Internal threading, sizing the bar to the hole

[WobblyHand]

I need to cut some 1.75-8 threads for a cast iron back plate. What is the general rule of thumb for bar size? Yes, bigger is better, but I am somewhat limited by common tool holders. My G0752Z has an AXA tool post. This limits me to 3/4" diameter for a standard boring bar holder.

I could make a boring bar holder that fits a bigger bar, might be able to squeeze in a 1" bar in an AXA format. (Have a CAD file for this, so adapting the design would be easy.)

Will I get a lot of chatter with a 3/4" bar? Would I be better off going to 1"? The depth of the threads would be approximately 1.91".

Was thinking of making the bar out of round stock, boring a round hole through it and inserting a 3/16" square HSS bit into it. Tool bit would be held captive by a screw. Would this kind of holder be fit for this purpose? It would look like this, but bigger.

This is a 1/2" bar that I made last year in an OXA holder. I have since replaced the SHCS with a setscrew to gain clearance.

 

[mikey]

Basically, a boring bar is not limited by the bore diameter, but by the bore depth. You can bore a huge hole provided the depth of the bore does not exceed the extension limits of your bar. If your bore depth will be 1.91" then a 1/2" steel bar will be fine, as will a 1/4" diameter carbide bar. Just remember that a steel bar can extend about 4 times its diameter and a carbide bar can extend about 8-10 times its diameter beyond the holder before losing tolerances becomes an issue.

Similarly, a Micro 100 solid carbide internal threading bar with a depth capacity of 2" will easily thread that hole. Or you can use any number of inserted tip bars. The old style bar you show will also work, although it will not be as accurate as a Micro 100 bar.

 

[mmcmdl]

I have him fixed up I think . I offered to lend him a bar and inserts if he needs them . To answer his question , sure that bar would work being it's a thru hole .

 

[mikey]

Okay, good, Dave. Coarse thread pitches like 8tpi tend to cause a lot of bouncing of the tool inside the bar. This makes a solid carbide bar much preferred over the "stick the bar in from the side" kind of deal.

 

[SLK001]

When I did my 2-1/4" x 8TPI for some cast iron nose protectors, I used an internal threading bar about 0.400" in diameter with threading inserts. I had no problem with chatter, etc, and everything went smoothly.

 

[mmcmdl]

I kept a Kennemetal top notch and have the full depth inserts if he wants to thread away from the chuck , we're swapping pms at the moment . I'll lend him the AXA holder if he needs it also .

 

[mmcmdl]

Better yet . I have a back plate already made up . Look in the for trade section .

Edit . Wrong size once again .

 

[WobblyHand]

Have some morse drills I can use to get the hole close to the right size. Also have a 1/2" carbide shank boring bar to bore to size. Unfortunately my Circle Machine bar is only a boring bar, and does not take threading inserts.

The 1/2" threading bar I made was shaky in a mini-lathe. However, that may have been loose gibs, though. Found that out later, but haven't retried inside thread cutting on the mini-lathe. But that's the backstory and motivation for the question. Wasn't sure if a 3/4 bar was stiff enough to get clean 8 TPI threads.

@mmcmdl and I have pm'd, so I hope to be able to move forward on my set tru backplate project.

 

[mikey]

Check to see if Dave has some Micro 100 threading bars. They are quite good and will last a lifetime with care.

 

[WobblyHand]

Check to see if Dave has some Micro 100 threading bars. They are quite good and will last a lifetime with care.

Which kind?

I have used Micro 100 solid carbide boring bars. It was great, up until it snapped. Did get another one, but am a bit reticent in their use.