Take a look at my build in progress. I'm in a hurry right now I will try to respond to some of your questions later if someone else hasn't answered them.
Hey folks. I'm looking to build a small mill for PCB work. I've been entertaining the idea for a year or two now but always lose confidence before starting the project. The last few days, however, I've become convinced it's possible to build a machine capable of doing fine trace work.
I do have a few questions and would also like some of you knowledgeable vets to confirm if some of my current conclusions are correct. I'll just run through them real quick in a list to save you some time if you're interested:
1) Frame Material: 80/20 aluminum--does this material work well? Does it end up rigid enough to to flex when using the larger joining plates? I was thinking of building a base frame out of 10 series double-wide 80/20 and the gantry out of triple-wide. I don't have the ability to weld aluminum and don't want to work with wood. Metal machines appeal much more strongly to my aesthetic, not to mention being stronger.
2) Base Plate: I was going to purchase a flat 1/2" thick aluminum plate to cover the base frame upon which the X-axis table would be secured and built.
3) Size & Gantry Design: I plan to have a fixed gantry and a moving table since my design only needs to accommodate 6x8 inches or so. I assume the fixed gantry will help with precision since the heaviest pieces are attached to it and it would be fully rigid. I figured I'd build an 18x30 table so that at the very least I could fit a 12x12 work piece after the space lost due to the ball nuts and such.
4) How do you secure drive screws to the frame? How do the motor/screw couplings work? These two details are the part I'm least sure about.
5) Drive Screws: I plan to use ball screws since I found an affordable source for them. They may not be necessary, but damn I think they're neat and would like to incorporate them.
6) Spindle Motor: Has anyone had any success with those cheap spindle motors on eBay from Asia? I know there's a thread about what I assume to be the same motors from 2008, but I was hoping for some more recent data. Would I be better served by a motor from Keling?
7) What do I need in order to actually attach a tool to a spindle motor? I'll be using 1/8" or 1/4" circular shank solid carbide tools for trace cutting and hole drilling. Do I need some sort of chuck or something else? How is it attached & centered?
8) Work Deck: Is there any advantage to buying say T-slot decking over solid aluminum plate?
Thank you for your time.
1- The 8020 stuff is pretty strong, and all bolt-on, making assembly easier. There are many builds here, even small format machines, that use the stuff.
3- Like any design there are tradeoffs. Each type of designs has both pros and cons. But for small machines, most I've seen are fixed gantry.
4,5- Drive screws are anchored at each end with bearing blocks. For ballscrews, one end is held captive; and journals are machined for both the bearings and coupling/pulley. On the other end, the bearing is also fixed to the screw, but "floats" in its block, allowing for thermal expansion. If you're direct-driving the screw, a coupling is used; the drive journal and motor shaft may be of different diameter, and the coupling can also take up some misalignment.
6- If I'm not mistaken, the Keling spindles are probably similar to the cheap Asian spindles on eBay. There are a lot here that use them with good results, but documentation could be a little shaky.
7- Usually there is a collet, which the bit slides into, and collet nut, which "tightens" the collet, holding the bit. You probably would get one with the spindle, but you could purchase more sizes as needed.
8- With an aluminum plate, you'd need to add threaded holes, or t-slots, to hold down work. The t-slots are nice and done, but making your own, you can customize it to your work at hand.
I would go for slabs of aluminium plate over 8020 extrusion any day, and did (please see my small CNC build); http://www.cnczone.com/forums/cnc_wo...ll_router.html (DIY hobby small plastics mill/router)
my machine is about 370x250mm cutting area.
The benefit of 8020 is less drilling and tapping, but at a much higher $$ cost and also 8020 is not ideal for geometry compared to custom cut plate.
If you have a good pedestal drill and can drill and tap holes it's not too hard to work with plate. You can buy aluminium plate pre-cut to rectangles you choose (for a small cutting charge), and you can even get it CNC cut to your own design special shapes if you shop around a bit.
Good information in this thread. Regarding plate aluminum, my main concern is workability. My current tool set is extremely limited, but I suppose if I can even get a half-assed mill going I can make proper parts to rebuild it with. From what I gather, that's actually pretty common. Is there any reason to go with a particular alloy? I can get 1/2" thick 5052 for quite a bit less than 6061 for example. I don't think it's as strong, but it'd still be darn stiff I imagine. Any reason to spring for 7075?
I'm still not clear on the screw mounting hardware. I looked for "bearing blocks" and all I see are things that cost hundreds of dollars each. That doesn't sound right. I'll be securing a screw above a plate. What hardware do I need, specifically?
Last but not least, where do you guys shop? I see a lot of very expensive outlets and finding affordable joints seems to be most of the battle. I need to be able to build this machine on a thousand dollars or less.
-from 2 months i have an 0.8kw 24000rpm spindle coupled with Toshiba inverter and could not be more happy, even at 1000rpm it has nice torque and is super quiet, cutting the material makes more noise
-i have a ground aluminum plate bead on my machine with pin holes and threaded holes and is definitely the best choice for a small router, if rigidity and workpiece securing is a concern. you can nicely stick to it a sheet of double sided tape like material and then can easily take it off
-with so small machine like your plan to construct, i would spend some money and buy some good cheap square HKS rails and shafts from automation overstock. this can make the real difference and your machine outstanding
I hope that supported round rails work as well. Before I saw your post, I'd already decided to commit to the project and spent money on linear motion parts. An Asian vendor on eBay (linearmotionbearings2008) has respectable prices for gear and seems to move a lot of product. Since shipping is expensive from overseas, I tried to find as much as I would need that could come from that same vendor.
Here is what I've purchased so far. Feedback on mistakes, tricks for how to use the parts, or reassurance is appreciated. After all, I put together a laundry list of parts based partially on what I've been reading about and partially on what sort of stuff I saw included in CNC kits. I'm not exactly what you'd consider competent at this stuff yet.
Screws & Ballnuts - $136
I've purchased 16mm grade C7 ballscrews. I hadn't actually realized what C7 meant until after the purchase, but hopefully it ends up being of sufficient precision. Since my mill is to be small, I figure there's a good chance.
Screw lengths are custom just for me! =)
X - 466mm (400mm / 15.74" travel)
Y - 366mm (300mm / 11.81" travel)
Z - 266mm (200mm / 7.87" travel)
Along with this come three ballnuts which were supposed to be anti-backlash, but the invoice doesn't say it so who knows. Hopefully I get what I want, because that part is sort of important.
Screw Bearing Blocks - $99
Three "BK/BF12" blocks are also in the package I built with locknuts and circlips. This is not something I know much about. I hope their utilization will be obvious once I have them in hand.
Rails & Bearing Blocks - $109
I've also purchased 20mm supported linear rails from the same merchant. These are the circular type and hopefully suitable. I see a lot of people using flat rails. I've also ordered bearing blocks to match.
Rail lengths are also custom. It's nice getting exactly what you want!
X - 450mm / 17.72"
Y - 350mm / 13.78"
Z - 250mm / 9.84"
I've got two of each rail length and twelve 20mm bearing blocks.
Couplings - $15
Three 6.35mm x 10mm flexible couplings are included. Honestly, I have no idea how to select the proper couplings, so I let the vendor do it. What dimensions do these measurements correspond to? They're cheap, so if these aren't the right parts I can afford to just get something else.
Shipping - $138
I'm looking at chassis metal now. I plan to take a cue from Roman and build the machine out of solid plate to keep things simple and rigid. I've found a good source for 1/2" thick 7050-T451 aluminum plate on eBay--the guy is selling it for less than I'd have to give for even run-of-the-mill 6061. The good news is that 7050 is similar to 7075 but even stronger. A lot of its song and dance also appears to revolve around the fact that it doesn't suffer from stress corrosion cracking like 7075.
If I go down this road, I have to ask: How can I join two pieces of perpendicular plate? That will be required for building the gantry and also for attaching the gantry to the base plate. Is it not destructive to drill into the edge of plate and thread it for say 1/4" bolts? If not, what tools do I need to accomplish that? Are bolts even going to be rigid enough? If it isn't feasible, how does one go about joining the two pieces? This part seems less obvious than my original plan of using 80/20.
yes, the 20mm round rails are good choice too. and a bit forgiving if not perfectly straight mounted, i believe they are the fastest also, and for the price can not be beaten. I am also contemplating round supported rails on my next build
It helps if you have a decent sized drill press. My pedestal drill is juts a cheapy $200 one but it is pretty tall, so I can put flat plate vertically in a vice and drill into the edge of the flat plate.
Like most drilling there are some tricks, like to use a good centre punch to locate the starting drill then use a pilot drill first, smaller than the final size. Also if tapping a thread in a deep hole in aluminium you can use a slightly larger hole which is easier to tap and less chance of tap breakage.
There are a heap of experts in the metal working machines section of the forum that can help with specifics. You might want to ask them about the 7050 material too, in case there is some issue with working with it (like it being hard to machine or hard to tap).
Good news. The linear motion stuff I ordered from Hong Kong arrived on Monday. Now that I have it all in-hand, I have a better appreciation for how to build this thing. The parts clearly are meant to be used in a particular physical configuration, so now I understand the relationship among the different types of bearing blocks, screws, nuts, circlips, etc.
One thing I'm surprised about is how stiff the ballscrews are. I thought they are supposed to be easy to turn even by hand due to the efficiency, but that's not the case. That must be the hidden cost of anti-backlash ballnuts, yes? There's certainly no slop in these parts. hehe
I'm looking forward to ordering more parts. I'm debating if I should order electronics and motors first or metal. I can't afford both at the same point in time. If I order the electronics first, I could build the machine quickly out of plywood and then use that shoddy machine to drill the aluminum for the real chassis. Plywood or not, it would still vastly outperform my ability to drill these holes by hand.