simple DRO scale mounting with ball joint linkage

[ironhoarder]

I'm about to mount scales on my Bridgeport clone. I've been contemplating various mounting approaches that will let me get the scales perfectly aligned so there is no binding as the table/saddle/knee moves.

However, I came across this:

http://bbssystem.com/viewtopic.php?t=1552

It seems like a good approach. If you can get miniature ball joints with no slop, then this rig would not require that the scale be perfectly aligned. Close would be good enough because the ball joint linkage would prevent destructive forces from being realized at the scale head.

I've found suitable ball joint components at McMaster for <$8, and some very cheap ones on ebay that are quite small (4-40 thread).

Assuming there is no slop, can you see any problems with this approach?

[psomero]

looks like overkill to me.

while it's not a trivial endeavor, it's easy to align a scale using a test indicator. just do it the normal way...

[Paul Alciatore]

.....
Assuming there is no slop, can you see any problems with this approach?

The possible problem I see is in this very question, "Assuming there is no slop...".

Trigometric errors will be quite small if the length of the ball joint assembly (ball to ball) is relatively long compared to any side motion that will occur. This ratio is the key element here.

But can you guarantee no slop between the two balls and any other mounting that is done? And will this stay that way over a long term?

I considered this same problem some years ago and came up with this:



It is simply two pieces of music wire clamped to a couple of mounts. One mount would be fastened to the mill table and the other to the back of the DRO's slide. About 2 - 4 inches between the mounts should allow enough sideways motion to prevent any binding or twisting of the DRO's slide.

Advantages:
It can be made fairly thin so it will fit behind the slide.
It is highly resistant to dirt or chips.
Music wire is dirt cheap.
Using two pieces of wire as shown should keep the two mount parallel so there is no twisting.
No precision parts needed.

You may need to experiment with the gauge of music wire, but like I said, it is dirt cheap.

[gnm109]

With 2 and 3 axis DRO systems selling down around $500 delivered nowadays, I don't think it's worthwhile to use the scales. Modern DRO's have large displays that are easy to read, extra features such as tool offset storage, circle programs, radius vs diameter resolution correction for lathes on the cross slide and many other features.

However, if you do use scales instead of a DRO, you might as well use ball joints.

[Circlip]

For no slop ball joints, look at the heavy duty units used in Radio control linkages for toy aircraft and boats etc. The U/J's used to couple the propshafts on toy boats also may be of use in lighter drive applications.

Regards Ian.

[Forrest Addy]

It aint that big a deal to dial in the scale references to 0.001 or so. Run the slider from a solid but tweakable connection to the moving part. That way you need no compliant mount. If uncertainty still goads you use a membrane attachment to accomodate residual misalignment.

Heim joints are great for in articulatd linkages lots but they are not in any way free of lost motion and their axial compliance is hard to compensate for - if there's means to do so in that budget DRO unit.

The installation in the link is one I would caution against emulating but I have to give the fellow who posted it points for workmanship and a valiant try. I'd be interested in results of the reverse zero offset test.

[Carld]

ironhoarder, if your using a standard DRO the reader head has to be mounted firmly in line with the scale. If your going to use a digital caliper for a DRO that may work BUT, when you reverse direction any slop in the ball joints will falsify the reading in the opposite direction as Forrest said.

If your gibs are adjusted correctly you will have no need in a flexible link.

[ironhoarder]

The scales I bought are branded iGaging and I'll be connecting them to a Shumatech display. I am outfitting X,Y,Knee,Quill for $129 delivered, plus the cost of the Shuma display head (don't recall- something like $125).

Slop in the ball joints is the question. I may buy a set just to see. I see small cheap ones on ebay, and maybe they are intended for remote control linkages. Don't know how precise these would be.

Forrest- when you mention a membrane- are suggesting a flexible strap that is rigid in one axis? Do you have a material in mind? Or maybe a simple shock mount or even a grommet mount would make for a compliant link.

Regarding the idea of the music wire linkage- I think I could use a single section of large gauge wound wire. I play pedal steel guitar and have every gauge of string. I could use a section of the largest gauge I have, which is .068". From playing with a piece of it, I can see using a 2-3" section of it and not having any buckling when in compression.

I agree that it will not be too hard to shim and jiggle the scale mounts and indicate them in. But as Forrest says, the uncertainty goads me

[wierdscience]

Not sure which scales you got from iGaging,but I bought a 12" model for one axis on my bench mill and I would suggest clamping them down and comparing the reading against a DTI before mounting.

The 12" set I bought has a .002-.003" intermittent error every .100" of travel.Good enough for wood working,not good at all on a mill.

[ironhoarder]

Not sure which scales you got from iGaging,but I bought a 12" model for one axis on my bench mill and I would suggest clamping them down and comparing the reading against a DTI before mounting.

The 12" set I bought has a .002-.003" intermittent error every .100" of travel.Good enough for wood working,not good at all on a mill.

This is the first bad thing I have heard about them, and I have heard many positive reports. Generally they seem to be regarded as a step up from the garden variety Chinese scales (all though these are either chinese or taiwanese, I'm sure).

I will do some tests as you suggest before I go drilling holes in my mill for mounting.