increasing the bending stiffness of steel pipe or tube?

Try a bit of KISS if possible and use a larger diameter tube.

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Thank you

MichaelG.

How about going down the pre-stressed route? Say a 1/2" thick washer a close fit at each end, each drilled 1/2" diam on their centres. Fill the void with your favourite goo (polyester resin/epoxy resin/sand and cement) tension the washers by torquing up the nuts on the HT steel studding holding the washers in to half the bursting point of the tube. Might need some belville washers to keep the tension on if the goo contracts when it goes off.

Frank

> Might need some belville washers to keep the tension on if the goo contracts when it goes off.

Might need some serious help from the 'how clean is your house' team if he miscalculates the bursting stress...

Neil

Tubes crush when bent so concrete should help but unfortunately shrinks when it dries and doesn't take kindly to being bent much either - that's why rebars are fitted in structures but that would make filling difficult in this case.

You will find that stiffness goes up markedly with diameter for the same wall thickness. If I remember correctly the width preventing the deflection goes up at a cubic rate were as the other one doesn't so diameter has the biggest effect in this case.

I'm pretty sure you could find simple sums in Machinery's if you want to work it out - almost bound to be in imperial though. They do work correctly.

John

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Edited By Michael Checkley on 06/06/2015 20:43:44

Sounds to me like a 50x5mm pipe is the minimum I should consider for a more serious setup, which is a pity, I have rather a lot of 35x5.

Can still do a light drill press though so it's not all bad news

The 50x5 would be a lot easier for welding internal bracing as well

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It may, or may not, be appropriate for the column of your proposed drill-press, but; a very good way of using the 35x5 would be to weld it in a cluster of three, so that the cross-section is like a clover leaf.

MichaelG.

A drawn tube(seamless) with a similar type of tube push or force fitted inside is no doubt stiffer than a thick wall tube, Back in the 1950/1960s when we built motorcycles in the uk some scrambles bikes were fitted with twin wall tube handle bars,ie a tube within a tube and they were very strong,dont know how they made the tube.

I think the twin wall handles started out as two thick tubes pushed together and then drawn out. Certainly some tubes were made like that. I don't think it was done for extra stiffness but for better shock resistance and to retain the original shape.

In the early 50s they may not have been able to get tube of a heavier gauge.

If the tube for the drill press were too light/Thin walled, I'd be worried about the tube kinking mainly at the attachment points. The material to use would be hollow bar, maybe a grade that could be Nitrided.

Ian S C

In bending, half of the tube is in tension and half in compression, so concrete isn't going to help a great deal, especially if there is nothing stopping it simply moving along the tube on the compression side.

Similarly, filling the tube with resin etc and compressing it will simply increase the hoop stress on the circumference (and very marginally at that - think of the stress in the studding vs the minimal stress you would generate in the tube itself) but do nothing for the axial / bending stiffness itself, even if it is somehow firmly anchored to the inside wall of the tube. Making it into a pressure vessel won't make it stiff although ultimately it would bring it closer to yield in bending.

I can't imagine how you'd weld these stiffening members inside the tube or why you'd want to. I hear you say it's just a discussion but as pointed out, the stiffness is related to the second moment of area ie fourth power of radius, so surely the lesson is that if your application needs more stiffness, go for a thicker walled (or solid) pipe. If you are determined to use up your thin walled pipe, it's going to bend something rotten before it collapses. Are you stuck on a desert island?

Merry

Murray,

I'm surprised, and a little disappointed that you didn't endorse my clover-leaf.

... It does make a very stiff section.

MichaelG.

On way of minimising the deflection of the 50mm tube would be to solidly fix both ends of the tube instead of a cantilevered beam (post) as is the norm with a drill press. The support would have to be well clear of the table to allow it to rotate around the shaft without hitting a support structure.

Using the same 500 mm length of 50mm diameter 5mm wall steel tube the deflection with a 35kg load applied in the centre 250mm from the ends would only be about .01mm.

However the design of the drill press has evolved over many years with thousands of design iterations to reach the present almost universal form. An interesting project to better it.

Regards John

I just did a quick check of a 50x5mm steel tube when filled with concrete. The maximum increase in stiffness (ignoring differences between the tension & compression sides) is only 10%. As mentioned above, the best way to significantly increase its stiffness would be to put it inside a bigger tube or get a solid bar.

Incidentally, a bar in the shape of a cross has the same stiffness in any direction. So a + and a x that would fit in the same tube would be equally stiff.

The drill press could be made like a black smith's wall drill, mine a 1"dia steel bar for the column, the top and bottom of these are bolted to the wall much as a lathe is bolted to a bench.

Ian S C

He might struggle to make a table that moves up and down it easily and accurately

Neil

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Neil,

1. That should be a trivial matter for a man with a shaper !!
2. I did say that it may, or may not , suit Ady's design for a drill stand
3. The original post was a general 'concept' question; the drill stand was introduced a 'ferinstance"
4. Ady says that he has a lot of 35x5 tube available
5. Murray's rejection of various 'internal stiffeners' was very valid, but my suggestion requires none.

MichaelG.

This is a common misconception, which is why I mentioned it. For a shape such as a cross with equal leg legs, the second moment of area is the same at any angle: do the math for a "+" and a "x" and you will see!