By Mike Smith
Generating the perfect hole
In the world of manufacturing, a hole isn’t just a hole because every shop addresses holemaking differently. One company may be machining material with special demands, while another is applying a custom designed manufacturing process. There is, however, one common challenge every shop faces with holemaking: generating a perfect hole is nearly impossible without the right finishing tools.
To effectively meet their customers’ holemaking requirements, shops must utilize precision finish tools, such as fine boring heads and reamers, to follow the drill operation and complete the job. While both fine boring heads and reamers can finish a hole, the design of each tool can have significant advantages over the other depending on the requirements of the finished component.
However, selecting the right technique and cutting tools for a specific holemaking operation can sometimes be difficult. Manufacturers must consider hole diameter and length, interruption within the hole due to internal cavities and the required straightness, size, tolerance and surface finish of the hole. Plus, having the right machine tool to get the job done properly is important.
Finish boring is ideal for short-run jobs and situations where more stock is remaining in the hole. When compared to reaming, finish boring is more flexible when machining different hole sizes using the same tool. Finish boring is also more forgiving than reaming if a shop is using a machine tool with a lot of runout at the spindle as an adjustment of the boring head can compensate for such runout. Additionally, precise hole positioning is less critical than in reaming because if a drill does not drill the hole straight, a finish boring head will true up the hole.
A finish boring head.
In terms of hole quality, finish boring heads typically achieve tolerances within IT5 and surface finishes above Ra 1 micron. The main drawback to a finish boring head is feed time as it only has one tooth for cutting. Also, with one tooth doing the cutting, tool life is generally less than with reamers. Once the insert wears, size and finish can diminish quickly.
Reaming works best with pre-machined holes that have small amounts of stock. When compared to finish boring, a reamer holds tighter tolerances for longer periods of time because the cutting is generally spread out over multiple flutes. Such characteristics also allow for faster feeds over finish boring heads.
However, when it comes to reaming, it is important to understand not all reamers are the same. Manufacturers must determine if they are machining a blind or through hole as the geometry of the reamer and the method of applying the coolant will be different in each situation. In blind hole applications, the coolant must come from the front of the reamer to flush the chips back out of the hole. For through hole operations, the coolant should originate from slightly behind the tip of the reamer to flush the chips forward out of the hole. In fact, coolant delivery is one of the most important aspects of the reaming operation. Coolant needs to have a high amount of lubricity, especially when using a padded reamer. High-pressure coolant is also important in chip evacuation. For example, if a shop is reaming a deep hole but only has external coolant and/or low-pressure coolant, this can cause poor chip evacuation and ultimately result in shortened tool life and a bad surface finish.
Furthermore, stock allowance is an important aspect of reaming. Too much stock allowance can result in chattering upon entry of the hole as this extra stock dramatically increases tool pressure. But even if the reamer does not chatter from the extra stock, the cutting area for a reamer is quite small, and the extra material passing over the cutting zone will quickly wear it away and decrease tool life. For example, in steel applications, it is a good rule of thumb to leave only 0.1524 to 0.2032 mm (0.006 to 0.008 in.) total stock for the reamer. On the flip side, when there is insufficient stock, the reamer will push instead of cut. There also may not be enough stock to clean up the hole completely from the drill operation.
Runout must also be taken into consideration because it can wreak havoc on the reaming process (as well as any other machining process). It’s important to have a newer, accurate machine tool as well as high accuracy toolholders, such as hydraulic or shrink fit holders, to help combat runout. When you are dealing with tenths or microns the process must be secure.
Furthermore, several manufacturers like solid reamers because they are easy to use. However, a coated solid reamer is only good for a certain hole tolerance – anything below an H7 tolerance generally requires an adjustable reamer. Adjustable reamers are very accurate but require setting to achieve high accuracy.
Because finish boring and reaming approach holemaking differently, there might be situations where shops need to apply both processes to successfully complete a project. In the past, Seco assisted a gun component manufacturer in a holemaking application that required true positioning and very tight tolerances. The manufacturer attained the tight tolerances through reaming, but first had to use boring to straighten the holes before the reamer could accurately do its job.
In another example, a customer was losing valuable productivity time by utilizing a finish boring head to machine 125 holes in each of the large plates that it was producing at high volumes. These holes had very tight tolerances, requiring an adjustment of the finish boring head after every seven holes due to insert wear. It turned out reaming was a better solution because the customer could feed a reamer five times faster than a boring head due to the reamer having multiple flutes. The finish boring head would have been a suitable option if the customer were only machining five holes in a 10-piece job. SMT
Mike Smith is product manager for Seco, Reaming and EPB.