# Internal Single point threading



## Alittlerusty (Dec 11, 2017)

This is my first attempt at single point threading ever, and may as well go big right out the gate.   I have 3 chucks for my southbend 16" lathe. 2 of them came with the lathe, a 6"3jaw scroll and a 9" 4 jaw independent.  These 2 chucks share a backing plate (spindle is threaded 2 3/8"-6tpi). It is a major pain swapping it back and forth so it's time to make another one. I've decided the new one will be for the 3 jaw.  At 2 o'clock on the 6" Chuck and the backing plate on the 9" chuck there are two punch marks and some dykem for orientating  the plate on the 6" chuck. I'll post more when I get to the shop.


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## Alittlerusty (Dec 11, 2017)

_The previous owner of the 6" chuck drilled and tapped the 4 holes with the Allen screws and the current backing plate registers on the inner circle next to them. This measures about 4", I'll get a better measurement later but I picked up a couple feet of 4.25" round from Craigslist and will use this for the backing plate.


_


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## BGHansen (Dec 11, 2017)

Sounds like a great project, learn a valuable skill and save yourself some time with future chuck changes.  If I was taking on the project, I'd measure the spindle threads via the 3-wire method or a screw pitch micrometer for starters.  Then turn an arbor to match the threads.  That way you can check your new backing plate threads when you start getting close.

Bruce


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## Alittlerusty (Dec 11, 2017)

I cut a 3 1/2" slug off the round stock (first mistake the spindle is only 1.75" long should have cut it around 2.25" to leave room for facing both sides).  Mounted it in the 9" 4 jaw because it has a 3" thruhole and then I can thread all the way thru. I got it running true within a thou and faced the end. Turned the o.d. Down abit to pretty it up. Drilled and bored the center to 2.200". Started boring a relief groove to 2.375"(threads on spindle don't go all the way to spindle register surface) and stopped.


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## Alittlerusty (Dec 11, 2017)

I agree totally BG now lol


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## Alittlerusty (Dec 11, 2017)




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## GarageGuy (Dec 11, 2017)

I learned internal threading from watching this tubalcain video.  It takes some practice.  I would do a few smaller pieces, then go for it on your backing plate.


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## Alittlerusty (Dec 11, 2017)

I too figured practice was needed so dug around scrap bin and found a candidate to try first. This came from a bulldozer bottom roller assembly


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## Alittlerusty (Dec 11, 2017)

Knocked the split bushing out and mounted in my 12" 4 jaw bored to size


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## Alittlerusty (Dec 11, 2017)

Next I took a 3/4" piece of round and cross drilled then filed the hole square to accept a 5/16" hss tool , drilled and tapped the end for a set screw, now a home made internal threading tool is born.


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## Alittlerusty (Dec 11, 2017)

Mounted the tool, proceeded to cut the threads and was giving to much chatter so I rotated the tip down abit and that helped a lot since I didn't have enuff clearance on the front of the tool.continued to cut the thread until the profile looked right and lo and behold it fits reasonably well.


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## Alittlerusty (Dec 11, 2017)

I had the tool mounted on my turret tool post which replaces the entire compound so I had to advance the tool straight into the cut. I will set it up with the lantern tool post and compound and advance the cut with the compound set at 29 1/2 degrees so the tool will only be cutting on one side but for a first attempt I'm quite pleased.


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## benmychree (Dec 11, 2017)

I HOPE THAT YOU MADE THAT BACKPLATE OUT OF CAST IRON ---- STEEL IS APT TO GALL UP AND RUIN YOUR SPINDLE.


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## Alittlerusty (Dec 11, 2017)

Hmmm, hadn't really thought this thru yet. The practice piece is steel and the piece I intend to use for the back plate is mystery steel which I assume is just hot roll. I certainly don't want to ruin the spindle threads. Going forward I think I'll make a male thread as BG suggested to check the fit of the threads without having to remove the chuck each time to test them and maybe come up with something else to make it out of. I know I could buy a semifinished backplate but am hoping to make one from scratch. Anybody know of something I could repurpose out of cast iron? Weight plates come to mind but not sure the centers are thick enuff. I would need 2" thick for the threads.


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## benmychree (Dec 11, 2017)

I would buy the semi finished plate, and yes, the thread gage is the thing to make to fit the threads, and when threading the backplate, do not try to fit the threads too tight, also the counterbore that fits the register diameter on the spindle.  I have made thread gages for all the lathes that I have owned it is time well spent.  As far as backplates are concerned, on some of my machines, I made a pattern with just enough extra stock to cleanup machine and had them cast at an area foundry.


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## FOMOGO (Dec 11, 2017)

Automotive/truck flywheels are generally made out of cast iron and should be pretty good quality as compared to weights, and you would already have your center bore accurately located. Should be dirt cheap at the junkyard or free if you know the right folks. Mike


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## Alittlerusty (Dec 20, 2017)

Ended up buying a semi finished back plate from shars. Turned it down from 8" to 5.25" faced it then turned a 4.000" register. Centered it on my rotary table and drilled the 3 mounting holes 120 degrees apart. Bolted it up and I'm getting about .008" runout.  I'm not sure but I think I either didn't get a nice square corner on the bottom of the register or the mounting holes are off abit and to tight pulling it off abit. The register seems to fit excellent with no play but could be made twice as long. The saga continues


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## jrkorman (Dec 25, 2017)

benmychree said:


> I HOPE THAT YOU MADE THAT BACKPLATE OUT OF CAST IRON ---- STEEL IS APT TO GALL UP AND RUIN YOUR SPINDLE.



Beginner here - I've seen items - collet adapters and thread protectors for example - made out of various materials.

Does your statement about steel apply? The spindle on my lathe is not something I want to screw up!

Thanks,


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## benmychree (Dec 26, 2017)

I STAND BY MY STATEMENT; DO NOT USE STEEL.


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## Armourer (Dec 26, 2017)

benmychree said:


> I STAND BY MY STATEMENT; DO NOT USE STEEL.


Hi Benmychree, Just out of curiosity how will it gall the threads on the spindle? I am a newbie and don't understand how/why it would. I am going to attempt a similar project and bought some steel to do it, not cast. Now I am rethinking my purchase...


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## benmychree (Dec 26, 2017)

Armourer said:


> Hi Benmychree, Just out of curiosity how will it gall the threads on the spindle? I am a newbie and don't understand how/why it would. I am going to attempt a similar project and bought some steel to do it, not cast. Now I am rethinking my purchase...


All commercially available lathe chuck mounts are cast iron, and there is a reason for that, which is that for one thing, crud and small chips are sure to get into the interface of the spindle threads and the backplate threads, no matter how clean we think they are, and we know how hard it can be to remove a chuck after taking even not-so-heavy cuts; if there is a piece of grit or small chip in that interface, it can start a gall that keeps getting bigger as we try to unscrew the chuck, damaging the spindle and backplate threads, and I have seen lathe spindles that have been damaged in this fashion.  Cast iron, being softer than steel can imbed these small particles, avoiding damage.
Thread protectors and possibly collet adaptors that thread on the spindle are OK to be made of steel because, for one thing they do not get jammed on by cutting force and impact that we see with chucks.  My advise is to save the chunk of steel for another project and buy or have made a cast iron blank.


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## Armourer (Dec 26, 2017)

benmychree said:


> All commercially available lathe chuck mounts are cast iron, and there is a reason for that, which is that for one thing, crud and small chips are sure to get into the interface of the spindle threads and the backplate threads, no matter how clean we think they are, and we know how hard it can be to remove a chuck after taking even not-so-heavy cuts; if there is a piece of grit or small chip in that interface, it can start a gall that keeps getting bigger as we try to unscrew the chuck, damaging the spindle and backplate threads, and I have seen lathe spindles that have been damaged in this fashion.  Cast iron, being softer than steel can imbed these small particles, avoiding damage.
> Thread protectors and possibly collet adaptors that thread on the spindle are OK to be made of steel because, for one thing they do not get jammed on by cutting force and impact that we see with chucks.  My advise is to save the chunk of steel for another project and buy or have made a cast iron blank.




Thank you for the reply! Makes total sense now, guess I will go back shopping again!


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## Billh50 (Dec 26, 2017)

I have never seen steel to steel gall. I have seen steel to aluminum and stainless to steel gall. But never steel to steel. And I thought chuck adapter and face plates were made from Cast Iron because it absorbs vibration and avoids vibration suction. Also is cheaper and faster to cast.


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