# Threading on mini-lathe, what am I missing?



## Skeet_Man (Sep 15, 2020)

Second time threading on my mini lathe today.  

Problem I'm running in to, both times, is depth of cut being too shallow.

Thread is 1/2-28.  Shooting for class 3 fit with major diameter of .498 (confirmed spot on w/ dial micrometer) and target minor diameter of .457

This should give me a depth of cut of .0205"  Lathe dials are standard read, so .001 on the dial is .001 in (removing .002 if turning OD).

Threading with the compound set at 29.5*, this means I should read .0235 on the dial to correct for the cosine.

I cut the threads to that, then went in another .001 on the crossfeed to clean up (which theoretically meant I should have been at .456 minor diameter).

The problem I'm running into is I'm still not deep enough to either form a V or screw on a test nut.  I even went another .003 or .004 and I'm still coming up shy, so it's like I'm not even close.  I touched off, did a scratch pass, ect so the initial depth setting was correct.

I put a dial indicator on the compound, and it's off .001 which i took into consideration so, on paper, my cut was correct if a touch over.

I bought a die as a way to hedge my bets, and ended up using that to finish the thread, but would like to remove that necessity in the future.  I realize the huge limitations of my equipment and I may just be running to an inherent issue with the machine, but thought I'd ask to see if someone can catch something I'm missing.  Checking with wires would probably be my best bet but I wouldn't use them nearly enough to justify the investment.


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## pacifica (Sep 15, 2020)

You could run a 1/2-28 die over the threads ,might be the least expensive accurate way to do this. 

Otherwise there are a number of variables that may be difficult to account for: machine rigidity, tool rigidity, springback, depth of cut, tool sharpness, ability to measure precisely, speed of cut. number of passes, experience.


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## aliva (Sep 15, 2020)

If you are using the compound to advance into the cut, remember the cutter will only advance 1/2 of what the dial reads because of the 29.5*


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## addertooth (Sep 15, 2020)

If you have flex in your tool, tool post, crossfeed, etc... this can make you come up a few thousandths short when under the load of cutting threads.

Your math looks good.  1/(Cos 29.5) * (desired depth cut of thread)  = dial measurement cut on crossfeed.  You show 0 .0235 ... whereas I calculate 0.02355, and would round up to 0.0236, and might go deeper by a few thousands If I want it to thread easily.  Also, finish matters.  If the finish is rough it turns up little tiny burrs, which will make the valley and sides of the threads "bigger" than what your dial indicates.


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## mikey (Sep 15, 2020)

I get 0.0268" estimated DOC with the compound. These estimates for thread depth are exactly that; estimates.  Those estimates are just to get you close so you can start checking your pitch diameter to get the fit you need. 

Much also depends on the tool, material, rigidity of the tool post/gibs/structure of the lathe, etc. You do not need a huge, rigid lathe to cut precision threads. I can cut Class 3 thread all day on a little Sherline lathe. However, your gibs and everything related to holding the tool rigid must be adjusted well. If I had to guess at the problem, I would guess that the support under the tool is moving.

You did not mention the kind of tool you're using. If you're using carbide then that requires more rigidity in the lathe. If you're using HSS then that requires a small flat at the tip and sharp edges. I am assuming you assured the tool was set perpendicular to the work. It might help to show us a pic of your set up.

Your goal is a Class 3A 1/2-28 thread and you're trying to get that without thread wires or a thread mic but a Class 3A fit requires that you hit a pitch diameter of between 0.4768 to 0.474". Without some way to read the PD, either wires or a mic, you cannot cut a Class 3A thread. And you are not going to get that with a die. 

Cutting precision threads is fun, an exercise in precision turning and hitting pitch diameters within a narrow band, but you need the right tools to do it. My suggestion is to buy a thread mic.


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## Asm109 (Sep 15, 2020)

What is the tip of your cutter look like?  Dead sharp or the flat called for by the thread standard?


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## higgite (Sep 15, 2020)

Is your compound for sure set at 29.5* from perpendicular to the work piece or 29.5* from parallel to the spindle axis? It should be the former.

Tom


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## ub27Rocks (Sep 15, 2020)

My additional suggestion is to set your compound at 90 and ignore the 29.5 stuff altogether. For 28 tpi it won't matter. Sandvik's Training Handbook Section C4 has an excellent description of the 3 methods, including depth per pass. Only in North America do ppl seem to persist with the 29.5 method, seems EU and elsewhere have moved on from that with manual machines.


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## tq60 (Sep 15, 2020)

Pro intent is also with cross feed and not compound

Our 1944 ish 16 in L&S has a locking compound that when locked properly comes OUT 2 turns.

The intent is you sneak up on final cut for work, threading primary function then you activate the lock or limit.

Now you can back off 2 turns and come in on next work.

However, for small or not rigid lathes the 29.5 process is to allow only cutting on one side of the tool.

Less stresses on the system.

Sent from my SAMSUNG-SM-G930A using Tapatalk


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## erikmannie (Sep 15, 2020)

I cut threads on a small lathe, and I would not want to do it without either thread wires or a thread micrometer.

I use these:






						0-1" Screw Thread Micrometer
					

Shars Tool




					www.shars.com
				




and






						Brown & Sharpe 599-4816 48-Piece Thread Measuring Wire Set: Amazon.com: Industrial & Scientific
					

Brown & Sharpe 599-4816 48-Piece Thread Measuring Wire Set: Amazon.com: Industrial & Scientific



					www.amazon.com
				




I hate Shars products more than anybody, but I have to admit that their thread micrometer is wonderful.


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## Masterjuggler (Sep 15, 2020)

Use some small drill bits or sewing pins as thread wires and compare against a factory made thread. Doesn't matter what diameter the pins are, you just need to get the same measurement as the factory thread. Not the class fit you're looking for, but that will at least give you a proper measurement of how far off you are from your desired diameter, without having to buy wires.

You said you're using a dial indicator against the compound though. Assuming that's accurate, which is usually safe to say, then the issue probably lies in the part (or the machine) flexing during the deeper passes. Run a few spring passes at the end until it stops biting, and minimize your part stick out. Using a sharp HSS tool might be better than a carbide tool for these passes.

EDIT: By the way, as far as the 29.5 degrees thing goes, it helps with chatter for a mini lathe. Especially if it isn't braced against anything and has stock bearings, it needs all the help it can get.


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## erikmannie (Sep 15, 2020)

You can get a thread measuring wire set for really cheap:






						HHIP 4200-0241 48 Piece Thread Wire Measuring Set: Amazon.com: Industrial & Scientific
					

HHIP 4200-0241 48 Piece Thread Wire Measuring Set: Amazon.com: Industrial & Scientific



					www.amazon.com


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## springer (Sep 15, 2020)

Lots of things here. I'm no expert, but I don't mess with the 29.5° stuff. Just feed straight in. Also, flex will be a huge thing. Are you doing any spring passes to verify you're actually cutting what you feed in? Even still, the last little bit might require further feed to get it to cut while have spring back if the cutter. Like ulma said, carbide will require more force. 

Also, a 3a fit will be a tight fit anyways. I've cut some to extreme tolerance on both sides and even tho they measure correct by the book, they won't thread on with out lots of friction which might make you think it doesn't actually fit.

Try running a triangle file down the root of the threads to clean them up and then a at file across the tops. .498 is large on the major diamter and might be interfering with whatever female thread you are using if they were cut with anything less than a perfect 60° point. 

You cam count numbers and measurements all day long, but in the end if parys don't fit, they don't fit. Cut them to fit and you're good no matter how much you dialed in on the cut or what the measurements actually read.


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## Masterjuggler (Sep 15, 2020)

springer said:


> Try running a triangle file down the root of the threads to clean them up and then a at file across the tops. .498 is large on the major diamter and might be interfering with whatever female thread you are using if they were cut with anything less than a perfect 60° point.


I didn't even think of that, but yeah, nothing about filing the top has been mentioned. The male and female threads aren't supposed to come to perfect V's, they should have small flats. The cutter will never have a perfect tip, so the thread valleys won't have perfect angles.


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## john.oliver35 (Sep 16, 2020)

Also, if this is in mild steel there is likely a burr built up on the OD that needs removed with that flat file across the top.


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## SLK001 (Sep 16, 2020)

How are you supporting your work?  Is there a center support via the tailstock?  How many spring passes are you making?  Does material still come off with your spring passes?

If you are NOT using a center, your work is probably deflecting from the tool pressure.  Without a center, your threads will be anything but class 3A, since the PD will vary greatly as you near the headstock end of your work.  You show no pictures, so we have to assume that you have a SHARP tool and it is set up properly.  

Trying to cut class 3A threads on your first try is an exercise in futility.  You should practice making 1A and getting it correct, then proceed to class 2A and getting that correct BEFORE you jump off to class 3A threads.  You state that you are shooting for a minor diameter of 0.457".  Minor diameters are dimensions that CANNOT be measured by most people (and, they don't really matter).  What you HAVE to be able to measure is your Pitch Diameter (PD).  How are you going about measuring your PD?


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