What Size For Threading?

[AR. Hillbilly]

I need to make a special stud. It needs 7/16" nc threads on one end and 5/16" nf on the other. I thought I'd use a 7/16 bolt and turn it down and use a die to thread the other. I have experience tapping holes and have a chart for what size drill to use. I have no idea what size to turn the bolt to to thread the other end. Is there a chart someplace I can copy to I don't have to keep asking dumb questions?

 

[4GSR]

For most threads, go with nominal OD, 5/16 or .3125" with a minus tolerance of about .005". This should be good for most of us.
In industry, we have calculations and machinery's hand book that give you more information.

You get it turn down to .312/.308", I would single point the thread to near depth, about .034". Then chase the rest of the thread with a die.
BTW: actual thread OD is .311/.302" I always go about .001-.003" big (.314/.312) and at the end of threading, I dress up the thread with file to remove burrs and a little emery. By the time I do that, the OD always wind up around .310".

 

[Ulma Doctor]

 

[AR. Hillbilly]

Thanks for the information. I really do appreciate it. I have one more question. How do I chuck up the existing threaded end and not destroy those threads while I turn down the other end? And for the poster above I haven't learned to cut threads with my lathe yet. I have a friend that may come help me but he's always short of time and I don't like to bother him. I may have a few questions before I get to this. It wouldn't be a large cost to ruin one in trying but before this I had no idea where to start. Would a grade 8 bolt be hard to turn and thread?

 

[Wreck™Wreck]

An excellent choice is to subtract the thread lead from the major diameter, this will get you close, if you have reason to make a very accurate thread buy a thread gauge or measure over wires, this is unpleasant in many ways however.

 

[T Bredehoft]

Put two 7/16 NC nuts on the big end and screw them together VERY tightly, then chuck on one of them in your three jaw chuck. If it wobbles, start with it chucked in the lathe, cut or turn the head off (holding on the unthreaded shank) and centerdrill the end. Then, when it wobbles, loosen the chuck, put a live center in the tailstock in the centerdrilled hole, tighten the chuck on one nut and go to work.

 

[Ulma Doctor]

Thanks for the information. I really do appreciate it. I have one more question. How do I chuck up the existing threaded end and not destroy those threads while I turn down the other end? And for the poster above I haven't learned to cut threads with my lathe yet. I have a friend that may come help me but he's always short of time and I don't like to bother him. I may have a few questions before I get to this. It wouldn't be a large cost to ruin one in trying but before this I had no idea where to start. Would a grade 8 bolt be hard to turn and thread?

i'd try another bolt rather than grade 8 at first.
a grade 5 will cut much easier and thread easier too.
stainless bolts at the box stores are generally grade 7 or thereabouts, and still relatively machinable.
grade 8 is harder and chews up tooling.
to be honest i haven't really seen the need for 5/16 grade 8 bolts, other than aircraft or military spec
a grade 8 proofs at 120,000 psi, grade 5 proofs at 85,00psi

do you need more than 70,000 psi tensile? in a 5/16 bolt
(i'm giving a kentucky windage guess that you'll have a 15,000 psi loss slop factor after machining for argument sake)

 

[brino]

I have an ER collet system for my lathe, with it I can grab onto the points of threads with good grip and no damage.

If you have not single-point threaded before here's a few things I have learned:
-make sure you use back gear; you want slow speed and slow feed!
(to cut right-handed threads you have to cut towards the head-stock, and you do not want to "crash" into the chuck)
-find/grind a 60 deg tool and use a gauge to get it "square" to the work, also be sure it is on centre
-you need to cut the thread in multiple passes to get it to full depth
-set the compound to 29 deg. and use the compound to advance for each cut pass; this way you only cut on one side of the 60 deg point, and you get less chatter
-be sure you have a small recess on the work to stop the tool bit in and the end of the pass
-if your lathe has a threading indicator use it to be sure you are "cutting in the same groove" on multiple passes.
(if you do not have a threading dial, then you need to keep the half-nuts engaged the whole time even when bringing the tool back towards the tail-stock for the next cut)
-set it up to what you think is right and then take an "air cut" without the tool touching the work, this will give you a feel for how fast you need to react to disengage the feed
-for your first real cut pass, make it very light, just scratch the surface, then stop the lathe and check it with a thread gauge or known bolt, just to prove you have all the feed levers set right for your required TPI

For the threading dial there are rules about when to use any number, or an even number depending on your lead-screw and the TPI being cut.......I can never remember them!
I just use the exact same number for each pass.....I may spend a bit of time waiting for my number to come around, but I spend less time looking up the rules!

-brino

 

[Charles Spencer]

Since you're just turning down the end and not single point threading, I'd probably turn it between centers. You could put paper or thin cardboard on the threads while holding it in the dog.

I agree with Mike (Ulma). I'd use a bolt that wasn't quite as hard.

 

[AR. Hillbilly]

I do have a collet set that has a 7/16" collet. There will be a 7/16" dead space/shoulder between the 2 sizes appx 3/8" long. I shouldn't get too close to the chuck. I do have a threading dial.