Slot Milling Depth Of Cut

4/12/2022by admin

conposhiaprud1973.netlify.com › ▄ Slot Milling Depth Of Cut ▄

I got a note recently from a G-Wizard user who wanted to know how to decide on best depth and width of cut when milling. It’s a great question. Most machinists, I suspect, use rules of thumb and habit more than anything else unless the situation dictates something in particular based on the dimensions of the feature being machined. They’re used to using some fraction of the cutter’s diameter or some figure that they got to some other way through habit (40 thousandths or some such is what they’ve always used). Perhaps their CAM program has a hardwired default that is a percentage of the cutter’s diameter.

But these values, while they have worked over time, are not necessarily optimal figures with respect to material removal rates, tool deflection allowances, or a host of other variables we might choose to consider. What’s a more systematic way to approach the problem?

By taking a depth of cut of 1D per pass, you may go as deep as the tool shank will allow. Good chip flushing with either air or liquid coolant is essential so you are not re-cutting chips as they fill the pocket. It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all. I was wondering what others normally consider maximum depth of cut ratio to diameter of end mill with a roughing end mill milling cast iron medium hardness?? End mill was average length with 1.5' LOC flute for 5/8 dia end mill. Normally i would use a depth of cut of.31' to.625 with a.625 dia roughing end mill for a full width slot feeding straight across G1.

First thing is we have two variables (width and depth of cut), so it’s hard to make progress unless we can nail one of the two variables down and focus on the relationship of the other. It’s usually pretty easy to nail down one of the variables based on the situation. Let’s divide our work into two categories:

– Slotting: I’ll generalize this to be any situation where the material to be removed is very close to the cutter’s diameter. It may be a slot, or it may involve interpolating a hole or pocket that’s only a little bit larger than the endmill’s diameter.

– Pocketing: Here again, I will generalize this to be any situation where the cutter’s diameter is quite a bit smaller than the dimensions of the material to be removed. That doesn’t mean there isn’t some inside radius or other feature that isn’t more like the slotting example, but for the most part, we have some room to work in. Note that profiling will be considered to be the same as pocketing for this discussion.

Okay, so now we have to take the task before us and decide whether it is closer to slotting or pocketing. The reason I’ve defined these two the way I have is that it informs our choice of which variable to work on first. If we are slotting, the cutting width is the first variable. If we are pocketing, the cutting depth is the first variable. Why?

When slotting, the feature is very close to the cutter’s diameter in size. We can’t take a 1/2″ endmill and use it to make a 1/4″ slot. In general, we want to use the largest diameter endmill that fits the feature, and then we pretty much have to make at least one cut that is full width. Once we’re cleared that cut, anything remaining is handled the way we would under pocketing. So, when slotting, we focus first on cut width and make that the cutter’s width to get started.

When pocketing, our limitation will be the smallest inside radius we have to deal with as well as the depth of the pocket. Remember, it may be advantageous to make two passes. The first with a cutter that has a diameter too large for the smallest inside radii we have to deal with. That’s a roughing pass that uses a larger cutter just to get done faster. The second pass is a finishing pass, and must use a cutter whose diameter is less than or equal to that required to reach into the smallest internal radius the pocket holds. Note that we can go around an outside radius (a boss) with any diameter cutter, it is the inside radius that limits us.

So, we pick a cutter that is either as big as the smallest radius, or we choose to go two passes and go with a larger cutter. Let’s leave the two pass issue aside for the moment, because figuring out when that is optimal can take a bit of trial and error. Its similar to think of one pass. Given that the cutter is chosen, we can choose just about any width of cut we want. So how do we nail down a variable when pocketing? On the slotting case, I like to nail down cut width. On the pocketing case, I prefer to nail down cut depth.

In general, we get a nicer finish if we cut the pocket in as few layers as possible. CAM programs are good at layering down into the pocket, so we can pick arbitrary depths of cut. If I can, I like to do it in one layer for a pocket that isn’t two deep. If not, I prefer the depths of the layers to be equal. In other words, I wouldn’t go down 1/4″, 1/4″, and then 0.19″ on the third layer. So pick a layer depth that satisfies that criterion.

Now, in both cases we have locked one of the two variables–slotting locks width, pocketing locks depth. We need to determine the best value for the variable we left floating based on the value of the one we locked. This is where the G-Wizard Cut Optimizer makes it easy. Enter the values you know for the cut and let the Optimizer figure the value for the floating variable.

For example, let’s suppose we need to cut a pocket that is 3/4″ deep in 6061 aluminum. The smallest internal radius is 1/4″, so we’ve decided to use a 1/4″ 3 flute carbide endmill. Here is the problem set up in G-Wizard:

Material, Tool, Tool Diameter, Flutes, and 3/4″ Cut Depth Entered…

Now we can invoke the Cut Optimizer just by pressing the “Rough” button:

As you can see from the red arrows I added, for a 3/4″ depth of cut, this endmill can handle no more than 0.1799″ width of cut when roughing. Let’s round that down and go with 0.170″

Press the finish button to see what sort of finish allowance we should have the CAM leave for our finish path and we get 0.0052″. That’s a pretty light pass, but 3/4″ is deep for this 1/4″ endmill. Here’s an interesting thought: if we reduce tool holder to tip length to 0.9″ instead of 1″, we can increase that cut to 0.0095″. That gives you an idea of how important it is to keep the tool stick out as little as possible. I’d be inclined to go with choking up on the tool and a finish width of cut of 0.009″ were this my job. The other thing to consider is two levels of finish pass. If we don’t mind taking two levels and still choke up on the tool, we can get 0.015″ width of cut for the finish. That’s about as much as I like to take on a finish pass.

The problem with this cut is its a little bit deep for our 1/4″ endmill. That’s a 3:1 ratio of diameter to depth. We can tell its straining because the max recommended widths of cut are so light. If I had a CAM program that made it easy to make the roughing pass with a bigger cutter, I would be tempted to jump in with a 1/2″ endmill (or maybe even larger) for the roughing pass and then go to the 1/4″ for finishing, but you get the idea.

The slotting case is pretty similar, except for that case, instead of trying to compute the width of cut, we want to use the optimizer to figure out the depth. For example, if we continue with our 1/4″ 3 flute, let’s say we need to cut a slot 0.300″ wide to a depth of 3/4″. Our plan is to cut a full slot 0.250″ wide down the middle, and then finish it up by cutting the remainder on each side. How deep can we make our full slot passes? Once again, dial up the initial parameters, and this time, hit the “Slot” button. For roughing, the Cut Optimizer tells us we can cut to a depth of 0.3466″ before we get too much deflection. Two passes at that depth will get us to 0.6932″ deep. That leaves 0.0568″ on the bottom for us to finish and 0.0259″ on either side for the finish pass. Remember, we’re not cutting a full slot for the finish pass, so we treat it just like we did our pocket to figure out the width and depth of cut.

That’s all there is to it. To summarize:

1. Decide whether you are slotting or pocketing.

2. When slotting, pick a value for width, and use Cut Optimizer to decide depth.

3. When pocketing, pick a value for depth, and use Cut Optimizer to decide the width.

If you approach the problem this way, you’ll maximize your MRR’s while minimizing your tool deflection as appropriate for either roughing or finishing. That’s a much more optimal approach than the old wet finger in the wind!

For more thoughts on cutting parameters when milling, check out the Milling Surface Finish page.

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begin with the two slot test

A biscuit joiner should cut a slot slightly deeper than half the width of the biscuit you’re trying to fit. This space provides some wiggle room, but still ensures that about half remains on each side of the joint. Here’s how to check your joiner’s depth setting. First, make a test slot. Next, stick in a biscuit and mark a pencil line where the biscuit touches the wood, as shown in Photo 1. Now flip the biscuit and mark a second line. Adjust the depth-setting dial on your tool so that overlap is about 1/8', as shown in Photo 2.

THE face-TO-edge BISCUIT JOINT

To cut the biscuit slots for the shelf, draw a pencil line across the inside face of the side and clamp the edge of the shelf against the line. Next, mark your biscuit slot marks on the top of the shelf. Resting the biscuit joiner on its base, place it on the side and cut the slot for the shelf as in Photo C. The base also serves as your reference when slotting the sides. Turn the joiner on its head, use the line engraved on the bottom of the tool to align it with your slot marks, and plunge it into the side, Photo D. Once cut, insert biscuits into the grooves and flip the shelf up on the location line.

Stick the board’s end over the edge of your bench so that the fence, not the tool’s base, sets the slot depth.

Balance the biscuit joiner’s fence on the end of the side panel to cut the grooves in the face.

Face-to-edge joints are the bread-and-butter joint for biscuit joiners. A biscuit joiner can cut the needed joints to assemble a cabinet in about as much time as it takes to drag a heavy sheet of MDF to your table saw.

Building a simple box provides a good opportunity to explore the advantages and disadvantages of fence- or base-guided slot cutting. Sometimes, you’ll have the freedom to choose the method, but sometimes the decision is made for you. Box-making uses both.

First, position the side and end panels together and draw lines across the joint to mark where you want a biscuit. (As a rule of thumb, place the biscuits about 6' apart.) When you pull the panels apart, you should see marks on the edges and ends.

To cut the grooves in the edge of the end panel, place the work flat on the workbench, as shown in Photo A. Adjust the fence so that the blade cuts a slot roughly through the center of your work. To cut the face grooves in the side, you’ll balance the fence on the panel’s edge as shown in Photo B.

Remove or retract the fence and use the joiner’s base to cut the slots in the end of the shelf.

Don’t move your wood; just flip your joiner so that the base rides against the shelf. Plunge the joiner into the side.

Biscuit Sizes

To cut grooves for most of the biscuit sizes (0, 10, 20), simply adjust the stop on your biscuit joiner. Smaller FF (or face frame) biscuits are available for the Porter-Cable biscuit joiner. To use these mini biscuits, you’ll need to switch out the standard 4' dia. cutter with a smaller 2' dia. blade.

THE END-TO-EDGE AND MITERED BISCUIT JOINTS

Biscuits are also an easy way to build mitered and end-to-edge frames that you would use for picture frames or for a cabinet’s face frame. In this case, biscuits are very useful because they are adding reinforcement to otherwise weak end-grain joints.

Start by selecting the right biscuit. An easy way to do this is to position the joint together then arrange one or more biscuits on top. Drawing a center line on a test biscuit can help you lay out the slots as shown in Photo E. Wider biscuits provide more strength (even if you saw off an end), but there are times when you want the biscuit to be hidden within the joint. When form is as important as function, use the Biscuit Sizing Chart on page 13. When picking a biscuit, don’t forget the wiggle room. The slot is usually about 3/16' wider than the biscuit you’re trying to fit.

When slotting narrow pieces, don’t hand-hold the stock. Because a portion of the blade will be exposed while making the cut, and since the spinning blade can “walk” if the tool or work isn’t properly held in place, it’s too easy to get hurt. Eliminating the risk of a visit to the ER is easy: simply clamp narrow pieces to a stopblock as shown in Photo F.

You can now align the center mark on the joiner with the marks on your stock and make the cut. After making the cuts, glue the biscuits into their slots and clamp the frame together. Once the glue has dried, you can slice off the protruding biscuit tip as in Photo G.

Position the bigger biscuit so that it won’t protrude into the visible inside edge of the frame.

Use clamps and extra backup, not your free hand, when cutting narrow stock.

Saw off the biscuit tip with a handsaw or using your table saw and crosscut sled.

Double-Biscuit Joinery

Biscuits don’t fare well when matched against other joints in wood-joint torture tests. Because biscuits are relatively short, it doesn’t take a rocket scientist to realize that this joint isn’t as strong as traditional mortise-and-tenon or half-lap joints.

An easy way to strengthen a biscuit joint is to cut two grooves, as shown at right. In seconds, you’ve doubled the gluing area of the joint. Try to leave a ¼'- to ½'-strip of wood between the biscuits to maximize available gluing surfaces and preserve the strength of the wood. (With careful spacing, you can use the double-biscuit technique on a ¾'-thick rail and stile joint.)

To cut the grooves, I prefer to use spacers instead of my fence. Referencing the joiner and working against my bench is fast, and for those times when I forget to cut a slot, easily repeatable.

face miters

Face miters are A way to hide end-and edge-grain when making solid wood boxes or plywood cabinets, but long miters are tough to glue and tricky to reinforce. A few biscuits can solve both problems. They can register the ends so that the corners can’t slip during clamp-up and strengthen the otherwise end-grain joint. A biscuit-reinforced miter is as strong as a full table saw-cut spline, but biscuits can be hidden within the miter for a cleaner-looking joint. (With the table saw, you cut the spline from edge to edge.)

To cut a slot in an angled edge, use your fence. How you cut the slots in the mitered face depends on the fence-adjustment flexibility of your biscuit joiner and your working style. If your router has a fixed fence (or if you prefer working with your workpiece flat against your bench) you will want to try making your cuts as shown in Photo H. Set your fence to 45°, adjust the cutter depth so that it doesn’t slice through the thinner tip of your miter, and make a plunge cut into the end.

Some joiners, like the Porter-Cable 557 (see the Buying Guide) have a two-stage fence that can reference the outer face of the miter as shown in Photo I. This design prevents corner alignment problems, especially if the stock thicknesses aren’t equal. The wrap-around style of this fence also makes it easier to hold the tool in place while making the cut.

Adjust the fence height or add a shim so that the blade does not cut through the tip of the miter.

When the fence is set to 135°, you can squeeze the joiner against the miter as you cut the biscuit slot.

offset joinery

There are times, as when joining a rail to a leg where you may not want a flush-fitting joint. Your biscuit joinser is equally adept at creating offsets. The trick is to use a spacer that the same thickness as your desired step back. The spacer-offset trick can be used with your biscuit joiner’s fence, or under the base of the tool, as shown here. (I think resting the joiner on top of a large hardboard spacer is alot easier than trying to sandwich the spacer between the work and biscuit joiner’s fence.)

To make the offset joint, mark the leg and apron for the biscuit slot just as before. Now choose a spacer that is the same thickness as your desired offset.

Position the spacer under the jointer as shown in Photo J, position the leg with the show face against the bench and cut the slot. To cut the matching slot in the apron, simply remove the spacer and slot the end of the board as shown in Photo K. Be sure the show face is touching the bench.

Slot Milling Depth Of Cutting

Place a spacer under the joiner to raise the slot and set the height of the offset.

Slot the mating piece without a spacer. The biscuits will create the offset on the workbench-touching face.

THE EDGE-TO-EDGE BISCUIT JOINT

You might want to think twice before using biscuits for solid-wood edge joints (See “Biscuits or No Biscuits?”). but biscuits are very useful for attaching solid wood edging to sheet goods, and also when you need to straighten out bowed boards. With practice, you may be able to biscuit an edge joint in less time than it takes to read how it’s done.

Slot Milling Depth Of Cut Concrete

To start, position the boards edge-to-edge, then make short pencil lines across the joints about every 8' for the slots, as shown in Photo L. Make the first and last slots at least 3' in from the end; that way, you won’t accidentally expose a biscuit if you trim the panel later.

Next, set the joiner’s fence height to cut a slot roughly in the center of the board. To ensure that the joiner’s fence is setting the slot height, and not the base, clamp the board so that the edge sticks off your bench as shown in Photo M. Let the cutter reach full speed before plunging it into the wood.

You’re now ready for glue-up. Because the biscuits swell quickly, do a dry assembly and make sure everything’s set before you open the glue bottle (Photo N). Remove any sawdust from the slots, test-fit your biscuits, and dry-assemble the panel. If everything fits like it should, you’re set to reassemble it for good with glue.

Slot Milling Depth Of Cut Rings

Keep the fence flat on the board surface when cutting slots.

Use a biscuit-slot bottle to quickly squeeze glue into the slots.