Laser cutting has been around for a long time and has been embraced by industry for the most part but I still do see some instances of designs that don’t really take advantage of all the things you can do with it. So I’ve come up with a list of a few tricks I’ve learned that you can do to save time, money and manufacturing effort. Maybe you’ll take away a new trick from this article and if you have one I didn’t mention, please leave a comment below. 🙂
One of the first big time savers I learned with laser cutting is the relative ease with which you can cut a thin rectangular hole in a part to correspond with a tongue on another part. Not only does it eliminate the need to measure out the position of the part but it also holds the parts together while they are being welded. I picked up this trick in my early days of FSAE from the chief engineer at the time.
The clearance around the tongue should vary depending on the application and thickness but I’ve found that in 90% of situations (mild steel between 1mm and 10mm), a 0.25mm gap all around the tongue works. If you’re dealing with material over 10mm, you might want to increase this a little bit. Also, if you’re using obscure stuff, you might find that the 6mm plate is actually 6.35 (1/4″) so double check. The depth of the tongue is another parameter that requires a little thought. If you’re going to fillet weld the parts together and you have clearance on the back side of the slotted part, you can make the tongue long so it protrudes through (The example on the right in the below picture). A long tongue holds the part together better and makes assembly nice and easy. Alternatively, you might want to make the tongue short (half the part thickness) so that you can plug weld in the slot. (The left hand example). This results in a weld that can be ground flat without losing (much) strength.
Below is a photo of a machine part with this trick done in 6mm thick mild steel. In the foreground you can see the bent part which has the tongues mated against the flat part that has the grooves cut into it. In the background, you can see the same assembly from the other side. Those lines of weld which are filled into the back of the groove.
Here’s an image of the dxf file I sent out for laser cutting in 2009. It’s 1mm thick 4130. The two pairs of grooves cut in the panel second in from the bottom left correspond with the tongues cut in the bit at the top right as well as with another gusset (not pictured). This part actually features a couple of other tricks that I’ll be talking about later.
Another use for tongue and grooves is controlling warp. If you’re having long strips cut, you may find that they come back from the cutter with a significant bend in them. This can be alleviated with good design. See the picture below for an example. It’s a quick sketch showing a long warped piece with tongues cut in it being retained in a big sheet with grooves cut into it.
There are some pitfalls associated with doing this. The more obvious ones are the tongues and grooves just plain not lining up due to design changes in one part not following through to the other part or just not being the right size. Some less obvious ways to screw this up occur when dealing with symmetrical or near-symmetrical parts. Below, a slightly asymmetrical part is shown with symmetrical tongues. It can be assembled either way. If you drawing is super clear, this may not cause any issues but a clear drawing and knowing you weren’t “technically” wrong is small consolation when you need to cut a machine apart to fix a mistake.
Incidentally, with the image above, another approach you can take is to make the part symmetric by mirroring that hole so that it doesn’t matter if the part is assembled the right way or not. Sometimes one way is better than the other. With CAD these days, it’s often easier to constrain your model so that the tongues and grooves are equispaced rather than to dimension them individually so I often find myself trying to preserve symmetry. Inevitably, though, a mounting hole is going to go in at the last minute and you may well forget to un-equispace those tongues and grooves. As such, I’m currently trying to get into the habit of making all of my tongue and grooves non-symmetric to avoid this happening.
Another pitfall arises when parts are being folded. Laser cutting is usually a 2D process, so the part you cut can usually be folded the “right” way or the “wrong” way, which would result in a mirror-image part. You can get around this by ensuring that the part is symmetric. Which brings us again to the conundrum I mentioned above- to symmetry or not? It’s very easy for your folder to misread a drawing and bend something the wrong way. I make an exception for folded parts. We’re engineers. We compromise.
Sometimes you need to butt weld two parts together. This can happen because of limitations in the size of the sheet. Also, some laser cutters will quote material cost based on the smallest rectangle that can contain the part- so a very large “L” shape part made from two rectangular parts butt welded together can save you a lot of money when your material cost is high. In this case, you might want to cut dovetails into the part. If you’re just doing this to roughly locate the part then a 0.25mm clearance is fine. If you’re trying to use this to fully align the parts, though, you might want to cut that clearance down because you can get quite a large angular misalignment from a relatively small clearance. Also, bear in mind that your boilermaker is going to grind a chamfer all around that with an angle grinder (unless you do some trick 3rd axis laser cutting which is probably going to be too expensive) so it just might be cheaper in the long run to have a straight line joint and align the parts another way.
Now, back to that upright piece I showed earlier. I’ve shown it below again so you don’t have to scroll. You can see the two long edges at each end of the net have some zig-zag kind of pattern cut into them. The idea of this was for them to lock together to make assembly easier- and it did- but bear in mind your weld will have a slight zig-zag to it. I’ve had boilermakers complain that I didn’t do this and I’ve had boilermakers tell me not to do it because they don’t like their welds to look crooked. So I guess what I’m saying here is, your mileage might vary. You might want to put in just a couple of locking tabs at each corner that can be ground off after tack welding but before seam welding.
Finally, you can see the stitches cut in between each section. These are so the part can be easily bent into shape by hand. There are situations where a part can’t be bent by a brake press or a folder, so this is a way around that.
On the topic of folding, most software will automatically put in fold lines when you export a part so you can etch them in to make your folder happy. Another trick to reduce folding cost is shown below. I don’t know what the proper term is but I call it scotch fingering because the parts break apart like scotch finger biscuits and because it sounds amusingly lewd. Some folders will charge per part so sometimes it might be a good idea to join several parts together so they can all be folded in one hit and broken apart afterwards.
Now I’ll put in a few random ones. Corner breaks. It used to be common practise for engineers to put a chamfer on every exposed corner so you don’t get a sharp edge that will cut or gouge unsuspecting operators. We used chamfers because they were easy to cut, grind, punch or shear. Straight lines were easier. Nowadays it’s just as easy to laser cut a curve but I still see a lot of chamfers laser cut into parts and to me it makes them look dated and lazily designed. It’s not any cheaper than putting in a nice looking radius so why do it?.
Below is a photo of a few parts that I was worried about tolerance stack on. So I slotted the mounting holes to give myself some wiggle room. Slotted holes are a brilliant arse-covering technique and I do it pretty often.
Edit: After a friend commented, I realised that I didn’t really write enough about this the first time. The part you can see is mounted by the slotted mounts you can see but is located by other, nonstructural parts (In this case, a pair of screw conveyor troughs). If position is important and your structure is not positively located in anyway, putting slots in everything just might make your workshop foreman/fitters want to kill you. Getting a few hundred kilos of steel in the right place with a crane and podgy bars is not easy. I guess what I’m saying here is that there’s a time and a place for doing this kind of thing.
Most laser cutters can’t cut a hold that’s a smaller diameter than the sheet thickness so you’ll just have to drill it. Make it easy for yourself, though, and laser-etch the hole centre.
Also, the cost added to etch on the part number is very little.
Another quick way of locating two parts:
Slots aren’t just good for covering your potential mistakes. You can also use them to standardise your parts. Below is a photo of a standard torque arm. It’s slotted so that it will fit any size gearmotor.