Machsupport Forum
General CNC Chat => Building or Buying a Wood routing table.. Beginnners guide.. => Topic started by: noob64 on February 05, 2017, 02:16:56 PM
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Home built Wood CNC
I just installed a 8mm acme rod on my Z axis and I am trying to set it up.
I don't have an anti backlash nut, not sure if this is my issue.
Nema 23 motor.
I have mounted a digital caliper to the carriage to measure my movement for set up.
I am measuring a 50mm distance using Z50 and Z0 for the movement and then measuring the distance and resetting each time.
Here are my measurements in mm:
up -.2
down +.36
up +.18
down -.01
up +.32
down +.24
up +.07
down +.35
I've done more cycles, but this illustrates my perceived problem
up off by ave.1925 range of +.32 to -.2
down off by ave.24 range of +.24 to -.01
both averages are less than ten thousandths.
Is this too great and inconsistency? What are acceptable tolerances for the different axis' in wood and hopefully aluminum at some point?
thanks, Mike
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Acceptable tolerances are whatever you find acceptable.
Having said that, you should be getting more consistent results.
Is everything tight mechanically?
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I ground a flat on the acme rod and added another set screw to contact the flat, so I think so. What is the standard repeated precision that I shoot strive for?
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What's keeping the acme screw in position? If it's supported by the motor, then you won't get any better, as the motor shaft will have some end play from the bearings.
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I used a pillow block. See if anything i the attached picture jumps out at you. I do have another motor I can try
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It looks like the only thing controlling the end play of the screw is the motor bearings and the flex coupler unless there is something on the other end that we can't see in the photos. You might want to look into a more rigid way of controlling the end play.
Mike
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There is a pillow block below the coupler. Maybe I need to add another pillow block at the other end
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What's holding the screw in place, just a set screw? Or the coupler?
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I installed a set screw on each end of the coupler and ground a flat on the acme thread
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So the coupler is carrying the weight of the spindle?
The screw should be fully supported, so that it can't move up and down.
I think what your seeing is the play in the motor shaft, in combination with the friction of the screw sliding through the pillow block
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I will take it apart later today and take some pictures. It is a home build with a lot of "opportunity" to improve
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Unless the pillow block is a press fit on the screw all it is doing is supporting the screw radially and not axially.
Mike
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It has two set screws on it
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I just checked a few sites and that style of pillow block is really designed to take radial loads. It is not really designed for axial loads so some end play is normal.
Mike
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I'll have to look into what the usual method of control is
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I cut a shoulder into the lead screw at both ends of the Z axis on my machine. Secure mounting of the pillow bearing blocks serves to form a "Clamped" lead screw. The only "Play" left is in the recirculating ball bearing nut assembly of the Z itself. I did have to experiment with different "Shims" in mounting of the pillow block bearings until I achieved movement without any binding. Also, I used the set screw hole of the coupler as the guide for where to drill an indendation into the lead screw into which the set screw is locked. Never had any slippage since! Engravings in particular require precise repeatable movements to within .001" I realize this post is almost a year old, but perhaps anyone else who may be building their own machine, may benefit from this comment. Have a blessed new year!