Supporting > Tooling & Machines

Pros and Cons of home shop CNC

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Jasonb:
Prompted by some comments on my James Coombes thread I thought I would start another for general discussions about the use of CNC machines in the home workshop.

The point raised was about how long it takes to machine a part compared to castings . This can have several different answers.

If the casting is readily available and assuming no hard spots, defects such as being undersize or having bosses that don't line up with hole positions then generally machining the casting would take less time as you may simply need to flatten the top & bottom and drill/tap a few holes if we take a typical sole plate or bed casting as an example. But if a lot of fettling and reworking is required then time differences will start to reduce

On the other hand if you are making something to your own design or replicating an old long out of production model then using the CNC to cut from solid will quite probably be quicker than making your own patterns, having castings done if you don't have your own facilities and then there is the actual machining to be added on so in this case the CNC is likely to come out the quickest. Though for those that do want to use castings the CNC is just as happy machining wooden, plastic or even metal patterns and can be quicker than traditional woodworking methods as well as very accurate.

It was then questioned how machining a part on the CNC compared to a built up fabrication. This is another where there is no easy answer as each part will be different and depending on what the user has to hand in the way of materials and machines will dictate what is best for them to produce the part in question. Some parts will be better suited to cutting from solid, others better built up with manually machined parts but a lot of the time a combination of the two works out to be the best option using the CNC for things that it is best at like cutting curved components for the fabrication, the lathe to do round parts and maybe the manual mill for simple flat, square and rectangular bits.

The CNC's ability to cut quite complex shapes from solid can also be an advantage where a specific material is required. For example it is easy to silver solder a steel fabrication together but what if you want that crankcase to be aluminium or cylinder head in iron both of which are not so easy to join. So the CNC will be faster than manually carving the part from solid.

I'm probably in a bit of a unique situation, I was originally approached a few years ago asking if I would like a KX-1 size mill. After giving it some thought I said now based on what I had seen at a couple of shows where the guys demonstrating their CNC machines seemed to take an age to perform the simplest of 2D operations that I could have done in a fraction of the time on a manual machine. I also did not fancy having to learn "G-Code" that I had heard about and it all seemed quite daunting.

Moving on a couple more years I was again asked if I would like a CNC, this time the larger KX-3, this one being a customer return. by this time I'd seen a bit of CAM in use and was more proficient at CAD so decided to take up the offer, after all it was only going to cost me a Mach licence and reconditioned PC so no great loss if I did not like it.

Most will have seen my Darkside thread about my journey into CNC and having used it now for a couple of years I'm glad I accepted the offer. Although I still enjoy using the manual machines the CNC is really just another tool and I tend to use it for the things it excels at and don't have any intension of going over to a fully CNC workshop though it does seem to work for others.

To be continued

Jasonb:
Carrying on with the time question there are also some other factors that should be considered.

At the moment you can't just go out and buy or download the code that the CNC needs to produce a part let alone a whole model. So you need to produce a 2D or more often these days a 3D model of the required part, not really an additional time addition for me as I was designing or redrawing existing designs to be built on the manual machines before I got the CNC. However if you are used to working from others plans then you will need to draw these up in CAD which depending on how proficient you are will add time to the overall picture.

But that is not all, once you have your part drawn you need to tell the CNC how to machine it, some may choose to do this by writing out the G-code themselves which depending on teh complexity of the part could be a few lines or thousands. However I think anyone venturing down the CNC route these days will opt to use some form of CAM which simplifies the production of the G-code. This is the way I have done it from the start and with all the items I have produced have not written a single line of G-code so don't let the thought of it put you off like it did for me.

Even with CAM you do still need to have an understanding of milling as the CAM although it may have some presets for various cutters and materials will need to have these tweaked to suit the users individual machine and the cutters you actually have.

Which brings us nicely onto another variable that has a big effect on how long a job may take on the CNC, which is the capability of the various machines. In the home workshop machines are likely to fall into three catagories. Gantry type router, converted manual or purpose made machine.

The gantry machines often fitted with small DC motors at the smaller end of the scale through routers to spindle motors at the top of the range. These can often be the cheapest option into CNC but are often of fairly lightweight construction so are not that efficient at cutting metal particularly steel or iron. If you do want to cut metal with these particularly the entry level ones then very light cuts are needed which means the cutter has to make many passes so the run time can become very long.

Converted machines suit quite a few particularly if you are able to build them up yourself. Being based on manual mills they will have the rigidity to cut all metals. Their biggest downfall is likely to be maximum spindle speed which will have an effect on feed rates for a given chip load. For example the majority of manual mills are likely to have a spindle that can run up to around 2000rrpm, maybe not too bad if you just want to profile a piece of sheet to a fancy shape with HSS tools but when it comes to using smaller diameter tools to machine complex three dimensional curved surfaces then these slow feed rates will see the converted machine needing to be run for a long time to complete a part. Though there are some benchtop mills on the market now with spindle speeds of 5000rpm which would be ideal for conversion and using carbide tooling

Purpose design machines are going to be the fasted option be that a new machine or an old one with updated software, drivers etc. These will generally be solidly built with spindle speeds of 5-10Krpm which allows faster feed rates and would reduce machining times to 20-40% of a typical converted manual mill.

Whatever type of mill CNC is being used in the home workshop it is unlikely to be able to perform like a commercial machining centre due to a combination of rigidity and power so metal removal rates will be less than they can manage. Comparing like for like size machines a CNC one is likely to be able to remove metal faster than a manual one not least because you can't physically wind the handles fast enough but also the tool is always on the move even at corners or other changes of direction. For example if you manually machine out a rectangular pocket you are likely to slow down your hand feed as you approach each corner watching the DRO readout, lock one axis , unlock the other and then go through the same again for the next side. The CNC will move constantly from one instant change of direction to the other and there is no risk of it overrunning which is all too easy on the manual machine. The added bonus is that you don't end up with the tool rubbing as it slows into to the corners which wears the tool.

Enough for now, hopefully a few others with CNC will chip in or if you don't have one feel free to ask any questions.

M

Jo:
Lots of words there JB :noidea: 

In the beginning: Some stuff for model engines was originally made with castings, some stuff would be a difficult casting (like a table with the fancy bits that chill) and they can be made up in layers. Some members make some amazing stuff by twiddling the handles on their manual machines (Crueby, gbritnell, Ramon to name a few). All models have other bits that are made using a lathe or milling machine manually.

And then the world changed... We gained CNC  :headscratch: We now have members like Vixen building really complex model engines using their stable of CNC milling machines  :paranoia: And others like Jason using his to make models which would previously been done by more traditional means. I can't recall anyone showing off their CNC lathe (I know Vixen was looking in to it). I assume these models also require bits made by traditional hand milling/turning


So what are the good points, the bad points and the ugly points of CNC? This is a comparison with other more traditional techniques for making a one off model for a hobby not as a business.

I know that Vixen claims his CNC can be very Artistic  :lolb: and applies features to his parts to keep him on his toes.

I am going to claim price is an Ugly point  ::)

Jo

Jasonb:
I'm sure I have seen Vixen's Emco based CNC lathe in action in a number of his posts

vtsteam:
I have a fairly eclectic view since I both cast and have some cnc facilities, but have never bought a casting. And that view is, I do what is pleasurable to me. I do love to cast metal, and I do love to make patterns, so I don't account the time involved as getting in the way of what I want to accomplish.

I can't say I enjoy CNC milling, but then I don't really enjoy hand milling much either. That's just personal. I do love working with the lathe, and I am quite certain I don't want to convert my lathe to CNC, because that would (for me) take the fun out of it and the challenge.

Because I have no call for production, repetition with the lathe isn't generally needed. I imagine if I were into production, all would be different.

I will say (and it takes a reversal of a former "attitude" on my part) that I did enjoy making my first pattern using a 3D printer this fall, BUT, I still did a lot of hand work with that, including filling sanding and painting. AND, I have no wish to eliminatethe extra time they involve either. Those are tasks that I put into every pattern, and I actually like them, and the final result.

What I liked about the 3D printed pattern was being able to make a relatively thin pattern with a hollow torroidal shape -- which would have been very difficult to execute in wood. I would never however want to stop making wooden patterns that are easier conventional shapes. I am through long practice, very fast at making and finishing a pattern, as well as casting. I can make a simple pattern, fill, sand, paint and have cast it in about 4 hours when I'm really interested. And I like all aspects of that. A small 3D printed pattern takes much longer -- at least the first one I tried did, but its complexity made it advantageous.

So, conclusion? Uh well frankly I like both oranges and apples, and I don't think one will ever replace the other for me because they are different. I do particularly enjoy manual skills, and do not want a computer to do things for me in general, but I also appreciate the option to do some complex things with assistance from CNC, as long as it doesn't take over as a substitute for my actually physically making things.

As for my opinion of others work in CNC. I absolutely enjoy seeing good work, by whatever method, and have been truly amazed at some of the models I've seen here. I know how much work, programming, foresight, research, skill, goes into making those models, and I honor them no less than I do pure manual works.

To me it's all about each person finding the niche they enjoy the most, and I'm frankly glad we're all a bunch of individuals with different joys in what we do. I am nowhere near as skillful as many people here, and yet I see that here, we all seem to appreciate the full range of abilities, experience, and personal interests we express in the works we show here.

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