Milling101

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See also: Grizzly G8689 manual mini-mill and Taig Mini-Mill

Milling is a subtractive process- this is sometimes expensive, if your stock is far from the final size. Note that a large "L" shape might be much cheaper if you use 2 pieces of bar stock and stick them together or bend into shape, for example.

NOTE: A thou is also frequently called a "mil". Not to be confused with millimeter.

Leadscrew is 16TPI. This makes it really funny to go by graduations on the handwheel when going >1 turn, and you'll just have to deal with it. That is, a single turn is 0.0625" so trying to go by handwheel graduations, 0.2" from where you are now is 3.2 turns, so 3 turns and 12.5 thou counted on the dial.

The graduations on the handwheel can be turned to zero it in any way you like.

Backlash (gear slop) is about 10.5 thou on X and 12 thou Y. Backlash not only affects the screw drive, but if you press hard on the table, it can move forward by that much. This will seriously affect "climb milling" (below). It's very hard to feel it on the table, but you can feel it on the handwheels. The gib locks will prevent that, but they lock the axis so it can't move at all.

Backlash can be avoided when you drive the screw in the same direction each time. This is not always possible.

Materials Capabilities:

  • Aluminum (love it!). Aluminum often benefits from cutting lube, but does not always require it.
  • Most plastics, melting is the major issue here. Sharp bits and, counterintuitively, HIGH feedrates can prevent that.
  • Wood
  • Brass- probably wanna use cutting lube
  • Bronze- probably use cutting lube
  • Fiberglass, if you have carbide bits. HSS dulls very quickly on FG.
  • Excessively soft materials- foam, rubber- can be really hard to clamp properly
  • Steel is HARD to work, you must understand work hardening issues to have any chance
  • Stainless- much harder than mild steel. There's multiple types of SS and one is not like the other.
  • Tool steel, spring steel- you'd have to be a glutton for punishment to even consider it
  • Copper- surprisingly difficult to work. Work hardens quickly, very gummy. Requires cutting lube.
  • Not acceptable: stone, glass, ceramic

Look up your materials' "Machinability Index", a guideline on how difficult the work is going to be.

Try to know your specific material.

  • 6063 aluminum is more "gummy" than 6061 aluminum, for example.
  • Extruded acrylic does not cut nearly as well as cast acrylic.
  • So many differences in types of steel under the name "stainless"

Work hardening is a problem in specific materials. After an initial stress, the material locally becomes much harder, making it wildly more difficult to cut, especially with any consistency, because you've now got a hard crust on top of softer base material. Primarily a problem with copper and some stainless grades. Possible solutions are climb milling (but the Grizzly can't do climb milling), avoiding multiple passes, or making sure the last pass is a big one which rips off the hardened crust without a problem. In the case of copper, you can use a torch to anneal the stresses away, but probably not while it's on the mill!

Lube/coolant/clearing:

  • Oil-based lube is not nearly as useful as water-based coolants. Or even water.
  • Lube can be spread on the part and bit before starting, and between passes as we go. You don't need a LOT. A lot of operations go just fine without it, most people mill aluminum without it.
  • Coolants reduce heat on the bit and material. Aluminum does not work as well when it's hot, and metal expansion throws off tolerance of the final part.
  • Some coolants are just mist. This is popular, and not all that messy. Usually water-based, which is easier to clean up and outperforms oil-based because water-based evaporates off hot surfaces and oil doesn't. We don't have that, maybe we can set one up.
  • FLOOD coolant is a gushing spray capable of flushing out chips as we go. It needs a collection scheme. We don't have that.
  • Chips which don't clear the cutting area can be a problem for a smooth finish, and can gum things up by being recaptured by the bit. Clear them. Brush, air, blow on them. Whatever works. Except reaching over there with the bit running.
  • You may need compressed air to clean chips out of a pocket. Be aware of the EXTREME danger of flying aluminum chips. Depending on pocket geometry, they can shoot right back in the direction you're spraying from. Eye protection for all in the vicinity.
  • Be aware some videos online about milling omit coolant for visibility, so you may see some brutal cutting operations somewhat "misrepresented".

Metal chips are very hazardous to electrical plugs (power strips in particular). The RACK SYSTEM is off to your left so don't spray metal hard in that direction.

Magnesium and titanium chips are a huge fire hazard.

As a Drill Press

The Mill can be used as a drill press - a really good drill press, BTW. Gib locks are there primarily for drill press operations, which prevents any backlash. Mills are far more accurate holes than large drill bits, they never "walk" as they go in, even on an angled surface.

The drill chuck is very poor for runout, and is only for drilling. Won't take the side forces of milling. NEVER use the chuck for holding milling bits.

Note that there's no protection against milling or drilling into the table other than the Z-stop. You must set this if you're trying to cut deeply so there is a risk of striking the table. To set Z-stop: Mount bit, leave mill off, position the bit over a spot where the bit can go down to the table without the stock getting in the way, plunge the quill as far as you want to go (and definitely above the table surface), bring the Z-stop up to the base of the Z-carriage and lock it there. Let it up and down a few times to be sure it stops where intended. Remember, the Z-stop will stop your quill so don't go thinking something is wrong and try to force it past it.

All internal box cuts are limited by milling bit radius. It's physically impossible to make an internal box WITHOUT rounded corners. Sometimes we achieve a square pocket by milling some extra out of the corners:

Clamping solutions

  • Clamping MUST be very stiff, esp for aluminum or steel. Visualize a hyperactive 2-yr-old with a #2 screwdriver and an aggression problem going after it. If you can picture it moving, it'll probably move during milling. It can't be able to lift, drop, rotate, or shift in either direction.
  • Failure of a clamping solution can cause chatter, which will result in poor finish, can chip the bit, or even send the work flying across the room
  • 90% of milling work is the clamping solution. This is hardly an exaggeration. Much of the skill learned is in clamping properly. You might spend an hour figuring out how to hold a part and 5 min actually milling it.
  • We have a clamping kit with T-nuts, step blocks, and top clamps, and two milling vices.
  • Top clamping simply won't provide access to the entire top at once. In some cases, clamps may be moved to an area you're done milling to expose the rest of it. We don't have a side-holding solution other than the vices.
  • Your stock size is limited by the ability to clamp to the table, rather than the "work box" (travel range of the leadscrews). However, it possible to clamp stock longer than the X-axis or Y-axis and only work on a part of it.
  • The mill is NOT bolted down. If you put something heavy on it offset, it can TIP OVER. You might start with something heavy on-center, then drive the X-axis down until it's NOT.
  • Rubber-faced clamping surfaces are NOT stiff enough for metal. Or even plastics sometimes, if the milling is aggressive
  • If you need to cut or drill all the way to the bottom of your stock, you must use a piece of SPOIL BOARD. MDF is very common for this. We've got tons of plywood and acrylic scrap which would also suffice.
  • Cuts which free an internal piece are a huge problem. Internal piece must be clamped before it goes free or it'll bang around like you won't believe, maybe go flying. Could break teeth on the bit, snap the bit, or bind it up and break a gear on the transmission. Most of the time we prefer to mill away internal pieces entirely if we don't need them.
  • When it's a drill press- and ONLY a drill press, WITH A DRILL BIT- clamping solutions are not critical. Well, no more so than any drill press operation. If you use a milling bit, EVEN IF you only intend to plunge, you MUST have a secure hold-down solution in place

Fear of Fly cutter grabbies. Don't wear loose things or gloves.

Feed and speed issues

  • Larger bits are run proportionately slower, because the speed of the tooth is higher.
  • More teeth should go faster to ensure the same chip size, but usually 2-flutes really are the best.
  • In many cases we must use multiple passes down the same path, going a bit deeper each time, to achieve a vertical wall.
  • Plunging deeply requires "pecking" to avoid clogging the bit with chips. They don't climb out of a bit very well, esp not a 4-flute. They usually don't coil up and out like on a drill bit. So, go in a little bit, pull it out all the way, drop back to where you left off, peck out a little more.
  • Too slow and you can melt/burn materials
  • You can work-harden some materials, which stops the bit or at least creates a horrible finish
  • Too fast is bad
  • Appropriate speed depends on feedrate
  • Chip size should be chips. Not dust or thread.
  • Feed and speed recommendations may be found online, but we're kinda screwed because the Grizzly has no numbers on the mill to quantify the RPM anyways.
  • This head always turns the bit CLOCKWISE. Just like a regular drill. If you're feeding the stock material towards you, and the work is on the left, this is CONVENTIONAL milling. If it's on the right, it's CLIMB milling. Here's a note of the differences:
  • On this mill, there is a lot of backlash, and this could cause serious jerking around in climb mode, esp on hard materials. I don't recommend it.
  • Cutting a deep channel is a chip-clearing problem. Don't go too deeply in one pass and clean out the chips between passes. If you're cutting a channel say 1.5x the width of the bit, don't cut the whole depth of one then the other. Cut one side, clean out chips, then the other, clean out chips, then step down. The space to the side will provide a place for the chips to go.
  • Same situation if you plan to cut out your "thing" on the right side of the stock and don't care what happens to the left. If it's a thick piece of stock, DON'T try a single-pass deep channel the width of the bit, it could clog with chips. Mill out something wider as you go.
  • Soft materials- plastic, copper, aluminum- can clog the bit and if the tooth isn't clear, it CANNOT cut with clogged teeth.
  • Generally this mill's speed is limited by its stiffness. Chatter starts to shake it around. Chatter can be reduced by locking unused axes.

Bit Types

  • HSS is cheap, wears somewhat quickly. HSS can overheat.
  • Carbide is now cheap. It's super-stiff and works great in the superior 2-flute geometry. Carbide does not wear easily, but chips easily on abuse like loose/chattering, dropping it out of the spindle when changing tools, dropping on concrete or tile, striking steel screws unexpectedly. Carbide really can't overheat.
  • Ball end mills can never flatten a surface without scallops
  • Endmills can flatten, but still scallop on a ramping surface
  • Ball end mills can scallop LESS on ramps, but it'll still be there
  • Bullnose is a combination of the two
  • Mostly, manual milling uses flat endmill. Smooth 3D shapes can't be made anyways.
  • Plunging is a major issue with many milling bits. Some bits are not designed to plunge at all, some 4-flute bits cannot. If there's any space in the middle not covered by teeth, it can't possibly plunge. Some 4-tooth have 2 teeth which meet in the middle whereas the other 2 teeth don't- this allows plunging. If you can't plunge, you're limited to starting from the side, or using a drill bit to start the pocket. All our 4-flute carbides CAN plunge, but never as aggressively as a 2-flute.
  • Single-flute creates a better finish on plastic and is wildly better at pulling chips out of the hole. Paul just ran an 1/8" bit at 1/8" depth through acrylic at 10K RPM and 35 ipm and it had no problem at all, and left a much smoother, clear finish on the acrylic. Single-flute is definitely great on plastic. I've seen mfgs say these carbide single-flutes work on wood and even aluminum. Which I'm skeptical of, but we DO have more than one of these if the experiment is not successful.
  • 1/4" is a common size. 1/8" is common. There's a ton of 1/8" carbide tooling with narrower flutes, but commonly flute length is 4x the diameter. So, "not very deep at all".

Don't break the gears. Don't stall the motor. Use low gear as opposed to turning down motor RPM If you can.

Motor starting procedure:

  • Plugged in
  • Turn RPM to zero (that's the trick)
  • Lift the yellow slam button
  • Flip the power toggle switch
  • Turn up th eRPM
  • The order isn't actually important EXCEPT the machine will latch in an error state if powered up without the RPM all the way down and clicked to "off". If you do this, just turn the RPM knob to "off" and turn back up again.
  • Emergency stop: hit the slam button, or toggle switch

Condition to leave the tool in:

  • Slamswitch closed, toggle off, RPM at zero
  • Bits removed and stored
  • Drill bits back in on tool shelf
  • Chips cleaned up, incl floor
  • Milling tooling should NOT leave the proximity of the mill. Only drill bits and the green vices may be moved.

Drawbar usage

Materials: the open-end box wrench, the funky curvy wrench, a tapping mallet

Removal:

  • DISENGAGE the fine-tuning adjustment on the Z-axis
  • LOCK the Z-axis
  • LOOSEN drawbar bolt on top, but don't unscrew it ALL the way. Just a couple of turns. We have to drive force through it, and must have threads engaged. Otherwise, only the end of the thread makes contact and gets dinged. It only needs to move a fraction of an inch to release its fit.
  • HOLD the tool you're going to remove, it will fall free.
  • Bang lightly (tap) on the top of the drawbar with wood or plastic. A metal hammer can mushroom the contact area.
  • Any tooling inside of a collet should fall out, the collet will remain screwed in.
  • UNSCREW the drawbar bolt the rest of the way and the collet will fall.

Installing tooling:

  • Select either the 1/2" or 3/8" collet or the drill chuck.
  • Place into the chuck
  • Hold it there while screwing the drawbar bolt into it
  • If it's a collet, place the tooling you want into it
  • Tighten the drawbar bolt with wrenches

Don't drive the X axis off the nut

If you break the tranny, fess up, it's no big deal, we've got 2 replacements.

Obtaining Metal Stock

  • Westbrook on Airport and Lamar
  • Metals4U on I35, north of Wells Branch on west side

Both are the same company and offer the same prices and stock. Aluminum is basically 3 categories:

  • Cut-to-order aluminum is about $0.445/cu in + $5 cut charge
  • Cut stock is stuff they've already cut, but still of valuable size. It's still $0.445/cu in, but no cut charge. Many modestly very useful sized pieces are only in the $10 sort of range, so even if you have to discard a lot because it's too big, it is still cheaper, and the scrap you cut off will always get used for something.
  • DROPS ("remnants") are pieces they've deemed too small or odd to be of significant value. Generally less than a foot long on rod stock, or plates might be several feet long but 6" wide. These get sold for $1.30/lb, which comes out to $0.12675/cu in.

Brass is considerably more expensive than aluminum, primarily due to the basic commodity value of its copper. Brass and copper is rarely found in the drops bin, but they do have a listing for it. Since it has a high recycling value, even drops carry a premium price. Brass is considerably denser than aluminum. The figure on the wall is a per-lb drops price, but due to the higher density, the price will be higher than the basic size of the piece suggests.

Tramming questions

Clothing

Metal chips

Cleaning

Lubrication

Protection of bits