# Accuracy of digital scales



## Captain Jerry (Dec 18, 2010)

My head hurts.

I spent the morning installing a 12" digital remote scale on my HF X2 mill. I spent the afternoon pulling my hair out. The scale is the $35 from Grizzly(Igaging). The installation was fairly simple. I know I got the mounting method from a HMEM post but I cant find it for the life of me.  The scale is mounted across the rear of the table where it is out of the way of gib screws and locks and robs only about ½ of Y travel. When finalized it will be fully protected by a U shaped channel bent up from roof flashing material. Only the bottom of the channel will be open so swarf should not be a problem and I dont use a lot of lubricant.

This was installed to replace a quickie/cheapie digital caliper set up that I was pretty happy with until discovered how inaccurate it was. It cost me a couple of screwed up parts before I figured out the error was in the caliper scale.

Once I got the new scale installed and working, I decided to test it using the same method that had showed up the inaccuracy of the caliper. I used an edge finder to locate the opposite faces of a 1-2-3 block. I have been told that the only really accurate way meaure is with a really good micrometer but even a cheap micrometer is better than a digital caliper. I havent figured out a way to install a micrometer on the mill table but my cheap micrometer says that my 1-2-3 block is indeed 1 inch thick so that will be the standard that I will measure.  By locating one edge, zeroing the scale and locating the opposite edge, I should get a scale reading of 1.200 (The edge finder has a .200 dia tip) but the results I was getting varied from 1.196 to 2.218. WTF! Following good advice from HMEM members, I cleaned and oiled the edge finder. I got better results, but I also got stiff neck from trying to see the edge finder from both sides. I couldnt use the feed screw dials as a check because of backlash since I had to approach the edges from opposite directions.  

More thinking, AHA! 

I bolted the 1-2-3 block to an angle block so that the upper edge of the angle block sits about 3/8 higher than the 1-2-3 block.  Now I could locate the edge of the 1-2-3 block, zero the scale AND the dial, raise the head and locate the face of the angle block. The scale should read 1.000 and the dial should read zero. The results were better but still not totally consistent. It must be the edge finder. 

More thinking. AHA!

Replace the edge finder with a DTI. Much more reliable and more readable. Thats what I did and I finally thought to take some pictures. Here is a pic of the setup.







Now I get very repeatable results.  The DTI and the dials agree and the scale display reads 998. This is 6 times greater than the advertised error of less than .001 in 3. While scratching my ###d I noticed the display start to jump around ending up at 1.598  Maybe weak batteries? Maybe bad installation.

Has anyone tested the accuracy of these scales with better results? Has anybody had whacky numbers jump up?  Tomorrow, I will install fresh batteries and double check the installation but for now I rigged up a 1 dial indicator and re-installed the caliper set-up. (it doesnt interfere with the scale). I ran a few tests and I am getting 1.000 from the dials, from the DTI, and the dial indicator. The Igaging scale reads .998 and the calipers read 1.002.  If I reverse the travel, all return to ZERO reliably. My head hurts.

Here is a pic of the full test setup






Jerry


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## Loose nut (Dec 18, 2010)

If you are constantly getting the same reading off of your dti and dials and a constant error on the scale then there are a couple of possibilities.

The scale has a repeatable error. Most of the digital scales have a error of +/- a thou and if you add in a rounding off factor it could give you your error.

the scale could be mounted wrong, at an angle to the table but it would have to be way off square to get that much error

There is some flex in the scale mounting allowing it to move a couple of thou. when you move the table. The brackets that are supplied with these scales aren't the best.

Did you allow for backlash, it sounds like you did but?????


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## Captain Jerry (Dec 18, 2010)

LooseNut

Thanks for the reply. I'm going to work on this tomorrow and I would like to hear from someone who has been able to test these scales and verify the advertised accuracy. You raise a number of good points and here is my thinking on those.

The advertised accuracy is .001" over 3" but I am getting .002" over 1" which may be as much as .006" over 3". Maybe this a bad unit and I should get it replaced.

I don't think that the mounts are that bad. I did use the supplied brackets for the scale to table but I made a custom mount for the sensor. I have checked the deflection by locking the table and pushing on the scale with my finger in various directions. If there is any deflection, it would show up on the display but there is none . I think its pretty rigid.

Is it straight and parallel with the table? I think so. As good as I can measure (Ha Ha). But if it were to be off line, the error would show as a high reading rather than a low reading, (1.002" instead of .998")

I have considered the effect of backlash and eliminated it by locating both faces while moving in the same direction. This shows by the fact that the dial agrees with the DTI and the Dial Indicator.

Tomorrow, new batteries and tighter control on the test procedure and data recording.

Jerry


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## Troutsqueezer (Dec 19, 2010)

Hi Jerry,

This may be the thread you were thinking of. http://www.homemodelenginemachinist.com/index.php?topic=9163.0


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## metalmad (Dec 19, 2010)

Hi captain
I will be following this tread with interest as a mill DRO is very high on my list .
I actually went to my supplier the other day and it was only after paying for them .was I told Hare and Forbes could not supply until feb.
 (maybe not then)
This was for the cut to fit style and was $78 Australian each
I wonder if Grizley would ship me a set ???
in the end I got my money back and went home with a headache
Pete


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## Captain Jerry (Dec 19, 2010)

Troutsqueezer

Yeah, thats the one. Except I got the 12" scale which goes the full width of the table. Have you been able to confirm better results than mine?


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## Davo J (Dec 19, 2010)

Pete
That sounds like H&F's. I went to buy 10 band saw blades, I put them on the counter and was told I could have 3 of the 10. I said they where on the shelf but the guy behind the counter told me they where bought in the morning but hadn't been taken off the shelf yet. After seeing the manager, it ended up he was wrong and they had piles of them that hadn't been put on the computer.


With the Grizzly scales, it could get expensive getting them from the US.
If you can afford it the Sino brand off ebay are only around $450 for a 2 axis air posted and the glass scales far more accurate than these types of scales. The 3 axis are around $550 posted.

Dave


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## metalmad (Dec 19, 2010)

Hi davo
I only have a rf 31 mill (round colume) and am a little concerned with spending more on the DRO then what the Mill cost me.
I bought it secondhand for $500 aust, cos it came with a very nice vertex universal deviding head ,clamping kit ,face mill and very s###y vise .Since then I have added a possilock collet set and a cheap RT.
At the moment I can not see where the money would come from for a better mill, so I do not know what to do yet, but yes I would love a nice 3 axis kit. 
(maybe one day) 
dead broke in Australia 
Pete


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## Tin Falcon (Dec 19, 2010)

I have used dros on pro machines but not at home. In my experience electronic calipers are not for dead on precision. I expect to be within .002 with calipers. I used to work in a shop that sometimes had tolerances of +.0003 -0.000 I used the electronics to verify I was close but mics to measure. (in a dark shop those Mic lines can be hard to see and you do not want o be of by precisely .025) 
tin


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## Captain Jerry (Dec 19, 2010)

Tin,

Thankfully, I don't have to work to tenths. After all, this is a hobby and I am only a machiner with no hope of becoming a machinist. I would be happy to be within .002" on most dimensions and .001" on the critical ones. Most of the professional people on this forum have weighed in at one time or another about the inaccuracy of calipers. Installing $500+ DRO equipment on a hobby mill dose not make sense to me, but if I did that, the first thing I would do is try to prove the accuracy. If these remote reading scales are not any more accurate than calipers, I just want to know. If they are less accurate, I want to know. And if tweaking the install could improve it, I want to know that too.

Jerry


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## Tin Falcon (Dec 19, 2010)

Jerry you are a wise man test the capabilities of of the tool and know what to expect. Good education for those interested in buying those DROs. I agree $500 for a dro on a $500 dollar mill does not make sense. I opted for the CNC route on my X-2. 
Tin


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## Loose nut (Dec 19, 2010)

If the error is linear IE: .002 over 1' .004 over 2" .006 over 3" etc then I would think the scale is buggered but if it is .002' over any measurement and is consistent .002 over 1" .002 over 10" then either there is some flex in the mounting or it is a simple scale error and can be adjusted for. You could always add .002 to your displayed reading. 

Remember you can be .0015 off and it may read .002" off (and still be within specs) if it rounds up. If you set it to metric, wash my mouth out for just saying that, it will read to a greater accuracy level for the same # of digits (.001 MM is .00039" roughly half a thou which you can't disply for the lack of digits in the display) and retry your test. See how accurate it is with the metric display.

Most DRO's specify something in the range of .008" parallelism in the mounting of the scales to be within the specified accuracy of the display, this is easy to obtain by using a dti when mounting the scale. It should be about the same for digital scales.


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## Captain Jerry (Dec 19, 2010)

LooseNut

Thanks for the input. I have been testing over 1" and I need to test over 3" and 6" as well. The metric setting is also a good idea. That will reduce the rounding error. I have been testing at the middle of the scale. I guess I should also move the test to either end and check for consistency of error. 

Any more suggestions? I'm off to the shop, a 10 minute walk and no internet.

Jerry


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## Loose nut (Dec 19, 2010)

A 10 minute walk, damn, is it really worth that much effort.


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## firebird (Dec 19, 2010)

Hi

I have similar DRO's fitted to both my x1 and x3 mills to the x and y axis. I get on with them ok and get reasonable accuracy but you have to watch them carefully as all mine have a tendency to jump and sometimes zero at the most inopportune moment. Having said all that I wouldn't be without them. The most used part for me is the ability to switch between imperial and metric. Both my mills are metric but my lathe (Myford) is imperial so when it comes to changing a part from lathe to mill its no problem. I'm afraid you get what you pay for and have to make allowances.

Cheers

Rich


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## firebird (Dec 19, 2010)

Hi

I don't wish to hi jack this post but I have just come across these 

http://cgi.ebay.co.uk/3-axis-digita...613?pt=LH_DefaultDomain_0&hash=item33636f21bd

Doe's anybody have any experience with them.

Cheers

Rich


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## bentprop (Dec 19, 2010)

Pete Metalmad,do you have a branch of Machinery house anywhere near you?
I bought a 600mm scale from the NZ branch,and it was the cheapest all round even with the freight to south island.This is the type with a separate readout unit.I'm very happy with it.


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## metalmad (Dec 19, 2010)

Hi Bentprop
Machinery house is Hare and Forbes in Aust, How much did u pay for yours? and was it the cut to length one? 
maybe I should deal diectly with them,rather then going to a suppler?
Sometimes Talking to someone face to face makes a difference.
Pete


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## Captain Jerry (Dec 19, 2010)

Bogs

The Grizzly (Igaging) scales are made differently. There is no groove in the back for the depth thingy and the capacitance strip seems to be potted in epoxy in the face instead of pasted on. There does not seem to be a bronze tension strip. The scale is positioned in the body of the sensor by hard felt wipers on all four faces which offers much less play than the spring strip. When mounted on the mill, there is little to no twisting of the head on the scale. It is rigidly fixed by the mounting. All in all, a much better setup than converted calipers and the wipers should eliminate swarf intrusion. A formed sheet metal shield will eliminate swarf and coolant and with the remote display it can be mounted anywhere. If it is less than accurate, I think it would be due to lack of precision in the scale or second rate electronics. I am just trying to determine if my unit is far enough out of spec to require a replacement. Before I ask for a replacement, I want to be sure that the problem is not me.


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## quinette7 (Dec 19, 2010)

Jerry -

I, too, spent yesterday installing a pair of Grizzly Igaging scales on my mill, but by the time I finished, it was too late to test them. I put a 24" scale on the x-axis of my Millrite, and I cut about 3 or 4 inches off of a 12" scale for the 8 inches of travel of the y-axis. After reading your post, I spent this afternoon testing the scales. I don't have a 1-2-3 block, so I used a DTI in conjunction with the spindle of my 1"-2" micrometer held in a vise.

I first tried the y-axis and found that 1.000" movement gave a scale reading of .994", with a fairly linear error reading when tested at .100" increments. My head was hurting, too.

But when I tested the x-axis, a 1.000" movement gave a 1.000" reading, and it was within +/-.001" at the .100" intervals.

Using a $30 digital level, I found that my y-axis scale was .7 degrees out of parallel with the y-axis ways. This scale is in an awkward location under the mill table, which made installation difficult, and I was only able to align the scale to .1 degree before starting to pull my hair out. But when I tested the y-axis again, it was now as accurate as the x-axis, so I'm going to leave it at that.

I haven't witnessed any randomly erroneous readings like you mentioned.

I hope yours is as easy to fix.

BTW - For other Millrite owners - The x-axis travel of my mill is 24.5" inches. By slipping each mounting bracket out slightly, I was able to get the 24" scale to read the full 24.5 inches, but that is about the maximum than they can be "stretched".

Bob G


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## rake60 (Dec 20, 2010)

When I was a working machinist most of the manual machines I ran were fitted with 
state of the art digital scales.

When it came time to hit a critical size the old mechanical dial indicator came out.

Digital scales are great for a quick roughing references.
That is their limit.

Rick


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## Captain Jerry (Dec 20, 2010)

Bogs

My logical thinking is hard at work so I have a few question. 

1. Are glass scales rigidly mounted?
2. Can glass scales be accurate if not mounted true and square?
3. If the scales are properly and rigidly mounted, what is the need for position compensation in the head?
4. Are expensive scales easier to mount properly than cheap scales?

I'm not poor but I am frugal (cheap) and after all, this is just for fun.

Jerry


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## Jasonb (Dec 20, 2010)

I can only second what Bogs says. I originally had the cheap scales on my X3, I then added remote readouts. Over the 3 years they were fitted they played up more and more so this summer I replaced the X&Y with glass scales together with a 3 axis universal console and have not had one problem with them and the other functions that teh console offers are a big bonus. As I write there is a long box under the xmas tree with another glass scale (I hope) so I can also do the z-axis.

Jason


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## Davo J (Dec 20, 2010)

Captain Jerry  said:
			
		

> Bogs
> 
> My logical thinking is hard at work so I have a few question.
> 
> ...



1) The scale itself it rigidly mounted and so is the reader head. 
2) The scale can still be accurate if not mounted true with the linear error compensation which is a program built into it. You can measure say a 1 2 3 block to determine if it's reading true and adjust it to suit.
3) Their is a small spring loaded mechanism between the scale and reader head to stop any damage (within reason) if the scale is mis aligned. This works if say one side of the scale was lower and as it comes across the reader head the spring will let the reader head follow it's proper position inside the scale. The actual reader inside slides up and down the glass strip on 5 bearings.
4) The glass scales are mounted with one bolt in each end and 2 bolts in the reader head (usually M4-5). The covers have a bolt every 300- 400mm along their length which is usually M3.

Dave


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## Captain Jerry (Dec 20, 2010)

Thanks for the good info Davo. Software trumps harsware every time. Apparently the linear compensation program and calibration software is the key difference. Well written software should be able to handle not only cosine error (alignment) but could also map point to point variations. That certainly justifies the price but for me it doesn't justify the expense. After all, my last attempt to send a rocket to Mars only missed by } { THAT much!

Jerry


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## mklotz (Dec 21, 2010)

Please start a new thread, Bert.


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