Inside threading, can you recommend some tooling?

Of course! Can't believe I didn't think of that. Oh well, I cut down a set screw in the lathe. Good thing, since the flat was ground on the top of the bit. Next time, I will try the screw from the end. Soon, I should have some square HSS which should give me more options.
 
Tried two things. 1) lowered the bit so the point was closer to center line & 2) used a different gear combination. The little machine shop gear calculator creates some settings that, at least for me, were physically impossible. Found a different combo that worked for 24 TPI. 60, 45, 40, 80. Checked the scratch pass with a gauge and it was correct. I also slightly extended the bit outwards. Think I will back that off as I got a tiny bit of singing. (High frequency chatter)

Things progressed ok for a couple of passes. Then it seemed everything had gone wrong. What really had happened was a bit of tearing. I then made a light pass of 0.002 at slightly higher spindle speed which cleared up the thread. Here's an inside right hand thread 24 TPI in 6061 made with a home made inside threading tool using a "left feed" and reverse spindle. Focus is not as good as I'd like, but at least it is a picture, and it did happen! :laughing:
PXL_20210805_145453483.jpg
Thanks for everyone's help. Now to make a matching plug. To match this I turn the male blank down to ID + 0.5/tpi ?
 
Aluminum is prone to thread tearing. I've tried several different tapping fluids and varying speed. Seems like a higher speed often improves the finish. It certainly cut the time. 300+rpm works. Requires feeding away from the chuck to be safe. Square bits work fine in round holes. The set screw against the flat of the bit keeps them from rotating. Only disadvantage is, you need to have the grind oriented to the flat since you can't just rotate the bit like a round one.

I've got a D bit grinder which makes sharpening somewhat easier. The grind is also smooth enough that I don't fool around with honing. It took me a while to figure out how to get all the angles set. Not all that hard once you've done it a few times. Using a commercially made bit as a reference helps. There is a lot of information about what exact angles each grind should be. I'm sure there has been lots of testing to come to those #s. If I manage to get in the "ball park" and the bit cuts OK, good enough.

I've got lots of 1/4" & 3/8" round, dull, carbide router bits to make into cutting tools for metal. HSS is a lot faster to grind so it is the choice for short term use. There is a lot of difference in various HSS formulations. Some grind easier but usually don't hold up as well. I don't see any difference in the cut quality of a sharp HSS VS a sharp carbide.
 
The matching plug I made is loose. Apparently my formula is wrong. It fits but could be a lot snugger.
Correct formula for male OD = ID of threads in tube + 0.866/tpi. It kind of feels like there's 0.016" of slop in the thread. Which is what the difference is between 0.866/24 and 0.5/24. No problem, I'll make another. That's why we practice and learn!
 
Made a reasonable plug cap for the inside thread. Using the formula in the previous post, turned it down to a few thousandth over the formula value. Externally threaded it. Now the thread fits properly and isn't loose. Knurled it to make it look a little nicer. First attempt at a straight tool knurl. May have over pressured it and got some flaking. Learned another valuable lesson about holding the workpiece. (That's we call screwups!) Had made a spacer for the lathe so the plug cap would just stick out of the chuck. Then used some aluminum shims to prevent marking the workpiece. Since the cap was not very thick, there wasn't much of the jaw holding the piece. While facing the piece it grabbed the cap, dug into it and pulled out the piece from the chuck and jammed it into the tool. Made some pretty deep gouges.

Decided to thread the piece into the lower half and chuck that into the lathe. Put an o-ring into the thread relief to act as a buffer. Then faced off the cap. The cap wasn't perfectly concentric, but it worked out ok for facing. With the cap screwed all the way in, the container should be waterproof.
PXL_20210806_003029733.jpgPXL_20210806_003450734.jpg
 
Aluminum threads against aluminum threads have a propensity to gall. When this happens it is like they became welded together. There are lubricants that help... molybdenum disulphide is one.
 
SLK001 said:
If you're trying (still) to thread left-to-right, it looks like your tool is not upside down (which it needs to be). If you cut that way, you did little cutting and a lot of dragging.
Click to expand...
Can you explain this, please? The lathe is in reverse, (CCW) the flat is pointing towards the backside and is upright. Why is this incorrect? Basically it is a LH tool, not a RH tool turned upside down.
 
Thanks for the tip on aluminum galling. The aluminum was only a test piece with stock that I had on hand.

When I get a chance, I'm going to do internal threading on 304. I have two ways to do it, either a LH threading bar with carbide inserts, or my home made tool (with HSS), which is ambidextrous! It can be right or left handed, depending on how I insert the tool. When I cut threads towards the tailstock, it was left handed.
 
Last edited:
WobblyHand said:
Can you explain this, please? The lathe is in reverse, (CCW) the flat is pointing towards the backside and is upright. Why is this incorrect? Basically it is a LH tool, not a RH tool turned upside down.
Click to expand...

Okay, I had to think about the geometry and you have it correct.
 
You must log in or register to reply here.
Back
Top