# Keep crashing internal threads on Grizzly 4003 lathe



## ridgeway (Sep 11, 2013)

I have no idea what I'm doing wrong or if there is something wrong with my machine or all above.  I have my set up just like the picture below.  I am doing 18 TPI with a minor diameter of 1.005" to screw over a gun barrel tenon.  I tried this 4 times, same thing.  I make several passes that cut fine and boom, threads get crashed!  I spend a lot of time turning, drilling and boring to prep for the threads.  Not sure if there is lash somewhere that is upsetting the travel?  I am engaging the half nut on the same number each time.  Each pass, I take .005" on compound.

Any tips of suggestions to try?  I'm at a loss.

I've can thread external threads no problem.  Below is pictures of 18 TPI on 304 SS and 1/2-28 on cold rolled bar stock.  They come out fine.


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## Bill C. (Sep 11, 2013)

ridgeway said:


> I have no idea what I'm doing wrong or if there is something wrong with my machine or all above.  I have my set up just like the picture below.  I am doing 18 TPI with a minor diameter of 1.005" to screw over a gun barrel tenon.  I tried this 4 times, same thing.  I make several passes that cut fine and boom, threads get crashed!  I spend a lot of time turning, drilling and boring to prep for the threads.  Not sure if there is lash somewhere that is upsetting the travel?  I am engaging the half nut on the same number each time.  Each pass, I take .005" on compound.
> 
> Any tips of suggestions to try?  I'm at a loss.
> 
> ...



I would suggest making sure to remove all the backlash is out of the compound and cross feeds. Also I would use an indicator with a magnetic or fixed base as a part stop.  I have used a zero to one inch travel indicator.  I know it is frustrating but must be some little problem causing such a headache.


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## Ray C (Sep 11, 2013)

Need to see the cutter and threading bar setup... Looks like you're bottoming out.

Ray


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## markknx (Sep 11, 2013)

have you checkedfor play in your compound, and cross slide


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## ridgeway (Sep 11, 2013)

I agree, it has to be something simple.  As for backlash, I do have some in both.  I am feeding toward me, so both compound and crossfeed should be tight against my stock.  

I am using a dial to stop plus .010" more so i dont bottom.  Who know, maybe it still is.  If I try again, i may cut a relief at the end with a boring bar. 

Ray...tooling is 1/2" Mesa threading bar with carbide insert.


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## DBlue (Sep 11, 2013)

.....


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## schor (Sep 11, 2013)

ridgeway said:


> I agree, it has to be something simple.  As for backlash, I do have some in both.  I am feeding toward me, so both compound and crossfeed should be tight against my stock.
> 
> I am using a dial to stop plus .010" more so i dont bottom.  Who know, maybe it still is.  If I try again, i may cut a relief at the end with a boring bar.
> 
> Ray...tooling is 1/2" Mesa threading bar with carbide insert.



I also believe your compound is incorrect, you need to either place it like in the diagram or you would need to turn it to the back side of the lathe at 30. (or 29)


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## schor (Sep 11, 2013)

More on what's happening.

I went out to the shop for a quick demo of how your crashing your threads. The position of your compound is incorrect and the forces of cutting the thread are pushing back against the compound causing the backlash. It's a bit hard to see the backlash in the video but just go try it yourself, push back directly towards the tailstock and you will see the same. 

[video=youtube;StzhjogT47g]http://www.youtube.com/watch?v=StzhjogT47g&amp;feature=youtu.be[/video]


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## ridgeway (Sep 11, 2013)

schor said:


> I also believe your compound is incorrect, you need to either place it like in the diagram or you would need to turn it to the back side of the lathe at 30. (or 29)



I did have it set on the backside.  I just threw the threading bit on my tool holder to take the picture.  Sorry for the confusion.


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## Ray C (Sep 11, 2013)

Here's a bunch of random thoughts because I don't see a smoking gun.

1)  I can't count the number of times I've done inside threading and didn't bother to change the compound to 30[SUP]o [/SUP]the other way.  It doesn't seem to make a difference because I do straight-in feed with the cross slide most of the time and when I do, I set the compound 90[SUP]o[/SUP] perpendicular.  If you are going through all the steps of manipulating the compound by a few thou here and there then yes, you should have it the other way.  I'm not advocating you do the way I do -just saying I seem to be successful at getting away with it.

2)  It looks like you're cutting brass or bronze which tends to make "powdery" swarf that easily packs up under the cutting bit.  A little bit of lube will go a long way and also, keep the bit clean (be safe now).

3)  Try making a spring pass (no additional dial-in) after every second cut.  Brass and Bronze are usually pretty cooperative and cuts clean but, it's worth a shot especially since there does not appear to be face or top relieve on that cutter.  -But I can't see it well enough to tell really.

4) Are you sure that bit is dead-on perpendicular and sure it's real tight in the bar, and is the bit centered?

5)  With inside threading, you need extra face relief because the curve of the piece is coming right at the base of the cutter.  If there is little to no face relieve on that cutter bring it up above center by about 1/8" or so -or better yet, grind some relief on that cutter.

That's about all I can think of today but, I'm kinda whipped from a headache at the moment.  If more comes to mind, I'll post.


Ray


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## ridgeway (Sep 11, 2013)

Guys...thanks a ton for all the suggestions.  I may give it one more try and keep in mind of the suggestions.

Ray...its 304 SS...i think its the lighting in my garage casting the tint.

I will cut a relief groove in the next on and maybe raise my bit a tad bit above center.  

I am thinking toward the end of the cut, I may inadvertently travelling in a few thousands before lifting the half nut and backing the bit away causing a disruptio.  Or the flow of chips following the cutter and bottoming out.  I dunno...frustrating.  LOL.  

No biggie with the part...i can live without it.  The purpose of this tool was to thread onto various barrel tenons and have indexing marks to locate where to engrave the caliber on the barrel so it lines up in the correct position.  That way I dont need to screw the barrel on the action, mark the location and take it back off


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## Ray C (Sep 11, 2013)

OK, first, with SS, it is not cooperative.  You must do spring passes on every second pass unless you have a beast lathe.  Trust me on this.  I do SS threading 3 times a week at least.  I'd venture to say this is the smoking gun.  Next best guess is the relief on the cutter.  Also, with SS, you must have dark cutting oil or it grabs like crazy.


Ray




ridgeway said:


> Guys...thanks a ton for all the suggestions.  I may give it one more try and keep in mind of the suggestions.
> 
> Ray...its 304 SS...i think its the lighting in my garage casting the tint.
> 
> ...


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## ridgeway (Sep 11, 2013)

shor...thanks for taking the time to make that video.  One of my tries, i did have my compound set wrong...actually twice.  LOL.  I also had 29.5° set on the wrong axis doing external threads...you can only imagine what they looked like, LOL.


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## DAN_IN_MN (Sep 11, 2013)

ridgway

From your info on your first post, it does sound like you know how to turn a thread and something is giving you the run for your money on internal threading.

Have you tried turning the same thread in mild steel?  How about cutting the same thread in SS without being a blind hole?

Like others suggested, you might be bottoming the cutter out and the carriage pushes the cutter back in the tool post so not it's not in sync with the thread groove. 

Another thing you could do, and it will take more time, is to shut the lathe off and manually turn the spindle with the half nut engaged and watch the cutter to be sure it lines up with the threads.  If it lines up, without disengaging the half nut, hit reverse and back up the carriage, advance your cutter for a deeper cut and make the cut.  Do this until you discover the cause for the cutter misalignment.  

I suppose after checking to make sure the cutter hasn't moved, you could engage the half nut using the threading dial.  I think you know what I'm trying to get at here.


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## ridgeway (Sep 12, 2013)

Dan, I crashed the threads 2 times in mild steel and twice with stainless.  Wasted 4" of good 304!

I don't think turning the spindle by hand in low gear would be a pleasant experience, lol.

Anyhow, i will try this again on mild steel.  I will bore to minor diameter.  Then I will bore a large relief inside and thread inside out.  I only need .750" worth of threads.  I should be able visually see what I'm doing atleast, lol.

Might not be for awhile, but I wi report back if im sucessful at this.


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## ridgeway (Sep 14, 2013)

Well guys...I am happy to report back that I suceeded in turning inside threads, sort of.  I set up to thread from the inside out.  I counter bored relief inside so I only threaded half the length, which is totally fine for this tool.  I did manage to break the carbide threading bit though.  Not sure why,  maybe chips or burr?  I was set up to make my last pass, engaged the halfnut and boom, broke turning bit the first three threads.  Good thing I had my hand on the crossfeed to back out in a hurry.  The break upset the tool bit and wasnt about to replace the bit and rechase the threads.  So I pulled the part out and almost chucked it into the scrap bin.  I looked at the threads and blew them out with air.  Gave it a whirl and it magically threaded.  Im like good and chucked it back into the lathe to counterbore the damaged threads away.  While I was at it, I went ahead and finished and put a super light knurl on it.  Threw a coat of cold bluing on it to help with rust.


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## wallyw (Oct 19, 2013)

After many years of threading and several years on my G4003 ran into a problem.  Threads were just not up to snuff.  Checked all the usual fixes.  After an hour or so found that the cap screw  opposite the halfnut engagement handle had loosened.  Tightened it.  Good threads again!

Wally


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