# Forstner bit and aluminium



## starlingwood (22 Jun 2021)

I want to use a forstner bit on a 10mm aluminium plate. 

Is it OK to do? 

I once used a mitre saw blade to cut a thin strip of aluminium but the blade got knackered. Since bought a blade suited for aluminium. I can't find such a comparison in terms of forstner bits.


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## Sideways (22 Jun 2021)

Not an experiment that I would try and I have no qualms taking carbide tipped router cutters and saw blades to aluminium.
My concern would be the wide cutting edge going downward into the alloy. That's a big cutting edge and forstner bits are not usually razor sharp.
I imagine you would need a very strong bit and a very powerful rigid pillar drill to have any chance at all. The cut would be ugly, a cheap cast bit might snap under the downward pressure and the drill might stall.
When you try, let us know how it went 
Then just make a circular template and do it with a 1/4" handheld plunge router, 1mm at a time using an 8 or 10mm bottom cutter just keep routing the circular groove down and down until you are through.
Chips everywhere but if you have a steady hand and a decent router, you can bore through 20mm plate eventually.

And in case you also think of using a holesaw, don't bother. Total fail. They just won't clear the chips.


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## Jameshow (22 Jun 2021)

I brought a 50mm hole cutter off eBay to cut gas drain holes in steel floors in a camper conversion. It worked a treat. Subsequently used it to cut holes in Alu sheet for a coupe of router planes I made.

Not cheap at £10 but well worth it!









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Cheers James


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## Sideways (23 Jun 2021)

You'll have fun getting that through 10mm plate James, but they are fine for sheet thinner than the depth of their teeth.
More expensive carbide tipped versions will even go through stainless sheet.


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## Ollie78 (23 Jun 2021)

Aluminium can be described as sticky and grippy.
A forstner bit is the wrong thing for it.

Ollie


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## novocaine (23 Jun 2021)

I drill 10mm aluminium (i wont state the grade) with a hole saw in the piller drill fairly regularly. By a starrett or equivlant hole saw and have at it, nice slow speed and keep the hole clean by frequently lifting the cut and clearing the chip. 
Its as close to the right tool for the job as you can get.

Edit to add, starrett do a deep cut bi metal hole saw designed specifically for this sort of work.


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## Jameshow (23 Jun 2021)

Sideways said:


> You'll have fun getting that through 10mm plate James, but they are fine for sheet thinner than the depth of their teeth.
> More expensive carbide tipped versions will even go through stainless sheet.



Well I did X2.

Taking it slow and clearing the bit regularly as suggested and by cutting from both sides. 

Cheers James


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## TFrench (23 Jun 2021)

starlingwood said:


> I want to use a forstner bit on a 10mm aluminium plate.
> 
> Is it OK to do?
> 
> I once used a mitre saw blade to cut a thin strip of aluminium but the blade got knackered. Since bought a blade suited for aluminium. I can't find such a comparison in terms of forstner bits.


Are you trying to achieve a through hole or a flat bottomed pocket?


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## TRITON (23 Jun 2021)

I've used the router set at its slowest speed, and a carbide bit on alloy sheeting 15mm thick and that worked great. Chippings were more chipping than dust, and did chatter a bit but it wasnt too bad and worked well. 
Was making a router top and needed a hole for the cutter to go through.


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## --Tom-- (24 Jun 2021)

I think the outer lips will struggle to cut in deep enough to get the main cutting edge to engage. If the cutter engages and the drill is going slow enough it would work for a hole or two I’d imagine. 
Aluminium drills nicely enough with a touch of lube - either specific drilling compound or a bit of oil. Drilled 12mm holes through 32mm Ali bar the other day and a bit of ct90 on each hole helped keep the swarf coming easily.

Up at first we sizes I’d use a holesaw though, with a relief hole added on the perimeter for chip clearance


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## Fergie 307 (24 Jun 2021)

Nobody has mentioned the most important thing in cutting Aluminium. Because of its low melting point it will become plastic at the tool tip. You will then get a nib of metal forming on the cutting edge, effectively blunting the tool. You need a lubricant to stop this happening. Paraffin is ideal if you have any, if not WD40. Just squirt a bit on every 30 seconds or so. Your tool will last much better, and you will get a cleaner cut and finish.


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## --Tom-- (24 Jun 2021)

Agreed (lube=lubricant)


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## starlingwood (29 Jun 2021)

TFrench said:


> Are you trying to achieve a through hole or a flat bottomed pocket?



I'm after a flat bottomed pocket.

The intended use for this plate is a mounting plate for my Incra LS positioner onto an MFT base. So it will have 4No 6mm holes in the corners where I will use quad dogs and M6 screws to secure to the MFT bench. The LS positioner will be secured to the alumnium plate by 6no M6 T slot bolts which will be secured underneath with a washer and bolt, therefore I need 6no 6mm holes and then say 6mm deep flat bottomer pocket to take the nut and washer (the underside of the plate site flush with a bench so T slot and nuts need to be recessed. 

Ive knocked up a quick with MDF for time being and put the aluminium one on back burner but do need to do as the MDF one wont last.


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## starlingwood (29 Jun 2021)

Thank you everyone, for your advice, when I get round to it I will report back on successes and lessons learnt.


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## Spectric (29 Jun 2021)

starlingwood said:


> The intended use for this plate is a mounting plate for my Incra LS positioner onto an MFT base.


Have you looked at one of these, Incra


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## --Tom-- (29 Jun 2021)

If it were me a 6mm hss endmill would cost less than £5 and work better than a forstner bit


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## starlingwood (29 Jun 2021)

Spectric said:


> Have you looked at one of these, Incra



Thanks, yes I bought one but had to send it back, it was great. However, I set my MFT holes out so the router was in the middle of two rows rather than in line with a row as needed for the Benchdog plate, I was gutted.


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## TFrench (29 Jun 2021)

Definitely get an endmill, it'll work much better than a wood cutting bit


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## Devmeister (30 Jun 2021)

Carbide router bits cut aluminum. Most foster bits are has. An endmill will work. I just began restoration of a wood shaper that I pulled out of Boeing Vertol. They used it to make aluminum helicopter parts. So it's not uncommon to use many woodworking tools on aluminum. Just make sure your carbide tipped or your using HSS meant for metal like a steel endmill.


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## starlingwood (30 Jun 2021)

The flat bottom hole needs to be about 20mm diameter as it need room to get a socket in there to tighten the M6 nut.

I looked at end mill bits last night and 20mm is common but so is the shaft so it wone fit in my drill press chuck. As Devmeister says I will try carbide-tipped Forstner bit and take it slow and easy with lots of WD40 and expect to throw the bit away after its done, it will be a one-off job after all.


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## Devmeister (30 Jun 2021)

You may try using a piloted spotting mill cutter. I got one that spots a 7/8 in spotting operation using a 1/2 in shank. The pilot helps you line up the spot with the hole. What your doing here is a classic spotting operation.


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## hawkeyefxr (7 Jul 2021)

I have used Parafin on aluminium before, it's thin like water and oily at the same time. When you have a hole stablished keep it filled up with parafin.
You do not say what aluminium you are using soft or hard, like Dural. Soft with be the worst to machine.
When aluminium is machined the work area is drenched with a constant flow of coolant.


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## starlingwood (9 Jul 2021)

I did it. Happy with the result. The forstner bit is not in too bad condition actually, the point has gone but the cutting edges still sharp, not a great shame to replace anyway. 


















Here she is in her intended use.




And at rest for the next time.


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