by Timothy Fara
Are you centered with your machining centre processes?
Machining centre manufacturers spend effort, design, and cost to make sure the spindle in the machine will operate on centreline. Spindle rotational accuracy as well as spindle alignment play an important role in cutting tool performance and productivity.
Cutting tool manufacturers use the latest materials and grinding equipment to assure the cutting edges are uniform in location, size, and tolerance to the centreline. This assures the tool will rotate on the centreline.
With these issues getting so much attention and focus, why do we pay so little attention to the accuracy of our toolholder and tool clamping products and the maintenance of the spindle rotational accuracy? The tool “connection” between the machine tool spindle and the cutting tool is as important as the maintenance of the jet engine. If airlines did not maintain the engines regularly, flying would be a lot more dangerous and expensive.
Studies have shown that for every .0001 in. the cutting edge runs out on TIR, the cutting tool edge life is reduced by an average of 10 per cent. This means a $100 cutting tool operates as a $90 one with just .0001 TIR and as a $50 tool with a .0005 TIR.
So how do you maintain the centreline of your manufacturing processes?
Spindle taper maintenance and care. On a daily basis, use a spindle wiper to ensure your spindle taper is free from fine metals caused in the machining process and passed through the coolant onto the toolholder and machine spindle during tool changes. The fines can embed in the spindle taper as well as the coolant and tram oil can be “squeezed” into a thin sticky layer holding the fine metal particles. This causes excessive toolholder shank wear and potentially excessive spindle taper wear leading to excessive TIR in the connection.
Spindle alignment. Since all spindles rotate and most are on a rotational axis, alignment in reference to the centreline of rotation is critical to cutting tool location “away” from the spindle face. Axial runout and the spindle rotational axis in reference to your table/fixture, determines the runout of your cutting tool edge and this determines your cutting edge tool life even before you start machining. Spindle alignment test arbors used in your PM (preventative maintenance) program will make sure your spindle alignment, as well as your spindle rotational accuracy is maintained. Test arbors will even help indicate when your spindle bearings are showing excessive wear and run-out.
The spindle drawbar. Maintaining this within factory specifications is critical to the life of spindle bearings, spindle taper surface, and toolholder taper accuracy. If a drawbar fails to maintain the proper load on the connection, metal removal rates are reduced due to chatter, cutting tool movement, and cutting tool edge location based on programming location. In addition, vibration between the “loose connection” and the taper interface will cause excessive spindle taper wear, fretting (a burning of the material surface due to high frequency vibration between surfaces), which damages spindle and tool taper material integrity. Pull force gauges should be part of your regular PM program to ensure that the machine manufacturer’s drawbar tension force is maintained.
Toolholder selection is important. Poor quality (low TIR accuracy tool holders) can push your spindle rotation TIR to .0005 -.0009 or higher, costing you valuable tool edge life and production rates. With the evolution of the tool holders from end mill holders through collet chucks to today’s shrink fit technology; the TIR has improved with every evolution/innovation of the tool holders in the market.
Maintenance of the toolholder system is critical to the accuracy of the assembly as well. What impacts the maintenance of a tool holder?
Proper torquing of the collet nut to properly grip the cutting tool shank in the collet. Under torquing the collet nut will allow the collet to grip the cutting tool shank with less strength and machining dynamics will cause the cutting tool to move in the assembly. Over torquing the collet nut will cause the collet to “twist” thus causing axial run-out in the tool assembly and collet fatigue reducing the clamping strength of the collet over time.
Ensure proper cleanliness when assembling the tool clamping system–removal of all chips and grime from tool taper and ID taper before assembly
Retention stud installation is important because improper torquing of the retention stud will cause distortion of the tool taper leading to taper contact reduction and axial run-out growth.
Handling of the tools from storage to machine carousel determine how well the tool taper is maintained. Improper handling of the tools allows the tapers to get knicked, dented, or scratched, which destroys the taper surface.
Two important facts to consider:
Average toolholder shank accuracy is three to five years before the taper is beyond standard manufacturing tolerances from wear and every day use. This means that the taper will have excessive TIR in it resulting in loss connection TIR.
Putting “used” toolholders on new machine spindles
reduces the life of the new machine spindle taper surface by 50 per cent because you introduce “poor” quality connections to a new connection accelerating the wear on the new machine spindle surface.
You can get a lot more out of your machine and machining processes if you are centered with your machining centre processes. This is where quality and price determine the outcome of machining performance and machining costs. SMT
Timothy Fara is managing director of Bilz Tool Co. Inc.