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Thread: Qualities of a good plumb bob

  1. #1
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    Default Qualities of a good plumb bob

    I've never used one for real in a project. I know what they do, but really have never seen them used. All the framers and deck people I would expect to use one use levels and laser levels.

    So I'd like to get some info from users of plumb bobs as to what qualities should a good plumb bob have? I want to make one on the lathe for a simple project. I am not going to try to get fancy, just build a good purposeful plumb bob.

    Material: Brass tarnishes so would need a clearcoat, is heavy. Bronze, cool but who's going to pay $150 for a plumb bob. Stainless, could work. Or do we care more about weigth than material, and who cares if it rusts or pits or tarnishes over time?

    Weight: What is a reasonable weight for a plumb bob, one that you would use most often?

    Size: Does size matter? Some women think so, but we're talking about plumb bobs.

    Shape: Does this matter at all, it's just a mass hanging on the end of a string, as long as the bob is reasonably balanced the point will be true.

    Interested in your thoughts.
    Steve The Drill Sergeant
    Check out MyShopNotes on YouTube.

  2. #2
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    Default Re: Qualities of a good plumb bob

    Steve

    I use an old surveyor's plumb bob. It is brass with a stainless steel tip and weighs about 2 pounds. The two most important qualities are alignment (symmetry) and mass. The tip needs to be pretty sharp also to make sure of where it is really indicating plumb and the string should be a braided type to keep the thing from spinning and thus gyrating. I don't think that the shape is particularly important.

    Ken

  3. #3
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    Default Re: Qualities of a good plumb bob

    Do you have dimensions of the bob and maybe a picture? Brass in 2" round at 12" is 11.6lbs, that wouldmean your bob is pretty big to be 2lb.

    Quote Originally Posted by KenL View Post
    Steve

    I use an old surveyor's plumb bob. It is brass with a stainless steel tip and weighs about 2 pounds. The two most important qualities are alignment (symmetry) and mass. The tip needs to be pretty sharp also to make sure of where it is really indicating plumb and the string should be a braided type to keep the thing from spinning and thus gyrating. I don't think that the shape is particularly important.

    Ken
    Steve The Drill Sergeant
    Check out MyShopNotes on YouTube.

  4. #4
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    Default Re: Qualities of a good plumb bob

    Steve,

    I don't have any pictures but the bob is 6" overall, 1 1/2" diameter at the biggest part and has some lead filling in it. I would say the lead core is about 5/8" in diameter. It is a heavy bugger but it was designed to be used on a transit or level outdoors so mass was important to keep windage effects at a minimum. Does that help?

    Ken


    Post script: I found this picture on-line and it is very similar to my bob but is made by General Instruments.
    Last edited by KenL; 10-25-2013 at 09:04 PM. Reason: Added information

  5. #5
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    Default Re: Qualities of a good plumb bob

    I have one of these but have never used it for anything. I know what it does, but have never found it helpful in any practical application. What do people use them for?

  6. #6
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    Default Re: Qualities of a good plumb bob

    I have a LV bob. I've used it a number of times. For example, I had to put some new posts in under an existing second floor deck. The beam had to go in one specific place to align with something else. So I chalked out where my beam would go, then bobed down from that mark to get where to dig the piers. I bobed again at the end to confirm saddle placement. Or another time I was adding a lean-to car port onto the side of a garage. There was siding, etc, in the way, making it hard to get a line square off of the garage in order to set the posts and beam for the carport. To solve the problem, I exposed the first truss (which had to be done anyway) and bobed down at several points. Then I just chalked a long line on the ground that intersected those marks, and voila: a line square off the building. Also, sometimes we end up having to fill in studs on gable ends. Not being able to do a traditional lay-out, a bob works well to get a mark for the stud.
    I think it's one of those things like a flush-cutter: people say "what is that good for?" And the standard answer is "for undercutting door jambs when doing flooring" which is true, but once you have one and use it and get familiar with what it can do, you start to see opportunities for it in your work all over the place.
    Last edited by callee; 10-26-2013 at 07:35 AM.

  7. #7

    Default Re: Qualities of a good plumb bob

    Quote Originally Posted by schor_ View Post
    Material: Brass tarnishes so would need a clearcoat, is heavy. Bronze, cool but who's going to pay $150 for a plumb bob. Stainless, could work. Or do we care more about weigth than material, and who cares if it rusts or pits or tarnishes over time?
    I would assume it should be non-magnetic.

  8. #8
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    Default Re: Qualities of a good plumb bob

    That's true and I should have mentioned it before ! The materials in my old Geotec bob are all non-magnetic.

    As to their use; they are used to locate points at a lower elevation relative to points at a higher elevation or vice versa as the case requires. This can be done with incredible accuracy when proper technique is used! The surveyor's bobs were used to locate the centre of a tripod mounted transit or level relative to known points so offsets from that point can be determined with great accuracy. Engineering and land surveys were and are specialities in the measurements world.

    Plumb bobs for carpentry are a sub-set of engineering measurements applied to architecture as explained by Ryan with his examples.

    Ken

  9. #9
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    Default Re: Qualities of a good plumb bob

    I've used the lee valley one to hang the casing for a set of French doors on our hallway closet - I know the guys who do this day-in day-out use a 6+ foot level or strait edge instead but boy did I learn a lot about plumbing doors in three or four planes.

    I can see it would probably be awesome for framing with some mentoring, my FIL taught me some cool tricks with string lines when he was putting has monster garage/shop together years ago.

    John
    Shut up, wretched cricket of doom...

  10. #10
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    Default Re: Qualities of a good plumb bob

    I think there are two different uses for a plumb bob, the first being transferring a point at one elevation to a point at a lower elevation. This is what Ken refers to and requires a symmetrical bob.

    The second is to cause a string to hang tightly to provide a plumb reference line. This is what John is talking about and just needs a blob of enough weight but any shape (plumb blob maybe?).

    In my limited experience I've found more need for the latter, but your mileage may vary.

  11. #11
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    Default Re: Qualities of a good plumb bob

    Hang a door with one and you may never use a level again.
    "Do it Right!"

  12. #12
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    Default Re: Qualities of a good plumb bob

    Surveyors used plumb bobs for chaining prior to the advent of electronic distance measurement.

    I use a plumb bob to set basement and partition walls. Two points on the ceiling near the wall ends, transfer to the floor with a plumb bob, snap a couple of chalk lines, and you won't get a wall location any more precise than that.

  13. #13
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    Default Re: Qualities of a good plumb bob

    I have the ubiquitous StanRee "brass" plumb bob. Its far from high quality, but I dont use it that much.

    Has anybody used the LV Flatbob http://www.leevalley.com/US/wood/pag...=1,42936,50298 ?

    I like the look of these Japanese Tajima retractable bobs, they are supposed to be real handy when setting doors, I can see the rewind feature being good, Im forever having to deal with knotted, twisted and unwound plumb bobs.



    Starrett used to make a mercury(and lead) filled plumb bob at one time. That would be pretty sweet, the mercury would really damp and steady the bob you'd think? But the darn environmentalists... Starrett #87 Its a beautiful thing

  14. #14
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    Default Re: Qualities of a good plumb bob

    I have the LV flat bob. Mrs.callee got it for me for chridtmas one year. It's a nice tool, but overall I have not found it very usefull. I found it unsuitable for most normal plumb bob jobs, as it's flat side made it too susceptable to winds. The string that comes with it is way to short for anything useful, and the little holder that the string winds around, it is supposed to clip into the bob, but I could never get it to stay clipped in. I upgraded to one of the regular LV bobs, and as I posted above, I like it, though I do sometimes wish it was a bit bigger and more substantial, and I too fantasize a bit about the starrett.

  15. #15
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    Default Re: Qualities of a good plumb bob

    In a past life I worked in survey we used to buy a rewind thing called a gammon reel which held the braided line .I still have my old survey bob which is attached to an old diecast evans chalk line box with about 30 feet of line, enough to plumb 2 stories.As for my mercury Starratt very battered and worn but you would have to work hard to get the mercury out.My problem is how to ship two 3ft mercury barometers from the UK . So far no one will ship the mercury .Your environmentalists have struck at me too!

  16. #16
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    Default Re: Qualities of a good plumb bob

    The enviro-nazis are a big help aren't they?

    I used to have a proper reel for my surveyor's bob but it got wrecked...darn it. Now I just use braided 100lb test fishing line on mine but I really like the idea of re-purposing a chalk line reel for it. Thanks for that idea!!

    Ken

  17. #17
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    Default Re: Qualities of a good plumb bob

    My buddy owns an elevator company, and won't use anything *but* a plumb bob....his problem arose from trying to get it to stabilize on a 200' + line....we're talking a whole day in some cases. Anyway.... Years ago his dad figured out that the shaft hole was the same dia as a pail they had in the shop.... And had a cool idea.

    at the bottom of the shaft they put that pail in *about* the right place, then fill it with mineral oil. They drop the bob into the pail and it steadies immediately. They then move the pail so the bob-line is centred, then trace around the pail.

    Useful information? Probably not....but a cool example of making it work
    I dream of a better world, one where chickens can cross roads without having their motives questioned.

  18. #18
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    Default Re: Qualities of a good plumb bob

    I've used plumb bobs throughout my career. I still use one today for chaining over uneven terrain. There is no laser that will do this for you and the total station driven solution is very expensive and has proven to be less than accurate at times. If you have 3 hands and want to struggle with a level be my guest otherwise there is the simple, elegant, time tested solution of using a plumb bob while holding 10 lbs tension on the tape. For this I use an 18 oz brass surveyors bob with replaceable hardened steel tips.

    We have big plumb bobs up to 15 - 20 lb custom machined for work on concrete wall and column forms. When I was an apprentice in Alberta in the 1970's there was one in every job box. They were good for all but the windiest of days. There were some days when, even if you could put the bob in a pail of water the wind would still act on the string giving you a poor reading. Those days we would pull the transit out and shoot the columns in plumb. Today with steel system forms that can be plumbed easily with a 4' level and so you don't need a bob very often but they are still used.

    I was taught to use a plumb bob to set door frames. You hang the line on a nail 4" off the face of the hinge jamb and have a spreader made up with a 4" offset marked on it. Wedge the head tight, adjust the bottom shims until the offset mark lines in directly under the point of the bob then adjust the rest of the wedges on the hinge side to the plumb bob line. You then swing the door and the strike side and head are set to the door.

    Today I use a cross hair laser for wood frames and a 4' level for hollow metal masonry frames. The cross hair laser is much faster to use and I can easily get the heads aligned in a given area quickly and easily.

    There are still situations in finishing where the plumb bob is the only tool that does the job. I had to do architectural panelling in a 3 storey elevator lobby a few years back where there wasn't any room to use a laser top to bottom. We did it the old fashioned way with plumb bobs.

    I have my layout kit that I use to lay out commercial interiors like stores. It includes line and dot lasers, laser measuring device, 100' tape, chalk lines in 3 colors (tajima of course) dry lines, 5 different colours of spray paint, black sharpie and a plumb bob because there are times when the tried and true always works ..... like when your batteries run out.
    Last edited by dave_k; 10-27-2013 at 08:40 AM.

  19. #19
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    Default Re: Qualities of a good plumb bob

    Quote Originally Posted by Rusty View Post
    Hang a door with one and you may never use a level again.
    You wouldn't say that if you saw how long it took me to get that first casing right (of course, I was fighting an existing wall that wasn't plumb, so I spent just as much time playing around with reveals as I did figuring out how to make a plumb bob work). That said, the doors don't have any tendency to open or close on their own so I think I got them close.

    Anyone want to guess how many engineers it takes to run a plumb bob (just one in my case, but I sure wouldn't make any money at it)
    Shut up, wretched cricket of doom...

  20. #20
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    Default Re: Qualities of a good plumb bob

    I did a lot of surveying early in my career on many types of job sites including aircraft alignment using a theodolite. The plumb bob is your friend in countless situations for accuracy and precision. That is why I still have and use mine, despite the advent of laser alignment.


    Ken

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