Hi Iain,

You may want to watch this video by the excellent Stefan Gotteswinter on indicator sag.

Here is a link **LINK**

I think the aluminium version would be fine but perhaps a steel sleeve to attach the rod.

Regards.

David

If being used as a “comparitor”.

The droop will remain the same, and hence irrelevant, if the measurement “force” remains the same.
Careful design of the clock will mitigate the variation of force with measurement displacement. .
That aside stiffness or rather lack of stiffness will frustrate you evey time you use it.

I second the tube solution..maybe with rod under tension down core

Arr..re read the op.

You already have the arm..you are just about the coupling.

Iain,

1. I'm very envious … wish I had seen that one.
2. Martin's remark about scaffold poles is spot-on [whether we are talking about arms, or brackets], but I would just add that circular cross-section is not necessarily optimum … if the loading is predominantly in one direction, then a 'deep' rectangular box-section may be more appropriate.
3. Yes: This indicator is unusually heavy, and minimising the incremental increase in weight will be advantageous … provided you can retain the necessary stiffness.
4. 'Stiffness to Weight Ratio' is what really matters here; and that can be varied by choice of shape or choice of material. Choice of shape has already been mentioned [and in-extremis, leads us to the monocoque shell] … Choice of material is interesting, in that most engineering materials will be found to have similar 'Stiffness to Weight Ratios' [so, a very thin steel shell weighs approximately the same as the thicker aluminium alloy shell of comparable stiffness] … Exotic materials such as titanium, and carbon fibre composites break out of this trap.
5. Using this as a 'general purpose' indicator would be [as my friends in Liverpool used to say] "like using a Rolls Royce to kill a Pig" … effective, but rather too good for the job! … Find a task that is worthy of it, and design your supporting bracket accordingly.

Sorry to witter-on … hopefully there is something of interest there.

MichaelG.

Not sure if it's of interest but there is a surface plate DTI base on ebay at the moment, 391426886831. A bit smaller than mine but should be adequate. Looks like it has it's own test bad as well. The dti would normally be sticking out the other way so they need to be heavy.

Surface plates are like anything else. They have a tolerance. It's usually possible to null the error out by reversing "the part" and seeing if the error in the part is still the same. Engineering optical flats are usually to a 1/4 wave. Optical vary. For glass you might find 25mm glass is better. Disks can be bought that are used to make telescope mirrors that have been cut from 25mm plate. Moulded ones are best avoided.

If you buy 3 telescope mirror blanks it's possible to make your own optical flat by testing on against another and using some simple algebra. Details are in some of the telescope making books. If the blank is 25mm thick I'd guess that the max dia to hold optical flatness levels would be 150mm.

One source of blanks is this one. You could ask how flat is flat. There are other sources even a couple in the UK.

**LINK**

The ground finish he supplies is pretty matte.

John

–

Iain, in the context of the thread, I think Michael's suggestion of 'armchair engineering' reading was quite reasonable. Certainly E and I are different, but E is simply a material property and having selected your preferred material it will then be I that determines how 'stiff' your structure is (i.e. what the effective 'E' of your structure is).

But I suspect you already know that…

Roger

Hey that's interesting!

Wouldn't measuring anything remotely bendy (like a rod supported at one end in a chuck) be rendered inaccurate by an instrument needing that level of force to get a reading?

Ta

Dave

Damn and blast it! Isn't anything ever easy?