Fun with Internal Threading

[Just for fun]

So, I decided to try my luck at internal threading. I had never used my boring bar before either and failed at hitting my numbers. I'm not sure what happened but I'm thinking I must have read my snap gauge wrong or didn't take a good reading with it. At any rate after drilling and the first couple of bores I read .802 took .020 ended up with .824, took .023 measured .850, took .048 measured .900, took .050 measured .983 WTH . I was shooting for .9515. Oh well it was on a test piece anyway. I went ahead and threaded it just like I was threading 1" x 20. My very first run at it I crashed my cutter into the end of the bore. I had it bored to 1.25" so I was planning on stopping at 1", I guess I had spindle going to fast as when I went to disengage the half nut it was stiff and took more force than I expected....... CRASH!!! Luckly no damage to cutting insert or anything else that I can tell. It did knock the tool post out of alignment other than that all seems OK.

From then on all went well. I slowed the spindle down to 110 RPM and at the 1 inch mark I would turn the cross slide in and disengage the half nut, back out, zero the cross-slide dial in 5 thousand on the compound and repeat.

 

[erikmannie]

So, I decided to try my luck at internal threading. I had never used my boring bar before either and failed at hitting my numbers. I'm not sure what happened but I'm thinking I must have read my snap gauge wrong or didn't take a good reading with it. At any rate after drilling and the first couple of bores I read .802 took .020 ended up with .824, took .023 measured .850, took .048 measured .900, took .050 measured .983 WTH . I was shooting for .9515. Oh well it was on a test piece anyway. I went ahead and threaded it just like I was threading 1" x 20. My very first run at it I crashed my cutter into the end of the bore. I had it bored to 1.25" so I was planning on stopping at 1", I guess I had spindle going to fast as when I went to disengage the half nut it was stiff and took more force than I expected....... CRASH!!! Luckly no damage to cutting insert or anything else that I can tell. It did knock the tool post out of alignment other than that all seems OK.

From then on all went well. I slowed the spindle down to 110 RPM and at the 1 inch mark I would turn the cross slide in and disengage the half nut, back out, zero the cross-slide dial in 5 thousand on the compound and repeat.



View attachment 416364

When I tried internal threading for the first time, I used a bushing cut from hex bar (thereby making a nut) so that the cutting tools would travel in to open space after doing their cutting.

How long was your safety (i.e. relief) groove?

I would be interested to see your setup with using the compound. I just used the cross slide with a lot of spring passes after each adjustment on the DOC. Using the compound is better as this reduces the tool pressure by 50% and makes better threads.

Did your setup look like this?



Here is another drawing:

 

[erikmannie]

I wonder what the argument is against using only the cross slide together with making smaller cuts. I guess because it would take longer.

One of my friends is 79 years old, & he starts the threads with single point, & then finishes the job with a tap or die. I have seen him do this, & it goes pretty quickly.

Like I said, I do a lot of spring passes between adjustments on the cross slide (or compound). I do these until I am removing zero material. This takes a long time.

 

[JimDawson]

To prevent the possibility of a crash when internal threading in a blind bore, turn the tool upside down, and run the lathe backwards. That way you are cutting from left to right, out of the bore rather than into it. I do this also when threading an external thread up to a shoulder.

 

[mmcmdl]

@Just for fun . When boring , the normal process is to take 3 or so cuts the same depth to establish bar spring etc . Just say .030 on diameter , 3 passes , take a reading , then you can rough it out within .100 and repeat . Taking cuts of all different depths will give you different readings . And we do thread out as Jim suggested , you just use the opposite hand threading bar .

 

[BGHansen]

When doing anything blind, like boring in a pocket or threading, I first "stage" the tool at full depth with the lathe off. Then mark the edge of the carriage on the bed with a Sharpie; that's my CRASH mark. I also adjust the tool stick out from the boring bar holder so the tool holder is about 1/4" from crashing into the work. That gives me a couple of visual references for stopping points.

If I'm boring, I'll stop just a hair short with the power feed and hand feed to depth. If threading, I'll disengage the half nuts a bit shy of my crash marks like normal threading. If it's an oddball thread that needs to be done without disengaging the half nuts, I'll stop way short of my crash mark (power off the lathe) the first few passes to get a feeling on how much further inertia in the chuck takes me past my power down point. When making passes, I'll power down a little short and rotate the chuck by hand to finish the threading pass.

Bruce

 

[OCJohn]

Subscribed. I’ve got an internal threading op in my near future and it will be my first.

Any holder/insert recommendations? I’ll be cutting M27x1.5 on a low torque 7x…

 

[mickri]

What type of lathe do you have? On most lathes a .020 cut removes .040. On some lathes the dials reference the actual amount of material removed. .020 removes .020. Your first 3 passes indicate that your lathe dials reference the actual amount of material removed whereas your last cut indicates your lathe removes twice the amount of material as is typical for most lathes. So you are off somewhere in your measurements.

I had lots of trouble getting accurate measurements when I was boring holes. I had trouble using the telescoping gauges and in reading the gauge with a micrometer. I was never able get accurate consistent measurements with a caliper. Had to use a micrometer. I found putting my thumb at the bottom of the hole really helped in rocking the gauge to get a consistent measurement as I pulled it out of the hole. The other thing that helped was using a stand to hold the micrometer.



When boring/threading blind holes I use a crash mark on the lathe bed for a visual reference like Bruce mentioned and I use a carriage stop. As the cross slide approaches the crash mark I put a piece of paper between the carriage and the stop. When I can no longer move the paper I disengage the half nuts.



I think that you are running your lathe way too fast for a beginner at threading. When I was learning how to cut threads everything that I read about cutting threads recommended running the lathe at its slowest speed. On my craftsman 12x36 the slowest speed is 28 rpm. At that speed I have plenty of time to disengage the half nuts. There is no rush to do things. I still run my lathe at that speed when cutting threads.

 

[Winegrower]

On most lathes a .020 cut removes .040. On some lathes the dials reference the actual amount of material removed.

A friend and I both have Takisawa TSL-800 lathes. Interesting that his cross slide dial references diameter, i.e. dial says 10, compound moves 5, while mine references radius, compound moves as far as the dial says. I find the mental problems are easier to solve on mine than his. Maybe he would say the same thing about his.

 

[Cadillac]

Thread away from the chuck it’s ALOT less stress. Theirs enough stresses with threading eliminate what you can.