Tuesday, October 9, 2007

The Lowly Parting Tool

I am going to focus a couple of posts on the care and proper use of the much maligned parting tool. Here is a turning tool that is most often abused, misused and treated as an necessity rather than a choice. The parting tool sets the diameters for all of the details in spindle turning but gets none of the glory of skewing a perfect surface or cutting a flowing cove. I have seen many turners grit their teeth while jamming a dull parting tool straight into the spinning piece, the vibration, noise , and yes, the smoke! confirms the lowly status of this tool.

I'll talk about the sharpening process later, now I'd like to pretend the tool is in top notch condition and focus on technique. Below, you see a 1/8" diamond parting tool (I am thrilled to find that Sorby still offers these). The diamond shape helps to ensure that the tool won't bind in the groove that it is cutting. The widest point of the tool is 1/8" which helps reduce vibration. As I've noted in earlier posts, vibration begets vibration, and by using the parting tool correctly, this initial rumbling can be avoided.

The key to cutting with the parting tool is to use a shearing cut as opposed to a scraping cut. This is best achieved by rubbing the bevel of the tool against the (fast) spinning work piece and slowly raising the back of the handle until the cutting edge makes contact. At this point a shaving will shoot along the top of the tool. Here is where many go wrong. Don't just continue lifting the handle. This will transition into a scraping cut as the diameter gets smaller. Instead, feed the tool into the cut by pushing forward. The result of this may be that the cutting edge lifts out of the wood and the tool will once again be rubbing on the bevel.
This is fine.
Once again raise the rear handle and engage the cutting edge. When done correctly, the rate of feeding the tool forward and the rate of raising the rear of the handle will keep the tool cutting high enough on the round to maintain a continuous shearing cut. Above is the correct position to hold the tool when cutting a large diameter. Below is the correct position when cutting a small diameter. Notice the different angle of the parting tool, but the relationship to the diameter is the same as far as the cutting edge is concerned. The skill is maintaining that correct angle as you cut from one diameter to the other.
It is definitely better to stop cutting by feeding the tool forward too much than to let the tool scrape. To make the whole process a bit tougher, wiggling a bit side to side as you cut will also help the kerf stay wide enough to reduce vibration.

Below is the scraping position, easy yes, but it will make it hard to love this tool. I have come to enjoy my parting tool now that I shear cut, grind often and give it that funny wiggle. Grinding is next.


DickL said...

I was initially taught to use a parting tool in the scraping mode. I later learned better (at least I think so). I have found that using a small scraping cut at first can eliminate some tearing. I'll be interested in your thoughts on sharpening.

Anonymous said...

Awesome! I know what i am doing wrong now. I am just starting to turn wood and my parting tool has emitted smoke before!