Monday, June 24, 2013

Recommended torque for jaw screws

Recommended torque for jaw screws should be applied while tightening the top jaws with master jaws.

Use screws of reputed make only. (Eg. TVS, Unbrako). Length of the screw should be as per GMT standards. Write to for a free electronic copy of GMT PHNC Instruction Manual.


                          SCREW SIZE                                                      TIGHTENING TORQUE

                                        M8                                                                         39 Nm

                                       M10                                                                        77 Nm

                                       M12                                                                       110 Nm

                                       M16                                                                       220 Nm

                                      M 20                                                                       400 Nm

Important Instructions:
If you use screws that are  longer than recommended size, the top jaw will not clamp with master jaw, as the clamping screw will rest at the bottom face of master jaw T-Slot. This will result in master jaw breakage.If the soft-jaws are purchased from outside source, ensure that the jaws meet GMT standards.

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Friday, June 21, 2013

Chuck Lubrication: What can go wrong if ignored?

What if you don’t lubricate as per schedule and don’t use the right lubricant?

 Improper lubrication results in;
  • Loss of Gripping Force
  • Component Slippage
  • Clogging, restricting lubricant flow and Jaw Stroke
  • Corrosion  

If the chuck is not lubricated regularly, the chuck body, wedge and the sliding parts will get corroded, resulting in high friction and loss of clamping force. The curve of the clamping force of a new lathe chuck, depending on the RPM, shows as a continuous curve. An improperly lubricated lathe chuck, shows a considerably reduced clamping force and shows as a dashed curve. This result in unsafe clamping of the work piece and the chances of falling out during machining is high. To avoid this, follow the lubrication and maintenance instruction provided in the chuck instruction manual. 

GMT recommends you to slide the master jaw through full stroke (clamping and de-clamping) to ensure uniform distribution of grease on sliding surfaces several times, without the component, after 500 jaw cycles. This helps to convey the lubricant to the sliding faces and maintain the clamping force longer.

For lubricating the lathe chucks, GMT recommends one of the following grease:
·         OKS  265            
·         ALTEMP  QNB50
·         MOLYKOTE  TP42

GMT chucks are equipped with several lubrication nipples for even application of grease. To ensure  even distribution of the grease inside the chuck, slide the master jaw through full stroke (clamping and de-clamping).

Remember, in extremely contaminated environment such as machining C.I work pieces a chuck can lose clamping force. In such cases, the complete chuck has to be disassembled, cleaned and lubricated. 

Chuck  with improper or no lubrication causes corrosion as shown in the photos below

                                                 CHUCK BODY                       BODY SLOT

                                                  WEDGE                                    BASE JAW

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Thursday, June 13, 2013

Chuck Lubrication: General Guidelines

Your lathe chuck is really a consumable. It wears with use. In addition, chuck wear results primarily from  lack of lubrication and improper usage and application. Burrs, coolant, chips, poor chucking practice all contribute to the problem. These can be overcome to a large extent by proper maintenance and lubrication. Poorly lubricated chucks wear out faster and become unsafe for use. Chucks can easily lose 50% of their normal gripping force and accuracy if not properly lubricated.

Frequency schedule for lubrication
Because of the widely differing job conditions, it is difficult to recommend a standard frequency schedule for lubrication. Lubrication once per shift with grease gun can be taken as a general standard norm.

Recommended Grease

All GMT Chucks are factory lubricated with GMT recommended grease and are ready to use.GMT chucks are provided with dust cover for better protection.

Under clean operating conditions, GMT chucks can be operated up to 10,000 cycles without dismantling & re-lubrication. To ensure extended service life and maximum performance ,every chuck should be dismantled ,cleaned and re-lubricated periodically.

Dismantling Frequency:
A new chuck must be dismantled after completion of  10,000 cycles.Then periodically after completion of every 20,000 to 25,000 cycles. If C.I castings are machined continuously , dismount once in a month regardless of no. of cycles. While applying grease by grease gun, slide the master jaw through full stroke (clamping and de-clamping) to ensure uniform distribution of grease on sliding surfaces.

Important Tip : The life of the chuck is considerably extended when it is well lubricated.

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