Chip loads for Hobby Machines in different materials

The chip load specification is for the cutter and not the machine; if the chip load is too low, then not enough heat is carried away, and the cutter can burn up. The depth and width of cut are the really important variables to get right for a hobby machine, not trying to remove too much material in one go.

I find a good starting point for chip loads is something like:

https://harveyperformance.widen.net/content/mrt84hhfmn/pdf/SF_809500.pdf

Where the values are taken as an 'upper limit', and running at half the chip load values in the table will be ok as a starting point. The machine loading can be calculated using something like the spreadsheet I posted here:

Feed, speed, deflection and machine power spread sheet

As the others have said, start being conservative, and listen to the machine

 

Chip load = Feed Rate / Flutes / RPM is mathematically the same as Feed Rate / (RPM x number of flutes). Yes, the chip loads in the tables are ideally for a cut-depth of one tool diameter in most cases, but a similar chip load value is also good for cutting a more shallow pass. Even with industrial machines, it may not be possible to cut to one tool diameter deep for thin tools that have a very long stick-out to cut a deep slot as the tool would bend too much with the loading. Therefore the tables are a bit 'theoretical' in that they assume that the tool has zero stick-out and would not deflect with the specified cuts.

If the stepover is less than 50%, it is the spreadsheet line which is adjusted for chip-thinning that is the important one that relates to the cutter cooling too. Hobby machines are often limited in the maximum feed rate that is possible, or the power available from the spindle, as well as the overall machine rigidity. The feed rate limitation can often be helped by using a single-flute cutter so that a decent chip load can be achieved with a not-too-fast feed rate. The single flute cutter also means that it is easier for the cutting chips to be ejected without coolant, which is really useful for aluminium and melty-plastics like acrylic.

The single flute cutter also has the advantage that as the feed rate is slower than a multi-flute cutter, the overall material removal rate is reduced. The spindle and machine power requirements are related to the material removal rate, so it is more likely the spindle and steppers can handle the load of a single-flute, while still keeping the chip load up to cool the cutter. Using a smaller diameter cutter and just doing multiple passes will also help to reduce the machine power requirements; it can take a while to surface a spoil board with a 1/4" end-mill when compared to a 1" face cutter, but the machine loading is much lower with the 1/4".

Using a smaller cutter to reduce the machine loading often also means that a slightly deeper cut can be taken per pass. If the cut pass depth is too shallow, then for a machine that is not very rigid, there is a risk that the Z axis will ride-up onto the surface of the workpiece and the cutter will just rub, getting very hot. By using a smaller cutter and a slightly deeper pass, then it is much more likely that the tool will bite-in and start to cut before the machine can flex; ramping into a cut can also make a big difference. Flex in the machine can also cause issues when entering and exiting a cut from the side as the cutter may be pulled into the work material and try and take far to big an initial bite. Using conventional rather than climb cutting for some tool paths can be worth a try if there is a particular entry/exit point where the tool is grabbing the work piece.

 

I have just realised, I missed properly answering your question regarding chip-load and radial and axial depth of cut. The chip load cooling effect on a cutter is related to the quantity of material ejected per second. For a given radial cut width, if say you had a 1/4" cutter and doubled the cut depth from say cutting 1/16" deep to 1/8" deep, then twice as much heat is generated, but also twice as much material is ejected. Therefore for changing axial cut depth from very shallow through to certainly one tool diameter deep, the ideal chip-load basically does not change. As you go deeper than one tool diameter, it gets harder to eject the chips, so the chipload may need to change a bit then. For the radial cut width, from slotting at 100% width down to 50% stepover, nothing changes in practice. As you go below 50% stepover, then the chips are thinner than ideal, and eventually, some correction needs to be made, but in reality anything between 100% down to 30% stepover can live with the same chip-load.

Where radial and axial depth of cut really matter is in how much force is applied to the end of the cutter. Higher cutting forces will need more power from the spindle, and also more power from the steppers to push the bit through the work piece. Often the load is not high enough to cause the steppers to stall, but it may make the machine flex, but especially may bend a small diameter bit. The spreadsheet calculates the deflection of the tip of the cutter and puts the value out as a percentage of the cutter diameter. Although most cutters can tolerate a 1% deflection, it can start to sound awful at that point, especially for a long cutter. Long cutters suffer from flexing far far worse than short ones, so where possible, keep the cutter length and stick-out from the collet as short as possible when cutting hard materials.

Therefore setting the rpm and feedrate to get the wanted chipload will ensure the cutter stays cool, but the radial/axial depth of cut needs to be chosen based on what the spindle, the bit, and machine rigidity can handle.