A different way of measuring while bending tubing
Yesterday I made my second small front rack top and it was the second time that I had trouble with the measurements for bending tubing (using a lever-style bender). While sitting down with a piece of scrap and the bender I figured out an alternative way to compute bends.
Swagelok has a the best manual for lever benders that I’ve seen. The information is good for any brand of lever bender (I have two benders, neither is made by Swagelok). Starting at page 13 there is a discussion on the gain calculations for figuring out where to place bends for making a polygon.
I’ve had problems with the Swagelok method ofcomputing gain because asmall mistake anywhere in the process can cause problems elsewhere. Yesterday I made either a computation or measurement error and madeone bend 1/2″ earlier than I should have. I ended up splicing twopieces together to recover. Since the Swagelok method has you map out all bends before you start any small errors at end bend will compound.
I figured out an alternative that is easier for me to use.Instead of marking only the end of a bend I’m marking both the start and end of each bend and measuring the straight section of rack between the bends. Using the bender radius I can compute the desired lengths of the straight sections.
In this example I make a roughly 5×7 rectangle (good for a handlebar rack) out of 1/4″ tubing using the Ridgid 404 bender. It has a 5/8 radius.
For a 5×7 rectange the long straight sections would be 7-(5/8)-(5/8) = 5.75″ and the short sections would be5-(5/8)-(5/8)=3.75″. To make life a little easier I rounded these up to 6″ and 4″. The rack will be 5 1/4″ by 7 1/4″, which is still a good size.
Using the radius of the bender I can compute the circumference of the bend that I’m making. For a 90 degree bend it is (pi*2*r)/(360/90). r is 5/8, so this gives me .98″.
The Process:
In this example I’m going to put the seam along in the center of one of the ~5″ sections.So I measure 2″from the end of the tubingand draw a line. .98″ from that lineI draw a second line. To bend I align the first line with the 0 point on the bender. After a 90 degree bendthe second line will mark the start of the new straight section.
I continue this process working around the rack. This photo shows us at the halfway point.
At the final bendIdo the same thing, but I also do something else to check my work. Using a square aligned with the center of the first piece of tubing I draw a third line in between the two normal ones. I put a S through it (square) making a $ sign. You can just barely see this in this photo, but it is clear in the next one.
The $ line will line up with the 90 degree mark on the bender:
I cut the tube to 2″ past the last line and bend:
Here is the final product:
It is about 5 1/4″ across (center to center). We used 4″ straight sections and the bend radius is 5/8″. 4″ + 5/8 + 5/8 = 4 10/8″ or 5 1/4″.
You’ll notice that I never had to measure any fractional numbers except for the bend circumference. I set your calipers up to the bend circumference and locked them in place. This let me use the ruler for the straight line measurements and the caliper for the bend marks.
The same technique should work for non-90 degree bends, you just need to figure out the circumference of the bent area. The formula is simple: (pi*2*r)/(360/degrees). So a 60 degree bend with a 5/8″ bender would be (pi*2*(5/8))/(360/60) = .65 (or 21/32nds). I’m going to be building some front lowrider racks with trapezoidal shapes next and will use this technique there.
Alex, thanks so much for posting this! You”d mentioned something about calculating bend lengths in an iBOB post and I”d never been able to figure out what was involved — this article clears things up nicely. My cheap Harbor Freight bender didn”t come with a manual, of course!
Alex,
Very nice explanation. Normally I don”t like photos as much as line drawings, but this is an exception. The photos are among the best I”ve ever seen for a howto project. The writing is clear and precise as well. Great work! Thanks,
Brent
Alex,
Our webmaster saw your post and forwarded it on to me. As the person at Swagelok now responsible for our tube bending training, I say “Bravo” to you for thinking up a method of bend layout that works for you.
Please allow me to point out that Swagelok offers two 1/4 in. hand benders; one with a 9/16 in. bend radius, and another with a 3/4 in. bend radius. The gain information provided in the manual reflects results observed using our benders. I can”t speak for the results achieved using our bend data with other bending equipment.
That said, I can suggest a few things that may help viewers of your blog with their bending projects (bends up to 90 degrees) regardless of the bender they are using:
1) Forget about gain. While it does have its purpose, it is usually more confusing than beneficial.
Make all bends with the reference end (the end of the tubing from where your first measurement originates) of the tubing to the LEFT of the tubing clamp. If you must make a “reverse bend” (a bend made with the reference end of the tubing to the right of the tube clamp), you will need to learn about gain.
2) Treat the marks on the roll support / slide (0, 45, L or 90) as you would a 0-90 scale. These marks indicate the tangent point for the angles 0, 45, and 90 (if the reference end of the tubing is placed to the LEFT of the tube clamp during bending).
Treat the R mark as a tangent point for a 60 degree angle. (by the way, yes, the R mark does have another purpose – but I”m not going there as discussing it brings up the subject of gain that I told you to forget about in step 1)
Scribe (or mark with ink) a line midway between the R and L (or 90 on some benders). This mark will indicate the tangent point for a 75 degree bend. Visually divide the area between 0 and 45 into thirds and scribe two marks to indicate tangent points for 15 and 30 degrees.
Example:
If you would like to make a 90 degree bend 2 in. from the end of the tube, make a mark at 2 in., place the mark under the L (or 90), and bend 90 degrees. The distance from the end of the tube to the centerline of the 90 degree angle will be 2 in.
Now, suppose a subsequent 45 degree bend is to be made 5 in. away from the vertex of the 90. Visually line up your ruler on the vertex of the 90 and make a mark 5 in. down the tubing. Line up this mark under the 45, make sure the bend is going to be made in the correct plane, and complete your bend.
If you had needed to make a 60 degree bend instead of a 45, you would have lined up the measurement mark under the R instead of the 45. Had it been a 30 degree bend, you would have used the mark you scribed between the 0 and 45 to indicate 30 degrees. 50 degrees? Line up the mark 1/3 the way between the 45 and R.
The drawback with this is that you must visually locate the vertex of the preceeding bend. It get’’s easier with experience, but it’’s not an exact method.
The biggest advantage is there is no math involved – and it works!
One last thing, I must caution you regarding the use of mathematical formulas. While I won”t argue that they work well for smaller tubing sizes, you will likely find that as the OD of the tubing you”re bending increases (1/2 in. and up), your results won”t always follow the formula. It’’s at this point that mistakes can get very expensive as well as frustrating. Believe me, I echo your desire to avoid wasting tubing (not to mention time).
I”ve recently developed a spreadsheet that provides bend data that accurately accounts for the variations from the theoretical values provided by the formulas.
Please use the following address to see data generated by this spreadsheet in our new Bench Top Tube Bender Users Manual (http://www.swagelok.com/downloads/webcatalogs/EN/MS-13-145.PDF)
In addition, I have also developed a system that allows you to use this bend data to accurately calculate the location for all measurement marks in a bend layout prior to making the first bend. It works for any bend angle up to 180 degrees in all tubing sizes supported by Swagelok benders. In limited testing it has worked for other equipment as well. I”ve used it to make layouts with as many as 14 bends. If you get into a bind, or begin bending larger tubing, please contact me.
Thanks, and good luck!
Mike Gagel
Swagelok Distributor Training Services
Alex,
It’’s pretty impressive that this guy would take the time to post such a detailed response. I know now what tube bender I”ll be purchasing in the near future.
Great pictures Alex!
Richard Long
Temecula, CA
RICX
Mike- thanks for your extensive comments, which I”m only beginning to go through. That a person like you posts something like that on a blog like this speaks volumes about Swage-lok. I thank you on behalf of the hobbyist bicycle rack-building community, however small it is.
Alex- cool stuff. way less complicated than what I have at my site.
you should try multipy the radius times the number of degrees times0.01745 then subtract this from two times the radius on your bender this will give you your gain when you lay out your tube subtract your gain from each bend
Do you have anything on progressive tube bending