Late in 2004, I decided to produce large woodblock prints -- larger than I was able working alone and using a baren... Of course, I'd considered using assistants, but prefer to work alone and on my own schedule. So I began to think seriously about what sort of 'press' I might like to use. I tested the method on some medium scale (32") blocks for "Anthony" using my 30 x 60 inch bed etching press and the very lightest pressure and it worked much better than I'd expected, so I began by researching existing large intaglio presses (I'd first looked into the possibilty of building a huge hydraulic plate-press, but it appeared too heavy and much too costly). Large intaglio presses are also quite expensive, and the best were specifically designed for very thin plates and didn't have sufficient room between the bed and the top roller to accommodate both my blocks and the vacuum plenum I require to hold them flat. None of the manufacturers seemed willing to modify their designs to my specifications -- that came as something of a surprise to me, but I understand their reluctance better now. There were also some significant paper handling problems to overcome while registering large papers single-handed (to be described in a future posting).
Because of space considerations in my studio, a moving bed press was undesirable (requiring a footprint twice the length of the bed, so I first tried to design a very stiff bedded press with a single roller. Deflection of the bed seemed to me to be an insurmountable problem with this method, so I decided a more conventional two-roller scheme would be easier to engineer. I ultimately decided to design a somewhat unique two-roller 'wringer' type press in which the bed would be stationary and the rollers could move back and forth across the bed. Because the rollers work in opposition, the bed could be very thin -- just stiff enough to support the block prior to printing. I got some 'help' from Doug Forsythe's $25 plans to 'build your own etching press' -- these made for a more confident start!
One of the most important decisions was exactly 'how' the rollers could be made to accurately move back and forth along the press bed. There are many options for 'linear motion' solutions -- after much consideration and comparison, I selected some off-the-shelf components from Bishop Wisecarver (BWC.com) who not only provided the excellent (and expensive) aluminum extrusions I chose for the frame components, but also the compatible carriage and rail system which will convey the rollers along the frame rails.
carriage and rail illustration from Bishop Wisecarver
After many false starts and much consideration about exactly how large I might want to print, I decided to dimension the press to enable printing full size blocks cut from widely available 4x8 foot 'standard' sheets of plywood.
drawing of the final press plan
Getting the measurements right, figuring out how long things should be, how to make everything fit together, especially in the drive section where a number of parts had to be coordinated was challenging! For example, consider the two sprockets and chain in the drive -- that has always seemed so simple... Until I tried it! There are a HUGE number of choices of chain sizes and sprocket sizes, and after figuring out what weight of chain would work, then there's the desired mechanical advantage, and finally, you can't just measure chain to the nearest 16th inch, the links are fixed, so you have to calculate how many links, then the shaft centers can be figured and fit exactly... LOTS of new stuff to learn and calculate -- I must have tried 100 different combinations of chains and sprockets before deciding on one set which would work... In the process, I gave up on the configuration which would probably have been best -- placing a light weight chain inside each of the long rails which support everything, then meshing a sprocket into that chain on each side to drive the rollers along... That would'a been very cool, but I wasn't patient enough to redesign EVERYTHING in order to make it all fit... Compromises, compromises...
detail of the plan showing the rollers, bearings, sprockets, etc... kinda hard to read, eh?
So eventually I decided on a design I thought would work using as many 'off the shelf' components as I could find, and making the rest. Because I thought I could machine aluminum myself using the same CNC machine (from ShopBot Tools) I've been using to carve large blocks since March, 2004, I designed many of the parts to be cut out of 1/2" aluminum -- here's the drawing from which the parts were machined:
aluminum parts machining plan
Below is a photo of the machining in progress -- it was more difficult than I'd imagined, and I broke a few bits out of ignorance, but if I have to do it again, I'll be able to accomplish it much more easily. Live and learn!
machining the aluminum parts
view of machine and completed aluminum parts
the completed aluminum parts along with some of the bearings, handles, etc to be mounted
OK, I got a little fancy and engraved a logo on the faceplate, then filled it with pigment laden epoxy and sanded it down flush -- kinda pretty!
The rollers were another challenge... The bottom roller had to be machined to very close tolerance in order to exactly engage the press bed. I was also concerned about potential deflection in the 1.25" center shaft and performed some fairly complex calculations in order to satisfy myself that deflection would be minimal under load. Eventually I settled on a design which I had bid at several local machine shops. My design called for four large 'washers' to be welded to the shaft and the inside of schedule 80 black pipe, then for the pipe to be turned to precise dimension. But it turns out that schedule-80 pipe is unsuitable for turning, as it contains stresses which won't allow an accurate turning to be produced, so I had to go with much more expensive seamless DOM (drawn over mandril) tubing.
the drawing for fabrication of the rollers
the BEAUTIFUL nickel-plated rollers themselves!
view of assembly area, the 10 foot long press bed is at the top in the background, the aluminum extrusion rails are at the left, the two nickel plated steel rollers are visible as well
Detail of the roller ends -- a local machine shop did a fantastic job fabricating these, even turning down the welds to make smooth chamfers! They are almost stand-alone works of art themselves!
So I am finally actually assembling the thing! So far everything fits perfectly, and I'm feeling optimistic that it's all going to really work! The rails and carriages have been assembled to the frame sides, seem to work as I'd imagined, and look good!
Assembly detail showing BWC carriage mounted to carriage rail and carriage rail mounted to press rail, and press rail attached to leg and brace.
Total cost has been about $6,500 (yikes! But still SO MUCH cheaper than even a well-used intaglio press of similar size)
To be continued (Mike Lyon -- 10/31/2005)...
¶ 10:47 AM
Having seen your progress with this project over the last few years and now, finally, almost to its completion, is amazing! Can't wait to see your exhibition!!!
Your friend for life,
Jon
: 12:51 AMQuestion: can those rails sustain the weight of the roller assembly across the breadth of that log span without bowing slightly? It didn't seem to me that there was any kind of supplementary support mid span, so I'm just curious.
YES, the entire weight of the two rollers (about 250 lbs.) is carried by the 10 foot long top rails supported only at the ends, and no matter how huge and stiff the beam, there will always be SOME deflection -- that's just physics!
The manufacturer of the extruded aluminum parts BWC.compublishesformulae and a chart in order to allow customers to calculate deflection of the various extrusion products in application.
There are three sets of formulae and graphic representations -- one for each of three simple beam configurations:
1) "cantilevered" where one end of the beam is rigidly fixed and the load applied at the free end,
2) "simply supported", which is the situation where the beam is simply laid across two pivot points and a load applied in the middle, and
3) "rigidly fixed both ends" in which both ends of the beam are 'rigidly fixed'...
This stuff was critical to me in designing the press, because I obviously didn't want the press to 'fold up' under the weight of the rollers. Because the rollers are supported on the outboard side of the rails, the height of the rail dictates the roller dimension, so the rails had to be a first decision, the dimensioning of the rollers dependent on the height of the rails!
see BWC chart -- the red line represents actual values for the press and indicates a bit over 1mm (1/24 inch) deflection of the rail under 125lb (600 Newton) load.
Yesterday, with my 200 lb. body sitting on the center of the assembled press rail, I measured actual deflection at about 1/16 inch, so it really is pretty stiff as the chart indicates -- much less deflection than I'd imagined! Even though it's not 'rock solid', it's plenty rigid for my purposes, and I won't accidentally bend the frame by sitting or standing on it even while the rollers sit in the center of the press.
-- Mike
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