Tuesday, March 2, 2010

Introducing a New RepStrap design: Auger

Introducing Auger
A New and Still Hopeful RepStrap Design

Well, having built a McWire I frankly am not satisfied with the performance. For large parts it print fairly decent even good. The smaller parts have been problematic. Mainly due to the course extrusion control. The smaller Items print better when I use multiply in skeinforge but it lengthens an already too long print time.  

I'm sure that I could probably make due with the McWire but I just can't stand the slow speed. I also think that I can do better starting with a blank slate. That being said I have changed my sights from building a Mendel to a design with the following attributes. 

*The mechanical parts should be available at the local home center (Except bearings, electronics and motors)
* Relatively inexpensive
* No printed parts should be needed anywhere in the build to get up and printing. 
* Keep the part type and parts count to a minimum.
* Speed should be comparable to the MakerBot. (I'm trying to do this without belts and pulleys)* Easy to make (although a small drill press is required)Hopefully this design will be something that would be good enough that it could be your first and last 3d printer if you choose not to make another bot.
So far I think I am on track. I have a sketchup model for the X and Y and am 80% through the build.

Here are the major build steps to get Auger printing from here:
* Mount the X and Y motors
* Join the motors the drive shafts
* Move the Mendel Z sled assembly to the Auger base plate and adjust the height.

Basic Construction
Auger uses a skate and rail system to glide the X and Y axis. #10 hardware is used throughout to hold the bearings in place and mount the skates to the platforms. The X and Y skate and rails are identical. The platforms are 7"x7" made from 3/16 inch luan plywood. The two are identical except for the holes to mount the upper stage to the lower. I think the X and Y sages could be made to work with a rack and pinion system but I am working on creating drive screws to move the stages.
The skate were inspired by the design found here at the GilesBathGate Blog.
They are made of 1"x1"x1/8" Aluminum angle. The skate bearings are inexpensive skate bearings ordered from VXB.  One skate uses opposing 45deg angled bearing to keep alignment while the other uses inline 90 deg to give glide to the other end. #10 hardware is used to hold the bearings in place and allows enough play to tweak in each stage making it level and square. Washers are used to space the bearings from the brakets. The Sketchup file has the ideal distance but I used a number of washers that was closest and that seemed like it was good enough. All the screws bite into tapped holes in the aluminum and medium (blue) thread lock was used because of the high vibe environment.
The rails are made of two parts. The same aluminum U channel that McWire had in its design is used as a base to hold 0.7 inch electrical conduit.  The conduit really inexpensive and is used to provide the rolling surface for the bearings and gives height between the sleds for the drive screws. This conduit is very rigid. We paint it and use it for curtain rods inside our house.  Dry wall or wood screws go through the bottom of the ply wood, through the U channel and grab the conduit to hold the rails in place.

The base is made of 1/2" ply wood. I'm actually using an old cabinet door that was in my scrap wood pile. Anything of that type would work. The main purpose of the base is to hold the rails and give a place to mount the Z sled. The Z sled is not designed yet but eventually will use the same design concepts as the X and Y sleds. In the mean time I will be using the Z sled from my McWire.

Drive System
I have had this wild idea that I could make a drive screw to move the stages. So far it has turned out pretty good but I have hit a few bumps along the way. 

The Jig
To create the drive screw I have come up with a jig. I call it my Augering jig. It basically consists of a platform that fastens the the base of my drill press. It has two rails that form a channel. The channel is adjustable to guide rods 3/4 to 1 inches. Two 1/4inch bolts are used to hold and clamp a bracket over the rods. That bracket holds a razor blade angled by a wedge. The wedge in the picture is 15 degrees. I have made it so the wedge can be swapped out for one of a different angle.  The bracket is tightened down so that the razor blade gets a good bite into the rod. When the rod is rotated the angle of the razor blade translates this into a linear displacement. This displacement drives the rod forward in to a bit that cuts a spiral grove into the rod. The jig works amazingly well. 

The Drive Screw

I first tried the jig out using a 3/4" poplar dowel. After a few attempts I was able to come up with a precise cut. At first I was turning it by hand but that got tiring. Then I drilled a hole in the end of the dowel where I fastened a 1x2 piece of wood. I centered it so I could turn it with two hands like a ships wheel. I was able to turn it easily but the rotation was not smooth because my hands had to trade positions. Then I attached the 1x2 at its end and steadied with my left hand and turning it with my right. I was able to keep constant pressure on it in a smooth rotation. I found out later that pausing had not ill effects on the cut but reversing did as there is no support in the opposite direction  for the razor blade and it flexed easily. 

I used some calipers to measure the variation in thread distance in one rotation and they were less than 0.005" of  one another from thread to thread. (It's really kind of hard to tell. I will have to measure the accuracy under operation.) Then thought it would be better if the rod was made of a lower friction material. I placed an order with McMaster Carr for some 1 and 3/4 inch UHMW HDPE.
 I tried cutting some this weekend but had some problems:

* The 3/4 inch rod I ordered was actually 0.81 inches in diameter or about 13/16".  All I have is a 3/4 paddel bit to make the nut. I cut threads in it anyways. perhaps I will pick up a bit this week.
* The HDPE has a low melting point and as a result likes to be cut slowly. My drill press on its slowest speed and an 1/8" bit still melted it as it cut. I was  able to cut the 3/4"  rod ok but the 1 inch rod melted too much and was not really useable. It seemed it was a slightly different material.
* The 1" inch rod had bulges in the diameter I think caused by the extrusion process.
* All the rods had a bend in them. I was able to correct this by hand.

If the razor blade had bidirectional support I think one could do gradually deeper cuts by going back and forth without too much loss in precision. I think that would be key to cutting plastics without melting.  The bit I was using is a downward cutting spiral bit.  A bit with 2 flutes and less surface area may help the melting issue as well.

Drive Nut
The dive nut is pretty simple in construction. I made it block by glueing two pieces of  plywood together and drilling a hole that matched the drive rod. I marked the diameter of the circle and transferred those marks to the top. I placed a 15 degree wedge under the block and I drilled a hole just inside the markings.  This made a hole that is parallel to the threads of the shaft.  I then put the 1/8" drill bit in smooth side first and cut it off flush to the top using the Dremel tool.  This made the thread of the nut.
On one of the nut and rod pairs there seemed to be some blacklash that is comparable to McWire's rod and coupling nut on the other there was hardly any at all. I will have to  wait for actual operation to see how good it is though. I have some more ideas on how to make these better that I will be exploring.