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How to mark 90 degree positions on round tube end?

JasonPAtkins

Hot Rolled
Joined
Sep 30, 2010
Location
Guinea-Bissau, West Africa
I think my brain may not be running at full capacity this afternoon. This tube (with stub shafts welded into either end) is going to be the central shaft for a trommel sifting rig. Accordingly, it will have support arms screwed to it along its length to support the mesh screen.

One shot trommel.jpg

In order to get things lined up, I need to get 90 degree marks on the face (end) of the tube, which i'll then transfer down the length of the tube by setting a stout piece of angle iron on the tube. The problem is, I can't figure out how I should be doing this, despite having what seems like plenty of measurement tools available.

Image2352952494092873692.jpg Image3217935350329109162.jpg

First, let me say that what I've got is probably good enough for my application. However, I'm annoyed that I can't figure out the "right" way to do this, if it really mattered that it was perfect, which again, it probably doesn't here.

Starrett center finder gets me the first two marks (call them 0 and 180 degrees), that's easy. However, I can't figure out a way to precisely get the 90 and 270 degree marks. The basic problem is that adding a second tool perpendicular to that first one has one or both of two problems. First, most of the tools (like another combination square) against the blade of the Starrett aren't flat, so their rule doesn't sit against the tube on both sides at once, and since the body of the square isn't small enough to fit into the tube (5.5" OD), they rock. The second problem is that all of them are depending on measuring the center point by eye on the Starrett rule, from which to send the perpendicular rule. So, it's easy to get a pair of points perpendicular to the first two, but I'm not certain they're centered on the hole - so I'm probably marking 88 and 272 degrees instead of 90/270 by being a little high, or 92 and 268 degrees by being a little low. I also tried using the framing square against the Starrett, which is flat, but still depends on me eyeballing the center point on the Starrett rule for vertical position.

Am I missing an easy solution here? I also can't understand why I wasn't able to find a technique for this on google. Is it so obvious to everyone else that it didn't even need to be explained?
 
Compass?
Set it in the center of the wall at zero and scribe an arc at 90 and 270.
Set it in the center of the wall at 180 and scribe the same arc at 90 and 270.
Where the arcs intersect will be your line.
 
My field method of doing this is to wrap a piece of paper around the tube. Overlap the ends. Cut with a utility knife so the paper is the exact circumference of the tube. Then fold the paper however many times is required. Then reinstall the paper on the tube.

They make "pipe wraps" that are a rubber ruler thing. I only work on small pipes / tubes so I've never found it useful. But perhaps I don't know how to use it.
 
YouTube

First start with bisecting a circle.
Then draw your diameter on a piece of paper.
Bisect it, giving you the cardinals.
Put tube on top of paper, center it up, and transfer your marks.

For a tube that is too long to handle"
Figure the circumference.
Divide the circumference by 4 (or however many divisions you like) and lay it out on piece of paper.
Wrap the paper around the tube, keeping it parallel with the bottom of the tube.
Pull the paper tight, and transfer your marks.

This is how I used to mark our underwater camera housing, blanks. From there, I would align my drill fixtures (off the cardinals) and build the housing.
 
I would normally do this on V-blocks with height gage on a surface plate, and a combination (or better) square to align the scribe marks, that you have drawn across diameter horizontally, to new vertical position all relative to surface plate. The part is sort of large for that type of treatment unless you have some big V-blocks and surface plate. On the other hand, it is large enough that you can easily resolve to sub-1 degree on the end by aligning marks vertically using a square on a surface plate.
 
Put a level on your centerfinder. Plumb is 0/180, level is 90/270. A 45 vial will get you 8 divisions. Or seamstress's tape and some pi.

A good framing square and torpedo level are pretty powerful layout tools. I'm partial to Stabila for shop/fab levels, in fact I just ordered 2 more.
 
i'm not good with cad but even i can easily command a circle & knowing the circle diameter i chose i can mark the 4 quadrants. print out and mark the pipe
 
Measure the circumference. Multiply by .25, .5, and .75 to get distances from the starting point to the quarter points.
 
Using only the items that you show, plus a bench vise to hold the tube:

1. I mounted the tube (PVC pipe) in the vise and leveled it. That level does not have to be exact.

2. With both the center finding head and the protractor head on the scale I set the bubble of the level on the protractor head to 0 degrees and scribed (OK, pencil marks so they would show up better in the photos) the first two divisions at 180 degrees apart.

attachment.php


3. I removed the protractor head from it's normal position on the rule and set it to 90 degrees.

4. This step requires 2 1/2 hands (3 1/2 to operate the camera), but somehow I managed.I held the center finding head on the top of the tube and rotated it around the tube until the bubble was again centered. Then I scribed the second pair of marks.

attachment.php


That makes four marks at 90 degrees to each other. How accurate is this? Well, with a good set of accessories for your square (Starrett should qualify for this) you can easily get down to below 1 degree error. With a bit of care and a sharp scribing tool held at the proper angle, you can probably get down to 1/4 or 1/5 of a degree (12 to 10 minutes). It is probably good enough for most purposes in the shop. If you need better accuracy you probably should use a dividing head or rotary table.

In case you have not noticed, the beauty of this method is that you can set the protractor head to produce any number of divisions that you may want:

3 divisions = 120 degree increments
5 divisions = 72 degree increments
6 divisions = 60 degree increments
7 divisions = 51.43 degree increments
8 divisions = 45 degree increments
9 divisions = 40 degree increments
10 divisions = 36 degree increments
etc.


Note: I know that the scale and the two accessory heads in my photos are cheap, HF models. They are part of a free gift that I got with another purchase and I do not trust them for any tasks where any accuracy is needed. I do not have a centering head for my Starrett square and my good protractor head does not fit any scale other then the one that I custom fit to it. I used them only for illustration purposes here as I wanted to only use items that the OP had in his photo.
 
I layout holes on plastic tanks 64" diameter to 142" diameter. Long tape measure to get circumference then divide it out. You measure it because the manufacturing tolerance on plastic tanks is +-2% or almost 6" on the 142" tank!
 








 
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