Rear mounting a parting tool ... if you can

[mikey]

If I were going to try this from the front I would make a tool holder that holds the blade horizontal.

 

[tony a]

The geometry is different if upside down.. Assuming we are talking about a traditional parting blade and holder.
Or is it? The same?

View attachment 367128

Using a carbide parting tool which is orientated horizontally so can be inverted without changing the geometry.
Sure this setup isn't as rigid as solid tool post mounted on the cross slide but it proved to me that the concept of upside down parting works and is more forgiving on a lighter lathe.

 

[Janderso]

Could you remove the screw then drill and tap the bottom to mount it in a rear tool post upside down?

The tool height adjusting bolt?
Setting to the correct spindle height would be challenging, if I understand correctly.

If I were going to try this from the front I would make a tool holder that holds the blade horizontal.

Ah, you want elaborate a bit on this one??

 

[mikey]

Ah, you want elaborate a bit on this one??

Sure. The tool holder you showed angles the blade up and is a very common holder. It works because the angle provides adequate back rake to enable the tool to cut more freely without digging in, at least ideally. It requires at least 7-10 degrees of relief under the cutting edge to clear the work. The problem with this holder is that it is very unforgiving if you are even a tiny bit off center height - too high and you rub; too low and you dig in and chatter. I suspect a lot of problems some folks have when parting is because they are not setting center height accurately.

Now, put a parting tool upside down in this same holder and your cutting edge has zero support. As the tool meets the work the cutting edge will be burnished quickly and the cutting edge dulls fast. I'm not guessing at this because I tried it to see what would happen. I did use a HSS tool so maybe carbide might last longer but it didn't work well for me. Then again, a series of one is not a valid experiment so try it and see what happens; I am certainly willing and hoping to be wrong.

Now, a tool holder that holds the tool flat or horizontal is different. It has adequate support with a 7 degree relief angle ground into the tool and it will cut, of this I am sure. The problem is finding or making a tool holder to fit your blades.

 

[Cadillac STS]

The tool height adjusting bolt?
Setting to the correct spindle height would be challenging, if I understand correctly.

Ah, you want elaborate a bit on this one??

Sure. You would place a riser under the rear tool post. That way it can get right height.

If you make a dedicated cut off tool post it would need to be high to get the tool in the right place. The tool post in back is on the cross slide with no compound under it so need to make up that space.

 

[ddickey]

If I were going to try this from the front I would make a tool holder that holds the blade horizontal.

Yeah horizontal.

 

[ddickey]

Don't forget the Eccentric Engineering sells a nice parting blade holder.

 

[mikey]

Don't forget the Eccentric Engineering sells a nice parting blade holder.

Forgot about these - thanks for bring it up. The EE holder allows you to hold a standard parting blade upside down in a regular QCTP tool holder. It is well worth a look.

 

[Karl A]

...
Try to also include a ledge that registers on the edge of your cross slide. This not only prevents the tool from turning; it automatically aligns the blade perpendicular to the spindle centerline so you will never have to mess with this alignment ever again.

Would a ledge that registers against the rear edge of the cross slide, instead of the left edge, work as well? A tool designed to have a ledge-machined-in-situ-at-the-rear seems easier to make.

 

[jwmay]

Would a ledge that registers against the rear edge of the cross slide, instead of the left edge, work as well? A tool designed to have a ledge-machined-in-situ-at-the-rear seems easier to make.

I didn't write this thread, but I made my ledge to register against the side of the cross slide as recommended. I didn't really think about it at the time. Just following directions. Then when it came time to mount the tool post, I discovered one good reason to put it there. If I had registered off the back, there wouldn't have been enough cross slide travel to bring the cutter to center. Of course, as you're being a thinker about it, I'm sure you'll design that out. But thankfully for me, the side register saved my bacon.