Accurate hole location

[Steven VineParticipant @stevenvine79904]

Thanks Jason. I have a wiggler and have used the pointy bit to pick up on line intersections, so I understand how that works. What I am not getting (call me thick) is the plasticine and pin bit. Going back to my wiggler, the pointy bit with the ball on the end will align itself to the centre of the collet, as the ball provides the centering.

Now, if I centre a rod in the collet, stick a bit of plasticine on the end of a rod, and then insert a pin into the plasticine, then the chances are the pin will not be in the centre of the rod and may be offset from the spindle axis. If the end of the pin that is in the plasticine is not central to the rod, then how does it pick up centre?

Steve

 

Edited By Steven Vine on 15/03/2017 10:51:30

[SillyOldDufferModerator @sillyoldduffer]

As per Jason's advice here's some pictures. I use dressmaking pins which are about 30mm long and have a big round bead on the head. I find this form of sticky pin with a loupe outperforms my 'proper tool'.

Pin with blob of soft modelling clay: some makes are softer and stickier than others. I don't think blue-tack is as good.

Pin stuck more-or-less centrally on the base of a milling cutter.

With the pin spinning at about 1200 rpm gently ease the sharp end into balance with a smooth rod. (Pencil, knitting needle, etc.  This takes a little practice.  Beware of the pin flying off if it's badly out of balance.

In this photo the pin is spinning at 1250 rpm with the sharp-end accurately aligned with the rotating axis of the milling cutter., ie centred.  The blur is due to the camera auto-focussing on the vice rather than the pin.  In real life the pin is steady.

Cheers,

Dave

Edited By SillyOldDuffer on 15/03/2017 10:59:51

Edited By SillyOldDuffer on 15/03/2017 11:00:19

[JasonBModerator @jasonb]

Steven, provided the top end of the pin is not massivelly off center it does not matter so long as you set the pointed end to run true

[Steven VineParticipant @stevenvine79904]

Thanks Jason and Dave. Looks like I will have to give it a practical go later today. I still can't get my head around it, though I do take your word for it.

If the fixed end of the pin is offset from the axis, then I expect the free end to swing out and away from the axis, and not swing inwards towards the axis. There must be some physics I am missing.

Steve

 

Edited By Steven Vine on 15/03/2017 11:41:29

[old AlParticipant @oldal]

The 'sticky pin' was a tool used by toolmakers and the like. In the toolroom that i in worked in, we used grammer phone needles (ok ,it was a long time ago). They were only about 3/4" long so your fingers got close to moving parts.

First you would dob your plasticine on the cutter, center drill, chuck, whatever. Then turn on the drill and mould it into a cone and then stick the pin into the centre when it was still going round and then centre the needle with your thumb nail. We could always get within 5 thou, usually better. (and only took seconds, every mill had a piece of plasticine stuck on it somewhere)

Their are better ways to do that now, its dangerous in the wrong hands and best forgotten

[Tony Pratt 1Participant @tonypratt1]

I wouldn't say the 'sticky pin' method is particularly dangerous, it's only a bit of plasticine & a pin after all. Just don't run it too fast & take care as always!

Tony

[JasonBModerator @jasonb]

Posted by Steven Vine on 15/03/2017 11:32:07:

If the fixed end of the pin is offset from the axis, then I expect the free end to swing out and away from the axis, and not swing inwards towards the axis. There must be some physics I am missing.

Steve

There should not be any swing, the plastacine is as stiff as the friction on the wobbler. Unlike edge finders where you want the end to swing out the pin is just nudged until it runs true and then stays there.

I tend to use the end of a steel rule to set the pin true. It could be unsafe if stuck onto a moving milling cutter as mentioned above but if you only do the nudging while moving and don't stick it to a cutter it should be relatively safe at sensible speeds.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 15/03/2017 10:51:45:

… I don't think blue-tack is as good.

.

Very true … it's because of the unusual dynamic properties of Blu-Tack

Ref. MEW 235, p61

MichaelG.

[Steven VineParticipant @stevenvine79904]

Posted by JasonB on 15/03/2017 12:30:10:

There should not be any swing ……

Ah,. That's the bit I was missing.

Steve

[SillyOldDufferModerator @sillyoldduffer]

The sticky pin / wobbler works like a spin stabilised rifle bullet. Spinning around an axis ends to even out any imbalance and the object centres on the axis.

In the case of a sticky pin, the head might well be slightly off-centre where it's attached, but it doesn't matter because the free end can float. Once the free end is spinning on the axis it stays there. I only mentioned the pin flying off because if you push the sharp end too far away from the axis centripetal force will take over and fling it off at a tangent. Perhaps my method is faulty!

Interesting to see how other people do it. I always stick the pin directly on the cutting tool, centre it roughly, and then wind the speed up to about 1200 rpm while using a rod to correct any gross sideways movement. I don't try and accurately centre the pin until it's turning fast. At higher RPM the pin seems to snap into position more readily. The method works well: BUT you do have to keep well away from the cutter and wear eye-protection in case the pin comes off. Am I doing it wrong (again)?

By the way I don't align the point of the pin to the work with the machine running! Once the pin is stable, I stop the machine, and then use the XYZ controls and a magnifying glass to position the work accurately under the point.

Dave

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