[stern-69]

Im trying to determine what size motors to use on a CNC PCB gantry type router (about 24" x 24", ground rod with lenear bearings and ACME lead screws direct drive). Problem I am having, and it mat be a really stupid question (Im a CNC newbe), but I have no idea what speeds are considered slow, fast or normal for ganty (axis) movement. Most of the calculators that suposidly figure out motor size ask what speed you want to run at (possitioning and cutting speed), but I have no idea what speed is good lol. I sure dont expect my machine to fly like some huge industrial ones, but also dont want to wait 2 weeks for a board to finish .... so

What are normal speeds when dealing with PCB isolation milling .... or better yet, whats the range (bottom slowest - highest for a small machine). At least if I have some idea of a speed range I can work out what size motors to get. Also, what is a typical (or nominal min-max) cutting force exerted when doing PCB milling. Once I get a handle on this I can at least try to figure out what size motors. Sure dont want to buy a 600 oz one when a 30 oz would do.

Thank you all

[Jay C]

Travels at 40 inches per minute are just tolerable on my 18x7.5" router. 60ipm would be nice, but I'm not a production shop so I live with it. Cutting speeds are dependent on your spindle, depth of cut, and end mill parameters. I typically mill PCB's at 20ipm

[stern-69]

Thanks, that gives me an idea of what speed area Im looking at (to give me an idea of motors and feed screws). So I am really looking at a speed range of between 20ipm and 100 ipm

[ger21]

Personally, I would aim for closer to 200ipm if possible. Regardless of how fast it is, you'll wish it was faster as soon as you start using it.

[blinkenlight]

It's always nice to have as much speed as possible, however, in my experience PCB milling is completely _dominated_ by the cutting speed (which will never be particularly stellar - you can't just whizz a needle-sharp tip around a piece of copper-clad and expect not to break it). At one point I did look at a simulator software that had a check-box to include or not non-cutting rapids in the simulation - un-ticking the box lowered the overall duration by something like 10% for me: it just didn't make any difference at all.

On the other hand, cutting speed will entirely depend on the specs of the tool you use, and quite a lot on the RPM of the spindle you have - my small hobby mill can do about 10000 RPM which just about gets me in the business; my friend has a 20000 RPM water-cooled one (better!) and people who really mean business are thinking more like 100000 RPM. Of course, using a really fast spindle and thus a large cutting speed, the rate of your rapids might begin to make a difference.

[ger21]

It's always nice to have as much speed as possible, however, in my experience PCB milling is completely _dominated_ by the cutting speed (which will never be particularly stellar - you can't just whizz a needle-sharp tip around a piece of copper-clad and expect not to break it).

Correct, but it is still a 24"x24" machine, and moving 24" at slow speeds becomes nearly painful after a while.

[stern-69]

Thanks guys. For the first rev of the machine Im using my dremmel as the spindle (I know its not accurate, but will let me "prove out" the whole project before dropping a bomb on a spindle, and the dremel runs at 35,000 RPM). I suppose the primary force limiting speed will be the cutter moving through the board, and I have no idea what it should be for PCB material (plus will probably be doing small aluminum bits, which Im think needs a lower spindle speed ?).
So, shouldn't I try to set the machine up design wise to shoot for a speed range that is usable ? I know non cutting jogs you want as fast as possible, but when the tool is cutting Im sure there is a speed range that is normal. I could use a lower pitch count on the lead screw to up the travel speed, but I assume that halving the thread pitch, doubling the feed speed, would double any accuracy error ?

Just want to try to get it set up with as accurate as possible feed and positioning while shooting for a range that will cover both Aluminum and PCB (majority of the work).

[Jay C]

Oh wow, the runout of a Dremel will kill your project. Look at something/anything with a better spindle like a Proxxon IB/E, Bosch Colt.

[stern-69]

Understand, and if thats the case I will probably switch to something else. At this point, getting the machine built and making sure it runes with all the bits is my priority, especially without dumping a ton of cash into it. If it all gos well and runs well then buying a real spindle of some type would be the only choice .... just want to make it and prove it first and then make it as best as it can be. (may not even use the dremel but instead mount a pen just to validate it.)

[inventor83]

I would stay in the 50ipm range. You need to go slow with pcb's, The only time you should start to consider going faster is when you have a precision spindle capable of over 40k rpms.

[stern-69]

OK thanks for the info. Now if I did have a real spindle, what would a maximum cutting speed be considered as. I obviously want to make this capable of running a real spindle, and assume that speed setting can be programmed via mach3, so I just want to make sure my actual gantry speed is capable of running at least as fast as cutting with a spindle (probably faster during jogs and positions). Just looking to make sure the mechanics will operate accurately at a good speed range, and once I have a good idea of that range it will allow me to beter select a lead screw pitch as well as the motors to use.

[inventor83]

If you want accuracy I would go with ball screws over lead screws. I would think that a 10mm pitch ball screw would be a good choice. Depending on your machine/bearings/rigidity/motors and controllers , you should at least be able to achieve 200ipm if everything is set up correctly. I would suggest that you invest in a Wolfgang engineering spindle also. Perfect for PCB's. The spindle is the most critical part of a pcb milling machine. Your runnout on the spindle will be the difference between nice looking circuit board, or one that looks like it was made by a 5 year old.

[Jay C]

inventor83, have you used leadscrews or ball screws on your cnc router? I don't think it's fair to compare the two. 200ipm or 1200" per second is great for travel and jogging, but I have never milled a PCB at that speed. Also, I have the Proxxon, Wolfgang, and Bosch Colt. The Bosch wins hands down. There are many more posts of issues with the Wolfgang (though I never really used mine) that I can't recommend it.

Stern-69, some other things to consider are solder mask and silkscreen legends. I've milled a few boards, and I use vitreous paint as a poor man's mask, but 2-sided boards are more trouble than I care to do, so most of mine are single-sided. Since mine is not a dedicated mill, it's nice having the variable speed of the Colt (and super-PID) to cut wood and plastic with the occasional PCB. Here are some of my videos.
Milling a ring light circuit - YouTube
Re-making a LED task light - YouTube

If you want accuracy I would go with ball screws over lead screws. I would think that a 10mm pitch ball screw would be a good choice. Depending on your machine/bearings/rigidity/motors and controllers , you should at least be able to achieve 200ipm if everything is set up correctly. I would suggest that you invest in a Wolfgang engineering spindle also. Perfect for PCB's. The spindle is the most critical part of a pcb milling machine. Your runnout on the spindle will be the difference between nice looking circuit board, or one that looks like it was made by a 5 year old.

[stern-69]

Thanks for the info. Still looking for a good source of ball screws IF the price is right. Problem with most US suppliers is the shipping to Canada can be more than the product (especially if you get dinged also for duty). Been debating the ball screw vs lead screw thing for a while (most post I have read state lead screws and ball screw can give the same accuracy, its just a friction thing. Also, seems a lead screw is FAR SUPERIOR to a ball screw thats crappy made. This has been my problem so far, as good ball screws from Canada can run around $150 or so, offshore one can be cheaper, but have to watch the quality. Will keep looking, but worst case I know I can use an ACME lead screw with anti-backlash nuts and get them to a very tight tolerance. Best case, I find some good ball screws I can use that doesnt add $600+ to a project I NEED to keep the cost down on.

As for a spindle, I wont buy one until I have a running machine that im happy with, as chucking a bad machine in the scrap will only cost my labor (steel and aluminum I have and got no cost), and if it goes bad I dont want a $600 spindle I have no use for (CNC controllers I can use on the lathe or mill if I screw up).

[inventor83]

inventor83, have you used leadscrews or ball screws on your cnc router? I don't think it's fair to compare the two. 200ipm or 1200" per second is great for travel and jogging, but I have never milled a PCB at that speed. Also, I have the Proxxon, Wolfgang, and Bosch Colt. The Bosch wins hands down. There are many more posts of issues with the Wolfgang (though I never really used mine) that I can't recommend it.

Yes I have ball screws. Lead screws are not meant for anything over 100ipm IMO or should be used for heavier machines. ball screws>lead screws. As far as speeds and feeds. I did say that getting a spindle that does over 40k rpm you should be able to achieve 200ipm. Speeds and feeds are proportionate no?

Maybe an air bearing spindle would be best? Seen a few on ebay once in a while for cheap.

[stern-69]

Thanks you for that info. I am leaning towards ball screws, as long as they can take forces. While it will be almost entirely for cutting PCB's, it will be 24" x 24" and set up to also cut aluminum on a rare occasion (more than likely with a different spindle or at least a slower spindle speed). If ball screws can take it them I think I will start looking for a place to get them and bite the bullet and spend a bit more. Will see how the search goes

Thanks again

[blinkenlight]

I milled my first PCBs on a machine with trapezoidal (ACME) screws (with an anti-backlash nut), and once I cleared up all else they came out perfectly decent; so at least I can say it's definitely possible. On the other hand, it's a workshop machine with a top spindle speed of about 3500 RPM (so I had to go slow as a snail...) and obviously I'd have preferred ball screws too if I had the choice.

[inventor83]

Thanks you for that info. I am leaning towards ball screws, as long as they can take forces. While it will be almost entirely for cutting PCB's, it will be 24" x 24" and set up to also cut aluminum on a rare occasion (more than likely with a different spindle or at least a slower spindle speed). If ball screws can take it them I think I will start looking for a place to get them and bite the bullet and spend a bit more. Will see how the search goes

Thanks again

Contact George from Xzero cnc. He can get you some ballscrews. He is in Toronto.

[stern-69]

Thank you for the info, I will do that

[lapoltba]

Most of this has been addressed but I will put my $0.02 in...

If you can afford it, go with ball screws.
1. They can be run faster.
2. They do not wear like a lead screw can.
3. The backlash is zero or nearly zero, and in some cases, can be adjusted.
4. They are much smoother than an acme lead screw.

As far as speed, you get the "it depends" from me. I have many years of experience running cnc milling machines and lathes, which are sort of similar to routing. While you do not use the max speed all the time (ie. while cutting/engraving), a lot of time is spent with the tool in the air moving from place to place. This is of course dependent on the efficiency of your g-code generation method. If your machine maxes out at 50ipm, and you have ineficient code, you will be spending most of your time "cutting air". If you can bump your rapid speeds up to 200-300ipm, even if you can't actually cut at that speed, you will be doing yourself a huge favor.

Remember that with stepper motors, speed = less torque. You may need to look at your acceleration values and tweak them, but if you can get to a high rapid speed in a reasonable time, you will be better off. One more if though.... This only really holds true if you intend to do larger boards. Rough guess is that the time would be X^2 or something as you go up in size. If you only want to do small boards, it becomes less of an issue.

As mentioned above, you will likely be limited by your spindle speeds before you have issues with your table speeds. I just had a guy at work mill a PCB on our Makino machine at 30k RPM with a .032" 2 flute end mill, and the feed rate was 0.002" "per flute, per rev". (120ipm

I don't know much about sources for ball screws and such, but check out UGRACNC.com. Their prices looked reasonable, but I have no experience with their products. I just put my first order in for a 800w spindle and some other stuff. Andy was good to deal with and was more than patient answering my questions.