PS: do you happen to know the calculation for determining the necessary kernel frequency?  I have search high and low and found conflicting reports.

Thanks!

By Oldham style I assume you mean just a solid piece with a bore and set screws to
attach / lock to the shafts. That is fine and easy to do.

I can't tell what type support you have on the Z. Basically there are two types, fixed and support.
If it is just a support type then there will be axial play since the screw is NOT fixed at that end.
Do you have info on the support?

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Remove the motor and coupling. Put an indicator on the end of the screw above the support.
Push up on the Z carriage assembly and note the movement. Make sure you are not lifting the x /y  linear slide from the rail.
If it is  a fixed type and preloaded you would have
very little movement ( ie; 0.0001" to .0004")

For now,

RICH

RICH,

The Oldham style couplers have a center piece of hard plastic, I assume to allow it to give (or break away) if some catastrophe happens, but not as squirrely as the style I have on there now.  They are made up of 3 parts total.  Like  the following:

http://www.ruland.com/ps_couplings_oldham.asp?gclid=Cj0KCQjw1a3KBRCYARIsABNRnxtK8nD8gPazSOpJwEocEMnJ-va7I0dhWuq2bf0DxIQDMpQayfq9X8gaAv-9EALw_wcB#about.

I could make something like this fairly easily.  Just need to raw materials - don't have it all in stock.

The thought of this all being caused by mechanical play is a little odd to me (although I also want to be sure that the mechanical side is correct!) - since the discrepancy always seems so uniform and cumulative.  Also, the strange issues I started having on the machine were also on the Z-Axis.  The thing went a little crazy and began hesitating.  I thought there was binding - but confirmed by hand-turning there was not.  Then when locating to WCS the Z-Axis would travel to the Machine zero point and make a weird noise, then sorta stall there, but think it was at WCS zero.  I reinstalled the software - and although it did not fix the issue we are discussing, it does seem to have stopped this weird behavior....for now

The support on the Z-Axis is provided by a BK20 fixed bearing block at the top.  It has the bearings seated inside the block, and the play is taken out by the 4-sided nut with setscrew that threads onto the end of the ballscrew.  I have attached a photo below of the type of bearing block I used.  

Thanks!

Just make a solid simple coupling to test and  eliminate that part as a culprit.

You have a fixed support, BUT, still do a check for any movement per last reply. May have some play and will know how much it
is contributing if any.

RICH

Will do.  I'll check the play when I disassemble for installing the new/test coupler.

Should be able to bang something out quick f I am just making a rigid coupler.  I have some aluminum round bar here in stock.

Thanks again!

Well RICH, I can't thank you enough!  I suppose the only thing left is to get your address so I can deliver you some whisky/wine/beer/chocolate/centerfinders.... Pick your pleasure or poison. 

Made a quick direct/rigid coupler, slapped it on there and the thing is repeating PERFECTLY!  Good grief I thought the stress would crush me!  Wondering if you have a suggestion for a more permanent solution for a coupler.  You said you were using custom ones with just Belleville washers as safety relief?  Do you have any photos?  I feel it's probably best to have some type of give for those "oh sh*t...." moments that may occur if Mach3 goes sideways.

I'm going to check the play even though it is working well now.  Just curious to see how she's holding up.  Forgot to do it when I pout the coupler on there - too excited to see if it fit right and helped at all. 

Really appreciate all your time and patience. 

Scott

Photo attached of the quickie coupler that I made on the lathe.

Glad you got it fixed. The coupler you made is fine and two set screws are better than one.
I would suggest that you put / file  a small flat on the motor and screw shaft
as the set screws will mark the them making it hard to pull the coupler off.
Just use the new couplerand nothing else is required to be done.

You said you were using custom ones with just Belleville washers as safety relief?

No, mine are plain Janes just like yours. The washer comment pertains to the
motor ( if you ever take an old one apart you will see the washers to keep the stater
in place.

I feel it's probably best to have some type of give for those "oh sh*t...." moments that may occur
if Mach3 goes sideways.

Any force up is taken up by the support bearing so would not worry it.

so I can deliver you some whisky/wine/beer/chocolate/centerfinders....

Thanks for the offer, but we moderators are highly compensated.

RICH  

Glad to hear you are well compensated - as well you should be! . Actually, I helped out with a forum for a bit - a thankless job, so please know that I am indeed VERY thankful!

My only concern with this rigid coupler is of course any radial alignment issues that would not be compensated for. I know I shouldn't have it to begin with - but I did have some slight irregularities with the work done at the water-jetting place.

I may be able to tweak the motor alignment to ensure a perfect fit. Then I wouldn't really be too concerned. Especially since I realized that while pretty nifty, Oldham style couplers do not offer any axial support. I suppose that's only an issue when the motors are powered off.   I'll
Check what kind of play I have, as you suggested.

Any higher quality flex coupler (Bellows) is ridiculously priced. At that rate - cheaper to risk it and replace the damn motor if something wears

The motor shaft has a flat already - and I will try to file a small flat only the ballscrew end as well.

Now that I've got that sorted, time to get back to the list!

My only concern with this rigid coupler is of course any radial alignment issues that would not be compensated for.

Idealy the Z axis components should be parrallel to the rails and perpendicular to the X Y surface.

Just need to play some with the mounting of the z components.

Motor mounted first, install coupling with support loose, ball nut loose / not attached to plate.Now tighten support. Check how
perpendicular the screw is to the X y. Rotate the screw by hand using the motors shaft (attached old coupling
to the motor shaft to make turning the screw easier). Check runout of of screw at the end. Move end of screw half of runout and tighten the screw nut. Loosen support and re tighten. Attach motor. Should be darn close to being alligned.

You can  work top down or bottom up or even work out from the middle.

Writing this is like try to tell someone how to tie their shoes!

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How I do it for a short rather rigid axis alignnment:

I use  torque indicators and they are very accurate.

1.Preload the support bearing. Check torque with nut finger tighting of nut.
  Continue tightening to add preload and check torque. As preload increases the
  torque required to turn the bearing increases. If the motor is say 100 in/oz
  you don't want to use up 50% of you motor torque. You want to minimize the torque
  and also have min axial play. Axial play results in backlash.
  There is no such thing as no backlash.
  Total backlash is the sum of each components backlash.
  If you isolate a component you can measure it's backlash.
  Note both the torque setting and the backlash at that setting.

2. Measure torque required to turn the nut. Note it and measure any backlash.
 

2.The support bearing is installed / mounted  and rather closely alligned to other axes.

3.The nut is installed / mounted.
  You will find the torque ( nut + support) is very close when the nut is located at the
  end, middle, and close to the support if installed correctly. Note any increase in torque.
  The increase will be due to axial misalignment and whatever attached load is being moved
  due to friction and weight.

4.Install the end support when the nut is close to to the end support. If fixed type do step
 #1 for it,but, usualy it is not.
 
5.Check total torque when axis is near fixed support, middle, and at end of longest  travel
  near the other support. All torques should add up to individual torques.

6. Re calibrate axis after motor is attached.

Now nothing is ideal, for instance, the screw may be not be straight. So you need to tweak
each component along the axis travel.In the case of a vertical installation it may just right
but the axis travel is not parrallel to another axis.ETC!

In the end one will have minimised backlash and the torque to move the axis having a
a positive affect relative to velocity, acceleration, and repeatability and component wear.

There are other way to do things.......and surely I missed something......

FWIW 

RICH