Finding the centre again

[WolfieParticipant @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

[WolfieParticipant @wolfie]

[David Clark 13Participant @davidclark13]

Hi Wolfie

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

regards David

 

[WolfieParticipant @wolfie]

Because it was too thin and also countersunk, there was nowt but a wafer and it wouldn’t have kept the drill straight.

[David LittlewoodParticipant @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

[WolfieParticipant @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

[JasonBModerator @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

[jason udallParticipant @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.

[jason udallParticipant @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

 

[jason udallParticipant @jasonudall57142]

sorry about typos

[Harold Hall 1Participant @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

[TerrydParticipant @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

[Billy MillsParticipant @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.

[WolfieParticipant @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.

[Ian S CParticipant @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

[jason udallParticipant @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.

[jason udallParticipant @jasonudall57142]

“hold original IN vise”

[MICHAEL WILLIAMSParticipant @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 .

[Billy MillsParticipant @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.

 

[lee hawkinsParticipant @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.

[Jim GreetheadParticipant @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

 

[lee hawkinsParticipant @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

 

 

[TerrydParticipant @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

[TerrydParticipant @terryd72465]

T

Edited By Terryd on 22/01/2012 21:57:04

[TerrydParticipant @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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