Need Help!- COAXIAL INDICATOR

[tjd10684]

OK i bought an admittedly cheep coax indicator from whole sale tool. i have noticed when i try to center parts even though it says im centered when i drill my hole it looks visibly off center. has anybody had this kind of problem with cheep coax indicators? is their any way to fix it? or did i waste my money?

[wildwestpat]

Suggest that you check that the dial gauge and reference surfaces are OK. Do this by holding the device in your hand and gently rotating the sensor mount. If the reference surface is clean and in good condition the dial gauge will show no deflection and the action will be silky smooth. The other thing to check is that the drill is not flexing and/or that the machine axis is not inclined. Also it is good practice to use the appropriate sensor arm with a co-axial centering gauge so that the extension matches the tool to be used.

These devices are common workshop tools.

Hope this helps - Regards - Pat

[tjd10684]

I will check the other things you mentioned but i tend to discount this statement.

Originally Posted by wildwestpat that the machine axis is not inclined.

only because i have tried using this to center a reamer on 2 lathes and tried to center a the xy on two mills and i seam to get consistently inconsistent readings.

on a side note am i correct in saying a shorter probe should give more accurate results.

[sld4121]

The shorter the arm, the more accurate the results yes. My "cheaper" version came with a conversion factor of sorts to assist with that. I must say though that it reads as well as my Blake co-ax.

[wildwestpat]

Hi

I don't see how you can get inconsistent readings from a device that is only measuring the deviation + & - from the true center. The device pointer flaps as the spindle is rotated and the error is reduced as the object becomes aligned with the center line of the indicator. I suspect the inconsistencies you are experiencing have their source elsewhere. Twist drils are weak and require the use of a center drill to start the hole correctly and to work up the sizes if a large hole is being drilled. There are short stuby drills that flex a bit less but the real problem exists with all twist drills is that the cutting lips may be unequall and this causes the drill to wander hence the need for an appropriately sized center drill to get things started.

I would always use the probe that operates at the same distance as the tol that is going to do the cutting as measured from the reference face of the co-axial device. The reference face is the one against which the 'Y' shaped probe holder runs.


There a number of sources that could lead to errors in the actual centering. I am going to assume vertical mill notation for X Y & Z

1. The device is duff. The holding part is not exactly at right angles to the reference surface. I think this would make it impossible to get a clean zero by adjusting X and Y but the device would still indicate center but with the pointer still flapping. I am assuming that you are rotating at very slow speed so that the pointer can keep up with the off centre wobble.

2. The Z axis may not be advancing at rightangles to the part / table. Check the tramming of the Z axis and correct any errors.

3. The tooling is flexing. Check with a DTI to see if there is any unwanted movement at the tool tip when applying firm finger pressure to the tool in various directions.

4. The longer the tool tip the greater the sensitivity of the indicator but the accuracy is virtualy independent of the probe length or off set - but longer = better ONLY if you are certain about points two and three above.

Suggest you go back to the mill and clamp a known god circular object to the table. A ball race with a clamping bolt through the bore would be good. Try and centre the ball race outer circumference using the X & Y using the shortest probe. the co-axial indicator should then give a very low order of movement (pointer flap). Having zeroed the indicator try leaning on the table to see if it is ridgid and the pointer flap is not increased. Repeat using the longest probe without touching the X & Y axis. This is a rough check of the tram of the mill provided the probe is running at the same point up the surface of the ground outer of the test ball race. There are of course unlimited variation on this sort of test al of which can be verified using a DTI

I doubt the co-axial indicator is at fault unless the speed of rotation is very high and the inbuilt DTI is unable to keep pace or the bearing surfaces are worn / sloppy which a visual inspection and running the plunger in and out would show by feel. I have used both very expensive ones and now own a cheap unbranded one. Yes the more expensive ones were better finished and the box a bit stronger.

A simialr test could be done on the lathe but the center of the tailstock is often offset with respect to the headstock. The center line of the head stock should be parallel to the bed in both planes and a dti on the sadle would indicate the errors. There is a lot on this site about how to correct both a mill and a lathe basic alignments.

Hope this helps and that you get to trust the co-axial device as it saves time over using a single DTI mounted on an adjustable arm.

Regards - Pat

[wildwestpat]

Hi Folks

Originally Posted by sld4121 The shorter the arm, the more accurate the results yes. My "cheaper" version came with a conversion factor of sorts to assist with that. I must say though that it reads as well as my Blake co-ax.

That table shows that the shorter arm gives less sensitivity i.e. less deflection as seen per divisionon the inbuilt indicator dial. The longer the probe the greater the deflection of the pointer but the accuracy is for all intents and purposes independent of probe length as the device is being set for minimum flap of the pointer which is why it is best to use the longest one that is convenient for the job.

If you want to measure the concentricity then use a DTI corrected for approach angle - a very tedious process and best done in the tool room. The co-axial indicators are intended for quick and easy centring on the machine with acuracy and speed. These instruments are robust and can take a lot of punishment. Hope this clarifys the use of the table.

Regards - Pat

[underthetire]

I bought 2 cheap ones for our service department. One worked as well as the Blakes, one wouldn't read even close. My blake has never let me down. We use them to dial in boring bar/turrets on lathes only though.

[christinandavid]

Originally Posted by tjd10684 i have noticed when i try to center parts even though it says im centered when i drill my hole it looks visibly off center.

Don't trust your eyeballs if they are out of calibration...

DP

[packrat]

[QUOTE=tjd10684;932748] i have noticed when i try to center parts even though it says im centered when i drill my hole it looks visibly off center. QUOTE]

Maybe it's because I'm OLD. Or maybe it's just a locality thing (where I learned the trade). But no one yet is making mention of this is not the way to do a job like this .

To put a hole in the center of a part you;
1. Make sure your vise, or the part (if bolted to table) is indicated parallel to axis movement.
2. You clamp part in vise and use a 'edge finder' to find edge of part. You always work from the solid/unmoving jaw of vise. Then move to center of part, found by dividing width of part by 2. Or going to dimension given on print.

Again maybe it's a old school thing, but don't we as a community owe it to all to teach correct methods?

[wildwestpat]

Hi Packrat

I agree that some basic help should be offered to those who are less well informed and have not been shown how by a master. However these co-axial centre finders are only of any use for centering holes or round objects - either holes or bosses. Hole centering is difficult on circular parts and this is where the co-axial indicators have a role to play as centering a pre machined hole or boss after removal using just a DTI is a pain as well as requiring persistence and skill.

Here is a link to a YouTube video that explains how these tools are used.

YouTube - Dial co-axial Centering Alignment Indicator

Hope this helps TJD.. decide if the indicator is duff or useable - Regards - Pat

PS. If the task is to center a square or squarish object than edge finders of one type or another are the instruments of choice and the co-axial indicator would suffer damage if used on surfaces with corners or rough surfaces.

[tjd10684]

Originally Posted by christinandavid Don't trust your eyeballs if they are out of calibration... DP

LOL i did recheck my eyeball calibration using pack rat's method

Originally Posted by packrat Maybe it's because I'm OLD. Or maybe it's just a locality thing (where I learned the trade). But no one yet is making mention of this is not the way to do a job like this . To put a hole in the center of a part you; 1. Make sure your vise, or the part (if bolted to table) is indicated parallel to axis movement. 2. You clamp part in vise and use a 'edge finder' to find edge of part. You always work from the solid/unmoving jaw of vise. Then move to center of part, found by dividing width of part by 2. Or going to dimension given on print. Again maybe it's a old school thing, but don't we as a community owe it to all to teach correct methods?

i appreciate the help and before this wonky indicator, the method you described is how i was finding the center of round stock on the mill. i got this tool to maybe make setup a little quicker and to make checking the tail stock alignment on the lathes easier but it looks like the old way is the best way for now. but im not giving up their were a few variables i may not have been controlling as well as i should such as surface finish and probe tip alignment. but if anybody else has anything they would like to pass along im ill ears.

[christinandavid]

I use a Verdict T8 clock for mostly everything, including centre/edge finding. It is on a jointed shank so you can use it directly in the spindle and touch on with relative ease (you only need to mount it on an arm if you want to clock a bore/boss larger than about 3.5", or to tram the head on the mill etc).

It can be used as an edge finder once it has been set against a known size, say a ring gauge, but what I tend to do is clock both sides of a part with a known dimension (mic it) - I set the mc readout and clock to zero on the first side then once I have set the opposite side the same the readout amount minus the part size gives me the diameter the clock is currently set at.

I would also suggest that touching on in four positions (N,S,E and W) by moving the head up/down is a more fail-safe way than sweeping the indicator around the bore/boss, which is something you may not be able to do with your indicator (don't know for sure 'cause I've never used one).

And, as mentioned previously, there are measures that should be taken to ensure your subsequent machining is sufficiently accurate position-wise.

DP