Review - Indexable Toolholder
Large Size Indexable Toolholder on the MiniLathe - Not Recommended
[April 10, 2014]
[Added note: April 14, 2014]
NOTE: As a result of this experiment (and a related post on an online forum) I was able to get some good information on using carbide tooling on small lathes. For links to that information look here (click on the link and scroll down to "Indexable inserts"): https://sites.google.com/site/lagadoacademy/useful-links#info-other
For some time now I have been curious to see how well a larger indexable toolholder would work on the 7 x 11 minilathe. Please keep in mind that this review evaluates the tool specifically with respect to use on a minilathe (which is certainly not its intended use), and should not be read as saying anything negative about the tool in "normal" use.
With that in mind, I recently purchased the Dorian MTGNR08-2A indexable toolholder (shown in the photo to the left), with the benefit of a 20% discount offer. This is a 1/2" shank height toolholder, so it should fit my QCTP toolholders. I specifically wanted to get the largest size possible, in part to see if the larger size would be more stable while cutting, as well as to try out a different style of insert.
You can read the details of my initial use below, but if you just want the short version, here it is: I would not recommend this tool for use on small lathes. While it's a great tool in its own right, it's just too big to be fully useful on a small lathe.
The first thing I did (after fitting an insert into the tool), was to make sure it would fit my QCTP toolholder (see photo to the left).
So far so good!
But the first sign of trouble emerged when I tried to center the tool - I could not get it low enough! This is because the indexable insert sits higher than the shank. I could have milled out the QCTP toolholder a bit more to compensate, but I was concerned that this would substantially weaken the toolholder.
The only other option was to mill off part of the insert holder. Using a carbide endmill, I was able to mill off 0.075" from the bottom of the shank. I took it slow, never taking more than 0.010" depth of cut.
This worked better than I expected (see photo to the left). The tool shank still had plenty of "meat" on it, and I was now able to center the tool.
As a quick test, I did some facing cuts on O-1 drill rod and brass (360) - the results are shown to the left. I got a nice smooth finish, although I did get a bit of chatter with the drill rod until I got a better feel for the tool.
Now I wanted to tryout the tool on a real project. I had a boring bar holder which I picked up at an auction recently. It needed a bit of work to bring it into good working condition. In the photo to the left you can see the tool as I bought it. I don't know what the previous owner was doing with it, but the shank was pretty beat up (the picture actually makes it look better - it was really rough) , and there were some screws missing from the dovetail clamp. Aside from that it just needed a bit of cleanup.
By the way, that twisted bit of metal at the bottom of the picture was some hand ground bit of junk that had been used as a boring bar!
I disassembled the boring bar holder and mounted the top part in my four jaw chuck (with some aluminum shims to protect the tool), centered it, and drilled a center hole so I could support the right end with a live center while I turned the shank to clean it up.
That's when I ran into the second problem: The indexable toolholder was too bulky to fit in the space left by the center.
So I decided to take a chance and do the cuts anyway using just the four jaw chuck. In the end, it took just two .005" cuts (and a couple of final spring passes) to get past all of the dings.
I finished it off with a bit of 240 grit abrasive cloth.
Here's the final result (photo to the left) - a good tool salvaged and a nice addition to the shop.
In Closing
As I said in the introduction, this is a great tool in its own right, but it's just too big to be fully useful on a small lathe. The tool cannot be used out of the box; it has to be "cut down" before the cutter can be centered. The large size makes it a tight fit for a number of cutting situations, and a "won't fit" for some others. On the other hand, as modified it's now a useful tool for me, so I'll keep it in my collection and use it when appropriate.
On balance, however, as a prospective purchase for a small lathe, you would be better advised to spend your money on a smaller indexable toolholder.