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Mills

This topic has 55 replies, 18 voices, and was last updated 9 December 2010 at 21:22 by KWIL.

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24 November 2010 at 17:10 #59251
Wolfie
Participant
@wolfie
I have a couple of end mills.

Now if I want to face off something do I use an end mill? And what happens where it slopes? Where your cut gets deeper at one end of the job? Do I need to change to a slot mill or am I totally barking up the wrong tree.
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24 November 2010 at 17:10 #5303
Wolfie
Participant
@wolfie
24 November 2010 at 17:40 #59256
Chris Trice
Participant
@christrice43267
You don’t have to mill something full depth in one pass. Yes, an end mill is fine. You can use either the end or sides of the cutter to cut with. You can’t plunge cut an end mill in the same way you would a drill bit for example. That’s what slot drills are for. You can use either to generate flat surfaces but the end mill having more flutes than a slot drill is a much stiffer tool. Cutting speed is VERY important. Find a table and check against the diameter of cutter you’re using. I’d recommend reading up on milling first though. There are a number of quirks that you’d do well to be familiar with before you dive in.

Edited By Chris Trice on 24/11/2010 17:41:58
25 November 2010 at 21:30 #59369
Sub Mandrel
Participant
@submandrel
One gotcha- four flute end mills used to make a slot tend to wander and make a slot wider than the tool. Two and three-flute slot drills are much less likeley to do this.

I did once understand the mechanism behind this but I’m getting tired…

Neil
25 November 2010 at 23:14 #59381
John Olsen
Participant
@johnolsen79199
Any of them will cut oversize in a light machine. This is part of the penalty we pay for not being rich. I have quite often started with the next size down, then take a few extra passes along one or both sies to enlarge as needed.

There is apparently a problem with a four flute cutter in a slot when it emerges from the end…I forget if it was GHT or Tubal Cain or someone else, but one of them had this happen a few times, then worked out that the forces that occurred as the cutter comes out the end allow one flute to catch on the end of the job, taking up the backlash and causing the cutter to break. This is related to the climb milling situation, and may not be a problem with big heavy machines with no backlash. But since I don’t own anything like that, I have always been careful to avoid using four flute cutters on slots.

regards

John
26 November 2010 at 01:34 #59387
Bogstandard
Participant
@bogstandard
To get accurate width slots, it is better if you first cut the slot out to within say 0.005″ of depth with a one size smaller cutter. This is where using both metric and imperial can come to the assistance of each other, say for a 1/4″ slot, go down with a 6mm one first. Then follow down to full depth with the right sized cutter, with plenty of lube and very slow feed.

By taking the fine feed last, you won’t get as much tool deflection, if any, to make the slot oversized.

The other way is to go down first with a one size under cutter, then take a skimming cut either side with the same cutter to bring the slot to correct width.

I invariably use the first method and get very acceptable results.

Bogs
26 November 2010 at 08:06 #59392
Wolfie
Participant
@wolfie
So do I understand from this that an end mill will cut sideways??

So what does a slot mill do, does this not make it redundant?

If I start with a solid block then that needs a space cutting, I make a hole with a drill, then insert correct size mill and start cutting sideways?

Edited By Wolfie on 26/11/2010 08:07:45
26 November 2010 at 08:17 #59394
Eddie
Participant
@eddie
Hi Wolfie

End mills start at the end of the material cutting an open ended slot.

Slot mills start in the material and cut a slot closed at both ends.

You can drill a hole and use an end mill but the slot will not be flat where you drilled the hole, except if you plan to have a slot right through the part.

The easy of using a slot drill to go through instead of drilling amd then aligning it in the mill can be a problem, one setup one task.

Eddie
26 November 2010 at 08:25 #59395
Anonymous
In the odd way of the world an endmill, despite it’s name, is intended to cut on the side, not on the end. What an endmill will not do is cut straight down. That’s where slot drills come in. They can be plunged straight down into the material, and so can be used to machine closed slots or profiles.

Regards,

Andrew
26 November 2010 at 10:31 #59407
Ian S C
Participant
@iansc
The important thing about a slot drill is , it will cut an accurate width slot for a key way, where as if you try to use an end mill for the same job it will invariably cut wide, ie., a sloppy fit. Most of my cutters are slot drills, I’m afraid I’m a bit of a cheap jack, I take what I can get. The only thing with the two flute slot drill is that to ge the same finish as an end mill you feed through at half the speed. ian S C
26 November 2010 at 11:36 #59412
Terryd
Participant
@terryd72465
Hi Wolfie,

Stub Mandrel (Neil) is perfectly correct in his statement about the wander caused by an end mill and that is the real reason for the difference between an end mill and a slot drill. Like Neil I once knew the theory but it is a long time since I was at Tech and working in the toolroom and I was more of a turner than a miller.

If you are cutting a cavity rather than a slot you can drill a hole and plunge the cutter but you can only take quite shallow cuts, on a hobby mill of medium size I would expect to take a maximum cut of around 0.5 – 1.00 mm depending of course on the material and size of cutter. The theoretical cutting speed is a good place to start but these are calculated for large rigid industrial machines, you have to learn what your own machine is capable of by trial and error, that’s where experience comes in. Start with light cuts and then increase a little with each pass until the machine complains. Don’t be hasty.

When you drill your hole to plunge, do not make it the same size as the cutter. make it slightly smaller then swap the drill for the milling cutter. you can then plunge cut with the end mill (but don’t move the job, have the table locked). To see how much smaller, look at the geometry of the end of the milling cutter, the centre has no cutting edges, that is the reason for the inability to plunge cut. The hole you drill must be larger than that centre.

When you say about a slope, do you mean the cutter is getting deeper into the work? How are you holding the cutter on the machine?

Hi Andrew,

I don’t think that you’re quite right about end mills not being designed to cut on the ends. They will cut very well on the ends that’s why the cutting edges are sharpened. What they will not do, as everyone points out, is plunge cut, because the cutting edges do not continue into the middle of the tool.

Terry
26 November 2010 at 11:49 #59414
Nicholas Farr
Participant
@nicholasfarr14254
Hi, this paradox about end mills not been able to cut closed slots in a material has bewildered me. In a previous employment in the dept’ I was in, there was an old timer machinist. He never had any slot mills/drills and all the closed keyways in shafts that he cut were done with end mills of the finish size. I fitted a hugh amount of these shafts in my time, with keyways cut in this manner and never experenced any over size. The keyways were always clean and flat in the bottom, they were however cut on a horizontal mill with the shaft mounted so the keyway was cut on the side. The mill was a Parkinson No. 2. I’ve never tryed this approach myself, but with all that I’ve raed about not using end mills in this way, I wonder how this would have been achieved.

Regards Nick.
26 November 2010 at 12:20 #59420
David Clark 13
Participant
@davidclark13
Hi There

You can get centre cutting end mills but he probably used a 3 flute end mill or FC3 cutter.

The correct way to cut a keyway is as follows. (How I was taught.)

Firstly, for a new keyway set up, use a new cutter. An old cutter may have wear on the end. that is to a shallower depth to that being cut. You don’t want a stepped keyway.

Plunge to depth with a slot drill, (raise the table to put on cut to full depth (not lower the quill which should be locked) feed along very slowly, I think about 1inch per minute or less for smaller cutters, preferably with feed. Stop feed at end of slot. This is not too difficult if feeding slowly enough. Lower table, job done.

We never used a roughing and finishing cut, straight in, along and out.

I believe slot drills are ground to cut about 1 thou smaller than required.

We did many shafts this way.

regards David

Edited By David Clark 1 on 26/11/2010 12:48:35
26 November 2010 at 12:48 #59422
Wolfie
Participant
@wolfie
Thanks gents, I was under the impression that end mills cut on their ends. It worked when I tried it anyway lol. But I didn’t go very deep

I’m acyually holding the mills in the lathe chuck, can’t afford a milling machine
26 November 2010 at 13:30 #59428
Nicholas Farr
Participant
@nicholasfarr14254
Hi David, he only ever had 4 fluted end mills and there was never any predrilling of any sort. I had watched him set up and start his machining process while waiting for a part for a breakdown job. At that period in time I never gave much though to the techniques, as I was not into any milling at the time. There were not many keyways cut below 1/2″ / 12mm and most shafts were 2″/ 50mm up to 4″/ 100mm.

Regards Nick.

Edited By Nicholas Farr on 26/11/2010 13:33:18

Edited By Nicholas Farr on 26/11/2010 13:36:09
26 November 2010 at 14:56 #59434
Terryd
Participant
@terryd72465
Posted by Wolfie on 26/11/2010 12:48:57:

Thanks gents, I was under the impression that end mills cut on their ends. It worked when I tried it anyway lol. But I didn’t go very deep

I’m acyually holding the mills in the lathe chuck, can’t afford a milling machine

Hi Wolfie,

the drill chuck is your problem with ‘slope’ because of the helical angle of the cutter it is trying to unscrew out of the chuck. This is a well known problem. it is possible to mill like this but only by taking very small cuts. A relatively cheap (but not free unfortunately) solution is to buy a plain shank cutter and an individual arbor. Such as these and these From Arc Eurotrade, note though they are for plain shank end mills not the threaded type.

But also please remember you can’t learn from scratch by trial and error, you need some basic machining books as a start and use the information available on the internet.

Terry

Edited By Terryd on 26/11/2010 14:59:13
26 November 2010 at 15:04 #59435
Terryd
Participant
@terryd72465
Hi Wolfie

Here’s a starter if you haven’t seen these videos you ought to watch them. They are made at the Massachusetts Institute of Technology (MIT for short). They cover many basic aspects of workshop processes, including milling. Watch them before you chop lumps out of yourself

Terry

Edited By Terryd on 26/11/2010 15:05:14

Edited By Terryd on 26/11/2010 15:05:38
26 November 2010 at 16:09 #59441
Geoff Sheppard
Participant
@geoffsheppard46476
I was always advised to avoid trying to cut a slot with an end mill with an odd number of teeth. As one tooth starts to cut, the cutting forces deflect the cutter away from that side. As that tooth moves away, another begins to cut on the opposite side and the deflecting forces are reversed. You would think that this would result in a slot that was narrow, but mine always seem to end up too wide.

The answer is to use a slot drill with an even number of teeth, then the cutting forces are balanced.

Is there any truth in this theory?

Geoff
26 November 2010 at 16:32 #59443
Martin W
Participant
@martinw
Terryd

I have just had a look at a couple of those MIT videos for my own education. There is quite a bit of useful information there but one thing I noticed was that the tutor was using both normal milling and climb milling procedures. While there was no problem doing this with a machine of that size and a small workpiece he did not draw attention to the potential problems which was a shame.

Thank you for the link to these demonstrations I will look at the rest later as they seem to be very informative, just need to translate the procedures from a large workshop to my very small one .

Best regards

Martin
26 November 2010 at 19:04 #59445
Nicholas Farr
Participant
@nicholasfarr14254
Hi David, here is a photo of a mock up of the set up for milling keyways that the old timer machinist I refered to in a previous post used to use.

Of couse he would have it clamped down. I can’t actualy try his method, because this machine is inoperatable at the present time.

Regards Nick.

Edited By Nicholas Farr on 26/11/2010 19:05:12

Edited By Nicholas Farr on 26/11/2010 19:08:01

Edited By Nicholas Farr on 26/11/2010 19:17:51
26 November 2010 at 23:19 #59452
Terryd
Participant
@terryd72465
Hi Nicholas,

just to emphasise, a normal end mill cannot plunge deep enough to cut a keyway due to the lack of cutting edges near the centre, it simply meets solid metal after a few thou. Your friend must have used a plunge cutter with a cutting edge which extended to the centre. They are available as specials, otherwise it was impossible.

Regards

Terry
27 November 2010 at 00:20 #59453
Nicholas Farr
Participant
@nicholasfarr14254
Hi Terry, this is the paradox, I’m understanding that is should not be possible and the reasons why. However, I never saw him use any thing else but an end mill when he cut a keyway with this set up and he did not have any slot drills or otherwise for cutting closed keyways. The only other cutters were the side and face cutters used on the spindle between the machine and the support arm. These of course leave the curve of the cutter at each end of the keyway, and are used over the top of the work piece. All of the cutters belonged to the firm we worked for of course, but they were of his choice.

Regards Nick.
27 November 2010 at 01:17 #59455
Chris Trice
Participant
@christrice43267
It’s not recommended but you can plunge cut an end mill a few thou, then feed it along back and forth then plunge it a bit further. Or indeed plunge it at the same time as feeding it along the slot. Not good practice but it does work.

The reason slot drills don’t cut oversize as much as endmills is because as one edge is cutting and deflecting the tool to the side, the other edge is trailing it 180 degrees in open space. With an end mill, the tooth at the front doing the cutting deflects the cutter sideways while the next tooth which is (typically) only 90 degrees away, is in contact with the side of the slot making the slot wider.
27 November 2010 at 06:17 #59458
Ian S C
Participant
@iansc
Arnold Thorps’ book “Vertical Milling In The Home Workshop”, its no 2 in the Argus Workshop Practice Series, it gives lots of tips on using the light duty vertical mill that most of us use, including depth of cut, and speeds in most of the commonly used metals. Ian S C

Edited By David Clark 1 on 29/11/2010 20:53:33
27 November 2010 at 20:31 #59502
Anonymous
Terry,

I agree that end mills are perfectly capable of cutting on the end. However, unless they are moving in Z, then all the cutting takes place with side flutes. I guess what I was trying to say is that they are somewhat mis-named. While they will cut on the end, that’s not what you generally use them for. You’ll have to be patient, I are an engineer, so naturally I struggle a bit with the English language, like what it is wrote.

I rather suspect that industry is moving away from end mills per se. Most of the ‘end mills’ I’ve bought recently are effectively three flute slot drills, even the roughers. I guess this has been driven by CNC requirements, where you need to enter the material at an angle, or a helical path in Z, in order to pocket out an enclosed area. Once at the correct depth you are essentially endmilling, but it helps to have a centre cut facility on the way in.

Regards,

Andrew
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