Info on thread dimensions.

[Old Croc]

Hi learned friends.
I have just bought a new Laguna wood lathe and the spindle thread is 1 1/4 inch X 8 TPI. I cannot find any thread table on the net that has this thread. It is the same form as my Whitworth thread pitch guage, but i would like to have the correct dimensions to make up some faceplates. I dont believe i can use the dimensions from the Whitworth or UNC charts as they have added an extra 1 TPI to make this thread. Hopefully thanks in advance.
rgds,
Crocy.

[Steamwhisperer]

Hi Crocy,
Just use the depth of thread dimension for 1" Whit.
Subtract two depths for your minor diameter.
8 TPI can be put on any diameter you like.

Phil

[welder]

It a UN 8 tpi same pitch but on any diameter.

[pipeclay]

As stated,once you no the pitch all the other dimensions stay the same in regards to the thread form.

[Pete F]

Internal is 1.1150/1.1297 for a class 3B, but confirm that off your spindle. I have some I need to cut myself so please let my know if these figures are wrong.

[Old Croc]

It a UN 8 tpi same pitch but on any diameter.

Welder, thanks for this, that is the info no one else had told me, most said it was just Whitworth.

Internal is 1.1150/1.1297 for a class 3B, but confirm that off your spindle. I have some I need to cut myself so please let my know if these figures are wrong.

Pete F, thanks for this info as well. I will not be back in the shop for 2 weeks, timber cutting and Drag Racing at present, but because of the info from Welder above, I did a new search on that and found this site,
http://www.gewinde-normen.de/en/index.html
This had not come up on any previous search and is one of the most informative sites I have found, you just pick which thread you want on the LHS.
I will give some feedback on how I go,
thanks again guys,
rgds,
Crocy.

[Pete F]

Nice site. I couldn't see that it would provide enough information in order to single point the internal thread if that was your intention.

I will be doing this very job quite soon, I just had a couple of other things come in the meanwhile I need/want to attend to first. The first is to make up some W20 arbours for both my Haimer and a Clarkson Autolock. However once they're done I'll get back to this.

For what it's worth, the process I'll be doing is to make an accurate dummy spindle and use that as a gauge for the chuck I'll make. While the threads don't necessarily need to be super accurate, I figure if I'm going to do it I may as well do it accurately. A dummy spindle can also be used elsewhere too. Measuring external threads over wires is very easy, albeit a bit of a juggle at times, whereas measuring internal threads is arguably easiest with a go/no-go gauge. In this case there won't be a no-go version of the gauge, and just care when coming up on size. Making the same could be worth considering if you haven't already done so.

[welder]

UN 8

[Pete F]

Croccy, I was able to whip up a dummy spindle nose tonight and thought I'd put up the process here for you as I'm guessing you'll need to do the same. Sorry my spindle is 1 3/4 not the smaller one I mentioned, but the process is the same.

First thing is to measure the existing spindle. Yes it "should be" 1 3/4, but always best to check these things. Here I'm using blade mics as they can get in just a little closer to the bearing cap. It looks a little in the photo like I'm measuring the thread, but it's instead the register.

Hercus 260 spindle nose.jpg

Likewise measure the dimension of the thread over wires, sorry for the crappy photos, but it's hard enough to juggle thread wires as it is, never mind one handed while balancing a camera in the other hand!

Hercus 260 spindle threads.jpg

Turn the blank down accurately, a tip to hit accurate dimensions easily is to have the compound set at 6 degrees. The vernier is then multiplied by 10.

Hercus compound.jpg

Before placing the blank in the chuck I mark the #1 chuck jaws with a punch mark. You can replace something back in to the chuck jaw bruises quite accurately this way.

Marking blank.jpg

I think I need to start another post due photo limitations

[Pete F]

At this stage you'll have a turned blank. If you want to actually use this, instead of just for a gauge, make sure you face the shoulder accurately, as it's this the backplates register against.

Turned Blank.jpg

This is how I actually turn the threads. This one is quite crowded as it's quite a large thread and not a lot of purchase in the chuck, so I'm backing it up with a centre. I normally have the dial indicator on the other side. Anyway, in practice it doesn't matter and it works well. I set the dial indicator to the thread depth and dial in until it's zero. It normally needs a little more than the theory says it should. The blue line is to give me a mark as I didn't have a runout groove and instead blended the thread in to "spindle" OD.

Threading.jpg

Once you're getting close, measure again over wires and compare to the original (above)
Dummy over wires.jpg

This is the end result
Dummy Spindle.jpg

[Michael G]

Interesting technique Pete - I've used thread wires for measuring threads but hadn't thought of using them for a work in progress. Are you using them to get to the same as the original thread or having measured the original, are you aiming for the "unworn version"?

One thing that I would add is to do a minimum clean up cut on the stock that you were not planning to machine. That way if you ever have to remount the thread accurately in a 4 jaw you have a surface known to be concentric with the thread. (One of those things that if you don't do it one day you will need it, whereas if you do you never will...)

Michael

[Pete F]

Hi Michael, I used the thread wires in this case to be able to replicate the spindle threads as accurately as possible, I got within 0.01 mm, so I'm going to call that "good enough"

As you know, thread wires are a bit of a PIA to use, but superbly accurate. Every time I use them I swear I'm going to spring for a thread mic! However 90% of the time I wouldn't bother, and would just take a little more off until the nut I had fit correctly. Using the dial indicator I can come up on correct size quite quickly, although often the thread will need a little more. In this case it was for a gauge so went to the extra effort. I'd only bother with the wires when I was very close, so I could see how much I was still fat and then take off the appropriate amount. IIRC I only used the wires twice while cutting.

Yes I will clean up the stock outside. I missed a critical detail and that was I put centres in both the ends for that reason and will drive the threaded part with a dog. I didn't see any advantage in doing so before hand as it would have meant a bit more mucking around to get a good finish right along the whole length, and wouldn't repeat any better than what I did unless I similarly marked the jaws. In theory yes, in practice I doubt it. I find the jaws will mark the stock when it's left as it is, to the point of putting small "bruises" in it that are easy to see, the chuck was done up quite tight. I marked the #1 jaw as shown, and find I can snug up the chuck then wiggle the piece and it will locate itself in these bruises. Once I'm satisfied everything is Kosher I'll clean up the outside and can do it in one setup. Fortunately I didn't need to do any of this in this case, but it's a good point and definitely something I was considering. I wouldn't do it in a 4 jaw however, indeed I have to admit I don't use mine all that much, just personal preference I guess, and where accuracy is required I'll instead turn between centres if I can. It seems that's becoming a bit of a lost art these days, and few seem to do it from what I've seen except in more specialist fields.

An error I made was in not measuring the amount of unthreaded spindle on the end of the threads, and thought I'd eyeball it. I don't think the 4 jaw will fit on this dummy therefore, as it has a very short register on the back plate. It's no big deal, but here you can see the difference between the two.

Hercus original and dummy.jpg

The reason, another critical omission was this was done on a metric lathe through transposing gears, so I needed to keep the half nuts engaged the whole time and stop/reverse the lathe. The lathe is programmed with a brake cycle, so it's impossible to stop at an exact point as might be possible by just disengaging half nuts. I could have done it on my other lathe (imperial), but I prefer this one. Although not as fast as I'd normally thread, I wasn't exactly crawling along either as I'm a little impatient and just want to get jobs done without taking all night! Obviously all the other dimensions I'm working to are metric too, as I do these days, but that's no biggie, and my calculator spits out metric by default anyway.

The next to do are the internal threads, and I'm not looking forward to doing them with the half nuts closed! At least I can have a good runout groove at the other end so will take advantage of that. There's no way I'd be able to see the tool, so I think what I'll do is work out some type of compliant "post" like a straw or similar I'll put on the bar, and when the post touches the work hit the stop button.*

I've now fully abandoned imperial and indeed think the system is a horse's that belongs in a museum alongside buggy whips, even though it was what I was originally schooled in! Sadly however there are still a lot of imperial threads around and this is where I pay for having such a basic lathe. Mind you, it used to be the opposite on the other lathe, where I'd have to dick around when cutting metric threads (far more common for me).

*Edit: on further thought, yes I will possibly do this as a backup, but will more likely use an electronic cutout on a limit switch. I completely forgot about that as it's in parallel with the machine's covers and so I disconnected it. However it will be perfect for this and will mean I can thread much faster. Where there's a problem there's always a solution just waiting to be found

[morrisman]

Pete F Thats a nice clean thread u cut . Did u use a HSS tool ? Is that leaded steel u used ? And did u use the 30 degree compound method ? When measuring my spindle threads, they always are a smaller diameter near the tailstock end, this is due to wear i THINK When making a test spindle as u did , would u use the unworn diameter as the reference ? Mike

[Pete F]

Mike, the tool was a HSS tool bit. I would have ground it myself I expect, but that was a while ago and just touch it up when required. I just quickly touched it up with a diamond stone before use, but it really needs a little more attention than that, and it's getting a little blunt in my opinion. They should be very sharp tools as relatively big cuts like a 8 tpi are taking off quite a bit of material with each cut. Sharp as in the cutting edge, and not the tip (root of the thread) of course. There are some tiny chips in the photo I should have brushed out before taking the shot, but I was more concerned about the cutting oil on the lens!

The material was 1020 steel.

Yes I use offset threading, and feel it provides much better threads on noodle lathes like my Hercus's (plural of Hercus? Hercies?? ) compared to plunging, especially on larger threads. I'm known however to be anti this 30 degree, and especially the 29.5 degree, BS. The chances of setting either angles precisely when peering down at little scribe marks on a compound are pretty much zero, and if more than 30 degrees is set the thread will be the wrong form. Anyone who thinks they can set 29.5 degrees needs to remove their hand from a certain part of their anatomy. It can be set to anywhere between zero (ie plunging) and 30 degrees, so I just set it to roughly 28 degrees, as that gives plenty of margin for error. The only caveat is to ensure the tool is square to the work. I grind my tools symmetrical and square, so I just need to align the tool post, and do that by pushing it up against the chuck, all very quick. Indeed photographing and posting these posts has taken much longer than the job itself!

Sorry I don't understand what you mean about an unworn part of the thread. If you mean the spindle thread, yes that would be wise to read the wires on an unworn part of the thread, but this lathe is relatively new, the spindle is hardened, and it's not like I'm changing chucks every 5 minutes. The wear would be minimal.

Pete

[Michael G]

The next to do are the internal threads, and I'm not looking forward to doing them with the half nuts closed! At least I can have a good runout groove at the other end so will take advantage of that. There's no way I'd be able to see the tool, so I think what I'll do is work out some type of compliant "post" like a straw or similar I'll put on the bar, and when the post touches the work hit the stop button.

My cheat for that is to put a piece of masking tape on the boring bar - when it is level with the "front" of the hole, I'm at depth.

Michael