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Spindle? deflection

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Muzzer19/09/2016 10:48:36
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Posted by Neil Wyatt on 19/09/2016 09:47:23:

recently there was a thread where someone was concerned taht a foot-long bar and a load in the tens of KG caused noticeable twist in a lathe bed.

In practice the torque being applied was far greater than would actually be experienced at the tool. It is easy to over-estimate the real world deflection and the best judge is how you actually get on in practice.

You can make a reasonable estimation using one of these calculators. The real buttock-clenching moments come during parting, when we are busy rubbing our stomachs and turning round 3 times with the toolpost upside down at the rear of the machine. These are the circumstances under which we see tools visibly moving due to backlash, spring etc and also when they tend to fail. From what I can see, the forces under these circumstances will be of the order of 10s of kg (100s of N).

It was in order to be reasonably scientific about this that I gave some thought to how you would apply a controlled load to the tool and measure its deflection in an accurate and repeatable fashion. Of course, the forces need to be applied and measured in the correct direction(s), ideally horizontally and vertically.

Murray

Mark C19/09/2016 10:51:39
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Murray,

I have seen these and they are available for sale (peanuts) without having to get all the rest of the scale assy if you go looking. The RS amps are about £40-50 I think but they are very stable and as I mentioned, will support multiple modes of operation. Perhaps outside the budget for messing about but it you go to the trouble of making a sensing system as you modelled, it might be a shame if it is let down by flaky sensors?

I usually (always) employ at least two arrays normal to each other (they often come in that configuration to save space and agro glueing two close by) to compensate for thermal effects - always an issue with resistive strain gauges.

It also requires a lot of thought implementing them in any system that measures in more than one plane as you have to take account of any coupling of forces to ensure you are measuring what you actually want.... again, another one of those "how hard can it be" jobs!

Mark

SillyOldDuffer19/09/2016 12:08:18
10668 forum posts
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Posted by Mark C on 18/09/2016 21:58:04:

Dave, funny what you can find when you go looking - especially in the right place!

Next time you go the shops (a nice big M&S store is ideal) stand between the columns on an upper floor with your feet flat and heels supporting your weight, if you wait for some people (big heavy variety are good for this) you will feel the floor bouncing up and down. You can then use this as an excuse to run out of the store to the nearest ground floor coffee shop and claim you need a strong coffee to calm your nerves (you could also insert pub and choice of drink if you are a brave man).

Mark

I can't remember if it was Wookey Hole or Cheddar but I was once asked to leave the caves after jumping up and down on a steel footbridge. I wanted to amuse my kids by making it bounce but the woman behind me had a panic attack...

Thanks for an excellent reason for never going clothes shopping again. It's obviously not safe if the floor moves.

Dave

Neil Wyatt19/09/2016 15:35:07
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Posted by Muzzer on 19/09/2016 10:26:03:

If you buy a set of bathroom scales rather than luggage scales, the amplifier thingy is designed to take 4 inputs and arithmetically sum them to work out the net weight applied but apart from that the arrangement is very similar. You can often find such scales on offer at places like Aldi, B&M etc for £5 or so (eg Salter brand etc).

Murray

I have one from an old scale that uses beams like an old analogue scale with a single strain gauge.

I keep meaning to fire it up as there are zero and span pots on the board I should be able to make it do something useful (although if I can get one pre-calibrated for £1...)

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