Another threading thread!

devils4ever said:
Okay. Now, that I've "mastered" (yeah right!) external threads, I've been thinking about doing an internal thread.

So, I have a few questions:
1. Does a boring bar give better results than just using a drill bit in the tail stock? Is the boring bar truer? More precise?
2. Do I place the compound at 29.5 degrees as with an external thread, but place it a about 7 o'clock instead?
3. Can I use an internal threading tool as a boring bar? I see no difference except for a deeper reach with the boring bar.

Thanks!
Click to expand...

To add on to the other replies:
1. A drill does not create a straight hole, nor is it round or precisely sized. You can bring it to the right size with a reamer but it will very likely not be straight. You will always be better off boring it straight, true and of the size you need.
2. I find it is not necessary to use the compound for internal threading. It is much simpler to set the compound to 90 degrees and cut the thread with the cross slide feed.
3. Some boring bars accept internal threading inserts as well as boring or grooving inserts; the vast majority of bars do not. The bar must be specifically designed for these multiple functions and they tend to be expensive. You can check with your bar maker to see if this ability is an option.

Internal threading is not as easy as external threading because unlike an external threading tool that can be kept short and rigid, an internal threading bar must be extended to accommodate the depth of the hole. Therefore, deflection is going to occur. Most internal threading bars are made of steel and have an extension capability of 4 times the OD of the bar. Keep this in mind; if you must go deeper, switch to carbide shanked threading tools.

Since we typically take light cuts, radial cutting forces tend to be higher and this will add to the deflection caused by the bar extension. Since we cannot use thread wires or a thread pitch mic, most of us will either use a manufactured or shop made go/no-go gauge for the tolerance class we need to gauge progress as the internal thread is cut. If you're just messing around or just need a class 1-2 fit then a hardware store bolt will work as a gauge. It is when you need a precise fit or when you must fit your part to an already made part that things get finicky and an accurate go/no-go gauge is needed.
 
I can tell by the photo your compound is set at the wrong angle. You want to set it to 59*. The idea is to have the compound set near parallel with the trailing flank of the tool. For right hand threads most of the cutting is on the left flank of the tool. The right flank just gets a skim cut. That is why the compound is set slightly off parallel with the angle of the thread. My South Bend compound is at 0* when it is perpendicular to the bed. On my Nardini it reads 0* when the compound is parallel with the bed. Unfortunately most instructions for threading fail to recognize that there are two different ways of marking the degrees.
 
Should have read all the responses before posting. Others have covered the issue very well.
 
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