# What Resolution On Dro Scales For A Pm-932?



## Jim Wilson (Feb 10, 2016)

I'm looking into putting a DRO on my PM-932.  The vendor I'm talking to ( looking at a Fagor DRO) - has different options available for the scales. 

I'm wondering if it's worth the money to put a higher resolution scale on the PM-932 - or is the mill just not capable of taking advantage of it?

Here's what the have for options:
- resolution .00005”/.0001”
- resolution .0002”/.0005”*
*


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## T Bredehoft (Feb 10, 2016)

.0002 is almost meaningless as far as a milling machine is concerned, IMHO any more is money spent without return.


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## Dan_S (Feb 10, 2016)

resolutions, is only part of the picture, you also need to consider accuracy, repeatability, & Hysteresis. 

This image below (from dropros) & webpage are  good starting points.






http://www.yuriystoys.com/2013/12/selecting-scales-for-a-hobby-dro.html


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## ch2co (Feb 10, 2016)

Dan,
You beat me to it, by seconds.


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## Muskt (Feb 10, 2016)

I have the "factory" one installed--It is great.  It reads to "tenths"----HOWEVER, & this is a big "HOWEVER"--after getting everything set, and then tightening the assorted locking devices, it usually changes anywhere from 2-4 tenths on mine.  It is relatively easy to hold to less than a thousandth, though.  

Jerry in Delaware.


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## ch2co (Feb 10, 2016)

whoops, did it again !  keep pushing the wrong thing.
To continue, 
It really depends on what you're planning on doing with the mill.   Yuriys toys site explains it well.
http://www.yuriystoys.com/2013/12/selecting-scales-for-a-hobby-dro.html


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## Jim Wilson (Feb 10, 2016)

Thanks for all the info everybody - I'm going to have to sit down and do a little reading tonite and educate myself.


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## wrmiller (Feb 10, 2016)

I was recently talking to the folks at DroPros about the EL700 DRO for my new mill, and they said that typically you don't see resolution below 5 micron on mills, but it is not uncommon to put 1 micron scales on lathe cross slides. Which is what I did. I'm putting the 5 micron scales on my new 935. FYI the 1 micron reader heads are a $100/ea. upcharge IIRC. Ouch...


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## ycroosh (Feb 11, 2016)

Jim Wilson said:


> I'm looking into putting a DRO on my PM-932.  The vendor I'm talking to ( looking at a Fagor DRO) - has different options available for the scales.
> 
> I'm wondering if it's worth the money to put a higher resolution scale on the PM-932 - or is the mill just not capable of taking advantage of it?
> 
> ...



Jim,
For lathe's cross slide I can [marginally] justify a 1 micron scale if the lathe is of "tool room" quality. I used on my Jen 1024 and it's a grotesque overkill. I got mine very cheaply, so it didn't matter than much, but I would not pay extra for one.
Here is some food for thought: cast iron has coefficient of expansion of 0.0000058 in/(in*degree F). Aluminum is double that, almost exactly. Glass can be between 0.0000022 and .0000050 (pyrex vs. plate glass).
Let's say your scale is 30" and temperature in the shop fluctuates between 65F in the winter (in my case) and 80 in the summer. That's 15F over 30 inches. Cast iron will have delta of 0.00261, glass - 0.00099. That is 0.00162" difference between how much scale and the bed expand (best case scenario). This basically means that in order to take advantage of the regular 5 micron scales the shop needs to be climate-controlled, etc.

Hope this makes sense

Regards
Yuriy


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## wrmiller (Feb 11, 2016)

ycroosh said:


> This basically means that in order to take advantage of the regular 5 micron scales the shop needs to be climate-controlled, etc.
> 
> Regards
> Yuriy



No, you don't need a temp controlled shop. My lathe or material is not going to see that much temperature swing during operation regardless of what temp it starts at (winter or summer). The DRO measures distance traveled from a point you select, for example the point at which you just contact a round bar with your cutter, and from there only reports how far you have traveled from that point. There is no fixed point reference from which all measurements are taken.

I can hold tenths tolerance on a part regardless of how chilly or warm my machine is.


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## ycroosh (Feb 11, 2016)

wrmiller said:


> No, you don't need a temp controlled shop. My lathe or material is not going to see that much temperature swing during operation regardless of what temp it starts at (winter or summer). The DRO measures distance traveled from a point you select, for example the point at which you just contact a round bar with your cutter, and from there only reports how far you have traveled from that point. There is no fixed point reference from which all measurements are taken.
> 
> I can hold tenths tolerance on a part regardless of how chilly or warm my machine is.


OK, here is an experiment for you: turn a piece of steel to exactly 1.0000", leave it in the garage for a few hours and measure it (down to tenths). Write down your number. Now put down the micrometer and warm up the pin in your hand for a few minutes. Measure again. Write down the number. Compare two numbers (and compare them to the 1.0000" you turned the part to in the first place).If you happen to have two micrometers that can measure don't to tents, try to compare the measurements you get from them as well. 
Let me know what you find out


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## wrmiller (Feb 11, 2016)

Apples and oranges...

I know quite a bit about coefficient of expansion and other material properties, thank you. And that is not what I'm talking about. And it has nothing to do with the accuracy of a machine or a DRO.

What I said was that I can hit a number regardless of how warm or cold my machine is. Your suggestion that a warm part is larger than a cold part has nothing to do with cutting a part to a specific dimension.


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## JimDawson (Feb 11, 2016)

I like my 1 micron scales, I can watch the tenths change!    Makes everybody think I have a really accurate machine  My shop is climate controlled too, cold in the winter and hot in the summer.


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## ycroosh (Feb 11, 2016)

wrmiller said:


> Apples and oranges...
> 
> I know quite a bit about coefficient of expansion and other material properties, thank you. And that is not what I'm talking about. And it has nothing to do with the accuracy of a machine or a DRO.
> 
> What I said was that I can hit a number regardless of how warm or cold my machine is. Your suggestion that a warm part is larger than a cold part has nothing to do with cutting a part to a specific dimension.



I'm not saying you can't hit *a* number. What I'm trying to tell you is that number has nothing to do with the actual dimension of the part. Just because that number happens to have four places after the decimal point doesn't mean that you can hold tolerances to a tenth.



JimDawson said:


> I like my 1 micron scales, I can watch the tenths change!    Makes everybody think I have a really accurate machine  My shop is climate controlled too, cold in the winter and hot in the summer.



Ha-ha. I guess you're right. "Climate controlled" doesn't specify which way the control goes.


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## Dan_S (Feb 12, 2016)

ycroosh said:


> I'm not saying you can't hit *a* number. What I'm trying to tell you is that number has nothing to do with the actual dimension of the part. Just because that number happens to have four places after the decimal point doesn't mean that you can hold tolerances to a tenth.



What i think he is trying to tell you, and something i agree with, is that a machinist who knows what he's doing knows how to deal/compensate for the fluctuations in temperature and thermal expansion. This is why metrology, has a reference temperature.


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## wrmiller (Feb 12, 2016)

Thank you Dan. 

I misunderstood the first comment and thought we were talking about how the expansion/contraction of the lathe doesn't justify a 1 micron scale, then we shifted to the dynamics of the material being cut, and then to someone's claim (not mine) to hold a tenth? Figured I'd just stop there.

It's all good.


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## Jim Wilson (Feb 12, 2016)

Thanks for all the responses , they helped.  Given the mill I'm putting these on , the type of work I'll probably do with it (likely not high tolerance stuff) - and the cost , I will go with the lower resolution scales and save a little bit of cash.


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## wrmiller (Feb 12, 2016)

You will like having the DRO. Makes many things easier and can help prevent the scrap pile from growing too fast. Above all, have fun.


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## jbolt (Feb 12, 2016)

Even with a DRO I always use test indicators for high precision work.


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## petertha (Feb 14, 2016)

Not sure about the PM mill but I have a somewhat similar Taiwan RF-45. One thing I noticed when I installed DRO with 0.0005" display is the simple action of tightening the table lock screws made the table moved like +/- 2-thou. (Nothing like a DRO to see what really going on!). The root of this evil is the torque action of the set screw can impart a slight linear displacement to the table itself. Then you chase your tail for a few more iterations trying to get it clamped to a DRO position. Also the gib strip tends to get marked up with little set screw arcs. Not a great design.

My solution was to make a ~0.5" insert part from brass rod. It goes in the set screw hole first & contacts the gib strip with a face machined to the same dovetail angle. On the other (set screw side) it has a 60-deg center drill relief. The tightening bolt is modified with a similar center drill & ball bearing locktited on the end. Now the screw pushes on the insert part with nill rotation & the matching angle face gives more perpendicular clamping force without all the unwanted junk action. Made a world of difference.

One thing I didn't think about was removing the part from the hole. Turns out I could expose the hole by moving the table. But in hindsight, maybe thread the end? Chewing gum on a stick?


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