I found something interesting in an old printer...

i took apart an old portable printer a couple months back, but just decided to take pictures of an interesting design element i found.

the print head was driven by a twisted 5mm rod similar to a ballscrew; perhaps even a ballscrew, but with no ballnut.

This is the underside of the carriage.




the other side of the carriage, at the top right parallel to the ballscrew, was just a 5mm smooth rod. as you can see in the pictures, the carriage has two tubes that the screw goes through, and the little metal nub is spring loaded by a small piece of copper that presses it into the threads of the screw.

it works very well, with no visible backlash. certainly not as well as a well machined and matched ballnut/ballscrew combo, but also certainly much much cheaper.

anyway, i just wanted to share this find with the rest of the community.

The most interesting part here is, it's apparently fine to run plastics directly on metal. I'm always a bit reluctant to this, as I assume substantial wear in such a configuration. Is this old printer a cheapo one, and how much has it been used?

Another nice bit is to use the leading screw directly to support the carriage. Reduces overconstraintment quite a bit.

---

Generation 7 Electronics	Teacup Firmware	RepRap DIY

Looks like its multi start, I was looking at some spiral brick ties the other day, which are very similar but stainless steel twisted, so may not be accurate but with feedback?.

---

Random Precision

You can run plastics directly on metal, but you need an appropriate finish on the metal, and a suitably abrasion-resistant plastic. Some plastic bearings have lubricant in them also.

what is multi start? the nub only runs through one of the channels; i just pulled the screw out so i could photograph the nub, but it never comes out that far. just wanted to clarify that.

it's from a printer very similar to this one - a cannon bj-10 portable bubblejet printer. it was very old, purchased used sometime in the '90's, but was very rarely used, if ever by us. the newer models of this series moved to belts.

here's another one, and if you click the other pictures, you can clearly see the screw.

IIRC, those printers used a bronze threaded rod (worm gear) and a mild steel follower formed into a thermoset plastic carriage.
The nubs do wear out after a lot of use, but you can also change them out.

On the whole, I avoid having plastic and metal sliding on one another. I know that theoretically if you have the right kind of
plastic and the right metal surface treatment that there should not be any problem, but I always use bronze bearings or
stainless steel inserts to keep metal sliding on metal.

the nub is about the same size as the tip of a ball point pen, which might work even better since it will roll. perhaps also flush out the ink and put in some light oil.

and yeah, i think some brass bushings would be in order if someone were to adapt this to reprap. this carriage is very light weight, so i doubt the plastic would wear out in the usable lifetime of the printer, but i don't think just plastic would scale up to reprap-type aplications

The ball point pin tip as a replacement nub doesn't work real good because the threads of the screw
wear out the sides of the tip and then the ball falls out, and then the tip collapses and gets stuck in
the threads. I know that one from personal experience. :-)
In the printer, it will extend the life about 100 pages before failure.

The nubs are easily made with scrap round bar from a broken 3.5 inch disk drive though!

The RepStrap/light cnc mill I'm making is going to use leadscrews, could this nub idea be used in that with enough accuracy and strength? Just asking 'cause while the trapezoidal thread is cheap enough the nuts are a bit pricey.

You should be able to use nubs instead of nuts without any major issues. Ideally the nub has the
same diameter as the thread pitch of the nut that would fit on the screw.
The easy way to do it is with a machine screw that is close to the correct diameter. Drill and thread a hole
through the print head carriage for the screw, cut the screw to the correct length, and then grind or file the
end of the screw into a nub. If you grind off to much, then you just start over with a new screw - if the nub
wears out, it is easy to just unscrew the old one.

Thanks for the tips criswilson01, I'll sure give it a try once I've got more parts together.

Just out of curiosity, how well would something like this work over a distance of say just over 2 feet?

The drive screw i removed from the printer is about 1 foot long, and only 5mm dia., so i don't think 2 feet would change the system much. you'd probably want a thicker drive screw.

The longest drive screw that I've seen on a machine was 10 feet long, 1 1/2 inches in diameter and made of stainless steel.
The carriage riding on it was also on casters in a track to keep the majority of the load off of the screw.
So you can use them longer distances, but be prepared to either over design it or break out the books on beam deflection.

Head positioning slides used this kind of drive screw in the old 3.5 floppies and in some old CD players.
Some Old 5.25" hard disks have a small rack and pinion other 5.25" hard disks have a sping steel type of belt arrangement.

All of these were used before multiple platers and the swing arm head positioner became the standard in hard disks.

---

Bodge It [reprap.org]
=======================================

---

BIQ Sanguinololu SD LCD board	BIQ Stepcon	BIQ Opto Endstop
BIQ Heater Block PCB	BIQ Extruder Peek clamp replacement	BIQ Huxley Seedling
BIQ Sanguinololu mounting	BIQ standalone Sanguinololu or Ramps mounting	Print It Stick It Cut it

My rep strap: [repstrapbertha.blogspot.com]

Buy the bits from B&Q pipestrap [diyrepstrap.blogspot.com]
How to Build a Darwin without any Rep Rap Parts [repstrapdarwin.blogspot.com]
Web Site [www.takeaway3dtech.com]