Locating the centre of a hole tool desired

[picclock]

I often do small jobs that require either the holes in the casting to be enlarged or other machining work needs to be carried out using the centre of the holes as reference points. Tuypical sizes for me are from 25 to 150mm. Currently I have found the best way is to machine a piece of Aluminium stock to an exact fit in the hole, centre drill it, then use my mill centreing tool (Starret - the pointy end) to get the correct position to clamp it onto the rotary table.

Thing is I seem to be getting a large collection of randomly sized bits of aluminium and I feel there ought to be a better way. 

Does anyone know of a tool which will do this with fair accuracy ?

Just off to turn up another one .. .

Many Thx in advance

Picclock

(If it exists I may be able to pick one up at the show on ay Ally Pally this weekend)

[Bluechip]

Picclock
 
I've centred 4" , dunno about 6" ... on a WM16. I think the supplied wands would do it though.
 
Ask them ???
 
http://www.machine-dro.co.uk/dial-co-axial-centering-alignment-indicator-metric.html
 
Dave
 
EDIT Bogger me! Just seen  it.. out of stock      ARC do them IIRC, not looked ..

[DMIOM]

Standard kit, if you've got the headroom is a Blake co-axial indicator (lower cost clones available from Allendale etc.).   Mr Todd of this forum is evolving lower-profile designs of co-axial indicator. 

Another option is visual - either a centreing-microscope or a miniature camera. With those, if there's a marking then you can pick it up; otherwise if its a bigger hole, traverse to one side, note reading on DRO or dials, traverse to other side and halve the reading to get the centre co-ord.

Dave

edit - cross-posted same time as t'other Dave !

[picclock]

OK I have an attachment that fits on the mill spindle with a gauge, so you can rotate it and find out how much off centre you are, but this is almost impossible to do properly as you have to take a reading, rotate the spindle then look at the difference and try to move the work in such a way as to minimise this. At the moment I plug the hole and use the starrett centre finder. Immediately I can see the magnitude and direction of any offset. So then I can tap the work which is loosely clamped until I get good alignment.

The edge finder gives an immediate visual offset of magnitude and direction which allows easy position correction, without any spindle rotation or dial reading - so its all good except for turning plugs to the correct size. 

Best Regards

picclock

[AdeV]

How about something like a very small 3-jaw chuck? Put the chuck "jaws" down in the hole to be centred, wind the "jaws" out until they are a snug fit in the hole (doesn't need to be a grip fit, just an accurate sliding fit); the back of the chuck would be accurately centre drilled to give you your hole centre.

The hardest part, I guess, of building such a device would be cutting the spiral track for the "jaws" to run in; so it might be better to buy the smallest "real" 3-jaw you can buy, adapting the jaws to suit this task & adding a back plate which is centre drilled.

[andyf]

Hi Picclock,

To get my round column mill back into registration over a small hole after altering the head height to accommodate a tool change I forgot to allow for, I made up this affair:



The conical end is retained by an internal spring, and slides around like the end of an edge finder. It is lowered into the hole, and X and Y are adjusted until I can't feel the join anywhere round the circumference. Fingertips are sensitive, and this gets me centred on the hole again within a thou or two.

If you don't need absolute accuracy, maybe you could fabricate versions with fat business ends on thin shanks, and truncated cones of varying major diameters to suit your 25 - 150mm holes. Mine is silver steel/drill rod and the cone is hardened, but silver steel in your size range would be expensive, even if you could find it.

Andy

[picclock]

@AndyF that's the same as the starret edge finder, but the edge finder only goes up to 8mm or so. Its a great way of getting the centre of smaller holes. I use this in conjunction with an aluminium disc that has been turned to fit the hole and centre drilled. I'm trying to do it without turning the disc.

@ Adev I have a 'real' 3" Lathe chuck that I have experimented with but its just not good enough. It was never very good as a chuck anyway which I why I use 4 jaw's now with a dti. The chuck centre varies substantially depending on the size of hole. I don't know if a better make would do the job either as you are working off of the wrong side of the spiral. The jaws are being forced inward rather than outward as they would be in normal use.

I keep thinking that it ought to be easy making something like this but I'm out of ideas, and I know you forum members are way smarter than I will ever be - hence the post

Many thanks for your thoughts

Best Regards

picclock

[andyf]

I don't want to labour the point, Picclock, but I was thinking that making two along the lines of this sketch would cover your range of hole sizes.



Andy

[picclock]

@ AndyF
Thats an excellent way to do it - it makes me feel stupid for not thinking of it before. Its much simpler that my other thoughts, and it will easily lend itself to having sections of the cone removed leaving either 3 or 4 flutes, which is sometimes useful for me. I had thought of something along the lines of the mechanism used in adjustable reamers, but I think simpler is likely to be better - especially in my case. I may need more sizes but I think the idea is very sound, especially if I make the shaft(s) out of a single piece of Silver steel, fixing the cone blanks then turning them, before parting off the cone part so that its all kept concentric.  The only slight downside I can see is that it references the edge of the hole which is often damages or chamfered. So I might limit the cone angle to 30 degrees to get round the worst of that.

I'll give it a go - its probably easier to make one of these than it is to get an exact fitting plug - or at least not much harder.

Many thanks

picclock


 

[andyf]

I hope it works!

Now I look at it again, my suggestion might need the cone jiggling a bit with your fingers as it is poked into the hole, to make sure it is sliding on the shank rather than tipping over and opeing up the join on one side. Once it's properly engaged with the rim of the hole, it should be OK, allowing X and Y movements to line up the join.

Andy

[picclock]

I received a PM from Mike Fox (drmico60) which outlines a potentially excellent method for hole centering.

I have an idea that I have been meaning to develop myself that could be useful for locating the centre of holes under the mill spindle. I thought I would bounce it of of you.

Imagine a small laser pointer mounted at a slight angle that is fixed to say an MT2 taper that fits in the mill spindle. With the mill rotating slowly the light beam will describe a circle that is always concentric with the axis of the mill spindle. The size of the circle can be varied in two ways. Firstly, by adjusting the angle of the laser pointer. This provides a rather coarse adjustment. The second way is to alter the distance between the table and the mill head.
Imagine now that there is a casting on the table with a hole. By roughly setting the circle described by the laser to the right size by adjusting the angle the table is adjusted to bring the hole concentric with the circle. The height of the head is now adjusted so that the circle lies exactly on the perifery of the hole and then to actually go down inside the hole. When this condition is satisfied then the hole in the casting must be exactly concentric with spindle.

I think that this could provide an easy way to set up a hole in a casting concentric with the spindle.

I hope this is useful to you.

Mike

I really like this idea, but suggested a twist or two when I replied :

That is truly and excellent idea ! How about if the mill is rotating quickly persistence of vision will see a circle of light which should make alignment a snap. If the circle size could be adjusted whilst the mill was turning the light size could be increased until it shone evenly on the inside of the hole or if it was not concentric it would light up one side of the hole interior.

Further thought indicates that if the laser was angled inwards by a degree or so then it would illuminate the top edge of the hole, forming a sort of light cone. To centre the hole one would increase the diameter of the cone until one side of the hole was illuminated or the hole was evenly illuminated on all sides.

Any further thoughts on this idea much appreciated.

Best Regards

picclock

[Jonny]

Easier if you have a dro.

If you can get away with a couple of thou and want to pick up from a hole just slap a drill in the same size.
Just keep pecking and watching for deflection as move the X and Y axis, absolute p easy.

If you put a laser or, wiggler or a light up centre finder in, it will have its own inherant runout, i rarely use these except if i dont want to mark soft items.
If use a wiggler of some description off a small bore your not going to see it centred unless split the difference using a dro or scale on hand wheels. It will work on the o/d got a brand new one some where never used in 10 years.

[Jonny]

Got one of those lasers cost me about £60 from California some years back, absolutely useless unless marking out plate work.
They need zeroing as well.

[AdeV]

Hmm, possibly even simpler - if you're not using a vice at least: Make one of Andy's pointy things, but don't part it off from its shaft. Mount the rigid pointy thing in a collet, bring spindle (NOT rotating) down onto casting so that point enters hole. Unclamp casting from table; allow pointy tool to move casting as you feed in, until the tool is now gently pressing the casting to the table. Lock quill, clamp casting down.

Even if you can't move the casting, you can still use the fixed pointy device, just wiggle the table around until the point snugly fits all around the edge. I might be tempted to add a strong light source in the nose of the point, when you can't see light shining out of any part of the hole, you know you're bob on.

[BillTodd]

How about a large radius low profile coax indicator....

Bill

[Fredbare]

The laser idea looks feasable on a flat casting, but would it work on one that was concave, or convex?

John

[picclock]

@Johnny
For small holes this isn't a problem - the Starret edgefinder goes up to about 8mm on the pointy end. My issue is with holes that are awkward sizes, like 39mm or, as I've just cut a plug for 49.3mm.
 @ Fredbare
The laser would light the inside top of the hole. The rest of the casting shape would not matter. Basically I see it working like this. Attach the gizmo and roughly position the work. Start mill and roughly centre the work under the circle of laser light. Reduce the light circle diameter until it shines on one side of the hole only then recenter the part. Carry on altering the size of the light circle and moving the part until even light is showing around the edges of the hole. Reducing the diameter of the light circle should result in the light circle going black evenly.

I think in some ways this is similar to using reflections on a gun barrel to check for distortion.

Its advantages would seem to be that no accurate machining would be required since the assembly rotates with the spindle ensuring a true centre regardless of runout. Very shallow castings can be used (one of the problems with the cone method(s). Only a moderately focused beam would be needed as it doesn't rely on centering something with the beam, just detecting the edge - so any cheapo laser should just about do.

The main problem I can see is that of adjusting the beam diameter easily whilst the mill is running. I will have a think about it and see if I can come up with a solution so that I can do some tests and see what accuracy can be achieved.

Best Regards

picclock

[Jonny]

Picclok you havent a dro?
Do that all the time what your doing but use electronic edge finder and have to keep rotating for runout. Cheaper and easier to use than a laser.

Lasers really hit and miss and as good as you eye sight. May well be 10yrs i tried zeroing a vice with one, the beam is too big and looks smudged. What you may think is edge could well be +/-0.3mm out. You never know or are undecided whats splatter or the real beam.
http://www.lasercenteredgefinder.com/main.html

[picclock]

@ Johnny
Hi - yes I certainly have DRO's - albeit fairly budget ones. On the mill they are on the X,Y and Z axis. If the part was held in a vice or a fixed clamp arrangement then using a DRO and an edge finder makes it easy.

However problems start with a rotary table. Often I need to enlarge or add/alter holes in order to fit repair replacement parts. So I mount my rotary table on the mill and zero it in under the spindle. Now here comes the tricky bit, I also need to centre the work under the spindle so that all the axis align.  Only then can I ensure that the hole enlargement or alteration will be concentric with the original hole. I need to do this with the X and Y axis locked so that the alignment between the rotab and the work is precise. Then if I enlarge the hole to drop in an insert or whatever the shafts or fittings will all line up as original (Hopefully  ).

Eyesight is a funny thing, and I have no idea why it evolved this way, but one of the this that eyes are very good at is comparing things that are next to each other, so if you have a circle of light that is darker one side than the other eyes are pretty good at detecting that sort of thing. Now if you take it one step further and have the circle of light so it can only just be seen, differences in light levels at the visibility threshold like that stand out like a sore thumb - so I am fairly hopeful that alignment to better than 1 thou will be achievable - I just need to clone myself in order to get the time for further investigation.

Best regards

picclock 

[Lew_Merrick_PE]

Pic --  Starting with Andy's sketch, make the tool shank (say) ø.500/ø12 mm and bore it out to (say) ø.375/ø9 mm.  Face the ends flat, true, and very smooth.  Mount (using collars) an appropriate radial bearing on the tool shank and make an adapter that will support a rod for your test indicator.  The test indicator will be used to measure the concentricity of the conical end.

Make the conical end such that you can insert (say, jam threads or the like) a ø.1875/ø4.5 mm rod that will pass through the tool shank.  Use a low friction (teflon, delrin, etc.) washer on each end of the tool shank.  Retain the conical end using a check-nut (call it ø.281/ø7mm) and a Belleville washer.

You can then have multiple conical ends for a given tool shank.  The rod connection will help keep everything aligned and the test indicator will tell you when the shank and conical end shank are concentric.

???

[Jonny]

Know what you are saying Pic, i do a lot of stuff by eye even down to microns but no way is a laser good enough with the beam spatter. Rather like some torches, you have a central pronounced beam or black spot and outside that loads of spatter.

Are these cylindrical objects in a 3 jaw on RT?
Or irregular shapes in a 4 jaw on RT?

[Ned Ludd]

Hi Guys,
Re laser circle projector, you are almost exactly a year late. There was one being demonstrated on the SMEE stand at last year's Ally-Pally show(this years coming up this weekend). To overcome the beam spread, it had a cap over the laser pointer with a 1mm hole through which the beam emerged. It seemed to work, so go ahead and make one for yourselves.
Ned

[picclock]

@ johnny
 irregular shapes clamped to the RT

@Ned Ludd
Thats really cool - I am going on Saturday, Snow permitting. I will check it out (assuming its there) and may even fork out some dosh if its not too outrageous.

Thanks for the info

Best Regards

picclock

[Ned Ludd]

Hi Picclock, I have sent you a PM.
Ned

[loply]

Am I missing something? To center the spindle over an existing hole, regardless of size, you just put an indicator holder in your spindle and compare the reading on your DTI from the four sides of your hole...?

I always have a DTI on an arm for this purpose, and for centering stock in the 4-jaw on the lathe...?

All this talk of lasers I don't get... It'll cost more than a DTI and won't be anywhere near as accurate...

[Ned Ludd]

Hi Loply,
I agree absolutely, if the hole is big enough to get a normal edge finder in. An edge finder and the halving button on my mill's DRO is the method of choice for the majority of my centring needs. The laser circle one was developed for such things as broken bolt centring, prior to drilling. As to cost, they can be built for price of a small laser pointer(<=£2 the last time I checked) and few pence worth of metal and plastic. Oh, and a few hours in the shop but that does not count, does it?
Ned

[Jonny]

Theyve got a monstrously wide laser beam those cheap and nasty ones, need at least 1/10 the size.
Used to use it to annoy the dog.

Just so happens got another expensive part dont want to replace and needs absolute to thou no runout 3 dimensionally picking up from an 8mm hole. Any runout when undercutting this 8mm hole will render scrap. Guess what i will be using again, i'll tell you an 8mm dia milling cutter.
I do have all manner and other means of finding the centre with lasers, wigglers, dti's and numerous electric edge finders.

Eye sights more than good enough to centre up in a busted bolt hole, done loads never failed even in a pillar drill.
Even drilled out a 1.5mm hardened shear pin in to a blind aluminium hole, cant beat number one eye ball.

[75Plus]

Typical sizes for me are from 25 to 150mm.

 

Joe

[efrench]

An appropriately sized ball bearing will also work well for centering on a hole.

[lockstocknbarrel]

Hi Picclock,

Try this on eBay.
$59.99 USD thats a good buy.
http://www.ebay.com/itm/Co-Axial-Centering-Indicator-CO-AX-250-Axis-Offset-/390533479727?pt=LH_DefaultDomain_0&hash=item5aed9bfd2f

How to on youtube.....................................

Regards Beagles