Question
I'm in the process of building a CNC router. I'm having trouble selecting the proper lead screws. What is the average/standard threads per inch and does most CAM software have a way to configure this? I don't want the computer to think it's moving an inch when in reality it's moving two inches.
Forum Responses
(CNC Forum)
From contributor B:
Your CAM software will create G-code that your router will follow. Your control software for the router will read this G-code and move the router around your table. Control software is configurable to each individual router just as you will need.
I think the more important question would be what diameter ball screw do you need? I would expect that the larger diameter ball screws would have a larger pitch, and that you must configure the software to account for that pitch.
Contributor T's thought on the diameter of the lead screw is correct, but the size of the screw does not relate to the pitch, except in a general sense. A bigger screw is stronger, though, and hence can spin faster without deflection. And since the pitch determines accuracy but takes more RPM to move an inch, you need a bigger diameter screw to get more speed, and also to get more accuracy (and torque I think) for a given speed. Need both? Go big, or go ball screw, which doesn't rely on pitch as much for the accuracy.
I'm in the process of moving and will not have my shop back until February. I do have access to my tools, but in an extremely limited space. So, I've got time to kill. The reason I need the machine is because I've gotten a contract to do the milling for a company that sells aftermarket guitar bodies. I'm going from a dozen a year to 50 a month overnight.
At first I was just going to build it to make my templates, figuring that I could make the machine for what a machine shop would charge for one template (I need 12), but then I was able to trade a guitar for some stepper motors that would otherwise be way out of my price range. Now I've got the power, may as well build the full-blown ShopBot type machine.
Quick follow up question: is it the control software, then, that is told a) the size of cutter in the tool and b) where the tool is now (i.e. reset zero)?
Most can provide rudimentary operator interface software as well. These are just a couple that I know off the top of my head.
I basically want to take my CAD drawings, pull them up on the machine's PC, tell it how much depth per pass and what size cutter, hit the GO button, and go make a sandwich.
Am I wrong that this is how it should work? If I do have to plot every step manually, I can live with that because I'm going to be making the same part for months once I get it going.
I think that you are admirable for doing it yourself. I think the software questions are worth asking, and the more you know about your machine the more you can do with it in the long run.
The deals on turnkey systems are just too killer right now.
Things to keep in mind. A) A Shopbot would probably be fine for your work. After 1-2 years or less you could always start making your machine, either upgrading the Shopbot or using what you've learned from it. B) Rack and Pinion is okay for the work you are doing, accuracy wise, and easier to find a good fire-used used unit. Trust me, if you can find a used unit with a servo drive, you'd probably be okay with a chain drive if you looked after it! The servo lets you run all day without worries about the steppers stalling and screwing something up. Doesn't happen often, but....
Comment from contributor A:
I am a design and manufacturing engineer for one of the largest guitar manufacturers in the world. I have a few thoughts on your project, and some suggestions. I think it is admirable to try to build your own CNC (I'm doing it now, too), but I think you need to examine your needs and skills a little more before proceeding.
I agree with you - I don't think any of the small hobby type machines, and to me this includes several models that go up to around $50k, are worth the money. Especially for what you are trying to do. I also wouldn't suggest buying a used machine before learning a little more about them, and seeing how the business goes. I've seen a lot of people buy a machine before they needed it, and that's why there are so many used ones on the market. Besides, lots of the big boys make most of their bodies without CNCs, too.
What are your reasons for going to a CNC? What equipment do you currently use to make bodies? You said there were about 12 bodies you need to make. What about routing configurations for pickups, bridges, etc.? What type of neck? Glued in or bolt on? Are the bodies carved, like a LP, or flat like a strat? How are your electrical and mechanical skills? I'm not trying to be critical, I just think there is a better, more economical way for you to go about this.
I don't believe any machine you build, especially with stepper motors, is going to satisfy you from a quality and accuracy standpoint. I think you would be better served using your time and money to invest in a good, used pin router. Something with a heavy table, good spindle, and easy depth adjustment. I would get a good used two spindle shaper and bandsaw if you get into necks, too. With just 50 bodies a month, you can rough out the blanks with a jigsaw. Since there are just twelve different bodies, you could buy or make some standard templates to do the body shape, control cavities, neck pockets, pickup pockets, etc. If you do your own CAD, and can save to a dxf file, someone out there would make them, I am sure. Things are tight everywhere, and most people with a CNC would love to make a little extra cash for running a sheet or two of patterns. I would suggest Baltic birch for the templates.
If you still feel like you need to build a CNC, check out the plans at machinetoolcamp.com. You buy the plans and they guide you step by step through the process of building a machine like you are talking about. This machine would be great for making templates, jigs and fixtures for your pin router. I just think it would take too many passes and be too hard on the machine to do the whole process. And you have to consider fixturing the CNC, too. You would definitely need a vacuum system to hold the parts down, and that could cost as much as the machine. There's also the problem of not being able to change tools without stopping the machine.
A good source of info on guitar building is an ebook by Jim Donahue titled Guitars: Design, Prodution and Repair. He goes into how different sized operations, from home shops to large manufacturers, do it. And it's not me - I don't work for Ibanez. It's not the clearest written book in the world, but it does have some great info.
Also check out Building Electric Guitars by Martin Koch. He has an ebook and printed version.
Just remeber CNC is just a means, not an end. It doesn't guarantee success, but it has doomed many startups when done wrong. Stick with the basics, be innovative with your fixturing and jigs, and in the long run you'll be much better off.
I made a 5' X 12' CNC that is capable of cutting a myriad of materials. In parts, I spent approximately 4 grand. What kind of machine could I buy for 4 G's? Maybe Shopbot or Shop Sabre. However, you cannot buy these machines in these sizes for this price!
I started to build the Shopbot according to their procedures, but using Greenley struts as the x and y axis rails is nuts. I departed from them after I had built the table (I would differ from their table design in afterthought). I purchased stainless 1/2 diameter predrilled rod with the appropriate aluminum supports and Ina bearings from Mcmaster Carr for the linear motion guides for the x and y axis. The x and y axis on my CNC are driven by a gear and rack system using 1050 oz/in steppers with anti-backlash springs installed between the two gears. I have an accuracy of .003 with a speed of 400+ ipm. The traverse (no load) speed is much faster. It moves! The harmonic ossicilations in a non-tensioned ball screw system will limit the top speed (whether loaded or unloaded to approximately 150 to 200 ipm whether you rotate the screw of the nut to accomplish linear motion). And most machines that I have seen that cost upwards of a quarter mill use the gear and rack system anyway.
