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Need A Centering Square: What Do You Guys Recommend?

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Nogoingback

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#1
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?
 

Bob Korves

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#2
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.
 

Bob La Londe

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#4
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.
 

Dave Paine

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#5
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.
 

Nogoingback

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#6
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.
 

CluelessNewB

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#7
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.
 

chips&more

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#8
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.
 
Last edited:

woodchucker

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#9
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.
 

hman

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#10
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.
 

royesses

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#11
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.
I have that PEC combo square you link to. I also purchased it from Harry j's about 3 years ago. Very good quality. Almost impossible to find the cosmetic defect. Very good buy if you don't want to spring for the Starrett version.

Roy
 

rock_breaker

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#12
My interpretation of your project would be to use layout dye on the end, use a small center drill to establish a center in the work in the lathe chuck; use dividers to establish the circle diameter; also with the dividers establish a line across the center hole and establish two points on the desired circle diameter. From there I think you would be able establish another line to intersect the circle at 90` to the original line. Note I never took Geometry is HS but am reasonably sure these techniques are possible.
With respect to a center guide I like the product recommended by CluelessNewB and if I didn't already have one that's where I would go. No affiliation to PEC or CluelessNewB.
 

Nogoingback

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#13
Thanks for all your responses. I decided to place an eBay bid on a older Union Tool 18" rule with a square and centering head. This will provide
interchangeable parts with an old 12" Union square that was my Dad's and that I've had for years. The starting bid was less than
what I would pay for the simple fixed square in the pic above so if I win it I'll be way ahead.
 

Bob Korves

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#14
Thanks for all your responses. I decided to place an eBay bid on a older Union Tool 18" rule with a square and centering head. This will provide
interchangeable parts with an old 12" Union square that was my Dad's and that I've had for years. The starting bid was less than
what I would pay for the simple fixed square in the pic above so if I win it I'll be way ahead.
Union Tool Co., Orange, MA, was in business from 1908 to 1975. They made rules, squares, gauges, etc. Nice tools. Take good care of your square and cherish it along with the memory that it came from your Dad. Tools like that are the most special ones we can have. I have a few Union tools myself, a couple of them from my Dad. Tools like that are kept in a special drawer, and soon will be in a Wedell & Boers wooden tool box from the 1920's, after I get it rehabbed some.
Edit: I am getting off topic again, need to get back to the thread...
 

Nogoingback

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#15
You're absolutely right Bob, it is a nice tool, and special because I associate it with my Dad. I use it frequently, and then put it
away carefully.
 

EmilioG

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#16
a centering square will only get you close to round piece of stock. Like Bob Korves wrote above, out of round and the lines won't intersect.
I had an iGaging centering square but sold it for this reason. My Starrett center square attachment worked just as well, or got me the same results with out of round stock. Better to start with a part that is turned using a DI on a lathe first.
 

kd4gij

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#17
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.

When in the 4 jaw chuck . mark it at each jaw. you will have 4 marks 90deg apart.

That said never pass up an opportunity to buy new tools. I would watch eBay and get a good combination square set.
 

Nogoingback

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#18
a centering square will only get you close to round piece of stock. Like Bob Korves wrote above, out of round and the lines won't intersect.
I had an iGaging centering square but sold it for this reason. My Starrett center square attachment worked just as well, or got me the same results with out of round stock. Better to start with a part that is turned using a DI on a lathe first.
I'm aware that that's a more accurate way to do it, but in this case I plan on getting it close, bolting it up, and then turning the OD. I've got lots of
material to work with: it's a 4 inch disc, about 7/8" thick, but it will be turned down to about 3.60".
 

Nogoingback

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#19
"That said never pass up an opportunity to buy new tools. I would watch eBay and get a good combination square set."

Yup, that's the plan.
 

EmilioG

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#21
Watch Ebay. I purchased my 12" vintage Starrett, then bought the other pieces as they came up. Protractor and centering square, all hardened steel versions, complete. I would use the finest tip scribe I can find. Most times, but not all, the lines won't intersect to the center, but will put you somewhat close depending on how concentric the stock is. The smoother the finish, the better the scribed lines. I use a vise when I can, to hold the tool/work.
 
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