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Author Topic: Machining internal threads on lathe.... Seeking advice...  (Read 494 times))

Offline Duane38

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Machining internal threads on lathe.... Seeking advice...
« on: February 18, 2021, 03:52:19 PM »
Here is the run down... 

Valve half to have 3/4" x 16 internal thread.

Compound set at 29.5 (yes, compound facing correctly), feeding inward to thread,  cross slide zeroed and backed away on each exit/backing out pass, thread dial (1-8, set at 1) .  Half nut never disengaged as to maintain position.

Bored the hole to the necessary diameter.  As I thread, the threads cut, but the bore grows and grows while the thread depth seems to fall short of ever reaching depth.  The hole just grows with seemingly shallow threads.  I check and check with a thread gauge and the threads never reach depth.

Any advice?
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Offline JuryRigger

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #1 on: February 18, 2021, 04:19:05 PM »
Have essentially zero experience single-point threading; but are you sure that your insert/bit is ground to the right angle and set correctly? Almost sounds like you have a wider angle than 60* going, so the thread height would never get any taller past a certain point-but the bore ID would.
Hope this helps; like I said zero experience here-only reading and thinking!
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Offline rsterne

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #2 on: February 18, 2021, 04:24:29 PM »
Are you advancing the compound feed and also the crossfeed BOTH? (ie both moving the point to a larger radius?.... That could cause what you are seeing....

You have to EITHER use the compound feed OR the crossfeed, but not both.... Using the crossfeed causes the point to cut on both sides, using the compound feed cuts only on one side of the tool bit.... As you said, a 29.5 deg. compound angle is usual for a 60 deg. point on the tool bit.... but then you should only be feeding with the compound....

Bob
« Last Edit: February 18, 2021, 04:34:00 PM by rsterne »
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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #3 on: February 18, 2021, 04:30:53 PM »
is the cutter fully relieved on the sides and front ?? the bottom of the 60 degree tool could e rubbing the wall of the tube and thats even more prevelent on smaller i.d tubes.
threading  internal i usually relieve the cutter a bit more than 60 degrees on the Z - side( chuck side ) .. So yes i end up with a 58 degree tool , but it always makes a perfect 60 degree thread ( or close enough  ;D
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Offline rsterne

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #4 on: February 18, 2021, 04:35:37 PM »
Yes, Rob, that is the most likely scenario.... the tool bit is touching below the cutting edge, where it should be ground for clearance....

Bob
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Offline Duane38

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #5 on: February 18, 2021, 07:08:12 PM »
Yeah, advancing only the compound in the thread. 

Using a thread bar which has flats for tool holder set screws.  The flats are arranged so the tool tip a few degrees down. 

Used this one before on 3/4 or 1 inch holes I'm pretty sure.  Maybe 1 inch holes.

This could be the problem:  tool too big for the hole, maybe. 

I'll fool around with it and further troubleshoot. 
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Offline Duane38

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #6 on: February 18, 2021, 07:10:35 PM »
Yes, Rob, that is the most likely scenario.... the tool bit is touching below the cutting edge, where it should be ground for clearance....

Bob

Thought of that, but the threads are super clean.  Wouldn't the threads show signs of roughness? 
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Offline Duane38

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #7 on: February 18, 2021, 07:12:44 PM »
is the cutter fully relieved on the sides and front ?? the bottom of the 60 degree tool could e rubbing the wall of the tube and thats even more prevelent on smaller i.d tubes.
threading  internal i usually relieve the cutter a bit more than 60 degrees on the Z - side( chuck side ) .. So yes i end up with a 58 degree tool , but it always makes a perfect 60 degree thread ( or close enough  ;D

You know, could be.  This is an indexable insert.  Suspect the sides are not relieved more.
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Offline sb327

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #8 on: February 18, 2021, 07:15:27 PM »
The only other thing I can think of is you are using an 11ir a60 insert tool. It has a pitch range from 16-48. If this is the case, the tool height adjustment can be off and the insert can start shaving the bore before reaching full thread depth. I have had trouble getting external threads to 16 pitch depth before tool starts to shave (with some cheap inserts).

You can check your insert against a 16 pitch bolt thread to see how close the flat is to the thread peak.

Dave

Was typing while you posted that
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Offline clarky

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #9 on: February 18, 2021, 07:16:30 PM »
Here is the run down... 

Valve half to have 3/4" x 16 internal thread.

Compound set at 29.5 (yes, compound facing correctly), feeding inward to thread,  cross slide zeroed and backed away on each exit/backing out pass, thread dial (1-8, set at 1) .  Half nut never disengaged as to maintain position.

Bored the hole to the necessary diameter.  As I thread, the threads cut, but the bore grows and grows while the thread depth seems to fall short of ever reaching depth.  The hole just grows with seemingly shallow threads.  I check and check with a thread gauge and the threads never reach depth.

Any advice?

Cutting tool is rubbing away diameter before full thread form is achieved because its not set to correct height...and the body of the bar is rubbing in the bore...even the tiniest unseen contact can do it..
« Last Edit: February 18, 2021, 07:19:45 PM by clarky »
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Online Rob M

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #10 on: February 18, 2021, 07:33:36 PM »
is the cutter fully relieved on the sides and front ?? the bottom of the 60 degree tool could e rubbing the wall of the tube and thats even more prevelent on smaller i.d tubes.
threading  internal i usually relieve the cutter a bit more than 60 degrees on the Z - side( chuck side ) .. So yes i end up with a 58 degree tool , but it always makes a perfect 60 degree thread ( or close enough  ;D

You know, could be.  This is an indexable insert.  Suspect the sides are not relieved more.

YEA , a lot of my china carbide insert tool holders have no relief on any side.. which goes unnoticed on external threads, not soo much on internal.
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Offline Duane38

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #11 on: February 19, 2021, 06:05:02 PM »
Okay...  Checked everything a third time over.  I suspect it is the insert.  The side profile really does appear to be "dang" near 90 degrees from the top. 

So, ordered another threading tool from LMS ground properly from HSS.  Will give it another go once the tool comes in. 

With that, may go for a bigger hole/bore so the valve doesn't have to be as long.
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Offline Duane38

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #12 on: February 21, 2021, 03:36:22 PM »
Thinking on threads...  Did some calculations...  Would it be safe to go with a 7/8-20 thread with an outside diameter .995? 

Valve body is 7075-T6 (female thread) and the valve exhaust end (male thread) will be 303 stainless.

Valve pressure 2-2.5kpsi.  I know that leaves a rather thin wall section....  May stick with 3/4 just to safe.  Don't want the valve popping apart.
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Online Rob M

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #13 on: February 21, 2021, 05:39:42 PM »
.995-.875 thread root =.060 wall in 7075, youd  be fiine .
thats assuming the threaded section is seeing pressure , if the oring is inboard before the threads, the 1502lbs could be greatly reduced.
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Offline Duane38

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #14 on: February 21, 2021, 06:19:46 PM »
.995-.875 thread root =.060 wall in 7075, youd  be fiine .
thats assuming the threaded section is seeing pressure , if the oring is inboard before the threads, the 1502lbs could be greatly reduced.

Yes sir.  Oring will be inside ahead of the threads..
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Offline rgb1

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #15 on: February 22, 2021, 08:34:58 AM »
Phil, what machines do you have......or have access to?
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Offline Duane38

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #16 on: February 22, 2021, 08:56:03 AM »
Phil, what machines do you have......or have access to?

Sieg X2D mini mill
Grizzly G0759 727 mill
Grizzly G0765 7x14 lathe
Grizzly G9972Z 11x26  lathe

Own these machines.
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Offline JPSAXNC

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #17 on: February 22, 2021, 10:43:16 PM »
Hi Phil, If the tool isn't cutting to depth it sounds like the radius on the front of the tool is to big, or the angle is somehow greater than 60 degrees. hth.
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Offline Duane38

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #18 on: February 23, 2021, 12:15:30 AM »
Hi Phil, If the tool isn't cutting to depth it sounds like the radius on the front of the tool is to big, or the angle is somehow greater than 60 degrees. hth.

Pretty sure it's BOTH!  Rats!  All good.  Have a proper tool on the way snail mail.  Tried to find where I got it from to track down thread specs it can cut.  Thinking it is for larger threads with a large root. 
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Offline Gippeto

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Re: Machining internal threads on lathe.... Seeking advice...
« Reply #19 on: February 23, 2021, 10:11:10 AM »
Hi Phil,

You're using insert tooling...assume the geometry on the insert to be correct. The "flat" on your cutter shank should be horizontal, insert will be tipped downward.

A shot in the dark here, but some lathes (like the Smithy I learned on) are not at the "30degrees" you NEED when set AT 30degrees. With some lathes, you need to set @ 60 degrees. Having this wrong, produces a mess similar to what you describe, and THIS was the "hump" that I struggled with learning to single point.

If you look at it from the top, does the compound actually appear to be on the correct angle to follow the desired thread profile?


Aside...good to see you still kicking around. :)

Regards,
Al


« Last Edit: February 23, 2021, 10:15:02 AM by Gippeto »
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