I have to drill a perfectly centred hole in the flat part of a cylinder.I don't know how to do but I thought something like this:
I want to take a piece of wood and fix it at the base of the drill press and then drill a hole of the same diameter as the cylinder. Then put the cylinder in the hole and drill it. In this way all the holes will be centred. Is this a good idea?
EDIT: The cylinder is 4 mm dia and 1.5 cm high.
If the hole is big enough to fit a bowl gauge in it (for instance, if you're making a cup), you can get a perfectly centered hole by mounting the cylinder to a lathe and turning it like you would turn a bowl.
As a general statement, your idea will work - though precision will be dependent on the quality of a) your drill press, b) the bits used, and c) the 'solidity' of your clamped jig, as well as being limited to higher tolerances by the nature of this approach.
IF the drill press has little to no play when extended, it will be better suited than a drill press with any discernable play. (Play is any ability to wiggle the extended shaft around.) Accuracy will decline in direct relationship to amount of play present.
IF the hole saw is very good quality the pilot bit will actually be centered in the hole saw itself, but that accuracy is relative - by nature of it's assembly, a hole saw isn't accurate if tolerances needed are small (under 1/16 or 1mm).
IF the drill bit used to bore the centered hole in the work is true (straight) and sharp it will lead in best, and I would suggest a brad-point bit to aid lead-in. But this is dependent on the material you are drilling into.
IF your holding jig is very secure and made of something that will not distort when the work is inserted and removed, it will improve accuracy.
To close this out, I recommend this approach only for medium to large items due to the inbuilt inaccuracy at small scales.
Do you have a lathe available? If so, here's my answer from a (similar question)
If you have a lathe available (you did mention the spindle being turned already), then you have a great way to drill a hole along the center axis.
You use a drill chuck (like this one at Rockler), to hold your drill bit. You chuck the spindle, rotate and move the tailstock towards the headstock to do the drilling.
Depending on how you chuck the spindle, it may be easier to put the spindle in the headstock and the bit in the tailstock or the other way around.
I've used this technique on fairly short spindle stock, probably wouldn't work well if your spindle was a 20" long, half-inch diameter piece, as the spindle wouldn't have sufficient support.
A quick image search found this picture to illustrate what I'm talking about, though it looks as if the material is brass:
I've done it as you suggest, and it works. Obviously, the dowel you're drilling has to be fairly short. You also have to make sure it's held securely so it doesn't spin in the wood that's holding it.
If there is only ONE workpiece:
If you only need to center drill one cylinder, then mark the center, and drill it.
How to find the center of a circle
If there are multiple workpieces:
Yes, your idea will work roughly, however you would be much better off using a v-block to hold the workpieces. The reason for this is that any circular fixture you make will not perfectly fit the cylinders, so they will move slightly inside of it, creating error. Not only is a v-block more simple to make, but you can hold the workpieces tight against it, so there will be no movement.
To make the v-block, just clamp two blocks to the table of the drill press:
To locate the fixture: in order to locate the fixture so that the center of the workpiece is under the drill, first mark the center of an example workpiece. This can be done with a center finder or with calipers. Loosen the clamps. Mount a needle in the drill press. Adjust the workpiece until the point of the needle touches the center mark on the test piece. Gradually tighten the clamps while maintaining the adjustment of the test piece until they are tight and the workpiece is still exactly centered. This procedure has to be repeated if there are sets of workpieces which are of different sizes.
Additional notes: a permanent v-block can be made by bolting two pieces of wood together. Also, if you have a slotted drill press table and the work piece is 2" or diameter or less you can use 1-2-3 blocks to make this setup. The advantage of 1-2-3 blocks is that they have threaded bolt holes, so you can bolt them together, then bolt them to them to the drill press table quickly. Also, if you have 4 1-2-3 blocks, you can bolt more blocks onto to the end to make a larger v-block.
For small cylinders as described (4mm dia, 15mm long) the premise in your OP will work, but the drill press used needs to be accurate.
My recommended execution of your idea:
Assumes negligible play in drill press and use of good brad-point drill bits. Drill press table should be as close to chuck as will still allow the exchange of bits.
Cut a piece of 18mm MDF about 100mm wide and 100mm shorter than your drill press table's diameter. Mark centre. Clamp down with centre mark at drill bit centre. Drill a 4mm hole to 2mm deep, and without unclamping drill your desired bore diameter right through the 18mm MDF.
Unclamp, set aside and retain. This is the top half of your jig.
Clamp a second piece of 18mm MDF to the drill press table (100mm longer than the first, which should equal your drill press table diameter). Clamp at very outermost available places.
Bore a 4mm hole to 11mm depth, then drill a 2mm hole clear through the MDF.
This second MDF chunk is the jig's lower half. It remains clamped in place until operations are complete.
Blow out any dust from drilling, and place first cylinder in place.
Place jig top half onto top of work, clamping in place using the extreme ends of the jig half.
The two jig halves will compress the work from top and bottom, preventing rotation while drilling.
Drill desired bore into work.
Unclamp jig top half, remove work by poking up from below through the 2mmm hole.
Blow out jig.
Repeat.
Since this is a very small object, go easy when drilling and don't try to hog it out all in one go. You'll get a good number of cycles out of the jig if you take care to remove and re-clamp gently.
