origin of odd decimal shaft sizes

About as close to 4mm as Damn is to swearing.

About as close to 4mm as Damn is to swearing.

--------------------- It's even closer to 5/32". Maybe the shaft was meant to be a running fit in a reamed hole? It's easier to make a custom OD than a small custom ID, though I've found second-hand reamers 0.001" over and under standard fractional size.

It doesn't seem to fall into the over or under by 0.001 range though. I can't find the motor now, but I think 0.178" is another small inch shaft size that I come across.

Maybe a wire gauge but I can't find a better match than the 4mm or 5/32" already mentioned even amongst obsolete wire gauges. Traditional Meccano shafts are 8SWG (0.160")(4.06mm). I got into work one day some years ago and a couple of the designers I worked with were discussing why another designer who was no longer with the company would have called up a spacer of 0.065", I pointed out that 0.065" was 16SWG and commonplace when the design was done, much less so after metrification in the UK, I don't know what they did to resolve the issue I left that to them as I was working on software.

Maybe a wire gauge but I can't find a better match than the 4mm or 5/32" already mentioned even amongst obsolete wire gauges. Traditional Meccano shafts are 8SWG (0.160")(4.06mm). I got into work one day some years ago and a couple of the designers I worked with were discussing why another designer who was no longer with the company would have called up a spacer of 0.065", I pointed out that 0.065" was 16SWG and commonplace when the design was done, much less so after metrification in the UK, I don't know what they did to resolve the issue I left that to them as I was working on software.

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The hardware store inch-size washers I buy come in a range of almost random thicknesses, which helps when I need a shim. I suppose they were punched from whatever sheet stock was available cheaply, perhaps because it was out of spec. Their aluminum and steel rod stock is also loosely sized and doesn't always fit 5C collets. I've had better luck with CRS shafting which machines much better, but it's not as easily available.

It doesn't seem to fall into the over or under by 0.001 range though. I can't find the motor now, but I think 0.178" is another small inch shaft size that I come across.

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There are bearing clearance standards, in fact lots of them, so pick the one you like.

I guess the first question would have to be is it really a "precision" shaft or just a piece of round bar? Even if it is precision shaft what are the tolerances of manufacture. Look at the notes on most any size/shape stock on the McMaster Carr website to see what I mean. Sometimes its right up on top, and sometimes you have to click on the individual product information link.

The next question is how much does the exact size matter?

It's precision and common on small motors shafts. They make matching radial bearings and reamers for this size. I just can't figure out where these bizarre sizes originated.

To be specific, C-frame fan motors have sizes like this as do some fairly expensive DC motors. If you just throw a ruler on these motors, you get darn close to x/32nds. If you have to make mating parts, you'll run into some serious problems.

In the metric world for shafts, they seem to have nice clean ODs even on ground and polished shafts, even in the sum 1/4" range. 2.0, 2.5 3mm etc. The sleeve bearings may have more clearannce, but no big deal there.

One servo motor in my hand has the shafts turned down to 0.1545" and it's no fluke. The shaft inside the motor and bearings are bigger and likely metric, but this number is no accident and common practice.

It's precision and common on small motors shafts. They make matching radial bearings and reamers for this size. I just can't figure out where these bizarre sizes originated.

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Motor manufacturers' catalogs might tell you.

Sometimes the shaft is unsersized to provide clearance in a "pricision" bearing. The shaft is 3.924mm - providing just under .003" running clearance in a 4mm bearing.

That's not unlikely; a job shop doing stampings will frequently have leftover parts of whole sheets of material, and can make a bit more money by punching out washers than from recycling the remainder as scrap for remelting.

hardware store washers are definitely the worst of the worst, if you're looking for any type of consistency. I make it point to show people the random thicknesses in of those things. They must be running the worn out dies on scrap until they finally fail. Might also be the junk from setting up the machines.

On the other hand, Lawson makes some real nice washers if you don't want to worry about sharp edges up or down. Some appear to be fineblanked.

My guess would be that manufacturers would buy either 5/32" material or a wire gauge size about 5/32" or just over, and then grind just enough off to get a true straight shaft, this size then became the standard.

Sort of why TGP is common in sizes like 3-15/16". Start at 4" and true it up.

Randy