# Finding The Center Of A Boss On A Small Casting



## tomw (Apr 16, 2015)

Dear All,

I have a smallish casting in my steam engine kit that requires I find the center of a boss to drill a hole. I am curious as to how folks approach this problem. The casting and the applicable drawing is in the attached photo. Sorry I cut off the drill notes.

Thanks,

Tom


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## Billh50 (Apr 16, 2015)

well if you could indicate 3 sides and make them all equal I would think you would be close enough to center. I have no idea how accurate the casting is but that is what I would do and then just touch the center drill to the part to see how it looks.


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## smoa (Apr 16, 2015)

I can't tell what that center hole looks like, but I would start by dialing the part in straight along one axis of the machine.

1. I would pick up the center or the existing hole using a dial indicator in a drill chuck.
2. Using a travel indicator on a mag base or DRO, I would move .75" to the center of one boss spot drill it just enough to make a mark.
3. Use the travel indicator or DRO move 1.5" back to the other boss and spot drill that one the same.
4. I would then use my dial calipers to double check the distance between the spots and that the spots are close to the center of the cast bosses.
5. If it looks good, drill one hole then the other.

You could try to dial into each boss instead of the center hole, but it's a rough looking casting. If you do that still spot each hole and check the distance between the spots before you drill.


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## randyc (Apr 16, 2015)

Use the centering head on your combination square.  Draw a couple of lines at at least two different angles - the intersection is the approximate center of the diameter.

If you don't have a centering head, a normal square can be used.  Measure and mark 1/2 of the diameter with the square at several different locations.  The center of the constellation of dots is approximately the center of the part.


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## NightWing (Apr 16, 2015)

Fractional dimensions can be achieved with eyeball orientation.


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## randyc (Apr 16, 2015)

After locating and center punching the diameter to be processed, this is a handy way to center up the work in mill or drill press:





A center finder like this one is almost always to be found in edge-finder sets, even the inexpensive imported ones.  (Note that old-timers used a "wiggler" to do this and that method also works well.)

The point of the center finder is lightly lowered into the punch mark and the part (or the X, Y handwheels) moved to center the mark under the spindle.  Running your finger nail down the side of the center finder, it's easy to tell when the end is misaligned with the body and adjust accordingly.

A DTI could be used for more precision but locating a center-punched location doesn't imply a lot of precision   The "fingernail" method is said to be able to detect errors as small as .002.  I've never measured the error myself.


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## planeflyer21 (Apr 17, 2015)

Does what you are attaching the casting to already have holes that can be transferred?


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## NightWing (Apr 17, 2015)

These methods are all well and good, but these are carpenter's dimensions.   Why overkill the project?


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## tomw (Apr 17, 2015)

Randy,
The parts are too small for that approach, at least for me. The large bosses (bossa? bossae?) are about 5/32 in diameter. I like the use of the edge finder to find the punch mark. Neat trick. 

Plane, I was planning on using these parts to transfer the holes to the other casting!

I also should have mentioned that I was trying to find the center of an end boss, then the hole on the other end is measured from there. The hole to hole distance across the larger bosses is the only important dimension. The highlighted part in the picture below is the one I am trying to machine.


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## Ebel440 (Apr 17, 2015)

You can just set a caliper to a little more then the radius and scribe 2 or 3 lines in the center and where they meet is center


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## chips&more (Apr 17, 2015)

Make a pod so that the flat bottom of the cleaned up casting fits snugly into it. Noting the X & Y  dialings as you make the pod. Strap/clamp the casting into the pod and make the holes. Another and better way would be to use an optical center locator mounted in the spindle. But not everybody has one of those in there HM shop…Good Luck, Dave


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## tomw (Apr 17, 2015)

Dear All,
A super accurate center is not required. It can probably be off a 64th or so and still look OK. The big deal is the distance between holes. Therefore I tried Ebel's technique first.

The difficulty with that technique is the fillet on the casting alters the distance the calipers move towards the center as I wiggle. But, with four passes I had an area of lines defining a center perhaps .05 across. Using my +2.75 reading glasses, I then lightly punched in the middle of that area. It seemed close, so I went ahead and drilled both ends. It actually turned out pretty well.

I think I will try the edge finding method on the next piece. 

Dave, please explain better your pod process. What is a pod? I probably should know that. Please excuse my ignorance. I am assuming it is not an alien pod. Which is would be great, as I have no alien pods (Cocoons or Alien variety).

Thank you all, even if not mentioned by name, for your ideas and help.

Cheers,

Tom


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## RJSakowski (Apr 17, 2015)

There was a YouTube video dealing with finding a center as you are trying to do.  A laser pointer was mounted in the spindle and the mill started at low rpm.  I does not need to be centered; inf act it works better if it is slightly off center.  The laser will light up a small circle on your workpiece and you can move  the table until the circle is centered on the boss.  The spindle is now properly centered.  It helps to have a laser with adjustable focus.  Most have some sort of adjustment if you can get to it.  It may mean removing the cover to access it.  
Refinements on the method would include providing for an adjustable circle diameter.   My pointer is 1/2" o.d. and my boring head uses 1/2" boring bars so I can mount the laser in the boring head and adjust the diameter to match the boss diameter.  It provides fairly accurate centering even on irregular bosses as you would have with a casting. 
Unfortunately the pictures I took do not accurately show what is seen because the laser appears brighter and the camera can't capture a full circle.


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## Bill C. (Apr 17, 2015)

I was going to add fractional dimensions usually are +/- 1/64th of a inch.  If the casting looks as good as the picture you shouldn't have any problems. 

 I don't know if you are using a milling machine or drill press.  I will assume a drill press for the following.  If it was my project.  I would use a sheet of sand-paper about 150 grit on a flat smooth surface so I could sand the bottom surface of the casting so it sits flat on the table. Then take a marker put a dot in the middle of the top surface of the boss.  Since it is a small area to work with use a caliper and mark three arcs to find the center. They don't have to be equally spaced on the outside diameter to work. The area in between is the center if they don't cross exactly.  Use a small center punch and hammer to prick the center. Set your calipers to the overall distance between the two holes and mark the second hole.  Prick punch the second hole.  If the centers look good then make the punched marks deeper.  You can use a small straight shaft center in the drill chuck to position the center drill. Use a small center-drill, #2 or #3, and drill out the marks.  That is how I was taught. 

The Best with your steam engine project.


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## randyc (Apr 17, 2015)

tomw said:


> ...The parts are too small for that approach, at least for me. The large bosses (bossa? bossae?) are about 5/32 in diameter. I like the use of the edge finder to find the punch mark. Neat trick...



LOL, sorry Tom, the scale of that tiny part eluded me.  There are as many different ways of locating the center and accurately determining the location of the other holes as there are people that can read your post   I have no doubt that you'll figure out a way of making it happen !


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## rafe (Apr 18, 2015)

I think I would approach it from the back side as it is flat. Dress the piece out back and sides to final finish, I would center the 1 1/2" dimension then the center from side to side. I don't think it is as critical as you think. I would verify that the threaded holes on your other piece are in fact 1 1/2".....HTH


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## tomw (Apr 18, 2015)

Thank you all for your help and sage advice. I ended up getting close enough for this model by using both Ebel's technique and a wiggler edge finder. Photo below.
	

		
			
		

		
	




If you know this PM model, you may notice the lightweight aluminum connecting rod instead of the original cast bronze. That piece taught me to lock my z axis on the mill. Hence the fully machined part.

Cheers,

Tom


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## Bill C. (Apr 18, 2015)

It looks good. Aluminum probably stronger anyway.


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## benmychree (Apr 21, 2015)

NightWing said:


> These methods are all well and good, but these are carpenter's dimensions.   Why overkill the project?


I agree, there is also the hermaphrodite caliper for this sort of work, it has one leg like a inside caliper and the other with a scribing point; bear the caliper leg against the boss diameter, adjust the scriber to approximate center, scribe a line and move the caliper leg around the part in 4 positions and scribe lines; their junction will be the center.


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## Dr. Duzlittler (Apr 22, 2015)

Have any of ya'll ever used a coaxial indicator? For finding center of round stock, I don't know of any quicker method. Just always remember to put your gear box on a very low speed or you'll likely damage it. I've had one of these for several years and it's my go to instrument for quick centering in the mill. Here's a stock photo that I found of one.


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## tomw (Apr 22, 2015)

benmychree said:


> I agree, there is also the hermaphrodite caliper for this sort of work, it has one leg like a inside caliper and the other with a scribing point; bear the caliper leg against the boss diameter, adjust the scriber to approximate center, scribe a line and move the caliper leg around the part in 4 positions and scribe lines; their junction will be the center.



I have seen these, but did not know their purpose. Thank you!


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## benmychree (Apr 22, 2015)

The Co Ax indicator is worth its weight in gold; it preforms internal and external indicating plus indicating from prick punch marks; it also is expensive!  There are knock offs to be had at a cheap price, but you get what you pay for ---


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## Boswell (Apr 22, 2015)

Will the Coax Indicator work in the example given by the OP?  What I am concerned about is that the outside edge of the boss may be the rough surface of the casting and I think the Coax indicator needs a smooth surface.


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## benmychree (Apr 22, 2015)

You are correct in that the Co Ax would not work for that application very well; If I were doing the job, I would layout the center lines, trying to center the bosses as best as can be possibly done to make them ALL as close to centered as possible.  One has to realize that this is not a perfect world and bosses may not always be at proper centers due to the foundry process being an imperfect art; that is what the layout process is for, to divide up the imperfect into something that is a compromise and allows a part that will serve its intended purpose.  This is quite a simple casting, so it should not be hard to accomplish as opposed to larger complex castings.  In the shop I apprenticed in back in the 1960s, we had a full time layout man, always busy, saving the machinists much grief and lots of spoiled work ----


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## tomw (Apr 22, 2015)

Also, CoAx indicators don't really fit well in my Sherline mill. This makes me sad (sad emoticon here, or not), as I have one. I am thinking of getting the longer column just for that reason.

I really like my Sherline stuff, but the vertical travel on the mill can be quite limiting.

Cheers,

Tom (the OP)


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