reinforcing pipe

[chrisk]

I am working on a scarphing jig that needs some piping that won't bend at all. I currently have 5'x 3/4" black pipe because I thought it was stiff enough not to give under the weight of the router assembly. However, there seems to be just enough bend from the weight to make an uneven line near the edges.

It makes a pretty straight line across the middle 2' which is good enough for now because I only need to scarph 16" wide plywood sheets at the moment. The lines are as straight as I've been able to do with a hand plane at least when the wood is put in the middle and under 2' wide.

If I move the plywood to the outside the cut is uneven because the pipe is supported on the outsides and there's enough of a sag in the middle for the lines to be more uneven then I am willing to accept.

I've thought about filling the pipe with cement in hopes that would keep it rigid across the 4' expanse. But, before I commit to that I thought I'd ask if there were other ideas. Maybe a different type of piping that is more rigid then standard black piping.

I'd appreciate any suggestions.

[Bruce Hooke]

The quickest way to improve the stiffness would be to go up in diameter. A small change in diameter would make a big change in stiffness.

Another idea would be to go with solid rod stock, or use a stiffer metal (stainless steel?, hardened steel?).

I have my doubts about how much good stuffing the pipe full of concrete would do, and it would be a pain to get it in there.

Before doing any of these things I would take a very close look to make sure that the culprit is indeed the pipe bending and not, say, the plywood flexing up just a hair for some reason, or a slight bend in the base on which the plywood rests or something else along those lines.

[Roger Cumming]

I agree that increasing the pipe diameter would be the best way to increase stiffness. Stainless steel is no stiffer than ordinary carbon steel. Stiffness of a material is directly proportional to "E", its modulus of elasticity. The E of steel is 3 times that of aluminum and hence 3 times stiffer. Filling the pipes would not increase stiffness as much as a small increase in pipe diameter.

[donald branscom]

It is easier to bend a solid rod than a pipe.
The reason is that the ID,area 1 X OD, area 2 = strength.
So when area 1 is zero(solid rod) it will bend easier.

Black pipe is used for gas lines higher grade than galvinized pipe.

Pipe is different than tube. Tubes come in different thicknesses for the same diameter. Pipe has different schedule #'s =thicknesses.
Tube is measured by the O.D.Only varies by .002 thousandths.
Pipe is measured by the I.D. Varies by .025 smaller OR larger!

Stainless is softer than steel (nickel content), BUT as soon as you bend stainless the metal gets work hardened.
A square tube is 16% stronger than a round tube.

[pipefitter]

There is always schedule 80. Also,you could get a piece of 3/4"x1/4" flat bar,round the edges just until it fits tightly edge ways inside the pipe to act as an internal strong back.

[donald branscom]

There is always schedule 80. Also,you could get a piece of 3/4"x1/4" flat bar,round the edges just until it fits tightly edge ways inside the pipe to act as an internal strong back.

Good idea pipefitter.
I had forgotton about that remedy.

[Gary E]

Many years ago the Machine Tool Industy wanted to reduce weight, so they tried to put the machines on a diet and replaced the CAST IRON base with Fabricated Steel... Well, that made things lighter, but it also replaced the vibration dampining properties of CI with those of the steel fabrications, they sometimes rang like a bell. So,,, umm... fill the base with CONCRET... ahh... that worked a little but shrinkage etc... So they came up with some sort of EPOXY CONCRETE... that worked better... but the cost... Cant win... CAST IRON is the BEST...

So... how about mixing up a little EPOXY and some sand and whatever other little particles of junk you have and fill up the pipe? It ought to make it stiffer... and dont everyone like a stiffer pipe?

[Canoeyawl]

Round tube makes a poor beam.
I know it may require a redesign but square tubing would be much more effective.

[ssor]

You are trapped. Larger pipe or thicker walls if you insist on round bar. 3/4 inch sched 40 pipe is about 1 inch O.D. 1 inch cold rolled round bar stock will be stiffer you can also buy hollow bar stock 1 inch O.D. 1/2 I.D. but it will cost you.

[boatbear]

Hmm, why not redesign your jig. It sounds like you are suspending a router on bars over the work. There will always be deflection over a 4' span. How about mounting your router on a wide base and running it along a 1:12 ramp which is clamped or screwed down through the plywood to a flat surface below?
Also, plywood is easy to scarph using hand held tools. The edges of the ply layers give a very clear indication as you plane or sand the scarph. Just keep the edges parallel and evenly spaced and you can't go wrong. I have found that a plane is the easiest, provided that you keep it sharp.
Charlie

[chrisk]

Thanks for all the advice.

I'll consider the various recommendations. I don't mind changing the design. That's half the fun of it. The jig is pretty simple so I don't have so big of a time or financial investment in it that I don't want to redesign it.

At the moment it's cutting good enough scarphs for up to 2' wide plywood placed in the center. That's all I need for now. So, after this project I'll consider modifications.

Thanks

[pipefitter]

Another consideration would be movable supports that stop just past the width of the planks you are scarfing. Could be as simple as wedging it with wooden blocks/shims or whatever.

[merlinron]

1-1/4" dia. X 1/4" wall chrome-moly go-kart axle.... won't bend or sag. 4ft. long and as straight as can be.....about 70-80 bucks. search for " bob's 4cycle" lots of sources there.