Simple toolholder setup to thread backwards

[KeithB]

One of the more common machinist's problem is threading up to a shoulder. The solution is to cut away, not towards a shoulder. When cutting standard RH threads the lathe spindle is turning CCW (viewed from them front) and the tool travels from the right side to the left, It requires a lot of eye-hand coordination to stop the lathe at the right place so as not to run into a shoulder. It is a lot less demanding to run the spindle in reverse (CW) and have the tool travel from the left side to the right. The problem is orienting the cutting edge of the tool in the right position, by (a) mounting the tool with the cutting face up in a position on the back side of the part, or (b) mounting the tool upside down on the near side of the part.

On my manual lathe there is no practical way to mount a tool on the far side of the part, (I can and often do that on the CNC lathe) That meant I needed to figure out a way to mount a threading tool upside down in my Aloris tool holder.

My solution was to make a rectangular block 3/4" x 1-1/2" x 3-1/2" from a piece of steel scrap. I drilled and tapped two 1/4-20 holes in it. Then I took one of our spare laydown type threading tool holders and drilled two 17/64" holes to match. Bolt the tool holder upside down to the block and clamp the block in the slot of an Aloris type tool holder and its ready to cut.

I didn't have time to try this setup but I know the carbide laydown inserts are capable of cutting beautiful threads so I don't expect any problems. The toolholder I used is a RH version and as can be seen it means that the cutting point is 3/4" from the left, It would be hard to cut up to a shoulder on the left so I ordered another toolholder only in a LH version. I will also have to buy left hand inserts but that's no problem



.

 

[Brad]

The simple way is often the best.
I often thread with the tool upside down and the lathe in reverse. The lack of stress makes it far better for me.

 

[Gary]

Here's how I plan to go with my threading;

 

[Slughammer]

Let us know how the LH vs RH inserts both work and what thread pitch you tried them on. I was wondering if the relief angles work with the pitch.

Are you feeding in with the cross slide or the compound on your manual lathe?

I tried the upside down threading and what I didn't like was the lack of visibility of the chip coming off. (But, I haven't given up on it yet).

 

[smokeywolf]

I was taught to always use the compound and always set it to 29-1/2 degrees. That way, each time you dial in you're taking the majority off the left thread surface and just a very slight skin cut off the opposite surface.

Can't even remember if I have lefty insert tooling for single pointing. I had numerous toolbits that I ground for right and left threading (that is, running the spindle in reverse and threading away from a shoulder). My favorite toolbit for that was a 3/8 x 1/2 Tantalum Tungsten toolbit that was about 4-1/2" long. I had one end ground for right and the other ground for left.

 

[KeithB]

I thread both ways. Like you, I was taught to use the compound for infeed and I do that on coarser threads. When I use full profile tooling I generally just feed straight in so the crest of the leading and trailing thread gut radiused properly. Some materials you just have to try it different ways.

 

[KeithB]

I got my LH threading tool holder from Shars. Modified it by drilling two holes in it to match existing holes in adapter block Had to raise tool holder block up about a 1/16" to get tool point at right height. I cut several ,threaded parts in both aluminum and mild steel. Everything worked fine, just have to pay attention when engaging the spindle to be sure its running in reverse..

The top tool is the LH one.