Chronos digital readouts

Recently, I bought a 24" digital scale from Chronos, which I fitted to the Z axis of my machine. It looks very similar to this one, only
longer:
http://www.chronos.ltd.uk/acatalog/info_SGU___.html
I'm somewhat annoyed to discover it has a 0.004" error in every 0.100" of travel! Is this normal for this kind of scale, or have I got a duff one?
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Ade.

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It appears that your digital scale is defective, accurate enough for a ruler, but defective as a precision digital readout scale.
It's time to find out how well the dealer handles defects and returns.
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AdeV wrote:

What machine is it you've fitted it too and how have you determined the scale is at fault rather than the machine.
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if it is really off by 1/2 inch per foot, it would be easy to prove this - put a piece of scrap on your machine and make a cut at 000, then go 12 inches and make another cut and then use a ruler to measure - if it isn't off by about 1/2 an inch, the error is elsewhere.

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David Billington may or may not have intoned:

It's fitted to a Bridgeport series 1. I spotted the error as soon as I finished the mounting adaptor, as it read 0.096" for one complete turn of the Z-axis handwheel (for some bizzare reason, my Z-axis is in inches, wheras X & Y are both in mm). A second turn yielded 0.184", and so on.
To prove it was the Chronos unit & not the machine; I faced a block of aluminium with a 1/2" end mill, then cut a 0.100" deep slot based on the handwheel reading. I then returned the mill to the start point, measured 0.100" on the Chronos, and cut a second slot, adjacent to the first. So I ended up with two "U" channels in the top of my ali. Used a depth mic to establish the depth of the channel; the handwheel was pretty well spot on 100 thou, the Chronos one was almost exactly 0.104"
I only did the test once, mind, and I can't remember now if I made sure I accounted for backlash. The Chronos unit seems to have about 0.010" of "backlash", compared to only 0.002"-0.003" of actual Z-axis backlash.
The scale is bolted to the main body of the machine. I made some aluminium pieces to hold rubber swarf covers (which replace the z-axis gib cover/felt pad scraper); into this is screwed an L-shaped piece which attaches firmly to the scale (it is sized to fit the base of the scale slider exactly AND it screws into place). Although it's aluminium, it's pretty rigid; and whilst I can believe that maybe the "backlash" comes from flex in the fittings, it can't create a cumulative error - and certainly not such a repeatable error.
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Ade.

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AdeV may or may not have intoned:

Duh, brain fart. 0.192" for 2nd rev, 0.288" 3rd, and so on.
No idea where .184" came from
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Ade.

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wrote:

Are you sure the scale is perfectly parallel to the ways? A 2 degree mounting error would explain the discrepancy.
BTW, I've used several identical-looking scales from www.shars.com with no problems (even after calibrating their length with a hacksaw) other than the cheap batteries supplied. http://www.shars.com/products/view/2401/8quot_Horizontal_Digital_Machine_Scale
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Ned Simmons may or may not have intoned:

I'm pretty sure it is: The mounting brackets (shown on the pic I linked to earlier) are exactly in line with the edge of the knee. If my trig is about right, then over 24" I'd expect to see roughly 0.8" of misalignment for 2 degrees? In fact, the scale is significantly longer than 24", it has nearly 30" of usable measure, but either way, if there's more than 0.050" difference between the top & bottom of the scale, I'd be very surprised.

It could be a duff unit, then.... I just wanted to make sure I'd not done something stupid; or that they were this inaccurate in general...
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Ade.

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wrote:

While I cannot speak for Chronos..most reader heads have internal dip switches that allow you to correct for scaling issues..which are quite common.
Check your manual
Gunner
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Gunner Asch may or may not have intoned:

Unfortunately, the unit arrived without a manual. I will see if Chronos can get me one on Monday.
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AdeV wrote:

I wondered originally if you had a Chinese mill but I have a series 1 BP as well and have never had any issue with the lead screw accuracy so it certainly sounds from your testing that the scale is at fault. I would contact Chronos and see if they will replace it. I've bought from them before but not had any issues with the stuff so haven't tested their problem resolution.
Some information on the scale operation here http://www.yadro.de/digital-scale/working.html . Maybe the etching mask is out on that batch for some reason.
BTW a company I know is having parts made in China and some of them have been outside tolerance by as much as 0.004" apparently, which it quite significant for the parts, so maybe they're using measuring kit from the same maker.
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David Billington may or may not have intoned:
[snippage]

I nearly bought a Chinese mill after most of the Bridgeport (and Bridgeport types) I was watching ended up going for "silly" money (silly=out of my range). I got lucky/unlucky with my machine: Lucky it wasn't too dear, unlucky in that it's got some interesting wear issues, and the head seized the 2nd time I used it. Fortunately, liberal applicastions of moly grease to the main bearing in the pulley block has restored normal - albeit noisy - operation.

Aye, I'll give them a go. I've bought a few hundred pounds worth of stuff off them recently, so I'm hoping it'll go at least moderately well...

It's either that, or the measuring unit increments by the wrong amount at each mark; which is I think what Gunner was getting at in his reply. Whilst it's tempting to go dip-switch diving, I'm not sure I want to take the thing apart until Chronos have had the opportunity to rectify...
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Ade.

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I'm fairly certain that there are no dip switches to adjust for errors in your digital scale.
I've had my digtal calipers apart, and they are esentially the same product as your digital scale.
There aren't dip switches in reader heads of any popular brand of DRO scales that I've seen, which have been most brand names (with the exception of 2 brand names that I can think of). All of the DRO sets that I've seen that have been manufactured since about 1980, have had error compensation settings entered into the DRO firmware by keypad entry.
There may have been some old models from the 1960s with dip switches in the reader head, but all the more recent designs have been manufactured to minimize the size of the heads (and the heads are liquid-tight sealed (no reason for users to open them).
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wrote:

Could this be a machine with metric lead screws and fudged imperial dials?
A large fixed percentage error would be difficult to achieve given the way they are made inside.
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Spehro Pefhany may or may not have intoned:

I did wonder that, what with the other axes being metric; but according to both the depth micrometer & the dial caliper, it's definitely an imperial leadscrew on the Z-axis...

On the other hand, it IS possible for the machine to be interpreting each division incorrectly... I've seen discussion of Heidenhain DROs having a scale adjustment inside, which allows you to adjust out any scale error. I can't see anything similar with the Chronos unit, and it came without instructions of any kind, so I'm a bit in the dark really...
I haven't yet verified it has the same error when reading metric, but I guess that's the next step.
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Ade.

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AdeV writes:

Seems like you should be checking against a dial indicator rather than the handwheels.
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Richard J Kinch may or may not have intoned:

I have measured & verified the error (at 0.100" at any rate) using a depth micrometer, as described in another post. Also, the handwheel on the Z-axis appears to be remarkably accurate. It's possible the X & Y wheels are accurate too, but I never use them - I have a DRO for those axis. The Chronos unit was supposed to add a Z-axis DRO, but I can't use it with a 4% error.
Yesterday, I checked the metric readings & found the same issue - i.e. a slot of 2.54mm (according to the scale) was 0.1035" deep (instead of 0.100"). Not sure where the 5 tenths went, maybe it gets more accurate as you use it...
Unfortunately, my depth mic only reads in imperial, which is why I chose 2.54mm...
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Ade.

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    Is it possible that you have it mounted imperfectly parallel to the travel? Such an angle error can cause it to read slightly high.
    Good Luck,         DoN.
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DoN. Nichols may or may not have intoned:

It's parallel to within less than 1mm over its full length (about 30"). I deliberately mounted it right at the edge of the knee, so I could use the edge as a reference. To get an error of the order I'm seeing, I'd need it to be between 1/2 and 1 inch out of parallel... which would be quite noticable I think...
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Ade.

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