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Sparrow Legs
21-09-2013, 10:48 AM
Hi Guys,

Initially I should point out that I don't have a CNC machine, nor was it my intension to buy or build one. However, circumstances of a recent project has made me re-evaluate my options.

I want to make a logarithmic spiral, (like a snail), from birch plywood. I have worked out and completed the design. It consists of 102 sections which are hollow cylinders with an angled top. Each section has a different diameter ranging from 48.5mm to 238mm. The heights range from 13mm to 38mm.

The design is completed, the disks have been nested for a ply sized 1200*2500. The file is an .STL ready for CNC routing.

10181

When contacting various companies for quotes to have this machined, they are over a £1000, excluding material and delivery.

Why does it cost this much? If the company does 15 jobs, then the machine is paid for?

Jonathan
21-09-2013, 12:23 PM
Why does it cost this much? If the company does 15 jobs, then the machine is paid for?

Because most companies are greedy and extortionate because they're trying to justify having spent a huge amount of money buying a commercial CNC router. I bet if you post this in the RFQ section here you'd get people quoting substantially less than that as they built their own machine. I certainly wouldn't charge anywhere near £1000 for that.

One thing that makes your parts more expensive is that you've got a large number of small parts to be cut out and they all need clamping down somehow when cut out. A decent vacuum table would do the job, but without that it's going to be very labor intensive.

EDIT:
Ok, forget most of what I said above. I just noticed you're cross section drawing which implies all the parts are tapered? If so then that will take substantially longer to machine with a 3-axis machine, as it's now requires 3D profile milling, or it requires a 5-axis machine. Either way that's going to add a lot to the cost which goes some way to explaining the high cost.

~andy
21-09-2013, 12:27 PM
It doesn't sound outrageous for a one off job. Don't forget all the overheads they will have, like somewhere to put it and someone to operate it.

Looks like a cool project anyway. You will have to post the results, when you get it made.

JAZZCNC
21-09-2013, 12:44 PM
Remember these are business's and they don't just have to cover overheads they also need to make profit for the rainy days, days like when £50,000 machine decides it's no longer playing ball and they call out a service engineer at £100hr. Or compressor that's running multiple machines expires and so on.!!

This is not greed it's good business sense and your job is more complex than it first appears, there's also the time and labour involved in laminating large sheets together.

Now your best bet cheapest way is to get someone to cut them out for you on a 3 axis machine then create the taper using jigs on either bandsaw or planer thickness-er.

Jonathan
21-09-2013, 12:55 PM
Ok, maybe I just charge too little. My point was if you find someone here who has made their CNC router as a hobby then it will be cheaper as, like me, they're not paying the same overheads that a business would have to.

Sparrow Legs
21-09-2013, 01:30 PM
Thank you everyone for your input.
For starters, a vacuum table or clamping isnít required. I have designed the cut-outs to have a top and bottom tab, so that the disk remain attached to the board when routing.
Regarding the cost, I am a builder with 10ís of thousands of pounds of tool; can you imagine telling a client that there is a rental change for the tools as well as labour, materials etc?
Horse for coursesÖ.
JazzCNC, I am not sure why you would be laminating large sheets together? You may have noticed the note that itís 18+22mm ply. This is because Iím in France and 40mm Ply doesnít exist here, but it does in the UK.
Also have you ever tried planning Plyboard across the grain? You canít, it will rip the layers apart.
I am going to take Jonathans advise and stick this post on RFQ.

JAZZCNC
21-09-2013, 02:00 PM
Regarding the cost, I am a builder with 10’s of thousands of pounds of tool; can you imagine telling a client that there is a rental change for the tools as well as labour, materials etc?
Horse for courses….

So did you have those tools given free.? . . . No they get factored into the job or they come from your profit. Either way the client pays for them whether they know it or not that's business.!!



JazzCNC, I am not sure why you would be laminating large sheets together? You may have noticed the note that it’s 18+22mm ply. This is because I’m in France and 40mm Ply doesn’t exist here, but it does in the UK.

Yes I did see that from drawing and presumed the quote the would have been based on this.?


Also have you ever tried planning Plyboard across the grain? You can’t, it will rip the layers apart.

Yes I used to make Bespoke furniture so I have and it's very easy to plane if you have the blades ground correctly and nice sharp plus don't be greedy on depth of cut.!!
Thou depending on the finish you require I'd be more inclined to make a Jig for bandsaw has it will be quicker and more controllable.

You could also make a jig and use a router with large bit.?

EDIT: Actually this could still apply with CNC and would actually be quicker. Cut them out, make a Jig at required taper to hold 5 or so and then use wide tooling bit using a normal surfacing toolpath. Will be much faster than 3D toolpaths.


I am going to take Jonathans advise and stick this post on RFQ.

Will be lengthy job has it's lots of 3D toolpaths with small step over but still should come in cheaper if done in a shed.!

m_c
21-09-2013, 02:03 PM
I'd love a 5-axis machine for £15k!

It is one of those parts that although you think would be simple to make, is actually quite difficult.
I'd go for Jazzs suggestion of get them done on a 3axis, but get the tapers stepped/waterlined, then finish on a sander. A jig on a decent (i.e. a floor standing one, not a stupid handheld thing!) belt or disc sander will remove the remaining material pretty quickly. Might not be the quickest or cleanest method, but it'll do the job.

Sparrow Legs
21-09-2013, 02:13 PM
Hi JazzCNC,

This isn't a project I fancy doing by hand. I have 102 disks to cut plus another 102 as I want to make a pair. This would be 408 circle to cut plus making a jig for a router.

The main reason for not going down that hand made route is the angles of the tops of the cylinders. A fair few are at different angles. This is to time-consuming as I work 60hour weeks.

I have started a thread on - http://www.mycncuk.com/forums/projects-jobs-requests/6599-can-you-quote-my-project.html#post49692

Sparrow Legs
21-09-2013, 02:25 PM
M_c, the quotes I received where based on a 3 axis machine. I suppose this is why I thought the price was a bit steep.