end mills and slot drills

[Ian Skeldon 2Participant @ianskeldon2]

Hi,

I have long been of the understanding that a slot drill and an end mill are two different things in so much as, a slot drill can be used to bore a hole into the face of a workpiece and this si not the case with and end mill.

I now have a requirement to create several bores with varing depths at a diameter of 8mm, I want them to have a flat bottom and I was thinking of using a slot drill in my mill to do the job so that accuracy would be assured. On googling for slot drills I saw no end of adverts saying for instance "two flute slot drill or end mill".

Can anyone please advise me if end mills can indeed produce a bore accurately without wandering and chatering?

Thanks,

Ian

[Ian Skeldon 2Participant @ianskeldon2]

[David Standing 1Participant @davidstanding1]

A slot drill will drill a hole because the cutting surfaces overlap, thus giving a cut for the full width of the hole.

A typical end mill will not do this, as the centre of the cutter has no cutting capability, therefore it will only work going in a horizontal direction, either as an end cutter, or as a side cutter.

[Ian Skeldon 2Participant @ianskeldon2]

Thank you David, I thought so, and yet several websites are describing one item but calling it by both names.

[David Standing 1Participant @davidstanding1]

Pure sloppiness I suspect!

[Brian HParticipant @brianh50089]

There are also some 3 flute cutters that will do both operations, just to muddy the waters even more!

Brian

[David Standing 1Participant @davidstanding1]

Posted by Brian Hutchings on 08/11/2017 22:15:04:

There are also some 3 flute cutters that will do both operations, just to muddy the waters even more!

Brian

 

Yes there are, I have some, but ONLY if the cutting surfaces overlap with they act as a slot drill, which is why I said a 'typical' end mill in my earlier post, as with a typical end mill the cutting surfaces do not overlap.

 

 

Edited By David Standing 1 on 08/11/2017 22:25:56

[Anonymous]

Posted by Ian Skeldon 2 on 08/11/2017 21:58:25:

Can anyone please advise me if end mills can indeed produce a bore accurately without wandering and chatering?

Yes (with a slot drill, not an endmill), I have done it, but you need a rigid setup to prevent chatter, especially at the start of the cut as the cutter is eccentrically loaded. The bores are ok, but they're not particularly accurate, compared to drilling/reaming. The other thing to bear in mind is that the hole will not be flat bottomed as slot drills and end mills are ground slightly hollow so the bottom teeth don't rub when conventional milling. It's only hollow by a degree or so; depends on the application as to whether that is a problem.

Andrew

[Chris TriceParticipant @christrice43267]

I use slot drills to bore holes but I've found that they run truest if you drill the hole first a little undersize so the slot drill has the minimum material to remove and the cutting force is equal on all two (three or four on an endmill) flutes.

[Tim StevensParticipant @timstevens64731]

Descriptions on e-bay (and other sources) tend to be overloaded with terms, so that a search will find them even if the name used is not quite right. And sometimes because the seller is just copying some else's advert for something fairly similar, and anyway is not a good English speaker. So you get switches described as 'momentary latching' when the two terms are mutually exclusive.

The answer I offer to your query is that the drilling capability depends on a cutting edge across the centre of the tool. Most slot mills (two flutes) have this feature, and some end mills (four flutes) do. The more modern three flute cutters generally do too, and in my experience are a good compromise for many jobs. Even if your cutter has the right sort of geometry is can be asking a lot for it to shove the central metal to one side through a deep hole. Drill a smaller hole first and save it the trouble.

Cheers, Tim

[Martin DowingParticipant @martindowing58466]

I would notice, that you will *not* get flat bottom hole using either of these. There will be a slight convex on the bottom because of geometry of both slot drills and end mills. The cutting edge forms about 1deg concavity, so drilling hole with it will produce approx. 1deg convex on the bottom of hole. I think, flat edged endmill/slot drill would produce horrible chatter and would also deliver oversized/irregular hole due to propensity to be dragged sideways while drilling that hole. You will need final grinding operation to produce said hole flat on the bottom.

Martin

[Tim StevensParticipant @timstevens64731]

It is not possible to machine (or grind) a flat bottomed hole. However near you get, irregularities will exist. So, it really depends on what is the purpose of the required flatness. We are not told this – rather the question was actually about straightness and roundness. Neither of these is possible for anyone equipped with super sensitive measuring kit, so what should we do – try to be helpful, or just say 'What you seek is not possible' ?

Cheers, Tim

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Tim Stevens on 09/11/2017 22:33:51:

… so what should we do – try to be helpful, or just say 'What you seek is not possible' ?

.

Eloquently put, Tim

MichaelG.

[not done it yetParticipant @notdoneityet]

Would finishing with a D bit achieve a reasonably flat bottomed hole, after using a milling cutter? Seems the best solution to me.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by not done it yet on 09/11/2017 22:57:47:

Would finishing with a D bit achieve a reasonably flat bottomed hole, after using a milling cutter? Seems the best solution to me.

.

Maybe!

It rather depends on the interpretation of 'reasonably flat' … and Tim's observation is crucial to this.

To put things into perspective: The theoretical height of the 'bump' produced by an 8mm [4mm radius] cutter would be slightly less than 0.07mm.

Whether this already qualifies as 'reasonably flat' and whether one could improve significantly upon it by a second operation depends on that interpretation, and on the performance of a 'real world' D bit.

Your approach could work … but I think 'production engineering' the job might be more appropriate.

MichaelG.

[JasonBModerator @jasonb]

Suppose you could use a boring and facing head with a small cutter to get a flat bottom.

Or if the parts are able to be held in the lathe then a small boring bar will give a flat bottom no problem provided depth is not excessive in both cases

[Neil WyattModerator @neilwyatt]

The answer to your original question is 'yes'.

As usual, all most of the other responses are based on speculation about other requirements of the job in hand.

So I'll join in…

Finishing with a D-bit ground straight across for > 1/2 the end will give you a flat enough bottom for almost all practical purposes of such a hole.

In the unlikely event you need a near perfect result, a could be obtained by using an undersize tool to mill round pockets.

Neil

[Gordon WParticipant @gordonw]

A slight change of topic- I need to put a couple of slots in some brass. I was going to drill and file but realised I did not have a small enough file so have thought to mill them. Will be 4mm wide and 15 deep in brass. Any recommendation for type of mill ? I'm guessing end mill in lathe spindle and work mounted on cross slide. Never done any milling this small and never in lathe.

[egaParticipant @ega]

This was a topical thread for me as I am building the Acute sharpening system – thanks Gary, and John Ashton – and the design calls for a number of flat-bottomed holes. The first was done with a small boring bar but, having reviewed and tested my stock of slot drills, I have decided to try the D bit approach as suggested by not done it yet. I realize that this will not produce a perfectly flat bottom but I doubt whether a slot drill will be much better.

ARC have some high performance slot drills which may be the answer.

Edited By ega on 10/11/2017 16:47:42

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