Threading on a lathe.

[Nic Scheepers]

Hi there,

Total moron here. I am a newbie to the world of twisting and turning metal. Although I have my ML7 lathe, and ZAY045 square column mill/drill for a time now, I am still stuck with the basics. I did come a far way down the line though but never-the-less still stuck at this point.

What point is that you may ask, well it is the !@$@#$^$ threading of a nut and a bolt. I "think" I have the actual turning of the threads inside and outside under the knee but turning them that they fit together is where I am totally lost at the moment.

I have a lot of scraps in the bin already, but it seems I just cannot get it right. This is not a question of give up and go and buy a tap and a die, but rather me that really wants to be able to do this before I go on.

OK, say for Instance I have turned down a piece of stock to 15.875mm(5/8"). I want to put a 26TPI thread on it. I found through literature that the full thread depth is 0.846mm. I go and turn a thread on this piece of stock 0.846mm deep. What I also do is take a minimal amount of the top for the rounding of the thread. Oh, I do not have a threading cutter so I am using a grinded 55 degree cutter.

I now take another piece of stock and bore a hole to the following calculation: Bolt outside orginal diameter - (0.846 * 2)

I now thread the inside of this nut. Guess what? the nut is way too small for my bolt. I know a lot of you may laugh but this is really frustrating to me.

Can anyone take me step for step through this dimensions thing so that I know how to work it out by myself in the future please. I have read this forum and some others but no where they actually give you the detail of this.

I must say at least my other turning skills has improved quite a lot with this excesize, but i want to move on now.

Thanks in advance.

regards

Nic

[keebler303]

i'm not terribly experienced on a lathe either but it sounds to me like you aren't giving it any clearance. You need the nut to be slightly bigger in order to fit over the bolt. The i.d. should be somewhat larger than the diameter of the bolt and then cut the same depth from that larger i.d. you are basically just putting in a little "slop". Practice makes perfect.

Good Luck
Matt

[Nic Scheepers]

Thanks Matt,

I figured that out, well sort off, my question is how much is the "somewhat bigger" you are talking about? How do I calculate that size for any threads I need to turn?

Thanks for the feedback!!

Regards

Nic

[Ryobiguy]

I'm not sure of the proper terminology, but think of looking at a tap drill chart where they list the percentage thread depth fit. For example, one chart I found from googling "tap drill" lists 75% fit for aluminum and 50% for steel.

It sounds like you need to bore a bigger hole for some clearance, and take off a little bit more of the one (or both?) of the threads so it's not too tight of a fit.

[M-man]

Ex M30x3

Pitch * 0.1mm = 0.3 .... 30-0.3=29.7 and this is the dimension you would want on the shaft before threading.

30-pitch will give you the bore of internal thread, dia 27 in this EX..

----------------
1"/26=0.977 metric pitch

15.875-0.977=14.9 Bore size.

0.977*0.1mm =0.0977

15.875-0.0977= 15.77 this is OD of shaft.

Threading depth depends of what kind of insert you use, but normaly there are tables from manufactors to get the right depth for their insert.

[mactec54]

Hi Nic Scheepers

I see that you are cutting a 55deg BSF 26 threads per inch the correct depth of cut thread on the bolt/shaft is .0246"/.6248mm per side or a total of
.0492"/1.249mm you can round the numbers off the Nut needs to have
a clearance on the core dia of your Bolt/Shaft of .0079"/.2mm when cutting
the nut keep trying the bolt/shaft till it fits.

Find a drill/tap chart for other threads something you may be able to find is a english hand book called ZEUS ( I have seen it on Ebay sometimes they do not cost very much )
or ZEUS precision it is in metric and english and covers almost everything you will ever need it gives you thread depth/core dia Etc this is the most used book in any shop if they have one I hope this is of some help

Found this on Ebay # 200111369889 For ZEUS hand book

[Geof]

Originally Posted by Nic Scheepers ... I am a newbie to the world of twisting and turning metal...... ......I now thread the inside of this nut. Guess what? the nut is way too small for my bolt. I know a lot of you may laugh but this is really frustrating to me....

Not only are you twisting and turning metal you are also bending it, elastically so it springs back. Tool deflection or workpiece deflection it is called. You might move the tool in to take a cut .005mm deep but really it cuts slightly shallower than that. On the two, intended to be mating, parts with such a fine thread the 'slightlys' can add up to be significant compared with the actual dimensions.

Don't let the book theory dictate to you let it guide you. You figured out the theoretical starting dimensions and thread depth but things did not fit. Add or subtract a tiny allowance for tool deflection; make your bolt thread a bit smaller and nut a bit larger. With experience you will find that after a while you can make a darn good guess the first time and come up with parts that fit nicely first try.

Even with years of experience and really fancy equipment cutting a thread from scratch to theoretical dimensions and getting it to fit perfectly the first time is as much a matter of luck as it is skill; and the phase of the moon and how may ladders you walked under last week . Even when things are running just fine you can get tripped up because you use a piece of 'identical' steel from a different bar and the properties are within spec but just different enough that the tool deflects differently.

Which is why you always check the part before taking it out of the machine and losing your alignment.

[Mazaholic]

Machinist Handbook
http://www.industrialpress.com/en/Ma...k/default.aspx
Look thru the book and find the thread you wish to make,it will have dimensions for OD and ID.
Set of thread wires.
If you turn the OD as per the handbook and are sure your threading tool is ground correctly,then use the wires to make sure the pitch is right.
http://www.threadcheck.com/pdf/3-wire-16-17.pdf
You don't need anything as exotic as whats show in the file,just a set of wires.
That will insure all of the parts have the same thread dimensions.
Thread the ID or "nuts" untill you get the fit you want.

[handlewanker]

Hi Nic, served my time on the diamond mines in Oranjemund, SWA in the 50's so I know your problem.
First you need a decent nut to act as a ring thread gauge when cutting outside threads.
If you can get a new bright mild steel nut run a tap through it to clear any burrs etc.
For want of a proper thread ring gauge a nut will have to do.
Mark it with the thread form and size, e.g. 5/8 BSW, or 12 mm etc, and put it in a safe place for use next time.
When you cut your outside thread, (Bolt), don't go full depth before "gauging" it.
The thread is cut using either the compound slide parallel to the job or the American way, where the compound is set over to half the thread angle.
Either way works well, just depends which you prefer.
When you're down to the last few thous, clean the top of the threads lightly with a file to give the point radius and then try the nut.
If it refuses to go on take another thou and do this for both flanks of the thread seperately, to give a clean thread.
Once the nut goes on it's done.
You can now cut the internal thread using the previous cut external thread as a test piece.
When doing the nut make the hole a "few" thou bigger to prevent the nut binding in the bottom of the bolt, as it's extremely difficult to round the bottoms of the threads internally.
You would need a thread chaser to do this and one can be made from a coventry die thread chaser brazed onto a round shank.
I always use a screw cutting tool with rake on the top, sloping to the right, and just feed the compound slide a few thou to the left for each pass.
For the final few thou I use a carbide tool with a flat top and cut the speed to 80 rpm with coolant at the trickle.
The carbide tool is one that I made in the 50's and I only use it for finishing. HSS works just as well but needs to be kept sharp for the finish cut.
The slower final speed and using a flat top tool with coolant will give a thread that looks like it was chrome plated.
I also prefer the compound slide to be parallel to the work.
The speed is 125 rpm for starting with the first two cuts .010" deep, then reducing the depth of cut to .005" with .001" on the compound slide.
The half-nuts are left engaged and the chuck reversed at the end of the cut.
If you have a fair bit of manual dexterity and have a thread indicator on the carriage you could disengage the half-nuts and wind back to the beginning, if you are familiar with this method.
When you cut Metric threads you will have to leave the halfnuts engaged anyway, if you've got an imperial leadscrew.
If you don't have a reverse mechanism for the lathe, you can fit a handle into the spindle bore at the end and manually wind the carriage back, flipping the gearbox or back gear into neutral to ease the load.
Ian.

[Nic Scheepers]

Gee Guys,

Thanks a lot for all the info. I have a much better feeling about this and am going to try it as soon as possible. What makes me feel stupid is that I was almost there, it seems according to your explanations, but I gave up too early and busted up the nut with some weird stuff I tried. I at least have the bolt still so I just have to make a nut for it again.

Hi Handle Wanker, always good to hear their is some guys from South Africa/Namibia around. Maybe we can change some ideas sometime.

Thanks again guys. But if anyone still want to pitch in I am all ears

Regards

Nic

[greg b]

Nick try using a thread chaser when you feel your close to the size of thread you need it just gives you that little bit of extra stock removal and helps true up the thread and gets you a nice neat fit.

[budP]

You also should know that you do not want to cut a thread with the backside of the cutting tool so you need to change the angle of the compound so that when you draw the cutting toward you cutting inside thread that it advances the tool forward into the stock.
budP