When using Hole Callout to dimension a slot in SOLIDWORKS 2009 or later, this is the type of dimension that is inserted. Method (b) The third method (c) provides the width and overall length of the slot in linear dimensions. This method is preferred if the slot has positional tolerances that use the boundary method (see ASME Y14.5M-1994 figure. Machining a groove or slot, often called full slotting, involves three machined faces: Slots closed at both ends are pockets, requiring end mills that can work in the axial direction Full slotting with an end mill is a demanding operation. The axial cutting depth should be generally reduced to.
Undercuts are something that every machinist, CNC or otherwise, will eventually have to deal with. There are a couple of ways to deal with them but I have a definite favorite that might be a good trick to take a look at.
What is an Undercut
An undercut is an area of a model where one part overhangs another part, creating a void in the middle that cannot be reached by a traditional 3-axis mill or router. That probably sounds like gibberish so here’s a picture-
That “hook” creates an area between it and the back plate that cannot be machined. You’d have to leave a section unmachined or break it into multiple parts and bolt or glue it together.
Slots Thru Machining Definition
It’s worth noting that an “undercut” is specific to a direction of machining. The part above only has an undercut if you machine it from the top or the back. You could flip it on it’s side to machine it but that’s not really a cure all- I could easily come up with a part where that is not possible. We’re using a simple part here to illustrate a point.
Special Tools
Special tools are sometimes used to machine simple undercuts. These are sometimes called “slot cutters” or “lollipop cutters” depending on their shape. Both tools feature a tip that is wider than the shaft. This gives the machinist some limited ability to reach into the undercut- but only by the difference between the tip and shaft diameter. For many uses this will be fine but it is certainly not a universal solution.
A big negative for these tools is that most CAM software does not support them directly. Because these special cutters are infrequently used, only the high-end CAM programs include algorithms that can utilize them. For the rest of us, special cutters require manual gcode, tricking the CAM program, or moving the mill manually. I’ve never had a case where their use outweighed their negatives.
It’s easy to see that the cutter above may not have the reach to machine the whole undercut in our example part. Surely there’s another way…
Think Like a Plastics Engineer
I spent years as an electrical engineer working with factories in China, ignoring the plastic and mechanical parts of my projects. One day a friend of mine showed me how he modified a part to eliminate a small undercut that would cause excessive wear in one of our molds. It was a common practice for plastic injection molding but it was not something that ever occurred to me- cut a hole in the back of the part for a tool to get in. In retrospect it was obvious but it wasn’t something I ever spent much time thinking about.
This access hole in the back of the part allows you to flip the part over and machine the undercut from the back. MeshCAM has a built-in “two-side job” type that makes this really easy. Below is what the front and back toolpaths might look like:
The red arrow shows where the tool is now able to access the undercut and remove that material.
A More Complicated Part
The same technique above can be applied to more complicated parts that would be much more difficult to machine in their “standard” configuration. Here’s one one of those, half a hinge:
The hinge is now easy to machine from two sides and would be easy to mold if necessary without slides or inserts. It’s not exactly going to work for a restoration job on an antique piece of furniture but it’s still a cool trick.
Play with Toys
If you have kids then you’re probably going through toys at a rapid pace, try opening a few up and looking at how they are designed. You will likely be shocked at the number of shortcut and tricks used to make complicated mechanisms that can be produced in simple injection molds without undercuts. Most of these injection molding tricks are applicable to the milling process as well.
Not a Cure for Every Case
Cutting a hole in a part is not going to work every time but it does work more often than you think and it’s one of the easiest ways to machine a difficult part.
noun
1North American A coin-operated gaming machine that generates random combinations of symbols on a dial, certain combinations winning varying amounts of money for the player.
‘Pachinko is a combination of slot machine and pinball game; players can control only the speed at which small stainless-steel balls are fed into the devices.’- ‘It's a very special relationship and I've seen it on both video game players and on slot machine players and the industry will say that gambling problems are purely from within the individual.’
- ‘As time progressed, computerised versions and all sorts of clever entertainments have been merged into the basic game so that a Pachinko machine is often more like a Fruit Machine or slot machine than a ball game.’
- ‘Delaware Park plans to incorporate a new player tracking component in its racing and slot machine gaming systems.’
- ‘I fail to see how risking your hard-earned money on the outcome of a sporting event, the random machinations of a slot machine, or the numerical suicide of a lottery makes life more interesting.’
- ‘At the conclusion of each level the player is presented with the chance to play a slot machine game for extra men.’
- ‘As a result, shares of the slot machine and video game manufacturer edged up from $33.48 to $34.63.’
- ‘I put some money in a slot machine, and I won my first try.’
- ‘They would, he warned, use bingo as a ‘loss-leader’ and encourage players to try their hand at more addictive slot machine games.’
- ‘This is not some sort of con game or high tech slot machine.’
- ‘In my opinion, nobody has ever had a gun held to his head and been forced to go into a casino and put money into a slot machine.’
- ‘You get what you pay for, Petal, and in the bar scene it is very much a case of putting the money in the slot machine and waiting to see if you win a prize.’
- ‘Players know that it determines the symbols that land on the pay line on a slot machine and the cards they're dealt on a video poker machine.’
- ‘As far as the slot machine goes, it really is an easily understood game, simple without being boring, with well laid out pay tables and easy to understand bonuses.’
- ‘Since the modern slot machine is programmed to select number series at random, no amount of finessing of the handle can change what has been decided.’
- ‘The basic premise of a slot machine is randomness to the extreme.’
- ‘I took a look at products from all the major slot machine manufacturers, and will give my impressions on new games over the next several weeks.’
- ‘I used to put minutes, quarter-hours, half-hours into this game, like ever larger sums into a slot machine.’
- ‘Collier is not alone in his fondness for the slot machine, an electronic poker game known locally as a ‘pokie’.’
- ‘This is my retirement money and if I want to put it in a slot machine, it's nobody's business but my own.’
- 1.1British A vending machine selling small items.‘The slot machine sells sex toys, erotic lingerie, massage oil and condoms.’
- ‘I remember on a rare trip to the seaside (the beach was out of bounds, full of barbed wire and gun emplacements) gazing at a long-empty slot machine that had once held chocolate bars and now appeared as a rusting icon from outer space.’
- ‘The reason I'm thinking about this, whilst I sit in the pub, drinking my pint - sorry, I'm in a pub - is because I can see a girl standing next to the slot machine, dressed in pink.’
- ‘The only other two in operation were DIY, where the hapless customer is expected to swipe groceries past an electronic eye and then fiddle about feeding credit cards or cash into a slot machine to pay for them.’
- ‘The ordinary idea of a slot machine logically involves the idea of a device in which some mechanism or other is set in operation by the coin, which in some way delivers the appropriate goods.’
- ‘If the government's central concern was the reduction in teenage pregnancies, the sensible thing would be to make the morning-after pill available cheaply - perhaps via slot machines in easily accessible places.’
- ‘A motorway service station may seem an odd place to remember 12 murder victims, but yesterday the bugler's Last Post called out amid the slot machines and coffee bars with haunting beauty.’
- ‘I suppose we ought to shut down every library, then find a way of building slot machines into books so that you have to keep putting 10p pieces in every few minutes in order to keep the pages turning.’
- ‘But let them run around cobbled streets, put old pennies in Victorian slot machines and try on historic costumes, and you'll find a very different reaction.’
- ‘And I finally found out during a visit to Old Penny Memories, a working collection of antique slot machines in Bridlington.’
- ‘The Bishop of Durham, the Rt Rev Tom Wright, says: ‘Prayer is not a penny in the slot machine, you can't just put in a coin and get out a chocolate bar at the bottom.’
- ‘Will you admit that you've been treating me like some kind of slot machine?’
