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Help please

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vtsteam:
I dunno, not super experienced with insert tooling, but have been using them more the last 6 months. "No chip breaker" in the one you referenced seems like it might need a lot of power and a stiff machine. All mine have chip breakers.

Lately I've been using a lot of DCMT070208 inserts for both cast iron and tough hot rolled steel. My present homemade lathe is stiffer probably than the Pultra. But I think I've also used them on the Gingery with okay results. They do have a fairly big tip radius.

However, there is no doubt in my mind that traditional lathe tool steel tools, properly sharpened, will be sharper than carbides (at least the ones I've used), and thus require less turning power and be easier on a smaller lathe. Properly sharpened is the key however The angles and the radiused tip have to be right and the honing fine. They won't stay sharp as long as carbides, especially if there is scale or casting surface skin, but they definitely will start sharper.

I also suspect the bolt steel itself as you and John mentioned will be harder to do a clean job on, but the fact that you can get a good finish on it with a graver seems to indicate it should be possible with conventional lathe tooling.

Dell:
Here is a picture of my setup but please bear in mind that I have set it up with the finished product, as I turned it down prior to milling the square, the second picture is the HSS bits I used and the carbide were the same pattern.
Dell
 

Dell:
 

vtsteam:
Tailstock:

Looks like you have a long extensions(s?) on the tailstock. That live center looks out of proportion. Try a short dead center for more stiffness. Keep the tailstock extension at a minimum and locked.

Steady rest:

That steady looks very light, and is located fairly far from the cutting location on the workpiece. That circular base makes zero sense to me. I'd make a new steady (they are not hard to make) that is solid, and keep it close to where you're cutting.

Workpiece: I assume you weren't cutting the thin end first as shown here, but did that after turning the midsection. Part stiffness and order of operations is important. But you did note that you had just set this up with a finished part.

Insert:

I can't tell anything about the insert from the pic (all pics are quite small). But again, I'd suggest using a regular hand-ground lathe tool of the largest cross section your toolholder can take. Probably 1/4" but if it can take 3/8", all the better for stiffness.

Toolholder:

That toolholder puts the location of the point of the tool far from the center of the toolpost. In other words produces a long overhang. I don't know if that is original to the Pultra. I'm guessing not.

Very small lathes are not improved in stiffness by miniaturized typical quick change tool posts. These tend to reduce lathe stiffness and increase twisting stress on the carriage and slides compared to simpler traditional toolholders. They are really suited to larger lathes.

I think an ideal toolholder system for a small lathe is the "Rose" type, which appeared quite awhile back in Model Engineer. Invented by a Dr. Rose who actually lived in the New Orleans in the US. These are very simple to make, and have the greatest stiffness possible on a small lathe.

Many of the modern "must haves" for lathes: insert tooling, quick change tool posts, live centers, and three jaw chucks are counterproductive for small lathes where stiffness is the greatest concern. Faceplates and collets, HSS tools, dead centers, and a simple short traditional tool holding system work better

The name of the game here is solidity, stiffness, short overhangs everywhere, sharp tools, and proper feeds and speeds.

Muzzerboy:
As noted above, these inserts have zero top rake angle, which is the last thing you need on a small machine. If anything, you can afford to run more top rake than on a "normal" machine, perhaps as much as 30 degrees or more. This results in lower forces and cleaner cutting. Even a large machine would struggle to get a decent finish in high tensile steel with a zero angle.

Personally I don't understand the logic of using HSS on indexable inserts when there are far better carbide inserts available. I'm also at a loss to understand how LMS can charge $11 per insert - but I guess if you don't ask you don't get.

For non-interrupted machining like this, you should try a CCGT060202 insert or similar (that's the metric nomenclature but the first 4 letters should be the same in the imperial system). The "G" indicates a higher tolerance which in practice means a polished (sharp!!) edge and the last digit indicates the tip radius - a smaller radius will result in lower axial and radial forces which can be helpful on a small machine. These are often described as "aluminum cutting" but they are also sold for "finishing" or "semi-finishing" in steel.

The more common CCMT inserts have a noticeably duller edge, which is a tradeoff between robustness and sharpness but makes life more tricky on a smaller machine.

If this doesn't work for you, grind some HSS tools with plenty of top rake and make sure they are razor sharp.

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