How can I keep a deeply drilled hole straight?

I'm making the steam valve for a model engine and have to drill a 6mm diameter hole through 44mm of mild steel. It needs to be decently straight because the valve piston travels in it and I don't want leaks or binding.

I've had trouble drilling deep holes because they wander. The hole starts dead centre on the axis of the work but on a bad day the drill comes out the other side as much as 2mm off course.

What's the best way of drilling a deep hole and keeping it straight?

Thanks,

Dave

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

Can't you bore it? Or at least bore as far as you can

regards Martin

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.

The the most important operation is to drill is a series of pecking actions and withdraw the drill very frequently to ensure there is as little accumulation of chips in the drill flutes as possible..

For fast drill removal release the tailstock clamp on the bed and pull the unit away from the job, much faster than winding off the feed each time. Peck some more and repeat until complete. Use coolant on the drill as well of course.

The ratio of more than 7:1 is high, but I have had success with that approach

Regards Brian.

.

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

As background reading, Machinery's Reference Series #25, 'Deep Hole Drilling' is a pretty good start, available here: **LINK**

Another thought is 'Can I change the design so it doesn't matter if the drill wanders?'. You will have to ream to get the hole circular, but it may be possible that if the centre at one end is correct, then it may be possible to make it so the valve does not have to be perpendicular to the face from which you drill.

Regards,

Richard.

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

When Jason says evenly ground, that means both lips the same length and angle.

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.

Thanks chaps, that's given me plenty to go on. I feel an urgent need to spend money on a variety of new drills!

I can do the job on either a lathe or a milling machine. Is there any advantage to using one rather than the other? I have a cheapo drill press too but I don't rate it highly.

Dave

Hi Dave,

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

One counsel of perfection is to have a rotating tailstock drill that operates at a slightly different speed to the lathe. That avoids any repeating resonances and any tendency to wander is constantly being offset by the rotation.

Just finished reading the link provided by Richard - very interesting. Very usefully it explains why it's best to drill deep holes with a lathe (spinning the work around an advancing drill) rather than with a drill press or milling machine.

Lot's of other good stuff in it, especially if you make 12" Naval Guns for Battleships!

Dave

On the lathe you should verify that the tailstock spindle is in line with the lathe spindle. I chuck an indicator in the headstock and sweep the tailstock spindle. Front to back adjustment is provided for in most tailstocks. If the tailstock spindle is low it will require shimming. You also should verify the lathe bed is not twisted. The intent is to make sure the tailstock spindle is concentric with the headstock spindle no matter where the tailstock is on the bed.

On the mill it is not a problem because the spindle remains in a fixed position (X & Y) with respect to the work. You can drill the hole undersized, then ream with a 4 flute end mill to staighten the hole, then follow with the full sized drill. I think this is as accurate as boring to size.

As others have said, the condition of the drill is of utmost importance.

For the initial hole I prefer a split point or 4 facet grind. There is less tendency to wander than a conventional grind.

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

Hmm, ask a simple question and it turns out complicated. For instance I hadn't twigged that the tailstock has to be accurately aligned. I wonder what I'll find when I measure mine! This job is harder than it seems, at least if accuracy matters. Probably a good thing it will be next week before I can cut metal - the delay will give me time to absorb the advice!

Many thanks,

Dave

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

First use a level to be sure the lathe bed is not twisted.

Hold a dial indicator in the lathe spindle. Sweep the bore of the tailstock quill to see if it is true to the lathe spindle. Most lathes have grub screws for adjusting front to back. It is also possible the tailstock quill is a bit low. That will require shims to correct. It is all a bit of a bother but essential if you want true holes.

I also suggest split point or 4 facet drills. They require less axial force to cut and are less likely to drift due to material inconsistencies. Some manufacturers make a heavy duty drill with a thicker web than normal. They are stiffer and less likely to deflect. They need a split point or 4 facet grind due to the thicker web.

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...