Project Logs

Collapse

• Design Sent for Laser Cutting

  agp.cooper • 07/12/2020 at 07:52 • 0 comments

  Laser Cut Design

  Sent off the design to get cut in 3 mm Acrylic. I have some acrylic glue (have not used this stuff before). The fumes are known to be carcinogenic!
  

  Here is the final work area:

  For homing, I have put 3 mm diameter holes in the far arm and in the base board to lock the home position.

  I have use an "I" section for the arms. Not that worried about twist with this design. The arms are offset vertically so that the I beams can "nest". I have include 1.2 mm diameter, alignment holes for the GT2 pulley:
  

  Note: The green outline is just to show the acrylic outline.
  

  Hopefully I have made no serious mistakes.

  Ah, I just thought of something, I have not check if the laser module will fit in-between the arm webs!
  

  Not as I hoped!

  Well I got back the laser cut parts. But the fit was very poor. The acrylic melts rather than vaporizes. So the I had to file the edges. During the test fit, it would go together but was very hard to get apart. I broken one of the parts. Also I glued one part upside down.

  I was not keen just to reorder the wasted parts. I am looking at buying some aluminium rectangular tube instead.

  I have ordered some new plastic hinges from another source (much cheaper!), I am hoping the quality is better.

  So just waiting for the parts to arrive.

  Not all bad!
  

  I glued the GT2 pulleys together, not too bad:

  They look a little rough but they engage the belt fine.
  

  Alan
  

• Adding an Offset Laser Engraver

  agp.cooper • 06/29/2020 at 03:35 • 0 comments

  An Offset Laser Engraver

  Mathematically it is just a polar adjustment (i.e. a radius from the right elbow with a rotation. For my design:

  • LL=207.020 mm
  • LA=-4.671 degrees (i.e. on the right)

  Mathematical Instability
  

  Before the transformation:

  After the transformation:

  Note the mathematical instability around the right arm pivot. It is due attempts to solve near zero angles with the Cosine Rule. It is not an issue as i is not inside the design operational envelop.

  Other Transformation Problems
  

  Here there are some transformations into the minus Y area:
  

  If I lift the minimum Y to ~36 then "minus Y area" transformation problems are gone (other than the bottom right "tick"):

  So now I have a stable mathematical model (if restricted to the working area above Y=~36).

  Update

  I was not that satisfied with the above solution. This solution looks better:

  I suppose it is time to look at the base board. Okay here is the base board:

  Too hard to solve the equations analytically so I just exported the "edge" points and fitted by eye the arcs. Then I fitted the largest possible 2:1 rectangle.
  

  Small Board Format
  

  Rather than use the full board, I am now looking at a small board format. The top edge of the board is the bottom edge of the work area:

  The small board allow 100mm behind the arm pivots.

  Homing

  The last SCARA I built used micro-switches to find the home position. It was not effective just to drive the arms into a stop position, as the SCARA was direct drive and used micro-stepping. It would just "bounce" off the stop position depending on the micro-step position. This time, it could be more effective if the stepper motor pulley is loosened and re-tightened,  then the home position is a power-on detent position.

  Stepper Motor Mounts
  

  Time to work on the stepper motor mounts.

  Added the work so far to the small board:

  The design is far from done but it looking okay (note, the arms do not show any stiffening flanges).

  I have to check how far the near arms need to fold back:

  Yep, pretty extreme, the left arm: 251.60 (i.e. 71.60) degrees and the right arm:  -52.25 (i.e. 52.11) degrees. I have only made an allowance of 58.11 degrees.

  Actually, if I constrain the SCARA to the potential work area above Y=188.0 mm then the there is no problem (i.e. UL=38.02 degrees and UR=26.67 degrees):

  Alan
  

• Bad 5 Bar!

  agp.cooper • 06/27/2020 at 02:10 • 0 comments

  Work Area

  I was looking at work area issues and like other SCARA types the optimum work area occurs when the arms are of equal length. No that important however to be exactly equal.

  Force Transmission

  For efficient force transmission I selected a minimum far arm angle of 45 to 135 degrees.

  This however is pretty inefficient with regard to work area. To improve the work area I adopted a square area rather than a 2:1 rectangle. Here is the result:

  Note: The arm lengths allow proper folding for "storage".

  The small work area pretty well explains why the this SCARA configuration is not in "common" use for router applications.

  Alternatives

  I have found a variant with a powered far arm:

  But this does not offer any real advantage over a convention (single arm) SCARA except it is more rigid (particularly for twist) and subject to less vibration due to the second arm.
  

  Aside, the above SCARA would benefit from increased work area if the spacing between the fixed arms was reduced. The bed lift mechanism should be replaced with a vertical bed lift mechanism. But I understand that the current lift mechanism can be fixed in software.

  The 30 degree 5 Bar SCARA

  Here is the 30 degree SCARA:

  Okay, I will have to proceed on the 30 degree SCARA basis.

  It all in software anyway, a laser could run at 15 degrees while a router could run at 45 degrees:

  And if I really have to I could power both far arms!

  Compromise

  I can use 45 degrees when out reached (need more force in this case) and 150 degrees (or more) when reaching inwards:

  Alan

• The Mathematical Model

  agp.cooper • 06/26/2020 at 09:24 • 0 comments

  The Mathematical Model

  I am not going to go into the mathematics. It is high school maths (Cosine rule and solutions to quadratic equations). You just need to send time (i.e. a week) working through it.

  What is important is what it does:

  What you can see here is a test of the model:

  • The white dots are the provided (test) coordinates, and the blue dots are the coordinates that were plottable by the machine (given the machine specifications).
  • The red dots that outline of the blue dots are the machine physical limits.These red dots are a one to one transformations of the white dots.
  • The large red rectangle is the largest 2:1 work area available for the given machine specification.
  • The other red dots are for the soft constraint (i.e. the 30 to 150 degree far arm angle limit). These enclose work areas that have a degree of rigidity. Well at least if the 30 degree angle constraint is a good number to use.
  • The yellow line shows the movement of the extreme top right point to the soft constraint boundary, and the arm positions (green). Demonstrating that the mathematics appears to be working.
  • The only thing missing from this model test are the left and right elbow positions (next time!).

  What Next?
  

  How much force can be transmitted through the arms using a 30-150 degree far arm limit? Should I use a 45 to 135 degree far arm limit?

  What arm lengths should I use to optimise the soft constraint work area?

  What driver motor spacing is best? The closer the spacing, the more work area, but what about force transmission?

  Why Truncate Unplottable Points?
  

  Basically, he motion planner needs the transformed constrained points to navigate around these infeasible areas:

  Consider the above drawing, although I don't have to plot the constrained points at the top of the page, but I do need to move the machine head via those points towards to first plottable point.
  
  

  Alan
  

• Abandoned and Restarted

  agp.cooper • 06/24/2020 at 09:36 • 0 comments

  Why I Stopped

  I stopped because I was waiting for 1.2mm diameter end mills for the gear wheel. As they take about six weeks from China fro delivery I went on to other things.

  I had moved on by the time the 1.2mm end mills arrived!

  I did have a look a the project a month or so ago, made some edits to the design, nothing exciting. The problem remained, the project was complicated and expensive.

  So I put it on hold.

  Another Approach

  While cruising the Internet, I came across the Maslow CNC project:

  Okay, it deserves a "WOW". Getting this technology to cut a 4ft x 8ft sheet of plywood is an achievement.

  Parallel or 5 Bar SCARA
  

  Okay, if they can get he Maslow CNC project working, I can get a parallel or 5 bar SCARA working (i.e. able to cut plywood). Here is an example of a % bar SCARA (http://fab.academany.org/2018/labs/fablabbottrop/group_week14_15.html):
  

  Why a 5 Bar SCARA?

  The main advantages of the 5 bar SCARA are:

  • Very simple construction.
  • The potential to use domestic hinges for bearings.
  • Heavy stepper motors are not on the arms.

  The main downside is:

  • Not a good design for the application of force.

  Possible solutions to the application of force:

  • Limit link angle to say 30 to 150 degrees (avoid near "straight" angles).
  • Use gearing.

  Other problems:

  • Need to use an offset laser or cutter if hinges are used.

  Here is a first drawing:

  And a close up of the motor gearing and hinges:

  Note: The gears are attached to the "near" arms.

  SCARA Mathematics
  

  Next is the Mathematics.
  

  
  

  Alan Cooper
  

• Update

  agp.cooper • 06/07/2019 at 02:38 • 0 comments

  Update

  While I have been waiting for the end-mills, drill bits, 6mm acrylic sheets and acrylic glue to arrive, I have been working on other projects. The orders have arrived and I have finished the other projects.

  Where was I?

  Here is may last plan for 6 mm thick acrylic:
  

  My last Section:

  May last layout for my CNC mill:

  The layout shows two slew bearings while the section has three bearings. The prototype bearing was quite good so I don't think I need the second bearing on the tower.
  

  I also have not really considered the tower and the base board.

  CNC Mill Parameters

  • Restarts: Add origin and maximum extent realignment holes.
  • Job Size: One piece at a time.
  • Feed Speed: 1200 mm/min (up from 600 mm/min).
  • Depth Speed: 150 mm/min.
  • Drilling: Peck.
  • Over Depth: 0.5 mm (down from 1 mm)
  • Spindle RPM: As low at I feel comfortable with, about 3/4 on the dial.
  • End Mill: Flat two flute 2mm diameter (better if single flute).
  • Cut Depth: 0.25 mm (if all works well this can be increased to 0.5 mm).
  • Compensation: 1.8 mm cut width for nominal 2 mm bit diameter (may need to be reduced).
  • Use conventional and climb cutting directions where appropriate. 

  Base Board

  The working envelop needs to be 2:1, the key assumptions are:

  • Arm 1 length: 200 mm
  • Arm 2 length: 150 mm
  • Arm 2 rotation limits 10 to 140 degrees

  I calculated the following:

  Dy	173.735	Working Area Height
  Dx	347.469	Working Area Width
  L1	200.000	Arm 1 Length
  L2	150.000	Arm 2 Length
  A1Min	-6.143	Calculated Motor 1 Range (Min)
  A1Max	115.600	Calculated Motor 1 Range (Max)
  A2Min	10.000	Motor 2 Range (Set Min)
  A2Max	140.000	Motor 2 Range (Set Max)
  Ox	-173.735	Bottom Left Corner
  Oy	128.598	Bottom Left Corner

  Here is the base board layout including the tower location:

  It fits on a 400 mm x 400 mm board. This layout has a effectively wider feet spacing.

  The normal layout is still 400 mm x 400 mm:
  

  Tower Design

  TBC ...

  Limit Switches

  TBC ...

  AlanX

• Working on the SCARA design

  agp.cooper • 05/25/2019 at 08:46 • 2 comments

  The SCARA Design

  I have spend a number of day on this but now the basic design seems "set":

  And:

  What is remaining?

  • The base board and tower.
  • The homing stops and switches.
  • And maybe a look at cabling.

  I am thinking of putting the controller on the first arm (to keep all the cables short, and only run the power and USB cables through tower and base board.
  

  The laser cutting service provider has published a recommended "kerf" of 0.21 mm for 5-8 mm acrylic. So the laser cut designs need to be adjusted for this (but not until I am ready to have the design made).
  

  At the moment the design fits on a 400 mm x 400 mm sheet:

  As I have said before, I will pocket the disks to fit the 6808 bearing with my 2.5D CNC mill.

  Another day and some more edits. Dropped the 1.25 mm alignment holes, I have some 3 mm pins so I can use the M3 bolt holes. Received the 2 mm end-mill bits and the nylon washers. Still waiting on the 1.3 mm drill bits. Moved the design less the pulleys to a 2.5D layout for my mill:

  
  Ordered two A3 6mm acrylic sheets (to suit the 2.5D mill) and a tube of acrylic cement.
  

  I will use the 400 mm x 400 mm x 8 mm Lexan sheet as a base board.

  AlanX
  

View all 7 project logs