Endmill vs. Slotdrill

[Nyrup BoeghParticipant @nyrupboegh48678]

Hi

I have just bought a small mill (Sieg X2+)

I have bought several books from the Special Interest Model Books series. They are verry good. Amongst the books i have bought is several by Harrold Hall. I am reading "Milling – a complete course" and "The Milling Machine". I have just made my first clamp pr. the instructions in the milling cource book.

I have 3 endmills: 3mm, 5mm and a 10mm. In the clamp I have to make a slot 8mm wide. I made it by drilling an 8mm hole in each end of the slot and milled out the in between using the 5mm and gradually widening the slot to 8mm. I am thinking of buying an 8mm endmill in a lokal shop in Brovst here in Denmark. The endmills I allready own i bought at Arceurotrade. But I am not shure which one to choose. The lokal shop has both 2 and 3 and 4 flute cutters. What is the benefit from each? What is the difference in practical use? Why are endmills cutting oversize and slotdrills not? How do they differ?

I have searched the forum and found this thread:

http://www.model-engineer.co.uk/forums/postings.asp?th=45899

but I do not find the answer to my question in the thread – at least I am not shure the answer is in the thread.

Does someone have a link to an explanation on the net or in MEW (I am a digital subscriber to MEW).

Kind regard, Nyrup.

[Nyrup BoeghParticipant @nyrupboegh48678]

[John CoatesParticipant @johncoates48577]

You need a 2 flute so it can plunge to the depth of the slot then cut the slot across. Normal 3 and 4 flute endmills are relieved in their centre and cannot plunge cut as the relieved centre doesn't cut any metal away. Some 3 flutes have a single long cutter to replicate this but a 2 flute is the normal slot drill

This is covered in Harold's book (WPS 35) on page 13

Welcome! Everyone on here has been really helpful as I have progressed from newbie to where I am now (still relatively newbie)

John

Edited By John Coates on 23/05/2012 08:25:56

[John Stevenson 1Participant @johnstevenson1]

Nyrup,

This is copied from a previous reply I made on another forum so ignore any names in it.

Let me try to expain about end mills and slot drills. Many people have their own description of these but I'm afraid no one is right any longer – even me !!!
First off lets go back in history and see what we had. There were 4 flute end mills and 2 flute end mills. Some could plunge some couldn't. Confused? well you will be.
The two flute ones were called slot drills in the UK and were ground with one cutting edge over centre so they can plunge cut vertically down and then traverse, hence the name. These are the ones on the right in JCHannum's picture.
The 4 flute end mills usually had a hole in the centre for clearance when grinding and so can only cut on the sides of the tool. Trying to plunge with one of these will leave a central pip that will break the cutter when you try to traverse.
Now when we got towards the end of the 20th century things started changing. We had better methods of producing cutters with fancy 5 and 6 axis CNC grinders. We started to see 3 flute cutters getting popular as a compromise between rapid metal removal and chip clearance. We also started to get the 3 and 4 flute cutters with one lip ground over centre like the two flute in the picture. This means that these can also plunge.
To cap it all the manufacturers put paid to any standards by calling their cutters all sorts of different names. Where a 4 flute cutter was called an end mill it can now be centre cutting and be called a 4 flute slot drill.
If you look in any cutter catalogue nowdays you will find that they call a certain type of cutter by one name on one page and another manufacturer calls it by a different name on another page! The manufacturers have moved the goalposts. The cutter descriptions we all grew up with in Chapmans and Moultreich <sp> has gone out the window with the advent of grinding techniques.
I took a 20mm solid carbide 4 flute end mill [ non centre cutting ] in for regrind last month and when it came back it was 4 flute over centre slot cutting cutter. What do I call this now?
The main thing to look for when buying cutters is the number of flutes you require and whether they are centre cutting or not.
I can see the time when the old 4 flute centre pipped type will become obsolete as most cutters are ground up from blank material nowdays.

[Ady1Participant @ady1]

The big advantage of a 2 flute I found is that you can resharpen it quite easily on a standard grinding wheel

[Clive FosterParticipant @clivefoster55965]

As John Stevenson says the first big distinction is between centre cutting and non-centre cutting which defines whether or not the cutter can plunge straight in like a drill. The second distinction is whether it will cut a slot true to width or not. A two flute slot drill will, a four flute end mill cuts oversize, a three flute centre cutting slot drill/end mill (as popularised by the Clarkson FC3 "throwaway" cutters) should cut to size and a five flute cutter might. Essentially cutting a slot true to size requires balanced cutting loads. Any unbalance causes the cutter to walk or oscillate from side to side during the process nibbling the slot wider. (Deep thinking about this will make your head hurt!)

Best answer with a machine like yours, which isn't really able to exploit the more sophisticated cutters where confusion really reigns, is to go all old fashioned and use two flute slot drills for slots and four flute end mills to cut along the sides of steps. Pay particular attention to the depth of cut both vertically and sideways when using end mills as small machines like yours aren't really stiff enough for book values on anything other than quite small cutters. As Ady1 says a two flute slot drill can be sharpened with unsophisticated equipment and will do end-mill duties just fine, albeit more slowly due to having fewer cutting edges and being a little less stiff. My view is "really sharp" beats "theoretically better" any day.

If bargain packs of three flute throwaways can be had its worth getting a holder, usually 6 mm or 1/4" diameter, as the attrition rate of small cutters can be high. Also worth picking up a high quality centre cutting carbide one (or two) should a bargain come your way. Reserve for get out of jail purposes like chomping out broken taps. Be lucky to manage more than a couple or three taps hence need for bargain price but when the alternative is starting over …..

Clive

[David LittlewoodParticipant @davidlittlewood51847]

The reason 4 flute cutters have a tendency to cut oversize is that when one tooth is at the front, cutting sideways, there is a side thrust which will deflect the cutter to the left (looking from behind). A 4 flute cutter will have a tooth at the left, which will cut into the side wall; a 2 flute cutter has no tooth there, so it won't. The "wobble" will also depend on the stiffness of the tool and the depth of cut.

The amount of the deflection will of course vary according to how hard the cutter is being driven forward (the feed rate) and this will vary – even under power feed, it is zero at start and stop. You may get away with it if the slot is open at each end, and your feed rate (and depth of cut) is exactly consistent. Otherwise, not.

I should give credit to G H Thomas for explaining this in one of his books.

David

Edited By David Littlewood on 23/05/2012 11:27:52

[HowiParticipant @howi]

Thanks for the info peeps, not into milling yet, but has made things a little clearer for me.

much appreciated

[Nyrup BoeghParticipant @nyrupboegh48678]

Thank you to all for your kind answers.

It is most helpfull. I think I will by a 2 flute endmill/slotdrill and try that. I do not take verry deep cut's on my small mill neither on my small lathe (Sieg C3). So far I have not had any problems and get the result I expected. But – then – the precision demands have not been wild in the small items I have made till now.

Kind regard, Nyrup.

[BazyleParticipant @bazyle]

When a 4 flute cutter tooth is just starting to cut in a slot hole advancing sideways there is another tooth 90 degrees in advance that is flat against the metal opposing the advance into the work and hence needing more force to push the cutter along. A two flute cutter has no such problem so is able to advance more easily when cutting over it's full width enabling deeper cuts with less stress.

A 4 flute is meant to be used to be used along the side or the work so that is is only cutting over less than half its width but the extra teeth mean faster removal and smoother finish.

3 flutes are a compromise for both tasks.

[David LittlewoodParticipant @davidlittlewood51847]

Bazyle,

I have some doubts about your explanation; each tooth of a 2 flute cutter must, for a given feed rate, take twice as much metal off per revolution compared with a 4 flute cutter. Also, I think (though I'd have to check) that 4 flute cutters may be slightly stiffer than 2 flute ones.

David

Edited By David Littlewood on 24/05/2012 14:34:46

[LambtonParticipant @lambton]

I tend to use 3 flute centre cutting cutters for all normal milling operations. FC3 cutters for small diameters and normal screw ended M42 HSS cobalt ones for 8, 10 and 12 mm diameter. I seldom use "traditional" slot drills and end mills now. The best cutter I have found come form Europa Tool Company.

[JonParticipant @jon]

I would tend to agree with that David.

4 flute are usually stiffer and dont flex like 2 fluters. The only problem with 4 flute is the finer chips and clearing them in a slot to size!

I use a lot of my milling cutters to drill and either two or three flute jobbies. Finishing cut i usually slap in a 4 flute and plunge down.

I use a lot of those disposable long FC3 from J&L, quite cost effective up to 6mm dia aroud £2.80 each. They outlast decent cutters at £10 each. Good for slots and great for drilling being 3 flute.

I have been importing from US certain carbide cutters the last two years. Theres something different they last, cut well and above all imported for the price of cheap and nasty chinese stuff. Check out foztigrenb on that site, reduced shipping costs for more than one item.

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