CNC Machine (Does Size Really Matter)

CNC Machine (Does Size Really Matter)

When I say Large CNC Machine I probably don’t mean this big but you can do some serious big shit with this machine.

Anyway you might be thinking “big machine” slow rapid, slow tool change etc etc. Well unless it’s really new and very expensive, it probably will be. But read on….

The Small Machine

So, you have a small machine brand new and its tool change time is just 2 seconds and the rapid is the speed of light.

Now let’s just imagine you are  making a part that’s fairly simple and uses 12 tools. Your cycle time is 5 minutes.

You finish machining it and check it all over. You give it to the “I just chewed a wasp” inspector and he passes it off first time.

Then you dance around the workshop like a schoolboy who just scored his first goal throwing your arms in the air.

Ok lets run one. Five minutes later. “Oh it needs another and another…”

So now you have to stay at the machine constantly loading it.

You then wet your pants cos there’s not even enough time to get to the toilet and back while it’s running. Not to mention the repetitive strain injury from loading a part every 5 minutes.

Large CNC Machines

On a large CNC machine you can load your part in several different presentations. In the example below the holes are being chamfered on the bottom of the part in a separate setup. Saves all that messy deburring.

You may think large machines are just for large components. Well yes that’s true. But…… I may have to disagree and ask you to think again.

On a large CNC machine you can fill the table with repeat parts. Or you could have many presentations of the same part.

The cube above could be machined on all the faces in one setup.

So this might take a while to setup but once it’s running you would get a complete part after each run. Not to mention that you could nip home and feed the cat and still have time to make the boss a cup of tea before it’s done.

Never Never Ever

Why would you never set a part up in the middle of the machine table?

Come on answer I don’t have all day.

So the boss comes along and says “Jack, stop that job you are doing and do this one instead”

Must have happened to you before.

So which one of the above machines do you want now?

Yes you got it. The one with the part set up to the right.

Easy because you can leave it all completely setup and when the boss finally wants that job it’s just a matter of calling the program and hitting the CYCLE START

Don’t forget to set the datums using G10 by the way.

So there is another advantage of a big machine.

You get to leave loads of shit on the machining table all ready to run.

Leave It Set

You might just leave your machine permanently with two vices and a table chuck all ready to use with the datums set. Oh and did I mention the fourth axis that’s just sat there in case I’m ever arsed to use it?

I Despair (Beam me up Scotty)

I was training this bloke recently and he said “oh we like to break every job down completely when we’ve finished, we take all the tools out and clean the machine”

Anyway I just smiled and muttered some profanity he couldn’t hear.

If you read this and think it’s you then it’s definitely not, I just don’t want any trouble ok.

I will agree on one thing only and that’s the cleaning of the machine.

Now look at this There’s Hope

Imagine your machine is big enough to get all these parts on the table.

When you pick up your spot drill it’s going to do some serious work (88 holes).

Eight parts in total..

Lets say the tool change time chip to chip on this big old machine is 10 seconds well now it’s actually 1.25 seconds because it did 8 parts.

10/8 = 1.25

Now what about the slow rapid. OK but it’s not really moving very far either is it?

If you watch this video you’ll see that where the spot drill ends the drill takes over in the same place. (No wasted movement).

With a bit of work you could make the rapid moves on this part even shorter.

Now imagine this on every tool. The cycle time saving is breathtaking. And when it’s home-time, load the machine up and let it run. Oh by the way if the next day you have a pile of scrap and a grands worth of busted tools it wasn’t my idea ok.

What if You only Have a small One

Little and often is the approach.

If you saw some kind of sexual innuendo here then I think you need to grow up. This is serious engineering shit and I don’t want your filthy infantile minds on my website.

Unless of course you want some training.

Small machine often have much less power.

Wave Form Milling as it’s called by Edgecam is a way of using a large depth of cut but small engagement.

You can remove metal very fast by this method and you don’t need a lot of power. Other CAM systems have a similar things but with a different names.

If you have large threads to produce rather than tapping, where you need a lots of power, consider thread milling.

https://www.youtube.com/watch?v=cDTpeLyU2TQ

Add an Indexer or Fourth Axis

Adding an indexer or fourth axis to your small CNC Machine will increase capacity and allow you to rotate parts to get to areas that otherwise you couldn’t reach.

Here’s a free advert for someone

New Small CNC Machine

Pros Small CNC Machine

• Shiny.
• Very fast rapid and tool-change.
• Small Footprint.
• Quick to set up.
• High rpm.
• Lowish cost.
• Warranty.

Cons Small CNC Machine

• Not many tools.
• Often low power.
• Not much space to set up extra parts.

Large CNC Machine (Maybe Old and not too Expensive)

Pros Large CNC Machine

• Make big bits.
• You can leave jobs set-up cos there’s room for other work.
• Make loads of bits at once, multi op or multi loading.
• Tend to have a lot more tool pockets.
• Usually more power.
• You can leave vices and chucks permanently on the table.
• Represent very good value second hand.
• Machine runs much longer unmanned so you can get on with re-thatching your cottage roof.
• Before you go home you can load up a full set of parts and let it run.
• One tool does loads of work.
• Great for “just in time” delivery because one complete part comes off per cycle.
• If it’s second hand you’ll need training.

Cons Large CNC Machine

• Cost shit loads of money new (more than a six bed-roomed mansion where I live)
• Slow rapid moves.
• Slow tool changes and pallet change times.
• Can cost an arm and a leg to maintain, find a good independent specialist to fix it.
• Can be very dirty.
• Have you got room for it?
• Watch the max RPM isn’t too slow.

More Things to do if You Have a Small One

Consider a fourth axis with a cube on it.

With a four sided cube you can increase your table capacity by an awful lot. Oh and you get to work on the side of the part too.

Manual pallet changers can be very cost effective.

Again these give you loads of extra capacity. They are actually cheaper than you think.

Conclusion

Let’s imagine you have £90,000 to spend on a machine. You could buy a new super fast small machine.

But, if you got the room you could buy a big second hand machine.

The new one will have a warranty and will be very fast. But what about the “old banger”. Well if you have room and you know someone who can fix it if it goes wrong …….

Thanks

If you feel you have been affected by any of the issues raised in this post or need CNC Counselling then contact me.

If you want to learn to program CNC Milling Machines

Siemens 840D Sinumerik training

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