Gunsmithing Internal threading question

crumpmd

crumpmd

Sergeant
Full Member
Minuteman
Oct 28, 2007
593
3
Huntsville, Alabama. USA
#1
When cutting internal threads, is it a good idea to use a left hand cutter and reverse the spindle direction to cut from the inside out?
I wonder if cutting with a right hand cutter up to a shoulder, blindly, may be done on the lathe with out crashing into the shoulder. I do not have a DRO but I can put a dial indicator on the carriage to measure travel during the cutting. I worry about being able to stop the feed in time. Also it may take 4 passes to cut the threads and each pass is an opportunity for disaster. I ordered some HSS to cut with so I can slow down. The carbide wants more speed than I am comfortable with.

And if you do cut from inside out, when you advance the cutter to make the next pass would you go back to the start point, start the chuck, advance the cutter and then engage the half nut on the fly, trying to catch the right start point.

I have a lot to learn, obviously, but the votech college only offers day classes and I can't quit my day job just yet. Maybe I can get a tutor.
 
#2
Re: Internal threading question

I have been threading as normal but stop the headstock long before reaching the shoulder and spin by hand from there. I run the bit in by hand and place a piece of tape on the chuck/spindle as a stop reference.

If there is a better method I am all ears. I am still a novice.

I saw one pic on here a few years ago where the guy installed a crank-handle on the outboard end of the spindle. Seemed like a good idea to me.
 
#3
Re: Internal threading question

Assuming RH threads, I think that's "backwards"? RH internal threads, wouldn't you start at the shoulder, and feed out?

I've heard of people reversing the lathe so they don't have to cut towards the shoulder on OD RH threads, and you also have to turn your cutting tool upside down.

But, I may be way off base....

Cheers,

Bill
 
#4
Re: Internal threading question

You just have to practice cutting them outside in. Practice on scrap a few times befor attempting a barrel. It can be tricky to disengage the half-nuts and run the tool in simultaneously, especially if you have just cut some external threads and have gotten into a rhythm of backing out. Stay focused and it will get easier. Please note that I am no pro, just some home shop tinkering.
 
#5
Re: Internal threading question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: WRM</div><div class="ubbcode-body">Assuming RH threads, I think that's "backwards"? RH internal threads, wouldn't you start at the shoulder, and feed out?

I've heard of people reversing the lathe so they don't have to cut towards the shoulder on OD RH threads, and you also have to turn your cutting tool upside down.

But, I may be way off base....

Cheers,

Bill </div></div>

Right you have to turn your tool upside down but still run the lathe counter clockwise. You can reverse the feed and thread inside out if you have to but you will have to make a cut out so you have a place to start. If your not doing a blind hole do it the regular way and just make sure you have plenty of room to stop. Either way it takes some practice to get good at.
 
#6
Re: Internal threading question

I use a left hand tool and run the lathe in rev. While cutting from inside out. I just set the carriage stop where I want to start in the hole and start cutting. Super easy and don't have to worry about destroying a 200.00 tool.
 
#7
Re: Internal threading question

The stop is a wonderful feature, for so many things. We always cut OD threads to the stop (with a relief grrove), and it came in handy for precision facing cuts....

Now I understand what you guys mean by LH/RH cutters. You can cut on "either side" of the ID, dending on which way the spindle is turning....

Still say that to cut RH ID threads on the "front" of the hole, you'll thread from headstock to tailstock (out), as described above. You could swap LH/RH cutter and put the lathe in reverse and do the same thing on the "back" of the hole. Make sure your spindle/chuck won't come off in reverse....

Cheers,

Bill
 
#8
Re: Internal threading question

I do things the regular production way. But I always have a 1" indicator on a magnet on the ways. I usually set -0- at a 3/4 turn on the indicator so I can see it coming up.

I also set up the compound so the main cross feed handle is just past bottom dead center so I don't have to worry about twisting it the wrong way. (Which has happened.) Of course cutting a groove at the shoulder just wider than the threading tool will give you a run out area. While I can get real close, it's ALMOST impossible to thread -0- to an indicator. I find it's always better to give myself an easy way out, if possible.

Remember the number you use to engage the half-nut.

You'll be suprised how much difference some practice makes. I would make a LOT of chips and get pretty good at it before I made the money cut on a real action.

But of course, you do what you wish. It's your money and your equipment.
 
#9
Re: Internal threading question

I appreciate the responses.

Until I have HSS cutters I won't be trying to turn anything by hand. I already tried that once and broke a chip off a $10 carbide insert.

So as far as starting your second pass..... I asked earlier if anybody tries to catch the thread dial on the fly or do you engage the half nut and then start the lathe with the cutter bit dialed in on the next pass?
 
#10
Re: Internal threading question

The insert shown is an inside RH insert or an outside LH insert, same/same. It's the only way to thread a blind hole in a lathe. When threading a blind hole without a travel dial I'd simply touch off at the bottom of my hole with my tool and then back it up .125" or so. Place a mark on your bed ways with a sharpie and use that as a visual stopping point. It works better than you think. You don’t need to thread all the way to the bottom of the hole, look at the threads in any muzzle brake. Ditch the HSS for threading, inserts are the only way to fly and the finish is superb.

These are the threads of Remington 700 receiver that I picked up and chased under power.


scuvk1.jpg


1230ehv.jpg
 
#12
Re: Internal threading question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: crumpmd</div><div class="ubbcode-body"> So as far as starting your second pass..... I asked earlier if anybody tries to catch the thread dial on the fly or do you engage the half nut and then start the lathe with the cutter bit dialed in on the next pass? </div></div>

Once set up, I'll start the initial threading pass with the tool set to fall out in the middle of the receiver. I'll kill power to the machine but leave the half nut engaged when the tool is close to the threads. I then spin the chuck by hand until the tool is just inside the part to be threaded. I adjust my compound and cross feed to get my tool tip centered in the valley of a thread and set all dials to zero. I disengage the half nut and get the tool out of the threads. I back the carriage out, turn the machine on, set all dials back to zero and make my first pass. Prior to any of this, I've already established the depth I want my tool going to and made reference marks to indicate such. My first pass is made with the dials on zero and the finish of the cut will dictate if I feed the carriage in for a deeper cut or not. When chasing internal threads, I never feed in with my compound, only the cross feed and only .001" at a time. Once I'm pleased with the cleanup I'll make several tool pressure cuts to get a good finish.
 
#13
Re: Internal threading question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: crumpmd</div><div class="ubbcode-body">So do you cut threads in one pass?
At what speed?
Looks good thanks for the reply. </div></div>

I thread at 200rpm, internal or external.
 
#14
Re: Internal threading question

I see. Thanks.

My current application is threading the inside of tubing and making a cap to thread into the tube. I am doing it this way so as to suspend a round bar in the center of the tube leaving .05in space between the bar and the tube to accommodate debris that could keep me from getting the bar out later. (I have a form 1 for this project in case anyone wonders.) The caps will have a contour machined into them to accommodate the bar and of course one of the caps will also be inside threaded like a muzzle brake.
 
#15
Re: Internal threading question

If your threading new, unthreaded steel tubing, I'd use the compound to feed in with. Dials still on zero. Thread your caps first using a gauge for either a class 2 or 3 A fit. Use the caps as a go gauge when you thread the inside threads and you'll have a matching class 2 or 3 B fit.

Loose fitting threads for what your doing will cause you to have a baffle strike in a bad way.
 
#17
Re: Internal threading question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: NotAGuru</div><div class="ubbcode-body">There is much easier way to go about this as well. </div></div>

Share the info
grin.gif
 
#18
Re: Internal threading question

Well what i would do is make a inner and outer piece. The inner will have one end that is slightly larger to thread. The rest turned down to the id of the tube. Then just put some threads into the tube deeper then needed. Drill a hole .04 larger then the bore size to avoid stikes. I would probly start with a larger piece and then use a mandrel to turn the od true to the id before threading it for the outside tube then the id for the barrel. Then its just a matter of cutting slots and you ready to assemble. I would put some weld on it insure that it stay together. Thats just my .02$.
 
#19
Re: Internal threading question

I take it you mean to have the rear-cap integral to the stack. Fewer parts to wiggle loose and cause a strike. That's how I'd do it too.
For what it's worth, I thread into the blind the good old fashined way. I run my threading tool in as far as needed, set an indicator, cut back into the material to thread depth, and wiggle each way about 15 thou. Then, when making a pass, I just disengage the halfnuts as close to 0 as possible, but i've got +/- .015 to play with. I'm just a hobbyist, so take it with a grain of salt.
CW
 
#21
Re: Internal threading question

I'd buy a tap for the ID threads, if doing on a manual.
Both ends would be done in less time than to chase one.
This is now your gauge.

Chuck up mat'l for caps
Rough face and turn.(You can go about .062 past where you will part.)
Finish face and turn.
Chamfer .010/.015 smaller per side than the minor of the thread.
Make thread relief undercut 1.00/1.50 times the pitch, and same dia as chamfer.

Practice threading on the OD, and if it screws up, all you have to do is part off and try again!

Carbide inserts are the way to go!
 
#22
Re: Internal threading question

Roscoe, nice set-up
Inserts are the way to go.
I-Internal< E-External, N Works for both good.

Crump, if you know how to single pass ID Threads in only four passes, please teach!

If you have to go slower than you planned, use some black oil.
It will help with the surface finish.
 
#23
Re: Internal threading question

Well I cut a piece of 1inch Al bar stock to practice on last night. I cut it just over 6 inches. Centered in the 3 jaw chuck I drilled a pilot hole then a 3.5 in deep hole, then turned it around and repeated. So I have a 6.25 inch bar with a 0.25 hole down the center.
I took the bar and turned it between centers to .890 and it will just barely fit into the tubing. I will turn a little more off it next week to give more clearance, perhaps .05 will do.

As far as threading, I really do not know how many passes it should take. I could easily take .05 at a pass using pretty high speed with the carbide insert bit(keep in mind I am working on Aluminum for now). But I would think it would be better to take more passes, and only cut 10 thou or less per pass and at a lower speed so I can control better what I am doing and not crash into anything.
 
#24
Re: Internal threading question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: crumpmd</div><div class="ubbcode-body"> As far as threading, I really do not know how many passes it should take. </div></div>

Do you have the peice that will be threaded into the threads you'll be cutting?

If so, use it as a gauge. Take cuts .005" at a time and use this formula to determine the rough thread depth. Stop cutting deeper prior to reaching the calculated depth.

For 16 or 18 TPI

1 / 16 = .0625 x .866 = .054"

1 / 18 = .0555 x .866 = .0481"

The thread depth for;
16 TPI is .054"
18 TPI is .0481

Just substitute your TPI with the 16 / 18 above. Watch the compound and stop shy of the depth, try your threaded peice for fit and only cut as needed for a fit.
 
#25
Re: Internal threading question

Here are a few pictures. There are practice pieces I am working on. You can see the one is a piece cut off of a barrel blank. The Aluminum is the part I turned and drilled. So far it is good and if the other machining on it goes well it will be the baffle.
IMG_0007.jpg


The bevel cuts on the end cap and baffle mate precisely.
IMG_0004.jpg


When I get the tools for internal threading I will practice a while on that. For now I will start cutting the spaces in the baffle stack.
 
#26
Re: Internal threading question

it looks to my limited experience eyes that your threading tool is dull or you have a loose machine. the metal looks to be torn off instead of cut. i'd work on those external threads till i had them down before i ventured into internal threading. you can at least watch how the chips are coming off of the external thread to help diagnose what is going right or wrong with the cut.

what type of external threading tool, spindle speed and oil are you using now?
 
#29
Re: Internal threading question

Those threads are torn, not cut. Aluminum wants to spin very fast and be cut with a sharp tool. If you are going to build your project out of steel i would practice with steel. Very very very very different to cut.

make sure your tool is just slightly BELOW center of your lathe bore. Make sure the compound is set to the correct angle (some lathes are marked 30 degrees off so yours might need to read 59.5), make sure the tool is set perpendicular to the material and only use the compound to advance the cut. Try taking lighter cuts the deeper you get. At first, you aren't removing hardly any material but the deeper you go the more material you will start to remove.
Leave yourself a couple thousandths for a clean up pass and do a spring pass to at zero when your done to get any taper out.


Makes me love my little hardinge lathe even more..:D
 
#30
Re: Internal threading question

I appreciate the comments! The threads in the closeup above are from one of my first attempts, the compound was indeed set at 0 degrees and I was taking 15 thou off at a pass. The tool is HSS bit that I ground myself(my first try at that too). I cut those threads at 70rpm using no oil.

So it looks pretty ragged! But I will get there.

A Hardinge would be great but I got the Grizzly G4003G for my hobby work. There are some unused lathes with a bit of tooling and some with stocks of steel bar that come up for auction from time to time through the military and I am on the lookout at Eglin, Pensacola and Keesler. I may one day own a real American lathe. For now I am learning on a machine that will give me good practice.
 
#31
Re: Internal threading question

If you are going to grind your own HSS cutting tools go to your local hardware and buy a stick of key stock. It costs about 15 to 20% of what tool blanks do. Get a good thread guage (fishtail) and grind both ends to fit the guage. You can also use this guage to set the tool in place to the work piece.

Like Glen noted above, put the nose of the tool just slightly below the centerline of the lathe. There are several ways to do this to varying degrees.

After making a touch up on the part, set the dial on the cross slide at -0-. Then make your advances on the compound after it is set at the proper angle.

Go to the proper "halfdepth" of cut and start checking it with a guage. (There are several charts giving the half depth of specified TPI standards. Machinery's Handbook is a good place to start.) Whether that is a nut or other threaded part or a measurement and thread wires. Practice a lot whenever you can.

Good luck.
 
#35
Re: Internal threading question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: heatseekins</div><div class="ubbcode-body">

make sure your tool is just slightly BELOW center of your lathe bore. Make sure the compound is set to the correct angle (some lathes are marked 30 degrees off so yours might need to read 59.5), </div></div>
i didnt mean the whole 29.5/30 degree thing the tool is being fed into the workpice at an angle that does not create a 60 degree cut thread it is creating a thread much greater angle than 60 degrees
 
#37
Re: Internal threading question

As Nick L was saying....wouldn't that be 29.5 degrees from perpendicular (perpendicular being marked as 90 degrees on your lathe), which should be 60.5 degrees on your lathe?

Unless I am all mixed up here, I would set that compound to 60.5 for a 60 degree thread.
 
#38
Re: Internal threading question

a lot of the import lathes seem to have their compound dial marked that way. i'd set that one to 59.5*. mine is set up the way the late in the picture is and i run it at 59.5 for threading.
 
#39
Re: Internal threading question

He's set up to cut at 60 degrees in the picture. 60.5 degrees on his dial would actually be 29.5 degrees.

Also, theres way too much material hanging out in front of the chuck.
 
#40
Re: Internal threading question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Rafael</div><div class="ubbcode-body">As Nick L was saying....wouldn't that be 29.5 degrees from perpendicular (perpendicular being marked as 90 degrees on your lathe), which should be 60.5 degrees on your lathe?

Unless I am all mixed up here, I would set that compound to 60.5 for a 60 degree thread. </div></div>

Yes.
Or maybe 29.5 coming the other way.
Instead of 60.5, 59.5 might be better.
 
#41
Re: Internal threading question

Well on that lathe a 90 degree setting would drive the tool directly into the work, if it was marked to 90 degrees.
This would mean that the tool was cutting the same amount on both sides....not the best.

If one spun this compound to 60 degrees, marked, the tool would only cut on the left side........certainly better than both.

Setting it to 60.5, marked, would achieve the half degree of right-side interference most people have suggested to me.

59.5 would mean that it was not only NOT cutting on the right side of the V, but that the tool was walking further and further away from the right side of the V, again causing problems.
 
#42
Re: Internal threading question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: wnroscoe</div><div class="ubbcode-body">

Also, theres way too much material hanging out in front of the chuck. </div></div>

i was too busy looking at the compound that i didn't even notice that. wnroscoe is absolutely correct, that is way too much stick out. choke up on your work so there is no unnecessary material stick out to keep your setup as rigid as possible. with that much stick out, there is a lot of leverage working and you may end up with chatter or even a taper cut/thread.
 
#44
Re: Internal threading question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Rafael</div><div class="ubbcode-body">Well on that lathe a 90 degree setting would drive the tool directly into the work, if it was marked to 90 degrees.
This would mean that the tool was cutting the same amount on both sides....not the best.

If one spun this compound to 60 degrees, marked, the tool would only cut on the left side........certainly better than both.

Setting it to 60.5, marked, would achieve the half degree of right-side interference most people have suggested to me.

59.5 would mean that it was not only NOT cutting on the right side of the V, but that the tool was walking further and further away from the right side of the V, again causing problems. </div></div>

i think you are incorrect. on that lathe, at 60*, the cutter would feed at the same angle as the right side of the thread. if you back it off to 59.5*, you are just cleaning up the right side of the thread. if you went to 60.5*, then you would not even be touching the right side of the thread.
 
#46
Re: Internal threading question

On the lathe pictured, 90 degrees, according to the scale, would have the compound perpendicular to the spindle bore. Now, subtract 60.5 degrees (the correct setting according to that scale for 29.5 degrees) from 90 degrees and you'll have 29.5 degrees. I might be wrong but, I was taught to thread at 29.5 degrees using the compound to feed in with, cross feed/carriage set at zero.

I know, it's AFU but sometimes, it just bees that way
grin.gif
 
#47
Re: Internal threading question

I amstill grinding high speed tools for the practice. I have ordered carbide insert tools for threading.
I set the compound angle at 29.5 and the tool is 60° grind and is set perpendicular to the workpiece just below the centerline.
This is set up to cut 60° threads. As far an I can tell.

I really appreciate the feedback and am not trying to stir up an argument. these threads are way better than my first attempt above. I suspect when I get the new tooling and start practicing with it the threads will look better.

Is oil brushed on during the cutting acceptable lubrication?
 
#48
Re: Internal threading question

after scratching my head over this, i have come to the conclusion that i have been doing it wrong all this time. i guess it doesn't really matter because i finish my thread with a cross feed finish pass but i will change my methods from now on.
 
#49
Re: Internal threading question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 300sniper</div><div class="ubbcode-body">after scratching my head over this, i have come to the conclusion that i have been doing it wrong all this time. </div></div>

I dont know, looks like you've got a pretty good handle on it. The end result and final product is the judge as to whats right and whats wrong. Your work looks pretty good to me.
 
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