On Thu, 08 Sep 2005 07:30:25 -0700, Grant Erwin
wrote:

mlcorson wrote:

I need to construct something that will slide occasionally inside a
2.5"x1/4" steel tube. I want to use a 2"x1/4 to fit inside a 2.5"x1/4"
Do they fit?

In theory yes. In practise, tubing is manufactured to precise outside
dimensions. The inside dimensions vary from manufacturer to
manufacturer, but are usually within +/- 1/32". It is this 1/32" that
determines whether or not they "fit" -- do not assume that they do.
I suspect that this variance depends on whether or not the tubing is
manufactured in a plant that uses metric measurments for production,
where wall thickness is measured in millmeters rather than in inches.
You may have to shop around for tubing from different manufacturers to
find something that suits your purpose.

I've only got piece of 2.5" to measure at the moment. In
theory looks like it will.
The length is approx 36", and the lenght of slide is only 5-6". I'd
like it to be a close fit but not too tight as it might bind. It's
going to be part of the frame of a 2"x72" belt grinder. Where the main
frame holding the axle will extend or contract depending on the grinder
attachments.
-Mike


I doubt it. Such tubes are welded, and there is a weld bead on the inside, and
this is usually the main problem with telescoping tubing.

You can use an angle grinder to grind out a notch on the inner tube to
make room for the weld bead on the inside of the outer tube.

If you don't have much to do, you should be able to clamp the work
piece into a vise and do it free hand.

For more serious production work, I've seen a jig made for this
purpose by replacing the saw head on a Dewalt DW708 Sliding Compound
Mitre Saw with an angle grinder and then using it to notch 20 ft.
lengths of square tubing. (You'd have to fabricate an adapter bracket
for mounting the angle grinder to the sliding rails of the saw frame.)

In this case, the sliding rails hold the angle grinder centered on the
tubing, while the 12" stroke allows you to work about a foot of tubing
at a time without operator fatigue.

You can buy
telescoping tubing, but of course it costs more money than regular square tube,
which is itself pretty pricey these days. What I recommend is to carefully fit
pads of 1/4" steel inside e.g. 3" square tube, just to make a rim around the
inside edge. This can be ground to fit over the bead, and welded on the end, and
the pad can be filed or otherwise shaped to just fit e.g. a 2" square tube. The
inner member should have a similar pad put on its innermost end, but on the
outside. This will make it ride straight, and will keep it from ever coming out.
You don't have to fit the pads perfectly in the corners. This is how they make
telescoping arms for boom trucks, and it works great.

GWE