Rear mounting a parting tool ... if you can

[7milesup]

So, my cross slide has no t-slots running across. Is there a way to make this parting method work in such a case? The answer is probably obvious but it's late here and I'm pooped.

 

[mikey]

For the new guys following along who may want to try a tool post like this, please keep in mind that the bottom of the slot that holds your parting tool MUST be on the centerline of your lathe spindle. So how do you determine that number accurately, and how accurate do you need to be?

The simplest way to determine your spindle centerline measurement is to chuck up a piece of stock - steel, aluminum or whatever is handy and dimensionally stable (not wood) - with about 2" sticking out of a 3 jaw chuck. Since this is a first operation, the OD you produce will be dead on accurate. Take a 0.010" deep cut to take off the skin, then 0.003" to size it, then about 0.001-0.002" and strive for a good finish. Measure the OD accurately and divide by 2. Use a height gauge if you can and measure from the surface of your cross slide to the top of the work piece you just turned and subtract that 1/2 OD. THAT is your spindle centerline. Write it down and put it somewhere you can retrieve it when you need it.

This number is the distance from the cross slide to the bottom of the blade slot in your tool post. Try very, very hard to cut that slot accurately, within 0.005" or less below but 0.000" above this number. If you can hit it, try to get the bottom of the slot dead on your spindle centerline number. Remember how we always have to try to align the tip of our parting tool to the spindle centerline? You will never have to do this again.

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.

 

[mikey]

So, my cross slide has no t-slots running across. Is there a way to make this parting method work in such a case? The answer is probably obvious but it's late here and I'm pooped.

If your cross slide bed is square and flat then you can drill and tap a hole for a bolt that will hold the tool post in place. If my lathe didn't have t-slots already, this is what I would do and it would be worth it.

 

[tony a]

I don't have a rear tool post but part with the lathe in reverse and the parting tool upside down on the regular tool post. Have a 12/36 Chinese lathe using 2mm carbide parting insert and haven't had a problem. I presume that if the tool is upside down then the physics would be the same in both front and rear mounting?

 

[NC Rick]

I don't have a rear tool post but part with the lathe in reverse and the parting tool upside down on the regular tool post. Have a 12/36 Chinese lathe using 2mm carbide parting insert and haven't had a problem. I presume that if the tool is upside down then the physics would be the same in both front and rear mounting?

That aspect of it for sure. Also, I think the act of taking the compound out of the equation will add rigidity. I was always taught to tighten the compound gibs in the most overlapping position when it isn’t needed. Depending on the geometry of the lathe, at a particular diameter there may be a lot more engagement of the cross slide ways during a cut with the tool on the backside of the cross slide. There may be another physical advantage but I don’t see it.

 

[ddickey]

My old lathe did not have t-slots. I made a base plate so that I could locate the screws where I knew they could go without interference with anything. I then mounted the block that held the parting to the base plate, worked very well.
FYI Hemmingway kits offers 2 rear post parting blade holder kits for sale. If you do get one of their kits remember you want to hold the blade in a flat plane, or parallel to the top slide.

 

[mikey]

I don't have a rear tool post but part with the lathe in reverse and the parting tool upside down on the regular tool post. Have a 12/36 Chinese lathe using 2mm carbide parting insert and haven't had a problem. I presume that if the tool is upside down then the physics would be the same in both front and rear mounting?

Forces should work the same as in the rear but you have more links in the chain up front. If it works, it works!

 

[Janderso]

Now, my intuition tells me that a rear mounted tool post with an upside down cutting tool acts as a fulcrum. Since the tool post is locked down solidly to the rear of the cross slide and since the tip of the tool is being pushed up as it contacts the rotating mass of the work, forces will be transferred to the rear of the cross slide. This motion takes up any clearance between the cross slide and the bed of the saddle and forces are thereby transferred down into the structure of the lathe.

This sentence beautifully illustrates the benefits.
Well done.

 

[Janderso]

I don't have a rear tool post but part with the lathe in reverse and the parting tool upside down on the regular tool post. Have a 12/36 Chinese lathe using 2mm carbide parting insert and haven't had a problem. I presume that if the tool is upside down then the physics would be the same in both front and rear mounting?

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

 

[Cadillac STS]

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

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