View Full Version : Help? First attempt at building CNC table

04-21-2003, 12:31 AM
My intention is to build clocks and other gear driven devices...to this end I need only a 2D machine with a relatively small capacity. My material will always be a plywood or laminated material, and in the neighborhood of 1/8" or thinner.

I've scanned over the archives, and found that small motors like the rotozip provide enough power to mill simple materials like wood, given a sharp router bit.

Now. Going through what materials I already have available to me, I realized that I have an old HP pen plotter capable of handling 8.5" wide paper, through legal size. (HP7470A)

Essentially, isn't this already a 2D CNC machine? Modifying it to take the additional thickness of plywood, rather than paper, would seem to be trivial, and the cutter head could be mounted on a flex shaft, with the motor "suspended" over the original plotter, which would eliminate its weight as a factor.

My concern is about the drag caused by the cutter head. It would be tremendous, compared to a felt tipped pen on paper, which is what this machine was designed to handle. Would this present a problem? Or do plotters have a means to measure the distance actually travelled?

Basically I want to know if I can do a conversion, or if starting from scratch would be a better choice?

Thanks ahead of time for any input you can provide. I have a feeling that it's going to be a LOT of help in the near future.

-- Chuck Knight

04-21-2003, 03:15 PM
Hi Chuck,

Nice to see you made it over to this place.

I believe that the plotter gantry was animated by means of a long toothed belt. You can make this stronger by removing this and running the gantry with a screw and nut system. This should make it alot stronger. You may also need to find stronger stepper motors, or maybe the existing ones can be sent more DC voltage to make them stronger. You can go as much a 4X higher than listed on the motor. You can NOT exceed the rated Amps.

04-21-2003, 05:23 PM
Thanks for the warm welcome -- it's appreciated. Why do I get the distinct impression that knowing about this place will end up being very expensive, and very interesting.

Now...back to my question.

I understand how to change out one drive system for another -- that's no big deal. But, how would you make sure that the commands it receives, are still translated in a 1:1 manner?

i.e. You send an HPGL command which tells the plotter to "draw" a line by moving the carriage 2 inches, or 800 steps or however it's indexed. Now, the controller internal to the plotter generates the necessary pulses that drove the toothed belt, that moved the carriage 2". How would you do the (mechanical?) conversion, such that the resulting movement remains the same?

-- Chuck Knight

04-21-2003, 06:33 PM
Oh, for some reason I thought you were going to remove the existing drives and replace them with "Step and Direction" drives like ones from Camtronics, or Gecko, or Stepperworld or many others out there. Doing this would then give you the ability to run various Step and Direction software which understands G-code just like the big boys using CNC machines.

04-22-2003, 12:07 AM
I'm not exaggerating when I say I'm a complete newbie WRT the entire concept of building a CNC machine. I'd love to do it, but have no idea where to start.

My initial idea was that I might be able to use the parts from the plotter, unmodified. Apparently this was wishful thinking.

So...can you recommend any good sites or even books that discuss the basic design parameters for a CNC router? Something that might help me get "up to speed?"

It seems that the basic goals are virtually identical to those of my plotter...but the mechanical requirements are much more significant. Is this basically correct?

Thanks again for your patience -- it's appreciated!

-- Chuck Knight

04-22-2003, 01:48 AM
Hi, Welcome to the forum!

Most plotters are too whimpy to run a dremel. To convert it is more work then building a whole machine. The machine pretty much needs to run on Gcodes and not HPGL. HPGL has no provision for a z axis. It has pen up/down but your plotter will be moving before your z will be down.

For plans for inexpensive machines go to www.crankorgan.com

Good luck, you will love this hobby!


04-22-2003, 08:19 AM
There has been a large discussion on this over here (http://www.rcgroups.com/forums/showthread.php?threadid=62920&perpage=15&pagenumber=1). It is well worth reading as it has lots of useful info. Hope it helps.

04-22-2003, 03:16 PM
I would not send anyone to crankorgan. Take a look at this site