 RPM and go as high as 60,000 RPM today. The need for drawbar force routinely now becomes evident!

As spindle speed demand goes higher, the need to check the drawbar pull force regularly increases as well. Too many manufacturing shops do not even check the drawbar force until the damage is already done. If your machines run one shift per day and 5-6 days per week you should e checking (and tracking) your drawbar force at least every six months (it only takes 5-10 minutes per machine to check the force with the proper gauge) and recording the reading to monitor the force decease. If your machine is running 2-3 shifts and 6-7 days per week, you should be checking and recording your drawbar force every 8-12 weeks. Today’s machining centers are being “pushed” to the maximum on RPM and radial load to maintain high production rates. this increased RPM and radial load are putting high stress on the drawbar thus potentially causing “earlier than expected” spindle and spindle taper damage.

Steep taper machines use a retention knob style drawbar and early pressure loss detection can be adjusted with a simple maintenance procedure to prevent spindle taper and tool holder taper damage that can only be repaired by regrinding a spindle taper (expensive and lost machine down time).

HSK taper machines use a drawbar the has “finger” that pull the “short” taper into the spindle taper socket. Machines with HSK spindles and tooling tend to run at higher RPM speeds than steep taper and with the shorter taper engagement,drawbar force is even more critical due the shorter leverage point the taper offers.

Symptoms of low drawbar force:

1) Cutting tool chatter (loss of cutting edge tool life)

2) Poor machine part finishes from chatter

3) Tool holder taper and spindle taper “fretting” - little brown burn marks near the gauge line of a steep taper holder. These burn marks are from a high frequency vibration (rubbing) because the spindle taper and tool holder taper are not fully engaged due the low drawbar force. This fretting can only be removed on the spindle taper by regrinding the spindle taper. The tool holder is basically “scrap” because the damage can not be removed without changing the taper accuracy

4) TIR accuracy loss causing additional cutting tool offsets to machine a part tot the desired dimensions.

Proper Drawbar Force based on international standards (confirm specifics based on machine make and model):

Spindle Run-out Test Arbor Instructions

A spindle can be measured either at speed (dynamically) or statically. The static measurement with a spindle run-out test arbor is significantly cheaper and easier, yet somewhat less accurate than dynamic measurement which will take into account heat, vibration, and centrifugal forces. Measuring spindle TIR is quite straight-forward with a spindle run-out test arbor and dial test indicator with at least 0.0005 in. / 0.01 mm units (0.0001 in. / 0.001 mm is preferred).

Measuring Spindle Run-out with a Dial Test Indicator

1. To start, load the spindle run-out test arbor into the machine spindle. If you suspect tools are not loading properly, testing drawbar force with a Drawbar Pull-Force test gauge is recommended.

2. Once you are certain the spindle run-out test arbor is loaded properly and completely seated, position the tip of a dial test indicator as close as possible to the center line of the test arbor. We recommend testing at 3 points on the length of the arbor. Make a reading at about 1 inch (25mm) from the flange, another point at about the halfway point, and the final one about 1 inch (25mm) from the end of the bar. The final one will give you the best indication of any wobble.

3. Position the the indicator stand on the machine table so that there is about 0.015 inch (0.4mm) of preload indicated on the dial. We do not recommend the use of your machines "Jog" button since an accidental rapid movement that exceeds the range of the indicator may damage its internal mechanism.

4. Rotate the test arbor in the spindle until you see a reading of minimum deflection. Now rotate the dial face to "0" and align as closely as possible with the indicator needle. Exact positioning can be difficult so don’t spend time obsessing on it, just make a notation of the final setting. Rotate the test arbor and spindle slowly until you find the point of maximum deflection. Remember that your hand pressure may be an influence on the indicator so remove it when making readings.

5. All that remains to be done is to subtract the initial position reading from the reading for maximum position to get the TIR for this spindle.

Spindle run-out

One of the leading causes of tool breakage, part finish issues, tolerance control, and machine registration issues is excessive spindle run-out. No matter how accurate your cutting tool or holder are, if your spindle is out of alignment or has spindle bearing run-out, your part processing accuracy will only be as good and accurate as the machine allows you to control. The simple definition of spindle radial run-out is how much wobble a spindle produces at the nose. Axial run-out is the measurement of how much play there is perpendicular to the axis of rotation. This reading is represented by Total Indicated Run-out (TIR), which means the distance measured between the largest plus measurement and the lowest minus measurement for a total indicated amount.

Spindle alignment should be checked every 90 days or no less than twice per year regardless of machine usage. It also should be checked after any “crash” or Spindle/tool holder contact with fixtures, part, or any object not being machined. Using a high quality, high precision certified Test Arbor is critical. Any abnormal alignment or rotational error should be corrected as soon as it is detected to improve machining processes but also protect spindle and tooling from excessive wear.