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Accurately marking centre line

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Wolfie24/09/2011 20:36:32
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502 forum posts
Hi all, back at the engineering after the summer (devoted to other hobbies) I could do with a pointer on this:
 
How do I accurately mark a centre line down a very narrow piece of metal.
 
I have some 3/16 square bar and I need to drill a couple of holes on a centre line. Now clearly the half way point is 3/32, but its too small for my smallest square??
John Stevenson24/09/2011 20:50:43
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5068 forum posts
3 photos
You need one of these.
 

El cheapo digital and a bit of work on the bench grinder or linishing belt.

John S.
Bogstandard24/09/2011 21:26:09
263 forum posts
I can fully attest that Johns precision scribing block/oddlegs work just fine.
 
I made a set when John first showed them, and they are used all the time, and are remarkably accurate.
 
John
Michael Gilligan24/09/2011 21:36:45
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23121 forum posts
1360 photos
Wolfie,

Another way ... Use a surface plate and a scribing block.

The surface plate need not be expensive; a bit of plate glass or mirror will do.

The scribing block can be easily made.

Put your workpiece on the surface plate
Set the scribing block to approximately half height and scribe a line.
Turn the workpiece over and repeat [scribing on the same face].

If your guess was good, there might be just one line
... More likely there will be "tramlines". These are equispaced about the true centre line. ... If they are close, it's easy to split the difference by eye. If not close enough, then adjust the scribing block a little, and try again.

It's quicker to do than describe.

MichaelG.
Tel24/09/2011 23:27:24
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157 forum posts
28 photos
Or you can make up a variant of this gizmo of mine - carefully done they are very accurate and accommodate a range of sizes.
 
[IMG]http://img.photobucket.com/albums/v332/laneranger/set5.jpg[/IMG]
Tel24/09/2011 23:35:46
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157 forum posts
28 photos

Adding pics to posts on this site is like trying to run through waist deep treacle!
http://www.model-engineer.co.uk/albums/member_photo.asp?a=7059&p=292309

Edited By Tel on 24/09/2011 23:36:48

Edited By David Clark 1 on 25/09/2011 09:18:39

Wolfie24/09/2011 23:37:42
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502 forum posts
Nice one gents, thanks
modeng200025/09/2011 07:25:52
340 forum posts
1 photos
Use a pair of dividers and scribe a line from both sides. With a bit of luck you will end up with one scribed line, if not the centre is the mid point between them.
 
John
Ian S C25/09/2011 11:24:19
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7468 forum posts
230 photos
Hi Tel, that looks interesting, is the bush for marking center hardened, and is it loose so that diferent sizes may be used? I'v got metal cut, ie., 4 pieces with holes at the ends, only need a hole in the center of one of them and its done. Ian S C
Tel25/09/2011 11:32:06
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157 forum posts
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 Nope, mine is not hardened, but I suppose it could be of advantage to do so. I only made the one bush, which takes a scribing point ground from a concrete nail, which tend to be very consistent size and finish wise - grind the points on the HH grinding attachment with a suitable collet in the end mill fixture adjusted to run with a little resistance - gets the point nicely in the center (which is the point, ain't it?) I'm pretty happy with the way it will automatically pick up the centre of the stock - not always the case with some of the other methods!
Ian S C25/09/2011 13:30:59
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7468 forum posts
230 photos
I was actually thinking of using it for drilling RSJs as first use. Ian S C
Terryd25/09/2011 16:51:56
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1946 forum posts
179 photos
Hi Tel,
 
Thanks for reminding me of that little tool. I'd forgotten the concept existed. I made my first one at Tech night school when an apprentice. I can hardly wait to get to my workshop to make one!
 
Thanks mate.
 
Regards
 
Terry
_Paul_25/09/2011 17:47:18
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543 forum posts
31 photos
Posted by John Stevenson on 24/09/2011 20:50:43:
You need one of these.

El cheapo digital and a bit of work on the bench grinder or linishing belt.

John S.
 
 
Thanks John what a simple and very effective idea, just made one now to Bogs design
 
Regards
 
Paul
 
(Apologies to the OP for the OT post)
Tel25/09/2011 22:25:41
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157 forum posts
28 photos
Now that's something I have been intending to do - a version of John's digital gizmo with a cross piece on the standing jaw to help keep the whole thing square to the workpiece - a thing that kept me from ever having much luck with odd leg calipers.
 
The idea of that thing I made has been around for a very long time - I made mine up as part of a set of marking stuff
 

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