So you have broken your bit.. let's talk about it

You’ve finally got into the “zone”, you have grand plans to make something magical, you hit Carve then moments later SNAP…. You broke a bit. If you planned this out well, you have a spare, if not, things got annoying very quickly.

We have all broken a bit of which this is an overview of our experiences and what to look out for.

Just a caveat, the achievable results published here are based on the bits sold by Adam's Bits. These are because we've gone through some extensive testing under various conditions. Also, what works for others, might not for you of which therefore Adam's Bits is not liable for bit breakages and undesirable consequences as a result of using this guide. OK, we got that out of the way, let's go!

Let’s start with the most common and obvious causes then move down to the more obscure. Treat this as a checklist if you are experiencing ongoing breakages.

Feeds, speeds & DOC

My favourite and best of all free app on your smartphone is FSWizard. Really easy to use and gives your realistic feeds and speeds with the ability to adjust variables.

Feeds - the faster you push the bit, the higher chance you have of breaking it. Use a calculator and start there. You will find that you will be able to push the bit harder after that but that should come with experience

Speeds - Match your router speeds accordingly. You are wanting to make chips to prolong your bit life but not too much! Rule of thumb, slow for wood, fast for soft metals, slow for plastics.

DOC - Depth of Cut/ Depth per Pass has a direct relation to your tool stick out/ flute length. The shorter the bit, the more rigid it will be when it is cutting under load. Rule of thumb, for cutting wood, aim for half diameter as depth per pass. Once you get more confident, move to full diameter per pass.

Small diameter bit

Check out my guide Choosing the right end mill for the job. It highlights the following process for choosing the right bit for the job

• Largest shank possible
• Largest diameter bit you can manage to the tightest corner radius
• Shortest flute length for the job

Did you know? - A shorter flute length can give you up to 30% higher feed rate? Use FSWizard and find out yourself

Jogging…. Yes… we have all done it

Not clearing high enough while moving your Y & X is a classic but by far my favourite…. 100mm on the X, 100mm on the Y and 100mm on the Z….oooops

Wrong bit for the task

Ever got those eBay bits from a Chinese seller that states you can cut aluminium with your 2 flute bit? Then wondered why it didn’t? There is a very good chance that they are dropshippers that also sell mobile phone cases and have no idea what CNC stands for. Then again, you get what you pay for.

Wood - there are few exceptions to what bit you can’t use for wood. You should be fine here.

Plastics and aluminium - The weapon of choice is a 1 flute. These materials are susceptible to “gumming” where the flute gets packed with the material you are cutting. A 1 flute allows for maximum evacuation

2 & 3 flute when slotting ineffective on aluminium - This one is an interesting point. As a rule of thumb, use 1 flute for slotting and 2 & 3 flute for profiling. You can use a 2 flute for slotting aluminium up to 6mm in diameter but anything above is dangerous. Using a 3 flute for aluminium slotting is a definite no. This is because aluminium is such a gummy material, it will clog your bits small flutes almost instantly.

Material and or waste-board is not leveled

By far my favorite cause of all time! Just like with depth per cut, if you plunge into your material at an aggressive depth, it will break. Same goes when you are thinking that you are cutting at X, but now X + a millimeter or two.

The set of questions I ask when someone rings to say they have broken their bits are:

When was the last time you levelled your board?

Your board should be levelled each time you install a new board

Periodically! You would be surprised the effect of moisture and humidity has effect on your boards level

Using screws to hold your material down creates nice 0.5mm to 1mm craters. Use screws sparingly and if you do use them, level them out with a sander or chisel before the next carve

Using plywood as a waste-board is not advisable. They tend to warp easily so stick to a thick 18mm+ MDF board

Is the bit breaking at the same spot each time?

Clamps do come undone as well as screws holding your waste board to your base. Check these regularly.

Hitting something it shouldn’t

Yes an obvious one and one that we have all experienced, hitting a screw, clamp or piece of material just after being cut out

Avoiding the use of screws and clamps is one way to avoid a breakage. There are holding methods best described in this article holding it down

Allow for a higher safety height when travelling. There is a reason why Easel uses 3.8mm as a safety height. Going any lower is “tickling the dragon's tail” so maybe get some experience before you try

Holding your material down with tape or tabs is advised as it’ll stop the part you cut out from wanting to pop out. Using a down cut router bit mitigates this

Holding your material down sufficiently is another consideration. If there is even a slight vibration, this will lead to early deterioration of your bit

Tool run out

Now we are moving into the more obscure causes. Tool run out is where your bit is not rotating on its axis perfectly. If a bit is jiggling or not tightened correctly, it’ll start to cut it’s own path and make all sorts of mess

Using the right sized collet sounds obvious but sometimes overlooked. Using a 6.35mm collet for example for a 6mm bit is not advisable, no matter how tempting

Sitting the bit in just above the flute cutting length is not only good for achieving a higher feed rate, but also increases rigidity substantially. I aim for 3mm above where the flute stops

Collet maintenance is also another good practice to instil. After and before each time you install a new bit, ensure the slits are free of material and there is integrity in the collet

Your machine has something loose or NQR

Checking to see that your CNC is in good shape is not only a good thing to do, but also another cause of breakages. If your CNC is vibrating, that is a sign there is something loose. Check that the following are tightened:

1. Wheels and eccentric spacers
2. Belts
3. Spindle holders
4. Nuts and bolts

One story that comes to mind was the shop that had a $400k CNC of which they could not return the ⅛” bit to the same hole and blamed it on the bit. The other was that they were breaking a 3mm 2 flute for aluminum constantly. I suggested a list of items but ended up refunding their unused bits. 3 months later I got a call from the owner saying that the spindle had busted bearings and was the cause for all their issues

Your X-Carve / XYZ-carve shakes violently when performing the last pass

A nuisance for belt driven CNCs such as X-carves with their Z axis is that you can jog your carriage straight off the bottom. Inventables do not install a bearing plate or stopper so it’ll do this quite easily. 99% of the time it’s not a problem but if you are using a short bit, have a thin waste board and you have set your Z makerslide really high, there is a chance your bottom wheels will no longer be in contact with the makerslide. Increasing the waste board thickness and dropping the Z rail will help this.

Compromised material

A less obvious one is foreign matter in your material. I have heard of customers buying cheap ply from overseas only to realise that there were staples and rocks pitted all throughout the material. Same goes for recycled material like pallet boards regarding forgotten nails.

I’d like to think I’ve heard of everything but I highly doubt that. If you have a great or obscure reason for your bit breaking, get in touch on the contacts page. I love a great story and will add it in to this article.