Erazz,

many thanks. PM sent.

Martin.

Originally Posted by erazz

I've started milling too! I hope to post pictures next week. It's extremely thin. The layup should be interesting.

What kind of machine are you milling with?

Greg

Erazz


PM sent

Thanks

Hi Guys,

Martin, I sent you the files. Roger just send me your email and I'll send the files - they're too big for the forum.

Greg, I'm milling on a friend's vertical machining center. Great things to have around

Erazz,

Thanks for the files. I've had a play with them - I opened them in Solidworks and then saved them as .STL files. The vertical fin file was then opened in Vectric Cut3D and the G-code generated. This G-code was then passed via Mach3 to my cutter.

The fin mould was cut out of MDF using a 1/4" end mill for the roughing cut and a 6.35mm cove core bit (sort of a ball ended bit - it is all that I had available) for the finishing cut.

The mould needs a little bit of hand finishing to remove the Z-axis errors in my machine and poor choice of bit selection.

Regards,

Martin.

I made a few errors setting up to cut the mould in my previous post, and would like some advice.

Erazz, I don't mean to hijack your thread with these questions, if you want me to move them to a new thread just say the word.

I thought that I had set the Z-axis zero point as the top of the workpiece, but it seems to have ended up a fraction lower than that. How do I ensure that I am exactly on the surface of the workpiece?

My attempt to cut the excellent design from Erazz is usable, but could be greatly improved with the right sort of cutter. The question is, which cutter? And what settings are needed for it? Stepover, depth of cut, etc.

Also, now that I have an MDF mould, what is the recommended way forward? A coat of polyester resin and some elbow grease to rub it back perhaps?

I hope you can help me.

Martin.

Originally Posted by heli_bee

I made a few errors setting up to cut the mould in my previous post, and would like some advice.

Erazz, I don't mean to hijack your thread with these questions, if you want me to move them to a new thread just say the word.

On the contrary! This makes the thread more alive. It has everything to do with the original subject. Besides I love seeing CNC work

I thought that I had set the Z-axis zero point as the top of the workpiece, but it seems to have ended up a fraction lower than that. How do I ensure that I am exactly on the surface of the workpiece?

My attempt to cut the excellent design from Erazz is usable, but could be greatly improved with the right sort of cutter. The question is, which cutter? And what settings are needed for it? Stepover, depth of cut, etc.

Also, now that I have an MDF mould, what is the recommended way forward? A coat of polyester resin and some elbow grease to rub it back perhaps?

I hope you can help me.

Martin.

I have a couple of ideas.

1) Don't make the mould directly from MDF. It's really difficult to sand and get the correct profile. Also, the MDF tends to swell a bit when you seal it. With a profile this thin you won't get anything near the correct shape.

Usually it's best to mill a plug from MDF and pull a fiberglass mould from it.

2) Whether you make a mould or a plug from MDF the process is the same. Mill the shape and leave ~0.2mm of material. Seal the MDF with epoxy. Brush the epoxy and use a heat gun to "sink" it into the wood. After 3-4 coats the epoxy won't sink any further. Then you put the part back on the router and mill the final cut with a very small stepover (0.5-1mm) and a new 10mm ball nose cutter. This should get you very close.

Since you already milled the part I suggest you try and seal the wood and finish mill it 0.2mm lower.

3) I noticed you cut the mould in parallel passes. This is not the best strategy. If your software has the capability you should try a pocketing strategy that follows the contour of the part.


4) In any case any mould needs a lot of elbow grease.

Keep going! The result is worth it.

Thanks for the compliments Erazz.

I used MDF as it is very cheap to experiment with, also my spindle is a fixed rpm version and too fast for dealing with metal!!

I will try the resin method, although I will have a go at using polyester as I still have a few kilos of it (about 10kg). If it works out, that's great. If not I will have wasted a few hours and others can learn from my mistake.

My software does not allow pocket maching in 3D it will only give the option of X-Axis, Y-Axis or at an angle of 135 degrees. It does allow the option of another pass at 90 degrees to the first though.

I can't easily re-machine the mould surface, but I will have go at another one, this time with an 8mm ball nose (that will allow me to drill the locating holes at the same time.

I have already redesigned, and cut out new Z-axis parts for my CNC machine, I'm just waiting for the trapezoidal nuts to arrive. That should sort out some of the roughness, and a smaller stepover should also help. Thanks for that info, by the way.

Just one question. Do you have any idea how well the cutter handles the resin coated MDF?

Regards,
Martin.

Originally Posted by heli_bee

Just one question. Do you have any idea how well the cutter handles the resin coated MDF?

Regards,
Martin.

I've done extensive mouldings in MDF using hobby routers and most woodworking cutters produce a very decent finish indeed. You will need to sand fill and polish it, but its darned accurate. few thou at most with a good cutter.
Id be tempted however for something like this, to prep up the machined MDF and then make a male resin cast and polish that, then take females off that.

Another possibility is to machine out of high density foam, fill that with casting resin and then toss the lot in a bucket of acetone to remove the foam, and then polish THAT.

Sort of 'lost foam' casting as it were.

Hi all,
after a little bit of thought, I decided that my best approach to finishing the moulds was to follw the advice offered.

To this end, I applied the first coat of EPOXY resin at lunchtime and used a hot air gun to help it wick into the MDF. All I can say is "Wow!" Does this stuff swallow the resin, or what. I mixed up 18g of West Systems epoxy and the mould took the lot.

When it is cured I will give it another coat, or two, and let you know how it is progressing. I will still try the polyester resin, but at least this way I will have a baseline to work to.

Regards,
Martin.

Great! Keep us posted.

Erazz,

I've had a play with the resin and a coat of white paint. It has proved the technique, however, the paint highlighted a machining fault. When I did the rough cutting, I think I took too much off in one pass. This led to a flat on the bottom of the mould. Photo to follow. Not really a problem as I will make another one and this time I will use a brand new cutter (cost GBP24).

One question I want to ask you about the mould. Is it a true airfoil with a zero thickness trailing edge, or is it modified with a 0.3mm (0.012") trailing edge thickness like Tom Siler prefers?

Regards,

Martin.

Hi Martin,

It's modified with 0.2mm TE. You can't close the mould properly if you don't have a place for the fabric to run off.

Hi all,

as promised see attached photo. The surface is actually a lot smoother than it looks. I stopped working on it when I realised it would take too long to put right. Actually, it would be more accurate to say it would take longer than I was prepared to spend on it.

Erazz,

I have had a bit of time this evening to play in the workshop. I re-wrote the cut files for the fin mould. This time I used an 8mm cheap dremel cutter, as I only had 6mm and 10mm in the good stuff. I played around with the settings in Vectric 3D and ended up with a range of 26 minutes to 9.5 hours to cut the mould!!

I decided to use the same cutter for the rough cuts as well as the finish cuts and saved them as a single, combined file. Running it through Mach3 simulation gave a total of 11 hours 16 minutes to complete.

I used 1mm pass depth and 7% stepover. I thought I would at least be able to see how the cuts looked before I stopped for the night. What I didn't plan on was the cutter only lasting 15 minutes before becoming blunt. Just goes to show, we need to use good quality cutters for this type of work.

Anyway, despite this I was happy with the quality of the cutting - whilst it lasted. See attached photo for the quality.

I will have to have a play with your mould file in SW to allow me to cut the dowel holes as a seperate entity.

Martin.