# How to find opposite sides of a cylinder



## Southtowns27 (Feb 18, 2015)

I need to drill two shallow holes on exact opposite sides of a cylindrically shaped piece of metal.  How the heck do I get them exactly opposite each other?


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## Cobra (Feb 18, 2015)

If you're using a mill, find the centre of each axis and the reference from the centre of the cylinder. 
To find the centre of each axis just use an edge finder along a line for that direction, lifting the edge finder to get to the other side.  Don't need to be on centre for either.


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## Wreck™Wreck (Feb 18, 2015)

You can only get them a accurate as *you* can measure them. The fastest and easiest way is to mill/drill the first hole and then mill a flat for a reference surface for the opposite hole. If this is not permissible either build a fixture to hold the part or use an indexer.


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## Shepherd (Feb 18, 2015)

Fixture...drill into a surface plate, install a ground rod.  Drill first hole in cylinder on a v block, then instal hole on the rod on the surface plate....


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## Southtowns27 (Feb 18, 2015)

I don't have a mill, only a lathe and drill press.  And I can't envision how that fixture is supposed to work...


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## Wreck™Wreck (Feb 18, 2015)

Southtowns27 said:


> I don't have a mill, only a lathe and drill press.  And I can't envision how that fixture is supposed to work...


Square fixture with a round hole for the part, put part through tool and drill first hole, rotate part and tool 180 Deg. and drill opposite hole, done.


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## Shadowdog500 (Feb 18, 2015)

How big is the cylinder? 

Wonder if you could clamp the cylinder into a v block.

Then put the v block into the drill press vise in one orientation to drill one hole, then put the vBlock in the drill press vise 180° and drill the other hole.   

chris


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## savarin (Feb 18, 2015)

Make a square block, scribe around 4 sides to ensure the line meets itself. drill through on the line to fit the cylinder dia.
Now drill through the other two faces to intersect the the first hole at the size you want the dimples to be.
Now drill from one end to the cylinder hole and thread it for a holding bolt.
Place cylinder in hole, tighten the holding bolt now use the other holes as a guide to drill the dimples.
I had to do just this last week
http://www.hobby-machinist.com/posts/274544/


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## Shepherd (Feb 18, 2015)

Southtowns27 said:


> I don't have a mill, only a lathe and drill press.  And I can't envision how that fixture is supposed to work...


Drill press will work fine...by drilling the peg into the plate, you have a "male" fixture..., set it aside and drill the hole on one side, then place that hole over the peg in the plate and clamp it down...picture the peg as if it were a drill bit, on its way right through the cylinder...may not be nasa level, but if the hole and peg are drilled accurately enough, it should get you pretty close....
Or, find a square tube that the cylinder fits snugly in and tack it with a welder and drill away...


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## Shepherd (Feb 18, 2015)

Or, Scribe a line along the length on either side using a height gauge set to half the diameter on a surface plate.....


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## BillWood (Feb 19, 2015)

Drill all the way through and then plug it with a short piece of dowel so that you end up with a short hole either side ?


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## janvanruth (Feb 19, 2015)

the way to go is to make the cylinder a part of something that hat two opposing parallel flat surfaces so you can turn it around without distrubing the orientation
clamping in a v block will do so
but there are other ways
it depends on the size of the cylinder
what size is the cylinder?


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## epanzella (Feb 19, 2015)

Center punch 1 hole. Cut a strip of paper longer than the circumference of the cylinder. Overlap it and mark it at the punch mark. Take the paper off and make a mark half way between the two spots that came together at the punchmark. Put the paper back on the cylinder and make a second punch mark at the half way mark you just made. The punchmarks will be opposite each other. If the end of the piece has been trued you can measure off one end to get them square. If not, you can stick it in a lathe and lightly scribe a line all the way around with a pointy cutter. Drill any way you want. This is how I made the fixture in attached photo.
Ed P


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## BillWood (Feb 21, 2015)

I've enjoyed reading the different ways of doing this.

Also enjoyed reading a previous post about getting 2 holes at exactly 90 degrees.

Any good books websites or previous threads that would help a newbie get attuned to this geometrical thinking ?

Bill


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## chips&more (Mar 2, 2015)

Hi, you need to tell us how big in diameter this part is? Because the method can differ depending on how big the thing is…Good Luck, Dave.


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## Fabrickator (Mar 2, 2015)

The most accurate way with only a drill press is to mount it to a square block fixture as previously mentioned.  Carefully measure the needed distance from the top/bottom edge.  Mark the first hole/high spot with a center finding gauge, or scratch the high point with a height gauge, or use a dial indicator to find the highest spot. If you have none of these, just use anything with a pointed tip to "very lightly" scratch the highest spot.  Center punch it and use a center drill to start the hole and finish drill it.  Flip the mounting block 180* and do the same thing.

Rick


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