Milling cutter damage – what am I doing wrong

[ChrisHParticipant @chrish]

Steve, I do actually have a DRO to fit to the Z axis, just not done it yet. Have found that if I haven't unlocked the quill completely then it doesn't move what it says it has on the dial.

Steve, you have the same mill as me, what do you find is a good target for cut per tooth as it were?

Chris

[ronan walshParticipant @ronanwalsh98054]

Pay little or no attention to these computer or smart phone "speed and feed rate calculators" you'll see, they are meant for high speed and high power industrial machining centres etc.

[GaryMParticipant @garym]

Hi Chris,

While you're figuring it out have a look at the video at the start of this thread on MEM.

**LINK**

Fascinating to watch if you're not familiar with the capabilities of modern CNC machines, like me.

Gary

[Danny M2ZParticipant @dannym2z]

G'day.

If you only wish to re-sharpen the four end faces, there is no need to go the whole hog of a tool and cutter grinder.

Harold Hall's book 'Tool And Cutter Sharpening' (Workshop Practice Series #38) shows how a square holder in conjunction with the simple grinding rest can be used to sharpen end mills to a very acceptable degree of sharpness. The simple grinding rest was my first major project and it is worth its weight in gold. Apart from precise lathe tools, one can sharpen screwdrivers, punches and the afore mentioned end mills.

Definitely worth a look.

* Danny M *

[JasonBModerator @jasonb]

Or you can grind right across the bottom at a shallow angle, so long as one of the cutting edges is at teh bottom it will work quite well for general roughing or for a first cut on Iron castings rather than risk a good cutter if there are hard spots.

The sides are usually still good if you have dameged the end so are quite usable for squaring off the ends of bars or sides of plate.May as well get your monies worth and use all the cutter

J

[Les Jones 1Participant @lesjones1]

A variation on Harold Hall's square holder is to use "Stevenson's ER25 Collet Blocks" (From Arc Euro.). I have bought a square one to sharpen my 4 flute cutters and an hexagonal one for my 6 flute cutters.

Les.

[ChrisHParticipant @chrish]

G'Day Danny,

I am on the case with what you suggest! When I say a toll and cutter grinder set up I meant a set up where I could grind up tools and cutters, not one of those undoubtably excellent but for me overly expensive and complex OTT set-ups.

In France last time I needed a small off hand grinder so I bought a cheap little 150mm dia one with the express purpose of using it solely to grind up tools when I returned to the UK. Then I read again just the other evening the Harold Hall book you mentioned "Tool and Cutter Sharpening" and saw the square holder and simple grinding rest and it those two items that are rocketing up my shed 'to do' list. So pleased to hear that you think so highly of it, justifies, if justification was needed, my decision to go down that route, and I am sure Harold will be pleased that you think so highly of it too. A materials list is being drawn up now, hope to crack on with it early next month – this month is taken!

Chris

[JC Uknz 1Participant @jcuknz1]

Thankyou for the book suggestion Danny .. have ordered it Have some blunt mills and it was amazing after not doing any milling for awhile how easy the 'new' mill went

Clarkson supplied a booklet with my cutter holder and since I suspect they think you are working with well cooled work/tool I treat the figures with great caution.

[ChrisHParticipant @chrish]

Jason – that is not a bad idea!

Les – it's worth a thought, but it may be a relatively expensive way, plus I would still need a tool rest.

Chris

[Les Jones 1Participant @lesjones1]

Hi Chris,
If all of your cutters are the same size than the square block holder is the best option. As I already had a set of ER25 collets I thought it was a worthwhile investment to save making a number of square and hexagonal blocks. I just adapted an old cross vice to use as a tool rest together with a modified cheap grinder. This is my lash up grinder. (Using one of Harold Hall's square blocks.)

There is another forum thread where Neil has put a picture of the tool grinder he has made by adapting something else.

Les.

[Anonymous]

A few random thoughts:

Despite the name end mills are not intended to cut on the end, but on the side flutes. That's why slot drills exist, for when there is a need to plunge into the work. The distinction seems to be disappearing in industry, as many smaller cutters, irrespective of the number of flutes, are centre cutting. The ends of cutters are normally hollow ground by about a degree so they don't rub when machining at constant depth.

Recutting chips sounds 'orrible and is bad for cutters. In my experience the resultant damage is small 'nicks' in the cutting edge, rather than breaking chunks out of the cutter.

My usual approach to milling is to decide upon a spindle speed based on material and cutter size and type (HSS or carbide). The I'll decide on the feedrate to use, based on the chip load I want and number of cutting edges. Finally I'll choose a depth of cut and width. Some parameters are determined by the geometry of part; if you're cutting a slot there's no choice but to cut the full width of the cutter. Depth of cut, and width of cut, also determine the power required. I don't normally calculate this in detail, but it's useful to have some indication; no point in stalling the machine! A useful rule of thumb is that 1 horsepower will remove 1 cubic inch per minute in low carbon steel.

Now as to the question of tooth loads. It all depends on cutter size, material and, oddly, width of cut. I'm still experimenting with this; but I have learnt that chip load is important. I've looked up a few CNC routines:

2mm HSS 2 flute cutter, aluminium, 4000rpm, full width, doc 1mm – feedrate 150mm/min, chip load 0.019mm/tooth

2mm carbide 4 flute cutter, 316 stainless steel, 3500rpm, width of cut 0.1mm, doc0.4mm – feedrate 70mm/min, chip load 0.005mm/tooth

12mm HSS 3 flute cutter, aluminium, 4000rpm, full width, doc 3.5mm – feedrate 900mm/min, chip load 0.075mm/tooth

10mm carbide 3 flute cutter, hot rolled steel, 2500rpm, width of cut 7.5mm, doc 2.2mm – feedrate 300mm/min, chip load 0.04mm/tooth

6mm carbide 3 flute cutter, delrin, 2500rpm, full width, doc 6mm – feedrate 800mm/min, chip load 0.1mm/tooth

With more experience I'd now up some of the feedrates above, especially for the hot rolled and delrin. I'm still learning about CNC, and tend to use higher feedrates on the manual mill, as one can quickly reduce it a bit if it doesn't sound right. With CNC it tends to result in 'ping' and a broken cutter.

One other point to think about; if the width of cut is small, then the feedrate can be increased, as the chip is thinner. On my manual mill, in aluminium, I often use the 'rapid' button for the finishing cut.

Regards,

Andrew

[John OlsenParticipant @johnolsen79199]

Thinking about this, it seems to me that any cutter that is detectably blunt to start with is probably too blunt to be using. Chris did say at the start that it was " the better of two" which does imply that it was less than perfect.

One useful way of checking how good an edge a tool has is to see it it will take a shaving off a fingernail. The problem of course for most of us is that we lack the means to sharpen a milling cutter even once we know it is not sharp. Actually you can give them a little hone with a hand held oilstone, but this has its limits.

John

[ChrisHParticipant @chrish]

Les – for you your way makes perfect sense as you already have ER25 collets. I don't, hence for me it would be more expensive! Plus it gives me more practice in the machining operations!

Andrew – thank you for your very comprehensive reply, I do appreciate all your information and advices given. The speeds and feed rates you quote for CNC are in my dreams but a million miles from what I could achieve on my manual machines, but your rule of thumb – 1hp can remove 1 cu.in. mild steel – is very helpful. Gives me a point from which to start my thought processes.

John – you are right, my cutters weren't perfect to start with, but without means to properly sharpen them you have to go with what you have got (unless a trip to the supplier can be arranged, and Senior Management agrees knowing what I am like once in there!).

Chris

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