Milling head or CNC router spindle for cutting aluminum?

I am currently building a CNC machine to make aluminum molds for my composite parts business and I wanted to ask you guys for opinions on which type of spindle is best for this purpose.

I am building the machine on a fairly tight budget but I have a choice of going with either one of the high speed Chinese CNC spindles off eBay (a 1.5kw or 2.2kw) or buying one of the 1/2hp low speed milling heads from Little Machine Shop. The decision is confusing me...

There are a lot of conflicting opinions on what set-up is optimal for machining aluminum. Some say I should buy the most powerful CNC router spindle I can afford and cut at higher speeds. Other say you get a better / more precise finish from a low speed milling head.

I have also read that you can't directly compare the specs of router spindles and milling heads because 1/2hp at 2500 rpm is worth more than 1/2hp on an 18,000 rpm spindle.

Both options cost around $350 all in. My main concern is the finish quality. Speed is not important to me at all.

Even though I'm building on a budget, it will still be a fairly rigid machine. The frame is all steel and aluminum. The rails are all THK and the gantry is carbon fiber and aluminum and quality ball screws are used for motion. I mention this because the machine will be rigid enough for the choice of spindle to matter.

What type of spindle would you guys choose for making aluminum molds from the options my budget allows?

I own a Techno 40" x 20" with servo motors and ball screws. The spindle motor that came with this machine (and which I am using) is a Porter-cable Router motor with a 3 1/2 horse power motor, and a variable speed control which maxes out at 25,000 rpm. This Porter-Cable spindle motor costs about $350. The next available spindle for this machine was a Perske spindle at about 10 times the price. This set-up that I have is very adequate for milling aluminum. The major drawback is that it is very noisy, and hearing protection is required. Conversation is almost impossible when it is running. The Perske spindle would be a lot quieter. The quality of the cutting action of the Porter-Cable is very good. I don't think the quality of the cut or finish with the Perske spindle would be any better.

I have learned that the bearings on the Porter-Cable router go bad after several hundred hours. However they can be replaced at a Porter-Cable service center for about $100. I have two of these identical router motors, so I can switch them out and send the one with the bad bearings in to the service center for replacement. This is not a job that you can do for yourself. I do not know about repair of the Perske spindle bearings, but unless they are water cooled, they will eventually go bad.

I understand that you can cut aluminum with regular routers. My question is more about which type of spindle is optimal for my needs given that I need a good surface finish and a spindle that will last.

The general advice I have received is to avoid using regular routers as they aren't designed for prolonged continuous use. They break quickly and, as you found, are very noisy. They also don't offer any real control over the torque. 3 1/2 hp would only be achieve at peak rpm and it would be a good bit less at a lower speed used for aluminum.

I am looking at either a dedicated CNC spindle controlled by a VFD or a lower speed milling head.

A spindle capable of 18,000 RPM or higher is excellent for aluminum milling, particularly with o-flutes. Router spindle FTW

Add another zero to your price range to get what you want. I use a Hitachi router with small cutters or the R8 spindle with larger cutters and lower speed. Both work, but the R8 with Tormach tooling is repeatable and easy to change while the router is neither.

Goemon said:

I am currently building a CNC machine to make aluminum molds for my composite parts business and I wanted to ask you guys for opinions on which type of spindle is best for this purpose.

I am building the machine on a fairly tight budget but I have a choice of going with either one of the high speed Chinese CNC spindles off eBay (a 1.5kw or 2.2kw) or buying one of the 1/2hp low speed milling heads from Little Machine Shop. The decision is confusing me...

There are a lot of conflicting opinions on what set-up is optimal for machining aluminum. Some say I should buy the most powerful CNC router spindle I can afford and cut at higher speeds. Other say you get a better / more precise finish from a low speed milling head.

I have also read that you can't directly compare the specs of router spindles and milling heads because 1/2hp at 2500 rpm is worth more than 1/2hp on an 18,000 rpm spindle.

Both options cost around $350 all in. My main concern is the finish quality. Speed is not important to me at all.

Even though I'm building on a budget, it will still be a fairly rigid machine. The frame is all steel and aluminum. The rails are all THK and the gantry is carbon fiber and aluminum and quality ball screws are used for motion. I mention this because the machine will be rigid enough for the choice of spindle to matter.

What type of spindle would you guys choose for making aluminum molds from the options my budget allows?

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I didn't know you could actually buy a spindle for a mold making machine for $350.00, I have been going about this wrong the whole time. Maybe I should replace all my spindles with them. I bet the Maatsura would run gooder with one of those.

R

Machine rigidity will affect surface finish mote then your spindle.

The only advantage I see for a low rpm spindle is that it forces you to lower the feed rate accordingly. And that's an advantage with cheap machines as the lower feed rate should allow them to move more smoothly.

2500 rpm would do ok if you don't run anything smaller than a 3/8" cutter. And the 1/2 hp, not much larger.

Disadvantage of the high rpm spindle is in drilling holes. 8k rpm is pretty fast.

Thanks for the info. I spoke to the people at Little Machine Shop too and they also advised that I would be better off with a higher speed spindle for aluminum. I was impressed with their honesty given that it meant advising me to buy someone else's product.

I'm going to buy a 2.2kw CNC router spindle with a 220v VFD.

For the guys that said you need to spend $3500 to get a spindle to cut aluminum molds, go to eBay and search for 2.2kw CNC spindle. These are more than capable of milling aluminum in the (low) volumes I need.

Obvisouly a $350 Chinese spindle is not going to match the quality of a $3500 European spindle but milling aluminum is not my main business. I make carbon fiber parts. The cheaper spindles will be fine for cutting one or two mold cavities a week.

As for determining the optimal speed for aluminum, I have become convinced that there is no (one) correct answer or none that anyone knows. Some say 8000rpm, some say 18,000 and everything in between. It's going to be a case of trial and error to find the best speed for my machine, end mill and aluminum alloy combo.

I am interested to see how using carbon fiber for the gantry structure will effect the limitations of the machine. It will be a good bit stiffer and stronger than other machines of similar weight.

Still, it leaves the question, why would they make a milling head for a benchtop mill that is only capable of 2500 rpm? I can't imagine many people buy a $600 mini mill to focus on cutting steel. I would have though it would be best to optimize it for aluminum.

Goemon said:

Thanks for the info. I spoke to the people at Little Machine Shop too and they also advised that I would be better off with a higher speed spindle for aluminum. I was impressed with their honesty given that it meant advising me to buy someone else's product.

I'm going to buy a 2.2kw CNC router spindle with a 220v VFD.

For the guys that said you need to spend $3500 to get a spindle to cut aluminum molds, go to eBay and search for 2.2kw CNC spindle. These are more than capable of milling aluminum in the (low) volumes I need.

Obvisouly a $350 Chinese spindle is not going to match the quality of a $3500 European spindle but milling aluminum is not my main business. I make carbon fiber parts. The cheaper spindles will be fine for cutting one or two mold cavities a week.

As for determining the optimal speed for aluminum, I have become convinced that there is no (one) correct answer or none that anyone knows. Some say 8000rpm, some say 18,000 and everything in between. It's going to be a case of trial and error to find the best speed for my machine, end mill and aluminum alloy combo.

I am interested to see how using carbon fiber for the gantry structure will effect the limitations of the machine. It will be a good bit stiffer and stronger than other machines of similar weight.

Still, it leaves the question, why would they make a milling head for a benchtop mill that is only capable of 2500 rpm? I can't imagine many people buy a $600 mini mill to focus on cutting steel. I would have though it would be best to optimize it for aluminum.

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You could run 100 000 RPM in aluminium if your machine has the balls to feed hard enough for it and keep up with the toolpath (also cutter and type of cutting dependent of course). You will see that most guys with proper machines will max out their RPM's, or suggest to, and run it as hard as possible for aluminium. If my machines went above 11 000 RPM I'd sure as hell run them up there on aluminium, but then again my machine would be feeding seriously hard to make sure that I am not up at those speeds just to say I can.

I have no idea what $600 will buy you because of currency difference but I am sure the only thing that might make me buy one would be if I needed an extra paperweight to hold down some scrap pieces of paper.

Here are a couple concepts you are missing. Cheap and good don't go together. Some spindles are not designed for axial load (drilling in hard material IE not wood) I had a 3 HP Columbo spindle that the manufacturer said was not hold up for drilling AL. A carbon fiber gantry is too light but you can build it and prove what I am telling you. When machining weight and rigidity is what you want. The cheap China spindle more than likely has no support if it breaks or even works at all. If you are going to make money doing what you have in mind don't waist your money on things that won't work and invest in something that will. If you are building the machine yourself you would be better off with an old machine with broken controls and add a new controller this way you have a machine designed to mill. From what I have read on here what you have planned will not work. Most of thee really knowledgeable guys on here are not even responding to this thread just from your title sometimes we really don't like the advice that was asked for. I hate to see people spend money on ideas that don't work. If it was possible to make a CNC machine for a few hundred bucks that could do what you are wanting to do the market would be flooded with them. Speed of the spindle is just part of an equation that requires the proper feed to make a proper chip to keep your tooling from breaking or burning up.
Good luck with your decision.
Mike

Goemon said:

Still, it leaves the question, why would they make a milling head for a benchtop mill that is only capable of 2500 rpm? I can't imagine many people buy a $600 mini mill to focus on cutting steel. I would have though it would be best to optimize it for aluminum.

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They are effectively "geared" differently. I have a 1.7kW HSD spindle and the biggest hole I can drill in AL is about .250" observing SFM limits and chipload rules of thumb.

CNC router spindles lose power as you reduce RPM. My 18k 1.7kW is actually a 500W spindle at the speeds I cut AL (7kRPM). Maybe I can drive it harder in AL, but I don't need to (small jobs). Drilling needs low RPM. Who would buy a mini mill that can't drill >1/4" in AL?

Surface finish: like said already, machine rigidity is more important than spindle. However, if you buy cheapo China spindle, good luck having next to no runout. I have a little machine shop mini mill (China) and it has runout on top of plenty of slop on the ways.

Another factor is heat. I'm not sure what type of lifespan guys are getting on their end mills cutting AL at 24k RPM. I suppose you can go super fast, but I imagine the cutters go quick. If you're not planning on flood coolant, even more to be concerned about.

I imagine you're making molds that have plenty of curvature. These jobs take hours to run. Again, end mill lifespan would be a concern to me.

If you buy the cheapo spindle, buy both options. neither will probably last all that long, then you can test both and have a backup.

Good luck.

I'm not worried about a lower cost spindle not being able to do what I need for one simple reason which I value above any advice: I have seen it work first hand.

Before starting my project, a member on another forum was kind enough to invite me over to see his diy bench CNC machine and he let me try a test job in aluminum so I could see what sort of results I could expect. I was worried about the finish quality being poor and requiring so much additional work that it made the whole thing pointless. I was satisfied with the results.

The machine I am building will be considerably more rigid than the one I saw.

For those who are stating that the Chinese spindles have no value, is this based on recent personal experience of trying one? It's not consistent at all with most of the advice I have received from people who use them. Almost everyone (who has used one) rates them well above using a consumer router and more than capable of cutting aluminum well enough for my needs.

Either way, it's not exactly a choice I have to make. I don't have the budget to spend $3500 on a spindle right now. My choice is to use a lower cost 2.2kw Chinese spindle, stick to using a regular router, buy a low cost 1/2hp 2500 rpm milling head or not build a CNC machine at all.

I currently make my molds manually out of carbon fiber. They work well but making them takes up too much of my time. I'm not looking to set up a CNC shop, just need something to make a few molds so I can focus on making product..

I looked at using a third party CNC shop but I could find none with prices that made sense for my business.

Anyway, I have the answer I was looking for. For cutting aluminum on a smaller lighter machine, higher speed spindles are optimal.

Even if the machine made just one mold and then broke, I would still end up ahead with the money I would make from the mold. It will make far more than one mold though.

Ah gotta love a lil Saturday afternoon free comedy...

I had to do my own research on this topic as I wasn't really able to understand most of the advice I was getting. Anyway... I understand it enough now, so... in case it's helpful to anyone else:


The correct answer to my initial question about choosing a spindle for making aluminum (6061) molds on a smaller benchtop CNC machine is a CNC spindle, not a milling head. Here's why:

With modern carbide and coated end mills, aluminum is best milled at higher speeds. Most lower cost milling heads are limited to less than 6,000rpm. Low speed milling is best done on large heavy machine cutting harder materials. There are high speed spindles on the milling heads of large machining centers capable of 30,000rpm but nothing affordable for a diy benchtop CNC machine. Not a single option.

I only found 3 options for low cost milling heads period. The best of them for aluminum is the Tormach 770 1hp spindle head which is sold separately. It reaches 10,000rpm and has the flexibility of an R8 spindle but I found better options for the money with CNC spindles.

With all that said, not all CNC spindles are equally suitable for aluminum. The part I struggled to understand is that the stated power tells you nothing about a spindles material removal capability unless it is linked to a spindle speed.

To put it more simply, a spindle that reaches 2.2kw at 6000rpm will be around 3 times more powerful than a spindle that reaches 2.2kw at 18,000rpm. A 3hp spindle spindle can have 2 or 5 times more torque than a 6hp spindle etc. The peak power means nothing on it's own.

Milling aluminum (6061) doesn't require nearly as much torque as milling steel but more torque is usually better than less. The higher the spindle speed, the more power you need to maintain decent torque. This is why large high speed CNC mills have crazy huge 40hp spindle motors (or more). Take a look at the Hass mini mills for an example.

The key point is that many of the 24,000rpm 2.2kw spindles on eBay have less material removal capability than the 500watt 2000rpm hobby milling heads sold by little machine shop. I ended up going for a 7.5hp CNC spindle which is about the max I can run from my 230v domestic 1 phase outlet.

The other thing worthy of note is that most cheap air cooled CNC spindles use a shaft fan. This means the level of cooling correlates to the spindle speed. This limits the minimum spindle speed to 10,000rpm which then limits the usefulness of an aluminum milling spindle. I found it better to shop around for a deal on a better quality spindle with a seperate electric fan. The Colombo RS line, for example.

Getting a deal on a refurbished unit meant that for not much more cash, I got a spindle capable of consistent 3nm of torque from 0 all the way up to 18,000 rpm. This means I can use both (smaller) high speed steel 4 flute end mills at 2000 to 6000rom and run 1 or 2 flute carbide (or coated) end mills at 16,000rpm (or higher). 3nm is not much in milling terms but it is enough to do what I need in 6061.

In terms of the difference between high end and cheap CNC spindles, this is what the manufacturers are incapable of articulating for some reason:

The main difference between cheap and quality spindles is the type (and number) of bearings used and the tolorances for concentricity. The latter is measurable with a dial guage. At $1500 spindle should run true out the box, a $300 spindle needs work...

Replacing the bearings on a decent CNC spindle can cost over $1,000 just for the bearings. The larger the spindle diamater, the more it costs for high quality bearings. Quality bearings capable of higher speeds also cost more which is why companies will pay $8,000 or $10,000 for a quality 40 taper spindle cartridge capable of 12,000rpm without the casting or the motor....

There is a tremendous variety of bearings at various quality levels and, in most cases, they are labeled with just a meaningless code. I don't know what all the codes mean but, as a general point, the quality brands use better ones. In real terms, better translates to more accurate cuts with less backlash and spindles that last longer.

It sounds like I am saying everyone should take out a second mortgage on the spindle. I am not. Based on what I have seen, it is best to choose a spindle that is consistent with the overall quality of the build. In other words, all that extra precision is probably wasted if it goes on a flimsy frame that isn't squared properly etc.

The longevity of the spindles is irrelevant because you can replace cheap spindle 5 times before you could replace the bearings on a Colombo.

I got lucky with a deal on a Colombo. In most cases, there is a higher chance on finding a good deal on other quality brands like Perske, Elte and, occasionally, on HSD units.

If you are buying used, make sure to find out how much life is left on the bearings before parting with cash. Used spindles are hard to resell so you don't want to get stuck with a Turkey. If the seller can't tell you, assume it's not a good deal.

A quality CNC spindle for milling aluminum should have dual front bearings (so 4 bearings total). Brands like Perske use single front bearings for their wood spindles. Make sure to look for their dual bearing models.

I have to tell you, every time I've looked into reinventing the wheel to "save money" or some other dubious metric I end-up going out and buying existing wheels because it's cheaper, better and faster.

You didn't say how big your carbon fiber molds need to be. If we are talking huge, then, sure, unless you have the money it might make sense to reinvent the wheel. Barring that, for anything that could be done on a conventional VMC I would not screw around. I'd buy a VMC, used if necessary. Hell, you could even consider converting a Bridgeport type machine to CNC (or buying a done-for-you clone or second hand).

Since you only need to do a couple of molds per week speed doesn't matter. My guess is you want good surface finish. This requires mass and structure, all the way from the ground to the tip of the cutting tool. Carbon fiber isn't a good material for building cutting machines. The reason companies use massive cast iron and epoxy granite structures is because they need both stiffness and the absorption of vibrations. Carbon fiber can't do that. The case is the same for aluminum, build a CNC rig using aluminum and it will ring like a bell resonating at various frequencies as the machine operates.

If your business is making carbon fiber molds go get more business and stop trying to reinvent the wheel. But the CNC machine you need. Decades of engineering at the hands of very experienced engineers have gone into making these machines.

I see lots of people trying to build their own. I wonder what the reality of using these contraptions might be after they are done? It's fun while building and solving problems but, what happens after that? I'm sure some make it work but I'll bet most anyone who has a real business needing machined parts eventually sees the light and buys a real machine.

martin_05 said:

I have to tell you, every time I've looked into reinventing the wheel to "save money" or some other dubious metric I end-up going out and buying existing wheels because it's cheaper, better and faster.

You didn't say how big your carbon fiber molds need to be. If we are talking huge, then, sure, unless you have the money it might make sense to reinvent the wheel. Barring that, for anything that could be done on a conventional VMC I would not screw around. I'd buy a VMC, used if necessary. Hell, you could even consider converting a Bridgeport type machine to CNC (or buying a done-for-you clone or second hand).

Since you only need to do a couple of molds per week speed doesn't matter. My guess is you want good surface finish. This requires mass and structure, all the way from the ground to the tip of the cutting tool. Carbon fiber isn't a good material for building cutting machines. The reason companies use massive cast iron and epoxy granite structures is because they need both stiffness and the absorption of vibrations. Carbon fiber can't do that. The case is the same for aluminum, build a CNC rig using aluminum and it will ring like a bell resonating at various frequencies as the machine operates.

If your business is making carbon fiber molds go get more business and stop trying to reinvent the wheel. But the CNC machine you need. Decades of engineering at the hands of very experienced engineers have gone into making these machines.

I see lots of people trying to build their own. I wonder what the reality of using these contraptions might be after they are done? It's fun while building and solving problems but, what happens after that? I'm sure some make it work but I'll bet most anyone who has a real business needing machined parts eventually sees the light and buys a real machine.

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Normally I would agree with you. I'm certainly no fan of reinventing the wheel when good options are already available. My time and patience is short. In this case, there were no affordable ready made CNC machines at the level of quality I wanted. I spent a considerable amount of time looking and there is not a single good machine between cheap hobby and high end commercial. Nothing with decent x-axis travel anyway.

There are no affordable (for me) third party CNC shops for aluminum molds either. I checked this first.

I didn't mention the size of molds because I wasn't asking for machine recommendations. I had already decided to build my own. I just hadn't decided what sort of spindle and motor I needed.
My molds are typically 30" x 7" and a max of 1.5" deep though. Too long for any commercial desktop CNC mill currently available.

Those old manual Bridgeport mills could cirtainly do some aluminum milling but they are old technology. They aren't fast enough to take advantage of modern carbide / coated end mills or to use the small diameter end mills required for precision cuts. They definitely have some advantages though. If I was milling steel it would have been a no-brainer.

Anyway.... I have no regrets with my decisions (so far). They were all well researched and, in most cases, I had little choice.


You're wrong about carbon fiber btw. It has great properties for milling machines and cf parts can be made as heavy as needed.

Carbon fiber parts are not automatically lighter unless you spec them that way. Compared to steel, aluminum or titanium, you can either decide to make them the same strength with less weight, or... the same weight with greater strength. As with any material, it's about how much you use. Aluminum is stronger and heavier than steel if you spec parts to be 4 times thicker.

This should be a whole new thread but as a general point, carbon fiber has superior vibration dampening properties, strength, stiffness, thermal stability and price point compared to cast iron.

Most of my frame and columns are made from a composite mix of carbon fiber, epoxy granite, carbon nanotubes and some embedded steel plates (for surfaces that need to hold screw threads). I have been delighted with the results. There are some fiberglass strands in the mix for volume too.

I never intended to make a super lightweight machine. As it currently stands, it is now a little over 500lb (without the spindle or electronics). That's not super heavy for a desktop CNC machine but it is significantly stronger and stiffer than any comparably sized cast iron machines. If it turns out that I need more weight, I will just add more weight...

I already have plans to use my cf / epoxy granite mix for my next build which will be a manual mill. I'd choose it over iron any day. Mainly because I don't think iron is an easy material to work with for a diy build.

Goemon said:

My molds are typically 30" x 7" and a max of 1.5" deep though."

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It would seem to me that this work envelope is well covered by a range of machines that can be had at a decent price. And quite a few of them can be had used for less than $20K.

Please note, I am not being critical of your decision. Everyone makes their own and that's that. I've been, twice. The first time I decided to buy a Haas VF3-SS and the second time a VF2.

I don't run a production shop. Just internal prototyping and engineering. I'm not even a machinist. I've learned from books, videos and mistakes and I am still learning every day.

Because of this speed doesn't matter to me as much as the ability to continue to focus on earning dollars through my work rather than taking gobs of time out of my twenty hour days to become a CNC machine design expert.

Now, if you truly needed a desktop machine that's a very different story. In other words, if you don't have the space or power to support an industrial machine.

I have designed and built several CNC routers for specialized applications at this really hot aerospace company that wants to go to Mars (I am contractually precluded from using their name...). Anyhow, in all cases the frame starts as a very solid 80/20 extrusion structure using their 3x4.5 in extrusions, sometimes filled with, well, stuff (again, can't say) to control vibration and resonance. The machines resemble Datron routers (DATRON High Speed CNC Milling Machines, Engraving Machines). When you include the engineering, components and fabrication required to produce a good finish with these home-brewed machines you always end-up in the $100K to $250K range, depending on what you might be doing.

Those old manual Bridgeport mills could cirtainly do some aluminum milling but they are old technology. They aren't fast enough to take advantage of modern carbide / coated end mills or to use the small diameter end mills required for precision cuts. They definitely have some advantages though. If I was milling steel it would have been a no-brainer.

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For about $20K you can have someone come over and CNC one of these for you turn-key with a decent control. Add to that a high speed spindle (they come in many forms) and you have a decent hunk of iron that will do a good job. It will certainly be far better than almost any homebrew solution I can think of. Heck, you could machine a sturdy aluminum mount for the very spindle you have selected for your home-brew machine, remove the Bridgeport head and attach it directly to the ram if necessary (or pay a machine shop to make you a mount out of a solid hunk of steel).

You're wrong about carbon fiber btw. It has great properties for milling machines and cf parts can be made as heavy as needed....This should be a whole new thread but as a general point, carbon fiber has superior vibration dampening properties, strength, stiffness, thermal stability and price point compared to cast iron.

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This would be an interesting thread. I don't know of any CNC metal-milling machines using any carbon fiber in their structures. I'd be interested in learning why that might be. Cost or performance?

Oh, key word DIY build, for a second I thought you were serious about carbonfiber being cheaper than cast iron?

Ask yourself one question before you post facts about cf vs iron price point of view.
Why are all machine frames made out of cast iron and not carbonfiber?
And I'm talking about real machines here, not DIY toys!

Just some food for thought, give it a good thought before you answer, and think about price per pound or Kg.

Marko

No point in trying to help someone who is convinced the crap that little machine shop sells is good. I guess Mazak, DMG Mori, Makino,and others of the same quality are just screwing there customers. How can they get 100000+ for a machine that any home shop moron with 350.00 could build.