Edge Finding Mystery

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carolinamark

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After saving up for a long time, I might myself a Bridgeport clone with a DRO. I've run across something weird going on when I use an edge finder or wiggler.

I have trammed the head of the mill to the vise and it is within .001" in both directions.

I have checked the DRO against the dials and it is working as it should in X and Y axis. i.e. 4.000" dialed off with the dials shows as 4.000" on the DRO.

I am using a 3.000" gage block to perform these checks.

I have used 3 different edge finders(2 with .200 end and 1 with .500 end from Fisher Machine and Brown and Shrap). When I picked up both edges of the 3.000 gage block in the y-axis the DRO shows 3.007 to 3.008 plus the diameter of the edge finder). This happened with all 3 edge finders. When I flip the block and check it in the x-axis it checks within .001" of the nominal 3.000" plus the diameter of the edge finder.

Now for the mystery. When I use a wiggler(chinese import) I get the exactly opposite results......meaning dead nuts in the Y-axis and .007" to .008" off in the x-axis.

The mill is brand new and the DRO was professionally installed. I've checked the scales etc. for loose hardware etc. and found nothing.

Anyone have any ideas?

Any help would be appreciated. Thank you.
 
Hello Mark,

Two things, have you accounted for the backlash in the screws and the dial readings should show half the probe diameter.
 
When I installed the DRO on my Bridgeport there was a calibration routine to do to tweak things after installation. Does your manual cover this? If not, this might be one thing to look into.

I can't explain why the wiggler would be different. I guess if I used four different devices and one gave me different results than the other three I would consider the fact that maybe there was an issue with that one device.

Good luck,
Ted
 
Charles Lamont said:
When you use the chinese wiggler on the x-axis, is the error plus or minus? (You didn't say.)
Have you checked the adjustment of the gibs?
Click to expand...
The error is plus. I.e. the Gage block measures 3.008. The Gibs seem to be properly adjusted.

Thanks
 
Try an electronic edge finder if you haven't already. If the wigglers that you have are not concentric in the collet that may also add error.
 
Try this; Set up a mag base DTI on the column. Zero it on the BACK of the fixed jaw. Zero the DRO. Now crank out the y axis 3+ inches and insert the 3" Gage block between the jaw where you zeroed the DTI then crank in the table till the DTI zeros out again. What does the DRO read.

lg
no neat sig line
 
larryg said:
Try this; Set up a mag base DTI on the column. Zero it on the BACK of the fixed jaw. Zero the DRO. Now crank out the y axis 3+ inches and insert the 3" Gage block between the jaw where you zeroed the DTI then crank in the table till the DTI zeros out again. What does the DRO read.

lg
no neat sig line
Click to expand...
Would this not be the same as checking the dro against the dials?
Thank you
 
So there is no backlash, The gibs are nicely set and the DRO's compare to the dials. Then the only remaining variables are the wobbler and the column. As you say three different wobblers being faulty would tend to eliminate those as the problem. That leaves the column. Is it vertical in both planes. Is there any runout on the spindle.
 
There could be discrepancy issue if your test block is not square to table axis. For example if you are edge finding in X direction & contact on left side of block, then come round to right side of block, you can expect variation if the block is angled looking down on the assembly, even with the same Y position. Error would be exaggerated if you indicated on opposite corners (example lower left side & upper right side). Or your vise may be influencing this if its holding the test bock & it is out. Bolt the test block square to the table to eliminate that potential variable.

Smaller error source but something to check. If the the column has nod or cant and you happen to be setting the quill ( including edge finder) at different depth, that might account for differences between tests. As long as you use the same quill settings for any one test, this wouldn't be an issue though.

Lastly lock your quill so there is no slop or float holding the indicator although the rotating kind should negate this. I prefer to hold the indicator in an R8 collet. I use a USA made 0.375 shank LED 'light on' edge finder & it repeats very accurately aginst DRO.
 
When using an edge finder - check that the object you are checking is not magnetic - nor has the edge finder become magnetised.

I once had all manner of problems with my edge finder because it had become slightly magnetised.
 
carolinamark said:
Would this not be the same as checking the dro against the dials?
Thank you
Click to expand...

No it checks the actual measured movement as measured by the gauge block against what the DRO says. Ideally this test would show you moved 3" according to the DRO. It proves your DRO. If you use your dials then you MAY measure the error in the leadscrews. You must have a known good devise, gauge block, to confirm the test. You can also do the same thing to prove tour leadscrew/dial accuracy.

lg
no neat sig line
 
Last edited:
When do a check with your edge finder and find it off loosen the collet don't take the edge finder out just give it a turn of 90 degrees and re tighten collet is it the same? Could be some nasty run out in your collet or spindle.
 
doc1955 said:
When do a check with your edge finder and find it off loosen the collet don't take the edge finder out just give it a turn of 90 degrees and re tighten collet is it the same? Could be some nasty run out in your collet or spindle.
Click to expand...
An edge finder should be insensitive to radial runout. Angular runout is a different story. The latter could make the edge finder appear fatter, but it would apply in both X and Y directions, and the angular error would have to be huge to get the kind of error numbers you show. How about a video showing the issue?
 
You are correct don't know what I was thinking. I have those moments every now and again. LOL
 
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