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
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
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.
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
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.
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.
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...
While I cannot speak for Chronos..most reader heads have internal dip
switches that allow you to correct for scaling issues..which are quite
Check your manual
"God grant me the serenity to accept the people
that don't need to get shot, the courage to shoot
the people that need shooting and the wisdom to know the difference.
And if need be, the skill to get it done before I have to reload."
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
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.
David Billington may or may not have intoned:
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
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
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
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).
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
I haven't yet verified it has the same error when reading metric, but I
guess that's the next step.
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
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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