Recently I purchased Mahr 801SL (4306950). I bought it for use in aligning bores in the lathe that are fairly long, aligning at each end round within .0005 . I have heard and read a little bit about cosine error and errors from changing the stylus. I am wondering does this only come into play while using a test indicator for surface comparator? Does anyone have any info so I may wrap my head around this as I have done some searching and reading. I dont see how turning the lathe chuck will cause the indicator to not read correctly no matter the angle(cosine). Thanks for your time guys, much appreciate it .
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- From: Lawrence London <lfljvenaura@gmail.com>
- To: machinist@lists.ibiblio.org
- Subject: [machinist] Test indicators and cosine error
- Date: Sat, 2 Mar 2013 22:24:29 -0500
http://www.practicalmachinist.com/vb/general/test-indicators-cosine-error-261440/
Plastic
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Test indicators and cosine error
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03-01-2013, 01:28 PM #2
Aluminum
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It matters not, if just trying to bring things into line. if you are using it to actually measure something (like the depth of a small shallow dent in a workpiece) a vertical indicator like an interrapid will only measure the distance difference correctly if in a certain orientation.
I seem to remember that for an interrapid I had it was 11-16degrees. I probably dont remember this correctly...
lever type indicators are great for some things, but I try and keep measurements to insturments that only measure. That said, indicators are great for measuring distances when doing scraping and aligning ways.
Indicators are king for doing alignments of all kinds. -
03-01-2013, 01:48 PM #3
Aluminum
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I agree with Nate, although I remember it as 12 degrees... A lever (test) indicator is for comparisons, rather than actual measurement in my book, so fine for centring a workpiece, checking height against gauge blocks etc, but measuring anything needs a plunger indicator and a precise 90 degree angle.
Just my ha'pennorth,
Dave H. (the other one) -
03-01-2013, 01:49 PM #4
Cast Iron
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Aren't the tear-drop shaped ends for preventing cosine errors on a DTI?
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03-01-2013, 02:27 PM #5
Titanium
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The dial indicators you are talking about are called Comparators. They do very well in comparing one surface to another surface. ie. A Jo Block to a part surface hopefully of the same height. That is a comparing process.
There are way too many variables in a comparator to actually measure something, as the needle point is waving around in an arc.
If you want to measure, use a micrometer or a caliper, they are designed for linear movement, like a yard stick.
Regards,
Stan- -
03-01-2013, 02:39 PM #6
Stainless
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Cosine errors on a DTI are about the correct angle of the stylus to the surface.
Anytime you use a DTI you should know the angle it works correctly at.
When changing tips you want to be very careful about length of the tip.
Swapping makes (and lengths) here can mess your readings up by a long way.
Using a different tip, one that still screws in just fine, it is possible to throw your readings off by a factor of 2.
Do not ask how I know this, it brings up very bad memories.
Bob -
03-01-2013, 04:43 PM #7
Diamond
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The cosine error is lowest when the stylus is almost parallel to the surface being checked. It's at is worst when the angle of the stylus is at 45 deg to what is being checked. The dial states the movement at the tip of the stylus, but you have to remember that tip travels in a circle. There, the indicator shows 0.001" at the tip, but movement would be 0.000707". If you're not changing the stylus angle and are only doing comparisons, it's not an issue.
I feel Bob's pain also.
JR -
03-01-2013, 06:01 PM #8
Plastic
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So when using to indicate in / align a part on a bore or round OD then you wouldn't worry about the cosine error?
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03-01-2013, 06:11 PM #9
Diamond
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Yup, use a DTI for relative measurements only (dialing in a part into concentricity on a lathe), not for direct measurements (using it against the compound to measure depth of cut on a lathe without or with screwed up leadscrew dials).
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03-01-2013, 06:14 PM #10
Plastic
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Thanks you guys. As always a wealth of knowledge and awesome.
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03-01-2013, 06:44 PM #11
Cast Iron
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those button head or back plunger indicators are awesome when it comes to lathe work by the way, like when you're dialing in a rough stock OD and such. Also when you have to reach between the jaws and a piece that doesn't stick our far enough to get a regular dial indicator on, the long needle attachment that extends from the front of the indicator is great!
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03-01-2013, 07:24 PM #12
Stainless
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To a degree. You wouldn't want the stylus at more than 15° to the work.
Originally Posted by Rustymic
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03-01-2013, 08:28 PM #13
Diamond
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"It's at is worst when the angle of the stylus is at 45 deg"
I would say, even worse when it's at 90 degrees - a direct plunge-in!
dial test indicators are null detecting devices. They're best used when the
answer you are trying to achieve, is zero deflection of the needle. -
03-01-2013, 09:48 PM #14
Stainless
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I always find this along with "dial test indicator is only used to show zero as a comparator" interesting. Originally Posted by Rustymic
If it says 2/10ths big I kind of want to know how much to adjust the machine.
Yes I know the "right" way is to zero on the part then go back to a adjustable master or stack blocks see what this number actually is.
Very few of my measurements come out exactly zero and I'll be dammned if I'm going to go back and dial in the height master to get a number if I'm making the same part all day.
I will zero my DTI on nominal and then I will trust it to tell me where I am.
Also when checking runout if it is not zero then how much runout do you have?
Understanding the error is key to using the gauge.
Doing a bit of math you can make them way sensitive once you get your head around how they work.
A bit of playing around with differing angles and a set of good gauge blocks will teach you a lot.
Bob -
03-02-2013, 02:21 AM #15
Hot Rolled
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I thought Fowlers were known for that, it was supposed to minimise cosine error. WRT Interrapids, two things: one, there are two different length styli, make sure the one being used matches the indicator (date of manufacture) else you'll have error that you can't quite figure out, and two, the stylus angle is different for the Interrapid compared to most others. Originally Posted by Peter.
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03-02-2013, 02:21 AM #16
Diamond
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Dave the only thing I take exception to is the word "precise" Originally Posted by Hopefuldave
Unless the person using the indicator has a severe eye fault then
perpendicularity is easy to see and any measurement innacuracy from a
slight deviance from 90º is probably less than the innacuracy of the
instrument.
Wow, do I usually use such long words?
Gordon -
03-02-2013, 09:47 AM #17
Plastic
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So after reading up more on the subject and seeing a test result a PM'er did with a DTI and a calibration machine, the cosine error makes a lot more sense. From what I gather if you have the stylus anywhere from 45-60 degrees your going to receive a error in reading. If you move something say .0005 it would actually move a hair less do the the error. So here is a thought; if you have the stylus set 45 - 60 and are using it for aligning a part in a machine. Is the part more accurately aligned in the machine having the cosine error? Or is this thought a little too deep and I should just be happy with limited dial movement and move on to cutting the part? Thanks for the knowledge as usual.
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03-02-2013, 12:24 PM #18
Cast Iron
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It would be a hair less accurate, as the dial would be moving a hair less for a given amount of runout. Originally Posted by Rustymic
I have a Bestest indicator, .0005" markings, with the LONG contact point (37mm I think, which is 1.436") and I love it. I dropped it one day and broke the contact point (luckily that was all due to falling on a piece of foam) so I bought a new contact point, and while shopping I decided to get a 1/2" long point as well. Now I can swap out to the shorter contact point to increase the sensitivity of my indicator. .5" / 1.436" x .0005 = .000174" per mark with the shorter contact point in. -
03-02-2013, 01:39 PM #19
Cast Iron
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You have that backward. The dial will show more runout than actual. Originally Posted by maxh
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- [machinist] Test indicators and cosine error, Lawrence London, 03/02/2013
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