Recently, I saw a video by Halligan on his experience with negative rake lathe tools on his South Bend 9 lathe. I have one, but it is probably not in as good condition as his. Also, I managed to pick up a nut can containing several beat up inserts. Most of them appeared to be negative rake types, mostly TNMG 3 and 4 series. They were used, but TNMG inserts have 6 edges, so they have a lot of life. Indeed, most of the inserts only had one or two bad spots, and some were nearly new. There are a lot of warnings on the Internet that these negative rake inserts are not suitable for small home shop lathes. It seemed that Halligan could make good use of his, although his top end speed was limited. So, I decided to put a few of them to work. I started with the TNMG 332 inserts, since they are smaller. The tool holder can only take 3/8" maximum tools, and most of the Ebay offerings for this insert are at least 16 mm or 5/8". Most of the western manufactured tool holders are 3/4" or 1". This is too large for the small South Bend. If it was determined that these inserts could do some work, a new tool block could be fabricated that could hold a larger shanked tool. But, it seemed to be premature to build one and buy a tool if it was unsuitable for the South Bend 9 class of lathe.
An experiment was called for. A 3/8" square piece of steel was hacksawed at an angle to produce a top with the required inclinations to set side and top rake to 5 degrees. It was cut down as a step in the top of the bar so that the edge of the insert would be level with its top. This left a somewhat weak nose, but it would only have to work for a few cuts, if the lathe could handle it. A punch spotted a drill hole to anchor the insert. It was found that a standard #6 screw anchors the insert well, especially if the clearance hole is drilled a size small. This test rig was simple and quick to make, and it was also inexpensive. It was tested on a scrap seized automotive bolt, which included a little interrupted cutting. This was done at the highest RPM setting that the lathe has, which is about 750 rpm. The feed rate was about 0.004" per rev and a 0.010" depth of cut was tried. This worked well, although the finish was a little rough. The finish improved with higher depths like 0.020" or 0.030". The lathe started grunting and chattering at 0.035" depth of cut. This did not work well. But, there seems to be a decent range that the insert cut well. Lots of hot blue chips! They can be used! This lathe can get larger depths of cut with HSS, since the negative rake tool has more resistance and cutting pressure. But, I was delighted to see that I could use the inserts for something other than breaking them up to braze on small tools. Now to work on a more decent set up to use them. Of course, there are enough HSS tools to keep the lathe well stocked, but at least this is something interesting to do in the shop, and besides it gave some insight that added to what was on the Internet,
An experiment was called for. A 3/8" square piece of steel was hacksawed at an angle to produce a top with the required inclinations to set side and top rake to 5 degrees. It was cut down as a step in the top of the bar so that the edge of the insert would be level with its top. This left a somewhat weak nose, but it would only have to work for a few cuts, if the lathe could handle it. A punch spotted a drill hole to anchor the insert. It was found that a standard #6 screw anchors the insert well, especially if the clearance hole is drilled a size small. This test rig was simple and quick to make, and it was also inexpensive. It was tested on a scrap seized automotive bolt, which included a little interrupted cutting. This was done at the highest RPM setting that the lathe has, which is about 750 rpm. The feed rate was about 0.004" per rev and a 0.010" depth of cut was tried. This worked well, although the finish was a little rough. The finish improved with higher depths like 0.020" or 0.030". The lathe started grunting and chattering at 0.035" depth of cut. This did not work well. But, there seems to be a decent range that the insert cut well. Lots of hot blue chips! They can be used! This lathe can get larger depths of cut with HSS, since the negative rake tool has more resistance and cutting pressure. But, I was delighted to see that I could use the inserts for something other than breaking them up to braze on small tools. Now to work on a more decent set up to use them. Of course, there are enough HSS tools to keep the lathe well stocked, but at least this is something interesting to do in the shop, and besides it gave some insight that added to what was on the Internet,