boring 20 mm diameter hole in aluminium

[David Brown 9Participant @davidbrown9]

I am trying to cut a round 25 mm diameter hole in some 25 mm thick aluminium disks. I am using a Sieg mini mill. Using an ordinary holesaw is not working. Probably because I have already cut smaller diameter holes in the discs so the holesaw drill has nothing to drill through to guide the holesaw.

I bought a Armeg carbide tipped holesaw, which was not cheap. This successfully cut through one aluminium disc but no longer works to cut through the other 3 discs I have. I am using plenty of cutting fluid.

Would a diamond hole saw work any better? I have a boring bar, but do not want to use this as I know from previous experience this will take a very long time if I use it. As on the mini mill I can only take very smal cuts each time.

Perhaps my mini mill is simply not powerfull enough? I should soon have access to a large lathe, but I will still need to know what to use to cut the holes.

[David Brown 9Participant @davidbrown9]

[not done it yetParticipant @notdoneityet]

Fill the current hole and start again with the hole saw? They are nog that accurate/precise and the finish imay be somewhat agricultural.

No lathe? Simple enough (if it will swing the disc) to drill and bore?

With only a mill, you could invest in a boring head?

Is it 20mm (as in the thread title) or 25mm as in the text of the thread? What’s wrong with drilling it? May need to step-drill, if your drilling capacity is less, but doable.

[Adam MaraParticipant @adammara]

Have a look at blacksmiths drill bits and work your way up.

[peak4Participant @peak4]

I wonder if your carbide tipped saw has chipped due to chattering.

If I'm using a hole saw like that to enlarge an existing hole, I mount the workpiece on a spare piece of hardwood.

Use the drill bit in the holesaw to drill into the hardwood, through the hole in the centre of your metal plate.
Remove the drill from the holesaw, and replace with a suitable length of round bar, the same diameter; I keep a previously broken drill bit and just reverse it.
Oil the hole in the hardwood and use the holesaw to make the new larger hole, as the centre of the is now supported in the hardwood to act as a bearing.

p.s. a diamond saw would likely clog, just use the HSS one, lube with WD40 , paraffin, or similar, and keep clearing the chips out.

Bill

Edited By peak4 on 26/09/2023 23:25:45

[Mark Davison 1Participant @markdavison1]

25mm thick with a hole saw (carbide or hss) sounds like a recipe for disaster, it will just clog. You will have move success with a rotabroach style cutter. The chip evacuation is far better. Make your own arbor from a soft ended blank.

[David George 1Participant @davidgeorge1]

Drill through with a drill the same size as the pilot drill in the hole saw firstly. remove the drill from the holesaw and replace it with a plain piece of bar. Next drill halfway through, making sure it is clamped or held in a vice. Use paraffin as a cutting lubricant. turn over and drill from other side. This should work.

David

[JasonBModerator @jasonb]

Given that the mini-mill does not have a great deal of power and may struggle to drive the larger blacksmiths drills even if working up in a range of sizes or the thicker kerf of a rotabroach. I would suggest stitch drilling out most of the waste with a 7mm drill bit which will allow the motor to run at a comfortable speed and not overload it or the plastic gears. Then take it out to final size with your boring bar(head? )

This is a fairly easy layout, I would drill the holes 6mm and then open out to 7mm to reduce the risk of the holes wandering as they break into the adjacent hole. Holes at 90deg are spaced 8.5mm from ctr, the ones on the 45angles are 6mm x 6mm from ctr which keeps it simple if using hand dials

[OldironParticipant @oldiron]

Posted by Mark Davison 1 on 27/09/2023 07:02:25:

25mm thick with a hole saw (carbide or hss) sounds like a recipe for disaster, it will just clog. You will have move success with a rotabroach style cutter. The chip evacuation is far better. Make your own arbor from a soft ended blank.

I agree. A rotabroach is a much better method. Plenty of lube ie wd40 or parrafin.

regards

[noel shelleyParticipant @noelshelley55608]

Rotabroach ! When buying get a centre for it – it will make positioning easy. but allow for it to move up as you cut. Noel.

[SillyOldDufferModerator @sillyoldduffer]

Good question.

I think this is a case of asking too much of the mill and cutters. Maybe also the Aluminium is soft and gummy, tearing rather than cutting, causing the motor to work extra hard.

Hole-saws are best for thinnish sheet metal rather than going deep, because swarf builds up in the slot, and having to drive lots of teeth puts a heavy demand on the motor.

Taking the cut slowly (lowish feed-rate) with plenty of paraffin or WD-40 and frequently stopping to clear swarf might help. About 500 rpm by my rule of thumb, but experiment with faster and slower. The mill's motor probably delivers more power and torque at higher rpm, but what suits the motor may not suit the saw.

So I'd change technique. Instead of trying to saw a large hole in one go, I'd twist-drill as big as I can – 13mm because I'm too mean to buy a Blacksmith's Drill.

Then bore the 15mm hole out to whatever size is needed with a boring bar. For this a boring head is needed:

Though they cost a few bob, boring heads are a valuable accessory. Boring does both deep and wide holes. As there's only one cutter, which can be adjusted to take very find cuts, it doesn't overload the motor. The holes are also considerably more accurate and straighter than anything done with drill or hole-saw. Main disadvantage is the time taken. If you want to make big holes quickly, a large powerful machine is needed to drive the cutter. Otherwise we have to slow down!

Jason's method is a good compromise: chain drilling lots of small holes removes metal quickly and then the boring bar only has to tidy up, which doesn't take long. The mill works inside it's comfort zone throughout, and swarf collecting and mincing in the slot isn't a problem.

Dave

[David Brown 9Participant @davidbrown9]

I will give a 24 mm rotabroach a try. I have a R8 mini mill. What adapter would I need so that the Rotaborach would fit it? The one I have found on Amzon says that it needs a pilot pin, SRA50 ejector pin. What is this used for?

I have a boring head and will use this if necessary. But I lack the patience to use a boring head on 3 holes because of the time it takes, unless there is no alternative. Plus, I have had problems making the holes too big with a boring head.

[Mike HurleyParticipant @mikehurley60381]

How come you get oversized holes with a boring head? These are very accurate normally, so might suggest you have another issue (wobble due to poor location / seating or even a small spindle issue ?) or you are trying to take too heavy a cut each time and you get springing of the cutter?

I admit they can be tedious to use manually having to keep stopping – adjusting – restarting, but the trick is to allow as little material as possible to finish the hole. As you already have one, thats no additional expense, if you haven't got blacksmiths drills then I personally would follow Jason's suggestion of drilling out a ring and even tidying up the remaining 'peaks' with a much smaller dia drill or even manually with a coping saw – it's only aluminium!. Finish with the boring head

I've recently completed something similar (25mm dia) in Mild steel using this method and it was straightforward (I used a coarse file rather than a coping saw though as it was steel!)

Mike

 

Edited By Mike Hurley on 28/09/2023 09:40:46

[noel shelleyParticipant @noelshelley55608]

The rotabroach has a 3/4" parallel shank with 2 flats at 90* apart.. The pin A) enables the cutter to be easily centred and also pushes out the core on breakthrough. Run it as slow as you can and lub. Noel.

[JasonBModerator @jasonb]

You are never going to do it quickly on a small mini mill due to the size of cut it can comfortably take. Using a large cutter that puts a lot of load on the slow running motor won't be quick in the long run when you have to spend time repairing the machine but I can see Ketan and the boys rubbing their hands now at the prospect of more sales of spare parts

If you can't size the hole then you have problems somewhere either rushing with big cuts that deflect things or tool geometry is rubbing somewhere. Boring head is more accurate and will give a better finish than any holesaw, rorabroach, blacksmiths drill, etc

You should be able to use the quill lever to feed the boring head fairly quickly, for best results use the fine feed for the finish pass

Edited By JasonB on 28/09/2023 09:55:25

[donkeyParticipant @donkey]

why not clamp down on a thick piece of ply then the drill will keep the saw true

bri

[Pete WhiteParticipant @petewhite15172]

Posted by David Brown 9 on 28/09/2023 09:08:32:

I will give a 24 mm rotabroach a try. I have a R8 mini mill. What adapter would I need so that the Rotaborach would fit it? The one I have found on Amzon says that it needs a pilot pin, SRA50 ejector pin. What is this used for?

I have a boring head and will use this if necessary. But I lack the patience to use a boring head on 3 holes because of the time it takes, unless there is no alternative. Plus, I have had problems making the holes too big with a boring head.

 

Thats good David, you can ask for advice but you don't have to take it, good luck.

Pete

I don't want to know, but do you know? is it 20 or25 mm dia holes that you want to produce.

Edited By Pete White on 28/09/2023 11:21:51

[Paul Mills 4Participant @paulmills4]

The thread on the arbor of my holesaw can just accommodate two hole saws, front one only has a a couple of threads, if for any reason I have to enlarge a hole using made a hole saw I simply screw the smaller holesaw on the front and use this as a guide, rather like the pilot drill , but as the smaller holesaw is not actually cutting there is no problem with it either being difficult to remove or stripping the threads.

Have to admit that I have only ever done this in thin materials

[Ady1Participant @ady1]

Once you get close with aluminium you can do a high speed single tooth whirry job for finishing, I've done up to 45mm in steel

Thing is a hobby lathe is much better for boring bigger holes, a hobby mill will struggle unless you get it just right

[MadMikeParticipant @madmike]

Do you have, or have access to a lathe? That would make he (w)hole operation very simple.

F you are close to he East Midlands the I could do it for you. There are possibly others on here who would help.

[noel shelleyParticipant @noelshelley55608]

If your near N Norfolk I will soon put a 24 or 25mm rotabroach through it for you. Noel.

[PeteParticipant @pete41194]

I've bored accurate holes a bit larger than 25 mm with a tiny Emco round column C5 sized mill in steel plate and without a boring head since I didn't have one at the time. I just used a left hand lathe tool with a braised carbide tip that was specially sharpened for the correct clearances in one of the usual fly cutter tool holders, then set my offsets to the increasing sizes with a dial indicator against the end of the tool tip as it was moved in or out by hand. With even a mini mill and a boring head through aluminum, it should be much easier and faster. Mark out and chain drill with the holes almost touching a bit under the finished bore size, knock the slug of waste out ,and then bore to finished size. Rotabroaches, hole saws etc are costly and can only do a single fixed size. And with thicker plate, I've never found any hole saw to work at all well since there not really designed for that type of use. With a small 2" / 50 mm boring head and the proper range of boring bars, you can precision bore any hole size from about .080" on up to a couple of inches even with a mini mill if your careful. With a bit more power and enough torque at lower rpms, the cross hole most boring heads have can be used for even up to 4" – 5" diameter holes in a pinch. Although the depths of cut does have to be reduced at those diameters. I have a much larger mill, industrial level boring heads and bars now, But for anything quite a bit larger than my drills can provide and to speed up the job, I'd still use that chain drilling method. For shops with manual machine tools, it's still a commonly used industrial technique for large precision holes in plate today, and I've seen it done many times. Manual machining is about precision, for larger areas of bulk metal removal prior to that machining, drills and saw cutting will beat any end mill or boring bar out there both in more efficient metal removal rates and tooling costs.

Boring to an accurate size is fairly simple, bore until the hole fully cleans up but still under size, measure and for example lets say it was .030" under. I'd set a .005" offset on the head, bore through and accurately remeasure to see what the tool did for size. Lets say it actually took only .009". I'd then off set the head to take the next cut at .0055" and bore at that setting. Remeasure and lets the say it did take .010". Offset the head a further .0065" and take the last finish cut. Measuring and offsetting the head over or even under, compensates for any tool deflection at a known depth of cut in that particular metal type so it's closely repeatable for the next cut, or even poor inaccurate feed screws and dials. On critical sized holes and even though the feed screws and dials are very good on my BH's, I still prefer setting the last few cuts with an indicator. It's less the about the tool and more about the technique and methods used since tool and bar deflection are always present. Boring with a lathe still requires that measuring and compensation as well. And for high accuracy bearing fits using a lathe or mill, I don't know how else you would guarantee getting any bored hole to a known and close tolerance size without doing so.

[David Brown 9Participant @davidbrown9]

I will try drilling some holes and then using a boring head. I may have time tomorrow, otherwise I will give it a go next weekend. Thanks for all the advice. Hopefully it will save me from spending more money and possibly still not succeeding.

I will should be able to access a lathe in a few weeks. So if I get totally stuck I will give that a try. I live on the London/Essex borders

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