View Full Version : Help with centering head problem

Chris Fazio
02-13-2003, 09:38 AM
About 3 yrs. ago I purchased a PEC brand 4 pc. combination square set. I didn't use the centering head until a few weeks ago. When I used it on a turned and faced piece of round aluminum and scribed a line, the line appeared not to be in the center of the bar. I then turned the head over and scribed another line which appeared to be about the same distance from the true center as the first scribed line but on the opposite side of center. I sent back the centering head to PEC and they sent me a new one.The new one did the same thing as the old one. I called them and told them this new one did the same thing as the old one. They were nice enough to next day ship me another one, this one came with a new rule also. I tried it last night and it does the same thing. I took the square part of the set and placed it into both centering heads which I assume should be a 90 deg. angle also. When I did this the square would rock inside the centering heads. If I hold the square to one side of the centering head I can see a gap between the square and the centering head on the other side. What I would like to know is am I doing something wrong or are these centering heads screwed up?


Ragarsed Raglan
02-13-2003, 09:47 AM
I always scribe two sets of tramlines (as you described) at 90* to each other then spot with an optical centre punch. It maybe that you are 'leaning out' too far with the scriber or your scriber is too thick (T.C.Tipped?) where it touches the rule, if you want a single line you got to 'tuck' a slender scriber into the bottom edge of the rule. I find most TC tipped scribers are pretty much too thick for this type of marking out work. Great for other stuff though!


Chris Fazio
02-13-2003, 10:43 AM
RR, I'm using a metal pointed scribe and I don't believe this is the problem because there is at least a sixteenth of an inch between the two lines. If I place the center head on the bar and scribe a line then rotate the bar 90* and scribe a line, rotate 90* and scribe a line,rotate 90* and scribe a line I end up with a square box around the true center. I try to get the point of the scribe as close as possible to the rule.


02-13-2003, 08:03 PM
Is the stock round to begin with? Clean off the rust/crud/chips/scarf/dirt/sand/stones/general debris and mike it at 45* intervals around the end where you are going to use your centering head. You'll be surprised just how out of round lots of stock is!
Is the end of the stock fairly square to the length? If its not then you will have scribe lines that will never ever be able to allign with each other regardles of the width of your scriber. Now if its off and you wind up with a nice little "box" on the end of your stock try this. Take your scale and connect the opposite corners of the little box and where those lines cross should be real close to the center of the stock.

Paul Gauthier
02-13-2003, 08:04 PM
If as you say the square rocked in the centering heads either the centering heads or the square is off. The fact that you have had several centering heads do the same thing will, of course, rule out the square. Hencethereforth I would say the centering heads are no good. "BUT" the fact that you have had several that seem to be off is a mystery. One person getting three of them would almost mean that all of their centering heads are bad.

Just as an exercise try setting your scale into the protractor and set protractor at 90* and check with your square head.

Another exercise.
Do you have a milling mach. or mill drill???? If so place centering head on table and lightly clamp it. Indicate one arm of the c/head dead zero, then indicate the other arm. It should also read zero.
You could even perform this check on a lathe.

Paul G.

02-14-2003, 12:34 AM
You can still use the centering head by scribing multiple lines (moving the head about the diameter of the stock). This is good practice even with high end equipment as errors in scribing (as noted by others) can cause problems. It is then a simple matter of centering from there.

This is a good example of why low cost tools are not always a good bargain. I have an old Fuller try square that is dead nuts and cost $2. For accurate work I have Starrett and Mitutoyo Cast Iron and Steel squares. My Mitutoyo 6" Square cost $9 (CDN) on eBay and it was new and sealed in the factory carton.

Jim Hubbell
02-14-2003, 12:41 AM
As I see it, the sq. hd. does not have to be exactly 90deg.but the blade MUST bisect it. I have an old set ( Union Tool ) I purchased about 40 some years ago that does the same thing. It is only about 25 or 30 thou. off so the "little box" is close enough for me. It would be interesting if a few of us tried marking as was outlined in the first post and see how many do the same thing.

Chris Fazio
02-14-2003, 09:22 AM
Thanks to all that replied to this question. Paul G.
Excellent ideas I will try them out tonight. I agree it's hard to believe I got 3 bad ones, that's why I posted this, I thought maybe I was missing something.
Ken G.
I turned and faced a piece of aluminum to rule out the errors that you mentioned.
This brand (PEC) was supposedly made in USA . It was cheaper than a Starrett or Mitutoyo but more than an import. A chinese import might have been better.
Jim H.
Your correct in what you said about the angle being off, as long as the blade is "centered" it should still work, hadn't thought about that.

I guess it's time to get a new set. Enco has a Brown and Sharp set on sale, how's their quality, comparable to Starrett and Mit.??

Thanks again,

02-14-2003, 10:38 AM
Try it on a much larger round piece. If the little 'box' you scribe with four successive efforts 90 deg apart gets bigger then obviously the head is not seating at the proper angle to insure the blade bisects the angle along its full length. If the box size stays the same then the edge of the blade is parallel to the line bisecting the angle, but offset by a small amount. That would suggest the seating groove in the blade is off by a small amount.
Those two different errors should also be revealed by simply trying to scribe the same line (lightly) by flipping the centerhead/blade combination over to scribe the second line. If you get two lines diverging from a common origin it's the angle problem. If you get two parallel lines then the seating groove is off. (You'll probably see a combination of those.)

02-14-2003, 10:41 AM
A combination square and centering head is not necessarily a precision piece of equipment. Accurate?, yes. Precision?, no.
PEC is an American manufacturer, and can be considered shop grade. Starrett and others will be better.
The fact that they sent several replacements three years after purchase speaks very well of them.
The main question is how accurate does your centering job need to be? As described, the very worst you will be off if the lines are 1/16" apart is 1/2 of 1/32", or 1/64". Not much in the overall scheme of things, your centerpunch can move this much.
If more accuracy than this is required, a height gage, V block and surface plate will give the best results.

Chris Fazio
02-14-2003, 02:32 PM

I hear what your saying about the accuracy, but then I have to wonder how they made 3 centering heads with approx. the same amount of error. Why could't they make 3 with no (or little) error? The books I read said the centerhead was used to find the center of a bar so I assumed they would be accurate. But because I am far from an expert, I asked this question to the people of this forum, many of whom are experts IMO, and again you didn't let me down. So it wasn't a wasted day today because I learned something with your help.

Thanks ALL,


P.S. lynnl I'll try what you suggested tonight.

02-14-2003, 03:59 PM
Chris, let us know the results.

Most of the PEC stuff I have I've found satisfactory (for my needs). However I did buy a PEC telescoping gage set from Enco that I've thought was pretty shoddy. In fact I'm not even sure it was PEC. When I ordered it Enco listed the "import" set at $15 or so, and the PEC at about $34 as I recall. I ordered PEC, but it has no markings on either the tools or the plastic case. That's been about 3 or 4 yrs ago. I now wish I'd sent it back for a refund. Not sure, since I don't have a known PEC set to compare, but I think Enco just cheated me.

Jim Hubbell
02-17-2003, 01:10 AM
After reading all of the replies I tested my Union Tool center-head again. I came up with a 10 thou. square in center. A magnified look at my scribe was a wakeup as too many sharpenings had made it way too blunt! When the taper was worked down I was able to scribe so that the first line blended into the second ( or vice-versa ).
Lots of good wake-up calls on this board!

Chris Fazio
02-18-2003, 09:45 AM
This weekend I fixed the centerhead. After looking at it for some time,and figuring out what needed to be done,I set it up in the vise on my mill/drill and using a shell end mill I corrected the problem. Now it's accuracy is within the width of a scribed line on both small and larger rods. It is now usable and I'm happy with the results. If I could make it work why couldn't PEC? Thanks to all for the help.

You have to watch Enco, I just bought a little base for my caliper to use it as a depth gage. The catalog said it was made in USA but when I received it the box said made in china.


Mike L
02-18-2003, 10:26 AM
Well, I just had to go and check the center head on a Starrett combo square I just picked up off eBay.

I drew 8 lines from various points around the edge of a piece of bar stock - all of them crossed at the center.

Now I can justify the $75 I spent.

Mike L
Amateur machinist, self-taught. I had a poor teacher, but I was a good student.