Hi Jeffrey,
Yes, a neat demonstration of eddy current braking (damping).
Given the aluminum structures the arm magnets are embedded in and the magnetic fields that are present the motion of the arm will be damped. I grant you that I was not looking at damping the motion of the arm. It is similar in results to the damping of the tonearm motion by the silicone bath surrounding WTA's golf ball, without the messy juice. From another perspective though I wonder how much motion can occur per unit of time to generate these braking currents where you have two very powerful magnets pulling on each other and resisting every force to move them out of axial alignment. I would think that this magnetic attraction would swamp any eddy current damping especially where the currents will be so very small, unless I am mistaken about being small.
I've placed small disks of aluminum and copper coins on a smooth table top and also suspended by a thread. Then brought a neo magnet smartly up to or past the conductive non ferrous disks and you can easily see them repelling each other. Key to this one is bringing the magnet up smartly. Move the magnet slowly enough and you won't see anything move. Yes, the galvanometer isn't sensitive enough.
Thanks for your input and the neat video link,
Bill
new design detail upgrade????
While looking for some more reference pics for my arm, I came across some new pics uploaded at OswaldMills (Schroeder distributor) of the Reference arm.
Looking at the new large pics, it looks like there is now a brass ring around the upper magnet mounted in the arm tube.
Frank... if you're lurking...and others chime in...
What would be the purpose / function? easier mountin of the magnet? string?Taming eddy currents??? etc......
http:\\www.oswaldsmillaudio.com
Jeff Davison
While looking for some more reference pics for my arm, I came across some new pics uploaded at OswaldMills (Schroeder distributor) of the Reference arm.
Looking at the new large pics, it looks like there is now a brass ring around the upper magnet mounted in the arm tube.
Frank... if you're lurking...and others chime in...
What would be the purpose / function? easier mountin of the magnet? string?Taming eddy currents??? etc......
http:\\www.oswaldsmillaudio.com
Jeff Davison
Last edited:
I'll most likely be going with a 3/8" o.d Titanium tube, tho I have a ceramic tube on the way to play with during the design phase.
On the Titanium tube for damping.....
It's 3/8" O.D. and 5/16" I.D.... Inside this tube will be an inner tube, a 3/16" O.D. 1/4" I.D carbon fiber tube which will have a series of "O" rings to keep the central tube concentric and to act as dampeners. The "O" rings will be evenly (or maybe proportionatelly) spaced along the length of the inner tube..
On the outside of the outer tube I may go with external dampening as well. A length of Polyolefin heat shrink tubing worked well for a SME 3009 I rebuilt and may use it here as well.
I'm working up some mechanical drawings to have a headshell fabricated, consisting of two parts... an end of tube part that will allow a cartridge carrier part to be adjusted lengthwise and alloe for rotation of the cartridge carrier to be rotated for the correct offset angle. The cartridge carrier will also be slotted for back and forth positioning once offset angle is set.
The cartridge carrier will have a small drilled and tapped hole for a finger lift rod around 1/16" dia and threded to match the hole... itwill be gently "ergonomically" curved. I'll be making the end of tube part out of 7075 Aluminum for it's strength seeing that it's a small part with small sections for a hold down screw to bear upon.
JD.
On the Titanium tube for damping.....
It's 3/8" O.D. and 5/16" I.D.... Inside this tube will be an inner tube, a 3/16" O.D. 1/4" I.D carbon fiber tube which will have a series of "O" rings to keep the central tube concentric and to act as dampeners. The "O" rings will be evenly (or maybe proportionatelly) spaced along the length of the inner tube..
On the outside of the outer tube I may go with external dampening as well. A length of Polyolefin heat shrink tubing worked well for a SME 3009 I rebuilt and may use it here as well.
I'm working up some mechanical drawings to have a headshell fabricated, consisting of two parts... an end of tube part that will allow a cartridge carrier part to be adjusted lengthwise and alloe for rotation of the cartridge carrier to be rotated for the correct offset angle. The cartridge carrier will also be slotted for back and forth positioning once offset angle is set.
The cartridge carrier will have a small drilled and tapped hole for a finger lift rod around 1/16" dia and threded to match the hole... itwill be gently "ergonomically" curved. I'll be making the end of tube part out of 7075 Aluminum for it's strength seeing that it's a small part with small sections for a hold down screw to bear upon.
JD.
Found a few off the shelf parts that may help with builds...:
1. The arm lifter I'll be using is a Rega part, available as a spare from Rega for btween $40 and $50.
2. The anti-skating knob I'll be using is a DIN 466 brass thumbwheel using an M5 threaded hole.. (about $.30 cents each in qty. 1) much cheaper than having one machined. There is a small radius between the head and the body, but that can easily be cut square and/or the top support piece can have the hole that it will fit into beveled or radiused to suit.
3. The top string holder (that fits into the anti-skate thumbwheel)is a vented set screw (grub) 10-32 x 1/4" long (10-32 is the same as M5)... the vented hole in the set screw (grub) is .042" diameter.
4. The string I'll be using is a fiber called "Vectran"... it is stronger and has less stretch than Kevlar. It's use is in ropes and actually astronauts space suits... about $5.00 for 50 yards (on eBay)
Frank... if this is too much info... I'll edit or delete the post (if not too much time has elapsed.. edit function goes away after a short time), just thought this may help some builders without giving "too" much away.
Jeff Davison
1. The arm lifter I'll be using is a Rega part, available as a spare from Rega for btween $40 and $50.
2. The anti-skating knob I'll be using is a DIN 466 brass thumbwheel using an M5 threaded hole.. (about $.30 cents each in qty. 1) much cheaper than having one machined. There is a small radius between the head and the body, but that can easily be cut square and/or the top support piece can have the hole that it will fit into beveled or radiused to suit.
3. The top string holder (that fits into the anti-skate thumbwheel)is a vented set screw (grub) 10-32 x 1/4" long (10-32 is the same as M5)... the vented hole in the set screw (grub) is .042" diameter.
4. The string I'll be using is a fiber called "Vectran"... it is stronger and has less stretch than Kevlar. It's use is in ropes and actually astronauts space suits... about $5.00 for 50 yards (on eBay)
Frank... if this is too much info... I'll edit or delete the post (if not too much time has elapsed.. edit function goes away after a short time), just thought this may help some builders without giving "too" much away.
Jeff Davison
Last edited:
Hi Jeff,
Nothing that I would delete, but the advice on the standard knurled brass antiskating wheel is prone to cause DIYers a little headache. The threaded hole in those is most often rather generously sized(slack!) and the suggested thread itself is quite a bit larger than what I use(M3). Same for the hole through the grub screw, 1mm is way too large! Besides that, the holes in standard parts like the brass wheel are often not perfectly centered...
Vectran is usable but only comes in "loose" fibres, making it difficult to lead through two holes that allow for NO slack.
Cheers,
Frank
Nothing that I would delete, but the advice on the standard knurled brass antiskating wheel is prone to cause DIYers a little headache. The threaded hole in those is most often rather generously sized(slack!) and the suggested thread itself is quite a bit larger than what I use(M3). Same for the hole through the grub screw, 1mm is way too large! Besides that, the holes in standard parts like the brass wheel are often not perfectly centered...
Vectran is usable but only comes in "loose" fibres, making it difficult to lead through two holes that allow for NO slack.
Cheers,
Frank
Any Rega distributor can help you as their arm-lifter is readily available.
I came across a dealer or two in the U.K. I got mine from a Dealer (Needle Doctor) in the US who has them drop shipped from the USA Disributor (Sound Orginasation). It would probably be less expensive to source them from a European source..
yep, Frank, I agree Vectran is a bit tricky to work, but my fingers are rather dexterious. ( play Flemenco guitar ). A hard twist of the fiber let me "thread the needle", so to speak, an a 4ply knot worked well for the .042" hole in the grub screw and just a drop of cyanacrylic adhesive binds the fiber together in side the well of the grub and to the grub itself. Testing has shown it is extremely well anchored and won't pull through. Tho not technically accurate.. looking under magification, the string looks centered.
Now when the thumbwheels come, we'll know for sure how concentric the holes are and see how tight the tolerences are... after all they are certainly cheap enough to play with.
Also it looks like the pull on the string from the magnets should be more or less a straight line on the string .. might not be "perfectly" centered, but probably somewhat workable and "close enough". within a RCH here or there (email me privately and I'll tell you what the unit of measurement RCH is ;^P ), .. the tricky bit will be when I try to anchor the sting in the arm... trying to work out a solution other than drilling thru the magnet.. I have some ideas I'm working out on paper i.e. AutoCAD
Jeff Davison
I came across a dealer or two in the U.K. I got mine from a Dealer (Needle Doctor) in the US who has them drop shipped from the USA Disributor (Sound Orginasation). It would probably be less expensive to source them from a European source..
yep, Frank, I agree Vectran is a bit tricky to work, but my fingers are rather dexterious. ( play Flemenco guitar ). A hard twist of the fiber let me "thread the needle", so to speak, an a 4ply knot worked well for the .042" hole in the grub screw and just a drop of cyanacrylic adhesive binds the fiber together in side the well of the grub and to the grub itself. Testing has shown it is extremely well anchored and won't pull through. Tho not technically accurate.. looking under magification, the string looks centered.
Now when the thumbwheels come, we'll know for sure how concentric the holes are and see how tight the tolerences are... after all they are certainly cheap enough to play with.
Also it looks like the pull on the string from the magnets should be more or less a straight line on the string .. might not be "perfectly" centered, but probably somewhat workable and "close enough". within a RCH here or there (email me privately and I'll tell you what the unit of measurement RCH is ;^P ), .. the tricky bit will be when I try to anchor the sting in the arm... trying to work out a solution other than drilling thru the magnet.. I have some ideas I'm working out on paper i.e. AutoCAD
Jeff Davison
Last edited:
pole pieces
Frank, or anyone else who've built the tonearm following Franks work...
I was going through the patent and have some questions..
The Patent refers to "high permeability pole pieces" that are place below the bottom magnet and over the top magnet.
Has anyone used such? Can the arm be succsesfully realized without them? are they vital? Frank, is there a special metal you use? I have some thin sheets of Mu-Metal that I can hole-punch to make discs that I can stack to get an appropriate thickness. Trying to find "off the shelf" shims or discs is almost impossible and going to a specialty house requires large minimum orders.
On the last point... I'm looking into out-sourceing the parts for my build to a mahine house for the head shell, arm-wand central piece, top and bottom support, central block, bottom block central support rod and bottom magnet holder. Doing one-offs are very costly as most of the cost is in set-up time.
If I have 3 of each part made the piece price comes way way down. example.. say one of the parts will cost $200 to make one, but 3 will cost $250, and 5 pieces of that part will cost $260. I have 6 different parts to have fabricated, so this arm will cost a "chunk-o-change"... The arm should perform better than any other commercially available arm for that money, but it sure would be nice to share the burden with others... I will not do this if it is not OK with Frank.
I will never share my CAD work or drawings I developed as that is one of the prerequisits I agree on with Frank and I wish not to violate any trust, nor will I ever sell parts once this project is finished.I don't want this to look like I'm selling parts, only splitting costs with a couple of others, not unlike a group buy. but the tonearm is propretary and patented and I wish not to in anyway be infringing or profiting. But what I would like to know from Frank, if it's ok to offer the other couple of each part I've produced to help offset some of the costs.
The parts I talked about above were quoted using www.Emachineshop.com.
In the meantime I'm trying to find a local "hungry machinist" to do cost effective one-offs for me so I won't have to go the more costly rout and avoid Emachineshop if possible. I've used them before for some auto related projects and they do fine work, albeit pricey.
a note to Frank.... if you'd like I can forward the Autocad .DWG drawings to you that I'm working from if you'd like.
Jeff Davison
Frank, or anyone else who've built the tonearm following Franks work...
I was going through the patent and have some questions..
The Patent refers to "high permeability pole pieces" that are place below the bottom magnet and over the top magnet.
Has anyone used such? Can the arm be succsesfully realized without them? are they vital? Frank, is there a special metal you use? I have some thin sheets of Mu-Metal that I can hole-punch to make discs that I can stack to get an appropriate thickness. Trying to find "off the shelf" shims or discs is almost impossible and going to a specialty house requires large minimum orders.
On the last point... I'm looking into out-sourceing the parts for my build to a mahine house for the head shell, arm-wand central piece, top and bottom support, central block, bottom block central support rod and bottom magnet holder. Doing one-offs are very costly as most of the cost is in set-up time.
If I have 3 of each part made the piece price comes way way down. example.. say one of the parts will cost $200 to make one, but 3 will cost $250, and 5 pieces of that part will cost $260. I have 6 different parts to have fabricated, so this arm will cost a "chunk-o-change"... The arm should perform better than any other commercially available arm for that money, but it sure would be nice to share the burden with others... I will not do this if it is not OK with Frank.
I will never share my CAD work or drawings I developed as that is one of the prerequisits I agree on with Frank and I wish not to violate any trust, nor will I ever sell parts once this project is finished.I don't want this to look like I'm selling parts, only splitting costs with a couple of others, not unlike a group buy. but the tonearm is propretary and patented and I wish not to in anyway be infringing or profiting. But what I would like to know from Frank, if it's ok to offer the other couple of each part I've produced to help offset some of the costs.
The parts I talked about above were quoted using www.Emachineshop.com.
In the meantime I'm trying to find a local "hungry machinist" to do cost effective one-offs for me so I won't have to go the more costly rout and avoid Emachineshop if possible. I've used them before for some auto related projects and they do fine work, albeit pricey.
a note to Frank.... if you'd like I can forward the Autocad .DWG drawings to you that I'm working from if you'd like.
Jeff Davison
Last edited:
Just a followup to my last post...
I found some Mu-Metal sheet (.01", .04" and .06" thick)that I'll be making into the pole pieces (highly permeable). I have acquired a jewelers "disc cutter".
I'll drill the small hole for the string support, then center the work-piece in the cutter ( I have a set of centering plugs for this purpose- very accurate) and punch out a disc of the appropriate diameter for the diameter of the magnet / magnet well in the arm body. I'll also be using the centering plug to make a little indentation so the knot of the support string fits flush.
For the bottom magnet carrier I don't need to drill the small hole, just puch out the metal for a holeless disc for the bottom pole piece.
I'm guestimating the thickness of the pole pieces to be between .05" anf .065"
Also just wondering if Mu-Metal is too permeable? or since it is more permeable than steel I could use a thiner section?
Jeff Davison
I found some Mu-Metal sheet (.01", .04" and .06" thick)that I'll be making into the pole pieces (highly permeable). I have acquired a jewelers "disc cutter".
I'll drill the small hole for the string support, then center the work-piece in the cutter ( I have a set of centering plugs for this purpose- very accurate) and punch out a disc of the appropriate diameter for the diameter of the magnet / magnet well in the arm body. I'll also be using the centering plug to make a little indentation so the knot of the support string fits flush.
For the bottom magnet carrier I don't need to drill the small hole, just puch out the metal for a holeless disc for the bottom pole piece.
I'm guestimating the thickness of the pole pieces to be between .05" anf .065"
Also just wondering if Mu-Metal is too permeable? or since it is more permeable than steel I could use a thiner section?
Jeff Davison
Hi Jeff, hello Bins,
Little time left before I fly to the RMAF so I'm keeping this short.
Please understand that I can't give away all the details about materials and dimensions of my arm. You simply need to experiment if you don't understand what the function of some of the arm's elements are.
The pole pieces are one example. If you knew what their purpose was, you'd realize right away how thick they need to be and whether Mu-metal, soft iron or C37 steel is the best choice. Hint: All three materials are suitable, but they are no MUST.
I do appreciate that you keep your drawings but, nevertheless, don't like the idea of "passing on" parts to lower the cost. After all, this is about DIY, not having others do the hard work(mho...). I'd rather sell you a part if you truly can't find a way to make it yourself or have it made for a reasonable sum.
After all, I do this for a living...
If you send me your autoCAD drawings I could check if there are any major flaws... but not 'til I'm back after the show in Denver.
Bins,
There are lots of materials usable to make counterweights: steel, tungsten, brass, bronze, even lead . Mine are not made out of brass...
But the skating compensation wheel is(on the Ref.arms).
The No.2 and DPS arm have a skating compensation wheel made from a different material.
Have a great weekend,
Frank
Little time left before I fly to the RMAF so I'm keeping this short.
Please understand that I can't give away all the details about materials and dimensions of my arm. You simply need to experiment if you don't understand what the function of some of the arm's elements are.
The pole pieces are one example. If you knew what their purpose was, you'd realize right away how thick they need to be and whether Mu-metal, soft iron or C37 steel is the best choice. Hint: All three materials are suitable, but they are no MUST.
I do appreciate that you keep your drawings but, nevertheless, don't like the idea of "passing on" parts to lower the cost. After all, this is about DIY, not having others do the hard work(mho...). I'd rather sell you a part if you truly can't find a way to make it yourself or have it made for a reasonable sum.
After all, I do this for a living...
If you send me your autoCAD drawings I could check if there are any major flaws... but not 'til I'm back after the show in Denver.
Bins,
There are lots of materials usable to make counterweights: steel, tungsten, brass, bronze, even lead . Mine are not made out of brass...
But the skating compensation wheel is(on the Ref.arms).
The No.2 and DPS arm have a skating compensation wheel made from a different material.
Have a great weekend,
Frank
interesting article on flux containment and why pole pieces are very helpful and desired..http://www.newton.dep.anl.gov/askasci/eng99/eng99423.htm
and another...http://www.newton.dep.anl.gov/askasci/eng99/eng99338.htm
a good read..
Jeff
and another...http://www.newton.dep.anl.gov/askasci/eng99/eng99338.htm
a good read..
Jeff
Last edited:
Got lucky today! Last week I contacted a supplier of Mu-Metal that sells "engineering kits" of a variety of thicknesses of mu-metal. I only needed a small piece and lo and behold, they sent me a couple of 6" x 8" sheets. One of them was .04" and the other was .06" What was real nice of them was they sent the sheets gratis. I emailed them back today and thanked them.
I tried the .06" sheet in my disc cutter, but it was too thick and if I banged away or used a hydraulic press to punch the disc, the metal may loose it's properties (since that would really stress the metal)... However, the .04" thickness was just perfect, so I punhed out a few.
The diameter I wanted was .375".... got within an RCH of that at .373", just .002" shy, I can live with that.
Since I didn't want to drill the mu-metal or make an indentation for the clearence of the support string knot (didn't want to loose too much of it's properties), I center punched a piece of .04" copper ( for an indetation), then drilled a .03" hole in the indented center, centered it in my punch with a set of centering plugs I have for this purpose, and puched out a perfect little supportt piece that will go just above the upper magnets mu-metal pole piece to anchor the string in the arm.
one little step at a time.
JD
I tried the .06" sheet in my disc cutter, but it was too thick and if I banged away or used a hydraulic press to punch the disc, the metal may loose it's properties (since that would really stress the metal)... However, the .04" thickness was just perfect, so I punhed out a few.
The diameter I wanted was .375".... got within an RCH of that at .373", just .002" shy, I can live with that.
Since I didn't want to drill the mu-metal or make an indentation for the clearence of the support string knot (didn't want to loose too much of it's properties), I center punched a piece of .04" copper ( for an indetation), then drilled a .03" hole in the indented center, centered it in my punch with a set of centering plugs I have for this purpose, and puched out a perfect little supportt piece that will go just above the upper magnets mu-metal pole piece to anchor the string in the arm.
one little step at a time.
JD
just the bottom side of the bottom, and the top of the top. Plan is to press fit the cylindical magnets into the arm tube and lower holder. Maybe a smidge of Loctite Green anaerobic retaining sealant around the magnet. Magnets from K&J Magnetics, N52 .375" diameter x .1875" thick.
JD
JD
Last edited:
mu-metal pole piece
Hi Jeff,
Your tooling has me green with envy. And a great score you made with the sample piece. A good size for allkindza stuff. But I wonder if you have thought out carefully what the pole pieces are really up to. As I see it from your descriptions your mu-metal pole piece is a simple flat disk of highly permeable ferrous metal a mere forty thousandths of an inch thick. It is positioned on the downlooking face of the magnet mounted in the arm. You have another small disk of copper between the mu-metal and the magnet. the copper disk is drilled/punched to provide the dimple for the suspension thread.
There have been some good links to explanations of pole pieces and their function in magnetic circuits in this forum recently. Now my question to you (and perhaps to me as well) is what is the function of a thin piece of flat mu-metal the same diameter as the magnet in this magnetic circuit? As I see it, it will allow the magnetic lines of force to pass virtually undistorted and unattenuated from one magnet to the other. It will make the effective surface of the magnet displaced from the true surface by the thickness of the pole piece. But that is the only function I can think of. Earlier discussion here dealt with the shaping of the pole piece(s) with respect to stability of the arm. Shapes such as barrel and domed and flat were all mentioned and if I remember correctly have been used by Frank Schroeder himself at one time or another. Now mu-metal can effect the effective short circuiting of a magnetic field. It is used all the time in magnetic shielding of transformers. In fact it is or has been common for shielding pickup cartridges. But to be effective it must enclose the object to be shielded. Even then there is some flux leakage through the mu-metal. There is no infinite attenuation. In fact the prime function is to provide a return path for the lines of force rather than stopping them dead.
So please think this through a bit more and come up with your interpretation of what the mu-metal disks are doing. I may very well be overlooking something which should be obvious. BTW you are very correct in not wanting to cold work the mu-metal. That hardens it and alters its high permeability parameter. After cold working it would need to be re-annealed which iirc is a difficult heat treating process.
So lets hear from you with your take on it.
Bill
Hi Jeff,
Your tooling has me green with envy. And a great score you made with the sample piece. A good size for allkindza stuff. But I wonder if you have thought out carefully what the pole pieces are really up to. As I see it from your descriptions your mu-metal pole piece is a simple flat disk of highly permeable ferrous metal a mere forty thousandths of an inch thick. It is positioned on the downlooking face of the magnet mounted in the arm. You have another small disk of copper between the mu-metal and the magnet. the copper disk is drilled/punched to provide the dimple for the suspension thread.
There have been some good links to explanations of pole pieces and their function in magnetic circuits in this forum recently. Now my question to you (and perhaps to me as well) is what is the function of a thin piece of flat mu-metal the same diameter as the magnet in this magnetic circuit? As I see it, it will allow the magnetic lines of force to pass virtually undistorted and unattenuated from one magnet to the other. It will make the effective surface of the magnet displaced from the true surface by the thickness of the pole piece. But that is the only function I can think of. Earlier discussion here dealt with the shaping of the pole piece(s) with respect to stability of the arm. Shapes such as barrel and domed and flat were all mentioned and if I remember correctly have been used by Frank Schroeder himself at one time or another. Now mu-metal can effect the effective short circuiting of a magnetic field. It is used all the time in magnetic shielding of transformers. In fact it is or has been common for shielding pickup cartridges. But to be effective it must enclose the object to be shielded. Even then there is some flux leakage through the mu-metal. There is no infinite attenuation. In fact the prime function is to provide a return path for the lines of force rather than stopping them dead.
So please think this through a bit more and come up with your interpretation of what the mu-metal disks are doing. I may very well be overlooking something which should be obvious. BTW you are very correct in not wanting to cold work the mu-metal. That hardens it and alters its high permeability parameter. After cold working it would need to be re-annealed which iirc is a difficult heat treating process.
So lets hear from you with your take on it.
Bill