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This is the rear-mounted parting tool holder for my Sherline lathe. It’s based on a design that’s been around for decades and I made it about 12 years ago, before Sherline offered one for sale. It is just a block of 6061-T6 aluminum and it cost almost nothing to make. Further on you’ll find drawings that will help you make your own. The design could probably be adapted for 7×10/12 mini-lathes, which are notoriously bad at parting-off.
Yeah, I know its beat up and ugly but it is extremely solid and functions perfectly. As you know, a rear mounted parting tool pushes the carriage down instead of raising it up so rigidity is markedly enhanced. My parting tool does not chatter, dig in, or deflect. It cuts cleanly in all materials I have used it on, at speeds averaging 2-3 times normal turning speeds.
The main features of the tool holder are
It accepts all 1/2-inch tall P-type blades, mounted upside down.
The bottom of the slot is on the exact centerline of my particular lathe. This allows the tool to be extended however far I need while still remaining at centerline. Canted designs do not do this. The Sherline OEM tool holder may not be on the centerline either so a shop-made one is a better option.
There is a ledge on the bottom of the tool holder that automatically aligns the tool perpendicular to the work. A single screw locks the tool to the carriage in a few seconds. The ledge eliminates the possibility of movement, even under very heavy loads.
Inverting the parting tool does several things
Improves geometry. Instead of being a zero-rake cutter an inverted tool has back rake, which assumes the importance of side rake on a turning tool. This not only reduces cutting forces but also greatly improves chip ejection from the cut and thereby also reduces cutting temperatures. Furthermore, the 5-degree side rake on a P-type tool also narrows the chip, improving chip ejection even more.
Improved oiling. On an upright parting tool the chips are piling up on top of the blade and carry much of the cutting fluid away before it even gets to the tip. With an inverted tool the oil gets to the tip first, further reducing cutting temperatures and improving both accuracy and finishes because the cut is no longer dry.
So, you have greatly increased rigidity, reduced cutting forces and cutting temperatures, improved oiling, and the ability to cut at higher speeds that leads to improved accuracy and finishes. With all of this I cannot imagine why a guy wouldn’t rear-mount a parting tool!
I also believe the improved geometry allows a blade to cut a larger work piece than expected. A P1N blade is usually used for work up to 3/4-inch OD but I use mine on work up to 1-1/2-inch OD, double what you would expect to be able to cut. I do alter the angle at the nose of the tool; I use 7 degrees instead of 5 or 10. I have found edge life to be greatly improved, while still clearing chips easily.
Here is an example of a cut made in 12L14 mild steel. The OD is 1-1/4-inch at the cut. Depth of the grooving cut is 5/16-inch and I made two cuts side by side to allow my turning tool to fit in there. Speed is 1200 RPM and I purposely extended the blade about 5 times more than needed for this depth of cut to see if I could induce chatter – there was none – note the finish inside the groove. This part was later parted off about an inch from the chuck at the same speed and came off cleanly. I assure you that I cut like this all the time, in all sorts of materials.I set cutting speeds based on how the feed feels. The tool should cut freely and easily but allow me to feel the tip in contact with the work while feeding at a pace I can keep up with comfortably. Because the tool cuts so well there is little fear of digging in or chattering so speeds and feeds are much higher than you would expect. Continue reading Rear-mounted Parting Tool Holder for Sherline Lathe
I’m a believer. As a new guy I totally sucked at grinding lathe tools. It’s almost painful to admit how many stubby, misshapen, multi-faceted, overheated and just plain ugly lathe tools I made back then. The amazing thing is that some of those tools actually worked as well as the pre-ground tools that came with my lathe. I started to believe those guys that tell you, “… just get it close and it will work.” Of course, I was too embarrassed to call myself a hobby machinist with those Franken-tools so I bought an expensive set of inserted tip carbide tools that I thought would make a major difference but was disappointed. They couldn’t rough as deep or finish as well as my ugly high speed steel (HSS) tools, at least not on my lathe, so then I believed those guys that tell you, “… stick with HSS on a hobby lathe.” Hey, desperation can make you mighty receptive, you know.
Fast forward 15 years and now I believe, after having ground many experimental tools, that the best lathe tool for a hobby-class lathe is a HSS tool with its tip geometry modified to reduce the cutting forces it produces, ground on a belt sander.
Since standard tools are intended for use on industrial lathes their geometry can produce cutting forces that are excessive at times, especially when roughing but this can also affect sizing and finishing cuts. To be clear, cutting force is that force produced by the tip geometry of the tool that must be overcome to make a cut. We can look at it as a continuous resistance generated by the shape of the tool as it is pushed through the material during the cut. A standard HSS tool has a broad, wedge shape and creates a lot of resistance (carbide is even worse), so we must dial down our control inputs (depths of cut, feeds and speeds) to use them. If we alter the tool so it has a narrower included angle it cuts with less resistance, thus lowering the rigidity and power needed to make a given cut.We still have to manage our control inputs but since the cutting forces are lower we would expect to be able to cut deeper, achieve better accuracy in sizing and finishing cuts, and finish better with greater ease before running into the rigidity and power limits of the lathe. And this is exactly what happens.
To demonstrate this I took a 0.050” deep cut in 12L14 mild steel on my admittedly older hobby-class manual mini-lathe using a very good quality inserted tip carbide roughing tool with a new insert, a sharp freshly ground high speed steel (HSS) roughing tool with standard tip geometry ground for steel, and the general purpose tool with modified tip geometry (not optimized for steel) that was ground for this discussion.
The carbide tool made the cut but chatter was excessive. Speed was as low as I could reasonably go and the feeding force was very high. The finish is really ugly due to all the chatter and the chips are tiny and powdery. Reducing the depth of cut to 0.010, the proper depth of cut for this insert, allowed it to cut as it should.
The HSS roughing tool did fair but there was a lot of chatter. Speeds and feeds had to be adjusted almost constantly to make it this far. Finish is rough but not excessive for a roughing cut. Chips are tightly curled due to the standard side rake. Reducing the cut to 0.030” allowed the tool to work much better.
The modified tool cut easily, speed was about 100 RPM higher than with the other tools and there was no chatter at all. Finish is much better compared to the other tools. The chips look more like loose shavings due to the sharper included angle at the tip.
Chips from the carbide tool, HSS rouging tool and the modified tool are distinctly different. You absolutely cannot fake a chip. They tell you exactly what is going on at the tip of the tool.
It’s not obvious at first but this video is about the Arthur R. Warner Company’s HSS indexable inserts. After seeing it I’ve decided that I would really like to give them a try. I know you can get a better finish with HSS bits but I don’t like having to stop to sharpen them. So I use carbide inserts. I get a pretty good finish with them and I can cut with a tip for hours before it wears out and I have to rotate the insert to a new one.
It looks like the Warner Company’s inserts give you the advantages of HSS along with quick and easy sharpening. When a tip on the insert gets dull you can rotate to a new one in seconds without losing your lathe bit’s position. And when all 3 tips become dull you can sharpen them by rubbing the top surface of the insert on a whetstone for a few seconds (see video at 2:30).
Unfortunately, they don’t seem to sell inserts that will fit my tool holders (TCMT32.52 for my 8×12 and TCMT 21.51 for my 7×12). So if I want to give HSS inserts a try I’ll have to buy a 5 bar turning kit for about $128 or somehow find the time to make a bar that will fit their inserts. The price of their kit is considerably more than what I paid for my imported insert holders but it seems reasonable for a quality American made product. Has anyone bought one or tried their inserts?
I think this is a brilliant idea. “Snub” made a ball turning tool and used a 4-jaw chuck as the base. He says it works perfectly and leaves an almost perfect finish. I don’t doubt him because of the weight and rigidity of the chuck and the large bearing he used as a pivot. His design also looks like it is quick and easy to make. Notice that he didn’t make a holder for the carbide insert. Instead took an existing holder and drilled it so he could bolt it to a piece of steel.
Unfortunately, you can’t use Snub’s idea if you have a small lathe. It would definitely be too tall for a 7x mini-lathe, and I don’t think it will work on my 8×12 even if I use a shorter 3-jaw chuck and somehow directly mount it on the ways.