designing my next printer, could use some input

Hi, all - I've decided that putting any more effort into trying to keep my Chinese prusa clone alive is just a waste of time. I'm going to start fresh with my own design.

The build area is going to be a 4mm plate of aluminum measuring 18" X 18". The design will be an ultimaker approach, in which the head moves along the X/Y axes in the top of the printer, and the platform descends slowly along the Z axis.

The 18" sq. bed will be heated by 4 - 200 watt plastic heater mats, and run from 2 - 450 watt 12 volt computer power supplies. I'll be using one thermistor in the center of the build plate, feeding that to a (probably generic Melzi) controller, and switching the load through a solid state relay. Not sure how I'm going to fasten the mats to the bottom of the bed, but Liquid Nails home product repair adhesive LN-201 seems like a possibility. Good up to 300 F, and will stick to both metal and plastic. I guess the other alternative would be to create a sandwich with another metal plate. More weight and cost.

I'm curious as to whether 8 mm rods will be sufficient to support the print head, or whether I should use 10 or 12 mil rods. I'd like to keep the weight/mass of the moving head as low as possible, but I don't want the damn thing to sag in the middle. Any thoughts?

I haven't decided on a Z axis dimension, but it will probably be in the vicinity of 24 to 36 inches. I think the primary consideration here is convenience in getting filament started; a 36 inch build height suggests about a 3.5 to 4 foot machine height, which is inconvenient to work on the top of when it's sitting on a counter. Outside of this consideration, I cannot think of any other Z axis limitation. Anyone got any thoughts on this?

I'd like to support the build plate in all 4 corners, but this is probably going to result in 4 stepper motors, which is a tad pricey. The other alternative would - I suppose - be one large-ish stepper motor driving a continuous belt around 4 shafts. (Assuming you can even buy a continuous belt. Don't know, but haven't seen one.) Anyone with thoughts on this, please chime in. How have others done it?

Actually, that turns out to be a real problem. I need a continuous circular belt, not a straight length of timing belt cut on both ends. Anyone know where to source that? I've come up dry.

I haven't decided on a frame structure. I was thinking copper pipe; In 1981 I made an entertainment center out of standard half inch copper pipe, 90 degree and tee fittings all held together with metal epoxy, and the damn thing is still as solid as a rock, 35 years later. My concern is mounting all the internal components. (But copper pipe, burnished to a satin glow and sealed with a clear epoxy, could look really cool.)

Any thoughts, please fire away. Thanks!

Charlie1

well I don't see why you'd use two psu's. why not just buy a better single one to start with ?
You can get a 650 watt for less than the price of 2 450s.
Why use 4 heat pads ? is it a size limitation ?

Rod wise, go for 10mm, if it's bowden and 12 if it's direct drive.

The only issue with z height I guess would be if it were a bowden setup. That's a lot of plastic to push through a tube, the flex and friction alone would make precision extrusion tricky. Direct drive, can't see any drive issues.

Don't understand why you'd need four motors or long belt thing if you have the build plate supported on all four corners.
As long as the bearing are good - you could probably get away with a single motor, and two height screws with the belt driving the two screws.
That would give you serious support and even travel over the whole bed. So four support columns at each corner and two screws center of the plate at opposing ends. Should be fairly easy to do.

Hi, Curious Aardvark!

Thanks so much for the feedback!

I wanted a larger build table than a standard 220 mm table; so I chose to go with a table twice the depth, and twice the width. That took me to 440 mm, and 18" is 457 mm so I just rounded up.

I could have had a custom heater pad made, but for a 457 mm sq. build plate it would have cost me about $140. Instead, I chose to go with 4 standard 220 mm heater pads, at about $8 each. The pads want 200 watts each, so that means I need 800 watts for the bed, and the rest for running the board and extrusion heater cartridge. I could have gone with one 900 watt power supply, but the cost per watt rises rapidly; two 450 watt power supplies are actually cheaper than one 900 watt power supply. 450 watt psus are actually the cheapest psu on the market, when viewed on a cost per watt basis.

Thanks for the feedback on the rod diameter. I like bowden feed, but I don't want to be restricted in the future, so I think I'll opt for 12 mm rods and bearings.

If I mount the drive motor for the bowden extruder at the top of the printer, on the back and half way between the left and right vertical support columns, then the greatest bowden tube length would be 20 inches plus a generous amount to allow the tube to transition from vertical to horizontal to vertical again. Call it 28 inches. I think that's not an impossible tube length. I could be wrong, which means I'd have to go direct drive, which means 12 mm rods are a good idea.

Some ultimaker-style designs use a single drive at the back of the build platform, and support the platform from the rear only. There might be chrome guide rods at the front to keep the build plate from shifting left or right, but all the weight is being supported from the back. I think that with a machine of the size I'm building - an 18 inch square build plate, and a maximum load of 1 kilogram of plastic sitting on it - supporting it from the back only is unwise. I think there could be problems keeping the build plate perfectly flat, and long term wear issues at the back screw drive. So I see a minimum of 3 drive screws being required: one at the center back, and two at the front left and right corners. But this means that those rods must be driven; and that means a circular drive belt driving all three rods at the same time, or three stepper motors working in unison. But modern boards (melzi, ramps, rambo) can drive a maximum of two stepper motors on the Z axis. This means that the problem can be most easily solved with one largish stepper motor driving a circular belt (a piece of continuous drive belt in the shape of an elastic band or a doughnut, as opposed to a linear piece of drive belt like a shoelace).

The problem I'm having is that I haven't been able to find drive belts, with the exception of the really tiny ones that ultimaker itself uses for transitioning the drive motor to the Y axis on the print head. It's only 3 or 4 inches in size, and I need something way, way bigger.

If there's a way of supporting the build platform in three points without resorting to stepper motors or drive belts, please tell me. Maybe I'm missing the obvious.

Thanks again, CA - I appreciate your help!

>Charlie1

Seems to me like the extra cost is worth it, unless you are shaking the couch cushions out to pay for this project. A quality 850 watt power supply with a single rail will likely last you a lot longer than two of the cheapest 450w units, and you can get one that is 80+ platinum certified for about $130. The efficiency alone would be a significant energy savings in the long run. You might be saving money now, but you will pay for it in the years to come.

http://www.amazon.com/Seasonic-PLATI.../dp/B00608FKN8

As an example, you can buy one of those used for about $120, and at 900w load they show a 90 to 91% efficiency. This is phenomenal.

Meanwhile, cheap off brand units with no ratings on them can dip down to 50 or 60% efficiency when you run them at their peak.