Lets assume,I know bad me to assume,you have to stack 6 blocks to reach 6". At +/- .0001 for each block, that would be +.0005" if all the were +.0001. Your micrometer has a tolerance of +/- .0001. if that runs to the full side your now at .0006. Don't know about your eyes but I can loose sight by +/- 2 to 3 marks by looking too long at the vernier scale, if I'm lucky I can get to +/- 1 mark. So now I have a possible error of about .0008 to .0009. That's assuming everything is to the plus side, it could also be the same to the low side, meaning tolerance band of .0016 to .0018, that is just for measuring the part. To get to +/- .002 for making a part is even harder. Because to hit an actual dimension of say 6.0000" +/- .0002 is near impossible for the home shop, because you can't measure that close to an actual dimension, nor can your machine cut a part that close, the graduations are .001 and the slop in the machine is probably .002"+, if you have
a good machine. so yeah I'm more than happy to work to .002 or +/-.001". Guaranteed you pass your part, that you measured, to someone else to measure with a different micrometer the will get a different dimension than you, if you try to measure to .0001".
Dave Patterson
odd_kins@yahoo.com
http://home.comcast.net/~oddkins/foundry_home.html
--- On Mon, 1/24/11, Rick Sparber <rgsparber@aol.com> wrote:
From: Rick Sparber <rgsparber@aol.com>
Subject: RE: [gingery_machines] Re: new article: How Accurate is Your Micrometer?
To: gingery_machines@yahoogroups.com
Date: Monday, January 24, 2011, 7:57 PM
Dave,
I think you are talking about how the error on each block combine to give the total error. My spacers are +/-0.0001" each. If I stack two of them up, then the worst case error is +/-0.0002". A more reasonable error would be the RMS error (root means squared error) which I figure is +/-0.00014". For 3 blocks the RMS error would be +/-0.00017".
My shop is around 60 to 90 degrees but am not sure what the temperature coefficient is for these spacer blocks. I would be surprised if it ended up anywhere need 0.002"
Rick
-----Original Message-----
From: gingery_machines@yahoogroups.com [mailto:gingery_machines@yahoogroups.com] On Behalf Of David Patterson
Sent: Monday, January 24, 2011 7:17 PM
To: gingery_machines@yahoogroups.com
Subject: RE: [gingery_machines] Re: new article: How Accurate is Your Micrometer?
Rick, when it comes to stacking gage blocks at 1" I won't make any measurable difference. If you stack blocks to get 6" you may not end up with exactly 6", as opposed to using a single 6" block. The reason is tolerance stack up.The error could be 6 microns per block for clean blocks through a little dust or cleaning solution film between them and the error would be greater. The blocks are accurate at 68 degF colder or hotter they change size. If you have a need to calibrate your micrometer, take it out and have it certified, then use it in a controlled enviroment. other than that if you can work to .002" your still splitting hairs. I'm happy with that.
Dave Patterson
[Non-text portions of this message have been removed]
http://groups.yahoo.com/group/gingery_machines/app/peoplemap/view/map
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