Need A Centering Square: What Do You Guys Recommend?

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I need to buy a centering square and wanted to know what you guys recommend.

It seems that my choices are either the adjustable Starrett sets, their lower cost Chinese copies, or a simple fixed square like this:

82285_R-1.jpg

These are available all over: Amazon, Victor Machinery etc. and probably would do the job for me. What kind of
accuracy can be expected from these choices?
 
Accuracy only helps to a point with laying out centers. Even after laying out a perfect center, which is only really possible if you have perfectly round work, it is still difficult to get the "usable" center as accurately placed as you had it laid out. So at some point there is diminishing returns. It also takes a fair amount of time to try to lay out a really accurate center. If the center is only to turn work down until it is round and coaxial, and you have enough excess diameter to start with, then lay it out quick and dirty and get to turning it to size between centers. After it is mounted between centers and turned to a true cylinder, then you can start doing accurate work with it. The reality is that little work is done between centers any more, most work is chucked on the O.D. and then worked from there. Working between centers is more accurate, but often slower, and the setups are often not as rigid.

All that is to say that I think a fairly simple centering layout tool is good enough for most real world work.
 
Those little centering angles that come on some combination squares work fine for rough work. Always make several lines. Not just two so if your stylus, marker, pencil, or scribe isn't perfect you can atleast circle the center up close. https://www.harborfreight.com/12-in-combination-square-set-62968.html Yes this cheap tool gets some use in my shop. I actually have some good quality high end machinist squares and center finders, but this thing is plenty good enough to get you close for a lot of work, and I won't cry if it gets dropped. Works better than good enough to mount something on a wood lathe.

On a metal lathe I find center with an indicator and a 4 jaw chuck. If the stock is "round enough" I just throw it in a collet. If I am turning it down first and I can do the job in one setup I don't bother.

On the mill I used to use a coaxial indicator, but I get good results faster on larger pieces (anything 1/2" or larger) by just using a clicker type edge finder and doing the math or on the CNC letting the controller do the math. I don't typically use a probe because I don't really trust any of my machines with that much stick out. Maybe its why I like the clicker better than the coaxial indicator too. Those have even more stickout.

Anyway, its really not rocket surgery. Determine your tolerance and find a tool that will get you there.
 
I have two fixed centre squares like the one you show, 1 1/2in and 3in. Mine are used mostly for woodwork to mark a centre on some round or close to round stock like in your picture. My squares were purchased from Grizzly, but likely made by the same manufacturer as some of the sites you mention. I mark the centre, then hand drill for mounting turning projects on my wood lathe.

For metal I typically just mount in the chuck and centre drill.
 
What is your application?

The task I'm working on now is to turn a 4" diameter aluminum round down to 92 mm. I've faced it off on both sides in a 4 jaw
chuck already, but need to mark it with lines at 90 degrees to one another so that I can drill for 4 bolt holes. It will be mounted
on a face plate and the OD turned to size. I'm not looking for the center, I'm trying to mark it for the bolt circle which only has to
be accurate enough to mount on the face plate. So high accuracy isn't important for this job, but as long as I'm buying one I thought
I'd ask and get something decent.
 
As a compromise between Starrett and the stuff from China check out Products Engineering , made in USA

Here is a link to one on the Harry Epstein site, poke around in the Machinists tools for other selections.

https://www.harryepstein.com/index.php/12-4-pc-combination-square-16r-usa.html

Personally I would avoid any with heads made of aluminum like you find at Home Cheapo and Horror Fright, even for home use they don't hold up.
 
Something decent IMHO would be a Starrett center square. But that will not give you 90°. Maybe try and just lay the face plate on your work and transfer punch it? When you faced it off scratch/draw a circle line on that face to help locate and reference for a punch mark…Dave

And I see Starrett center heads with scale for under 10 bucks all the time in my travels.
 
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I have a Starrett square with a centering head.
If you have a lathe, just stick the piece in the lathe and use your tailstock to mark the center while turning, or dimple it with a centering bit.
 
I've not used the type of centering square you showed in your post. That one looks a bit small to span a 4" round.

I use the centering head on the combination square (per CluelessNewB). For your application, I'd make one long mark all the way across the round, then rotate and make a short tick mark at center. Then get out your trusty plastic triangle to make a line thru the tick mark, perpendicular to the first line. Given that "high accuracy isn't important for this job," this should get you going.

PS - the PEC combo square that CluelessNewB linked to is likely a very nice, quality tool ... and at a good price. Their PEC tools are generally factory seconds (cosmetic defects), with the PEC name obliterated. But fully functional and useful otherwise.
 
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