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DIY linear tonearm
DIY linear tonearm
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Old 26th February 2020, 06:52 PM   #3501
niffy is offline niffy  Europe
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Hi Dahlberg,

Those look very nice indeed.

Niffy
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Old 26th February 2020, 07:54 PM   #3502
niffy is offline niffy  Europe
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Hi Warrjon,

For the core it is best if you can use end grain balsa. This is where the grain runs vertically between the two skins rather than along the length of the carriage. Used in this way balsa is pretty much the best core material available. I purchased a block and cut my own end grain pieces. The front and rear sections of the arm are also built around balsa cores. The bottom section is solid carbon fibre, this was done to add more mass lower down so the centre of mass is better located.

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Old 26th February 2020, 11:13 PM   #3503
ppap64 is offline ppap64  Canada
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Hi Niffy

For your carriage, what thickness are the balsa lams... does having more or fewer glue joints affect overall stiffness materially ?
On the bottom, all the blocky elements (buttresses) are solid carbon fibre ?
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Old 27th February 2020, 03:15 AM   #3504
niffy is offline niffy  Europe
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Hi ppap64,

I had the carbon fibre I used specially laid up by the supplier. It has woven outer skins and a unidirectional middle layer that is normally aligned along the length of the carriage. Unlike normal preprepared carbon fibre the outer woven layers are orientated at +/-45. This actually increases the longitudinal, lateral and torsional rigidity of the end structure. The carbon fibre is 0.75mm thick. The end grain balsa core of the top plate is 4mm making the whole thing 5.5mm. The buttresses are again balsa cored with a skin of carbon fibre on either side. The front and rear blocks are both balsa cored. There are internal carbon fibre components, especially in the rear section that supports the counterweight and adjuster, which are not visible. It is definitely preferable to keep any piece of carbon fibre as continuous as possible. I did end up using quite a large number of separate pieces in order to add extra bracing in key locations.
I normally quote the vertical resonant frequency, which is the lowest fundamental of the system, as about 19.4khz. The actual calculated fundamental was 26khz. I quote at only 3/4 of this to allow for deficiencies in manufacture. It does mean that the actual fundamental will definitely be above my ageing audio band ~15khz.

Niffy
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Old 27th February 2020, 08:07 AM   #3505
warrjon is offline warrjon  Australia
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Hi Niffy,

The carriage I posted is only the top section. The bottom was not glued on. I did this to test fit the wheels, which was a good thing as this is when I picked up the error in the m3 holes for the VEE screws. I have made a new top section.

I could not get balsa thick enough to use it as you did. I would have had to laminate it from multiple thinner sections. Which I thought would negate the weight advantage.

The top is 0.5mm carbon fibre with 3mm balsa core. The top section of CF I laminated 2 bits at 45deg to increase rigidity, making the top section 4.5mm
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Last edited by warrjon; 27th February 2020 at 08:10 AM.
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Old 27th February 2020, 09:34 AM   #3506
niffy is offline niffy  Europe
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Quote:
Originally Posted by warrjon View Post
Hi Niffy,

I could not get balsa thick enough to use it as you did. I would have had to laminate it from multiple thinner sections. Which I thought would negate the weight advantage.
I had to buy a 2by4 plank about 3foot long. I only used about 30mm of the length. Not the most economic use of materials.

Niffy
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Old 27th February 2020, 02:00 PM   #3507
nocdplz is offline nocdplz
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try the "samba" wood (here available in every brico-center, in planks and profiles >2-3mm). A bit heavier than the quarter grain balsa, but a lot stiffer and far cheaper. Eeasier to cut without burrs with a blade or a curcular saws

c
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Old 27th February 2020, 07:13 PM   #3508
niffy is offline niffy  Europe
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End grain balsa makes an excellent core material because it has a very high comprehensive rigidity to density ratio. Another materials that are commonly used are plastic or resin foams such as polyurethane and honeycomb aluminium or ceramic such as aerolam. Other wood types can be used but, like balsa, work best end grain.
I've never seen it used in this application but have a suspicion that end grain bamboo would work very well as a core material although I think that it would be better suited to other locations such as the sub-chassis or plinth.
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Old 27th February 2020, 08:35 PM   #3509
dahlberg is offline dahlberg  Sweden
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Is there an optimum weight for the "sled" or should it just be as light and rigid as possible ?
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Old 27th February 2020, 09:36 PM   #3510
niffy is offline niffy  Europe
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A heavier carriage will better control the movement of the cartridge body at audio frequency especially in the bass. A lightweight carriage will load the bearings less and have lower friction and less lateral tracking error. There will also be a maximum weight depending on the type of bearing used. All of these factors have to be balanced. I chose to aim for a carriage weight of 55g. A lower compliance cartridge may prefer a heavier carriage though I would be hesitant to exceed 75g with my current bearings. It's all a balance of compromise.

Niffy
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