Internal threading on a lathe, part 2

Now that I have external and internal threading tools ground at proper angles, I decided to thread and custom fit two parts. I'm not working off of standard diameters(hence custom), but have decided on 16 TPI (turns per inch) as the thread pitch, with the standard 60-degree acme threads.

The first thing to do was change the gears in the headstock of the lathe to give 16 turns on the chuck for every inch that the carriage moves.

Then I turned down the diameter of the male part and bored out the inside of the female part so they would mate with the thickness of the threads in place. I left a shank of sorts on the male part to give a defined area for the threads to be cut.

Usign the external threading tool, I threaded the male part, and then used the internal threading tool on the female part. The quick-change tools made the process so much faster - it was amazing.



The internal tool takes a long time to cut because deflects a fair amount. The shavings also tend to ride in the threads and you need to blow them out after every cut.

At one point I thought that it had been ground incorrectly, because they looked squarish. I was worried that perhaps the tool was rubbing the newly cut threads. It's actually hard to see the internal threads - there exact profile is nearly impossible to judge as the pattern creates a rather frustrating optical illusion. I used a thread gauge to check, and it seemed to indicate that I was doing okay.



In the end, the parts screwed together with very little play. I was very happy with this - I was half expecting to break the tool or ruin the the threads by engaging the half-nut at the wrong point or disengaging it during the cut.

Internal threading on a lathe, part 1

I was right, the internal threading tool was the hardest to grind of all. I could have ground all of the other tools I have made before in the time it took me on just this one. Of course, the first time doing something is always the longest. I sacrificed a broken parting tool to make it.

The difficulty stems from a few things:

First, you have to remove a gargantuan amount of metal. The most, easily, out of any tool. Grinding dust gets everywhere. I also had to dress the grinding wheel several times during the operation to bring back its cutting ability with the diamond tip dressing tool.

Second, because the tool is thin and not much metal is left, the tip heats up very rapidly. So you must be more mindful of cooling it. This is less of a burn hazard and more of loss of temper/hardness problem. I once read somewhere that modern tool steels don't suffer from hardness loss after excessive heating from prolonged grinding. This statement assumes one thing: that we are grinding a normal tool with plenty of surrounding metal, which will conduct it to your fingers and become unbearably hot, thus requiring a trip to the water cup. A tool with a thin shaft will NOT do this fast enough and it is easy to get the tip of the tool orange hot, with NO HEAT INDICATION on your fingers. This will ruin the temper on the tool tip, as indicated by an extreme softness of the steel even after it has been cooled in the water cup. If this is the case you must grind off the soft spot until hard metal is again reached.

Third, grinding the inside angle of the tip is not easy as it requires holding the tool at an extremely awkward angle that puts your hand frighteningly close to the spinning wheel. I have not yet come into contact with a spinning grinding wheel, and I don't ever want to suffer the experience.

The end result:

It definitely aint pretty, but it works.

Toolage

Today's update includes the arrival of a few new tools!

1.) Quick-Change Tool Post



The QTCP was smaller than I had orginally thought, but works great! It came with 4 attachments: 2 normal tool holders, a cutoff tool-holder, and a boring bar holder. Once mounted you can switch tools in a matter of 5 seconds. Also, the tool individual tool heights can be dialed in with a nut so that no shims are required to get the tools centered on the work!

2.) Carbide boring bar set

Grinding boring bars from tool blanks takes a long time (even more so than parting tools), so I ordered one of those inexpensive boring bar sets. The feature steel shanks with carbide tips brazed on. There are multiple lengths in multiple thicknesses. This allows you to select one that is the thickest possible for a given hole to minimize tool deflection.

3.) Cutoff tool

Cutoff tools (parting tools) are also time consuming to grind because a lot of material needs to be removed to make one. So after all that effort either it breaks or its not long enough to do the job. (sounds like the voice of experience) This cutoff tool is nice becuase it can be adjusted to multiple lengths and if the tip breaks off you quickly regrind it.

4.) Center Gauge

A center gauge is required if you want to be able to grind blanks into threadcutters. You need a precise 60 degree angle, and its not easy without a reference. So you grind a little, check the tool, grind a little, and check the tool again. It's also useful for keeping the tool perfectly perpendicular to the work.

Grinding and some threading

Finally (successfully) cut some threads on my lathe! They are 24 tpi on a peice of aluminum.. and some random diameter since I am just trying to get the process down.

The hardest thing to figure out was the gearbox changes to drive the leadscrew at the correct speed. Since this is a mini-lathe there aren't any of the nice gear selector levers or anything. So I poped off the gearbox cover and fiddled with it until I figured it all out.

The first time I tried I wasn't using the threading indicator, I was just trying to back up the leadscrew between each pass so as not to lose my position. The tracking was off - I suspect that changing the leadscew direction offsets the alignment. So I just turned down the botched threads and started over at a slightly reduced diameter.

This time I actually used the threading dial indicator, which works perfectly! Duh. I put a light line on the peice, stopped the carriage, pulled back on the crosslide to clear the peice, cranked the carriage back to past the end, set the tool .001 deeper, and engaged the leadscrew at the designated mark, 1-8 in this case. The tool tracked perfectly! I repeated 5 times, and ended up with this:



I didn't make the tool sharp enough so I ended up with squarish threads, but the boint is, it worked! By the time I was done I was shaking from excitement. I don't have a center gauge yet so its hard to grind the tool at 60 degrees, but I'll get one before I try any serious work. The next logical step is internal threading, which should not be harder save grinding the tool. Here are all the tools I've grinded so far:



From left to right: A cartridge rim cutter, a parting tool, a ~60 degree external threading tool, a boring tool and a double-sided roughing/finishing tool. An external threading tool would look a lot like the boring tool but be thinner to provide relief for the little cutting "finger" that would need to be ground at 60 degrees. It seems like it may be the hardest tool to grind of all :)