[gmoreno_cnc]

Dear all,
I need your advice to decide what to do... I need to buy a CNC Router that can mill Acrylic Glass and Aluminum, to produce Wing Molds for RC Airplanes. The working space should have a minimum of 2000x600x200mm, and the most important thing for me is to have an extremely high precision (wing molds are going to be produced, so the airfoils need to be as accurate as possible).

My budget is no more than 2.500-3.500 €, so I am not sure if there is a KIT in the market with my requirements, or I can do it myself within the costs that I have in mind.

Please let me know your comments.
Many thanks!

GM

[Jim Dawson]

Define ''extremely high precision'' That is a subjective value. To me it means +/- 0.0005 mm or better, it might mean something else to you. Cost generally increases as a function of 1 + the cube of the number of decimal places.

[gmoreno_cnc]

Hi all,
Thank you for your reply. For the things that I'm going to manufacture, I will need at least a tolerance of +/- 0.005 mm or better. If it reduces the price drastically, it would be OK with a tolerance of +/- 0.01mm

The working space should have a minimum of 2000x600x200mm. My budget is no more than 2.500-3.500 €, so I am not sure if there is a KIT in the market with my requirements, or I can do it myself within the costs that I have in mind.

Please let me know your comments.

[Jim Dawson]

The short answer is: No, it's not possible to buy or build a machine that would hold +/- 0.005 mm or better over those distances (or any distance really) for 2.500-3.500 €, it may be possible in the 1.500.000 € range. Just the high precision ball screws and measuring devices will far exceed your budget.

Repeatability of +/- 0.01 might be possible in the 100.000 € range, +/- 0.05 mm might be possible in the 15.000 € range.

The controls and on machine measuring devices are going to be a big factor in the repeatability of the machine, as is the stability of the machine.

The other problem that you have is how to measure the parts after they are produced. Without a very expensive CMM and climate controlled inspection room there is no way you can qualify the parts.

For the accuracy that you require, I would buy a used VMC of the needed size and start rebuilding it. No kit router will do what you want. You might be able to find a heavy industrial router and rebuild it. But there is no way to do what you want within your budget requirements. I was able to take a used industrial router and add controls that would allow me to hold +/- 0.05 mm over about 300 mm, but the cost of the controls was about 5.500 €, and that was just the cost of the controls hardware not including custom software to run it.

You are either going to have to increase your budget, or find someone else who already has a machine that will meet your requirements and will do the work.

[peteeng]

Hi GM - If you come back to 0.01 or 0.05mm and use tooling board not aluminium then a router could be made to work. But the budget is still a ???. What you are describing is a toolmaker grade mill. Very expensive and needed to be in a air con room. The other thing with wing moulds is that with a 3 axis machine you will need to hand finish them due to the stepover leaving "waves". To do wings with no waves or minimal waves requires a 5 axis machine so a flat bottomed tool can be used to produce very smooth surfaces. So a 3 axis machine producing moulds needs hand finishing so that potentially throws out your tolerance as soon as someone touches it with grit. I suggest you find a contractor and get some moulds made to gain some experience with surface finish and tolerance, then move onto your own router/mill. Peter

[gmoreno_cnc]

Dear Jim and Peter,
Many thanks for your quick reply. Now everything is clear... I will then subcontract to mill all the wing molds, which actually are the molds in which I need accuracy.

For the small pieces, which I don't need them to be as accurate as the wings, I still would like to buy a small router of about 800x400mm to mill small parts for the plane in wood, carbon and aluminium. Do you think I can buy any machine or Kit of this size that can mill aluminium, wood and carbon within the 2.500 € - 3.500 € range?

Thx!

[peteeng]

Hi All - Is there a kit company in Europe like Avid cnc in USA? GM look at Avid site to find out about costs etc. Peter

Avid CNC | CNC Router Parts

[gmoreno_cnc]

Thank you Peter!! This is exactly what I was looking for...

Any other options in Europe? I have found https://www.cnc-step.com which offers very nice machines at an affordable price

[peteeng]

Hello GM - In regard to composite moulds. If your parts are carbon fibre then you shall need to make CF moulds. Same as if they are glass composite you will need to make glass moulds. Composite parts exotherm and require post cure. If for instance you make a CF wing part in an aluminium mould they have a 24e-6/m/(m.deg) expansion delta. So a 2000mm mould will expand 2000x24e-6x45= 2.16mm at a 70deg C cure if the mould started at 25deg C. But the CF does not expand very much! so then the part cures 2mm bigger then it should be. So you need to look at your materials and processes to ensure you don't get unexpected results.

So you would need to cut a mould from tooling board then take a CF mould from this. Same as glass make a tooling board master then take a FG mould from the master part. Peter

[routalot]

Hello GM - In regard to composite moulds. If your parts are carbon fibre then you shall need to make CF moulds. Same as if they are glass composite you will need to make glass moulds. Composite parts exotherm and require post cure. If for instance you make a CF wing part in an aluminium mould they have a 24e-6/m/(m.deg) expansion delta. So a 2000mm mould will expand 2000x24e-6x45= 2.16mm at a 70deg C cure if the mould started at 25deg C. But the CF does not expand very much! so then the part cures 2mm bigger then it should be. So you need to look at your materials and processes to ensure you don't get unexpected results.

So you would need to cut a mould from tooling board then take a CF mould from this. Same as glass make a tooling board master then take a FG mould from the master part. Peter

It gets interesting when somebody tries to calculate the thermal expansion of tooling block for pre-preg tools.I have known several instances where somebody went to great lengths to ensure that the mould and hence the component would come out the "correct" size and were disappointed.Two of the factors being that heat doesn't transfer across the vacuum inside the bag at all well-and there isn't too much conductivity along the fibres of the breather cloth-and secondly the tooling block isn't a great conductor.The consequence is that the core of the tooling block doesn't see anything like the temperature rise of the surface and expands accordingly.

One keen young fellow spent a bit of time calculating the "correct" allowance based on a small sample test.To his disappointment,I took him to see a mould for a previous version of the same item and asked him to read the length indicated on a known good rule-which he did.I then suggested he looked up the inspection report for the master pattern, from which the mould was made, and compared the numbers.He did that too and found that within the limits of a Mk 1 eyeball,they were the same.Small sections will behave differently and you need to know in advance from your own tests of equivalent size jobs what to expect.

[gmoreno_cnc]

Thank you for your kind reply.
I didn't know that the aluminium was expanding so much related to the CFK.

My plan was actually to mill the positive molds in Plexiglas (PMMA), and then produce with the milled positive molds (plugs) the negative molds in Carbon Fibers + some layers (the final ones) in Glass Fibers.

Many thanks for your post... very much appreciated.

[gmoreno_cnc]

By the way, what's your opinion about this CNC Router of 720x420x110mm? https://www.cnc-step.com/cnc-router-...20t-ballscrew/ It has a repeatable accuracy of +/- 0.01mm which might be ok to me to produce small pieces or molds, and the price is about 3K.

From the same German manufacturer I have found these ones, which was the kind that I need to produce the 2000mm positive molds... accuracy is +/- 0.01mm but the price is between 16k - 20K.:

1. https://www.cnc-step.com/t-rex1224/

2. https://www.cnc-step.com/raptorx-sl-...traverse-path/ -

[peteeng]

Hi GM - plexiglass is a poor master material for various reasons. It changes size due to temperature and humidity and it is poor to machine. Look up tooling board this is the best path. Plus I think you will find buying slabs of acrylic to be very $$$ intensive.

Routalot & GM yes there is a lot of smoke and mirrors with estimating thermal expansion of various things. For instance aluminium has a heat transfer coefficient of 270 something vs plastics and epoxy at 5 something. The recommended soak time for Al is 1hr per 1" thick at temp so you have to leave plastic there a long time to get to temp 25mm into the structure!! So this temp delta in the structure can warp and change things unexpectedly.

Plus GM don't mix your fibres in the mould as glass and carbon have very different thermal expansion ratios... Peter

[gmoreno_cnc]

Hi Peter, thank you so much for your reply.

The reason for using the Plexiglas was to get a very smooth and shiny surface in the plugs, in order to get a "mirror" looking finish when doing the negative molds in Carbon Fibers... once the molds were produced, these Plexiglass Positive Plugs were going to be thrown away.

My only objection to using the Tooling Boards, even the ones of High Density (over 1,3 kg/m3), was that the material was more porous than the plexiglass, so when doing the molds, unless using a pore sealant, the finishing for the negative molds were not going to be "mirror finish". I'm not 100% sure if this is like this or not...

In any case, I am still considering to produce the negative molds directly CNC'ed in a tooling board of 1.2 - 1.4 kg/m3, and use this tooling board directly as the mold to produce my pieces (vacuum bagging). The epoxy resin that I will use, cures at 20-25 Celsius degrees for the first 24 hours, and then at 60 degrees for 12 hours. I guess I could use the Tooling Board directly as the mold itself... (which I was not able to do with the Plexiglas).

These are some Tooling Materials that I was considering:

[peteeng]

Hi GM - The tooling board will have to be finished to use as a mould. The board supplier will have a finishing system. This will be a sealer and a tooling gel coat. These are applied like automotive finishes then polished to the level you want.

Traditionally producing a negative surface (female) is difficult as its hard to fair. Usually a male surface is made that represents the actual part eg the wing. Being convex means it can be faired with flat tools or springy flat tools.

Tooling Boards – Meury Enterprises

Duratec Application Techniques - ATL Composites mould finishing pdfs

The marine industry uses MDF its mainstay on many projects. being cheap you can start there and learn the processes. Start with the block and rough cut it to say 1mm proud. Then saturate surface with epoxy and let cure. Then finish machine to size using a small stepover. Then use automotive products to seal and coat surface. Hand finish thru the grits to 2000G and it will be very close to done. A quick buff with some polish and it will be a mirror finish. Wax and make a mould or go direct to part. Any composite supply store will have a surface finishing system and lots of info on how to do it. Peter

[gmoreno_cnc]

Thanks for the info Peter. Really appreciated.

[HuguesP]

IN EUROPE, i would look at sorotec,

[gmoreno_cnc]

Thank you, the website from SOROTEC has many interesting things at an achievable price. Very interesting...

What would be the best place to "learn" about the right way to choose the End-Mills?

For my project finally, I´m going to subcontract the wing molds, but I need to buy the Milling Cutters.... I don't know where to start.

I need to buy Milling Cutters for these materials that I will be using for my molds:

1. Polyurethane PU Tooling Block / Modelling Board of 1,2 kg/m3 density

2. Alluminium


Many thanks,
GM

[routalot]

I have to say that I have never seen tooling block as light as 1.2 Kg/m3,in fact even the lightest PU foam I have experience of was 35 Kg/m3.I would expect tooling block to be closer to 650 Kg/m3 .

[peteeng]

Hi GM - airs density is ~1.2kg/m3 so I imagine you mean 120 or 1200kg/m3 Merry Christmas. Your local engineering supplies will have machine tools, find out the reps contact and get in touch with them... Peter