Finding center

[macardoso]

Don't be afraid of counting the handwheels on your mill, DRO's are great, but by no way a necessity. You can even hold sub .001" tolerances if you are careful. As I understand your project, you have a round bar of steel, and you want to drill and tap a hole through the side of the bar, roughly in the center of the part. Here would be the way I would approach it in your shoes...

1) clamp the part (vise is ideal but there are plenty of other options)
2) put a 1/4" diameter rod in the spindle using a collet or drill chuck (use the back of an endmill or a dowel pin).
3) spin the spindle to make sure it isn't wobbling all over the place
4) using the handwheel which moves the 1/4" pin along the length of your part (X axis?), gently touch the left face.
5)Don't move the handwheel, lift the quill of the machine to clear the pin from the part.
6) Rotate the collar on the X axis handwheel to show 0.
7) You need to account for the fact that you touched the part with the side of the pin, not the center. To do so, divide the diamter in half (0.25/2 = .0125). You now need to offset your axis this amount. Rotate the handwheel one full revolution and then .025 more (probably 25 small ticks on the handwheel). The centerline of your pin is right above the edge of your part. Rotate the collar to read zero again.
8) use your calipers to measure the length of your part (lets say it reads 1.500"). Divide that in half
9) You need to move 0.75 inches. Your handwheel will move the X axis .100" per revolution (edit: common for smaller milling machines, make sure it applies to your machine, it could be .2 or .25 per rev as stated above). So to move the distance to the center of your part, rotate it 7 times stopping on the 0 mark. Move an additional 0.05 (50 ticks of the collar).
You have now found the center along the X axis
10) Now using the Y axis hand wheel touch the 1/4" pin to the inside of your vise jaw (it equates to the edge of the round part).
11) Measure the diameter of your part (lets say 1.000"). Divide it by 2. Subtract the .125" to account for the amount you are already inside the vise. you have .375" left to move
12) Zero your Y axis collar, and move the axis 3 revolutions plus .075 (75 small tick marks) towards the center of your part
You have now found the center along the Y axis
13) Lock the X and Y axes, put your center drill in your chuck, and drill the part

This description did not account for backlash in the machine, but it won't matter for this part. Get the hang of finding center as you will still need to do it with the DRO's. Setup of the part is at least half the battle if not more.

Feel free to ask any questions you may have.

edit: Corrected my numbers to match the size of part you described.

 

[RJSakowski]

There is another simple way without using a "V" block. But it requires that you are able to hold the work piece in a vise square to the drill table. That method simply uses a rule or other thin flat metal plate balanced on top of the round material. To do this you trap the rule between the work and the drill point and then move the work piece until the rule is level. This method works best using the mill where you can get accurate fine movements.

It helps others to help you if you can post pictures of what you are trying to do.

I had thought of this as well. It is essentially the same as a method for find the center height for a lathe tool.

Rather than the drill point , I would suggest using a sharp point. A drill has a flat web at its tip which would create some uncertainty. I would use an old drill shank or dowel pin sharpened to a point.

I was uncertain as to the sensitivity so I ran the experiment. Using the flat of my 1/2" parallel, and my 1-2-3 blocks and a height reference on either side, was able to find the center to within +/- .002" on a 3/4" round. This was about the limit of my ability to see the level. Without the height reference, about +/- .01". In determining the center, I lifted the point, repositioned the round, and lowered the point.

This would be an excellent way to center a round on a drill press where you have no means of controlling or measuring movement.

 

[BaronJ]

Hi RJ,

Thank you for your notes. I do agree with you regarding using the drill, I tend to forget that not all drills are ground four facet. Since making my four facet drill grinder, I've made a point of going through all my drill sets and regrinding them. The net result is that there isn't a flat worth worrying about on the drill tip.

I would recommend that people have or make a drill rod center for just this use, if hardened makes a handy punch.

For the benefit of those looking at the last picture I posted, there are two edge finders shown. The one in the middle is my version of the Starret Edge/center finder, 1/2" shank and 4 mm probe. The one on the right is a simple ball race of 1/2" diameter pushed onto a piece of 1/4" drill rod. I do have a laser one that I made as well.

If anyone wants details just shout.