July 2009 - Countersinks and Chamfers
Pro Tips - July 2009
Countersinks and Chamfers
Last month's newsletter discussed bilateral vs unilateral tolerances. This month we are going to cover a fairly simple topic - countersinks, chamfers and the tolerances of these types of features.
Fig. A: Expensive countersink callout.
Even high quality countersink tools are notoriously unreliable on the dimension from their tip to X diameter along their cutting edge. This makes dialing in the tool more challenging the first time and after any tool replacement. Any additional time adds costs to the part. The higher tolerance that you need to hold the diameter to, the longer it will take to dial it in, and the more closely it will need to be watched while running the parts.
Fig. B: Diameter vs Depth relationship.
If the countersink is being used for a flathead screw, it might be possible to make the tolerance looser, possibly by removing a decimal place as shown in Figure C. In this example, the countersink tolerance with one decimal place is .5mm or approximately ±.020": a very generous and inexpensive tolerance. If you need to ensure the fastener head is not proud, then make the countersink a little deeper. To really ensure it is never proud, put that exact note on the drawing and specify the part number of the fastener that will be used. Having a "GO" gage is always a positive and quick way to check a feature.
Fig. C: Inexpensive countersink callout.
For machined chamfers that are not on holes, consider the use of the feature and tolerance it accordingly. Again, many engineers forget to loosen the tolerance and the shop is stuck dealing with a more challenging feature and overprocessing the manufacturing; ultimately, the customer is stuck paying for it.
Part of the Month
Every month we feature a really cool part that we have made recently. July's Part of the Month is this beautiful saxophone mouthpiece ligature. It is a bit of a pat on our own back, as this is a part of our own design and sold under our own Theo Wanne brand. Two months ago Pro CNC and Theo Wanne Classic Mouthpieces merged in order to seamlessly work together and make the worlds best saxophone mouthpieces and accessories.
Pro CNC Inc · 445 Sequoia Dr. Suite 113 · Bellingham, WA 98226
Inside the US, call 866-4PRO CNC. Outside the US, call 360-714-9000.