Dro resolution and other ?s

[Naiveambition]

Alright I've had enough!! I've been looking into a dro system for my zx45 mill. Been all over the net reading info, and as usual it is a cluster bomb of info that is hard to understand.

So.... Is resolution just the amount of numbers on the screen that it is capable of reading.? Why would I need tenths if my mill won't do it. I seem to be happy if I can hit the thousandths with a zero.
Most systems seem to offer different resolutions, so if I can change it, where does my repeatability come in to play. Some state that glass scales are in the .001 area. Why would I need glass scales if the simple caliper style like igaging also state .001 accuracy. .002 over six inches seems really bad to me. As most of you know this is enough to make or break a slip fit.

I know that my machine is not the most accurate mill, but if I add dro error to the mix, it just seems to make the case worse.

As of now I'm looking at a dro pros kit. I like the price point, but even more expensive systems offer the same resolution numbers. Am I missing something here?
I would like a 3 axis kit but doesn't seem to be appropriate for my style of mill. So i am looking at a 2 axis kit for the table and a mitoyo vertical scale for my z axis.
I'm not keen on the many options for mounting glass scales. Don't know part name but the huge part at the bottom of the quill attached to my depth stop :hDe: Is always in the way. Especially when using rotary table in vertical position, so with the many options for mounting dro scales just seems to complicate this problem.

I would love for someone to explain in laymens terms what I'm looking at. Meaning if your family member asked the same .?s you would say this is what you need and don't mess with this one blah blah blah...

 

[Blogwitch]

By the tone of your post, it sounds like you are trying to talk yourself into a cheaper method of fitting a DRO to your system. A wrong way to look at things. It is the same as fitting square instead of round wheels to your car.

I have no experience of your mill, but first things first. If your mill can't achieve and hold accuracy, then it is of no use trying to get accuracy by using a DRO system, you will be chasing your tail continuously. THE MILL HAS TO BE SET UP TO IT'S BEST POSSIBLE LIMITS FIRST, only then can you succesfully fit a good quality DRO. It is like fitting a rough old monkey wrench with a DRO and expect it to read as accurately as an expensive digital mic.

If you have a true system of X,Y, Z plus a quill on your mill, then you require a 3 axis system then say as I have done, a scale to give a reading for the quill. Little would you believe it, my Z1 and Z2 readings are the most critical parts of my DRO reading system, they are the ones that give me most of my accuracy, for depths of cut. By careful use of the two, I can easily achieve accuracy of 0.0002" if so needed.

I don't use super systems on my machines, just good quality workhorses. Sino heads and readers on my mill and lathe (interchangeable between the two on the display heads) and soon a Sinpo dedicated grinder unit on my surface grinder.

This is a shot of my quill setup and display head on my mill.

The scale fitted to the quill gives me accurate drilling depths and a way of quickly expanding or reducing daylight under the cutting tool, fine adjustments are done with the Z axis part of the DRO.
The display head shows my three axis of the DRO, plus at the top LHS is a display head for the quill scale. No batteries used at all, the scale and it's display run off a dedicated power system that a friend made for me, no more chasing stuck or varying scale displays half way through a job.

So basically, get the basic machine right, and only then think about showing and using it's accuracy by the use of a DRO, and asking the sorts of questions you are asking above.

John

 

[10K Pete]

Let me approach this from a different angle. What is accuracy? IMO it's the
ability to move the table off a position, then return it to that same position
+/- some error. Or move from a position to another +/- some error.

With a DRO you're concerned with the ability of the scales to read within that
error by some degree. The DRO reads the table scale position relative to the
scale reader mounted on, say, the saddle. If you want to be able to get within,
notice the word--within, say .001" then the system must be able to read to
+/- .0005" to hit that area.

Resolution is the ability of the system to actually measure the movement of
the table. In other words if the table actually moves .001" then the system
must be able to read to that fineness. If you're trying to hit that .001" area
and the system can only resolve (again notice the word) .001" then you will
have a tough time.

So think about the two things: Accuracy and Resolution. You should be able
to make some sense of a systems capabilities in achieving your goals.

Hope this helps,
Pete

 

[Naiveambition]

Hi John, as usual you are a wealth of information. Price point is somewhat a factor. I don't want the cheapest on the block but not the most expensive either. The problem with the z axis on the 45 series mills is to use a column mounted scale , I would only be able to crank the head up and down the column, and this defeats using my quill movements. Also needing to lock the gibs results in a slight head movement. So following what others have done, as shown in pic, would create other clearance problems. This is what I'm trying to avoid with the mounting options. I also need to clear my gearbox changes which creates more issues mounting a glass scale here. In truth there is really not many options for this mill. Most column mills have a factory dro installed which seems to be the best option. With the mitoyo scale I can solve many of these issues.

So as regards to resolution, the scales are rated 5 microns which I'm sure is fine for me. But reading other install posts, they seem that real world translation is .001 to .005 repeatability. Even the cheapies claim this, and some expensive ones too. One even stated, that it means the number of decimal places used on the dro screen. As I stated I know I can't hit the bullseye but I want to be close as possible for the least amount of money. Many others have used dro pros and are happy , I just want to know what it is I'm reading.

 

[Naiveambition]

Whoops sorry fellas , you must of posted while I was writing the last one.

The scales at 5 microns have a resolution of .00002. This is awesome if that is the movements. I'm just trying to decipher where others have stated the .001 to .005 as repeatability. My harbor freight calipers used for scratching lines are better than .005.The dro pros website does not list too much info on repeatability, so this is my area of concern.

 

[10K Pete]

Think of Repeatability this way: If the table moves from one place, say your
'zero' point, to another an inch away the DRO is supposed to show that the table
moved exactly that inch. Now if you move the table back to the zero point will
the DRO actually read that inch or will it be off a little? Remember, we're acting
as though we have a perfect way to measure that inch when we move the table
and we're comparing the DRO to that perfect master measuring device. Will
The DRO come back to the same place and actually show that it did? Over
and over, time and again?

That's repeatability. The number stated by the manufacturer as the repeatability
is how far off the DRO is in measuring that movement in comparison to the
'perfect' standard.

It might help to note that there is no such thing as 'perfect' measurement. You might get to the resolution of a single electron but there is still a variable. The DRO scales
and reader can only resolve (there's that word again!) to a certain fineness and the DRO can only calculate and display so many digits to the right of the decimal point.
So there is some rounding off that occurs and that happens for each measurement you
make. That can add up. So the Repeatability and Resolution must be kept to very small
distances so any errors stay really, really small for practical purposes.

Hope that helps,

Pete@Metrology is Fun

 

[Nick Hulme]

If you have a column and a quill there are "adder" units available for glass scale DROs which take 2 scale inputs and integrate them to one input for the DRO,

- Nick

 

[RonGinger]

I bought a DRO Pro system for my lathe. I think it is well build, the English manual is a great help. When I ordered they were out of stock on the long scale I wanted so they offered the higher res one for the same price. I assumed that was a good deal and took it. It may not have been- that gives me 5 digits past the decimal, and the last couple just jump around with the slightest vibration. I have thought about simply placing some masking tape over those digits.

For my G0704 mill I bought a 2 axis one off ebay for about half the price of DRO Pro. As far as I can tell it was made in the same factory- same nice heavy metal box, nice scales, and it works fine. The manual is the typical Chinese version of English, but I have found it to be correct, if hard to follow. I have not done any real test measurements, but my parts come out to the size I read on the DRO when I measure them, so that is all that really matters.

In both casees the size of the scales is a problem. On the lathe it limits how close I can get the tailstock into the job. On the mill the scale blocks the table clamps so the table can jump around a bit. If I move the scale to the back of the table it limits the Y travel. I guess you just have to live with that.

I would never give up the mill DRO. I often wish I still had my Trav-a-dial on DTI on the lathe the DRO is not reaally a big enough improvement for its price.

 

[Nick Hulme]

Ron,
I had the same issue with the cross-slide scale on a Myford Super 7 and went for 2 linear bearings on rods and mounts outboard at the back to create a self-contained linear slide to mount the read head with the scale bolted to the rod end mounts, it's robust and will take a beating without damage to the scale or read head and can be removed with two screws and a wing nut.
Regards,
Nick

 

[zoltan]

Check out this page to better understand resolution, accuracy, and repeatability:
http://www.yuriystoys.com/2013/12/selecting-scales-for-a-hobby-dro.html

Do yourself a favor and check out TouchDRO. It's a DRO solution which uses a tablet as the readout and interface and is fairly scale agnostic. Since it's a software solution it's also upgradable and the developer is still adding new features. I have a 8" tablet dedicated as my mill DRO and aside from running TouchDRO it's also a browser, holds various charts and documents, and plays music. The TouchDRO interface is also much more intuitive and feature/information rich than a standard hardware DRO.

You can see it running here on my X2:

 

[petertha]

I went through a similar process on my King RF-45 mill. Don't get discouraged. It might take some time but the pay-off will be once you have it installed it makes milling so much more enjoyable (and I would argue accurate). Just keep collecting pictures & installation posts (ideally feedback/review) of DRO installations most representative of your style of mill & dimensions. Even if you have to have to mock it with wood based on catalog dimensions so you are comfortable with plan of attack.

The XY scales can be a bit of a head scratch because it involves trade-offs. On one hand you want them out of the way & protected so you don't bang into them or interfere with other setups or movement range. But, that can also make mounting them more challenging when it comes to tapping holes & making mounting brackets. Glass scales are a bit bulkier than some other systems, but sizes vary by vendor & at least the trend seems to be smaller. Some scales are slimmer & come in more incremental sizes (another big consideration). Newall has 'tubes' which are quite compact & relatively easy to mount & align. But they cost more. I piecemealed my (newall) system together from ebay sales but if I had to do it again today, there seem to be several good, reasonable $ offshore solutions. Choose wise, but choose once IMO.

Don't worry about 0.0002" display. Most boxes let you specify the digit resolution. Some prefer to see 0.001 on one axis and 0.0005" on a different axis. Is it truly measuring that? Can it return to that position with same precision? Is your end mill finish or hole centering really cutting that resolution? These are the realities.

But, what amazed & shocked me was DRO faithfully recorded my table movement by 2-3 thou just applying a smidge of torque to the gib strip lockdown. Or pushing & actually slidng the table when gib strips & anti-backlash nut improperly adjusted. DRO faithfully record these deviations even though the lead screw handles are stationary.

Other considerations on the "45" mill style Z-axis: try not to cantilever your measurement device too much because vibration & flex can distort measurements. Some people use vernier or digital gauges & don't bother plugging in an encoder to a 3-axis box. But if you can use one box & accommodate the scale, some things are a bit easier. Also try to think of a way to still integrate 'stop' for the quill. Each mill variant is a bit different in their casting so it influences the DRO setup. But having a functional stop is still nice.

 

[purpleknif]

I like my I gaging readouts on my XJ 9512. Cant beat them for the price.

 

[Blogwitch]

Cant beat them for the price.

That is the main problem, they are cheap.
Nothing wrong with that if you are a bit cash strapped, but if you can afford them, glass or magnetic scales just can't be beaten.

I struggled for a few years with scale type DRO's and displays, and spent more on keeping the damned things going than a proper DRO would have cost me.

About 6 or 7 years ago when I upgraded my machines, I vowed I would never go back to that type of system, so invested in glass scale heads instead. Besides being used and abused, they haven't missed a beat, despite there being 7 read heads at this time. I can turn off for weeks and when turned back on again, the part I left in the vice or chuck is still perfectly set up datum wise on the display.

The difference between the two systems is like the difference between chalk and cheese.

For anyone having trouble with read heads covering up gib side adjusters or locks, especially on a lathe, I came up with this idea which could be used on almost any dovetail slide.

http://www.homemodelenginemachinist.com/showthread.php?t=24109


John

 

[petertha]

.., I came up with this idea which could be used on almost any dovetail slide.
John

Exactly what I did & in fact utilized your link! The dovetail angle matching wedge made a huge difference to minimize that irritating drift while tightening. When I examined my gib strip prior to this mod it had a series of little arc bite marks from tightening using the factory setscrew method. I stoned those flat, replaced with the wedge/bearing ball assembly. Life is good now, much more positive clamping.

One small recommendation & this is machine specific. Consider a way to retrieve the wedge if it's deep in the threaded hole & otherwise inaccessible. I made mine out of brass so couldn't coax out with a magnet. Soft or leaded steel might even be ok because its face to face contact with steel/CI gib strip?

 

[Jhawk]

I have the Grizzly G0755 which is similar to the rf45. I bought my 3 axis dro from the dro store (in Hong Kong I believe) and am very happy with it. They include fast shipping and I had it in a few days. I installed the z axis on the front of the head and it measures the quill travel. That was the hardest axis to install. The x and y were fairly easy to mount. Repeatability is excellent. A lot of times I start indicating 0 at the left edge of my vice then mount my part flush with that side and edge. It makes using the mill a much easier job. Like the others gave said, not very many home machinist are going to or are capable of machining something to the tenths. So the .0005 +- resolution scales are more than adequate and work great. There are soooo many benefits, center finding, hole patterns and so forth that become easy jobs with a good dro.

 

[Naiveambition]

Since doing more reading and decision making. I decided to go. With the dro pros 3 axis kit with glass scales. I looked strongly at the magnetic scales . The resolution was mostly the same from a home shop point of veiw Although the 3 axis magnetic was 4- 500 more. I would of liked the mounting options ,but price won out in the end

Now onto the dreaded z axis mounting. I would like to have it mounted on front of machine as a few others have done. The quill twist will need to be addressed on mine for the scales to be useable.

If I run into too many issues I may go with the quill dro' mounted separately , but I figured it would be cheaper than wanting to upgrade to the 3 axis instead of two later on.

 

[Blogwitch]

Hi N,

This is a vid of a chap fitting a DRO to your type of machine, he does rattle on a bit at times but the job does get done rather easily, and not too long a watching time, 16 minutes. As you can pick up during the vid, he has also fitted a scale unit to the quill to use as Z2, like I showed you on my machine.
Lots of ideas on how you could do it to your satisfaction on your machine.

[ame]https://www.youtube.com/watch?v=wpJj6h2tilk[/ame]

Hope it helps

John

 

[Blogwitch]

Peter,

If you read my post completely, you should have noticed I used a normal steel ball bearing loctited into the end of the brass slug, so that it could be extracted with a magnet.

John

 

[kvom]

I have some veteran Newall DROs on my Bridgeport and lathe. While I attempt to zero the 10ths digit I don't worry if it's not. Since the BP has a knee I don't need/use a spindle readout. It did come with one made from a caliper, but I found it interfered with the spindle stop and removed it.

 

[petertha]

Quite right John. Incomplete & clumsy wording on my part. I also locktited the ball on the wedge segment... only to then discover once in it was a stainless ball on brass wedge so non-magnetic!
Carbon steel ball & would be the ticket.