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Post by Deleted on Jan 4, 2019 14:36:51 GMT
Hi guys
I'm after a little advice, I have a 7/32 x 40 ME thread that I'm not happy with, it seems that I invertedly machined the stock bar too small before cutting the thread with a die and thus when fitting the nuts they are too sloppy. Rather than have to remake the parts concerned (not a five-minute job) I'm toying with the idea of cutting smaller threads over the 7/32 x 40 and fitting smaller nuts to match. There is no stress involved on these threads so being slightly smaller will not cause any issues.
So my question for those who know threads much better than I is what size would be my best option, not so much size but what's the closest thread form. I was thinking 5mm which is just slightly smaller in Dia but wanted to ask those more in the know first..
regards
Pete
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don9f
Statesman
Les Warnett 9F, Martin Evans “Jinty”, a part built “Austin 7” and now a part built Springbok B1.
Posts: 960
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Post by don9f on Jan 4, 2019 16:04:42 GMT
Hi Pete, a standard 5mm thread is 0.8mm pitch which is approx. 32 teeth per inch, a bit coarser than your existing 40 tpi. That probably doesn’t matter, but a slight problem is that the root diameter of the existing 7/32” thread is probably already smaller than 5mm, so the threadform might not be perfect. If this doesn’t matter either, then that would be a good choice.
Other alternatives could be staying imperial, and recutting at 3/16” x 40 in which case you should get a good thread, or 2BA with a pitch similar to the 5mm at approx. 31 tpi.
Finally, could you just make a new nut with a tight thread and get rid of the slop that way?
Cheers Don
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Post by simplyloco on Jan 4, 2019 16:06:40 GMT
Agreed. Use part of a taper tap until you get the required clearance. John
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Post by Roger on Jan 4, 2019 17:14:36 GMT
Hi guys I'm after a little advice, I have a 7/32 x 40 ME thread that I'm not happy with, it seems that I invertedly machined the stock bar too small before cutting the thread with a die and thus when fitting the nuts they are too sloppy. Rather than have to remake the parts concerned (not a five-minute job) I'm toying with the idea of cutting smaller threads over the 7/32 x 40 and fitting smaller nuts to match. There is no stress involved on these threads so being slightly smaller will not cause any issues. So my question for those who know threads much better than I is what size would be my best option, not so much size but what's the closest thread form. I was thinking 5mm which is just slightly smaller in Dia but wanted to ask those more in the know first.. regards Pete Hi Pete, Can you post a picture so we know what we're looking at? My first thoughts are to Silver Solder one of those nuts that are sloppy onto it and cut the thread again.
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Post by Deleted on Jan 4, 2019 17:58:27 GMT
[/quote]Hi Pete, Can you post a picture so we know what we're looking at? My first thoughts are to Silver Solder one of those nuts that are sloppy onto it and cut the thread again.[/quote]
Thanks for the advice guys..
Roger, here's the thing...This has been bugging me for some time and I was going to leave 'as be' but know that I should fix this. The parts in question are the 'trailing crankpins' so not a simple thing to change, especially as they are loctited to the wheels. The nuts/caps do screw on but are too loose for my liking, I was going to live with it as they are pinned anyway but I know that I'm not going to be happy unless I address this problem. I'm trying to avoid having to force the crankpins out which is probably the sensible thing to do, it would then be relatively easy to do as Don says and machine down to the next size of 3/16 ( I may still do this but worried about upsetting the wheels and damaging the paint)
There is enough meat on the pin for 5 mm, or at least it looks like it, of course, the issue is being able to cut a new thread squarely onto the old thread? I could possibly make a jig that could hold the die square to do this, something like a sleeve to slide over the pin that also holds a die tight to cut the thread, this may be my best option, or should I say 'of least risk to causing damage to other parts'.
I could just say, leave it and it will be held by the pin but I don't want to do this, I don't like 'unknowns' that may give me grief later.
what would you guys do?
Pete
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Post by Roger on Jan 4, 2019 18:30:00 GMT
Hi Pete, Can't you screw cut some nuts that are a better fit? If the thread isn't deep enough, you can cut them deeper if you use a split die.
If you can't screw cut a nut, how about making a Silver Steel tap using the split die to make it smaller than the standard pitch. Cutting three or four flutes on the mill would be all you need, followed by careful cleaning up and then hardening and tempering.
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uuu
Elder Statesman
your message here...
Posts: 2,800
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Post by uuu on Jan 4, 2019 18:33:03 GMT
I'm with this (and don9f and simplyloco): make nuts to fit the thread you have.
Wilf
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don9f
Statesman
Les Warnett 9F, Martin Evans “Jinty”, a part built “Austin 7” and now a part built Springbok B1.
Posts: 960
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Post by don9f on Jan 4, 2019 18:41:08 GMT
Hi Pete, now we know it’s the crankpins, I myself would definitely try to make nuts to suit. I have a miniature internal screw cutting tool out in the workshop....I’m going to brave the cold and have a look right now to see if it will cut a thread that small.
Watch this space!
Don
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Post by Deleted on Jan 4, 2019 18:46:26 GMT
Thanks, Wilf So I need a 7/32 taper Tap?... where would I find such a thing, does such a thing exist?... Or are we saying use a standard taper tap and not engage it fully into the drilled bore, surely I'd need to drill a smaller bore too?? or is this the plan? Cheers Pete edit: thanks Don, our posts crossed...
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Post by chester on Jan 4, 2019 18:54:45 GMT
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Post by Deleted on Jan 4, 2019 18:57:41 GMT
Hi Chester Yes, I know they are in 3's, taper, second and plug.. I was just wondering if we were talking about something more elaborate but think I just misunderstood what was being said, that's been the norm for me in recent weeks.. I do have the correct tap if we are saying just use that until it fits? Kind regards Pete
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Post by simplyloco on Jan 4, 2019 19:00:51 GMT
Hi Chester Yes, I know they are in 3's, taper, second and plug.. I was just wondering if we were talking about something more elaborate but think I just misunderstood what was being said, that's been the norm for me in recent weeks.. I do have the correct tap if we are saying just use that until it fits? Kind regards Pete Yes!
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don9f
Statesman
Les Warnett 9F, Martin Evans “Jinty”, a part built “Austin 7” and now a part built Springbok B1.
Posts: 960
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Post by don9f on Jan 4, 2019 20:04:55 GMT
I’ve checked and I think the smallest internal thread this miniature tool could cut is about 1/4”....however with a bit of sacrifice, it could be ground down a bit and could probably just about do 7/32”. Anyway try the new nut with a tap first..... Cheers Don
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Post by Deleted on Jan 4, 2019 20:07:03 GMT
Thanks, Don... will do...
Pete
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Post by Roger on Jan 4, 2019 20:09:26 GMT
If you want to screw cut a nut, you could also sacrifice a tap, grinding way all but one tooth which will have the correct form and clearance.
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Post by Deleted on Jan 4, 2019 20:18:09 GMT
Cheers Roger, that's an idea... I'll do a few tests to see what size fits best, then I'll have to machine up new caps as they are a little more involved than plain nuts.. such is life... I didn't really notice until I first fitted the caps and it's been bugging me ever since. Yes it still worked once tightened up and once pinned would be unlikely to give any trouble, but it's really not worth taking the risk, best to sort it now before getting too involved in something e4lse... cheers Pete
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Post by RGR 60130 on Jan 4, 2019 20:40:08 GMT
The taper pin in double shear, together with even a slack thread probably gives you as much holding force as you have on the front crank pins. When running, any side forces on the nuts should be minimal and short lived.
Reg
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Post by Deleted on Jan 4, 2019 20:49:46 GMT
Hi reg
that's what I was first thinking when discovering the problem, I then put it out of my mind while carrying on with the build, it's just that it's bugging me and so I'll do what I can to fix it...
Cheers
Pete
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stevep
Elder Statesman
Posts: 1,070
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Post by stevep on Jan 5, 2019 9:05:58 GMT
I'm with Reg. The nut is only to stop the coupling rod falling off. It's not subjected to high loads, and if it's pinned as well, I don't think you should be too worried.
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Post by Deleted on Jan 8, 2019 16:14:12 GMT
Afternoon guys Well, I remade the new caps today following the advice given (thanks all), regarding not fully tapping the thread using a taper tap. To play safe I tapped the thread in stages from either side of the cap until I was happy with the fit, I can now fully engage the thread on the crank pins requiring the use of a spanner for the final tightening as the cap locates tight against the pin shoulder, so in all I'm much happier than before although not totally as I now have 2 machined parts in a scrap bin, that's a first for me, I hope it's not a sign of things to come... The last job to do with these caps is to cross-drill them to accept a 1/16 taper pin. I don't own or have ever used any of these so a few questions. 1: where can I get 1/16th taper pins? 2: I have 1/16 collet pins, are these any good? 3: If using a taper pin, what size drill is used, do you drill an undersized parallel hole and use a suitable broach to open out one end, or are there special taper drills for this job? thanks in advance for any answers, here's a picture to show one of the new caps in situ. Many thanks Pete edit: Err.. ok I've found 1/16 taper pins and a taper reamer, so I get how this is done now, or hope I do... is there a chart that gives drilling sizes when talking tapers?
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