Thread: Stiffening Post on Drilling Machine.

Attached is a graph whipped up at work (quiet day, niggling question - you know the one). Equations are from Glieck (8E)
It's for a piece of steel pipe/bar 100mm in diameter, 2 metres long suspended as a cantilever. There is a point load on the end - around 15kg from memory and the rest is self weight. The ID is as a percentage of the OD - really it could just be mm as it is 100mm OD

Attachment 202821

The key point to note is that as the bar is hollowed out the deflection due to the point load goes up slowly until the wall thickness becomes so thin that the pipe can not support itself as well and then shoots up. The deflection due to the distributed load is a gentle decline. Adding together gets a minimum roughly around an ID of 60 to 75mm. Note that this will change depending on the size of the point load.

So, Grahame's engineer was correct - sort of. A pipe with an ID of 3/4 of the OD deflects less if used as a cantilever because of the effect of self weight. If the bar/ pipe was used in a vertical position then only the point load curve applies, in which case solid is better. Stuart's comment is important, because he is essentially asking is less deflection the same as more rigid? It's only words but rigidity implies a stiffness and the value of Young's Modulus (which is a property that typically represents stiffness) remains the same.
It works this way because the middle bit of the bar/ pipe is carries only a small proportion of the load and it is a balancing job between the extra mass that middle bit contributes and the amount of the load that that part carries. The outside/ inside/ inside/ outside bit is a little confusing - I think that might be part of the explanation at the time that has not been completely remembered. I remember funny looking diagrams that I was shown once with inside & outside diameters but there was a whole lot of theory behind it that got a bit baffling...

Michael
(Boy can I waffle after a drink or two...)

You "Boffs"
I asked a simple question and has led to this graph.
I will investigate the heavy walled pipe. But I am guessing that the concrete will be more affordable but thank you all for the interest my question has generated

Hi Micheal,
Thanks, thats ###### brillant!


Hi chambezio,
Unless you take measures to be sure the concrete doesnt shrink as is curers I doubt its going to do much good.

Stuart

I don't think concrete is going to do much apart from promoting internal corrosion. It will definitely shrink and not contribute to the stiffness and if it did it would eventually crumble from vibrations. All that is an educated guess.
If you want to increase the stiffness significantly at low cost, find or shape a pieve of flat steel - somewhere between 3/8" and say 3/4" thick - and fitting as tightly as you can manage it to fit in the bore of the column. Position it in the direction of the bending force (i.e. fore-aft) and hammer/press/push it in, then tack weld at both ends to ensure it stays facing that way.
I think that will make a much bigger difference than concrete, epoxy granite or anything short of expensive very-thick-wall tubing or solid round.
Cheers,
Joe

Originally Posted by jhovel

I don;t think concrete is going to do much apart from promoting internal corrosion. I will definitely shrink and not contribute to the stiffness and if it did it would eventually crumble from vibrations. All that is an educated guess.
If you want to increase the stiffness significantly at low cost, find or shape a pieve of flat steel - somewhere between 3/8" and say 3/4" thick - and fitting as tightly as you can manage it to fit in the bore of the column. Position it in the direction of the bending force (i.e. fore-aft) and hammer/press/push it in, then tack weld at both ends to ensure it stays facing that way.
I think that will make a much bigger difference than concrete, epoxy granite or anything short of expensive very-thick-wall tubing or solid round.
Cheers,
Joe

Another suggestion - put a length of high tensile threaded rod up the middle, welded to a plate at the bottom, and then preload it from the top. I *think* this will add stiffening to the column.....

PDW

Well I have listened to all you learned men and have made some phone calls both near and far (hence the merry-go-round.) I need 70mmdiameter X 1200 long.
I have asked for heavy walled tube or solid. I have been told that 70mm tube is not a standard so I am now waiting for a couple of mobs to get back to me.
I have had "Oh, we don't carry that! You could try..." Gee I hate chasing stuff. I found on the net a crowd in Newcastle who make "precision" pipe and solids, so maybe they might come good.
On the off chance....does someone know of a source for the post?

One quote I got today
71OD X 36ID carbon steel $195
70 OD 4140 $230
Plus freight of course.

Well now thats sorted, are you sure its the column thats the problem?
Have you thought about adding a backstay instead?

Stuart

Originally Posted by Stustoys

Well now thats sorted, are you sure its the column thats the problem?
Have you thought about adding a backstay instead?

Stuart

Not to mention thinking about what's going to flex/break next, after you beef up the column.

Bottom line is, that is a very lightweight radial arm drill designed for lightweight tasks. If you need a stronger machine, buy one.

PDW