Please recommend a micro adjustable DTI holder

[awemawson]

Recent alignment work on the lathe and it's turret has revealed a hole in my measuring equipment - a micro adjustable DTI holder.

I have a couple of the Noga style articulated arm holders that are very good so long as you lightly lubricate the internal locking wedges, but these only get you 'into the ball park'. What I'm after is a similar holder but with a micro-adjustment facility.

Now in the past I've had the style (picture below) where a clip / spring clamps two plates together that are riveted onto the ends of bars, but all that I've had were of nasty construction - the riveting works loose, or the clip flies across the workshop at the least opportune moment.

So educate me - what is out there that combines the benefits of  the articulated arm style, and also has a well engineered micro-adjustment that doesn't fall apart in normal use?

[chipenter]

Stefan Gotteswinter made an adjustable indicator stand with micro adjustment

its in two parts .

[PekkaNF]

Noga indicator stands are economical, but not that great.

It all depends what you do, greater public is not aware how flexible indicator stands are until you need to measure machine and rotate the indicator. Few pretty good demostrations on web:

Richard King demonstrating indicator sag.

Stefan Gotteswinter - Machineshop Physics: Indicator sag - Durchhang von Messstativen

Andrew is no doubt familiar with this, hence the question.

Do you have a shop that has Fisso stands and you can try them? Personally I would not try anyting lower quality than these if I need anything better than bog standard stand:
https://pmtshop.de/messzeuge/messstative/fisso-messstative/fisso-base-line/2936/praezisions-magnet-messstativ-fisso-lxs25-base-line-hoehe-277-mm


An my answer is to keep all arms as short as possible and avoid fine adjustment alltogether, if there is a way. Sometimes adjustement is good, I have used two ok andother was russina with flexture and separate push/pull screws - you loosen one screw, then adjust with one and finaly lock with the another one. One is fisso flexture type, and this one is not thet great if you need to rotate the stand but works really nice if you need exact fine adjustment, it's sold as a spare of Strato µ-Line fine adjustment:


I think I have seen these under 100€, still not cheap, but you shoud try how it works, whole better than standard fisso/mahr and different league than noga.

https://pmtshop.de/messzeuge/messstative/fisso-messstative/fisso-zubehoer/385/fisso-praezisions-feineinstellung-strato-line-8mm/m8


In priciple a stiff flexture on the stand (instead of of at the end of the arm) should be fine with stubby arms, but I don't have experience on those.


Do you have something spesific measurment problem in mind?

Pekka

[awemawson]

Pekka,

Yes Richard did that demonstration when we had the scraping class here the other year. The flexing and sag is not really surprising when you consider the length of the arm and the fact that the DTI will show a few microns movement.

What triggered my interest recently is that I am repeating the measurements and adjustments that I made on the Beaver Turret for alignment. I am getting a discrepancy between sweeping the bore of an axial holder to set centre, and the mounting face of a radial lathe tool holder that implies that the turret rotation relative to the Curvic Coupling isn't spot on. Now it is possible, but I think very unlikely, that it has moved since I did the alignment. So I want to set up a very rigid measurement system. The real issue is that to keep the DTI mounting arms short and rigid the turret has to be right up against the chuck making access neigh on impossible to read the DTI. To over come this I intend to use my electronic Tessa gauge so that the indication can be remote, but I need to improve it's mounting. Hence the question!

[PekkaNF]

I don't get that exactly, if it is like I think there are two similar type measurement problems:

Bore and arbour on the same line---this is often soleved by installing close fitting bushing (pin) to bore and maching diameter pin or arbour, then arbour and bore (pin) measurements are clocked with DTI. DTI does not rotate, arbour usually can be rotated with relative ease.

Another measurement problem I am aware is alingment of the two shafts (gear coupling assembly ot such) that used to be abiach, before laset aligment units came common. Measurement guy had to lug along heavy suiticase of measurement fixtures. Smallest one had a prism that rode atop of the shaft, tightened with a strap (or chain) and had pipes to mount DTI, something like this:

That one looks the part, excluding the "bolt" that mounts the DTI. We used 20 mm hydraulic pipe, sag was neglible in 150 mm extension....tested that on proofing bar, because one mechanician did not believe on sag with his easier magnetic stand

I am pretty sure you have used or seen one used. Am I right?

if unknown/flexible mount was used the trick was to rotate the assembly and use another DTI to measure "sag" - it is the arm/mount that sags and not the DTI....but this takes some experience and an analythical mind set.

[awemawson]

Pekka,

The bore of the tool pockets is brought into alignment by adjusting the relative positions of the tool disk and it's locating  half of the Curvic Coupling - they are bolted together but have a largish rotational tolerance to allow the adjustment. This rotation sets the correct 'Y' axis (height) alignment (there is no Y on this lathe but that's the direction we are correcting). Having set this rotation and bolted the Curvic up tightly, X will now be out of kilter but then it is a matter of moving X until the tool pocket is in alignment in X, then telling the controller the 'grid shift' on the X axis for ongoing use.

Previously I had set the tool disk rotational position just by tweaking it until a radial lathe tool holder had it's tool mounting surface aligned to X axis movement, which 'should' get the rotational position correct, but it's actually more important to get a tool pocket adjusted to be exactly on centre line.

So the plan is to swing a DTI mounted in the chuck round the inside of an axial VDI 40 tool pocket adjusting the tool disk until I get identical (but opposite) readings on top and bottom of the pocket, lock the disk up, then move the X axis using the controller until the left and right reading are also equal and opposite. Then I can set the X 'grid shift' parameter, which will be half the error value in microns now displayed on the control having moved it from where it thought X=0 was !.

[PekkaNF]

OK. Starting to "get" it...I thought that it would be possible to put identical diameter pins to WDI mounting pocket and chuck - and to compare their position difference with a DTI moving in Z-direction (rotation axis). Easy on manual lathe....but this has bed slanted and not that easy to use sling mount for DTI

A challenge.

[awemawson]

Pekka,

I already started down that track : The other day I turned a precise 10 mm spigot in the lathe leaving it in the chuck and mounted a 10 mm dowel pin in a 10 mm VDI 40 end mill holder. I bored and reamed a 10 mm hole in a bit of hexagonal brass that slid on the pin, and if in alignment should slide from the dowel to the spigot.

This was to check alignment when I suspected that it was 'out', and it is !

[PekkaNF]

If you fit similar pin into chuck, there is no way to mount DTI to compare this chukked pin H/V positions to VDI mounted pin position?

Pekka

[awemawson]

Sadly not Pekka. Also I don't have a collet Chuck on this lathe, and the current hard jaws are showing 8 thou run out, so it's a case of either turning a spigot as I did before and leaving it undisturbed or buying some soft jaws and boring them to suit.

Unlike a manual lathe you don't have exposed ways to slide a DTI holder along as all is covered by robust Swarf guards, which is why I made the sliding sleeve.

Once I've found time to sort out a rigid Chuck mounting for the DTI it should all work out ok I think

[PekkaNF]

I though that slant bed would be a problem, guards off box way model would have been somewhat doable, but ball/roller ways needs pins and stuff.

Bringing the DTI into spindle brings spindle bearings into play....You got 100mm round steel or aluminium and turn/mill one hell of the rigid holder to be chucked into the chuck and dovetail or spigot ont that face. Special, but rigid.