Mechanical Resolution for Mold Making

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Hello All. My name is John and last month I put a deposit down on a Nomad 883 Pro (YAY). I’m interested in making some molds for plastic gears and was recently reading a mold making guide that “recommended” a cnc machine should have a mechanical resolution of 5 micrometers for good mold making - Mold Making Guide. The stated mechanical resolution of the original nomad 883 is 0.001" (25.4 micrometers). It looks like Warren had some good luck with making a mold of a gear on his Nomad 883 and so I’m confident this machine can work reasonably well for me. Saying that though, I would like the resolution to better match the specifications listed in the guide I was reading.

How much more mechanical resolution can be expected with the updated Nomad 883 Pro and are there any aftermarket changes that can be made to better increase mechanical resolution?

FYI, The machine used by the person who wrote the model making guide is the Roland MDX15 and that has a 0.00625mm/step resolution (although I’m not sure what mm/step infers vs. straight mm resolution).

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Resolution is one measure of a machine (i.e. accuracy). Precision is also required. They combine together to yield the machine repeatability. What can one repeatably do every time?

Step resolution (at the stepper) doesn’t simply translate to repeatability at the tool. There are a lot of moving parts involved, each adding uncertainty. Steps are translated via gears to actual motion. Each axis adds it’s own uncertainty.

5E-6M is ~0.0002". That’s beyond what a Nomad 883 or Nomad 883 Pro can offer.

A CNC machine that has repeatability of less than 0.001" is rare, specialized and expensive. Machining at the 0.0001" level is… challenging. A simple desk - for a “desktop machine” - may wiggle too much! A specialized mounting is usually necessary.

The Nomad is an open loop (positioning) machine. There is no feedback that the tool is where it is supposed to be (a closed loop). It is limited to what one can achieve with those components. The 0.001" is about as good as it gets.

Closed loop positioning provide gains in repeatability. Fancier gearing provides more. Even then, 0.0001" is about as good as it gets.

Rigidity - sheer MASS using strong materials is necessary. Steel and lots of it. The Nomad is massive - for its size - but it is Aluminum.

Then the fun really begins… float the machine in the air, isolated from the ground. Don’t forget a closed temperature controlled chamber with very tight hysteresis. Add exotic feedback - laser reflective range adjustment. The list goes on.

$$$

I would check into that requirement. If that is really what you require, you better look at much bigger budget than for a Nomad! Don’t forget an operating budget either. :wink:

The Nomad will not disappoint… one just has to be honest about its abilities.

mark

P.S.

Yes, the Nomad 883 Pro should be “better” than the Nomad 883. How much? We don’t know yet. A factor of 5? I severely doubt it. 25X? No way.

Super high quality collets will help too - lower TIR (runout).

Choosing end mills from sources that specialize in fine machining will help. Solid carbide, micro carbide steels, exotic coatings help a little bit.

Temperature controls will help a bit.

Really good control of swarf removal, lubrication will help.

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Thanks for the in depth response Mark.

I understand what you are saying about step resolution not being equal to mechanical resolution. It seems like the step resolution has more to do with stepper motor quality/type and driver programming.

I also understand how to calculate theoretical resolution (i.e. how many steps per inch the machine can perform). How does a manufacturer calculate the real life mechanical resolution of a machine under a reasonable load though?

Being new to this it’s hard to say what I’ll actually need until I get the machine and start using it :smile:, but it is about the nicest machine I could justify buying. It looks like a Roland SRM-20 (replaced the Roland MDX-15) costs about $4,900 and that is completely out of the budget.

Thanks for your ideas on how to preserve mechanical resolution. I’m planning on building a rigid table for the Nomad to sit on, an additional enclosure (I’m anal about noise) that might also control temperature better, and using high quality end mills/collets for when I need high resolution. The lubrication may be tricky though. For swarf removal I was thinking of pointing a high powered aquarium air pump at the bit, we’ll see how well it does.

I’ll see what I can do when I get the machine!

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Hi @JohnM
Yes, I’ve done quite a lot of milling machineable wax, which I then use to pour a silicone mold, and then make resin castings of gears and other mechanicals.

I think a question for you to think about is what type of size gears are you making? Mine are about 0.5" diameter and larger… and that works great… but if you are making little micro-gears for tiny planetaries or watch pieces, etc. then that is a whole different category.

The resolution of the Nomad is great for my type of larger items… I guess it depends on how small you need to go.

Warren.

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Blowing air works well! For fun, look at a vortex tube - you can cool the work while you blow away the swarf. Air, especially cold air, is a lubricant.

Few CNC machines and mills actually know their repeatability limits; nor do the vendors want to state them if they are known. Resolution is easy to state and difficult to relate to actual, usable data - repeatability. Repeatability is what counts in real use. Often, this has to be measured.

If you’re working in soft materials - Renshape, wood and such - air safety is an issue. We can talk about this too.

The Nomad is a very nice “sweet spot”. For the money, you could do some things better but you would have to leave off other things.

mark

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Thanks Warren. I won’t be making any watch pieces certainly. Small planetary gear boxes would be nice, but I’ll just have to see how low I can go.

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Those Vortex tubes are cool, hehe! I had no idea what one was until you mentioned it, this youtube video helped too. I suppose for repeatability I could do a circle, diamond, square test and perhaps I can get an idea of resolution by measuring the smallest arc I can accurately make.

Air safety is very important to me. I will be using the machine inside the house with my wife and daughter around and I would be devastated if I made any of us sick. I understand wood can release spores, viruses, and general respiratory irritants in the very fine dust cnc machining produces. I am hoping to reduce the risk from wood by using a shop vac with a hepa filter (is this enough?). I looked up an msds for Renshape and couldn’t find much info about RenShape460_MSDS.pdf (955.6 KB). I saw the discussion regarding air quality in relation to milling FR4 pcb boards and am looking into a festool vacuum. Anyone have any opinions on the smaller model of festool vacuums such as the CT MINI HEPA or MIDI? I like the compact size of MINI and MIDI vacuum and I don’t think I would end up emptying it out that often, especially if I had a cyclone upstream of the vacuum. The other vacuums seem too large (I live in a smallish apartment).

I like the Nomad because it seems very refined for the money. It looks nice too :smile:.

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As a Festool owner (i.e. a CT26) I would say that you don’t want the MINI. I took a long and hard look at Festool and decided against the MINI.

The CT26 seems to be the sweet spot for small machines. The connectors are superior, the size isn’t that much larger, and you have options for hoses that provide better vacuum.

I would consider getting the Oneida cyclone for it. That way you’re not going through expensive HEPA filters quickly. I can help with things Festool and Oneida… I have an Oneida too. I think I posted a picture of it somewhere…

A HEPA filter with 0.3 micron rating meets or exceeds both US and EU requirements. It’s what a firefighters mask uses.

Yes, there is no replacement for measuring the actual machine do real work.

mark

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I really wanted to avoid the larger size of the CT26, although I understand the importance of having good accessories. That kind of advice is invaluable. Thanks! I’ll look into the Oneida cyclones, it looks like their dust deputy would pair well with the festool vacuum. How often do you change your HEPA filters when using the Oneida cyclone?

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The Oneida that stacks with the CT26 is the one I have. The separate works too, just not as nice a stack… and perhaps more space.

Remember to empty the cyclone OUTSIDE since it will contain 98+% of the particles. The value of the cyclone is the reduction in expensive filter replacement costs.

How often does one have to change the HEPA filter? That depends on what you machine and how often you do it. More wood and Renshape, more frequent replacements of the HEPA filters.

I do more than a fair amount of MDF and wood and I can go many months, sometimes a year, without a filter replacement. Without, I could go through a filter every few weeks/months. YMMV.

mark

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Good to know. At $60 per filter that could get expensive without a cyclone!

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It’s more like $80 from many sources. Definitely shop. YMMV.

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For what it’s worth I have the Festool Midi vac, and it fits great in my tiny Brooklyn apartment. All Festool vacs have the same suction power, I believe, and the same HEPA filtration, so it’s really just about what form factor suits you best - and if you’re catching all your particles in a cyclone you’ll probably never manage to fill the bag anyway - whatever model you buy.

EDIT: between the Mini and the Midi, I can see no reason to choose the former, unless you had a specific height space you were trying to put it, eg under a seat or something.

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Even more of a reason to get the cyclone. At $80 a filter it would only take 3 filters to equal the cost of a dust deputy

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Thanks for your input MrHume. Space is a premium in my apartment. I’ll have to consider the pros and cons carefully of the different models.

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I believe all of the HEPA filters are rated the same - 0.3 micron - but the larger machines have larger filters. With a cyclone in front the larger filters should last longer. Their marginal increased cost is well offset by the longer life.

@MrHume is correct about the ~110 CuFt/Min being the same across the models. Plenty with good tubing and smart usage for safe air handling.

All of the Festool offerings are excellent and air safe dust collectors. Add a cyclone in front - or stack it if you able - and you’ve got quite a combination.

I will say that for the CT 26 and up, the connectors and tubing options are a bit nicer than the smaller machines. The tubing is pricy but excellent. The stacking feature allows the Oneida cyclone to sit on top, saving horizontal space.

mark

P.S.

The dust head STL files have been posted! They are for the Nomad so some modification will be necessary for the Pro - but this should be easy.

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Good info. That dust head looks very functional, when my machine comes in I’ll see if I can make the necessary design changes to make it work on the Pro.

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Yup! Cyclones dramatically extend your filter life. The payback when using HEPA filters is quite fast.

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+10 on the cyclone. by far the best tool in my shop, even over the nomad… (well, its darn close)

+1 on a HEPA filter

+20 on good air movement through an air filter in the workspace. I just built a very simple air cleaner (450 cfm duct fan, stacked 14"x14" filters, simple enclosure) and that has really helped me breathe well whether CNCing or using my miter saw (my shop is in my garage, when the cars aren’t).

The Dust Box for the pro should just me a matter of determining the location of the spindle and relocation those holes in the top and bottom plate. I can make available those STL files as well if someone can get me the measurements.

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Good to know. I hadn’t thought about including an air purifier in the room, I’ll have to look more in to it. I Lug all my tools outside when I need to do cutting with my saws, but I won’t be doing that with the Nomad! When I get my Nomad I’ll post some measurements if someone doesn’t do it before me.