Discussing the merits and uses of different types of mill bits
|Stuart Munro 1||16/04/2021 17:11:45|
|76 forum posts|
I'm a relatively new member, and a very new engineer. 67 years young with all of a year or so playing with metal. I'm gradually getting to grips with topics such as the use of different types of mill bits but want to test my limited academic knowledge alongside real world experience. This section has many references to suitable mill bits but I thought pulling this experience together would help apprentices like me.
To start the ball rolling, can I put raise a few mistakes that I've made and elicit any thoughts, from 'what an idiot and clearly no engineer' (both alas true) to what I should have done/used.
1/ Cutting Brass: I make small brass and aluminium components. I read that Uncoated HSS is the right way to go for brass and this was great initially. But I was not cutting deep enough to get a good finish - getting chatter. As my confidence grew I cut faster (moved the table into the brass faster with a deeper cut) and this seemed to work better until the bit dug into the brass.
So I now read that the cutting angle of the flutes is important. Can anyone explain this and suggest an approach to milling brass?
2/ Indexing: I moved onto aluminium and found this to be much harder to mill with regular hard judders as the tips of the mill bits broke. I've purchased mill bits that are stated as correct for non-ferrous metals and I've been very careful to fix the work piece securely but still get the problem. Is indexing the way to go and is this realistic with a MT1 taper shaft? I've experimented with indexed tools on my lathe (Sherline) to avoid sharpening HSS tool bits and this cuts efficiently, but not with a very smooth finish!
3/ Clamping the tool: generally confused about the advantages and disadvantages of Weldon bits to tool holders vs the range of collet types available. Perhaps it's the clamping of my bits to the mill that is my weak point?
I'd very much welcome any input from experienced engineers, and questions from other trainees like myself.
|Stuart Munro 1||16/04/2021 17:16:24|
|76 forum posts|
ps. from this site, I've had some very good advice on different types of aluminium and their suitability for milling; I've bought aluminium from GLR Kennions listed as good machinability so I'm confident the error is with me, not the metal.
|Ian Parkin||16/04/2021 17:21:16|
931 forum posts
For alloy lubricate copiously with wd40 to stop tip build up
Edited By Ian Parkin on 16/04/2021 17:21:56
|Martin Connelly||16/04/2021 17:49:27|
1705 forum posts
Do you understand the difference between climb milling and conventional milling, it sounds like you may be climb milling on a machine that is not suited to it (juddering). There is a guide that can be reached from the home page but here is a link anyway.
Also as Ian said aluminium and its various alloys can easily clog up the flutes or stick to the cutting edge which causes problems. The fewer flutes the better with these metals, you can get special cutters with one flute but mostly two works ok, three at a push and avoid 4 flutes. Use polished cutters not coated as well. As soon as the cutting action starts to change stop at a convenient point and check for build up on the cutter. Some people suggest paraffin as a lubricant for aluminium but the smell is quite strong and you probably don't want a build up of flammable fumes in a workshop.
There are rules of thumb for cutting depth based on the tool diameter and you should learn about speeds (rpm) and feeds (thickness being cut by each pass of a cutting edge) to avoid either rubbing or overstressing the tooling and machine. There are online calculators to help with feeds and speeds.
|Stuart Munro 1||16/04/2021 17:56:59|
|76 forum posts|
Ian, thanks for this - I'm advised that Brass does not need lubrication and went on to treat aly alloy the same. Error no.1!
Can anyone explain the importance of the angle on the flutes (I think its called the helix angle). It seems that for Brass (I've not looked for Aluminium alloy) that mill bits with the correct helix are quite expensive so it would be useful to know why this is important to help decide if the expensive bits are worth it?
The same question presumably applies to Aluminium, but equally a different angle. If so, is my choice of 'non ferrous' bits too general in that one type does not suit both brass and aluminium? Perhaps my 'one fits all' bits are not appropriate for Brass nor Aluminium.
Argh, number of flutes, helix angle, coated or uncoated, type of shank.....now to lubricate or not. That's before discussing SPM, feed rate etc. You engineers know an awful lot of interesting stuff. I can read a lot on the 'net' but would really like to know what really works and why.
Thanks in anticipation
|Howard Lewis||16/04/2021 17:58:01|
|4741 forum posts|
What speeds and feed rates are you using?
It is assumed that the work is rigidly clamped, and that your machine has minimal backlash on all axes, with the unused ones being clamped.
Similarly, cutters should be clamped as firmly as possible with minimal overhang.
Feed rates should be based on cutter speed, number of flutes and feed per tooth.
With HSS cutters, taken from my Apprentice Training Notes; think in terms of:
For soft Brass Roughing, 200 - 300 fpm Finishing 600 - 700 fpm
For hard Brass Roughing 150 - 200 fpm Finishing 200 - 300 fpm
Aluminium Roughing 400 -500 fpm Finishing 600 - 700 fpm
FEED per TOOTH Face Mill Slot Drlll End Milll Slitting Saw
Free Cutting Brass 0.022" 0.013" 0.011 0.005"
Hard Brass 0.009" 0.008" 0.005" 0.002"
Aluminium 0.022" 0.013" 0.011" 0.005"
|Howard Lewis||16/04/2021 18:03:24|
|4741 forum posts|
Climb milling will give improved surface finish, ON A MACHINE DESIGNED FOR SUCH WORK, with backlash prevention devices.
Hobby machines are NOT!
You are likely to have the cutter grab and pull the work into it with the risk of damage to both.
The work should travel so that the motion opposes that of the cutter tooth.
|Tony Pratt 1||16/04/2021 18:37:27|
|1503 forum posts|
Always use sharp cutters for Ally & brass & Ally needs lubricating to stop what is called 'pick up' where the cut material sticks to the cutter, on Ally I personally use Paraffin or even cutting oil at home but when I worked on CNC machines we used a water soluble cutting fluid.
20443 forum posts
I think you may be getting mixed up with helix angles for drilling brass where a low or no helix angle is desirable to stop the drill bit being pulled into the work, this does not really apply to milling cutters.
Cutters made specifically for aluminium will also work well on brass and bronze. These tend to have a higher helix angle which is mainly used to help remove the larger volumes of swarf that can be produced from these faster cutting materials.
I think you mentioned that you also have a Sherline mill so quite light and only able to use relatively small cutters so some of those chip loadings mentioned above will need to be reduced as the small cutters are not as strong. For the same reason your depth of cut and approach (sideways depth) will also need to be less than larger machines can manage.
Climb milling can be used with care on most machines but on the lighter ones it is best kept as a very light finishing pass rather than a way to hog out the majority of the material.
Edited By JasonB on 16/04/2021 18:48:02
7131 forum posts
Well I like to keep it simple! I use the same coated HSS cutters on steel, brass and Aluminium, simply adjusting feed rate, depth of cut and RPM for best results.
Two flue cutters, aka slot drills. can plunge and cut slots. The big gap between flutes helps eject swarf. Four flute cutters are for flattening and edge cutting - they remove metal faster and produce a better finish provided the swarf can get out of the way. They can't plunge (usually). Keep meaning to try some 3 flute cutters, which are a compromise between 2 and 4 flutes, but never got round to it.
Coated cutters have a thin covering of something extra hard like Titanium Nitride to extend tool life, but Aluminium tends to weld to it, spoiling cut and finish. Not a big problem if spotted because the Aluminium can usually be scraped off, or dissolved in Caustic Soda solution, though having to stop does delay work.
I don't lubricate brass or cast-iron. Aluminium benefits from a light oil such as paraffin or WD40. On steel I use neat cutting oil, mainly because I have an irrational fear of suds causing rust. Or I go gently and don't lubricate at at all.
RPM, DOC and feed-rate are all a rule of thumb. Recommended industrial parameters are chosen to balance tool-life, metal removal rate, and economic use of power. They are more aggressive than most hobby machines are comfortable with, so back off! Don't expect a Sherline mini-mill to perform lanything ike a hefty Horizontal milling machine
I determine RPM by dividing 10000 by the cutter diameter in mm. This is about right for mild-steel, increase for Brass and Aluminium. Slow down by half for cast-iron. Depth of cut, up to 10% of the diameter of the cutter, feed rate fairly fast. It's important to cut rather than rub. I adjust DOC and feed-rate by ear; I like the mill to sound as if it's working, but not labouring. Reduce rpm if chatter occurs, and increase DOC and feed-rate so it sounds right. The exception is finishing cuts, which can be much lighter or climb milled, but avoid climb milling whilst taking deep cuts because 'our' machines are too weedy!
All this assumes the material is a machinable alloy rather than unknown scrap. Many alloys don't machine well, and some are downright evil! Learn on the right stuff so you know what to expect before experimenting with scrap just in case your junk box is full of rubbish. Mine was!
ps I don't think the people I buy cutters from offer different helix angles! As I understand it, they are optimised for roughing or finish, or in between. I think mine are all 'in between'. I doubt helix angle matters unless something special is in hand.
Edited By SillyOldDuffer on 16/04/2021 19:45:07
|Martin Kyte||16/04/2021 19:51:04|
2346 forum posts
For creating flat surfaces on aluminium as opposed to slots and steps I suggest you fly cut . I use an insert for aluminium in a fly cutter and produce a mirror surface cutting dry.
|Andrew Johnston||16/04/2021 20:58:54|
6009 forum posts
It's more dependent upon material. Cutters intended for aluminium often have high helix flutes. But they are only advantageous if one can run at the high speeds and feeds required. Similarly for tough materials cutters can have esoteric coatings and variable helix angle and/or flute spacing to reduce chatter. Again one needs a mill that is capable of utilising the features.
I don't regard my Bridgeport as being able to make proper use of the fancy cutters, so I don't buy them, other than polished cutters for aluminium.
|old mart||16/04/2021 21:13:06|
|2908 forum posts|
Assuming you have only a small ammount of backlash on the Xaxis, say 0.01", 0.25mm, you can get away with climb milling using shallower depth of cut than conventional and applying a little braking from the locks. This is useful for the finish cut which will be smoother.
|Stuart Munro 1||16/04/2021 22:02:02|
|76 forum posts|
Thanks everyone. I was aware of climb milling and plan to mill conventionally except a very light cut to finish where I find climb milling is preferable.
As to milling speeds, for a novice without someone looking over my shoulder its quite unnerving pushing the feed rate up yet I theoretically understand that the balance between Surface feet per min (Have I got the term right), the cut depth and feed rate sometimes needs me to increase feed rate. As an alternative I reduce the RPM.
Dave - your 'feel the right rate' view makes sense to me which I hope comes with practice.
But I've not been lubricating on my mill; I have an oil/water mix a friend gave me for a bit of iron I turned on m lathe - also a Sherline. But might try the WD40 - seems to be akin the the wild west elixir cure all but for engineers!
I clearly misunderstood the whole helix angle thing but got he coated vs uncoated bit right - although I still don't understand quite why - important to my satisfaction not to the machining as long as I follow the 'rules'. Is it that somehow uncoated cutters are sharper because the coating itself thickens the cutting edge? But without this the sharp uncoated cutters will blunt quickly. This moves me towards heap (I see Arc Euro and ACCU selling 2 flute 8mm uncoated bits for £3 - £4. Use one for a few days and throw it away!
I don't want to hog every ones time so if there are any other ex office dwellers moving into a useful pastime in retirement perhaps you have some questions. If not, any thoughts on holding the bits; I use tool holders and weldon mills but most people use collets. Why?
|Andrew Johnston||16/04/2021 23:10:50|
6009 forum posts
Not me. On the Bridgeport and CNC mill I mostly use the Tormach TTS system. My two main holders are 6mm and 10mm side lock, used with carbide cutters with no flat. They're simple and have a short overhang so one can run the cutters hard, note the blue swarf:
For cutters other than 6 or 10mm I use ER collets, this one is 4mm:
I have a selection of Autolock style chucks, mainly used with HSS cutters, imperial and metric:
I also use sidelock holders with larger cutters (1" slot drill in this case) for convenience:
I have been known to use a milling cutter in a drill chuck. Personally I use whatever holder I think is suitable for the job in hand and don't worry what other people do.
Modern carbide cutters with coatings are pretty darn sharp, and will slice a finger just like a scalpel - I speak from bloody experience. For aluminium I use uncoated and polished cutters, as it reduces the tendency for the swarf to stick to the cutter. For other materials I use mid-price cutters with TiAlN, or similar, coatings. I suspect my mills will not run fast enough to use the more esoteric coatings - so I don't waste money on them. Neither do I machine the really tough materials and 'orrid materials, like inconel. The coatings generally provide a thin, but very hard, surface to counteract wear and pitting from the swarf. But as always the devil is in the detail. With TiAlN the aluminium helps with lubricity meaning the swarf slips more easily. but the coating needs to be very hot for the aluminium to do its job. So running a TiAlN cutter at slow speeds doesn't make use of the coating.
|Stuart Munro 1||17/04/2021 07:14:14|
|76 forum posts|
Thanks all. So to summarise for anyone coming to this blog in the future.
We have mainly focussed on Aluminium alloys as the object material. To mill this effectively:
1/ Use uncoated polished two flute mill bits. The consensus is that these are both sharp and as effective as possible at removing swarf.
2/ Nevertheless, lubricant is advisable, paraffin is effective but pungent and runs the possibility of fumes building up. WD40 is another option. The lubricant is unlikely to be enough to remove all swarf so it is advisable to check the bit periodically and manually remove swarf.
3/ The exact type and grade of aluminium alloy is important. Some cheap aluminium is very difficult to machine. Source the aluminium carefully from a reputable supplier (see elsewhere on this site).
4/ Speed of cutting; Remember the relationship between RPM and Surface Feet per Minute (SFM). Lower diameter bits must rotate faster to achieve the same SFM. My Sherline mills came with a chart of recommended SFM for various materials (including 3 grades of Aluminium) so presumably all milling machines are provided with such information. Howard Lewis summarises similar information above. Take care to get the feed rate and depth of cut right. Some commentators mention that experience allows them the 'feel' and hear the mill to know that they have this right. My reading indicates that the swarf coming off - with aluminium in particular - is indicative of a good cut if it 'spirals'.
5/ Conventional milling vs climb milling. Smaller machines tend not to be as rigid as say a Bridgeport and can not hold a piece and bit firmly enough to climb mill unless it is a very shallow cut aimed at producing a fine finish. The bulk of milling should be conventional where the feed across the surface of the aluminium is against the rotation of the bit.
6/ Holding the bits in the mill; it seems to be personal preference which method of securing the bit is used when 'hobby' milling. High speed industrial work and/or very high precision may require specialist collets. On my Sherling a tool holder with side screw holding a weldon shaft is efficient - similar to but much smaller than Andrew Johnsons Bridgeport Tormach TTS system in the prevoius blog.
7/ Some discussion on specialist coatings of mill bits; TiAIN can be useful but at high speeds as it needs to be hot to work well, smaller and/or non industrial use will be unlikely to be sufficiently fast to achieve the heat needed. More esoteric coatings will generally suffer in the same way. The conclusion is that for the hobbyist cheaper uncoated and polished bits are preferable.
Many thanks everyone for your contribution
20443 forum posts
1. Although a lot of aluminium specific cutters are two flute I would not rule out using more flutes provided the cutter was sharp and uncoated. About 50% of the time on the manual mill ill use 3-flute cutters on aluminium and even on the CNC maybe 75% of the time I go to 3-flute carbide high helix cutters depending on the size of cutter being used and the type of cut. I don't push my machine hard enough or have a high enough spindle speed to really produce the amount of swarf.
2. Yes lubricate. That smell from Paraffin has it's advantages as it will mostly evaporate out of the swarf and off the machine where WD40 can leave a sticky film. It's not been mentioned but if going with WD40 then get the 5lts can and pump spray as it's a lot cheaper than aerosol cans.
3. It's not just cheap aluminium that can be hard to machine as you can pay good money for know grades that are not free machining. More a case of knowing what grades machine well and suit the particular job and then buying them rather than using "unknown" material.
4. Very few milling machines will have any suggestions for cutting parameters included with them
5.Yes on small hobby machines keep climb milling for light finish cuts
6. Holding can be a personal thing, although I do have side lock holders I seldom use them as I find the ER more versatile and rather than having to keep raising and lowering the head to take sidelock cutters out of the spindle I can hold all sorts in the ER such as edge finder, drills, small flycutter, milling cutters, etc.
7. I don't give much thought to coatings apart from trying to avoid them on aluminium but sometimes its a case of a particular cutter not being easily available without coating so may have to make do.
|Tony Pratt 1||17/04/2021 08:43:37|
|1503 forum posts|
Sharp 2, 3, 4 flute cutters are fine on Aluminium. In fact if you need to do a deep pocket or similar 2 flute 'slot drill' cutting edges are generally not as long as 'end mills' so they may not be an option.
20443 forum posts
Luckily you can get long series aluminium specific cutters with long flute lengths not just extended shanks that will have far longer flutes than a "standard" endmill if you do have a need for deep pocketing. I was down 4.5D during the week and still had a bit of length to spare on an 8mm cutter (36mm deep slot)
7131 forum posts
In reverse order:
Reading about how all these variables interact is intimidating. Don't worry because starting by rule of thumb and experimenting reveals what's needed and it soon becomes second nature. Theory is essential to making money in cut-throat production but absolutely not vital to producing results at home. The hobby is a mix of art and science and skill is more important than theory.
Edited By SillyOldDuffer on 17/04/2021 11:02:32
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