LocherInc
Plastic
- Joined
- Aug 1, 2011
- Location
- Lancaster, OH
Hey everyone. We're trying to get a little more active with the online community and this forum. We get a lot of questions about tapers as far as how accurate they need to be, how to tell if a spindle can be ground in-place, issues that can stem from a worn taper, etc. So I did a small write up with some general information for everyone. Please feel free to reach out if anyone has any questions. Take care.
Why Spindle Taper Maintenance is Important
Spindle tapers are typically an overlooked PM item on a machine tool. If there hasn’t been a wreck or clamping issues, there isn’t always a lot of thought about it, but the spindle taper is a wearing surface. Typically, what you will see as the taper wears is a condition called “Bell-Mouthing,” where the front (Major Diameter) side of the taper slowly opens up, costing you tool rigidity. A lot of times this won’t show up in just simply checking taper or tool runout. What bell-mouthing causes is tool instability due to the major supporting area being worn away. You’ll see shorter tool life, tool holder wear, and eventually it can get to the point where runout is no longer satisfactory or you’re having issues making cuts with the same kind of tool pressure. You’ll typically start to see fretting on the tool holders or taper surface. Fretting is a gold/copper colored pattern caused by high frequency vibration of the tool holder in the taper pulling metals to the surface. It can also indicate drawbar issues.
Where people can get into trouble is trying to address taper issues using hand tools like die grinders or other setups that aren’t designed to hold the types of tolerances required for a spindle taper. This happens a lot after a wreck to try and get the machine back into production. It is not recommended to use any kind of abrasives to try and polish the taper down. It’s a common misconception, but a spindle taper has the same tight manufacturing tolerances as the bearing journals on your spindle shaft. Being off even .010/.012mm or .0004-.0005” is completely out of spec. It requires the use of master plug gages or qualified air gages to hold the angle correctly. Incorrect angle, poor surface finish, or high spots can lead to premature wear of the taper and if too much material is removed, the spindle could be scrapped and need to be pulled for rebuild or replacement.
A correctly reground taper should not need to be ground yearly, unless under extreme cutting conditions, but periodic inspections should be done. These inspections include inspecting taper angle, measuring taper runout, inspecting runout using a precision test bar at the spindle face and 250-300mm from the spindle nose, measuring clamping pressure, and measuring gage line dimensions.
How do you measure “grind stock” or if a spindle even can be ground? A good base line measurement to use is “gage line.” This can be measured using a tool holder. Depth mic from the front side (part side) of the flange on the tool holder down to the spindle face. The amount you can grind and move this dimension will vary by machine and the drawbar setup, but a good rule of thumb is that you can be about -.015” before you’ll start to run into clamping pressure problems. That ultimate dimension will be determined by your clamping pressure. All machines utilizing CAT holders are going to have a .750” gage line. BT toolholders have a varying gage line and I’ve attached a .pdf with those dimensions. Machines utilizing Big Plus or other dual contact tool holders will again have a different gage line that will appear to be far minus as they are designed for a simultaneous fit. When grinding for dual contact, the face will need to also be ground to insure proper tool clearance. The amount of material removed should then be documented to track material removal. There are occasional outliers to these rules. When a spindle is rebuilt, some vendors or even OEM’s will not always grind and chrome plate the taper back to a standard gage line but set the drawbar up to work wherever the taper cleaned up. This isn’t a responsible practice as it can create issues with tool changers unless the machine was designed with that gage line in mind for clearance.
Why Spindle Taper Maintenance is Important
Spindle tapers are typically an overlooked PM item on a machine tool. If there hasn’t been a wreck or clamping issues, there isn’t always a lot of thought about it, but the spindle taper is a wearing surface. Typically, what you will see as the taper wears is a condition called “Bell-Mouthing,” where the front (Major Diameter) side of the taper slowly opens up, costing you tool rigidity. A lot of times this won’t show up in just simply checking taper or tool runout. What bell-mouthing causes is tool instability due to the major supporting area being worn away. You’ll see shorter tool life, tool holder wear, and eventually it can get to the point where runout is no longer satisfactory or you’re having issues making cuts with the same kind of tool pressure. You’ll typically start to see fretting on the tool holders or taper surface. Fretting is a gold/copper colored pattern caused by high frequency vibration of the tool holder in the taper pulling metals to the surface. It can also indicate drawbar issues.
Where people can get into trouble is trying to address taper issues using hand tools like die grinders or other setups that aren’t designed to hold the types of tolerances required for a spindle taper. This happens a lot after a wreck to try and get the machine back into production. It is not recommended to use any kind of abrasives to try and polish the taper down. It’s a common misconception, but a spindle taper has the same tight manufacturing tolerances as the bearing journals on your spindle shaft. Being off even .010/.012mm or .0004-.0005” is completely out of spec. It requires the use of master plug gages or qualified air gages to hold the angle correctly. Incorrect angle, poor surface finish, or high spots can lead to premature wear of the taper and if too much material is removed, the spindle could be scrapped and need to be pulled for rebuild or replacement.
A correctly reground taper should not need to be ground yearly, unless under extreme cutting conditions, but periodic inspections should be done. These inspections include inspecting taper angle, measuring taper runout, inspecting runout using a precision test bar at the spindle face and 250-300mm from the spindle nose, measuring clamping pressure, and measuring gage line dimensions.
How do you measure “grind stock” or if a spindle even can be ground? A good base line measurement to use is “gage line.” This can be measured using a tool holder. Depth mic from the front side (part side) of the flange on the tool holder down to the spindle face. The amount you can grind and move this dimension will vary by machine and the drawbar setup, but a good rule of thumb is that you can be about -.015” before you’ll start to run into clamping pressure problems. That ultimate dimension will be determined by your clamping pressure. All machines utilizing CAT holders are going to have a .750” gage line. BT toolholders have a varying gage line and I’ve attached a .pdf with those dimensions. Machines utilizing Big Plus or other dual contact tool holders will again have a different gage line that will appear to be far minus as they are designed for a simultaneous fit. When grinding for dual contact, the face will need to also be ground to insure proper tool clearance. The amount of material removed should then be documented to track material removal. There are occasional outliers to these rules. When a spindle is rebuilt, some vendors or even OEM’s will not always grind and chrome plate the taper back to a standard gage line but set the drawbar up to work wherever the taper cleaned up. This isn’t a responsible practice as it can create issues with tool changers unless the machine was designed with that gage line in mind for clearance.