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PostPosted: Tue Nov 16, 2004 2:24 pm 
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The DELEATED thread requesting a source for 7/16" studs had a post in it that promps me to post this one. I will be the first to admit that if I can avoid threading in a lathe I will. My personal experience at lathe cutting threads has been hit and miss with too many times the latter. I commend those that can do this work rounteenly.

Anyway it was suggested that a grade 8 bolt could be used for stock for the stud by cutting off the head and I guess presumely the incorrectly threaded end. My question is, do folks actually lathe cut threads on Grade 8 bolt material? Seems like that would be difficult on hardened stock.

In a pinch, when I needed a stud and couldn't find one long enough to do the job nor could I find a bolt of the right size and thread with a long enough threaded section to cut off for the job, I purchased long Allen set screws. Example most recent was a 1/2" - 20 tpi. by 3 7/8" long. The 4" Allen set screws I was able to pick-up locally worked well for the job. The fact that they were hardened was also a plus as was the cost @$4.00 each.

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PostPosted: Tue Nov 16, 2004 2:37 pm 
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I've single point threaded grade 8 bolts many times. Sometimes, I've bought extra long ones to make bolts with full thread length. I only use carbide insert threading tools for such work.

You just need some practice with your threading in the lathe. It's not something to be afraid of. Take your time until you are comfortable with it.

I would much rather chase the threads than use a die any day of the week. (Or use a die head with chasers.) IMHO, dies are only good for cleaning up buggered threads.

If you need help, just yell.....

Andy Pullen

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PostPosted: Tue Nov 16, 2004 4:42 pm 
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I'm not so good at threading on the lathe but look at is as a learning experience. I don't know about a hardened bolt tho. I certainly cannot thread with a die, even when I start it in a tailstock holder it goes crooked every time (I would like to know why) and so I use the lathe. Handy for large threads too. I hope Pat Miles sticks around and isn't to upset about something. Scott.


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PostPosted: Tue Nov 16, 2004 5:49 pm 
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A couple issues I have had extending the thread on grade 8 bolts.

First off these are a rolled thread. A center drilled end will be required to support the free end. The rolled thread process may not always be on center or leave the bolt very straight. As the rolled thread is ended mid bolt at the transition, those threads are somewhat incomplete. This will be the start of your newly cut thread. If the bolt is bent, that transition will be hard to match for pitch diameter, which can force you to sacrifice full thread strength.

It may help to run a good quality HSS die just prior to finish pitch diameter on the lathe. This can help correct that transition better than a single point approach.

The material is at about a 48Rc so it is still cold workable. Sulfur based thread cutting oil is best. Keep the chip load low. Only advance the compound about .001-.002 per pass. Initially the tip of the bit is not much of a match for the hardness of this steel. Taking more than it can handle will snap it off easily.

Matching the flank is a challenge and each pass should be verified and compensated for tool lead at the flank the compound it set to follow.

I just did a set of four jack screw leveling bolts that were metric thread on an english lead screw lathe. Talk about a chinese fire drill. It takes so much longer when the carriage has to be reveresed so the pitch lead is not lost.

DC


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PostPosted: Tue Nov 16, 2004 7:07 pm 
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A couple more thoughts on threading bolts from the penut gallery.....

Quote:
If the bolt is bent, that transition will be hard to match for pitch diameter, which can force you to sacrifice full thread strength.


Speaking of strength......I wouldn't consider a rethreaded Grade 8 bolt to be still full strength - you've likely cut through the hard outter skin and as a result sacrificed strength. The intended application will dicatate if this is a real concern or not.

Quote:
Only advance the compound about .001-.002 per pass.


Hey Doug you are far more patient than I am - I usually will be taking cuts in the .005" to .010" range and don't start with the .001" passes until I get CLOSE. I also calculate (or look up) the total thread depth (that I am cutting) so I know when I am close - it has proven to save my butt on parts that HAD to pass outside inspection more than once. Not that THAT would apply to rethreading bolts [img]/ubb/images/graemlins/shocked.gif"%20alt="[/img]

Quote:
Taking more than it can handle will snap it off easily.

Assuming I am understanding your point - which is start gently - which realy is good advice, but my experience there has been slightly different. The beginning part of the thread provides little tool load since you are not taking a "full swath" so to speak - as such I find myself taking fairly aggressive DOC's in the beginning (.020" or more all depending) then as I go and hear the machine labor slightly I take progressively lighter cuts - more along the .005" to .010" range stated earlier. Untill the nitty gritty when I get into the .001" range like you stated.


Quote:
Matching the flank is a challenge and each pass should be verified and compensated for tool lead at the flank the compound it set to follow.


Yep that's always fun!!! My biggest gripe there is that it can be confusing when you are wanting to set you dial handles in a certain position - too much fiddling around sometimes. Rechecking each pass sounds like a lot of work - I usually set it pretty close on the first pass - I stop the lathe mid cut (of course it's a dry run so I am not cutting yet as the tool is .100" or so OUT) so what I get is the slop out of the geartrain - then It's a matter of getting out the loop and dialing it in - little here a little there- aside from tool load/spring it should be VERY CLOSE now. Rest your dials and away you go!!!


I have to wonder since our methods are fairly different what type of cutting tool are you using?? I am using a "momax" cutter if memory serves me. Much more durable than HSS but not as brittle as Carbide.

Not long ago I had a BUNCH of studs to thread for my HOTROD [img]/ubb/images/graemlins/grin.gif"%20alt="[/img] (ok a "bunch" for me was 20 pcs) I needed to put fine threads on one side and coarse on the other. I found some TGP shaft laying around so I commandeered it and started to form a plan. Initially I thought I would just single point them, but the quantity steered me away from that. In the end I wimped out [img]/ubb/images/graemlins/crazy.gif"%20alt="[/img] and used threading dies. Since I lack a die stock that I can use in the lathe I chucked the die into the lathe chuck and the part into the tailstock chuck. Hookey maybe, but it worked great. Like you I used LOTS of sulfur based oil and marked each piece for a thread length (magic marker) - zing zing zing - it was done in a fraction of the time I could have single pointed them.

FWIW my buddy made his studs out of rethreaded bolts - and I remade a few of them for him when they snapped. Granted this is no high stress application, but it was enough to break his - mine are still going strong. Stronger is not always better [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]

Which leads me to another point - in another post in this thread there was mention made of using set screws for studs. Most industrial grade set screws FAR EXCEED grade 8 and as such are substantially harder and of course substantially MORE BRITTLE. We learned this the HARD WAY where I work when we had them start to fail in the field. Can you say "retrofit" [img]/ubb/images/graemlins/crazy.gif"%20alt="[/img] and you say "expensive" [img]/ubb/images/graemlins/shocked.gif"%20alt="[/img] Yikes.

Well here's the problem - set screws are designed to be used in compression NOT tension - when we use them as studs and we tighten/ tension them -they don't like it- [img]/ubb/images/graemlins/frown.gif"%20alt="[/img] as a result set screws are HARDER and more BRITTLE than Grade 8 bolts -this can lead to failure depending on the application. [img]/ubb/images/graemlins/mad.gif"%20alt="[/img]


Oh well I've drooned on ad on long enough. OOPS!!!! [img]/ubb/images/graemlins/crazy.gif"%20alt="[/img]

As always YOUR mileage may vary!!!!!

Time to go work on HOTRODS!!!!!!![img]/ubb/images/graemlins/grin.gif"%20alt="[/img] [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]


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PostPosted: Tue Nov 16, 2004 9:02 pm 
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Jacin,

I tried a .005 pass at first. It made it about half way along the unthreaded section and snapped the tip off. Another attempt did the same. I was using a hand ground carbide bit. Possibly too much relief, which reduces the support under the cutting edge. As to my knowledge grade 8 bolts are through hardened so I would not gather them to have a skin other than the black oxide coating. The difference in cutting raw stock at 20-30Rc compared heat treated 48Rc makes it very difficult to get a chip started and keep it constant. That presents another problem with bent bolts, because one side will pose a heavier load. Easier to error on the side of precaution than to grind another tip and go through the whole setup again. This stuff is tough, but gummy to an extent...if that makes sense?

The reduced strength of the thread is due to incomplete threads at the transition. A few of those threads have less peak material. Nothing one can do about that. Nature of the game.

When no threading die is available and that forces the single point process to make do. If the bolt is bent, your newly cut thread is on a different center than the transition. A nut may not run along that transition without cutting the pitch diameter undersize. That makes for a weaker thread. Same result as using an oversized drill for tapping. Machinery handbooks show how significantly the percentage of thread is effected when less material is available to support full torque. Not that all applications require that, but I felt it worthy of a mention.

I had some suspicions on why I had to reset the flank every so often. I think it was actually pushing the bolt along the jaws, slipping or some other physical anomaly. My bit was not moving, so I gave up for the sake of efficiency as my associates were waiting on me to get the machine back in production. I really did not concern myself with in feed limits since I had proper pitch diameter threads there as an indication where I should not continue past if I could help it.

Right you are, on the durability of hardened bolts. Reduced ductility does not take kindly to flex and will fatigue much quicker. Good point on the set screw attributes. The jacking screws I had made were to replace set screws for injection mold machine levelers which had cracked over the years. It was a small machine, but the allen socket was too small to be used without flexing the wrench. Besides needing to replace a couple longer ones to compensate the floor drop, I wanted them all the same size allen.

As another side issue for extended threads of this nature. Use liberal amounts of anti-seize as a preventative measure to galling. Grade 8 bolts do not always surface finish well. The bolt itself may not gall, but whatever it screws into can.

DC


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PostPosted: Wed Nov 17, 2004 12:35 am 
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Location: Onalaska, WA USA
Quote:
Speaking of strength......I wouldn't consider a rethreaded Grade 8 bolt to be still full strength - you've likely cut through the hard outter skin and as a result sacrificed strength.


I agree, it will no longer be as strong, but It's a little more involved than that. Because the threads are rolled, not machined, the material at the thread is stronger than it would be if it was machined because of grain flow. When you machine new threads, you interrupt the grain structure, plus you leave sharp areas (stress risers). Net result: Weaker material. Incidentally, grade 8 bolts are medium carbon alloy steel and are hardened through, not case hardened. The interior is not softer.

Quote:
Quote:
Only advance the compound about .001-.002 per pass.


Quote:
Hey Doug you are far more patient than I am - I usually will be taking cuts in the .005" to .010" range and don't start with the .001" passes until I get CLOSE. I also calculate (or look up) the total thread depth (that I am cutting) so I know when I am close - it has proven to save my butt on parts that HAD to pass outside inspection more than once. Not that THAT would apply to rethreading bolts [img]/ubb/images/graemlins/shocked.gif"%20alt="[/img]


Quote:
Quote:
Taking more than it can handle will snap it off easily.

Quote:
Assuming I am understanding your point - which is start gently - which realy is good advice, but my experience there has been slightly different. The beginning part of the thread provides little tool load since you are not taking a "full swath" so to speak - as such I find myself taking fairly aggressive DOC's in the beginning (.020" or more all depending) then as I go and hear the machine labor slightly I take progressively lighter cuts - more along the .005" to .010" range stated earlier. Untill the nitty gritty when I get into the .001" range like you stated.


In soft materials, you may be right (aluminum, leaded brass, etc.), but hard materials put a tremendous stress on the fragile tool tip, which leads to premature tip failure. Assuming you are trying to cut (external) threads to spec, the root quickly loses it's form, creating a too large minor diameter. I've re-sharpened more threading tools for that reason than I ever have from wearing them out.

Looks to me ol' Doug has done his homework here. Shallow passes initially are far better. Once you have the cut established and the flank of the tool is taking the cut instead of the point, depth of cut can be deepened. The first pass is the worst, because the chip generated comes from both sides of the future thread and tries to occupy the center. Tip pressure is at its greatest, so the tip really suffers.

The hardest material I've ever threaded was some heat treated tool steel, 58 Rc. Very slow speed, light cuts, exceptionally sharp tool, and only finish cuts. Gage was heat treated after roughing, and threads were cut on an almost new EE Monarch. Turned out great, thanks to the ability of the EE to creep and still have HP. The gage was a few inches in diameter (maybe 4"). Don't recall size now, considering it was over 40 years ago. Had to turn the threads, we had no thread grinder.

Harold

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 Post subject: Not Upset At All!
PostPosted: Wed Nov 17, 2004 1:32 am 
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Scott,
Just a short post to say that I am not upset in the least. Reading the posts to this thread tells me that even the experienced folks are challenged with the task of threading a grade eight. At this early point in my hobbiest career I have limited tooling. I don't see the point of buying a carbide insert threading tool to try and make one stud that I can buy for under $3.00. Yea, the guy was a little harsh but there lots of helpful and friendly folks (Victor) who will keep me coming back.
Thanks,
Pat


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PostPosted: Wed Nov 17, 2004 7:36 am 
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When I make fully threaded bolts; I cut off the rolled thread and put centers in the bolt and then cut it. It's worked fine.

AP

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PostPosted: Wed Nov 17, 2004 9:01 am 
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Quote:

I agree, it will no longer be as strong, but It's a little more involved than that. Because the threads are rolled, not machined, the material at the thread is stronger than it would be if it was machined because of grain flow. When you machine new threads, you interrupt the grain structure, plus you leave sharp areas (stress risers). Net result: Weaker material. Incidentally, grade 8 bolts are medium carbon alloy steel and are hardened through, not case hardened. The interior is not softer.

Hi Harold, yeah I stated this rather poorly - and given the standard terms of through hardened bolts (grade 8) and case hardened bolts (grade 5) my statement was quite misleading (unintentionally of course) what I was so poorly trying to get at was similar to what you stated. Buy maybe I don't understand it correctly - I was under the impression that when you roll the threads you are actually compressing the grain which results in an increase in hardness/strength - I also thought the entire bolt (shank) was rolled as well. I know this sounds contrary to the industry standard description of grade 8 bolts being through hardened - but I think that is only an easy way to help us distinguish them from other grades. Admittedly I could be all wet on this. I just seem to recall taking some grade 8 bolts and turning them down and recall that they seemed definitely softer in the core than on the outter skin - maybe I had a grade 5 confused with a 8 - I will turn a 8 down and experiment - I never claimed my memory was any too good. [img]/ubb/images/graemlins/crazy.gif"%20alt="[/img]

AS far as the threading DOCs I again used some bad words there - well not words so much but terminology - I stated specific DOC's when the numbers I was thinking of were really "off the dial" which on my lathe is OD and NOT DOC - aside form that I do stand behind those numbers - or at least how my memory remembers them (it's not like I rethreaded a bolt last night - which if I did maybe I would recant [img]/ubb/images/graemlins/blush.gif"%20alt="[/img])
However I do agree that my VERY first cut is light - I do that out of habit for other reasons - never knowing it was saving my butt for chipping tools - however I have always been far more aggressive than taking .001" off each pass at least until the very end when I take repeated cuts without changing the dial at all to minimize tool pressure as a big variable.

I won't profess to having threaded tons of grade 8 fasteners so I won't profess any real knowledge, and likely I misunderstood some of Doug's points - I was under the impression that he was suggesting taking cuts of .001" or .002" for the entire process. I have cut enough grade 8 bolts to know I wouldn't do that - [img]/ubb/images/graemlins/blush.gif"%20alt="[/img] For I am patient, but not that patient. [img]/ubb/images/graemlins/blush.gif"%20alt="[/img] I'd have to push it untill something made me back off - be it a chipped tool or something.

What a good thread - I think I learned soemthing AGAIN!!!!! Wow it's catchy.

Thanks guys


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