By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Forum House Ad Zone

Finding the centre again

All Topics | Latest Posts

Search for:  in Thread Title in  
1 2 Next > Last >>
Wolfie20/01/2012 18:58:28
avatar
502 forum posts
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

David Clark 120/01/2012 19:03:43
avatar
3357 forum posts
112 photos
10 articles
Hi Wolfie
Why did you not spot through the hole with the correct size drill?
regards David
 
Wolfie20/01/2012 19:09:13
avatar
502 forum posts
Because it was too thin and also countersunk, there was nowt but a wafer and it wouldn't have kept the drill straight.
David Littlewood20/01/2012 19:38:35
533 forum posts
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
Wolfie20/01/2012 19:59:59
avatar
502 forum posts
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

JasonB20/01/2012 20:23:07
avatar
25215 forum posts
3105 photos
1 articles
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 udall20/01/2012 20:36:03
2032 forum posts
41 photos
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 udall20/01/2012 20:56:38
2032 forum posts
41 photos
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 udall20/01/2012 20:57:19
2032 forum posts
41 photos
sorry about typos
Harold Hall 120/01/2012 21:40:29
418 forum posts
4 photos
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
Terryd21/01/2012 00:11:35
avatar
1946 forum posts
179 photos
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 Mills21/01/2012 01:18:08
377 forum posts
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.
Wolfie21/01/2012 09:31:31
avatar
502 forum posts
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 C21/01/2012 10:24:04
avatar
7468 forum posts
230 photos
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 udall21/01/2012 10:41:14
2032 forum posts
41 photos
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 udall21/01/2012 10:42:29
2032 forum posts
41 photos
"hold original IN vise"
Billy Mills22/01/2012 00:12:39
377 forum posts
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 hawkins22/01/2012 18:16:28
7 forum posts
5 photos
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 Greethead22/01/2012 20:10:39
avatar
131 forum posts
8 photos
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 hawkins22/01/2012 20:35:06
7 forum posts
5 photos
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
 
 
1 2 Next > Last >>

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Sign up to our Newsletter

Sign up to our newsletter and get a free digital issue.

You can unsubscribe at anytime. View our privacy policy at www.mortons.co.uk/privacy

Latest Forum Posts
Support Our Partners
cowells
Sarik
MERIDIENNE EXHIBITIONS LTD
Shopping Partners
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest

 

Donate

donate