View Full Version : Cutting Threads

05-10-2009, 10:33 PM
I'm new to machining. I am trying to learn how to cut threads in mild steel. When I cut the threads they appear to be galled, very rough. I am not using any lubrication or coolant. Also using a carbide insert type bit.

My procedure is to turn the shaft down to the maximum diameter, then start laying threads about .005 per pass. I am turning down to the minimum diameter using the cross slide.

Should I use the compound slide instead?

My guess is that I should be using a HSS bit with a radius that matches my thread root and use a coolant. I am hoping somebody can point me in the right direction.


05-10-2009, 10:59 PM
Hi Jb, what grade of steel are you using? Softer steels like rebar or 1018 are very hard to get a good finish on. Also use some thread cutting oil. Most prefer the dark cutting fluid like you find at Home depot or Lowes. I just use the Lps gold that i have. Try cutting threads in some 12L14 or some 1144 stress prof and you will probably have better results. Also feeding with the compound set at 29 degrees tens to help in that the cutter does most of the cutting with the leading edge instead of both edges.

05-10-2009, 11:07 PM
I assume it is 1018 CRS. I picked up some cutoffs from work to practice on.

Bill Pace
05-10-2009, 11:42 PM
Its very difficult to get smooth threads on cold rolled, they will be quite gnarly looking. A good sharp HSS bit carefully set as precisely as you can get it and at least some type cutting oil with light cuts will give you the best results, but will probably still not be 'pretty threads'.

Mike Burdick
05-10-2009, 11:52 PM
.... I am not using any lubrication....

I'd say that is the major source of your problem! Get some dark thread cutting oil at a True Value Hardware store, Lowe's, or Home Depot and use it liberally!


05-11-2009, 01:50 AM
I'm new to machining.
...then start laying threads about .005 per pass. I am turning down to the minimum diameter using the cross slide.

Should I use the compound slide instead?


I am also pretty new at this but a couple of things I do.

first .005 (or more) is fine to start with but towards the end it may be too much. might want to back down to .0025 or .001 at the end depending on the pitch that might be a pretty deep cut.

using the cross slide, the tool is cutting on both front sides of the cutter.
this might be your main problem. j

if you can use the compound and just cut with the leading front edge of the cutter it should be easier on the lathe.
(it is doing half the work the two edges had to)

my process is turn to diameter zero both cross slide and compound dial
(which I don't always do since mine are not adjustable)


advance compound (less and less as thread gets deeper)
engage carriage
disengage carriage
back cross slide off so tool beyond major diameter
crank carriage back before start of thread
advance cross slide to zero

you might want to wait and see if I get blasted for promoting
some threading blasphemy but if I do we will both learn something

05-11-2009, 01:57 AM
Adding to that blasphemy..

Compound at 29.5 (or so) degrees, last couple of light passes use the cross side only to clean up both edges of the thread.

I've found it easier to thread from the back and run the lathe in reverse. That way I can thread at higher speeds away from the chuck. Less stress on me and better finish.

Jim Hubbell
05-11-2009, 03:02 AM
If you are not threading into a groove you definately want to back off cross slide BEFORE the lead screw is disingaged. Keeps the tool point from snapping off. Plenty of good cutting oil and light cuts as mentioned already.

Your Old Dog
05-11-2009, 05:29 AM
My sins were cutting tool not perfectly on center and too much unsupported stock sticking out of the chuck. I use a very small container with thread cutting oil and brush it on with a 1" wide paint brush. I also didn't have my lathe adjusted up tightor snug enough on the gibs.

(when I absolutly have to get it right, I have a handle I screw into the end of the quill and turn the lathe by hand. This is an idea I got from Evan)

Forrest Addy
05-11-2009, 07:21 AM
Use oil and a hand ground HSS tool. Yeah I know. Tha pre-ground carbide once are so convenient and hand grinding a HSS a hassle but it does work better.

Any kind of oil will do but the best is either bacon grease for mild steel (honest!!) or Rigid heavy black pipe threading oil. Later on you'll accumulate a selection of cutting ouls and coolants like Tap Magi, Chesterton, mineral lard oil, etc but in general don't thread or use form tools dry.

05-11-2009, 11:33 AM
And... at a scraping class I attended last year...Forrest also recommends Sheep fat for storing tooling.

Forrest was green before it became a buzz word.:D

But this is all good. I'm not "allowed" to eat bacon, but now I can buy it and say "it's for my threading". Ah.. the thought of sizzing bacon fat on my tool make my mouth water.

05-11-2009, 01:19 PM
This is a pretty good starting place on threading.

05-11-2009, 06:14 PM
When I started cutting threads 8 years ago, I made threads on plastic pipe until I got a feel for it.

Then I made threads on steel.
I showed them to my brother who had been cutting threads for 25 years.
He said I was doing everything I could do wrong as wrong as I could do it.

I remember finding out that the .015" cut at the threshold of chattering was a problem. He said take off .001, .002" or .003" per cut.

Now my threads look good from 3 feet away, but under magnification, I still have some tearing.

My brother's threads look like jewelry under magnification.

Maybe some of us were meant to only make functional threads and not fancy threads.

J. R. Williams
05-11-2009, 06:54 PM
Too items that help - SHARP tool and plenty of the old reliable high sulfur, black, cutting oil. The old Tap-magic with the high chlorine base fluid will help the cutting. A rigid tool mounting and a good lathe can help teh quality.


05-11-2009, 10:42 PM
Thanks "ALL" for the input! I'll give her another go with oil, compound rest, and HSS bit. I will add results here tomorrow.

05-11-2009, 11:00 PM
There have been several lubes I have used with success, bacon fat one of them. The every present dark threading oil, LPS gold, re-li-on is good but high dollar, Castrol Moly-dee rocks but is not cheap.

05-14-2009, 06:36 PM
#1. Recognize and accept the fact that as tight as you can adjust everything, your lathe will still have a little spring in it. Don't believe? Prove it to yourself! Take a cut, then repeat the pass without advancing either cross or compound slide. Notice the little shavings? One of the best machinist in my local area operates a big CNC machine, and when cutting threads he finishes the threading process by having the lathe make FIVE passes with no advancement of the tool! Of course the speeds at which a CNC machine can operate certainly makes the threads smoother, but this repeated passes with same position of the cutting tool also helps.

#2. For us guys cutting threads with the manual lathes, Jake tought me to take light cuts off the top of the threads with a file! This takes most, if not all of those tiny burrs off the tops of the threads. When my calculations say I should be finished, I use this file method before I try a nut on the threads as a guage. I've found out the hard way that If I try the nut/guage before I file away those burrs, I'll get a false read, and wind up cutting the threads too deep!

All the above recommendations above regarding cutting oils are good. The old black (and ugly) pipe thread cutting oil is certainly good stuff. ANYTHING IS BETTER THAN CUTTING THREADS DRY! ! ! !

Now, with all these guy's inputs, go have some fun cutting threads!


05-14-2009, 10:29 PM
One trick I use is to go over the thread (while rotating) with one of those grit impregnated green rubber stickss... works wonders...

05-14-2009, 11:58 PM
At the full length of thread, (Z axis) use a narrow cut off tool to make a cut to depth of thread + a little. (blocking in the thread)
Chamfer front of part.
Set top slide to 29 degrees. (cuts on lead, wipes on trail)
Mount cutting tool in tool holder using a Fishtail Gage.
Move carriage to the part.
With crosslide, (X axis) offer up tool to work (somewhere mid span) and touch OD. Set crosslide to read "0". (left hand)
Retract tool (X axis) w/ crosslide. (left hand)
Move carriage to different position(somewhere midspan).
Retract top slide from previous position. (right hand)
Return crosslide to "0" position. (left hand)
Turn top slide handle until tool contacts work. (right hand)
Set "0" on top slide.
You now have "0" "0".
Retract crosslide. (left hand)
Move tool to the right (or in the open) of the thread.
Move crosslide to "0". (left hand)
Return topslide to "0" position. (right hand)
Move top slide desired amount of infeed.(.010 or ?)(right hand)
Engage half nuts.
As you approach the clearance of the blocked out end of the thread, shut the machine off and let it coast to a stop.
If the tool is not in the blocked out clearance, pull the spindle through by hand until it reaches desired position.
Retract crosslide beyond "0" (left hand).
Disengage half nuts.
Move carriage to the right, past start of thread.
Return crosslide to "0" position. (left hand)
Move top slide desired amount of infeed.(.010 or ?) (Right Hand)
Repeat until desired depth of thread is achieved.
There is a law of even or odds on a feed dial. I never bother with it. I just hit the same number.
Use a file to chamfer the crests of the thread.
If the nut fits almost yet not all the way, take another pass or two at the same numbers.
After this, just use lapping compound and flip the nut.
It will work great.
Use plenty of lube. Cutting oil, threading oil, old motor oil, new motor oil.
1018 is tough to thread well.

usn ret
05-15-2009, 05:14 PM
Something I use to finish threading is a Craytex medium grit bar. I wipe away all lubricant and run the lathe in reverse holding the Craytex bar against the threads to burnish and finish the threading. I also use the Craytex to finish edges in other operation on the lathe. Gives a nice polished surface.

Cliff :cool:

Don't use force, get a bigger hammer!!!!

05-17-2009, 09:31 PM
I finally got a chance to try some of these tricks and am very satisfied with the results (for a new guy)! I used a small hss bit, cutting oil, compound slide movement, and very small cuts (.002 per pass).

I'll have to say that I learned a whole lot more than threading on this exercise. I was making some knobs with 3/8-16 threads. I had to scrap two of them because I bent the shafts during the parting operation. I thought I would be fine since I was turning b/w centers. Clearly I should have re-chucked before cutting off.

Thanks again for all of the good advise.


05-18-2009, 08:43 PM
Jb-mck get with Bill pace and come over one Sunday evening .I will show you how to cut threads and any thing else you want to know.

05-18-2009, 09:43 PM
Thanks for the invite. I will definately take you up on it. I am tied up this weekend but what about the next Sunday? I will drop you and Bill a note when it gets closer.