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Internal threating

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Larry42

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#1
I'm new to metal machining. Been trying to do an internal thread about 5/8" diameter (OD of shaft threads is 0.743) 16tpi. Material is 4130. Carbide insert (new). Using a 4 jaw chuck, dial indicator centered (0.0005+-, 0.001 OA), 60rpm, cutting oil. I get some tearing but it seems like the metal also wants to flow up as I cut. I bore to the proper finished size, make multiple passes until the thread looks fully formed. By then the ID has decreased to the point where the thread will not fit on the shaft. The top of the threads has become sharp. I've tried using a boring bar to just take the tops off but then they tend to mush over, making it a no go.
Each time I take the part off the lathe to try a test fit I find it difficult to get it back on the lathe to where the treads will cut in the same groove again.
What is the proper way to do this? How do you measure an internal thread accurately. I can do that on an external using wires. I think it is time to make an external threaded test piece to match the part so I can test the fit on the lathe. Of course that introduces another variable.
 

markba633csi

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#2
It is tricky to pick up the thread again, but it can be done. You just need to keep track of the backlash- yes making a test piece would be a good idea
The more you do this the easier it gets- hard to believe but true :) Add some extra light on the work, more is always better
mark
 

Larry42

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#3
Then there is the issue o me centering the work in a 4 jaw. Takes me forever. Then I watch Abom79 do it in 15 seconds.
To bad my 3 jaw isn't dead on. Tomorrow I'll make the test part.
 

T Bredehoft

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#4
Make the test part. Be very careful, don't rush it. Verify the tool is really square with the axis of rotation, use thread wires for diameter. Make it equal the male part.
 

benmychree

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#5
I studiously avoid insert threading tools, I use form relieved HSS tools made by Bokum Tool Co. for internal threading, they cut freely and do not raise burrs to any significant extent; these tools are sharpened only on top, which does not change the geometry, For external threads, I use Aloris threading tools, again, sharpened only on top. One should never remove a part to try the thread, unless it might be a external thread, such as for a gage held between centers and driven by a dog. Any fussy part to be internal threaded, I would definitely make a plug gage to try the fit.
For each of my lathes with threaded on chucks, I have made a plug gage to thread any new chuck backplates or other accessories.
 

markba633csi

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#6
Don't worry about youtube prodigy- I can't do it in 15 sec either
 

vocatexas

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#7
I'm not even going to tell how long it took me to center a piece in a 4 jaw the first time, but I will tell you, the more you do it, the faster you get and the easier it seems.
 

Cooter Brown

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#8
you can try making a thread gauge. Cut the same diameter thread on the OD of a piece of scrap. so you can test your 4130 part before you part it off.
 

JimDawson

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#9
Sounds like your tool is rubbing rather than cutting. Make sure you have enough clearance below the tool bit and that it is on center.
 

homebrewed

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#10
I found that 4130 needs sharp cutting tools to get a decent finish. Carbide did not work out very well for me, but I haven't machined 4130 all that much (I stick with aluminum if at all possible). I'd try an HSS tool, and, as Jim indicated, make sure the tool isn't rubbing. It is easier to correct this with HSS, just grind the bit as needed.

I also want to mention my first experience with cutting threads (external ones). I didn't know much about the procedure so I just plunged the tool bit straight in with the cross slide, rather than setting the compound over to the correct angle and advancing it. As a result those threads were pretty crappy (thank goodness it was a test piece). The only reason I mention this is your comment in your OP on being new to machining, so I am offering my own learning experience.
 

Larry42

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#11
When I started out, I set the compound to 29+-. Then watched Joe Pie's YouTube video on threading. Made perfect sense. At anything less than a perfect 30* the tool will not be taking a big enough bite to cut on the one side. Result will be rubbing, burnishing, heating. On external threads I have good results, cutting tool upside down and cutting away from the chuck. No crazy fast moves required. I've been cutting all of my external threads that way and get good results, if I'm using metal that machines well. This morning I sharpened my HSS internal threading tool. Put a bit more relief on. Now to make the test part and see if I can sneak up on a fit for the internal part.

If I get the internal threading down, my next project is a spider for the out-board end of the spindle.
 

Larry42

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#12
Success! Turned out I couldn't use wires because the threads were to far back in the machine. Turned several female threaded parts out of scrap until I got one that fit. Then turned an external threaded part that fit that. Then threaded the good part to fit the sample. What a lot of messing around! But the new part fits quite well. Have it setup in a collet block to mill the wrench flats. Need to buy another 3mm end mill to cut the keyway. Dropped the one I had and could not find it.
Next project is one my wife has been "suggesting" for quite a while. 30' of porch rail with turned balustrade. Going to make it out of cypress.
 
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