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Old 12-11-2019, 09:13 PM
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Default Why do they do this/

I have noticed that very often a metric shaft is an oddball figure. For instance, a 15mm shaft is very, very, seldom 15mm. It will be some thing like 15.12mm

Today once again it bit my ass. A guy called up and wanted a taperlock shaft size of 3/4" and brought me a 5/8 taper. So I did it, as they were panicking for it. Of course it did not fit because after I had to drive over and measure it, the thing was 19.28m. Why call it a 19mm shaft if it is not? That .28mm is about 9 thou, and it can't be hammered on the shaft....they tried that.
I've got some 18mm stock that is also not 18, but 18.06mm. I have had bearing suppliers insist a 25mm bearing is 1", when I brought it back because it was .9842.


At least in Imperial, when we say a shaft is 1/2" inch we are not shittin' about it.
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Old 12-11-2019, 09:57 PM
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Originally Posted by Ironman View Post
I have noticed that very often a metric shaft is an oddball figure. For instance, a 15mm shaft is very, very, seldom 15mm. It will be some thing like 15.12mm

Today once again it bit my ass. A guy called up and wanted a taperlock shaft size of 3/4" and brought me a 5/8 taper. So I did it, as they were panicking for it. Of course it did not fit because after I had to drive over and measure it, the thing was 19.28m. Why call it a 19mm shaft if it is not? That .28mm is about 9 thou, and it can't be hammered on the shaft....they tried that.
I've got some 18mm stock that is also not 18, but 18.06mm. I have had bearing suppliers insist a 25mm bearing is 1", when I brought it back because it was .9842.


At least in Imperial, when we say a shaft is 1/2" inch we are not shittin' about it.
If it's not a measurement with two digits to the right of the decimal, then it is a welding and cutting measurement, not a machining measurement.

That being said, there are some pretty sad digital calipers on the market today that pretend to be accurate measuring tools. Those will bite you in the butt too.
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Old 12-11-2019, 10:29 PM
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"At least in Imperial, when we say a shaft is 1/2" inch we are not shittin' about it."

That's why I like to stick to Imperial. The mm stuff always bites me in the ass.
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Old 12-11-2019, 10:38 PM
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Originally Posted by rock6.3 View Post
If it's not a measurement with two digits to the right of the decimal, then it is a welding and cutting measurement, not a machining measurement.

That being said, there are some pretty sad digital calipers on the market today that pretend to be accurate measuring tools. Those will bite you in the butt too.
I'm happy with 2 digits to the right as long as they are zero's.

I tend to machine in inches, but rest assured that if you are doing repair work on anything you have to be dealing with metric equivalents. I measured out the shaft as .7950 with the Starrett mike..And that's a metric 19mm shaft, that is not exactly 19mm.
I only trust the mititoyo caliper, and not even then in critical stuff.
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Old 12-11-2019, 10:55 PM
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...At least in Imperial, when we say a shaft is 1/2" inch we are not shittin' about it...
If you buy Imperial dimensioned T & G (turned and ground) shafting in 1045 or 4140 it's usually right on the money. Cold finished 1018 shafting--which is very commonly used for PTO shafts and a variety of other "lower level" uses--is usually a couple thou undersize so it easily fits pre-machined yokes, sprockets and sheaves. It's meant to be that way so you know what to expect.

Even though we are a "metric" country I buy very little metric shafting but when I do it usually comes from a "real" steel supplier and measures dead nuts to what it's supposed to be. I have noticed that some of the metric stuff that comes from a "farm store" type place can be off a bit. I have no idea why and I also have no idea where it comes from but it has "surprised" me a couple of times--and yes, it's usually stuff supplied by the customer...
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Old 12-11-2019, 11:06 PM
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Originally Posted by LKeithR View Post
If you buy Imperial dimensioned T & G (turned and ground) shafting in 1045 or 4140 it's usually right on the money. Cold finished 1018 shafting--which is very commonly used for PTO shafts and a variety of other "lower level" uses--is usually a couple thou undersize so it easily fits pre-machined yokes, sprockets and sheaves. It's meant to be that way so you know what to expect.

Even though we are a "metric" country I buy very little metric shafting but when I do it usually comes from a "real" steel supplier and measures dead nuts to what it's supposed to be. I have noticed that some of the metric stuff that comes from a "farm store" type place can be off a bit. I have no idea why and I also have no idea where it comes from but it has "surprised" me a couple of times--and yes, it's usually stuff supplied by the customer...
Farm store metric is shady stuff. Always undersize, so it spins in the bearings.

And so is Browning. This was the input shaft on a Browning box. The output is 30.06mm and the great white hope is to find a 30mm shaft to slide into the hole.

Come on, Bubba, jump in here and tell me why they do this.
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Old 12-12-2019, 04:23 AM
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its a conspiracy led by Big Inch, they're trying to keep the sensible measurement system down.
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Old 12-12-2019, 07:50 AM
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its a conspiracy led by Big Inch, they're trying to keep the sensible measurement system down.
https://en.wikipedia.org/wiki/Big_Inch
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Old 12-12-2019, 09:23 AM
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Well, it is probably caused by Global Warming. No one else seems to know why metric does stupid things.
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Old 12-12-2019, 10:10 AM
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It’s cheaper material, especially if bought at big box stores. They want to sell at certain price point, and the manufacturers will adjust the tolerance level to their price point.
+- .010” is a lot different than +-.001” especially.

We all complain about cheap shit from China sold at Walmart and other stores, but has been mentioned here several times that the factories in China have some great high precision tools that they made, but they can produce a lot more stuff that is cheaper if they can and need to cut corners.

The one thing you can count on in when pricing or having a job bid on, is the winner that has the cheapest price usually does not have the best quality.




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