hand forging a plumb bob...

[elkdoc]

Hey folks...

I'm wanting to forge a plumb bob for my brother (a carpenter) for Christmas. I'm assuming that it'll be no big deal. Forging and filing to a precise and centered point shouldn't be that difficult.

However, are there any thoughts on how to forge a place to attach the string? I'm wondering if I could just forge a little "rat tail along one side of the top and scroll it over making an upside down "U" shape.

I've also thought of drilling a hole down through the center of the top, and then drilling a large diameter hole through the side to pass through the end of the original hole, making a "T" shaped hole. This way, the string could be passed through the top and out one side, knotted, and then allowed to pull back through to the center, with the knokt holding the plumb bob on the string.

Your thoughts?

[irnsrgn]

elk,plumb bobs work because the weight is equal all the way around and the point on the bottom is in the exact center. All you need is weight and I think fullering a piece of pipe would almost automatically give you the point if you are careful, and you would have the round shape to help it keep from swinging in the wind, the weight is also there to some extent, and if you make it kinda long in the center section and then fuller the top again then cut off and work this end flat you can fine tune the point in a lathe and also drill and thread the top end for a 1/4 inch bolt that you have drilled a hole thru inserted the string and tied a knot and then screwed into the bob top.

[ThomasPowers]

If you have the lathe I'd just lathe the whole thing---you might weld up a pattern welded steel billet for looks a nice twist pattern would be pretty or twist some wrought iron and etch after lathing.


If he uses it mainly for verticals than the bob doesn't have to be too accurate but if he is trying to transfer points then it should be as accurate as possible.

Thomas

[elkdoc]

No lathe here...

I actually just forged the end of a railroad spike into a round point, and then put a twist in the spike. I cut off the end, and I'm annealing the whole thing tonight. I'll either punch or drill a hole through the side, and then drill a hole from the top. Then I guess I'll use my trusty bader grinder to grind the point smooth.

It'll be interesting to see how it turns out.

[Strine]

Ahhhhh the plumb bob; most useful tool in a surveyor's kit. Having been one (a surveyor, not a plumb bob) for 35 years I can shine some light on the subject. Essentially everyone is right so far. It's a weight attached to a string which hangs vertical. A good one should be about 16 oz in weight, carrot shaped (pointy end down) with a hardened steel tip. It should be weighted evenly about the central axis which should be the line from where the string leaves the bob to the point at the bottom. Survey suppliers that I've had anything to do with usually only stock the brass models. Oh, and there should be somewhere on the bob to wind the string onto. There could be nothing worse than a length of string floating around in a tool box or carpenter's nail bag

But it is more than a weight on a string, and if we're not talking accuracy a brick would suffice. It's a make do hammer, a digging tool for looking for elusive survey points, a chisel for making "chisel cut in concrete", a toy to swing around and play with during slack moments, a source of string in an emergency, a sighter to look at through the theodolite, a device useful in setting the theodolite exactly where you want it, a drawing instrument for making mud maps in the dirt, a dog deterent, a paper weight for holding down plans layed out on the bonnet (hood) of the survey vehicle and there are many more applications that slip the mind momentarily.

Elkdoc I Hope this gives an insight into what a plumb bob could be used for. Whatever you make, of absolute importance, whether it be for accurate survey work accurate carpentry or just a bit of bush carpentry, make sure you supply a straight bit of string and what ever you do avoid string that hangs off vertical.

[6013]

make sure you supply a straight bit of string

Can you suggest a source for "a straight bit of string"? Is it a special brand that is starched something? And how do you transport straight string that is over maybe 3-4 meters in length?

I ask as most string available here is quite limp and sold in either a ball or coil. :wink:

what ever you do avoid string that hangs off vertical

"String that hangs off verticle" - is this labeled as such, or is it some kind of special string, maybe a factory blemish, or factory second that does not meet standards for being straight string? :shock:

This has me puzzled. I thought the upper attachment point of the string would always be directly above the attachment point to the weight. The weight would then rotate to align the balance of the weight in reference to the point of attachment. In other words if you attached a meter of flat bar to a limp string (the type we are accustom to using) the string being attached at the center point of the end of the flat bar, the opposite end would hang down verticle. If attached to the center point of the length (half meter) the flat bar would hang horizontal, with the string in the same verticle position as before. And it attached between the end and the mid point, the flat bar would hang at an angle,with the string in the same verticle position once more. In all three instances the string should hang from the top attachment point in a verticle direct line down.

This brings up another question. :idea: If you were to look down the string from the top to the bottom, what would the string point toward - the center of the earth? I am going to guess that a brass plum bob is used so as not to be influenced by the magnetic center of the earth or any magnetic fields in the area.

So, based on the assumption we use a properly designed plum bob with the attachment point of the string and the plum bob pointy end, both on a common center line, the string (straight string or limp string - but not an inferrior string that hangs off verticle) points from the top attachment point to the center of the earth forming what is referred to a "plum line". This would then mean that no two plum lines were parallel as they meet at the same point - the center of the earth. Imagine cutting a pie from the edge crust to the center, and the cuts being the plum lines. And no it does not have to be a plum pie as any type pie will do for the example.

This would seem to be a concern as the greater the distance between the plum lines, the greater the error that would be encountered. Is there a formula to factor in for calibration of plum lines that are plum but not parallel by the nature of the process?

And no, I don't have too much time on my hands, it is just that inquiring minds would like to know.

One last thing: is the name "plum bob" the common name for the higher priced instrument called a Plum Robert ?

[HWooldridge]

I think Strine meant not using string that might have a "set" because that could influence where "Plum Bob" points.

My grandfather had a steel bob - it was hollow and the string went inside. A cap was screwed on and had a hole for the string to exit. I think they use brass now because it's easier to machine and needs no finishing to prevent rust.

The pie illustration is good and it would be relatively easy to calculate (on something with scientific notation to take care of the zeroes) the amount that two lines are out of parallel for a given distance apart. A triangle is formed so plumb line A will point to the center of the earth as will plumb line B. I did some quick calcs without checking any reference material (so I might well be wrong) but two points one degree apart on a radius of 4000 miles to the earth's center should be 68 miles from each other so for all practical purposes, two plumb lines within any reasonable distance apart can probably be considered parallel. What say ye, Strine?

[Strine]

In relation to the being sure you supply straight string with your Chrissy prezzy; liken it to sending the apprentice chainman down the street for said plumb bob and asking him, while he's there, to purchase a can of striped paint. Such paint is often required for painting the plastic tape used to surround important survey marks in the hope they will be protected from blind dozer drivers. It must be emphasised to the lad that he should not get the chequered paint as that is used by the police for their crime scene tape.

In relation to the turn this thread has taken viz; that plumblines are not parallel and that we have suddenly become very concerned about the effect this might have in a civil engineering sense a couple of points should be noted.

The novice surveyor might well assume that a plumb bob points to the "centre of the earth". Posts up until this point seem to suggest that this centre is a defined point somewhere down in the hot, sloppy part of the earth, and no matter from what part of the earth you hang your bob, it points to it. A plumb bob will point to the centre of mass or centre of gravity of all the stuff below it. In other words it aligns itself with the direction of gravity for the particular location. For those that wish to be further enlightened as to where a plumb bob might point it would be pertinent to find out the difference between the "Geoid" and the "Spheroid". Basically the former is the actual shape of the earth as defined by how the oceans lay - lumpy and bumpy on a grand scale as well as on a small scale where the lumpies and bumpies are just waves or swell. The latter is a mathematically derived "best fit" of the Geoid. This allows for the simplification of computations in developing maps and the like.

Having made these rash statements and you all believe I'm telling porkies, consider this. We can generally say a bob hangs vertical lest the wind is blowing on it. Gravity is doing its thing for sure, just as it does in making the surface of a body of water flat. I use the term "flat" hesitantly and only to make the illustration, for we all know that if it was flat we would have lost a lot of sailors over the years. I should have said level, because "level" is defined as being at right angles to vertical. So what happens twice every day at the nearest beach to your location. Correct... the tide comes in. It doesn't come in because the earth tilts and all the water sloshes over to your side. It comes in because the centre of gravity changes. Water never lies "off level" it always lies level, i.e. at right angles to vertical. If it did lie off level my kids could have had a swimming pool in the backyard which is sloped. But hang on, vertical is defined by our Chrissy prezzy plumb bob. What a dilemma, not only does a plumb bob point to different centres of the earth depending on where you are, it changes through out the day.

Just to make things worse. Have you ever noticed that having spent a good few minutes stopping your plumb bob from swinging it starts swinging again as soon as you turn your back. Believe the apprentice when he vehemently denies touching it as it is just another earth science thingo to worry about. It's not that fickle gravity this time but earth rotation. The bob is continually trying to catch up to where it's supposed to be given that it should be hanging vertical but someone keeps moving the goal posts.

Finally, congrats on the comps HW you're spot on except the concept is misplaced. The Sears Tower (we need to talk about this size structure since the average chook house is far too small to warrant such in depth discussion) is 1451.1 ft tall and let's say 360 ft wide at the bottom. Had the tower been merely a rectangular block (some architects are that boring) and the corners aligned with Elkdocs plumb bob on the end of a really long piece of string, the top floor would be about 2" wider than the bottom floor.

The moral is that the Earth is one big lump of stuff and what we do in our own little sphere of interest whether its a sky scraper or an outside dunny is infintesimally small in comparison. The design of the plumb bob is therefor open to your own interpretation, go wild.

Mmmm that was fun :D

[Ten Hammers]

The last building I built was squared with a sky hook and a bolt on the end of a piece of baler twine. I just meaeured the string for distance between the top and bottom of the corners ( sky hook = chunk of angle stickin out from the top of the pre-built wall). Worked pretty good but was NOT going to be checked by an engineer other than me). I have also just used the chalkline for a bob ( it's designed for that) in other cases. Hand forging a bob would present challenges. This is good if you need challenges but I got plenty for now. I do enjoy reading the technical side of the conversation though. Work to get done for now. Regards....

[GobblerForge]

Elkdoc
Your Idea sounds like the one I made some years ago. It works great and is all hammer and tongs. If your looking for that as a gift. The rat tail is perfect in that it lengthens the bob and is adjustable. The string always wants to hang in the same place so after hammering as close to center as you can, gentle persuation is all that is needed to adjust so the point stays in one spot as it spins. The handle keeps the string neat and stores the bob. Hope this helps. Brad

[elkdoc]

Thanks folks!

Strine, that was perhaps the most interesting reading I've done in awhile. Alot of geophysical concepts that I was aware of, but never seen together and from a survey accuracy standpoint.

My plan remains this: Next time the forge fire is hot, I'm going to punch a hole near the top through the side. Then, I plan to mark the center of the top and drill it on my press and in a vise. I plan to clean up the taper on my belt grinder, as close to center as possible. Then, I guess I'll hang the bob from a string and let it spin, and mark the sides with a felt marker where I need to remove steel to keep the point centered as it spins. In my mind, this should make it as centered as possible.

Although it most definately won't be precise enough for Strine to survey and align the sides of the Sears Tower ( :wink: ) it should be functional for a homebuilder.

I'm considering using this one as a prototype to keep around the shop and then welding up a billet of junk steel damascus and forging another out of it for a gift. If not for Christmas, then maybe for his birthday in January. That'd be pretty swanky! A damascus plumb-bob! It'll be the jewel of his tool belt and he the envy of the worksite. :lol:

[Glenn]

Good grief a fellow can not even blink without you folks geting all technical. Best laugh I have had in a good while, while being informed and learning the proper ways of how supposibly simple tools work. Thanks.

Well I need to go recalibrate my 2 pound smithing hammer to an extra decimal place so I can now do precision work instead of just beating on iron.

[frogvalley]

So what happens twice every day at the nearest beach to your location. Correct... the tide comes in. It doesn't come in because the earth tilts and all the water sloshes over to your side. It comes in because the centre of gravity changes.

I thought the moon was involved in that. Maybe it's just lunacy.

Blacksmith=Lunatic

LMAO

[elkdoc]

:shock: :lol:

[Strine]

The rat tail, as I understand, it is a great idea and would cause a lot of envy in the office. If I made one I'd have to make twenty :?

I assume, Gobbler, you'ld get say a length of 1" round bar and taper it to a point. The finished length would be such that you end up with the weight you're after. Then you'ld apply a few wobbles and curly ques aiming to get the point directly under the string. Spinning the bob is a great way to set the point in the right spot...wish I'd thought of it.

Elkdoc, I wonder if, instead of drilling a hole from the top and a hole from the side to meet somewhere inside the neck (hopefully) could you not drill a diagonal hole from the top until it pops out the side. It would be a lot easier to thread the string. For a bit more value adding you could tie a fancy knot on the end of the string, say a turks head or mini hangsmans knot.

And one last tip, the rim of the hole where the string enters is a wear point so be sure you polish off any cutting edge.

I can't wait to see the pikkies

Which reminds me, can anyone else see Brads pictures?

[GobblerForge]

Strine. I was wondering if anyone could see the pics too. I do believe you are correct that the bar was about 1" diamiter. It's been used alot and is dirty and beat up from the tool box but it has searved me well for some years. I thought the origianal post started about wanting to hand forge one so I put the pictures up. I don't think there is anything other than hammer and tongs to make this. I'm sure there is no grinding or drilling. There might have been a little filing of the tip to make it sharp. As I said earlier, The long tail with the little scroll gives a good anchor point for the string and allows for fine tuning to center the point. I think one would be very attractive in damascus.. What do you think? Brad

[Strine]

Re your pictures Gobbler, I tried the site shown in the picture properties but ZoneAlarms Security spat it back as a "hacking and proxy avoidance" site. I allowed that type of site to get the pictures and they all appeared. Nice looking plumb bob with more character than the run of the mill clinical looking brass job.

I was thinking more along these lines

[GobblerForge]

Strine. Those are realy inovative. Not realy my style but I like the concept.Brad

[elkdoc]

looks as though we're on our way to a "chile pepper bob". Very cool!

[6013]

Strine, great response to my question. Thanks. Now can you tell me where I can locate a container of plaid paint ?

[GobblerForge]

6013. It's on the shelf, right next to the muffler bearings and left handed smoke shifters. :wink: