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

Discussion in 'A BEGINNER'S FORUM (Learn How To Machine Here!)' started by Bill Kahn, Jun 24, 2017.

  1. Bill Kahn

    Bill Kahn United States Iron Registered Member

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    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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  2. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    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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  3. mksj

    mksj Active User H-M Supporter-Premium

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    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.
     
  4. T Bredehoft

    T Bredehoft Active User H-M Supporter-Premium

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

    wrmiller Chief Tinkerer H-M Supporter-Premium

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    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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  6. Eddyde

    Eddyde Active User H-M Supporter-Premium

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    +1 DRO
     
  7. NCjeeper

    NCjeeper United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Nothing to add but howdy neighbor.
     
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  8. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    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.
     
  9. Bill Kahn

    Bill Kahn United States Iron Registered Member

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    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
     
  10. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    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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  11. ronboult

    ronboult Australia Active User Active Member

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    Hi Bill
    Does your Y axis show the same error?
    Ron
     
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  12. T Bredehoft

    T Bredehoft Active User H-M Supporter-Premium

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    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.
     
  13. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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  14. Buffalo20

    Buffalo20 United States Active Member Active Member

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    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
     
    Last edited: Jun 26, 2017
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  15. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    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.
     
  16. T Bredehoft

    T Bredehoft Active User H-M Supporter-Premium

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    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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  17. Buffalo20

    Buffalo20 United States Active Member Active Member

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    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.
     
  18. Bill Kahn

    Bill Kahn United States Iron Registered Member

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    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
     
  19. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    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..."
     
  20. ronboult

    ronboult Australia Active User Active Member

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    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
     
    Last edited: Jun 27, 2017
  21. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    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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  22. ronboult

    ronboult Australia Active User Active Member

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    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.
     
    Last edited: Jun 27, 2017
  23. Bill Kahn

    Bill Kahn United States Iron Registered Member

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    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.
     
  24. joshua43214

    joshua43214 Active User H-M Supporter-Premium

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    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.
     
  25. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    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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  26. Bill Kahn

    Bill Kahn United States Iron Registered Member

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    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
     
  27. joshua43214

    joshua43214 Active User H-M Supporter-Premium

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    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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  28. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    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.
     
  29. Buffalo20

    Buffalo20 United States Active Member Active Member

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    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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  30. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

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    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.
     
    ronboult, RJSakowski and tweinke like this.

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