Cutting Tools for Precision Machining
“Precision machining” refers to maintaining tighter than normal geometric tolerances and superior surface finishes across a batch of parts. Not every machine shop is capable of achieving this level of performance. Others may have the CNC machine tools needed but don’t see it as something their target market requires.
At Impro we’re proud of our ability to provide precision machining services. We achieve this through investment in precision machining technology, skilled manufacturing engineering and CNC programming, and by focusing on the cutting tools. In this blog post we will address how the last of those, cutting tools, impacts precision machining.
How Precision Differs From Regular Machining
There’s no official definition of what constitutes “precision”. It depends on the parts being machined and their application, plus the materials and machine tools used. Machining specialists will probably agree that precision machining refers to tight tolerances and exceptional finishes, while leaving exact limits open for discussion.
Adding some quantification, these specialists will most likely agree that on linear and circular dimensions a tolerance tighter than 0.0004” (10 µm) requires precision machining. Likewise, flatness better than 2 µ” (0.05 µm) and roughness less than 1 µ” (0.025 µm) would count as precision. On very large parts though, such tolerances may be unachievable, while for very small parts, (those that are micromachined,) they are unhelpfully large.
An important aspect of precision is repeatability. A skilled machinist may be able to maintain tight tolerances on a single part, regardless of machine tool type, age and condition. However, doing so on a large batch would be extremely difficult and very slow.
The Cutting Tool Influence on Precision
Two aspects to consider are the cutting tool itself and how it’s mounted in the holder, cutter or tool post.
The main problem with cutting tools is that they wear and can chip. In a well set up cutting operation thermal expansion is rarely an issue because most of the heat is carried away in the chip.
Wear and chipping are addressed through choice of tool material. High Speed Steel (HSS) may be used for some machining operations, but most are performed with carbide inserts. Coatings on these reduce friction and wear. Titanium carbonitride coatings (TiCN) in particular offer good wear resistance.
Tool mounting affects precision in two ways. First there’s the deflection of the tool post or holder under cutting loads. The more rigid the mounting the less the tool tip will move, which leads to higher precision.
Second is the matter of tool presetting. This is the process of ensuring the tool tip protrudes from the holder by the expected amount. Any deviation from it will remove more or less material than the CNC programmer expected. (An alternative to very precise tool presetting is to determine tool offset values and enter these into the CNC machine controller.)
Presetting is especially important where more than one cutting insert is mounted in the toolholder. Large face mills are an example. Here, if one insert protrudes more than the others it will cut a series of grooves into the surface being machined.
Cutting Tools For Higher Precision
Two cutting tools that increase the accuracy and finish of features machined previously are grinding wheels and reamers. Grinding however, needs expensive machine tools and adds significantly to machine part cost. Reaming, done to improve the finish, roundness and size of a previously drilled hole, can be done on a CNC milling machine where it increases floor-to-floor time slightly.
Another approach is to use specially-manufactured cutting tools. This is sometimes done in turning where a stepped tool can produce two different diameters simultaneously. This results in much higher concentricity between the two diameters but does not increase operation time.
The same approach can be taken in cylindrical grinding by plunge grinding with a profiled wheel.
A Source For Your Precision Machining Needs
Precision machining – meaning maintaining tight tolerances and excellent surface finish throughout a batch of parts – requires close attention to every facet of the machining process. Cutting tool selection and preparation is part of that effort, and can have a deleterious impact on part quality if not conducted with enough care.
With modern, rigid CNC machine tools and deep machining expertise, Impro has the precision machining capabilities required by the most demanding customers. To learn how we could handle your machining needs, contact us today.