Tearing when Threading

T

timbertoes

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The problem that never goes away it seems.
Of the Half-dozen or so threads made....rough, rough. Tearing I am 99% sure.

Stainless steel. 416r and an unknown stainless. spark-tested, and magnetic.
also have seen it with common steel alloy.

Rough burrs and "strings" of steel.
Starts with the first cut at just those few-to-5 thou.
Leading edge of threads
top of threads are rough too.


HSS inserts, flat and postive. brazed Carbide
29 to 30 and inbetween.
tool at center.
60 rpm.
approx 1" dia work. 14 to 16 tpi.
Work properly and ridgidly held in 3 jaw and in tailstock
Vipers Venom oil, all over.
fishtail used carefully.
enco 13x40 lathe
lock the compound gib duringthe cut.
Can't lock the cross. DRO does not indicate any crossfeed movment.

would love to hear any and all advice/theorys.
I am getting a Wixey digital protractor to check that 30. I can eyeball with a 30-60-90....but..thats eyeball
kinda need the wixey anyway.
 
A rough thread usually indicates a lack of side clearance on the tool, or the tool is set too high. In either case, the tool rubbing on the work below the cutting edge is causing the rough finish. When you grind clearances on the tool, don't forget to allow for the helix angle of the thread. The leading edge needs more clearance and the trailing edge less clearance. You may want to bump the speed up as well. Unless you are cutting a very large thread, 60 rpm is very slow.

Tom
 
thanks,
All Tooling is store bought. Warner for HSS.
new sharp edges used, and any build up honed off with last diamond sticks.
I tried a 180 rpm a coule of times during the process.
It did seem better for those cuts.
But it was scary fast for me.

Now this was a long piece with large relief, so fast rpm practice is OK.
But I also do a few gun barrels, so now we have a much more reflex needed operation, and a $300+ piece of steel.
the last barrel was a 4140 type, and it did come out better than this metal that on the lathe now.

I cant help but wonder, when the Top of the thread is also rough, is that a possible clue to one particular error?
Will change the pulley/belt to get 90rpm, I think I can handle that.

- - - Updated - - -

Hmmmm...
My compound is set at 29 to 30 degrees off of:

"Zero" but my "Zero" is with the compund at 90 degrees to the cross slide.

I saw this elsewhere:

"start with the compound right on top of the cross slide with it's handle directly above the cross feed handle. I am calling this zero just in case your lathe is marked differently. Then move the compound 30 degrees to the right from that starting point. Now re-align the tool with your thread gauge"

?????????
 
The rough top comes from metal being dragged up off the flank by the rubbing tool. I would suggest grinding your own tool rather than rely on an insert which may or may not be suitable for the thread you are cutting. Different threads require different clearance angles so there is no one size fits all. grinding your own tool will also help you understand how the geometry of the threading tool works. Also, don't get hung up on the whole 29º thing. Just set your compound to 30º, cut your thread nearly to depth, and finish up using the cross slide. That will negate any error in setting the compound and copy the shape of the threading tool. Feeding with the cross slide also makes it a whole lot easier to finish the thread to size because your dial affects pitch diameter of the thread directly.

Tom
 
its really soooo very embarrasing to make this mistake.
I have read about the difference in lathe's compound marking so many times.. and to top it off, when I was making 11degree facing cuts for crowns, I made it the _correct_ 11 degrees by counting backwards from the 45degree mark -when the compund is perpendicular to the work..
At least this thing is going to stay in my head now. I am also going to make an real effort to quit eyeballing and assuming things, and make comparasons to whats in the machinist handbook.
 
timbertoes said:
its really soooo very embarrasing to make this mistake.
I have read about the difference in lathe's compound marking so many times.. and to top it off, when I was making 11degree facing cuts for crowns, I made it the _correct_ 11 degrees by counting backwards from the 45degree mark -when the compund is perpendicular to the work..
At least this thing is going to stay in my head now. I am also going to make an real effort to quit eyeballing and assuming things, and make comparasons to whats in the machinist handbook.
Click to expand...

Don't be embarrassed at all. There are a million damn things to keep in mind all the time, and so many subjectively-referenced numbers.. There are people who will search this now and appreciate your sharing your process of figuring it out.

Let us know if setting your compound 30 degrees from your crosslide helps.

And if not, grind some more relief in the front of your tool and see. Let us know!


Bernie
 
Hehe, I did the same thing my first time; even though I had JUST watched a video from somebody who complained about doing the SAME thing his first time. At least I was able to figure out my mistake pretty quickly because of the video, so it was usefull ;)
 
lol, will put a new piece of steel in the lathe tonight and start over. mebbe I'll have a pretty pic to show :)
 
Success :) funny how well things work, when done correctly. and although not 100% perfect, the threads are very nice and smooth to the touch.
Since there was no mark for setting the compound 90 deg to the work/0deg with crossfeed, I set it with a good square and popped a cold-chisel mark for 0.

Now to make the mating part :) with inside threads... I can only hope my 3 wire measurment was close enough, its nice to know they would be in the range for correct depth.
thanks for the replies and entertaining my needy post :))
 
timbertoes said:
.
thanks for the replies and entertaining my needy post :))
Click to expand...

Your post will be helping other folks in the future. While it is a "rookie" mistake, it was a good lesson learned and other than a few test pieces, nothing was damaged.

Congrats on a good thread :drink2:
 
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