Milling ma­chines — part three


The Shed - - Contents - By Peter Wood­ford Pho­to­graphs: Ge­off Os­borne

Re­mov­ing ma­te­rial — which cut­ters and what ma­te­rial?

The rea­son we bought a milling ma­chine was to make, re­pair, or mod­ify com­po­nents and to do this we have to re­move ma­te­rial. Hopefully from the right places. We have a va­ri­ety of cut­ters avail­able to us to ac­com­plish this in­clud­ing:

• drills

• ream­ers

• taps

• coun­ter­sinks

• end mills

• slot drills

• sit­ting saws

— to name a few.

Once your com­po­nent is se­curely clamped (see Part Two of this se­ries, Show­ing Re­straint, in Is­sue No. 83 of The Shed), en­sure that the cut­ting tool is held cor­rectly in its chuck or col­let holder, keep­ing tool over­hang or stick­out to a min­i­mum. Be­ing rigid and sta­ble ap­plies as much to tool­ing as it does to the work­piece. Drill chucks are not de­signed to take side loads in­duced by milling and are ex­clu­sively for drills. Milling cut­ters should be held in a col­let chuck or other suitable holder.

Cut­ting tools

There are three kinds of ma­te­rial that the most com­mon cut­ting tools are made from for the home work­shop: high-carbon steel, high-speed steel, and tung­sten car­bide.

High-carbon steel (HCS)

As the name sug­gests, cut­ting tools made of this steel have a high carbon con­tent, typ­i­cally be­tween a min­i­mum of 0.8 per cent and a max­i­mum of 1.2 per cent. This amount of carbon al­lows heat treat­ment to give a suf­fi­cient de­gree of hard­ness, which will pro­duce a good cut­ting edge. It means that cut­ting tools can be pro­duced from gauge plate or sil­ver steel and then hard­ened and suit­ably tem­pered. Typ­i­cally cheaper drill sets are made with HCS.

Note that over­heat­ing of th­ese tools will cause fur­ther tem­per­ing of the tool and a loss of hard­ness.

High-speed steel (HSS)

This is so-called be­cause cut­ters made from this ma­te­rial could be run much faster than the carbon steel cut­ters pre­vail­ing be­fore the devel­op­ment of HSS. For gen­eral use in the home work­shop, HSS is the most use­ful and cost-ef­fec­tive cut­ting-tool ma­te­rial. The main al­loy added to HSS is tung­sten. But HSS also has an amount of chromium, with smaller amounts of molyb­de­num, vana­dium, and cobalt. The ad­di­tion of tung­sten alone does not pro­vide the high­tem­per­a­ture char­ac­ter­is­tics. A bal­ance of all the in­gre­di­ents al­lows for very high tool tem­per­a­tures with­out the cut­ting edge los­ing any hard­ness. Typ­i­cally HSS drill sets are more ex­pen­sive and end mills and slot drills will be made of HSS. As with all things in life, you gen­er­ally get what you pay for. HSS cut­ting tools come in many grades and es­tab­lished brand names will be bet­ter qual­ity than an un­branded cut­ter.

Tung­sten car­bide

Tung­sten car­bide cut­ting tools (also known gen­er­ally just as car­bide cut­ting tools) are made of solid tung­sten car­bide or, in larger cut­ters, re­place­able tung­sten car­bide in­serts with other trace el­e­ments, and some­times a wear-re­sis­tant coat­ing on the sur­face.

They are pro­duced by a process called ‘sin­ter­ing’, where the tung­sten car­bide is used as a pow­der that is moulded un­der high pres­sure and tem­per­a­ture. Cobalt is in­cluded in the mix to help the par­ti­cles ‘stick’ to­gether.

Th­ese very hard cut­ters are able to deal with very hard ma­te­ri­als and ex­tremely high tem­per­a­tures, al­though they are

more likely to chip if not held rigidly.

Tung­sten car­bide comes in many dif­fer­ent grades to suit ma­te­ri­als to be ma­chined so the cost of keep­ing all the dif­fer­ent grades makes this very ex­pen­sive for the garage engineer.

All cut­ting tools, no mat­ter their shape, will have clear­ance an­gles and a cut­ting an­gle called ‘rake’ or ‘helix’ to shear (cut) the ma­te­rial be­ing ma­chined. This an­gle varies de­pend­ing on the ma­te­rial be­ing ma­chined. In pro­duc­tion ma­chin­ing, cut­ting tools spe­cific for the ma­te­rial will be used. For garage en­gi­neers, gen­er­alpur­pose cut­ters and drills will get us by. If you are mak­ing hard work of it next time you are drilling a hole, have a good look at the cut­ting edges and check that you have a clear­ance an­gle be­hind the cut­ting edge.

Rake an­gle

The rake an­gle of the cut­ting tool is of­ten de­ter­mined by the ma­te­rial be­ing ma­chined or the ma­te­rial the cut­ting tool is made from.

Pos­i­tive rake is the most com­mon of the rakes you will find. Most HCS and HSS drills will be pos­i­tive rake, as will HSS and solid car­bide end mills and slot drills.

Some ma­te­ri­als ma­chine well with neu­tral rake — the most com­mon in the home work­shop is brass.

The wide­spread use of car­bide tool­ing and the strength of car­bide in com­pres­sion make neg­a­tive-rake tool­ing within in­dus­try and its high ma­te­rial-re­moval rates very de­sir­able. Rigid ma­chin­ery, tool hold­ing, and high spin­dle power are re­quired to get the best out of car­bide tool­ing. A milling cut­ter with gen­eral-pur­pose car­bide in­serts can still be used in the garage work­shop to take on hard or hard­ened ma­te­ri­als. Just be aware that it will be work­ing your spin­dle bear­ings hard.

Neg­a­tive-rake car­bide tool­ing is very com­mon in in­dus­try but its use in home works is lim­ited be­cause of the higher power needed to drive neg­a­tive-rake tool­ing and the rigid­ity re­quired in the milling ma­chine.

In­dus­trial cut­ting tools can also be made from Stel­lite, ceram­ics, and in­dus­trial di­a­monds, among other ma­te­ri­als.

Cut­ting speed

Now a lit­tle bit of maths comes in when cal­cu­lat­ing the spin­dle speed for the drill or cut­ter. The spin­dle speed is de­pen­dent on both the ma­te­rial be­ing ma­chined and the cut­ting-tool ma­te­rial. This is quoted in many en­gi­neer­ing hand­books as sur­face speed in me­tres per minute (also writ­ten as me­tres/min or m/min), or in older pub­li­ca­tions as feet per minute. We will con­cen­trate on me­tres per minute. Aca­demics do not like the use of the word ‘per’, but it is very com­mon in the every­day lan­guage used here.

Cut­ting speed is calculated as the dis­tance the cut­ting edge of your

Fly cut­ter at speed

Milling a slot with a three-flute end mill. Work­piece is held rigid within vice jaws

Note that cut­ter on fly cut­ter juts out from the body

Ex­am­ples of cut­ters. (Top row) Sec­ond from left: fly cut­ter; third and fourth from left: ball-nose cut­ters; third and fourth from right: ream­ers; the cir­cu­lar cut­ters are side-and-face cut­ter (top) and slit­ting saw (be­low). (Bot­tom row) Far left: long-se­ries end mill; third from left: rip­per end mill for high-vol­ume ma­te­ri­als re­moval; fifth from right: re­flex, or cor­ner ra­dius, cut­ter.

Cut­ters (left to right): four-flute cen­tre­cut­ting end mill, four-flute end mill, three-flute cen­tre-cut­ting end mill, and twoflute cen­tre-cut­ting slot drill (slot drills by def­i­ni­tion are cen­tre cut­ting)

A cen­tre-cut­ting end mill plunges into an alu­minium block

Clogged end mill

A cen­tre-cut­ting mill pro­duces a clean cut hole in alu­minium block (left), while an ordinary end mill tears and de­forms ma­te­rial (right)

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