Author Topic: Play in Z axis  (Read 208 times)

Stonemull

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Play in Z axis
« on: August 24, 2017, 08:26:51 AM »
Trying to come up with a way to eliminate this movement, preferably without rebuilding it from scratch :)

I think if this could be eliminated then aluminium could be machined with care and careful control of feed rates and cutting depths and direction.
As you can see, the 8mm rods flex, the narrower machines might not be as much of a problem so perhaps the easiest solution would be to reduce the width, or perhaps adding a removable brace or two and working with a reduced feed at times, I am not going to need 300mm of X when machining metal.
This is with about a kilo or so of force, maybe a bit more, the z axis can easily generate this by itself.
The problem with milling is that this flex means the tool can be 'sucked' into the workpiece with traditional milling or forced away in climb milling, leading to chatter, crap surface etc.

https://youtu.be/oYIbTNR8rJc
« Last Edit: August 24, 2017, 08:29:06 AM by Stonemull »
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Lob0426

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Re: Play in Z axis
« Reply #1 on: August 24, 2017, 12:15:31 PM »
Larger rods would help. But it does not look like you have enough metal there to move up to 12mm. Shorter would help. A removable brace sounds like a workable idea to me.

The Z rail on the sherline is pretty hefty, yet you can easily deflect it enough, by hand, to measure easily with a dial indicator. Everything deflects under a load. Its part of the game, getting the best you can with what you have.
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Stonemull

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Re: Play in Z axis
« Reply #2 on: August 24, 2017, 07:50:48 PM »

I agree, if the forces could be transferred to underneath the 3 rods instead or torquing them, it would be a lot more rigid in the direct vertical direction, still prone to movement in the Y plane though.

I thought I might try some 3D printed 'blocks' that wedge between the rear 2020 and the rods first. Or a pair of largers block that wraps around the rod and 2020 and I can slide it out of the way mostly.

I have an old 1970ís model 1000 sherline lathe btw, I used to have the Z axis for it too but it was stolen many years ago.
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Lob0426

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Re: Play in Z axis
« Reply #3 on: August 24, 2017, 08:32:15 PM »
My mill appears to use the weight of the motor/spindle to help dampen things.

Those older Sherline Lathes seem to hang in there. I see old ones still being used all the time online!

A couple of things that might help, if you have not already tried them, is to buy some extra linear bearings for your rods and test fit them. And of course double check that all fittings are tight. I bet you have already tried these!
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laser_cutter

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Re: Play in Z axis
« Reply #4 on: August 25, 2017, 06:38:49 AM »
Updated:

The bottom rod flexes. There may also be some slop in the bearing in the Z assembly.

My solution(s):

Place wheels or something on the back side of the Z assembly perpendicular to the X. These wheels would lightly glide along the gantry groove and absorb the flexing.
« Last Edit: August 25, 2017, 07:55:36 AM by laser_cutter »

laser_cutter

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Re: Play in Z axis
« Reply #5 on: August 25, 2017, 07:59:08 AM »
Or get something like this, the linear rail would attach the the gantry


Stonemull

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Re: Play in Z axis
« Reply #6 on: August 25, 2017, 09:38:56 AM »
Heh, I have had a couple of SBR12 kits in my shopping cart for a few weeks. I had thought of that option.
it needs a rebuild to the gantry for that. or a lot of glue. That means the Z axis needs replacing as well as the X axis.
Then if I was going to rebuild the gantry then I would make it run on 3 or 4 rails instead of 2 and place the spindle motor between them, I reckon it would be quite substantial then which means it needs a better spindle motor, so I might as well design it for a larger diameter motor :)
WIth an improved X and Z axis then I think the outside  rails could do with an upgrade and I guess the Y axis would also do well with an 12mm rails.
So yeh, that is the scenario mentioned in post 1 "preferably without rebuilding it from scratch" as it is now a DIY machine.

But .. that I think might be the next project, just make a solid unit from scratch.




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laser_cutter

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Re: Play in Z axis
« Reply #7 on: August 25, 2017, 12:05:22 PM »
Heh, I have had a couple of SBR12 kits in my shopping cart for a few weeks. I had thought of that option.
it needs a rebuild to the gantry for that. or a lot of glue. That means the Z axis needs replacing as well as the X axis.
Then if I was going to rebuild the gantry then I would make it run on 3 or 4 rails instead of 2 and place the spindle motor between them, I reckon it would be quite substantial then which means it needs a better spindle motor, so I might as well design it for a larger diameter motor :)
WIth an improved X and Z axis then I think the outside  rails could do with an upgrade and I guess the Y axis would also do well with an 12mm rails.
So yeh, that is the scenario mentioned in post 1 "preferably without rebuilding it from scratch" as it is now a DIY machine.

But .. that I think might be the next project, just make a solid unit from scratch.

I would say then place blocks behind z axis that would close the gap between the Z assembly and gantry