Shaving a part is primarily done to ensure that proper tolerances on a part are maintained due to reasons such as head locking issues and cutting dynamics. Shops now expect tooling to produce error-proof machining, maintain step and lateral relativity, reduce grind stock and eliminate secondary operations.
One driver for shave tool use is a desire to produce very low surface finish requirements that previously dictated grinding, an expensive secondary operation. Given the proper shave tool, this finish requirement can be accomplished. Some shops that once used ground bar stock because they had to roll threads on the bar diameter now shave those diameters on standard bars. A massive and sturdy shave fixture can free up a position on the machine and eliminate the pre-grind operation. If secondary grinding is unavoidable, using shave tools to control the diameters of the multi-spindle produces grind stock removal that can be predictable, thereby reducing grind time, grinding wheel wear and material cost. Understanding the needs of the customer is the first priority.
And what about the part? What are the critical dimensions on it? What impact do these dimensions play on the overall effectiveness of the part in its final assembly? It is important to understand the historical problems encountered on a part to best conclude what and where to shave. Proper use of shaving technologies can reduce containment issues and resolve 8Ds, CPk, irreversible corrective actions, and zero defects / PPM woes. Properly applied shave tool technology will save money on the cost of the tool and improve the overall quality of the part. Once part considerations are analyzed, the application to the machine begins.
It has been said that even a good tool on a bad machine cannot make a good part. However, in most cases, a good shave tool on a bad machine can make an acceptable part. Profit margins are increasingly sliding downward due to competition, customer cost reduction initiatives, reduced labor skill, and rising material and operational cost because often maintenance on machines has been overlooked. An extensive evaluation of the machine is required to truly determine what type part you will make. Statically speaking, six sigma at 5 microns total tolerance will never be achieved on a sub-standard machine, even if you have the best shave tool available. Do not expect an old machine to hold tight total tolerances. Many of the multi-spindle machines being manufactured today are capable of holding ±0.002 inch tolerance with a form tool. This tolerance range used to require shaving on older machines. These newer machines can statistically hold 5 microns with the right shave tool. Six sigma results are no longer a dream.
The right shave fixture is important in determining shaving parameters. Customization of the shave fixture goes a long way in determining the amount of stock to remove, the length of the form being shaved, the aggressiveness of the shaving operation and achievable tolerances. Risking substandard shave systems can jeopardize intended results. Careful design elements can help exceed standard tooling parameters to help solve problems such as chatter. This is where experienced tool designers with knowledge of part and manufacturing specific information are invaluable.
Shave savings you might consider:
- Continuous Quality Improvement
- More Aggressive Cycle Times
- Elimination of Secondary Operations
- Reduction of Scrap on Older Machines
- Improved Efficiency
- Cosmetic Improvements
Every part has its own idiosyncrasies. Basic consideration should be given to material, depth of cut, roll diameter, speed and feed rates, and SPC dimensions. Cutting different materials demands geometry or stock removal tailored to the specific material. Some materials have a greater tendency to chatter if the old school 0.004 inch material per side is being removed due to the nature of the cutting action and chip reaction. The range in part diameters being shaved needs to be analyzed with regard to on which diameter the shave roll is placed. In extreme cases, a formed shave roll will pay dividends. Statistically critical dimensions on the part should have shave roll support to ensure adherence to SPC requirements. Relativity of multiple critical diameters is a function of the tool’s accuracy
The cutting profile of the tool ultimately controls the precision of the part dimensions. Tighter tolerances must be applied to the tool dimensions. Additionally, different part materials may require unusual geometry on the tool. In many cases, additional top rake is needed to achieve better part finishes—and in those the roll position may need to be altered. Hones, polishing and coatings have varying degrees of success on shave tools. Successful shaving hinges on the type of coating applied; the thickness of the coating; where the tool is polished; how much hone; and the material being cut. An aggressive cycle time modifies accepted machining norms. When processing a job, it is best to know the capabilities of the tool and fixture you intend to use.
Shaving Time, Saving Time
Cycle time saving processes can be achieved by implementing a shave tool. Fast spindle speeds and rapid feed rates in the forming positions often distort finishes and part dimensions, which can be corrected by using a shave tool. Shaving a part that has a taper will bring that part back into specs. Operator friendly fixtures with necessary adjustments minimize downtime. Time is money, and with the ability of most new ABMs to accelerate cycle times, shaving is usually the answer to producing a part more quickly with quality. Today, the process engineer is like a conductor of an orchestra. He or she must know the individual strengths of each component in the collective assembly of the machine. With proper direction, a multi-spindle machine can produce parts like a symphony. Shaving is the crescendo.