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View Full Version : Good way of marking the center line of flat stock?



ldn
06-27-2010, 01:44 AM
Hi Everyone,

Does anyone know of a clever way of scribing a line down the exact middle
of a piece of flat CRS stock?

I've got a method that works in theory but it requires a lot of very fiddly alignment and then three hands to scribe it. :)

Oh and by the way, I've started working on my milling attachment. However I may have to start over again because I've had a lot of trouble drilling the holes in the right place. I got my stock marked and punched well enough but on my first attempt the holes were all over the place.

On my second I used a center drill to start and had the same problem.

Then I discovered that my cheap Harbor Freight benchtop drill press has about 3/32" play in the spindle!

So I moved over to the lathe to do the drilling, and that was very precise. But with having to jury rig faceplate clamps and align the punch marks it took about 2 hours to drill 3 holes. Faceplate clamp setup for your amusement:

http://imgur.com/EVhN3.jpg

Here is a shot showing how far off the hole was when I used the drill press. The punch mark was centered on the line, and the center drill started off in the punchmark.

http://imgur.com/JzBh5.jpg

dp
06-27-2010, 01:47 AM
Too many square brackets on your image links. Remove the ones before and after the http://... string. :) Never mind!

I use a compass from both edges at two points along the edge to scribe intersecting arcs at two places then draw a line between them.

winchman
06-27-2010, 01:53 AM
I use a square with a ruler blade that's held in place with a thumb nut. After measuring and checking the CL mark from both sides of the stock, I slide the square down the side while holding a scribe against the end of the blade.

dockrat
06-27-2010, 01:59 AM
Generally I would use my height guage to do this. Set stock on side and measure full width then set and lock the guage at 1/2 that reading and slide the scribe on the guage down the length of the stock

ldn
06-27-2010, 02:02 AM
Too many square brackets on your image links. Remove the ones before and after the http://... string. :) Never mind!

I use a compass from both edges at two points along the edge to scribe intersecting arcs at two places then draw a line between them.

You'd think a programmer could get it right the first time! :o

I've used that method too but in this case the stock is only 3/4" wide so I didn't think it would work well. I think that it also requires precisely measuring in an equal amount from each side to place the compass point, which it what I was trying to avoid.

Now that I think about it, I've got a tool for marking model airplane hinge slots that is exactly what I need -- if it wasn't made of out nylon and too imprecise.

It is a short bar with 2 pins at each end. In the middle is a hole for a pencil. You just place it on the stock, twist it so the pins touch both sides of the stock, and the center hole lines up automatically.

Maybe I could make something like that out of metal with a hole for a scriber... Sounds like making it would require more precision than I am currently capable of.

ldn
06-27-2010, 02:05 AM
I use a square with a ruler blade that's held in place with a thumb nut. After measuring and checking the CL mark from both sides of the stock, I slide the square down the side while holding a scribe against the end of the blade.

What a coincidence, that is almost exactly what I did... I held the scribe against on of the tick mark indentations.

oldtiffie
06-27-2010, 02:06 AM
Get an adjustable square from a combination set:
https://www.machineryhouse.com.au/images/11880.jpeg

Set the rule to project as near as you can to half the width you want to "centre". Ie for 4" bar, set the rule projection to 2"

Put the square against one side of the 4" bar and place a scriber against the end of the rule. Drag the square along the side face of the 4" bar while holding the scriber onto the end of the rule.

The scribed line will be close to centre.

Now change he square to the opposite side of the 4" bar (do not alter the ruler setting in the square) and repeat as before and draw another line alongside the first line.

The centre is dead between the lines.

Adjust the ruler so that its end is exactly mid-way between the lines.

Scribe two more lines as before.

Those lines if right on each other are dead on centre. If not dead on centre, no matter as the centre is still dead between those lines which are now closer together.

Re-try if necessary.

It is a process of (re)iteration where the "error" gets successively less/smaller.

I usually leave two lines just a bit less than a 1/64" apart as it makes it easier to measure to or centre-punch than a single line does.

It is very easy to get the "hang" of, and once you have it, it is very easy, very quick and very accurate to use.

ldn
06-27-2010, 02:15 AM
It is a process of (re)iteration where the "error" gets successively less/smaller.

I usually leave two lines just a bit less than a 1/64" apart as it makes it easier to measure to or centre-punch than a single line does.


Thank you, that's exactly what I was looking for, a method that naturally finds the center and doesn't require precisely lining up the scribe with a tick mark on a ruler.

Walter
06-27-2010, 02:39 AM
Height gauge then center punch, do a ton of stuff this way. Old Rockwell drill press, spindle has play side to side, still does the job quite well.

dp
06-27-2010, 02:43 AM
You'd think a programmer could get it right the first time! :o

I've used that method too but in this case the stock is only 3/4" wide so I didn't think it would work well. I think that it also requires precisely measuring in an equal amount from each side to place the compass point, which it what I was trying to avoid.

I push the work against a straight edge so the compass has something to work against, but you can scribe a witness line parallel to each edge and use that, too. It's one of the best uses I've found for my odd leg calipers.

djc
06-27-2010, 03:04 AM
I use a compass from both edges at two points along the edge to scribe intersecting arcs at two places then draw a line between them.

Could you elaborate on this please.

At the first place you make your intersecting arcs, unless the line joining the two centrepoints of your arcs is perpendicular to the long axis of the bar, neither arc intersection point will lie on the centreline of the bar.

Similarly with the second place you try.

You will have four points defined by arc intersections, two of which lie one side of the centreline and two lie the other side. Pick any pair of these and join them up; none of the lines so produced will lie on the centreline of the bar.

One way to minimise the problem is to use a square to scribe a line across the bar and then make sure your compass point is on that line.

dp
06-27-2010, 03:08 AM
I'll lay up an example tomorrow and take pictures. It's pretty easy to set up.

Timleech
06-27-2010, 03:08 AM
Use an oddleg caliper, guess at the setting and mark from both sides. You'll see how much the error is, adjust the caliper to reduce it, try again. Continue until you have it as good as you need.
Same principle as Tiffie's method.

Tim

dp
06-27-2010, 03:11 AM
Use an oddleg caliper, guess at the setting and mark from both sides. You'll see how much the error is, adjust the caliper to reduce it, try again. Continue until you have it as good as you need.
Same principle as Tiffie's method.

Tim

Yes - that's a good approximation and can get you really close if not spot on. One more step with a compass and straight edge, though, and you are actually spot on.

Peter.
06-27-2010, 03:35 AM
I use tiffie's method, works very well.

Timleech
06-27-2010, 03:59 AM
I use tiffie's method, works very well.

It needs three hands, though ;)

Tim

Peter.
06-27-2010, 04:31 AM
Not if you use the end of your caliper. Gets the lines very close first time too.

strokersix
06-27-2010, 05:34 AM
Use a cheapo caliper as a scriber. Mark off from both sides will eliminate parallax. Exact center will be between the lines. Takes all af 5 seconds.

This has to be the most used tool in my shop second only to my coffee cup. I bought a $20 Chinese dial caliper about 20 years ago and sharpened the tips a bit. It's been a great value. I have nicer tools but honestly I rarely get the nice stuff out.

Timleech
06-27-2010, 05:37 AM
Use a cheapo caliper as a scriber. Mark off from both sides will eliminate parallax. Exact center will be between the lines. Takes all af 5 seconds.

This has to be the most used tool in my shop second only to my coffee cup. I bought a $20 Chinese dial caliper about 20 years ago and sharpened the tips a bit. It's been a great value. I have nicer tools but honestly I rarely get the nice stuff out.

I seem to remember SirJohn showing a cheap digital caliper which he'd ground up to make, in effect, swanky oddleg calipers.

Tim

jackary
06-27-2010, 05:42 AM
Use a digital caliper with a sharpened edge (diamond file on the outside tapered edge) to scribe the line. Measure with the caliper, divide by two, set the caliper to half the measured dimension and scribe. Digital calipers are probably cheaper than odd leg dividers etc. and can be sharpened many times.
Works for me and is quick.
Sorry Stroker Six did not see you said the same thing in an earlier post.
Alan

ngriff
06-27-2010, 06:50 AM
i've used a couple of the above methods with good results. quickest for me is to use the ghetto canadian tire special digital calipers. set the exact size you want and use the tip on one side to scribe. get some bluing ink so you can see the lines better and not dig the tips in too much.

if you are going to setup this part on a face place how about making a simple fence to place against the one side of the part? clamp in place, take off one side of the clamps temporarily and put a piece of bar you can bolt in place against the part. i would suggest that you put holes in your fence to bolt thru the face plates slots. once you're affixed re clamp, drill, then unclamp slightly and slide the part forward to your next hole. will make this job quicker and more accurate.

good luck
nathan

speedy
06-27-2010, 07:11 AM
For marking out I will use the method that Mick mentioned, or odd leg (jenny) calipers, or the finger/thumb/pencil carpenters method.
As long as I have two equidistant parallel lines I can do the rest with the eyeometer, autopunch and centre punch.
Drill part depth pilot hole then measure for central and distance before continuing.

If I wanted better accuracy then cheap home made toolmakers buttons would work Ok as well.

John Stevenson
06-27-2010, 07:35 AM
I seem to remember SirJohn showing a cheap digital caliper which he'd ground up to make, in effect, swanky oddleg calipers.

Tim

http://www.stevenson-engineers.co.uk/files/oddlegvernier.jpg
.

fasto
06-27-2010, 11:18 AM
I use my Starrett 120X dial caliper. It's made for marking out lines. It has brazed carbide jaw faces:
http://i451.photobucket.com/albums/qq232/fasto_tt/Misc/th_Dsc01276.jpg (http://s451.photobucket.com/albums/qq232/fasto_tt/Misc/?action=view&current=Dsc01276.jpg)
You might think they'd be fragile and chip easily. In the 15 or so years I have been using it, I haven't had any problems.
Mitutoyo make a similar caliper.

Paul Alciatore
06-27-2010, 12:47 PM
Generally I would use my height guage to do this. Set stock on side and measure full width then set and lock the guage at 1/2 that reading and slide the scribe on the guage down the length of the stock


Ditto. If you are the least bit unsure, flip it over and repeat the scribing. The two lines should be one.

Paul Alciatore
06-27-2010, 12:58 PM
Could you elaborate on this please.

At the first place you make your intersecting arcs, unless the line joining the two centrepoints of your arcs is perpendicular to the long axis of the bar, neither arc intersection point will lie on the centreline of the bar.

Similarly with the second place you try.

You will have four points defined by arc intersections, two of which lie one side of the centreline and two lie the other side. Pick any pair of these and join them up; none of the lines so produced will lie on the centreline of the bar.

One way to minimise the problem is to use a square to scribe a line across the bar and then make sure your compass point is on that line.

The only way to make this method reliable is to first scribe a a line across the bar that is squart to the sides. Then, the point of the dividers that is used for the center of the arcs must be on the ends of this line. This will put both of the intersection points of the arcs on the center line.

But the height gauge method is both fast and accurate. It really can't be beat in either category.

doorknob
06-27-2010, 01:20 PM
http://www.stevenson-engineers.co.uk/files/oddlegvernier.jpg
.


Slightly OT, but how does that marker pen compare to using Dykem?

What do you use to remove it? Acetone?

Has anyone used a blue Pilot Jumbo Permanent Marker to do the same thing?


http://ecx.images-amazon.com/images/I/41io1IfW3eL._SS500_.jpg

dp
06-27-2010, 01:30 PM
I have to confess to not having a height gage so that is not in my solution set. Fortunately there are a number of very accurate alternate solutions. And there's just something cool about odd leg calipers :D

Edit: Here's an image I cobbed up showing the result of using an odd leg caliper, dividers, and a straight edge. No rulers or micrometers needed. The objective is the vertical line in the center.

http://metalworkingathome.com/images/centerline.png

darryl
06-27-2010, 02:00 PM
To automatically center a mark on flat bar regardless of width- make a simple jig. Take a piece of flat bar say one inch wide and 1/4 inch thick. Drag one edge across some sandpaper on a flat surface to clean it up. Then take a sliding square and set it for roughly half the width- not critical. Scribe a line. Put three punch marks on that line, being careful to get the center mark exactly centered. Drill the two outside marks for a press fit for some pins. These should stick out about 1/8 inch or so on one side, and about 1/4 inch on the other side. Music wire or drill rod works good for the pins, or whatever hardened pins you can find. Roll pins would work good also. Just make sure if you use roll pins that the open edge is facing away on both ends when you insert them. Drill the center hole for a good fit to a center punch. You might even make up your own punch from drill rod or music wire. That's what I do.

Place this jig over the flat bar that you need a centered mark on. Rotate it til the pins both touch the sides, and the central hole is exactly centered on your flat bar. Slide it along til the hole is where you want the mark, then punch.

You can use either side of the jig, depending on the thickness of material you are working with. Two or three of these simple jigs will cover any width of flat bar that is common. The closer the pins are to the width of the flat bar, the closer to an end you will be able to get a centered mark. Making the jigs is probably best done on a mill where you can dial in the hole locations.

Peter.
06-27-2010, 02:15 PM
Clever bastard.

I had to read that twice to get it but it's a great solution!

metalmagpie
06-27-2010, 02:56 PM
I use a square with a ruler blade that's held in place with a thumb nut. After measuring and checking the CL mark from both sides of the stock, I slide the square down the side while holding a scribe against the end of the blade.

Me too.

If you're trying for accuracy on a drill press or cheap mill, scribe your lines with a height gage on a surface plate, then center punch with an optical center punch, then find that punch mark with a bigger punch and deepen the punch. The drill bit should find the punch mark and drill right there. I like to use a wiggler to try to locate the deepest part of the center punch right under the spindle. Then I drill with a newish 1/8" bit, then drill to size (unless you're on a flimsy drill press where heavy drilling pressure will flex the table, in which case step drill). Or drill one size under and ream.

There's only so much accuracy you can get on a loose, flimsy machine, but it's helpful to know how to get the best you can out of it.

metalmagpie

Your Old Dog
06-27-2010, 04:15 PM
http://www.stevenson-engineers.co.uk/files/oddlegvernier.jpg
.

John's Idea is a joy to use. Made mine to look like his and the guiding jaw never slips . Not only that, but I can hold the caliper level to the plane of the work and it never slips. I measure the width of the stock, divided by 2, set the caliper and scratch it down the stock. A great way to mark where you want to mark everytime.

rohart
06-27-2010, 06:48 PM
I doesn't matter how loose your drill press is if

a) there is a good strong centre punch mark,

and

b) your table is square to the drill press axis,

and

c) your work is held firmly and square.

That's my way of looking at it.

And I don't bother with all these sophisticated methods for getting the punch in the right place. I have two punches. One is a proper hardened punch, and the other is a sharpened hard brick nail fitted into a holder that I use as a first marker/punch.

I hold a ruler on the work and very gently punch with punch #1. Then out comes the magnifier, and with the ruler I gauge how far off I am. If I'm close, I'll use punch #1 or #2 to move the mark. If I'm 20 thou off I'll re-mark.

When I've got it spot on, I mark quite heavily. Then I'll remove the crater of the punch mark with a file, and check again. Punching at an angle can move the punch mark 10 thou easily so adjustments are OK. But If I've adjusted, I'll punch harder, and then clean up.

That allows a 1/8th drill to start cleanly, but you musn't anchor the work to the table. You must be able to move it as you see the drill move slightly when it hits the hole it's making. You only need do bolt the work down when you're up to a 3/8 drill.

I'll go up in 1/8ths at a time, tso 1/8th, 1/4, 3/8ths etc., with cutting oil, a drop every 1/4 inch of penetration of every drill. I can get four holes in a square within 10 thou most of the time, and my drill's no great shakes.

Do spend some time getting your table square in both directions first though.
Then the hole does the guiding.

ldn
06-27-2010, 10:12 PM
Thanks everyone, lots of good ideas and creative solutions here.

Ken_Shea
06-27-2010, 10:17 PM
As darryl mentioned, here is a picture.
Called a Center Scribe :)

http://www.google.com/imgres?imgurl=http://web.ivenue.com/riversidemachine/images/centerscribe.JPG&imgrefurl=http://www.riversidemachine.net/item598449.ctlg&usg=__OPXxb5TIggUFVrE1XuQtOLQHcUM=&h=480&w=640&sz=41&hl=en&start=1&um=1&itbs=1&tbnid=GRK0Oncb7Do3CM:&tbnh=103&tbnw=137&prev=/images%3Fq%3Dcenter%2Bscribe%26um%3D1%26hl%3Den%26 sa%3DN%26tbs%3Disch:1