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Centre finding

Problems finding centre

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Cliff Tandy 224/04/2020 07:29:43
10 forum posts

Hi all I’m getting a little frustrated trying to find the centre of a piece of square stock. A few years ago I bought a secondhand amadeal mill with a 2 axis dro. I have not used it much but lately have needed to drill holes along the centreline of a piece of 6mm square bar. I got myself a cheap edge finder from eBay and am finding the first edge, moving to the second edge then entering. The results with the edge finder are very consistent, but the hole I drill is always off centre. I have ordered a new starrett edge finder but with the current situation am not sure how long it will take to arrive. Has anyone else ever encountered this issue?

Adrian 224/04/2020 07:42:50
96 forum posts
19 photos

Hello Cliff,

This sounds like it may be back lash in the lead screw . Not a problem in itself . Having established the edge position carry on winding the lead screw in the same direction to take you to centre . Don't change direction. All should be well .Hope this helps.

Adrian.

JasonB24/04/2020 07:42:53
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20880 forum posts
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Are you talking about centring a 6mm x 6mm end of the bar or just wanting to drill across it?

Either way with a DRO best option is to find first edge and zero that axis, then do the opposite edge and half the DRO's reading to get the ctr which will be OK for drilling the edge. If the end then simply repeat this for the other axis.

Make sure your vice is clocked true and that the DRO has no play in it's brackets.

Gary Wooding24/04/2020 07:50:06
848 forum posts
214 photos

A friend had the same problem, it turned out to be that the DRO mounting was loose.

not done it yet24/04/2020 08:41:27
6078 forum posts
20 photos

Perhaps the problem is actually backlash on one axis? - especially if one of the dro axes is the vertical.🙂

Martin Connelly24/04/2020 08:56:55
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1789 forum posts
190 photos

Check the dro is secure as suggested above then check bed movement with a dial indicator compared to the dro.

With a vertical surface fixed to the bed mount the indicator to read zero against the vertical surface and zero the dro. Move the bed away from the indicator then back to zero on the indicator the dro should also read zero or very close.

The difference between the dro reading and the indicator reading is the backlash between the dro and the bed movement. This should be very small for most dro setups but things like loose gibs or worn ways can cause some backlash in the reading. If it cannot be adjusted away then it needs taking into account.

Martin C

Clive Foster24/04/2020 09:03:14
2735 forum posts
100 photos

Are you locking the slides before drilling?

With an older, lightweight, mill there are many potential issues from wear, imperfect adjustment, not quite tight enough components or even, as its a Far East import, over wide tolerances leading to more play they you'd ideally like in certain areas. Factor in the almost ceartainly imperfect technique of a novice user and finding the source of the error is like walking through a minefield blindfold.

Locking the slides will at least ensure that the machine stays where it is put during the drilling operation which will help narrow things down a little.

Its worth setting up something larger and putting holes at the quarter position as well as the middle. If they are all out by the same amount in the same direction odds are its a technique error rather than machine. But then you need to sort out whether its purely you doing it wrong or whether your technique cannot cope with machine or set up errors. Sort of situation where you get an expert friend come in to help out and he promptly just does it right "Whats problem, mate." Which is irritating. Especially when he walks you through it and you get that D'oH moment.

#1 for Garys advice. Check the simple and obvious stuff. Whenever I've spent hours on a proper puzzler the cause has invariably been something so simple that it was not checked on the "It can't be that" principle supported by "I know I did it right".

Clive

Sakura24/04/2020 09:32:31
46 forum posts
1 photos

If you're using a wobbler, are you halving the diameter of the ball and zeroing the readout before entering width dimension?

Oven Man24/04/2020 09:59:33
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131 forum posts
18 photos
Posted by Sakura on 24/04/2020 09:32:31:

If you're using a wobbler, are you halving the diameter of the ball and zeroing the readout before entering width dimension?

If you are just looking for the centre the ball diameter cancels itself out and can be ignored. Or am I missing something?

Peter

magpie24/04/2020 10:08:12
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477 forum posts
92 photos

Cliff, you don't mention what size holes you are drilling. I have, in the past, found that small drills will 'wander' if not guided by a centre punched workpiece. Small standard length drills are the most prone to do this, so try the double ended short series drills if you don't want to centre punch.

Dek.

JasonB24/04/2020 10:17:40
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Posted by Oven Man on 24/04/2020 09:59:33:
Posted by Sakura on 24/04/2020 09:32:31:

If you're using a wobbler, are you halving the diameter of the ball and zeroing the readout before entering width dimension?

If you are just looking for the centre the ball diameter cancels itself out and can be ignored. Or am I missing something?

Peter

It does if touching both sides which is best with DRO, If using handwheels then only come from one side so you need to touch edge then move by (end dia + work width) / 2

Ian Johnson 124/04/2020 10:24:55
355 forum posts
101 photos

I had similar issue with my 'cheap edge finder' it has what I thought was a 4mm diameter stem but after drilling a few misplaced holes I measured it and it was actually 3.9mm diameter, so I could never get a accurately placed hole unless I compensated for the error. I soon bought a better quality edge finder with 4mm stem which is 4mm diameter on. Problem solved.

Ian

DC31k24/04/2020 11:41:47
506 forum posts
1 photos
Posted by Ian Johnson 1 on 24/04/2020 10:24:55:

I had similar issue with my 'cheap edge finder' it has what I thought was a 4mm diameter stem but after drilling a few misplaced holes I measured it and it was actually 3.9mm diameter

But you were doing a different thing and using a different technique, so how is it relevant to the OP's problem?

It is misleading effectively to tell someone to buy new kit, when the particular fault you describe in your own kit is not germane to his actual problem.

If you are finding the centre of a bar using the correct technique (i.e. picking up both sides and halving the reading), you do not even need to know the diameter of the edge finder.

not done it yet24/04/2020 12:09:21
6078 forum posts
20 photos

Looking at the recent amadeal machine threads and that the machine was already second hand, it is also possible that the machine tram is not as it should be, leading to holes (particularly if not spotted) going off line. Needs checking out as part of the diagnosis of this phenomenon. This could be exacerbated by use of ‘less than fully sharp’ drills, wobbly chuck or other ailments, too.

Edited By not done it yet on 24/04/2020 12:10:54

mgnbuk24/04/2020 13:04:24
1001 forum posts
69 photos

it is also possible that the machine tram is not as it should be

Column to table squareness could also be an issue - something that is rarely mentioned as a check. Frequently the obssesion with "tramming" to the exclusion of all else recommends shimming the column to correct a tram error, but doesn't recommend checking that the column is square to the table afterwards (or beforehand come to that).

Why does this matter ? If the edge finder is short & the head wound close to the part to set the datum, moving the head upwards to fit a longer drill with the column not square will move the spindle centreline away from the datum - holes will not be drilled in the expected place.

Nigel B.

not done it yet24/04/2020 13:48:59
6078 forum posts
20 photos

I was referring to tram on either axis, not just in one.

mgnbuk24/04/2020 14:04:24
1001 forum posts
69 photos

I was referring to tram on either axis, not just in one.

Spindle squareness (tram) & column squareness to the table are different checks - usual practice is to set the column square to the table in both planes (X-Z & Y-Z) using a precision square, then check the spindle alignment to the table with a trammel check. If the spindle alignment (tram) is out, then the adjustment would not be made altering the column to table squareness.

Nigel B.

Roger Vane24/04/2020 14:38:13
100 forum posts
18 photos

Hi Cliff

I had a similar problem a few years ago, and traced it to run-out of the (big name) drill chuck that was holding the edge finder. On testing the runout on the chuck I found it to be around 0.004" TIR, which was playing havoc with accurate centring, even with an up-market edge finder.

Certainly it's worth clocking the chuck to check TIR - you may be shocked.

Cliff Tandy 224/04/2020 21:11:10
10 forum posts

Hi all, thanks for the replies. I think my technique is fairly sound, and I always lock the x & y axis once centre is found, I also spot with a centre drill before drilling the holes. I have checked the scales for tightness and they are fine as are the gibs on the x & y axis. The y axis is good from side to side but when I checked forward to backward it is leaning forwards running out 5 thou over 3 inches. I will undo the.column tomorrow and shim with some old feeler gauges. Again thanks for the response and I will update when done.

Vic24/04/2020 22:32:03
2847 forum posts
7 photos

Centre drills aren’t for starting holes, you should use a spotting drill. A few of mine.

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