Dial Test Indicator Flaw

[Zirok]

Hi

I am doing a project using dial test indicator.

As seen in Image 1, I have clamped the part in such a position that it is 45 degrees to X & Y axis.

Now as shown in Image 2, I go 20 mm offset from the zero point (Zero Point is shown in blue color) & move in Y axis until reading is shown on dial.

On similar grounds, I go to 20 mm offset from zero point in opposite direction (see image 3 ) & move in Y axis until reading is shown.

The machine movement in Y axis from Zero Point should be same in both the conditions as mentioned above. But there is variation of 0.453 mm in both the positions (Refer Image 4).

In short, the dial test indicator needs feeds of different values to create the same reading on dial.
We have ensured that the dial clamped is perfectly parallel to Y axis, so there is no issue of contact Point Error.
Can we conclude the Dial as A unbalanced Dial ?
I think this problem has never been noticed by anybody in history till date.
How can this happen & what should be the error & how to compensate that error. ??


It is definitely not a Co-Sine Error (Refer Image 5). It is clearly visible that the angle of lever arm is almost 90 degree to the part.

To know more, I had a broken Dial Test Indicator & then disassembled all the stuff to know actually how the dial mechanism works. ( Refer Image Internal Assembly )
As you can see In Image 6, how the mechanism works. It doesn’t seem to be a unbalanced dial. It is a simple mechanism & logically there should be no error.


Need help

[Brepins]

Not sure what your tring to do here find center?

If the x movement + or - the 20 mm doesn't start from the center it will read different.

If you are tring to find x center tram left and right points on the part.

[Zirok]

I used a electronic edge to pickup 4 points (as shown in attachment) & calculate the interestion point with help of autocad.

See, my trial is not to align workpiece. My trial is to know & check that test indicator should show accurate absolute measurement when the feed direction is from opposite sides, but not at right angles to the swivelling pivot.

My feed direction is 45 degrees offset to the swivelling pivot direction.

[wildwestpat]

Hi Zirok

As you have found out by pulling a DTI to bits there is nothing in them that can give rise to large errors. They do have errors but these are related to the precision with which the gearing is manufactured. It is good practice where ever practical to use the DTI as a method of comparing measurements rather than taking a measurement using the dial as a substitute for a micrometer or other precision device. Yes there are good quality DTIs with good linearity over their measurement travel but these are the direct plunger / rack type not the lever type where the mechanical advantage of the lever describes an arc. As you have indicated the error due to the approach arc is going to be small and not of the order of 0.4mm you are measuring.

How are you measuring the plus and minus 20 mm traverse of the work piece? Assuming you have the work mounted on a vertical mill I am guessing that you are moving the work by operating the lead screw to move the table. This would work only if you keep turning the lead screw in the same direction and have started the traverse further out than the first measurement point. If as I am suspecting you have reversed the direction of traverse you are now measuring with the backlash of the machines axis included in your measurement method and the backlash will be higher than any other source of error. (Backlash is the slop in the traverse mechanism such that one turn clockwise does not get taken out with a following turn back of one revolution.)

The 0.4mm error equates to a plus minus error on the traverse of some 0.1mm which would be a bit high but not unreasonable. The craft skill of a machinist is to allways work with backlash in mind and to approach all measurement and reference edges from the same direction. I also strongly recommend that you try and fix reference edges and zero all other dimensions to those edges.

You coud mount the DTI in the spindle (motor OFF) and check the backlash of the machine traverse and if I am right there will be an explanation for your apparent difference in the DTI readings.

Hope this helps - regards Pat

[Zirok]

Hi pAT

Thanks for your Reply.

Well the answer to all of your questions is I am Using a CNC Mill Machine. A CNC mill is designed to be consistent within 0.008 mm or the most 0.020 (when backlash considered).


I have taken trials considering all the counteractions to overcome backlash.

The problem is -

When a force (feed) applied from 45 degree left to the dial, the amount of table displacement to indicate reading on dial is different as that compared when the feed is applied from 45 degree right.

How can this happen.

[wildwestpat]

Hi Zirok

Good about the quality of the machine as this narrows the problem down a bit!

The lever type DTI as you will have noticed uses a pivot / lever to magnify the input before the main gearing. These instruments are designed for detecting small movements at the probe tip. They are not designed to detect any side ways motion but the application of a side thrust can alter the reading or wreck the instrument if excessive. The DTI should allways be at right angles to the direction of motion or very nearly so.

In your case the error you are seeing might be due to the side thrust on the pivot in the DTI that you are using. Think about lost motion due to the probe tip being moved sideways. Suggest you repeat the test using a plunger type DTI as these are a bit more robust. Withdraw the plunger whilst traversing and release to check the clearance. Some of the better quality plunger type DTIs have a mechanism to lift the plunger when access is a bit restricted.

Hope this helps Regards Pat

[Ashish B]

Best of Luck

[Zirok]

hI pAT,

Thanks for your Reply.

I completely agree that these instruments are designed to take perpendicular forces to the swivelling arm.

I also agree to the point that lost motion exists due to sideways movement of the probe.

But Pat, as a dial is balanced dial. Any forces on either side should have a common lost motion factor & so the same amount of displacement should provoke common readings on dial on either sides. But if that's not the case



&

If its my dream to make the trial on a Dial Test Indicator only, then i think need a correction factor.
Correction factor in a sense, i need to take trials by feeding in 45 degree from the left hand side untill zero on dial. Than accordingly shift the approach angle from the other side & offset untill the displacement difference to provoke equivalent readings on dial.

I know that's somewhat tricky, but I hope the correction factor behaviour is same for all types of dials.

[Brepins]

maybe it's me but pic 2 and 3 look like the indicator points aren't hitting the same points could be camara angle illussion. just in case double check part is still on 45 sometimes when you clamp it moves.. in this case clockwise.

[wildwestpat]

Hi Zirok

The reason for mentioning the lost motion was that unless the pivot is perfect the ball ended stylus has a degree of freedom that is at right angles to the correct sensing direction. This in the case of your 45 degree approach will lead to the ball end moving away from the work piece and riding down the 45 degree sloping surface resulting in an error.

Can you set up the lever DTI with the ball ended arm pressed against a surface of a surface plate or similar. Then measure the deflection at the ball ended lever at right angles to the correct sensing direction.

What I am suspecting is that the ball end moves sideways and thus records a different reading as it slides down the 45 degree face presented for measurement.

Also it might be interesting to check the repeatability using a ground work piece that is presented at right angles in both cases in place of the 45 degree angle of the present work piece.

Sorry to be so long winded but I do not trust lever type DTIs for anything other than checking for run out where total deflection is minimised as part of the adjustment routine for centering cylindrical work.

As an aside in CNC work it is normal to use a refference for each axis that is approached at right angles. If the reference edges are not part of the work piece then a tool makers button or similar gives the table an absolute refference from which to calculate true possitions for cutting.

If the DTI is absolved then the block has probably shifted 0.645 degrees clockwise and should be reset.

Regards Pat

[ImanCarrot]

Why don't you measure it approaching from the same direction, say X+ve- you could do this simply touching off one face, come up a bit in Z and placing a 3,2,1 block or a slip gauge against the face of the other side and touching off that? Does that make a difference?

[Zirok]

hI All,


I completely agree to the point that DTI is designed to make perpendicular measurements & not sideways. Also the play & friction is different on either sides.

This trial is to validate that does a DTI Provided consistent results when forces act from a non perpendicular position on either sides. If this trial suceeds, then we will be able to make absolute measurements also with the Dial Test Indicator & will be a landmark achievement.

A Idea -

I think we need to add some balancing weight on to the dial stylus ( not on the sphere shaped portion ) but on the portion as shown in figure 45. By compensating in this way, we would get consistent results as we are compensating for side thrust & play error. I know this will require stringent trials to create the equation of balancing weight v/s the side thrust error.
Any Suggestions of how to add weight


Thanks, for your superb multiplication of knowledge.