Machining A T Slot In Mastercam
All the aspects of high speed machining can be a real challenge to wrap your head around. Filtering through what’s real and what’s just a buzzword can be a chore in and of itself.
Contour Machine flats to finish the horizontal face on the bottom. Download demo files Please note: To purchase HSM Performance Pack it is a condition that the customer already has licenses for the add-ons Mastercam 3D Mill and Mastercam Multi-Axis. T-Slot Milling Cutters 3mm Depth 25mm Cutting Dia 8 Flutes HSS Shank Diameter 12mm T Slot End Mill High Speed Steel Keyway Knife for Shallow Groove Processing On The Side. 4.5 out of 5 stars 2. KEO 71140 High-Speed Steel T-Slot Milling Cutter, Uncoated (Bright) Finish, Weldon Shank, 2-7/32' Cutter Diameter, 12 Teeth, 1-1/4' Bolt Size.
So is peel milling a technique worth learning?
Definitely.
Peel milling is an approach that uses high feed rates, low radial depth of cut and high axial depth of cut. It relies heavily on the principle of chip thinning, using a tool path that maximizes tool wear along the entire flute length.
Trochoidal milling is a particular type of motion – a circular high speed maneuver that is excellent for carving out deep slots and other narrow features. Incorrectly used, though, it can waste a lot of time.
Ok, that was pretty packed with information. Let’s break that up and use a few diagrams to explain what’s going on, how to do it properly, and how to know when your application justifies it.
Peel Milling Principles
The basic idea here is to use a small stepover, usually around 10% of the tool’s diameter, but max out the Z depth.
Of course, when using this method in the real world, you’re best off consulting the cutting tool manufacturer for recommended cutting parameters, but these numbers are usually pretty realistic.
For example, if you have a 0.500″ endmill, you’ll cut at a depth of 1.0″ but with a stepover of 0.050″. Compare that to a standard approach of cutting with a depth of 0.250″ and a stepover of 70%, or 0.350″.
Now if we look at the area of cutting engagement, we can work out what kind of stock removal we can get.
For the standard milling approach, we’re cutting an area that’s 0.250″ x 0.350″, or an area of 0.0875 square inches.
For the peel milling approach, we’re cutting an area that’s 0.050″ x 1.000″, or 0.050 square inches.
At this point, it’s not too impressive, is it? Peel milling doesn’t have nearly the same engagement as the traditional approach.
But there’s a secret sauce.
Peel milling can take advantage of something called chip thinning.
If you take a look at the size of chip that you get with such a low radial engagement, you’ll realize that it’s actually super thin. What this means is that you can crank up the feed rate to get a normal chip thickness.
According to those sketches, you can cut the material with a 70% higher feed rate and have the same chip thickness as the “traditional machining” model.
Now let’s convert those previous examples from 2D area to 3D volume.
To do this, we’ll need to add some material data to come up with realistic feed rates. Let’s say that we’re cutting 4140 HTSR. We’ll use a cutting speed of 400 SFM for standard machining. Let’s see what that stock removal looks like.
For conventional machining, the RPM works out to 3200 RPM. We’ll use a feed rate of 0.003″ per tooth, using a standard 4 flute endmill. This means that we’ll be feeding the cutter at 38.4 inches per minute.
Taking that same cut of 0.250 deep x 0.350 stepover, our 2D cut is 0.0875 square inches. To convert that to cubic inches per minute, we’ll multiply that by our feedrate.
0.1875 square inches x 38.4 inches per minute = 3.36 cubic inches per minute.
Now let’s compare that to peel milling.
To maintain the same 0.003″ chip thickness at 0.050″ stepover, the feed can be increased to 0.0051″ per tooth. Another perk of peel milling is that the RPM can also be bumped up.
So let’s increase that RPM to 500 SFPM, using a 0.0051″ chip per tooth. This works out to 4000 RPM, and a feed rate of 81.6 IPM. As mentioned previously, the 0.050″ stepover and 1.000″ depth of cut works out to an area of 0.050 square inches.
0.050 square inches x 81.6 inches per minute = 4.08 cubic inches per minute.
That’s about 15% faster material removal than the traditional approach. But the advantages of peel milling don’t end there.
Maximizing Tool Wear
One of the real disadvantages of the traditional roughing approach is that wear is very concentrated to the bottom of the end mill.
Using that same previous example, what would our endmill look like after an hour of work?
The bottom 0.250″ would be worn out, and the top 0.750″ would be totally fresh. Not really maximizing the use of the tool, is it? Now aside from the cost of the endmill itself, add the down time of the machine operator swapping the tool for a fresh endmill. Overall, it’s just not an efficient method.
Now compare that to the tool that’s used for peel milling. What would that tool look like after an hour? Instead of all the wear focused on 25% of the tool, the wear would be evenly distributed across the entire flute length.
Also, because of the small radial engagement, the individual flutes are actually in the cut for less time. Let me illustrate.
To put it simply, the actual working time of the cutter’s flutes is about 1/3 of the time for peel milling compared to traditional milling.
Does this mean that your cutters last 3x as long? Not always, in my experience. But I do tend to improve tool life noticeably, especially in hard to machine materials like titanium, inconel and cobalt chrome.
Trochoidal Peel Milling Toolpath
In the previous examples, peel milling was used to clear out easily accessed material. But what if you need to cut a deep slot? Can you still use the principles of peel milling?
Definitely. This is where trochoidal peel milling comes in.
To begin, let’s look at what a trochoid is.
The Trochoid
Here’s the basic concept of a trochoid:
Imagine swinging a weight over your head at the end of a rope while you walk forward. That basic motion of a point rotating around a center and moving forward is a trochoid.
Here’s an illustration of the motion:
What you essentially get is a carving motion. For toolpaths, the forward advancement is low, but the “spinning” motion is done at high speeds. This is what keeps the cutting pressure light.
Here’s an example of what one of these toolpaths looks like in a CAM system:
As you can see there, rapidly feeding the tool forward with sweeping motions can maintain a consistent radial engagement.
Chip Clearance
Trochoidal milling is a really effective approach for tight areas where chip clearance would be an issue.
With traditional milling, it can be difficult to evacuate thick, heavy chips from deep features like pockets. This means that these chips will be cut again as the tool feeds around within the feature.
This additional, uncontrolled cutting of metal chips not only adds substantial wear and tear on the cutting tool, but it also adds instability to the operation. A buildup of chips can unexpectedly snap the cutter in half.
Trochoidal milling is an excellent way of clearing chips out of deep features. Instead of the thick, heavy chips of conventional milling, chips from peel milling are long, slender and light. That means that coolant pressure or an air blast can easily clear them away from a workpiece.
One thing worth noting, though, is that peel milling chips are made really fast. This means that your coolant or air pressure needs to be very reliable. If there are any hiccups in delivery, things will go south fast.
What’s Required for Peel and Trochoidal Milling?
I know a lot of guys that have tried this with standard tools that are intended for traditional machining. Then when something blows up, it’s (in their opinion) because peel milling is a gimmick and doesn’t work.
I can assure you that it works very well, but you need to make sure that you’re using the right tools and equipment.
Tool Holders
Peel milling puts a lot more pressure on the tool, and has the potential of being a nightmare for vibration. Your tool holder needs to be really solid to be able to handle this high performance milling.
What does this mean?
Don’t use weldon shank or ER collet end mill holders.
Seriously, you will pretty well never have great results with these. They just don’t hold the tool firmly enough and dampen vibration. Even if you use a 48″ cheater bar.
Aside from the really high speeds and feeds that you’ll achieve with peel milling, tools have a tendency of pulling out. This is because the teeth are hitting the workpiece so aggressively that the helix of the flutes will push the cutter down and out of the tool holder. Weldon shank holders and ER collets do very little to resist this.
My favorite option for this is a good quality hydraulic tool holder, although I’ve also had very good success with shrink fit systems.
What I like about the hydraulic tool holders is their versatility and vibration dampening qualities. They don’t have the tendency to ring at certain frequencies like other systems do. And, to fit up an endmill of a different diameter, you only need to change the sleeve.
Shrink fit systems are also excellent. The clamping force of the steel body once cooled down is insane, so tools clamped in this way are really rigid. The disadvantage to this is that you need a system for heating the holders up to change the endmills. This can be a pretty hefty startup cost, so it’s a bit of a commitment for shops to gain this capability.
One technology worth looking in to is Safe-Lock from Haimer. This is a really amazing way of stabilizing carbide endmills and making it essentially impossible for cutters to pull out from the tool holder. You’ll notice that you can push the tool much harder with a system like this.
Even though Haimer is the one that holds the patent to that technology, they make it available to other tool manufacturers through license, so you will see this offered by other brands.
Specialized Cutters
Here’s another interesting aspect of peel milling: The chips are always slender and thin.
We can use this to our advantage when it comes to cutter selection. This isn’t so much an absolute requirement, but it will help you take advantage of the high performance capabilities of peel milling.
The traditional approach of thick, heavy cuts meant that the tools needed a large amount of space between flutes for chip clearance. These chips needed somewhere to go, so large gaps needed to be designed into these cutting tools to accommodate them.
With peel milling, you don’t need that massive chip clearance.
What this means is that two things are possible:
- The flutes don’t need to be as deep on a cutter, so the core diameter can be substantially thicker. This means that you can use a much stronger tool and push it significantly harder.
- You can use more flutes. Instead of a standard 4-flute endmill, many tools designed for peel milling have 6 or 8 flutes. This alone can potentially double the possible feed rates.
Now considering the example we had mentioned before comparing traditional vs peel milling, we can see that with specialized tools, we can get significantly more efficiency out of peel milling.
This is why many shops have been able to double their material removal by investing in specialized equipment.
High Speed Mills
If you have a tired, old machine from the ’80’s, you might not be too impressed with it’s interpretation of peel milling.
Especially for things like trochoidal milling, you need a fast machine. And not just maximum feed rates, either.
A machine needs to be able to handle hard acceleration and deceleration, or else you’ll never get up to speed. You might notice on your machine that when you’re doing tight corners, the feed rate that’s displayed on the controlled is often constantly changing, slowing down on sharp turns.
Your mill needs to be able to make small, fast movements accurately. For most machines, you can punch in a line of code that will change the modes – from high speed mode to exact stop mode and everything in between. In high speed mode, it needs to be able to honor the programmed toolpath without significantly overshooting.
Otherwise, you’ll have a lot of broken tools when your machine ends up overshooting and pushing your tool 0.080″ into the material instead of the intended 0.050″.
Aside from having fast, responsive servos to handle sharp changes in direction quickly, your machine needs to be a fast thinker.
For high speed machining, you might end up with programs with millions of lines of code. If your machine controller can’t read the code fast enough, then all your potential efficiency from peel milling will be lost while your machine tries to figure out its next move.
Machines that are well equipped for high speed machining will advertise a high “block look-ahead”, often something to the tune of 10,000 blocks.
What this means is that the controller will read 10,000 blocks ahead of the current block to be able to “plan out” the most efficient way of hitting this toolpath within the allowable tolerance. If your machine doesn’t have this ability, then you might find that your feed rate never actually hits what you programmed it to be.
Cam System
This is fairly straightforward. In order to benefit from peel milling, you need to have software that is capable of it.
The good news is that peel milling has been around for a while, so the large majority of decent CAM packages will have something to offer.
They’re not all created equal, though. Some allow you to have a tighter control than others. If you’re looking at a CAM package for your shop, take your time and see how good both the CAM toolpaths and the postprocessed results are.
The reason that I say to pay attention to the postprocessor is that with peel milling, you will have a lot of small movements. You want this to be calculated accurately as arc commands (or splines if your machine has a Siemens controller) without rounding issues. If you have to resort to using G1 line motions because you keep getting errors on your machine controller saying that there’s a problem with your G2 and G3 values, you won’t be a happy camper.
Really, this is becoming less and less as a factor as peel milling becomes more mainstream, but even still, it’s a factor to pay attention to.
When is Peel Milling Practical?
There’s a reason that this isn’t the only operation you can select in your CAM software: while it is a great solution, it can’t be applied to everything.
Peel milling works best when you can really sink your tool deep in the material. In other words, if you’re doing shallow pocketing, you’ll be better off with another strategy.
Slotting
Trochoidal milling is very often the best solution for slotting, but again it really does depend on the feature geometry. For example, the slot might be so deep that the only reasonable way of cutting it is either with a slitting saw or an EDM. If the slot is 0.050″ wide x 1.000″ deep, no endmill will help you.
I find the sweet spot for trochoidal slotting to be where you can maximize the depth of an endmill that’s between 50-75% of the slot width. In other words, a half inch endmill with a trochoidal toolpath will be an excellent choice for a slot that’s 0.75″ wide and 1.000″ deep.
Pocketing
This is an excellent choice for deep pockets. The thin, light chips are easily blasted out of the pocket with and air blast or high pressure coolant, and the material removal rate is very high if you can use the full flute length.
Hard or Exotic Materials
When I’m roughing titanium, I use one of two methods: plunge milling or peel milling.
The material removal rates of plunge milling in titanium are second to none, but the leftover scallops can be a pain to clear out. They often require an excess of semi-finishing operations before you can run your finishing tool to complete the feature.
Peel milling, on the other hand, can often rough and finish in a single operation. Especially for deep and narrow features, it’s hard to beat.
Peel milling really shines when it comes to hard and abrasive materials. Since it distributes wear along the entire flute length, it can be a great solution in applications where notch wear or chipping is common. Here are some materials where you should seriously consider using peel milling as a practical way of roughing:
- Titanium
- Inconel
- Cobalt-Chrome
- Hardened tool steels (above 50 Rc)
- In general, any material that is work-hardening, abrasive, or hardened.
When Peel Milling Doesn’t Make Sense
When you can’t use a good amount of your flute length, then peel milling probably isn’t a great option.
For example, if you have a shallow slot – let’s say 0.750″ wide x 0.375″ deep – you’ll be better off going the traditional route of using a 3/4″ endmill and burying it.
If it’s an exotic or hardened material, peel milling might make sense even if you can’t use much flute length. Sometimes the solution is to simply use a smaller cutter. Or look into other strategies like hi-feed cutting (low Z depth, full cutter width stepover, and very high feed).
Ultimately, you’ll get a good feel for it once you’ve tried it a bit. Maybe try programming the toolpaths a few different ways and see which one seems to be able to achieve the best cycle times. Just make sure that your machining conditions are set appropriately for each strategy – don’t use standard feed rates for peel milling.
There you have it, you’re ready to take on the peel milling world.
Do you have any tips and tricks for high speed machining? Or do you have any questions about peel milling? Share them in the comments below!
Introduction
Workholding is the generic term for any device used to firmly hold your workpiece while machining it.
Closely related terms are “jigs” and “fixtures.”
A fixture holds your workpiece while it is being cut. A jig holds the workpiece and also guides the cutter. Given CNC, there’s little need for jigs as the g-code guides the cutter, so the term is largely related to manual machining.
“Fixtures” are colloquially workholding solutions that are custom made for a particular part or situation.
There are two components to workholding:
– The actual workholding device, such as a milling vise.
– The method of locating and securing that workholding device to your machine. This includes the ubiquitous T-Slots but goes on to include modular fixture plates, 4th axis solutions, and much more.
We’ll go through the various methods of locating the workholding devices and then follow up with a description of your choices for workholding devices.
But first, let’s talk about why Jigs and Fixtures are important and how to know when you need to make a special jig or fixture.
Jigs and Fixtures: Show Me the Money
There’s an old saying among machinists–fixtures are where you make your money. If you’re good at making fixtures that save time, you’ll turn a bigger profit. Or so the saying goes.
But is it really true? Is it always true? Will every job benefit from fancy fixturing?
I put together a Free Fixture Calculator that makes it easy to find out whether a fixture is a good idea for your job or not. It will even figure out what the savings (ROI) will be if you build the fixture by comparing two different workholding options.
The free fixture calculator make sit easy to compare scenarios, and it even has one click options so you can factor in:
- Repeat Runs: What are the savings once the fixture is made and the customer reorders another run?
- Modular Fixturing: Setup time is saved when you can just drop a fixture on the pins with a modular fixturing plate and run.
- Pallet Changer: Perhaps the ultimate is a pallet changer that allows you to load/unload a pallet while the machine is working on another pallet.
I’ve also made a video that walks you through the savings that are possible from fixtures and demos how to use the free fixture calculator to determine what the savings on particular scenarios might be:
I want to give a special shout out to Dave Bishop over at MiteeBite. Dave and the MiteeBite gang have forgotten more about fixturing than most of us will ever know, and he gave me some great suggestions to improve the fixture calculator. Also, if you’re doing job cost estimation, be sure to check out our G-Wizard Estimator software. The Fixturing Calculator started out there, and there are many other useful calculators and functions in the software to help with job cost estimation.
Now that you have the means to see how jigs and fixtures can benefit your work, let’s dig into the details of how it’s done!
Positioning for Workholding Devices
T-Slots
T-Slots are the most common method of positioning and holding down your workholding solution…
T-Slots are by far the most common way of positioning and holding down your Workholding solution. They are simple, robust, and they work. To attach something to a T-Slotted table, use T-Slot nuts and suitable studs or other fasteners that fit the nuts:
T-Slot Nuts…
While they are common, they have some disadvantages relative to other solutions. Aside from the fact that T-Slots can collect chips and other debris, their biggest disadvantage is that it’s hard to get your vise or other Workholding fixture back onto the table in exactly the same place and orientation. This can result in extra work every time a machine needs to be set up with new workholding for a new job. Over time, the cost of that inefficiency can be quite large.
Just imagine, what if instead of having a tool changer and a tool table you had to dial in each tool every time it was used? Wouldn’t that be a huge productivity block in your machining workflow? Well, setup time can be a big productivity block too, and T-Slots are not helpful for that.
There are some solutions that’ve been tried to make them a little better:
Truing the Slots
We can check the T-Slots of machine to make sure the run parallel to axis motion by sweeping them with a Dial Test Indicator (DTI). If they don’t, we can mill them true. A lot of folks hate the idea of intentionally milling their table, but if the T-Slots aren’t true and you need them to be, it’s hard to see what choice you have. But there is a choice that involves using something other than T-Slots as we will see shortly.
Keyed Vises and Fixtures
If your T-Slots are true, you can install keys on the bottom of vises or fixture plates that line up with the T-Slots. You can also install keys in the T-Slots that line up the edge of a plate or vise base. Doing so can save you quite a lot of time tramming vises and such, and it isn’t hard to do, so it is definitely worth considering.
The trouble is, these kinds of solutions will help with one dimension (typically the short dimension of the table is the Y dimension and is aligned perpendicular to the slots), but we still have the problem of positioning along the axis of the T-Slot.
Fortunately, there is a better way in the form of Fixture Sub-Plates (also called Tooling Plates).
Fixture Sub-Plates, Tooling Plates, and Modular Fixturing
Fixture Sub-Plates (also called Fixture Plates or Tooling Plates) are plates that are installed on top of a T-Slot table to provide a new way to position and secure Workholding. A typical Tooling Plate looks like this:
A typical Tooling Plate…
Tooling Plates typically use a grid of holes that alternate between holes for precision dowel pins for positioning and threaded holes for fasteners. If this grid is precisely positioning (or even if it isn’t and the positions are precisely known) you have a very repeatable way to install Workholding on the plate. The dowel pins provide precision location that is repeatable to perhaps half a thousandth. Imagine being able to drop a vise mounted on its own fixture plate with dowel pins and fastener holes onto a Tooling Plate and have it be repeatable to half thousandth. If all your fixtures can drop onto the Tooling Plate you can change the machine over to a new Workholding configuration very quickly indeed. The savings in time adds up to pay off the cost of such a system very quickly.
With an air ratchet in hand, a vise can be installed on one of these plates within a minute or two. A CNC Machine can be reconfigured in 5 or 10 minutes for a completely different job. And the skills required of the machine operators, as well as the potential for mistakes, are greatly reduced if the fixtures don’t have to be carefully dialed in each time. There are advantages for the creation of modular g-code too, because it can rely on the positioning grid of the Tooling Plate.
If accuracy of more than half a thousandth is required, it is often better to use probing together with selected g-code parameterization to correct for the remaining error. You could try to dial things in more precisely by hand, but a probing solution can rely on things being nearly right to determine the last little bit of error correction that needs to be applied in the g-code itself. For example, it’s possible to apply rotation to the g-code based on a probes results to tram (that’s aligning things to the axis motion) very precisely an almost right situation.
Tooling Plates are typically made of either Cast Iron or Aluminum, though there are steel ones available too. They can be purchased or made from scratch. For a complete guide, be sure to visit our Fixture Plate page.
Modular Fixturing
One other thing Tooling Plates help facilitate is Modular Fixturing. Once we have a fixed grid to rely on, we can purchase ready-made fixturing components that will fit the grid. This can save quite a lot of cost versus having to fabricate everything to some custom arrangement.
Ball Locks and Other Quick Change Tooling Plate Solutions
By now I hope you’re seeing how much Setup Time could be saved by using Tooling Plates. What could be better? Why there are at least two different ways to improve on the theme: quick change tooling plates and pallets.
With a Quick Change system, the time required to deal with the dowel pins and fasteners is reduced through some kind of integrated solution that allows precise position and locking very quickly. One such is the Jergens Ball Lock system:
Ball Locks are a Quick Release system for Tooling Plates…
With this system, you get precise positioning and secure holding with 4 Ball Locks. Just align the plate to the sub-plate (which has the Receiver Bushings and is mounted to the table), drop the Ball Lock Shanks into the hole, twist the bolt on top of the Ball Lock Shank, and you are done. Twisting four bolts and no fooling around with dowel pins or extra fasteners is really quick and easy. Jergens talks about 30 second fixture change times which is very fast indeed.
Pallets
The next step up are Pallets. Think of them as power operated Tooling Plates whereas everything else we’ve talked about has been hand operated. A typical machine with Pallets allows you to be setting one or more Pallets up while the machine is working on another. A Pallet change operation cycles the old Pallet outside of the machine’s milling area and brings the new one in. This minimizes the time the machine has to be offline and allows Setup to be done in parallel with machining.
Some machines have what are called “Pallet Pools”, which allow multiple pallets to be set up in advance and scheduled to run. A Pallet Pool can allow a machine to run unattended for quite a long time and can be a useful building block for total automation.
Pallets are typically only seen on Horizontal Machining Centers and some high end Vertical Machining Centers. They’re definitely a full-on production feature that is fairly expensive, so the cost has to be justified by the need ot keep the machines churning out parts constantly.
4th Axis, Trunion Fixtures, Tombstones, and Tool Columns
Sometimes it is helpful to be able to apply another dimension to our thinking–in this case a 4th Axis. In CNC, a 4th Axis is commonly a rotary axis that is aligned to spin along an axis parallel to one of the machine’s other 3 axes. On Vertical Mills, the 4th Axis is frequently parallel to X or Y, and is laid down. On a Horizontal Mill, the 4th Axis is also parallel to X or Y, but it is standing up. Both methods work great, but the horizontal mill’s standing 4th axis frequently has more clearance available since the work is never trapped between the table and the axis.
From a Workholding standpoint, a 4th Axis can be used to bring new orientations into play for two purposes:
1. It allows access to more sides of the part so machining can continue without having to flip parts around by hand.
2. It allows access to more parts which may be arrayed around the 4th Axis.
To find out more about these uses for Workholding, check out our excellent series on 4th Axis Basics.
Is Manual Machining Faster than CNC for Simple Parts?
Workholding Solutions
Having seen how we’re going to position and attach our Workholding Solutions to our milling machine, let’s delve into what types of Workholding are available.
Milling Vise
A pair of milling vises sit side by side on a machine table…
It’s not too much of a stretch to say that the most popular Workholding Solution today is the Milling Vise. While there are many manufacturer’s of such vises, the poster child is Kurt who shipped their first “Kurt Vise” in the 1950’s.
For much more in-depth coverage of Machinist Vises, see our Complete Guide to Machinist Vise. It’s chock full of great info.
Plates, Plate Fixtures, and Clamps
As useful as vises are, they have a sweet spot for part size. They have a difficult time with really large plate work, though as mentioned you can move the jaws to the outside positions for medium-sized plates. And, they can also be less than optimal for very small parts. Sure, you can fit multiple parts in a set of Soft Jaws, but the space between adjacent vises and the space required for the vise mechanism makes it hard to fully fill the table with as many small parts as you might otherwise be able to.
It’s hard to achieve this dense packing of smaller parts with a vise, but a plate fixture makes it easy. This one uses Mitee Bite Pit Bull clamps. The effect is not unlike tiny milling vises designed to fit each part just right.
When its time to machine large plates or a great many small parts, it’s usually time to pull the vises off the table and use a workholding solution based on clamps.
Step Clamps
The most common type of clamps are called step clamps because they have little steps machined on them. They’re commonly used with T-Slots, although you can also use bolt them into a Tooling Plate. Here are some typical Step Clamps:
Step Clamp holding down plate, step block supporting end of clamp, and bolt goes through T-Slot Nut…
A typical Step Clamp Set…
Pictured is a typical Step Clamp Set. It can be handy to stock up on a an extra set so you’ve got more clamping parts to work with. By stacking the Step Blocks and using longer bolts, you can clamp workpieces that are quite tall. When using step clamps, keep the bolt close to the workpiece rather than the step block. Also, it can be helpful to angle the clamp down on the part by raising it up a step or two from level. You may also want to put a shim of soft material between the clamp and the workpiece if you want to avoid marring the workpiece. Soda can makes a great shim for this purpose if you slice out some strips with your snips.
Toe Clamps
Step Clamps grip the top of the workpiece, which is sometimes inconvenient because you might need to machine the area being gripped. Toe Clamps grip the side of the workpiece to give you full access to the top of the workpiece. There are a wide variety of different styles available:
This toe clamp moves the clamp down a ramp when tightened to press against the workpiece…
These Mitee Bite Edge Clamps have an eccentric bolt head that forces the hex against the workpiece as you tighten it…
Tab Supports: Workholding Via CAM Software
What if your CAM software could solve some major workholding problems for you, wouldn’t that be neat?
Turns out some CAM software can help out with workholding quite a lot by automatically creating tabs. Here’s a video of MeshCAM’s facility for creating tab supports to help machine a part that has to be machined on 2 sides and is awkward to hold otherwise:
Using MeshCAM to add tab supports so you can machine a part on 2 sides…
We offer MeshCAM at CNCCookbook because it’s one of the easiest to use CAM packages there is.
Double Sided Tape, Glue, Wax, and Low Melting Point Alloys
Some workpieces can be very difficult to hold because they’re too thin or because their shaped so there’s just no way to clamp on. Some solutions for these situations are in the form of Double Sided Tape, Glue, Wax, and Low Melting Point Alloys.
The Glue needs to be something that will release when needed. For example, Super Glue releases at a particular temperature just like LocTite. The fumes from it are toxic, so take care to release with plenty of ventilation. Double sided tape can work great, especially for really thin materials.
Wax and Low Melting Point Alloys (typically Bismuth alloys) can be used to embed the workpiece and create grippable area. When the machining is done, one can melt the wax or alloy away and save for reuse.
Vacuum Fixtures
Need to apply uniform pressure to hold a part down? Maybe a vacuum fixture is the answer. Sea level air pressure is 14.7 lbs per square inch. A vacuum fixture pumps a vacuum under the part so that air is pressing down on every square inch of the top with 14.7 lbs of pressure. That can create considerable holding force if there is enough surface area regardless of the shape of the top or how thin the material may be. We have a nice article on how to create your own vacuum fixtures that’s very popular. For more in-depth information on vacuum workholding visit our Vacuum Table Page and our DIY Vacuum Table Page.
Perhaps the biggest disadvantage of vacuum fixtures is their hold-down force is limited by surface area–that means small parts can pop off relatively easily. When cutting forces exceed the hold-down force a vacuum table can pull, the part pops off and is generally ruined. This is a common problem for vacuum table users, especially for smaller parts that don’t have much surface area. Until now, fixing the problem has been hit or miss, but our G-Wizard Calculator software now has the ability to limit cutting forces to what your vacuum table can handle. For more information, check out our article on this special feature.
Chucks and Collets: For Round Parts
While we typically think of machining round parts on lathes, there are many times when you may need to mill them as well. If you’ve got a Mill-Turn machine, it may not be necessary to put them on a mill, but if you don’t, or if you just need to work on some round parts on the mill, you can use the same workholding solutions Lathes use. Just bolt or clamp them down to your mill table. For example, use a 3 jaw chuck or a bank of collet chucks.
Lathe Chucks are particularly common on 4th Axes because we often start from round stock.
Every now and then we put round parts on the mill because it’s just downright faster. Consider this setup for machining round parts::
I never would have guessed it, but Geof on CNCZone says the following 4-axis mill set up was able to machine these aluminum bars to length, ensure the faces were square, and drill and tap a hole faster than he could do it in a lathe. You can imagine each of those round parts being a 5C collet and suddenly you fit quite a few more parts on your 4th Axis. There’s probably room to do them on all 4 points of the compass on a rig like this.
Expanding Mandrels, Arbors, and Studs
Toe Clamps are one solution to keeping the workholding away from the milling, but we can do even better using expanding mandrels, arbors, or studs. The idea is to put an expanding cylinder in a hole on the underside of the workpiece and expand it to lock the workpiece in place. Once that’s done, you can access the workpiece from every direction except the bottom without encountering the workholding (do remember where the mandrels are though so you don’t have one in the middle of a pocket waiting to be hit!).
Here is a fixture using expanding studs:
Turn the bolt opens the expanding stud so it can clamp the workpiece…
There are a wide variety of these kinds of gizmos available for your workholding needs. They’re particularly common for lathes, but as we mentioned, you can use lathe workholding all day long provided you find a way to mount it to your table.
5-Axis Workholding
5-Axis workholding, like most things 5-axis, is a whole other world. I won’t go into any detail here other than to say you need different kinds of workholding when you can access a part from virtually any direction. It becomes more challenging in this kind of workholding that the workholder not get in the way of milling the part.
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