Finding centre of a hole by wiggler

I have watched these instructions several times and I am still puzzled at what appears an error in the method. I am sure everyone knows what I am talking about, but I will recap the instructions:

1) Find edge of the hole at a point A using an edge finder (wiggler was used with a 0.250" ball). Set your Y axis to zero. 2) Move to point B which is diametrically opposite to A on the Y axis. Use wiggler to find edge at point B. 3) Determine the distance from A to B by counting the turns of the handle and add whatever figure corresponds to the partial turn on the dial. 4) Divide this figure by 2. 5) Move to AB/2 on the Y axis. Lock Y axis. 6) Repeat the procedure on X-axis. The point arrived at is the centre of the hole.

My problem with this method is that it ignores backlash. If backlash is B and the AB distance is the true diameter of the hole then the actual dial reading is going to be B+AB. On reversing to reach the half-way point on the Y-axis the backlash has to be taken up again (I assume the same amount, B) but now we are moving the table by a distance of [(B+AB)/2]-B. Thus the actual distance the table travels is going to be (AB-B)/2, i.e the centre is going to be off by half the value of the backlash.

I would be happy if anyone pointed the fault in my reasoning or outlined the correct procedure. I know there is indicators but that is cheating :-)

Thanks,

Reply to
Michael Koblic
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I think the instructions simply require YOU to take the backlash into account. Before I got a DRO for my old mill, I had to keep a notepad beside the mill all the time, recording the numbers for going left and right the included calcs for the backlash. I thought that everyone did that before the days of ball screws and electronic "linear scale" counters.

Pete Stanaitis

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Michael Koblic wrote:

Reply to
spaco

This was a Part 2 of an instructional DVD series. In Part 3 they sort of mention that the backlash "cancels out" using this method. Not according to my calculations. Even if one goes past the mid-point and returns to it from the same direction there is still going to be half a backlash error (in this case half a backlash too far).

Theoretically one can ignore the backlash if one a) knows the diameter of the hole accurately and b) performs the measurment *exactly* on that diameter. The benefit of this method is supposed to be that one does not have to measure exactly on the diameter as the mid-point of the measurment will always lie on the X-axis diameter. I suppose one can compensate for known backlash but can one really measure backlash accurately? Does it change over time?

Reply to
Michael Koblic

if you're working to this accuracy you're over due for DROs. You'll find its like turning the light on in the room after working in the dark for years.

But, it sounds like you have valid points on the backlash issue.

I've only used a wiggler for really accurate placement after first getting within a few thou by other means. This would lessen backlash concerns.

Karl

Reply to
Karl Townsend

When you approach the sides from within the backlash affects both edges symmetrically, as though the hole was simply larger. Consider the limiting case where the edge finder fits snugly in the hole. If you 'find' opposing edges the dial readings will be + and - half the backlash from the true center.

This assumes the backlash is constant. It may not be for a large hole on a worn milling machine. This old brain needs another cup of coffee after visualising all that.

I use the pointed wiggler and a magnifier to get close, then an indicator if the job justifies it, which is rare. I can usually get close enough quickly by centering a 6" ruler* over the hole and moving the wiggler point to the 3" line. The lines are 0.003" wide and I can just about center on them.

Jim Wilkins

  • Some call them scales, but to me a scale measures weight.
Reply to
Jim Wilkins

This is why real milling machines (and not a mill drill) have dials on BOTH ends of the X axis table. You set one to zero going in one direction and the other to zero going the other direction. And yes this assumes that the screw is worn evenly and the backlash is constant.

And as been said the answer to the original question is it DOES cancel out.

And as also been said, get a DRO and forget about worn screws and backlass forever "und jus vatch das blinken glowie lamphenfiber".

--.- Dave

Reply to
Dave August

DRO certainly works very well in this regard, indeed.

i
Reply to
Ignoramus15648

Ah, but this DVD series relates *specifically* to the mini-mill.

I spent a long afternoon today trying to prove/disprove my assertion: Being an engineering mutt I did everything at least 10 times. In the end, using a dial indicator, I was able to show (conclusively I think) that I was correct. The centre was consistently out by about 0.010".

Interestingly I spent a lot of time yesterday on the 'net trying to find out if anyone else has commented on the same issue. Not a thing!

Some interesting findings as a by-product of the trial:

1) My edge finder (the cylindrical one) seems more accurate than the ball wiggler (the 0.250" one) - by about 0.005". 2) Cheap digital calipers from Canadian Tire are surprisingly accurate - I got the same inside dimension using them, a dial indicator and a telescopic gauge with micrometer (also cheap).

It seems that lacking a DRO, more dial indicators, preferrably ones with travel longer than 1" would be a good thing.

Reply to
Michael Koblic

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