Milling cutters choice

Mike jumping in if Dave will allow me.

Brass does like a sharp "edge" to the tool and the stoning action is not done to remove the sharpness of the cutting edge by rounding it but to alter the geometry do that the edge is more vertical preventing the drill bit from grabbing.

If you imagine the drill laid on it's side and compared to a lathe tool the helix will be like a lathe tool with a lot of top rake. Stoning the edge will modify it to be like a lathe tool with zero top rake. Where the front of the tool meets the top it still needs to be crisp and sharp.

Personally I never bother stoning a drill for brass or buying (new) slow helix ones but I don't use blunt drills either.

Edited By JasonB on 08/02/2021 12:02:47

What I would challenge Dave about is the coating making the surface of the tool smoother. In the case of aluminium specific cutters and even non specific an uncoated cutter is preferable as the smoother surface of the bare metal caused less problems with material build up on the too.

Wrong!

Coatings are thin, on the order of a few microns to a few tens of microns. They alter the cutter dimension as they are an extra layer that is deposited onto the cutter by vapour deposition.

Regarding drilling copper alloys stoning the edge of the drill has never worked for me. All it does is ruin the drill. I prefer to use slow helix drills. I have a small collection of common imperial sizes, which is sufficient for me. If using an ordinary drill to enlarge a hole by a few thou then snatching isn't going to be a problem.

Andrew

A couple of thoughts:

The flutes of a drill bit were designed following years of experience, and as steel was the material most often presenting difficulties, they were designed so that the stiff swarf would be removed automatically in a spiral. This meant that the rake was rather sharper than perhaps ideal, but it worked well enough to be accepted everywhere. In brass, the swarf is a quite different problem - as most brass tends to be leaded (etc) and breaks into flakes or splinters. So, reducing the rake for drilling brass makes sense, as it gives the reduction in grab without adding swarf problems. Cutters and taps for steel tend to have spiral flutes for similar reasons - which don't apply to tools for brass.

Cutters made for aluminium alloys tend to have even more rake, and this also makes sense for two reasons. First, the material is easier to cut and so the strength and rigidity of the cutting edge is less critical, and second, the sliding action of the metal over the tool face tends to shear off any build-up of semi-welded alloy. The fact that a smooth surface offers less 'grip' to adhering build-up is a further reason for a distinction in cutters for Al.

Or am I guessing way off the mark (again) ?

cheers, Tim

I think the high helix angles on Aluminium specific cutters also help with swarf clearance. Due to the fact you can run faster then you are making more chips in a given time assuming chip load stays the same so they need to be cleared faster which the helix helps with particularly when cutting pockets, slots or into tight internal corners.

Also with the method of using a fairly low approach Ae (sideways depth of cut) and high perpendicular cuts Ap (vertical depth of cut) the higher helix angle ensures at least one flute is always in contact with the work surface resulting in smoother running.

The also happen to work very well on other non ferrous metals like brass and bronze so almost the opposite of the slow helix drills!

at my first job ,where cast iron and brass were the main materials plus some ali and nickel silver,usual practice was to grind the rake off the cutting edge of a drill known as backing off by all the workers when doing batch work ,for a few off we just used the drill as it was,also no one bothered to back off drills under a 1/4 inch dia. The backing off was always done on a grinding wheel smaller sizes would be done freehand on a T & C grinder using a convenient thin wheel, A stone was never used, all too likely to get a polished but rounded off edge,grinding produced a sharper edge . On cast iron a 3/8 whit tapping drill would last for at least 300/400 hundred holes.On brass,I have seen a casting thrown over 40 feet when it lifted out the jig as the operater failed to back off the drill,of course someone had to chalk a cross on the floor where it landed.Bronze needs some back off as brass does, phosphor bronze has similar characteristics to steel,to cut it needs rake ,and will spral in violently when opening up a smaller drilled hole,its best drilled in one go ,or the work clamped to the drill table and a firm grip on the drill lever,avoid a vice if possible the work can be lifted from the vice jaws or the vice and the work can be lifted and spun. When drilling phosphor bronze in one go there is tendency for the drill to bind in the hole,one way to cure this is to slightly drill the hole off centre,ok it does produce a slightly larger hole but thats better than a broken drill. Neer saw anyone back off a milling cutter ,as milling work is held securely and there is more control on the cutting spindle,one mod to conventional cutters when used on cast iron (Clarkson Deadlock cutters or shell mills) was grind off the sharp point where the cutting helix met the face of the cutter ,about 3/16 would be ground back at 45 degrees on each tooth to give a better finishing cut and a lot longer tool life.

So that's why one sees a lot of Dedlock cutters ground thus on Ebay.

Andrew

Have to say the knowledge and response on this forum is very impressive indeed, Model engineering is totally out of my comfort zone but with a little more time on my hands I’m determined to tackle the steep learning curves and my goodness it’s a steep curve!
If only all forums were this well run and informative

Thanks all.

I usually purchase my cutters from an industrial supplier rather a dealer of hobby supplies. For steel I prefer a TiN coated HSS mill. For aluminum it is uncoated with polished flutes. These are often listed as aluminum specific. Avoid any coating that contains aluminum like TiAlN. Aluminum will bond to it. I don't do much brass but when I do I start with a new HSS uncoated end mill. I keep my brass tools segregated so they are only used on brass.

I have no objection to the more advanced coatings for end mills except cost. The work I do doesn't warrant it.

I do use a few uncoated carbide end mills, Mostly when I am in a hurry to get rid of a lot of metal or for long stick-outs.

I have a few insert mills, a face mill with 45* cutting angle and SEHT inserts, usually the uncoated polished version for aluminum, A 90*cutting angle face mill with APKT inserts, uncoated and polished, a milling tool from Banggood that uses APMT pr APKT inserts. I think it is a about 20mm dia. With the shank shortened it makes a superb milling cutter with a 90* approach angle.

Edited By JasonB on 09/02/2021 20:16:53