So why don’t you drill from the bottom face?

Being worried in general about tramming or misalignment errors is similar to Chicken Licken worrying about the sky falling on his head.

Measure the errors you have and assess their actual significance over the 100mm depth of the block.

8mm is not a conventional clearance diameter even for a close fit 5/16″ nominal item (too small). See:

Imperial Clearance Hole Sizes

If the holes are to fasten two components together, and you cannot dril with the item inverted as suggested above, drill both together. Then, to some extent, neither the absolute position of the hole entry nor the exit is critical.

When I want a hole as straight as possible I start with a stub drill then move onto a jobber and you may even need an extra length one for that depth of hole. Being short the stub drill is less likely to wander and it’s hole will help giude subsequent drills.

Withdraw the drill often to stop any build up of swarf pushing the drill off line and use paraffin or WD 40 to stop tip build up.

Well, strap in!  Any hole deeper than than 3x the drill diameter is a deep hole, liable to drift off.  100/8mm = 12.4, so four times over the limit.  Six straight holes is a challenge!

Suggestions:

• Drill in a lathe.  Holes are allegedly less likely to wander when the work rotates rather than the drill.
• The work and machine should be as rigid as possible.  Drill presses aren’t as rigid as mills.
• A pilot should be started with a centre or spotting drill
• Set the main drill accurately at a right angle to the job.  (Gerry said “The block will be squared up as best I can.”  Pay close attention to that! )
• Use sharp correctly ground HSS or carbide drills, and make sure they stay sharp throughout!  Both sides ground exactly the same, so both cutting edges are balanced.  One of my books says hand sharpening isn’t good enough for this, no matter what skilled men believe!
• The drill should be firmly held in a good chuck, collets are better than Jacobs
• Ensure a good start by shortening the drill shank (or pass it through the collet).  This reduces the chance of the drill bending.   If the initial hole is straight well beyond 3x diameter, it will tend to keep the drill’s helix straight as the hole gets deeper.
• Speed is important:  a bit lower than theoretical
• Lubrication essential – neat cutting oil in Aluminium
• Don’t force the pace – it increases the chance of wandering due to the helix unwinding.
• Absolutely vital to remove swarf – peck!  Consider a parabolic drill – they remove swarf better

When it matters Industry don’t expect twist drills to cut straight holes.   One approach is to drill a smaller hole and correct by boring.  Another is to use a ‘Ratio Drill with Cooling Ducts‘;  these are extra stiff, and blow swarf out by force feeding lubricant to the tip through the body.   Or, a gun drill when a ratio drill isn’t good enough.  £££

Once took me three attempts to successfully twist drill four straight 6mm holes through a 60mm deep block.   The air turned blue whilst my wallet wept…

Dave

Hi, rotating the work won’t guaranty the drill will follow a straight path, and if the drill has already wandered slightly, the drill will just follow the wandering. I do much the same a JasonB says.

Regards Nick.

you could try a drill with a secondary point angle they are supposed to help with long hole drilling

“The hole will be a clearance hole for an AN 5/16 bolt”.

So why are you worrying? They are clearance holes. Drill them a bit oversize and let the bolts find their own happy medium. At least 1/64 over, or even 1/32. Or you could go even more if needed.

Do as you already suggested and start each hole with a spotting drill to say half-inch deep so that hole acts as a guide for the longer drill bit.

Drill from the side that has to match the holes in the existing bracket.

You explain this block is a spacer between two components. Any way to align those to each other around the spacer, e.g. by aligning faces in the same plane? If so the holes could be quite generously clearance diameters.

Another approach would be, use two 50mm thick blocks. I take it you are drilling these on a milling-machine so can set the distances accurately, whether using a DRO or the handwheel dials.

The more usual way to ensure alignment on massive lumps of metal like this, is dowels, needing only fairly shallow but reamed holes. The bolt-holes can then be quite generous fits.

Careful marking out and then drill from both sides using the techniques as described above. If only clearance then just make it big enough to get the bolt through. Noel.

In the past when faced with similar jobs the easiest way to ensure the correct exit position is to drill the hole pattern on the one side using coordinate location. Centre drilling and using something like a 4 or 5 mm drill, drilling to half the depth of the block.

Turn the block over and working from the same edges layout the hole pattern again using the coordinates used previously. Again centre drill and drill 4 or 5 mm to break into the existing holes. Open up with a 7.5 mm drill and finally with an 8 mm drill. The existing holes act to guide the larger drills and reduce the wander. Entry and exit points are in the right place.

Yes it takes longer but it is guaranteed to give what is wanted.

Regards

Gray,

Obviously stiffness over such a long length relative to the diameter is one of the most important factors here along with correct technique. My suggestion therefore is to follow Jason’s advice and start with a stub drill and then switch to a quality ( dormer or similar ) crankshaft drill  ( as used to drill long small diameter oil  holes in crankshafts) . These drills have a thickened web in order to improve stiffness. Correct cutting speed and feed is also important in order to achieve good cutting and prevent rubbing , the latter of which results in excessive pressure on the drill point which in turn results in wandering. You should therefore drill not only as fast as you are comfortable with  , but more importantly maintain a feed of 0.004″ – 0.008″ per revolution and to do this with a repeated short cycle pecking action which will allow the essential lubricant ( paraffin in this case ) to flush away the resultant chips . I hope you will find this information of use .