Trouble with the z axis

I have just finished building a mendal with a RAMPS 1.3, and using Sprinter firmware controlled by RepSnapper .
All works as it should except for the z axis, I can’ calabrate it to move a selected distance by altering this line
float axis_steps_per_unit[] = {80, 80, 3200/1.25,20}; in the Configuration.h file. I am using 1.8 degree motors set to 1 /16 step.
With 3200/1.25 set the z axis only moves ¾ of the selected distance,both the x and y axis move the correct distance.
Can anybody tell me what to try
Tony

are you missing steps?

getting different move results for the same distance in gcode?

how are your motors wired?

Is it a Sells Mendel or a Prusa? The Sells has 8/21 gearing, but that would give a bigger error.

Are you definitely using metric studding with 1.25mm pitch?

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[www.hydraraptor.blogspot.com]

nophead Wrote:
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>
> Are you definitely using metric studding with
> 1.25mm pitch?

I was wondering the same thing. I bought a Metric Prusa kit from a seller in the the USA. It came with supposedly metric M8 nuts, but they don't fit standard M8 threaded rod. I am not sure, but I am guessing they may be M8 Fine threads with a 1.0 mm pitch, instead of M8 Standard threads with 1.25 mm pitch. ( See [en.wikipedia.org] for more information.)

Is it possible that you have 1.0 mm pitch rod and nuts? If you don't have any tools to confirm thread dimension (just as I don't!) try taking a close-up photo of a piece of rod butted up against a metric ruler (or put them on a photocopier), and count the number of turns in 10 mm.

Hope this helps!

Edited 1 time(s). Last edit at 08/25/2011 08:56PM by julianh72.

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Follow my Mendel Prusa build here: [julianh72.blogspot.com]

Buy a 8mm 1.0 nut if it fits then you have a tool for checking thread pitch of 8mm 1.0 bolts threaded rod or studs'.
My hardware store carry's metric fine thread nut's and bolts as does my auto parts store as 8mm fine is a common fastener on
auto's and motor cycle's some bicycles too.

If you building robots there is no reason to not acquire tools that will make your job easier.

One can order a set of thread gauge's from Snap-On or Mac Tools or Harbor Freight or Sears.
as well as taps and die sets that come with thread gauges or buy some thread chasers.
a set of them is not that expensive with the added use of cleaning up the little nicks and burrs
one some times get on bolts and rod's etc

metric fine lead screw rod will lower the gear ratio less holding power needed my prusa has such rod
i found it at a place that supply's race car builders. it is much straighter than other rod's i found.
one must spin it faster to get the same movement(speed) as 1.25 thread pitch.
and bigger steps for the same distance
so no real advantage over the 1.25