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Drill chuck runout

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Robin Graham16/03/2010 18:12:10
1089 forum posts
345 photos
Hello again.  I'm trying to cross drill 4mm holes into 30mm diameter by 7mm thick brass discs (they are bobs for a system of coupled pendulums).  Sometimes it works, more often it doesn't - the hole turns out maybe 0.1 to 0.2 mm off the centre line between the faces of the discs.  I need to do quite a lot better than this.
I think the problem is due to runout in the chuck(s) I am using.  Although the drill press is a cheapo chinese job, the spindle seems to run pretty true - runout measured on the part of the arbour taper which engages with the chuck ranges from about 0.03mm down to 0.01mm depending on the arbour.  However, if I mount a piece of 10mm silver steel rod in the chuck the runout on the rod (about 10mm from the jaws) is 0.15 to 0.3mm depending on the chuck (I seem to have four - two anonymous chinese jobs, a Porta and a Rohm).
My questions are these: (a) are these numbers reasonable? (b) if I buy a fifty quid (nigh half the cost of the drill press!) Jacobs chuck (Chronos) am I likely to see a significant improvement? and (c) is there any way of improving matters by software (ie brain)  rather than hardware?  On the last point, I suppose what I mean is that if I could persuade the bit to start biting where I want it to, it would probably carry on OK - but I haven't yet found a way of making that happen.
 
Regards, Robin

JasonB16/03/2010 18:47:32
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25215 forum posts
3105 photos
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How are you actually marking out teh hole and holding the work, if your dot punch and following centre punch marks are off by 0.1mm no Drill chuck will solve that. If you have a mill you can drill with that or clamp the discs to an angle plate on a vertical slide and drill on the lathe, both ways you can use an edge finder and then feed the job to align the ctr.
 
Starting with a centre drill and then a stub drill will help the hole off better than going straight in with a normal jobber length drill.
 
Jason
John Haine16/03/2010 19:43:07
5563 forum posts
322 photos
Robin,
 
Can't help with your query but would like to know more about what you are making, having an interest in this field.
 
John.
Nigel McBurney 116/03/2010 19:57:59
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1101 forum posts
3 photos
hi the only way that you will achieve a tolerance of .1 mm on hole position is to make an accurate drill jig. A good chuck on a poor machine will not improve accuracy and even with a good english drilling machine achieving plus minus ,1 mm is not easy.  4mm drilled for 32 mm deep  is 8 times diameter and even if the drill started correctly it will wander and not come out the other end in the correct position. For a few off its a job for a good milling machine, Why not drill the hole first in a wider blank and then machine the blank to size so that the hole is central.I am assuming that you have a lathe.
Versaboss16/03/2010 22:25:34
512 forum posts
77 photos

Robin, to be honest I don't think that's a job for a cheap drilling machine, unless you have a means to position the workpiece exactly (eg. a x-y table). I also don't think that a Jacobs chuck is much better than a Röhm, at least I suppose it is not a chuck salvaged from an old hand drill?

But to give you some constructive ideas:

- use a center drill, best a so-called NC center drill (the Slocombe (?) drills are for doing centers to put a lathe center in)

- use a self-centering drill, either a 4-facet or one with a split point. These have no chisel edge which can wander around on a flat surface, but a really sharp point in the center.

- doing it in the lathe would be a good idea also; either by first centering it in a 4-jaw (and then only releasing one jaw to change the workpiece), or mounting it with suitable shims and stops on the cross-slide.

I hope this can be helpful

Greetings, Hansrudolf

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