# PM25 DRO and Hand wheel off by 1 part in 1000?



## Bill Kahn (Jun 24, 2017)

I just got my PM25MV.  Lots of great beginner experiences. (Amazing how hard it is to indicate the head rotation to 1 mil over 18". But I got it.  The nod on the head over 8" is 3 mil, but seems like that adjustment is not for a beginner.)   But it is all set up and I am enjoying it way too much.  Life is good.

A question:  I spin the X handle 210 times.  Exactly.  With backlash taken out.  DRO says I have moved the table 21.025". So, the DRO and the mechanical handle differ by 25 mil in 21 inches.  About a mil per inch.

Which is right?  More trustworthy?

I tried to indicate, but over the 1" of my dial indicator the 1 mil difference is in the noise of indicator (guess now that I have spent $2000 on the mill I really should spring for more than $15 for a proper indicator.  But it is what I have now.)

Any thoughts as to why they differ?  (I have taken readings every inch.  And the effect is cumulative.  Not like all 25 mil comes in at one end.  Right through the center of the X-travel you can see the mil difference every inch slowly accumulating.

(And, I have no idea at all if I will ever do anything requiring .1% accuracy.  On my drill press with HF cross-feed vise I have been almost fully happy with 1% accuracy. )

Ahh, a man with one watch knows the time.  A man with two doesn't.

Thanks for any thoughts.

-Bill


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## RJSakowski (Jun 24, 2017)

Asian mills and lathes often use metric threads on their lead screws but have Imperial dials.  They use the approximation of 1mm = .03937" is approximately .040".    However, In your case the error is much closer than that.

At .1000"/revolution of the lead screw, this has to be an Imperial lead.  There are no metric thread leads close to it.  My guess is that the lead screw was cut on am etric machine and they approximated the pitch, figuring that .1% error was good enough.

I would trust the DRO over the dial.  Particularly if you have the magnetic or glass scales.  The scales are precision made and, in my experience, come with a calibration document. 

There are a variety of ways that you can verify the DRO accuracy. An edge finder and a micrometer calibration standard come to mind.  You can machine the ends of bar and measure with a micrometer.  The diameter of the end mill will need to be taken into consideration but that could be accommodated by making two cuts, one at one inch and another at five inches and calculating the difference between the two measurements.


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## mksj (Jun 24, 2017)

DROs have a specified accuracy, but usually with glass and magnetic scales it is more noise or fluctuations as opposed to cumulative.  So I would not expect to see a fixed change for every X revolutions of the handle. I had something similar in my previous bench top mill, and the error was very close to what you have. The problem was they used metric pitch feed screws and imperial dials. This was for all axis. But it could also be a error in the feed pitch. Per above, I would trust the DRO.


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## T Bredehoft (Jun 24, 2017)

Ditto, I would (and do) trust the DRO.


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## wrmiller (Jun 24, 2017)

Another vote for trusting the DRO. I typically check the accuracy of a new DRO with a gauge block, or a 1/2/3 block of a known (and measured) dimension, and a edge finder.

I don't even read dials on handwheels anymore.


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## Eddyde (Jun 24, 2017)

+1 DRO


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## NCjeeper (Jun 24, 2017)

Nothing to add but howdy neighbor.


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## Bob Korves (Jun 25, 2017)

The problem would have to be a lead screw that has the incorrect lead per revolution, or the DRO is not parallel to the axis, causing cosine error.  I understand that some DROs are adjustable for inconsistencies, though I have never looked into that.  The real question is whether either of the two is correct, and the way to test that is to use gage blocks or other trustworthy measuring standards to check against both the lead screw and the DRO.


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## Bill Kahn (Jun 25, 2017)

RJSakowski said:


> Asian mills and lathes often use metric threads on their lead screws but have Imperial dials.  They use the approximation of 1mm = .03937" is approximately .040".    However, In your case the error is much closer than that.
> 
> At .1000"/revolution of the lead screw, this has to be an Imperial lead.  There are no metric thread leads close to it.  My guess is that the lead screw was cut on am etric machine and they approximated the pitch, figuring that .1% error was good enough.
> 
> ...


RJ, Thank you for your thoughts.  Yes, the hand wheel accuracy is way better than saying 25.4mm per inch is about 25 mm per inch.  But I took your suggestion about how to directly measure given .1% error is still in the noise of my 1" $15 indicator.  I have a 1-2-3 block.  Cheap but my $25 digital caliper says it is 3.000" long.  So, I have some confidence in it.  I used an edge finder on it.  My reproducibility was .5 mils.  Over the (presumably correctly known) 3.2" (.2 extra for the edge finder diameter) the DRO was off by -.4 mils (3.1996", which is in the domain of reproducibility noise) and the hand wheel scale was off by 4 mils (3.204" which is way outside reproducibility noise).  

I find this simply fascinating.  I was vacillating on whether to get the $500 DRO upgrade on a $1500 mill.  How hard is it to read dials--that seemed just fine for me in high school 45 years ago.  I had no idea that the hand wheels were off by so much.  I am most glad I went with the DRO--doing a .1% correction on every precise location would have gotten very old very quickly.

It strikes me .1% for real machining is actually a lot.  OK, I am coming from a drill press with a $50 HF cross feed vise.  So .1% is way better.  But why would they make something so close but clearly wrong?  (Feels like the "uncanny valley" of almost human robot faces--quite disturbing.)

I don't see how this could be a bad apple--the screw seems to actually have the wrong pitch.

Yet the rest of the machine seems perfectly well built and designed.  And the company Precision Matthews seems very responsible--they have been very good not just with pre sales support but also post sales support.

I wonder if others with a PM25MV with DRO can replicate my findings.

Also, given I am a beginner, I am still reserving some probability for this is all just neophyte, not machine,  trouble.

Thanks again for your edge finder suggestion.  

-Bill


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## RJSakowski (Jun 25, 2017)

It is a common practice when cutting threads where you don't have the proper gears for an exact pitch to pitch one that is close.  As an example a 10-32 thread is fairly close to a M5 - .8mm (4.83mm dia. by.794 mm) and the 10-32 screw would engage an M5 - .8mm nut.  Not the best practice as threads are only engage for a short distance and prone to stripping under load but if there is no other option, it will work.   But over 100 threads, the error  would be .63mm or .025".  If the manufacturer couldn't cut Imperial threads with their lathe, they just might fudge it and cut something close.  The lead screw nut, being relatively few threads, would fit even if its pitch was correct.  For all intents and purposes, the difference would be unnoticed by the end user as it is unlikely that anyone would measure the length by counting dial rotations.  For my part, when I got past ten, I usually lost count. Then along comes someone like you who counts out 210 turns.

I would guess that if the lead screw were cut on a CNC lathe, someone could have errored in the programming.  With Chinese manufacturing who knows?  

At any rate, you have a DRO now and as others have said, you will soon ignore the dials completely.  My Tormach mill is entirely by the DRO as there are no dials and virtually all moves on my mill/drill are by the DRO.


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## ronboult (Jun 26, 2017)

Hi Bill
Does your Y axis show the same error?
Ron


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## T Bredehoft (Jun 26, 2017)

I just did the test on my PM25. I cranked the X handle 40 times, the DRO reported 3.9992. I would expect ±.0002 reading the dial, so the error is .0006 ± .0002.

Is this the same a s 6 mil, I'm not familiar with that unit of measure, sounds metric.


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## Bob Korves (Jun 26, 2017)

https://en.wikipedia.org/wiki/Thousandth_of_an_inch
Mil can be a bit confusing, because it can also refer to a millimeter or a milliliter.


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## Buffalo20 (Jun 26, 2017)

I guess I'm missing something, you turn the handle 210 times (each turn .100") and the DRO says 21.025".  You moved the table 21" (by the dial) and the DRO says 21.025",  so your off .025" in 21", not 25 millimeters, I would think over 21" of travel, being off by only .025" would be great.

If it was a millimeter per inch, that would make the error reading in the area of 19-21 millimeters (full travel) or about 13/16"

Again maybe I'm reading this wrong


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## RJSakowski (Jun 26, 2017)

Buffalo20 said:


> I guess I'm missing something, you turn the handle 210 times (each turn .100") and the DRO says 21.025".  You moved the table 21" (by the dial) and the DRO says 21.025",  so your off .025" in 21", not 25 millimeters, I would think over 21" of travel, being off by only .025" would be great.
> 
> If it was a millimeter per inch, that would make the error reading in the area of 19-21 millimeters (full travel) or about 13/16"
> 
> Again maybe I'm reading this wrong



My tired 40 year old Taiwanese mill/drill reads 18.396 over 18.4oo by the dial for an error of .004".  The error  increases to .008" mid travel where the wear is the greatest.  I would expect better than .025" error  from a new mill.


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## T Bredehoft (Jun 26, 2017)

Thus "mil" tends to be used more than "thou" for the thickness of plastic sheet, while "thou" or "thousandths" tends to be used when discussing machined dimensions. [Wikipedia]  
I hadn't thought of that. OK. I got it now.


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## Buffalo20 (Jun 26, 2017)

RJ,

As I don't do this for a living, I thought an error of about .001% over a span of 21 inches would be considered excellent. That being said, after reading your post, I called one of my friends who runs a small shop that specialized in stainless steel and other more exotic metal repair and parts for the local drug companies. He said the calibration company, that comes twice a year to his shop, runs a somewhat similar sounding tests, 5 times on each machine and get an average travel error. He said the 70s era Wells-Index (completely rebuilt in 2012) is off about .045" over 36" or about .001% and the 3 year old Bridgeport, has an error of about .031" over 36", or about .0009%.

As I didn't witness the OPs test or witness the calibration procedure, that the friend said they seemed very similar, I can't say it is, but according to him, he didn't think it was excessive error. I'm not trying to be argumentative, I'm looking for information. Obviously I need to do some more reading on the subject.


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## Bill Kahn (Jun 26, 2017)

Buffalo20 said:


> RJ,
> 
> As I don't do this for a living, I thought an error of about .001% over a span of 21 inches would be considered excellent. That being said, after reading your post, I called one of my friends who runs a small shop that specialized in stainless steel and other more exotic metal repair and parts for the local drug companies. He said the calibration company, that comes twice a year to his shop, runs a somewhat similar sounding tests, 5 times on each machine and get an average travel error. He said the 70s era Wells-Index (completely rebuilt in 2012) is off about .045" over 36" or about .001% and the 3 year old Bridgeport, has an error of about .031" over 36", or about .0009%.
> 
> As I didn't witness the OPs test or witness the calibration procedure, that the friend said they seemed very similar, I can't say it is, but according to him, he didn't think it was excessive error. I'm not trying to be argumentative, I'm looking for information. Obviously I need to do some more reading on the subject.



I am sorry for using a non-standard, for machinists, term of "mil".  I am still learning the lingo.  But folks did figure out that I meant .001" or, I gather from the guidance (most politely and indirectly phrased) a "thou" or a thousandth of an inch.  It seems that other PM25MV's do not have this problem as Tom B only saw an error (with respect to his DRO) of .0008 in 4" or .0002" per inch (what I would call .02%).  And his was in the reverse direction of my .001" per inch (.1%).

Hearing a Bridgeport was off by .031" in 36" gives me comfort.  I guess that is how accurate mills are.  Though I don't understand the source--the number of threads per inch seems invariant for a given machine.

So, before DROs, how did folks make accurate parts?  Off by .001" per inch, or like .01" in 10" seems like would be material for many sorts of work.  Did machinists use correction multiplications and purposely target 10.01" if they actually wanted 10"? 

My Y-axis has a similar property.  In 5.000" of hand wheel the DRO said 5.0045". So, just a hair less than .001" per inch instead of the X-axis which is a bit over .001" per inch.  Interesting they are not the identical error.  (The difference is well beyond the noise of the measurement).  They simply must have used the same screw thread set up.

I do not see how sine error of the DRO could be it--the sine of 5.7 degrees is .001.  That is a very large angle--10x or even 100x what seems possible based on visual inspection.

Machining is certainly filled with puzzles.  Yes, I will use the DRO.  But goodness, my curiosity has be spiked.

-Bill


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## Bob Korves (Jun 26, 2017)

The sine of 5.7 degrees is .1
My point is that you are trusting one unknown accuracy scale to test another unknown accuracy scale.  "Man with one watch knows what time it is..."


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## ronboult (Jun 26, 2017)

Sorry Bob but if the DRO scale was correctly installed ( scanned with a TDI) the angle error from the tilt of the scale is way below the error of the machine, or the DRO. perhaps Dan should run his DTI along the scale to check. Also the fact that the Y Axis shows the same error is very suspicious.

Bill this raises three Q in my mind
What has PM got to say about this error?
Is this error acceptable even in a cheap Mill? Would other forum members be happy with such a machine?
And finally if you had not purchased a DRO how would produce accurate work on such a machine?
Cheers
Ron


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## Bob Korves (Jun 27, 2017)

It is not only cosine error that can make the two scales diverge.  The lead screw can be made with an inaccurate lead.  I have not messed with DROs, but I think I understand that there is a way of reconciling errors by using the on board software to correct the scales.  Not just from cosine error, but also from inaccuracies of manufacture, perhaps with the factory tooling like that which graduates the scales.  Cosine error can be introduced from all axes, as well.  My point, which everyone seems to miss so far, is that what some are calling inaccuracies have not been tested against known calibrated and certified standards.  When a guy has a Starrett mic and a cheap Chinese mic, and they give different readings, the Chinese one will probably be blamed, but it is all wild ass guessing.  You must start with something you know is correct.  My first thought for the mill, and I think it would be usable, is to simply test against some gage blocks that are in current calibration and certification and see how they match up.   A stack of 4", 2", 1", and .500" would add up to 7.5000x(?)", long enough to see if error is accumulating.  The stack could calibrate against the mill table travel stops and then the DRO and the dials could do the same.  This is an easy test to do, and the test equipment required is fairly widely available around the machining community.  There may be better and easier ideas out there.  One thing is sure, don't call something inaccurate if you have not carefully compared it with something of an order of magnitude greater accuracy, and certified to be so.


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## ronboult (Jun 27, 2017)

Bob you have hit the nail on the head. The whole point of the thread is that there seems to be a problem with the accuracy of the lead of the lead screw. While your comments are correct, the other side of the coin is that Bill has attempted ( in my view validly) to eliminate  or account for any errors. The consistency in the increase of the error over 21" and the fact that it is the same for x & y axis strongly suggests that the error is in the pitch of the lead screws. My experience is that the errors in modern, even cheap glass scales, are an order of magnitude less than the difference between what Dan is measuring between his lead screws and the Dro scale. Modern glass and magnetic scales are easily equal in accuracy to your Starret mic.


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## Bill Kahn (Jun 27, 2017)

Bob Korves said:


> It is not only cosine error that can make the two scales diverge.  The lead screw can be made with an inaccurate lead.  I have not messed with DROs, but I think I understand that there is a way of reconciling errors by using the on board software to correct the scales.  Not just from cosine error, but also from inaccuracies of manufacture, perhaps with the factory tooling like that which graduates the scales.  Cosine error can be introduced from all axes, as well.  My point, which everyone seems to miss so far, is that what some are calling inaccuracies have not been tested against known calibrated and certified standards.  When a guy has a Starrett mic and a cheap Chinese mic, and they give different readings, the Chinese one will probably be blamed, but it is all wild ass guessing.  You must start with something you know is correct.  My first thought for the mill, and I think it would be usable, is to simply test against some gage blocks that are in current calibration and certification and see how they match up.   A stack of 4", 2", 1", and .500" would add up to 7.5000x(?)", long enough to see if error is accumulating.  The stack could calibrate against the mill table travel stops and then the DRO and the dials could do the same.  This is an easy test to do, and the test equipment required is fairly widely available around the machining community.  There may be better and easier ideas out there.  One thing is sure, don't call something inaccurate if you have not carefully compared it with something of an order of magnitude greater accuracy, and certified to be so.


Just a small reminder, earlier in this thread it was suggested I try to figure out how to use a known standard.  My (cheap) 1" indicator was not long enough or accurate enough to resolve .001" of error over 1".  But I do have a (cheap) 1-2-3 block that my (cheap) calipers tells me is 3.000".  Per the suggestion I used my .0005" repeatable edge finder.  And it is perfectly clear the DRO appears to be accurate and the hand wheel is not.  I am below beginner status with a few weeks of instruction in high school 45 years ago and now about 20 hours and 10 days of experience with my first mill.  So I have complete confidence someone who actually knows stuff could figure this out--after all, it is just engineering, not philosophy. I did call Matt at PM to ask about this.  He (and Greg) are very nice.  And have been most helpful.  He told me to take out the backlash (yeah, I had that figured out). And to trust the DRO (same as this thread).  But he did not offer any explanation.  My sense is he sees this as just some silly beginner blunder, and goodness, I certainly have made those too.  And so not worthy of his attention.  Actually, probably the right intuition.  I have little confidence in my own work here.  But I keep repeating the same steps and keep seeing (basically) the same results.  So I will (while using the DRO) treat this as one of the wonderful mysteries of life and the universe for the by and by.


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## joshua43214 (Jun 27, 2017)

I will toss in my thoughts as well.
I very much agree with what folks have already stated, modern glass scales are very precise (they will repeat a measurement consistently). They are not however always accurate. The Chinese seem to have their manufacture down pat, and even the cheap scales measure precisely - their real issue is poor construction leading to early failure. All scales must be calibrated - this is part of their installation.

.o25" error over 21" is way too much for sin error as well. The scale would have to be off by over an inch.

As already stated, unless you are calibrating to a known standard, the calibration is meaningless.
I have as much faith in the handwheel to be precise as I do an un-calibrated scale. I would probably be more inclined to trust the hand wheel than the scale.

Also, keep in mind that readheads and DRO's have a bandwidth. It is easy to move them faster than they can read, and they drop signal. Quill DRO's are especially prone to this. read heads will also produce error with sudden acceleration or deceleration, which is something that will happen very easily when turning the handwheel 210 times.

You are in a somewhat difficult position of not having the right tools for the job yet. I suspect your digital calipers probably do measure the length of your gage block pretty accurately, but you need a very precise test indicator mounted to the machine to use it. I would not trust a cheap import 0.001" dial indicator to measure error in this.

To the best of my knowledge, all DRO's are able to compensate for sin error.
The typical method for measuring error in the scale is as follows:
Measure a test block ~3.000" long to an accuracy of 0.0001" (you need a mic for this, a caliper averages too many errors).
clamp a 123 block to the table, and ring the test block tightly up to it.
Mount the test indicator to the machine head, and advance the table till the indicator reads 0.
remove the test block and move the table until the indicator reads 0 in the 123 block clamped to the table.
The difference in measured vs actual movement is the error.
Refer to the owners manual on how to correct for sin error.

Since I gather you do not have either a mic that can measure 3" to 0.0001", or a test indicator, I suggest you trust the handwheels more than a dial caliper and an import 0.001" indicator.
I suggest you use a cordless drill to run the table 21" by the dials at a rate of about 10"/min. This is an acceptable movement rate that will minimize noise and other errors that scales are prone to. 

For what it is worth, I used to own a PM25 and it was pretty much dead on. I don't recall if I ever tested it over 21", but it did travel exactly 3" by the hand wheel when measured with a Mitutoyo test indicator. They may have changed lead screw sources since then, so my experience may not be valid for you.


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## Bob Korves (Jun 27, 2017)

Bill Kahn said:


> Just a small reminder, earlier in this thread it was suggested I try to figure out how to use a known standard.  My (cheap) 1" indicator was not long enough or accurate enough to resolve .001" of error over 1".  But I do have a (cheap) 1-2-3 block that my (cheap) calipers tells me is 3.000".  Per the suggestion I used my .0005" repeatable edge finder.  And it is perfectly clear the DRO appears to be accurate and the hand wheel is not.  I am below beginner status with a few weeks of instruction in high school 45 years ago and now about 20 hours and 10 days of experience with my first mill.  So I have complete confidence someone who actually knows stuff could figure this out--after all, it is just engineering, not philosophy. I did call Matt at PM to ask about this.  He (and Greg) are very nice.  And have been most helpful.  He told me to take out the backlash (yeah, I had that figured out). And to trust the DRO (same as this thread).  But he did not offer any explanation.  My sense is he sees this as just some silly beginner blunder, and goodness, I certainly have made those too.  And so not worthy of his attention.  Actually, probably the right intuition.  I have little confidence in my own work here.  But I keep repeating the same steps and keep seeing (basically) the same results.  So I will (while using the DRO) treat this as one of the wonderful mysteries of life and the universe for the by and by.


Ordinary 1-2-3 blocks are often pretty accurate, but are still not a known reference source.  I 'think' your work is probably giving you useful information, but it is still a guessing game without using something that is in current calibration with a known and properly tested standard.  Yard sticks are pretty interchangeable, but comparing things in resolution of .001" and smaller starts to get more fussy if you really care about your answers being confidently correct.  Operator errors can also enter into the equation.  It takes some learned and practiced skills to use precision measuring equipment while getting repeatable and trustable results.


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## Bill Kahn (Jun 27, 2017)

Bob Korves said:


> Ordinary 1-2-3 blocks are often pretty accurate, but are still not a known reference source.  I 'think' your work is probably giving you useful information, but it is still a guessing game without using something that is in current calibration with a known and properly tested standard.  Yard sticks are pretty interchangeable, but comparing things in resolution of .001" and smaller starts to get more fussy if you really care about your answers being confidently correct.  Operator errors can also enter into the equation.  It takes some learned and practiced skills to use precision measuring equipment while getting repeatable and trustable results.


I find myself agreeing completely with you.  I do not have the skills nor equipment to really figure this out.  Nor the theoretical background to be creative in hypothesizing the possible sources of the problem.  I guess that is why no one pays me to do this stuff.  Just a new hobby I am fascinated by.  But my third aluminum cube came out better--not really obviously lopsided at least.  I am going next to try for a cube of pre specified size--1.4 cheap caliper inches on a side.  Whatever those units are for real I do not need to actually care about.  My only counsel to those on this list is do not plan to ever do a joint project with me thinking our parts will fit together.  -Bill


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## joshua43214 (Jun 27, 2017)

There are some extremely good machinists on YouTube. We all have our favorites.
For the technical aspect of machining,
I cant give enough thumbs up to:
Joe Piezinski, he is a professional of very high caliber, and gives some extremely good methods for solving many common and uncommon machining problems
https://www.youtube.com/channel/UCpp6lgdc_XO_FZYJppaFa5w

Jon Lecoyer (ThatLazyMachinist), is a retired tool and instrument maker who goes into great detail (almost too much at times) on topics a wide range of machining topics. He probably has the best videos on tool profiles and sharpening - he actually explains what happens to the metal as it cuts.
https://www.youtube.com/user/THATLAZYMACHINIST

Stefen Gotteswinter is a professional prototype machinist who sold his CNC gear to focus on the art of manual machining. He does many projects focused on improving shop tools
https://www.youtube.com/channel/UCY8gSLTqvs38bR9X061jFWw

ROBRENZ is a professional instrument maker, he makes the tools that tool makers use.
https://www.youtube.com/channel/UCn4U3aEr6L2nLe1m_3as6JQ

For pure machining porn, nothing beats Clickspring, who is a hobby clock maker and has easily the highest production value of all the YouTube machinists. Very worth watching because he discussing how he solves problems for many common machining problems.
https://www.youtube.com/channel/UCworsKCR-Sx6R6-BnIjS2MA

My personal favorite is Adam Booth, just cause he is Adam. He is a create guy and a talented machinist.
https://www.youtube.com/channel/UCw3UZn1tcVe7pH3R6C3Gcng

Pretty sure all of the people above have addressed machine set up at some point or another.


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## Bob Korves (Jun 27, 2017)

Bill Kahn said:


> My only counsel to those on this list is do not plan to ever do a joint project with me thinking our parts will fit together. -Bill


One of the greatest joys of hobby machining, where we only need to make parts to fit our own parts, is that we can often do just that, cut to fit (and paint to match!)  There is absolutely nothing wrong with that approach.  When you have to mesh with the rest of the world, life becomes more complicated.  Still, if you make a part on the mill to fit a part made on the lathe, and you cannot get a good measurement or trial fit while it is clamped for cutting, then you start to see the value of having your tooling and measuring equipment (and skills) making parts that will fit by design and execution, rather than by cut and try.


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## Buffalo20 (Jun 27, 2017)

Wait a minute - you mean people actually make parts without taking out of the mill or the lathe 15-20 times??........................


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## Bob Korves (Jun 27, 2017)

Buffalo20 said:


> Wait a minute - you mean people actually make parts without taking out of the mill or the lathe 15-20 times??........................


I learned pretty quickly that if you can do the work in one setup, it usually comes out accurate and parallel.  Take it out and put it back in again and all bets are off.  In fact, it is just about guaranteed to be off.


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## tweinke (Jun 28, 2017)

In response to the OP When I got my PM727 I had to keep telling myself I didn't buy a K&T or such and that for dollars invested am very happy. It does seem odd to have error like stated but it could be the graduations on the collar too, in respect to spacing? Of course I can argue that out in my head and get the answer no if you always stop on the same mark because a revolution is a revolution. Sorry stupid post.


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## Eddyde (Jul 1, 2017)

This thread piqued my curiosity as to the accuracy of my own mill. I have high quality Taiwanese made Bridgeport-esque  clone fitted with an Easson DRO with 1um glass scales. I tested the DRO against the dial at 1, 5, 10 and 20 inches, indeed there was a consistently progressive error at around .00015 at one inch, .00075 at five inches, .00145 at 10 inches and .00295 at 20 inches. so it looked like a lead screw error. However, when I tested against a 4" precision gauge block with a DTI I get about 00055 error, which is consistent with the above error, so it does indeed look like the DRO may be off! I will try further testing and see if I can find and confirm the exact source of the error...


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## Wreck™Wreck (Jul 1, 2017)

Eddyde said:


> This thread piqued my curiosity as to the accuracy of my own mill. I have high quality Taiwanese made Bridgeport-esque  clone fitted with an Easson DRO with 1um glass scales. I tested the DRO against the dial at 1, 5, 10 and 20 inches, indeed there was a consistently progressive error at around .00015 at one inch, .00075 at five inches, .00145 at 10 inches and .00295 at 20 inches. so it looked like a lead screw error. However, when I tested against a 4" precision gauge block with a DTI I get about 00055 error, which is consistent with the above error, so it does indeed look like the DRO may be off! I will try further testing and see if I can find and confirm the exact source of the error...


.00015" in 1", you hit the jackpot there my friend!


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## CluelessNewB (Jul 1, 2017)

I'm not sure who first said it but I remember this quote from high school shop class "If you have one ruler you know how long something is, if you have two rulers you never know".


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## ronboult (Jul 1, 2017)

With errors like  .00015" in 1" I would have thought that one is approaching the stage where thermal expansion due to the heat generated by machining starts to raize its head and needs to be taken into account.


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## Bill Kahn (Jul 1, 2017)

Eddyde said:


> This thread piqued my curiosity as to the accuracy of my own mill. I have high quality Taiwanese made Bridgeport-esque  clone fitted with an Easson DRO with 1um glass scales. I tested the DRO against the dial at 1, 5, 10 and 20 inches, indeed there was a consistently progressive error at around .00015 at one inch, .00075 at five inches, .00145 at 10 inches and .00295 at 20 inches. so it looked like a lead screw error. However, when I tested against a 4" precision gauge block with a DTI I get about 00055 error, which is consistent with the above error, so it does indeed look like the DRO may be off! I will try further testing and see if I can find and confirm the exact source of the error...


That sounds great to me at .00015" per inch systematic error.  So, a 20" part might be wrong by 3 thousandths.  -Bill


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## Bill Kahn (Jul 1, 2017)

Eddyde said:


> This thread piqued my curiosity as to the accuracy of my own mill. I have high quality Taiwanese made Bridgeport-esque  clone fitted with an Easson DRO with 1um glass scales. I tested the DRO against the dial at 1, 5, 10 and 20 inches, indeed there was a consistently progressive error at around .00015 at one inch, .00075 at five inches, .00145 at 10 inches and .00295 at 20 inches. so it looked like a lead screw error. However, when I tested against a 4" precision gauge block with a DTI I get about 00055 error, which is consistent with the above error, so it does indeed look like the DRO may be off! I will try further testing and see if I can find and confirm the exact source of the error...


I missed a key idea--what is "the dial" against which your tested the DRO?  I am guessing it is a known 20".  What do you have that is a known 20" to an accuracy of like .001" or such?  Thanks.


Bill Kahn said:


> I just got my PM25MV.  Lots of great beginner experiences. (Amazing how hard it is to indicate the head rotation to 1 mil over 18". But I got it.  The nod on the head over 8" is 3 mil, but seems like that adjustment is not for a beginner.)   But it is all set up and I am enjoying it way too much.  Life is good.
> 
> A question:  I spin the X handle 210 times.  Exactly.  With backlash taken out.  DRO says I have moved the table 21.025". So, the DRO and the mechanical handle differ by 25 mil in 21 inches.  About a mil per inch.
> 
> ...


OK, Two weeks into my PM25MV ownership and I am now looking at the Z axis.

First, One full 360 degree turn of the Z hand wheel moves the head by .083". Now, I ask, .083?  Where ever did that come from?  I have considered maybe it is actually 1/12th of an inch and it is really .08333.  But no.  The marking shows exactly .083.  The extra .3 thou would be quite obvious and it is not there.

Second, the Z accuracy is somewhat worse than the X and Y accuracy.  I moved the Z exactly 5.000" as established with my 1-2-3 blocks and as my caliper measures as 5.000".  The Z hand wheel showed up as 60 turns (of .083" each) plus .0115" for a total of 4.9915".  So an error of .0085".  Or .0017" per inch.  50% more than the X-Y error.  If I guess that one turn really is 1/12th of an inch, not .083, then the error would be even greater--.0023" per inch.  The other direction.  But as the dial shows .083 I do not think thinking it is really 1/12th makes sense.

The DRO Z accuracy is better, but unlike the X-Y where the DRO is basically correct, the vertical DRO showed 4.99764". For an error of .0024" in 5" or .0005" per inch.

I am most happy with the PM25MV.  I have nothing to compare it to, but whatever expectations I did have are being well met. The DROs seem fully adequate to meet any need I can imagine having--if some year (decade) I decide I want to start working to 1 thou accuracy I will 1) need a totally new set of skills 2) need all new ancillary equipment and 3) need a higher-end mill.  All three seem to be well balanced now and more than adequate for the next dozen projects I am planning out.

But, .083" per turn?  Any thoughts?  (For all I know all mills have that standard?  Maybe some throw back to ye merry old England strange length unit?  I have no idea.)

-Bill


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## higgite (Jul 2, 2017)

I googled it and .083" seems to be a one of the standard pitches for certain sized Acme lead screws. I don't have a clue why, but there it is FWIW.

Tom


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## WoodBee (Jul 2, 2017)

I have read this thread and am sucked in to the issue. I agree that it would be great to have dials on our machines that are "correct". I have no idea how accurate professional machines are, an I have no doubt our hobby machines are less accurate. I also know professional machinist can produce very accurate parts on less than accurate machines. My thoughts are they use the dials on the machines to get dimensions in the ball park, then measure with accurate tools to know how much difference they have between the part and the needed dimension. The resulting cut will then be a small cut, so the inaccuracy in either dial or dro will result in a tiny error.
I know I use this technique when making furniture (which I am much more knowledgeable about) on woodworking machines which are not accurate at all, an easily work with 0,1mm tolerances. ( My "accurate measuring tools" being a tape measure and calipers )
When working metal I am not quite so knowledgeable, so I haven't ventured into the problems described in this thread, but I am almost sure proper technique is an important issue here. And I agree with the notion that you need known accurate measuring tools to do accurate work in any case.
Peter


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## RJSakowski (Jul 2, 2017)

If your PM25MV head is anything like the head on my old mill drill, my dial reads .085"/revolution.  The drive for the spindle quill is essentially a rack and pinion, the distance per revolution of the mail crank being determined by the pitch diameter of the pinion gear.  

I measured the distance separating 13 teeth as 104.3mm or 8.023mm/pitch.  Assuming that the manufacturer cut a nice even pitch length, that would give the rack on my mill  a pitch of 8mm.  One full turn of the main crank moved approximately 3.14" or 80mm so my pinion gear would have 10 teeth.  

The fine adjust wheel is a worm gear drive.  Each crank of the wheel moves the engaging gear one tooth.  It took 37 turns of the fine wheel to travel 79.7mm by the DRO so the gear on my worm drive must have 37 teeth. This calculates to a distance/rev. of .08482".

The nice thing about gears is that all of the numbers are ratios of relatively low whole numbers.  The only used assumption here is that the rack was cut to "exactly" 8mm pitch. Forty turns of the fine wheel traveled 86.21mm  for 2.155mm/rev or .08485"/rev.  This agrees fairly well with the predicted value of .08482"/rev.

Note that the manufacturer would not be likely to put 84.8 divisions on the dial but would round it to the nearest whole number of 85 divisions.

Finally, I did notice differences in DRO readings depending on whether the quill was locked or not.  This is not surprising to me as for years, I have tightened the quill lock slightly when drilling to precise depths.  I did a quick check of the calibration of the DRO using calibrations standards for my micrometer set and the was some deviation but generally in the range of a half a  thousandth.


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