# Cutting Low Tpi Threads



## BillWood (Jun 8, 2015)

I often see warnings regarding cutting low tpi threads, for example a 3 tpi thread off an 8 tpi leadscrew.

I also see that cutting these threads with a manual crank on the leadscrew or on the lathe headstock is an acceptable workaround.

What precisely is the problem please ?

Is it a bit like going up a steep hill in a small car in a high gear ?

Where will "failure" occur ie broken plastic spur gears on a chinese lathe, broken metal gears on a SB9" or equivalent or better, jammed spur gears, broken/jammed half nuts, stalled motor, broken leadscrew ?

Or would the belts simply slip if the load was too great ? Assume a South Bend 9" 60 years old.

Bill


----------



## higgite (Jun 8, 2015)

I don't think the caution has to do so much with load on the lathe. You can take as light a cut as you want.  It’s more about tool speed and operator reaction time. Imagine cutting a 12 tpi thread at your lathe’s slowest speed. Now imagine cutting a 3 tpi thread at the same spindle speed. The carriage will be moving 4x faster. That full second that you had to react to stop the spindle or retract the tool or disengage the leadscrew at the end of the 12 tpi thread is only 1/4 of a second at 3 tpi. Hence, the hand crank to prevent crashes. Or cut the threads from left to right. Others may have a different take on it.

Tom


----------



## Wreck™Wreck (Jun 8, 2015)

Large depth of cut 3/16" or more, that is a lot of tool in the work, also the pitch is larger then the spindle on most micrometers and requires the use of a gauge block + wires to measure.
Good Luck


----------



## brino (Jun 8, 2015)

higgite said:


> Or cut the threads from left to right.



At first blush that looks like a great solution.........unfortunately you end up with left-handed threads!!!

You'd hafta run the lathe backwards with the tool upside-down.....but make sure nothing would unscrew first.....

-brino


----------



## higgite (Jun 8, 2015)

brino said:


> At first blush that looks like a great solution.........unfortunately you end up with left-handed threads!!!
> 
> You'd hafta run the lathe backwards with the tool upside-down.....but make sure nothing would unscrew first.....
> 
> -brino



Yep, that is correct. I just figured how to set up for left to right cut was a different topic.

Tom


----------



## 4GSR (Jun 8, 2015)

I did cut a odd pitch thread close to a 1/2 pitch many years ago on my 9 South Bend lathe.  It was about .090 deep and about .240 root width.  That thread was about 1 1/2 turns long about 1 1/8 diameter.  Ran it in back gear low as it would go.  Came out nice !


Sent from my LG-V410 using Tapatalk


----------



## tertiaryjim (Jun 9, 2015)

I had a gaul start while threading 4tpi and realized that by measuring the degree of rotation of the gauled section I could determine my reaction time.
It's surprising how fast one can move.


----------



## BillWood (Aug 28, 2015)

Hello,

Well I did it.

Worked fine.

Cut 8 tpi at 50 rpm on a piece of cast iron (gym weight)

See attached.


----------



## eeler1 (Aug 28, 2015)

DANG, internal no less.  very cool!!


----------



## brino (Aug 28, 2015)

tin foil to catch those chips....good idea!
-brino


----------



## Ulma Doctor (Aug 29, 2015)

i hadn't considered using iron weights for turning material...


----------



## atunguyd (Aug 29, 2015)

Can I ask why weights would need an internal thread on them? 

Sent from my GT-I9505 using Tapatalk


----------



## mcostello (Aug 29, 2015)

He wants screw on weights!


----------



## Doubleeboy (Aug 29, 2015)

Nicely Done!!  If you love your lathe you might want to consider covering your ways and using a vacuum while turning cast iron.  Cast iron dust is about the fastest way to wear out a lathe that I can think of.  If it gets into your gearboxes or carriage it will cause premature wear.  Even a spanking new lathe with fresh wipers will let cast iron grit into delicate areas.

cheers
michael


----------



## BillWood (Aug 29, 2015)

atunguyd said:


> Can I ask why weights would need an internal thread on them?
> 
> Sent from my GT-I9505 using Tapatalk



It started as an experiment - I needed to learn how to cut internal threads on big flat plates before reaming out and rethreading a couple of  11tpi faceplates to fit my 8tpi lathe nose.  Wanted to sort out the logistics of the cutting sequence and workholding before possibly making a mess of a Myford face plate whilst converting it to 8 tpi.


It seems to be finishing ok, which is a bonus, so now I will finish then nicely and see if I can slot them on the mill and do something constructive with them ie sacrificial face plates, table for pillar tool ?,  table for something else, cut in half and use for a sharpening jig trunnion ? Base for a small tool/jig/fixture of some sort ?

Maybe I could trepan the thick rims off the edges and use them as wheels or cogs or pulleys on something ? A lot of metal is sacrificed off the rim to get the entire wheel flat

Would like to see if I can get a better finish on Mill carbide facecutter vs lathe HSS toolbit - am a  mill newbie just bought it a few weeks ago.

I got some other weights a few days ago for $2.

Some people have reported that weights can be full of nasty uncuttable rubbish metal but it looks like I got lucky on this particular one.

I made a big effort to clean up as well as I could after a session due to the reported risks of nasty abarasive materials in the skin and also in the general dust - I am aware that some people have used dust extractors when machining cast iron.

Bill


----------



## Bill C. (Aug 29, 2015)

Cast Iron can be dirty and gritty.  Try to protect the ways either remove any oil and grease or use covers. If I read your post correctly you are cutting internal threads.  The last time I cut threads with a boring bar I turned the bar over so it cuts on the back side of the hole.  Doing so lets you see what is going on.  

A quick tip if you divide 1 inch by number of threads will give you how far the threading tool will travel in one revolution.  Ex: 1.000"/ 20 tpi = 0.050 per rev.  

The Best with your project,


----------



## drom68 (Aug 30, 2015)

Nice job.  When you mentioned low TPI threads I thought of ACME threads.  Nothing really different except the tool.


----------

