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

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.
 
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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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.
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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.
 
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.
 
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.
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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.
 
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.
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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
 
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.
 
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
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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.
 
Wait a minute - you mean people actually make parts without taking out of the mill or the lathe 15-20 times??........................
 
Buffalo20 said:
Wait a minute - you mean people actually make parts without taking out of the mill or the lathe 15-20 times??........................
Click to expand...
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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