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.

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

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

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

regards

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

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

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

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

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

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