Milling

When milling, how do you know how much to remove in one go ? is it as simple as 'it doesn't sound right' ?

As far as I have been concerned with milling in the back-shed it has always been a matter of 'guestimation' in both vertical and horizontal milling. There are treatises available on the subject because all materials have a cutting speed and it is possible to calculate the optimum rate of removal taking into account the number of teeth on the cutter and its diameter etc. but as with lathe speeds these are usually 'industrial' speeds were rate of production is the paramount consideration.

I don't know which milling machine you have, I have a Senior knuckle-head for vertical milling which is far too slow for cutters much less than 3/8" even on its fastest speed. I have a Harrison M1 for horizontal milling which is good for taking big bites out of noggins but its all guesswork for as far cutting speed is concerned. To me, if it sounds right and looks as if it is doing the job then it is right.

So in summary, I would suggest the smaller the cutter the faster the speed and the lighter the cut with progressively lower speeds for larger cutters. The depth of cut and rate of feed will always have to be a matter of judgement for me as clearly I am not going to plough a 3/16" end mill into a lump metal at a slow speed because it is obvious to me it will just break.

I am also interested to learn how others approach the matter.

That is going to be determined by the rigidity and power of the machine, size and type of cutter, material, type of cut.

As a simple starting point if using the full width of a cutter then D/4 that's 1/4 of the diameter depth of cut. If using the side then 1D deep x 0.1D sideways.

It should be easy to see that if using a 25mm cutter in steel then that would be a lot of material to remove in one go so if you have a mini-mill it won't do that but it should cope if it were a 5mm cutter, more so if cutting aluminium

Amount to remove is based on the volume of metal that can be removed per kW. Power varies, dependant on the material being cut, have a look at the production cutter suppliers for information such as Sandvik. Then when you have a ball park figure you can start at 50% and see how your machine goes. Less rigid machines will remove a lower of volume than the calculated, more than likely a lot less. There are lots of figures out there but the way to learn is to do it for yoursel.

I'm with Chris Crew on this one. Speeds and feeds are fine, but you might need coolant etc for best results and if its just a hobby you can afford to back off from the industrial rates.

My mill has a lot of backlash in the table feed, so driving it too hard results in horrid noises esp with horizontal milling, so I take a cut, increase the DOC for the next pass and repeat till the mill tells me its not happy, back off a bit and I'm at the setting for a good material removal rate.

For spindle speeds, I tend to leave the horiz spindle at 100rpm and the vertical at 850 which covers most of the work I do, but I have vfd speed control so slowing it down on the fly is easy.

What is often the most important thing is to make sure the tool is cutting and not rubbing and not overheating. If you are producing filings or smaller then you are probably not cutting the material away, more likely rubbing it away. This drastically reduces the life of the tool. There are also maximum speeds for the cutting edge. Going faster with, for example, high speed steel, than the recommended maximum for the cutter and material combination will probably cause overheating and reduce the tool life. Flood coolant is going to help if you are close to maximum but usually it is better to be below the maximum. These speeds can be found with a simple Google search for speeds and feeds calculators to find the maximum RPM then pick something about 50%-75% of it. The feed rates are a moot point if you are hand feeding so go for discrete chips not dust of fine filings. Note that a 4 flute cutter needs to be fed at twice the rate of a 2 flute cutter to get the same chips. With experience of your machine and what you are doing you will learn if you can go faster or slower as the work requires.

The problem with the listening to the sound method is that with some jobs there may be a lot of re-cutting of chips in the work area and this will make a lot more noise than a job where the chips are flung away from the tool.

Martin C

I generally go by what seems right at the time, cutters vary in their cutting rates. Be careful with small diameters, it is hard to tell whether you are overloading before it breaks. Yesterday, I was pleased to find that a 10mm solid carbide roughing cutter exceeded my expectations when cutting steel to a shoulder. The depth was 9mm, and I gradually increased the ammount cut each pass up to 3mm at 2000rpm, both ways. The cutter worked better in climb as the swarf was not a problem and the backlash is fully controlled. I had been contemplating using a shell mill, but the 10mm was quicker. Roughing cutters have wavy sides, like a thread and cut smaller chips, I only have one carbide one.

Duffer's Patent Rules of thumb:

• For HSS cutting mild-steel, RPM = 10000 / cutter diameter in mm. So a 10mm cutter would spin at 1000rpm. For smaller diameters as fast as the mill will go: 1mm wants about 10000rpm, and my mill can only do 2500.
• Side contact area, up to 15% of cutter diameter. Opinion varies, some say 20%. More than 20% is likely to cause loss of accuracy due to flexing the cutter.
• Feed rate, adjusted by ear. Make sure swarf isn't being minced , and increase the feed until the motor is heard to be working, but not stressed.

These should get the cut into the the right ball-park, but be prepared to experiment. Reduce speed for Cast -Iron. Most other metals prefer faster than steel, and least x2 RPM if the cutter is carbide and the machine can do it.

Dave

Is that for all types of cut Dave? seems rather low for side cutting, as I said above 100% of D is often used and many makers tend to give figures based of 1.5D (150%) I'm happy with 1D inmost cases but do sometimes use more

For slotting where the full width is being used I tend to go upto 25% but a lot of makers give data for 50%. Note they don't give data for using 4-flute cutters to do slots as that can cause over width slots.

Having metioned cutting speed, no one sems to have sias much about feed rates, or direction.

For End mills, a feed of 0.002" ( 0.025 mm ) per tooth would be about right for HSS tooling.

The feed rate is calculated on the basis of (Number of cutter teeth, x rotation speed x feed per tooth. )

With regard to direcvtion of feed, for hobby machines, climb milling is best avoided. Hobby and older machines do not have backlash elimination built in.

In climb milling the work is fed in the same direc tion as the tooth of the cutter. But this is liable to take up the backlash in the feed and drag the work into the cutter; so is best avoided.

The better and safer way is to feed the work so the feed opposes the direction of the cutter tooth (I me , a head on collision, where the cutting forces try to push the work away from the cutter..

HTH

Howard

Poor choice of words on my part, but at least no-one would break anything as a result of the advice! By side cutting I meant slotting, which levers the cutter.

Rules of Thumb are just that. Mine get safely near the right answer in most circumstances - a starting point, unlikely to be optimum. Always necessary to experiment a bit because machines, tooling, work-holding and materials all vary with circumstances.

Productivity is interesting. Anyone else noticed Jason works much faster than me, even though we happen to have similar equipment?

I estimate about 20x faster, and it's not exaggerated false modesty! Many reasons for that, and one of them might be my preference for lower metal removal rates. Not the major reason though; I'm inexperienced and self-taught, and my skills are part-developed because other interests steal time from metal-working. More, I mostly do development work, rather than production, which coupled with inexperience pushes me into spending far more time planning and setting-up than cutting.

Not so bad now, but early on I wasted a lot of time due to mistakes, misunderstandings and sub-optimal cutting sequences. Experienced workers minimise these. They're far more likely to find the best route for making several different types of cut whilst minimising work-holding adjustments, tool changes, moving the table farther than necessary, and re-referencing to maintain accuracy.

I also have a messy workshop, which wastes time when stuff needed by the workflow gets lost in the clutter. My tools and materials are semi-organised, so tap drills might be in the tap drawer, or in the drill drawer. Or in the machine, left on the bench, dropped on the floor etc. I know it's inefficient, it makes about 1 in 5 sessions frustratingly slow, but on balance I enjoy muddling through. My productivity would be zero if I was a perfectionist as well! My other faults mean how fast cutting removes metal is the least of my time issues.

And when cutting my main goals are accuracy and finish, not making swarf quickly. Cut, don't rub, and don't flog bendy hobby machines.

Dave

Edited By SillyOldDuffer on 22/05/2023 09:47:56