Finding the centre again

This topic has 30 replies, 15 voices, and was last updated 23 January 2012 at 16:45 by Martin W.

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20 January 2012 at 18:58 #82567
Wolfie
Participant
@wolfie
Heres another one. When I was making new jaws for my machine vice, I had to transfer the jaw screw holes to my new bit of metal. Ok so I clamped old and new together and marked the place. But how do I find the centre of the marked hole?

I pretty much did it by eye and got close enough but i would have been happier with it exact.

I know about transfer punches, but the original jaw was too thin (and countersunk) even if I had any.

Edited By Wolfie on 20/01/2012 18:59:27
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20 January 2012 at 18:58 #5882
Wolfie
Participant
@wolfie
20 January 2012 at 19:03 #82568
David Clark 13
Participant
@davidclark13
Hi Wolfie

Why did you not spot through the hole with the correct size drill?

regards David
20 January 2012 at 19:09 #82569
Wolfie
Participant
@wolfie
Because it was too thin and also countersunk, there was nowt but a wafer and it wouldn’t have kept the drill straight.
20 January 2012 at 19:38 #82570
David Littlewood
Participant
@davidlittlewood51847
Wolfie,

Why does it need to keep the drill straight? Surely it only needs to get it to the right location, and the drill keeps the drill straight. Might be easier if you put it on to the new piece upside down so the edge of the hole is at the top.

David
20 January 2012 at 19:59 #82573
Wolfie
Participant
@wolfie
I tried that, the old jaws were only a couple of mm thick so thin that the drill would have had time to wander before biting.

Edited By Wolfie on 20/01/2012 20:00:40
20 January 2012 at 20:23 #82579
JasonB
Moderator
@jasonb
Put a bit of packing between the jaws and your piece of work and then find a drill bit that is a good fit in the bottom of the CSK hole. Hold this vertically and twist with your fingers, it will leave a mark where you want it. Blue or run a marker pen over the area first to help show up the mark

J
20 January 2012 at 20:36 #82580
jason udall
Participant
@jasonudall57142
ok as to ‘finding center of existing marked circle’ as apposed to copying holes.
Might be fiddly at this scale but the center finder used for bar ends will work. See use in tool clamp project mew186
As would marking arcs from four points on the circle.
20 January 2012 at 20:56 #82583
jason udall
Participant
@jasonudall57142
lets assume the part is rectangular

to measure define a hole measure te wall from hole to each edge.

measure diameter of hole.

the centre is those “wall” measurements plus half the diameter.

thus id dia. is 10

and wall at top is 12 center will be 17 from top

simularly for othee directions.

sorry if stating the obvious
20 January 2012 at 20:57 #82584
jason udall
Participant
@jasonudall57142
sorry about typos
20 January 2012 at 21:40 #82596
Harold Hall 1
Participant
@haroldhall1
Wolfie

Make a bush, outside diameter a close fit in the hole in the part being copied. Inner hole a close fit on the end of your centre punch. Place the existing part, countersink towards the new part and centre punch through the bush. With the old part mounted that way there is no risk that the bush will slide up the countersink creating an error

Harold
21 January 2012 at 00:11 #82602
Terryd
Participant
@terryd72465
Hi Wolfie,

Assuming you made the jaws reasonably symmetrical, turn it over and spot through or use a spotting punch into the vice body,

Regards

Terry

Edited By Terryd on 21/01/2012 00:12:31
21 January 2012 at 01:18 #82606
Billy Mills
Participant
@billymills
I think that the original question was how to find the centre of a scribed circle. With a pair of dividers put a leg on the circle then guess the diameter and scribe an arc roughly around the centre. scribe another arc from a third around then another arc from 2/3rds around. you should then get a very small ” triangle” enclosing the true centre. ( I should have called the triangle a cocked hat)

This is an approximate method, you don’t have to be very precise but the “triangle” is very quick to draw and makes the guess very much better.

An accurate method. Chuck a point in the mill then place the point roughly over the middle. wind out the X axis past the circle then wind back to be over the circle, note the dial reading then carry on to the other side of the circle to get a second reading. Halve the readings to get the centre of the chord that you have just gone across and wind the x axis to the middle. you are now on a diameter of the circle.

Now do the same on the Y axis to find the middle of the tangent to the chord which is the centre of the circle. You do need always to work in the same direction to avoid backlash. drop the z axis to make a tiny centre mark.

Another version… draw a line across the circle intersecting the circle each side- roughly on the diameter. Set the dividers to the two crossing points, draw an arc from each point to intersect in two points each side of the line. join the crossing points with a straight line- this line is now on the diameter. From the two points where the diameter crosses the circle draw two arcs to intersect then join to find the true centre.

Billy.
21 January 2012 at 09:31 #82611
Wolfie
Participant
@wolfie
Thank you everybody I should have thought of one or two of them myself. I dunno how accurate the marking arcs method would be when the circle is only 5mm across mind.
21 January 2012 at 10:24 #82616
Ian S C
Participant
@iansc
Wolfie, even if you had thought of some ideas your self, by asking the question you may help some new commer with a problem he did’nt know he had, every bit of information might help someone. Ian S C
21 January 2012 at 10:41 #82617
jason udall
Participant
@jasonudall57142
Ok. Not withstanding how usefull knowing the dimentions would be.
If all you want is to copy a part .
Assume new plate is same size. And you have acces to milling mc or drill press.
Hold original vise
Chuck drill blunt end out
Or better yet drill blank all of same size of original hole
Now position rod/drill in hole. Clamp up part. Clamp down vise.
Recheck drill still fits hole
Remove part replace with new piece. Chuck drill say pilot drill
Drill pilot hole ….etc.
21 January 2012 at 10:42 #82618
jason udall
Participant
@jasonudall57142
“hold original IN vise”
21 January 2012 at 10:47 #82619
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
(1)

You can after just a little practice locate the centre of a small scribed circle within 5 thou and sometimes better just by eye .

You need a single ended scriber with a nice sharp symmetrical point . Place the scriber point best guess in the centre of the circle and look at it from various directions . When your eyes get used to looking at the work you will see that the radial distances from scriber point to circle are all different . Adjust position of scriber point as nescessary . When you can no longer distinguish any differences in the radial distances the scriber is centred and more accurately than you would ever think possible . Just tap scriber gently to make a pin mark and follow up with a small centre punch .

Bench magnifier can help you see clearly if needed but process is more accurate if you can do without .

(2)

Just a note on appropriate levels of accuracy in engineering .

There is usually no point and sometimes no meaning to using setting out and machining methods which are more accurate than the intrinsic accuracy of whatever you are working on .
22 January 2012 at 00:12 #82676
Billy Mills
Participant
@billymills
Would agree with Michael that you can locate the centre of a small circle with some precision with practice however when a newcomer asks how to find a centre it is kinder to give some methods which also work with much larger circles where visual estimation fails.

Billy.
22 January 2012 at 18:16 #82713
lee hawkins
Participant
@leehawkins46887
It may not work on something as small as maybe down to 6mm or something, but any other size circle it’s perfect.

Get yourself a piece of paper+compass, get the diameter of your circle , draw the circle on the paper, then cut it out nice and accurate, fold it twice , you then have a perfect segment of the circle with a nice point, put that on the workpiece you want to find the center of, accurately lining it up, now where ever that point is will be spot on the center

Maybe most people already know this way.

lee.
22 January 2012 at 20:10 #82726
Jim Greethead
Participant
@jimgreethead
That is a good one Lee, and even better because you don’t need to fold the paper; just use the centre where the compass points marked it..

Stick the paper circle on with a bit of spit and centre punch the compass mark.

Jim
22 January 2012 at 20:35 #82732
lee hawkins
Participant
@leehawkins46887
Yes you can do that Jim, OK for maybe just one off, but if you center punch more than a couple of times hole enlarges and I find accuracy disappears, I get what your saying though.

lee
22 January 2012 at 21:54 #82740
Terryd
Participant
@terryd72465
Posted by lee hawkins on 22/01/2012 18:16:28:

It may not work on something as small as maybe down to 6mm or something, but any other size circle it’s perfect.

Get yourself a piece of paper+compass, get the diameter of your circle , draw the circle on the paper, then cut it out nice and accurate, fold it twice , you then have a perfect segment of the circle with a nice point, put that on the workpiece you want to find the center of, accurately lining it up, now where ever that point is will be spot on the center

Maybe most people already know this way.

lee.

We do,

but 5mm dia is a bit small for such methods, even more accurate is to draw the circle in a drafting program and draw the centre lines with the finest lines possible.

T

Edited By Terryd on 22/01/2012 22:02:54
22 January 2012 at 21:55 #82741
Terryd
Participant
@terryd72465
T

Edited By Terryd on 22/01/2012 21:57:04
22 January 2012 at 22:00 #82742
Terryd
Participant
@terryd72465
Posted by Wolfie on 20/01/2012 19:59:59:
I tried that, the old jaws were only a couple of mm thick so thin that the drill would have had time to wander before biting.

Edited By Wolfie on 20/01/2012 20:00:40

Hi Wolfie,

You only have to mark the centre with the drill point, not let it bite, just a gentle rub of the drill point will do if you coat the jaw surface with marking blue or a permanent felt tip pen. Then you just have a tiny circle to find the centre of – much more accurate.

Regards

T
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