I'm far from an expert but I've always drilled holes undersized then bored to correct diameter although deepest holes I've drilled/bored have been 50mm in depth, reamers are another possibility.

Edited By Curtis Rutter on 14/09/2017 16:04:39

Drill undersize, bore and ream is the answer. Another way is to start the hole with the drill and then use a D bit. It is a pain, as you need to clear swarf far more frequently than with a drill, but it will get you a straight hole.

Andrew.

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I will take a stab at this.

Presumably it needs to have a decent finish to it as it has a piston of a type sliding. Sounds like a vital part so a bit of time invested will not be an issue.

If it was me I would peck a 4mm drill through. withdrawing, cleaning and lubricating often.

I would then go through as far as possible gently with something like a 3 fluted milling bit of about 5.5mm. You may need a long series one. This 'should' run a true hole. – even better would be one of about 5.8mm if you can get one.

If a 5.8mm milling bit is not available enlarge the hole to 5.8 ish with a drill bit. If the milling bit made a true hole this one will follow it. – Drill bits need to be of good quality, ground true and of course sharp.

Finally run through again but this time with a 6mm reamer and lots of lube to give a nice finish to the bore.

Nick

Edited By Nick_G on 14/09/2017 16:32:51

Are you drilling with a drill press, lathe or mill?

Start with a decent spot drill hole or ctr drill if you must. Then a slightly smaller stub drill, 5.5mm would be my closest and drill as deep as that will go then change to regular jobber drill and if even longer than you need change to an long or extra long series drill. I don't really like using small pilots as they are more likely to wander so that's why I would go with 5.5mm rather than say 4.0mm

Then go to say 5.8mm and finally ream.

In all cases drills must be sharp and evenly ground, clear swarf regularly and use a cutting fluid

That's right. And it's important not just to withdraw often to clear chips, but also feed gently to allow both lips to cut. Forcing the drill through hard will accentuate any effect from uneven length lips, and it's pretty hard in 6mm. to get both identical to very close tolerances.

For the hole to wander, the drill body has to flex, so the start hole from Jason's short, stiff stub drill – if you've got one – will also help a lot by giving straight support to some of the full-length drill when it's going on into the deeper reaches of the 'ole.

Hi Dave,

Would the lathe not be a better bet all round? drill presses are not great for that kind of job.

First a few general points:

1. Forget the drill press

2. If the lathe is used the tailstock needs to be precisely aligned with the headstock, if the mill is used the head needs to be properly trammed. If either machine is mis-aligned all bets are off

3. Use quality drills

4. A reamer is used to give a good finish and accurate size, it will do nothing to straighten a wonky hole

5. Use proper material which is uniform, hard spots or inconsistent material will push even a quality tool off centre

Which machine is used depends to some extent on how easy the part is to hold. The mill is probably simpler and quicker. On the mill I'd use a 6mm spot drill to make a deep centre. Then drill through 5.8mm, no point in faffing around with smaller drills. They're less rigid and more likely to wander off. My standard metric drills are Dormer A002 four-facet ground drills. Then I'd ream 6mm, half the speed, twice the feed. I can't see why tiptoing around with drill feeds would have any effect on the propensity to wander. It's surprising just how hard you can push even quite small drills. At 5.8mm I'd be aiming for several thou per rev. In this recent exercise I drilled a 1.3mm hole through ½" (12.7mm) of steel, so about 10D:

Turning the pulley through 180° and the drill still went through. That means one, the hole is centred on the shaft (hooray for the DRO) and two the drill didn't wander.

Using the lathe opens up more options. In the simplest method I would use the same as the mill; spot drill, drill 5.8mm and ream 6mm. The next stage of paranoia would be to spot drill, drill 5.8mm with a stub drill, or even a carbide stub drill for extra rigidity. Then drill 5.8mm with a standard drill and ream. The ultimate method is to spot drill and then drill undersize with a stub drill for 10mm or so. Then bore to exactly 5.8mm, then drill the remainder of the hole 5.8mm and finally ream.

Of course for larger holes one would drill and bore. But a boring bar for a 44mm deep, 6mm diameter, hole would be one special (aka expensive) tool.

Alternatively Nick's idea of a three flute centre cutting slot drill works well. It has to be three flute though. In my experience two flute slot drills create a very accurate slot when milling, but are all over the place when using them as a 'drill'.

So accurately aligned machine tools and quality tooling, what could be simpler?

Andrew

At the risk of going a bit OTT for a hobby forum. As above but solid carbide tools are worth considering, and with respect to Andrew's posts and in particular the 4th point. I wonder if this is less certain when using a solid carbide reamer. I am increasingly of a mind that these are somewhat akin to a boring bar. Which means the accurate alignment of e.g. the tailstock would then be even more critical.

Steve

what are you drilling through? drill a hole through an oversize lump of steel, then machine the rest so the hole is in the middle…

Or if you have enough space put a bigger hole in the part and sleeve it.

regards Martin

It is embarrassing that I posted twice. I am getting old and probably a bit soft in the head. At least I gave the same advice twice. I think my affliction is called CRS: can't remember s….