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Arm Board No. 4

This is still the layout pattern supplied by Brian Kearns.  The main difference between this arm board and the previous arm board No. 3 is that the top plate is now a 1/8 inch thick piece of Carbon Fiber.


above photo #1: It always starts with a plan.  In this case the plan is a drawing.  Using the sketch Brian sent, I worked up a quick CAD drawing.  The CAD drawing is useful for supplying a full scale ink jet print out.  The Print out is then carefully cut with exact-o knife and straight edge.  The 'cut' print out becomes my template for sawing, drilling and filing my parts into final form.  In this case the work is 2 dimensional flat pattern style.

2) cut_template.jpg (94288 bytes)

above photo #2: The cut template for the top plate lies on top of the material.  I used small strips of drafters masking tape to hold the template in place while tracing the cut lines in permanent ink.  In the case of the black Carbon Fiber, the cut template was referred to several times during the cutting process.

3) sme_assy_cf3.jpg (79856 bytes)

above photo #3: The pieces just before assembly.  The base pad is still Phenolic.  The .062 inch thick aluminum washers are still glued down to the sub-chassis from previous board mountings.


4) sme_assy_cf4.jpg (72924 bytes)

above photo #4: With the base pad in position, the 1/64inch thick lead sheet now visible.*  As with board #3, the lead sheet is flatly glued to the Phenolic base pad.  It is intended to serve as a damper between the base pad and top plate. Three sheet metal screws secure the base pad to the sub-chassis.*Note. the lead sheet was not found to be helpful and was eventually removed from this version of the armboard.


above photo #5: Armboard is assembled and awaits the SME tonearm.   Four screws to hold the carbon fiber top plate firmly to the phenolic base pad.  This time I'm using #4 countersunk flat head sheet metal screws to anchor into the phenolic.   

6) sme_cf2.jpg (42297 bytes)

above photo #6: Tonearm is mounted, and aligned.

7) sme_cf1.jpg (48427 bytes)

above photo #7: Suspension is adjusted.  Platter is even.  Top plate of arm board is level to plinth.  Bounce is vertical.  Nothing rubs.  Sub-chassis floats freely.


above photo #8: The classic Carbon Fiber look.

9) sme_cf6.jpg (42948 bytes)

above photo #9: Playing some vinyl

Listening impression:

Compared to the previous armboards tried in this exercise, this one is the magic bullet.    I heard more frequency extension at either end. The leading edges of notes, called transients, seemed to attack quicker.  More fine detail comes into view.  Soundstage, wide and deep. Midrange instruments appear to contain a natural timbre.  Vocals work well.  I think I'll leave it like this.  I would rate armboard #3 a fairly close second place to this one only losing out in terms of bass extension. 

About the carbon fiber material:

This material is already laid up and cured into a hard plate.  Rigid and strong like steel but light like plastic.  The manufacturing process ensures that the material has excellent flatness. 

This particular piece of CF cost $9.00 from McMaster-Carr. pn# 8181K629  It is offered as a 'sample'.  The sample comes as 3 x 6 x 1/8 inches. Happily, their sample size makes a TD160 arm board.  Many other sizes are available at drastically higher prices.

Comments about cutting Carbon Fiber plate

Safety precautions:

Eye protection. Wear a mask.  The graphite dust generated by the cutting process is extremely fine and gets into everything and up your nose.  I made an effort to control the spread of this dust. I also kept the shop vac near by and used it frequently.   


Keep your drills sharp.  Dull drills tend to  heat up and de-laminate the material.  Larger holes can be pilot drilled in stages to keep cutting pressures small.  Choose drills designed for cutting metals.  High speed steel drills are good.  Cobalt drills, better.  The material does wear the drills out fairly soon.


Use an abrasive encrusted rod or blade the same as is used for sawing fiberglass. 


The cross sections file easily with metal cutting files.


Comments about cutting grade LE Phenolic

Use metal cutting tools. The material cuts cleanly and easily but with significantly more resistance against the cut than does common hardwood.  Softer than aluminum, harder than wood.  The cutting process releases a strong ammonia smell that burns and waters the eyes, irritates the throat and offends the nostrils.  Don't breath the fumes.  Otherwise, it's just unpleasant.


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