Thank you For your answer!
I am glad you like it. I would assume this has tolerances that are in the tight range of what’s called for recommended shaft by new way.
It is very cheap and they cut to length which is what I need. I will just for the fun keep looking if I can find anything better and next week I will buy the bushing and shaft and then start some solid work modeling for the supporting parts.
The 20mm bushing is 51gr while the magnesium armwand is 22gr which is a total of 73gr.
I have to build the joint structure around the housing to hold wand and counterweight.
Provided I need a counterweight of 10-15gr (which I have no idea since I don’t know the leverage nor have I performed any calculation yet so I hope not to be too far) I would already be at 88gr.
I have no idea how much the joint part with counterweight shaft will weight but even if I am optimistic, I doubt it can be any less than 20gr, which brings to a total whooping mass of over 100gr.
I wonder if this would still be acceptable or not given that a rough calculation to center the resonant frequency to the sweet spot of 9-10Hz would call for a total mass of no more than 80gr.
Frankly it is almost impossible to achieve this low weight even before factoring in even the mass of the wiring and air tubing which which is not transcurable I am sure.
I don’t know what Kuzma claims for their arm but ain’t going to be lighter than mine unless they are using a smaller bushing but if doesn’t look like they are employing a 1/2” air bushing on their design, maybe I am wrong.
I am glad you like it. I would assume this has tolerances that are in the tight range of what’s called for recommended shaft by new way.
It is very cheap and they cut to length which is what I need. I will just for the fun keep looking if I can find anything better and next week I will buy the bushing and shaft and then start some solid work modeling for the supporting parts.
The 20mm bushing is 51gr while the magnesium armwand is 22gr which is a total of 73gr.
I have to build the joint structure around the housing to hold wand and counterweight.
Provided I need a counterweight of 10-15gr (which I have no idea since I don’t know the leverage nor have I performed any calculation yet so I hope not to be too far) I would already be at 88gr.
I have no idea how much the joint part with counterweight shaft will weight but even if I am optimistic, I doubt it can be any less than 20gr, which brings to a total whooping mass of over 100gr.
I wonder if this would still be acceptable or not given that a rough calculation to center the resonant frequency to the sweet spot of 9-10Hz would call for a total mass of no more than 80gr.
Frankly it is almost impossible to achieve this low weight even before factoring in even the mass of the wiring and air tubing which which is not transcurable I am sure.
I don’t know what Kuzma claims for their arm but ain’t going to be lighter than mine unless they are using a smaller bushing but if doesn’t look like they are employing a 1/2” air bushing on their design, maybe I am wrong.
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Hi Stefano,
The sweet spot of 9-10hz only applies to the vertical motion of the arm. It is set at this frequency to keep it safely above the frequencies at which warps occur and below the audio band. Lateral motion of the arm isn't affected by warps. It is affected by eccentricity that occurs at a much lower frequency, 0.55hz. Ideally laterally the arm/cartridge should be tuned to 4.5-5hz.
Over on the DIY linear tonearm thread I posted a reply to diyaudio member Icsaszar detailing some of the reasons why it is better to have a higher lateral effective mass. See #2923.
DIY linear tonearm
You will constantly hear people saying that vertical and lateral effective masses should be the same. What you will never hear is the reason why they should be the same. I have yet to hear a cogent argument as to why this should be. One of the advantages of the linear tracking arm is that it naturally has a higher lateral mass.
Unless you are using a high compliance moving magnet cartridge you shouldn't have any problems (quite the opposite) with a 100g moving mass. Personally I am using a high compliance cartridge, 22um/mN, so have a carriage mass of 55g to give a lateral tuning of about 4.5hz.
Niffy
The sweet spot of 9-10hz only applies to the vertical motion of the arm. It is set at this frequency to keep it safely above the frequencies at which warps occur and below the audio band. Lateral motion of the arm isn't affected by warps. It is affected by eccentricity that occurs at a much lower frequency, 0.55hz. Ideally laterally the arm/cartridge should be tuned to 4.5-5hz.
Over on the DIY linear tonearm thread I posted a reply to diyaudio member Icsaszar detailing some of the reasons why it is better to have a higher lateral effective mass. See #2923.
DIY linear tonearm
You will constantly hear people saying that vertical and lateral effective masses should be the same. What you will never hear is the reason why they should be the same. I have yet to hear a cogent argument as to why this should be. One of the advantages of the linear tracking arm is that it naturally has a higher lateral mass.
Unless you are using a high compliance moving magnet cartridge you shouldn't have any problems (quite the opposite) with a 100g moving mass. Personally I am using a high compliance cartridge, 22um/mN, so have a carriage mass of 55g to give a lateral tuning of about 4.5hz.
Niffy
Hi Stefano,
Kuzma uses a 20 mm air bushing. I can’t understand why they can achieve 80 g at all. My latest arm has short arm wand and fewer parts than Kuzma’s. But it is 89 g which is 9 g more than I designed.
Anyway, I think as long as you can control the total mass under 100 g, it should be ok.
As Niffy correctly stated in his post, you need to think about vertical arm/cartridge resonance frequency. I would suggest to draw the your arm accurately and decide what material you are going to use. So, you can calculate vertical resonance frequency. That will give you a basic estimate of vertical resonance frequency. Niffy knows how to calculate resonance frequency better than I do. He may give you some guidelines how to do it.
I will suggest to use magnesium for the center weight part.
Jim
Kuzma uses a 20 mm air bushing. I can’t understand why they can achieve 80 g at all. My latest arm has short arm wand and fewer parts than Kuzma’s. But it is 89 g which is 9 g more than I designed.
Anyway, I think as long as you can control the total mass under 100 g, it should be ok.
As Niffy correctly stated in his post, you need to think about vertical arm/cartridge resonance frequency. I would suggest to draw the your arm accurately and decide what material you are going to use. So, you can calculate vertical resonance frequency. That will give you a basic estimate of vertical resonance frequency. Niffy knows how to calculate resonance frequency better than I do. He may give you some guidelines how to do it.
I will suggest to use magnesium for the center weight part.
Jim
In fact my question for Niffy was: in the terminator style arm, you have a vertical mass that is completely different from the horizontal mass as the vertical motion is done by pivot method being completely decoupled from the slider while the horizontal mass is the slider plus everything else.
Conversely on the fully air bearing arm i.e. the ones that have all three axes powered by air, to me, it seems that the vertical mass can't be any different from the horizontal mass, is this possible?
The more I think about the mechanical system and the more I get to the conclusion that the mass that the cartridge sees in the vertical plane is still the entire air bushing wand and center joint piece with counterweight and same as the horizontal.
Could you please explain a little bit more why the two masses thus the lateral and vertical frequency are different in such a system?
Conversely on the fully air bearing arm i.e. the ones that have all three axes powered by air, to me, it seems that the vertical mass can't be any different from the horizontal mass, is this possible?
The more I think about the mechanical system and the more I get to the conclusion that the mass that the cartridge sees in the vertical plane is still the entire air bushing wand and center joint piece with counterweight and same as the horizontal.
Could you please explain a little bit more why the two masses thus the lateral and vertical frequency are different in such a system?
Are you talking about the center piece that connects wand to the bushing and counterweight?
Yeah I don't understand how Kuzma can have a total mass of 80gr either. The only think I can think of is if they are using magnesium parts. The want I obtained is only 22gr because of the magnesium material.
My question is: Are there machine shops that can CNC magnesium?
I only have worked with aluminum so far I don't even know if CNC can easily machine these parts from a full block of magnesium as I could do with a full block of aluminum.
Do you know if there are shops that can do this for prototype and a reasonable price?
You can use magnesium for the center piece. For counter weight, you may use aluminum thread rod as Kzuma's or carbon fiber tube with set screw as VPI's. I use lead shots as counter weight so I can save some unused weights. Each lead shot weights .1 g. It is not too convenient to use lead shots. However, I can save some weight.
For an arm with long arm wand, it is almost impossible to achieve similar vertical and lateral resonance frequencies. For an arm with short arm wand, it is possible. I have tested my arms. Both vertical and lateral resonance frequencies are very similar. This mutes the criticisms about linear arms although I am not 100% sure the reasons why vertical and lateral resonance frequencies should be same.
The shop I passed information to you may be able to work on magnesium. I simply don't see the reasons why he can't do. Magnesium is softer than aluminum. I think he doesn't have CNC machine. His shop is an old fashion shop but it is good enough. CNC machine service is usually expensive because set up fee is high.
Jim
For an arm with long arm wand, it is almost impossible to achieve similar vertical and lateral resonance frequencies. For an arm with short arm wand, it is possible. I have tested my arms. Both vertical and lateral resonance frequencies are very similar. This mutes the criticisms about linear arms although I am not 100% sure the reasons why vertical and lateral resonance frequencies should be same.
The shop I passed information to you may be able to work on magnesium. I simply don't see the reasons why he can't do. Magnesium is softer than aluminum. I think he doesn't have CNC machine. His shop is an old fashion shop but it is good enough. CNC machine service is usually expensive because set up fee is high.
Jim
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I will look for magnesium I can check with the contact you gave me. I do however prefer to have all CNC precision machining though more expensive.
From my initial search I have noticed that only few shops deal with magnesium as it seems to be toxic material and requires a particular processed to handle the chips.
Regarding machinability I am not sure which one is easier I read magnesium has higher tensile strength than aluminum so does it mean it's easier to work?
I was thinking of having the rod that holds the counterweight as one piece block with the center piece worked on 5-axel CNC and not having a separate rod to screw or glue in the center piece. I just thought if the center piece is a one piece including the counterweight, this will bring more rigidity on the full structure especially on the counterweight and micro-vibration.
I don't know the cost yet I have to work on the solidwork model which I will use for quoting. So depending on quotes I might fold back to the idea of having the rod for counterweight separate.
I think it will take a while to design everything especially considering it is not my expertise. I always start from a pencil sketch and then go into modeling.
Right now the concept I have decided to move forward thanks to the advice I received is:
Moving bushing concept, 20mm new way air bushing with high precision stainless steel rod.
Magnesium long armwand which I have already acquired.
Center piece one piece magnesium block which includes the counterweight shaft.
Arm has to have the following adjustments:
overhang
azimuth
level for the steel shaft
VTA
Regarding resonant frequencies I would like to understand a bit more. From what my small understanding is, lateral and vertical frequencies don't have to be identical, there is no reason for that.
out of center vinyl will cause lateral motion which is on the 0.5-1Hz range so a 3X resonant frequency will be more than enough margin, while vertical has to be outside of the audible band so below 20Hz and a 2X margin is very fine there and outside of resonant of the deck itself which hopefully is low enough so a 10Hz is ideal for vertical.
Is my understanding very far from the actual truth?
From my initial search I have noticed that only few shops deal with magnesium as it seems to be toxic material and requires a particular processed to handle the chips.
Regarding machinability I am not sure which one is easier I read magnesium has higher tensile strength than aluminum so does it mean it's easier to work?
I was thinking of having the rod that holds the counterweight as one piece block with the center piece worked on 5-axel CNC and not having a separate rod to screw or glue in the center piece. I just thought if the center piece is a one piece including the counterweight, this will bring more rigidity on the full structure especially on the counterweight and micro-vibration.
I don't know the cost yet I have to work on the solidwork model which I will use for quoting. So depending on quotes I might fold back to the idea of having the rod for counterweight separate.
I think it will take a while to design everything especially considering it is not my expertise. I always start from a pencil sketch and then go into modeling.
Right now the concept I have decided to move forward thanks to the advice I received is:
Moving bushing concept, 20mm new way air bushing with high precision stainless steel rod.
Magnesium long armwand which I have already acquired.
Center piece one piece magnesium block which includes the counterweight shaft.
Arm has to have the following adjustments:
overhang
azimuth
level for the steel shaft
VTA
Regarding resonant frequencies I would like to understand a bit more. From what my small understanding is, lateral and vertical frequencies don't have to be identical, there is no reason for that.
out of center vinyl will cause lateral motion which is on the 0.5-1Hz range so a 3X resonant frequency will be more than enough margin, while vertical has to be outside of the audible band so below 20Hz and a 2X margin is very fine there and outside of resonant of the deck itself which hopefully is low enough so a 10Hz is ideal for vertical.
Is my understanding very far from the actual truth?
I used to make some of the magnesium parts and felt magnesium was easy to modify. I understand magnesium may be a fire hazard if it is machined due to heat. If magnesium is not available, I would suggest using 2024 aluminum alloy. I would not recommend using 6061 aluminum.
For adjusting the level of shaft, you may take a look goniometer tilting stage as reference. I use two goniometer stages for two of my arms. I can't think anything better for adjusting the level of the shaft.
A tonearm is a spring system so it has its own natural resonant frequency. A tonearm’s resonant frequency should be under 20 Hz which is a low limit of music information in most cases. Ideally, it should be higher than 3-4 Hz, which are frequencies of eccentricity and higher than 5-6 Hz, which are the frequencies of warp. So, the ideal range of tonearm resonant frequencies should be about 9-10 Hz or even higher 10-11 Hz.
Here is a youtube video I did to test my cartridge/arm resonant frequency. You can see the arm mis-tracked at about 5 Hz. There was a rapid rise in sound level. It caused distortion. It indicates that my arm/cartridge resonant frequencies are within about 5 Hz.
YouTube
Jim
For adjusting the level of shaft, you may take a look goniometer tilting stage as reference. I use two goniometer stages for two of my arms. I can't think anything better for adjusting the level of the shaft.
A tonearm is a spring system so it has its own natural resonant frequency. A tonearm’s resonant frequency should be under 20 Hz which is a low limit of music information in most cases. Ideally, it should be higher than 3-4 Hz, which are frequencies of eccentricity and higher than 5-6 Hz, which are the frequencies of warp. So, the ideal range of tonearm resonant frequencies should be about 9-10 Hz or even higher 10-11 Hz.
Here is a youtube video I did to test my cartridge/arm resonant frequency. You can see the arm mis-tracked at about 5 Hz. There was a rapid rise in sound level. It caused distortion. It indicates that my arm/cartridge resonant frequencies are within about 5 Hz.
YouTube
Jim
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My current machine shop in China confirmed they can machine magnesium so I can do the center piece in magnesium.
For the kick I have inquired them if they are capable of producing the following thing:
magnesium hollow cylinder rod
20mm outer diameter
+0/-0.01mm tolerance
6-10Ra surface finish
1.5mm wall thickness (high precision on inner circle is not required)
300mm length straightness of 0.01mm on the entire length
I am sure they are not able to do that, but in case they are the rod would only weight about 40gr which I could match with one 20mm air bushing and attempt doing a moving rod concept instead.
But like I said I am sure they are not able to produce these very tight tolerance and surface finish especially using magnesium.
I will keep updated to what they answer.
For the kick I have inquired them if they are capable of producing the following thing:
magnesium hollow cylinder rod
20mm outer diameter
+0/-0.01mm tolerance
6-10Ra surface finish
1.5mm wall thickness (high precision on inner circle is not required)
300mm length straightness of 0.01mm on the entire length
I am sure they are not able to do that, but in case they are the rod would only weight about 40gr which I could match with one 20mm air bushing and attempt doing a moving rod concept instead.
But like I said I am sure they are not able to produce these very tight tolerance and surface finish especially using magnesium.
I will keep updated to what they answer.
very interesting video but is that 10Hz? Looking at the frequency response and listening to the tone that seems to be high frequency.
Could you explain what test record you have used for that and what program?
Hi Stefano,
With any type of linear arm, be it mechanical or air of either type, the lateral effective mass is the same as the total carriage mass including cartridge and counterweight.
The vertical effective mass is somewhat more difficult to determine. All of these arms pivot vertically about an axis. Parts of the arm a long way from the pivot move a greater distance compared to parts close to the pivot. Those parts closer to the pivot contribute less of their mass to the total effective mass.
Vertical effective mass is actually the polar moment of inertia of the arm about the pivot as seen by the stylus.
Think of a conventional pivoted arm. The total moving mass of the arm including the counterweight can be several hundred grams but the effective mass is measured in grams or tens of grams.
Normally linear arms have much lower vertical effective mass than lateral. It is however possible to change vertical effective mass by changing the layout of the arm. It can be increased by using a smaller counterweight set further from the pivot.
The type of bearing used has no effect on effective mass.
Niffy
Thank you very much for your explanation. That is why you stated earlier that a 100gr total mass will be fine given that lateral resonance will be anyway away from frequency of warps and concentricity of the disc, while vertical equivalent mass will certainly be less thus placing the frequency likely in the desired range.
Ultimately I assume you are implying that to know the exact frequencies the only way to do it is by measuring them with a test record.
Ultimately I assume you are implying that to know the exact frequencies the only way to do it is by measuring them with a test record.
Hi Stefano,
The ideal carriage mass will be dependent on the compliance of the cartridge. 100g would be ideal for a cartridge of 10um/mN, anything between 7-16um/mN is fine at this mass. It is preferable to calculate effective masses, both vertical and lateral, during the design phase of building as gross errors will be very difficult to rectify after the arm is built. Test records can only be used after you have built the arm.
Calculating lateral effective mass is easy as it is simply the mass of the arm. Vertical effective mass is a bit more tricky. I found a spreadsheet invaluable when designing my arm. I have previously posted instructions on how to calculate vertical effective mass on the DIY linear tonearm thread but don't have the page or post numbers to hand.
Niffy
The ideal carriage mass will be dependent on the compliance of the cartridge. 100g would be ideal for a cartridge of 10um/mN, anything between 7-16um/mN is fine at this mass. It is preferable to calculate effective masses, both vertical and lateral, during the design phase of building as gross errors will be very difficult to rectify after the arm is built. Test records can only be used after you have built the arm.
Calculating lateral effective mass is easy as it is simply the mass of the arm. Vertical effective mass is a bit more tricky. I found a spreadsheet invaluable when designing my arm. I have previously posted instructions on how to calculate vertical effective mass on the DIY linear tonearm thread but don't have the page or post numbers to hand.
Niffy
Thanks for the explanation. My Clearaudio Goldfinger Statement has a compliance of 15um/mN which is considered medium/high compliance thus technically requires an equivalent vertical mass of less than 80gr.
I would like to look at the spreadsheet you have posted and yes your post has evolved in several pages where it makes it hard to find a specific thing.
I can assume that the equivalent mass to be less than the total mass of the entire system, so I assume 80-100gr to be still suitable for a 15um/mN cartridge.
I am going to look if I can find your spreadsheet I would be interested to be able to calculate it before building anything.
In the case you happen in the meantime to find the link to it, please let me know.
Thanks!
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Hi Stefano,
Sorry for causing a bit of confusion. I didn't actually post a spreadsheet, I just recommend using one. Every type of arm will be made of different parts so it is difficult to produce a universal spreadsheet without making it over complex and confusing. What you need to do is work out the effective mass that each part of the arm contributes then add all of these together to get the overall effective mass. The more "parts" you divide the arm into the greater the accuracy. The spreadsheet helps to keep track of all the separate parts. If you know the density and dimensions of the parts and how far they are from the pivot (which you should know if you are building the arm yourself) the spreadsheet can calculate the effective mass for you. Any changes of material or dimensions can be easily made without having to manually recalculate everything.
When I say "part" I don't just mean a complete component but a small piece of the component. For instance you might divide the armtube into 5mm long segments each being classed as a part. If you can't find the original post where I detailed how to do this I will try to explain it again for you. The maths is quite simple and shouldn't be a problem as long as you are familiar with how to use a spreadsheet. It might take a while for me to get this done as I am in the middle of moving house.
Niffy
Sorry for causing a bit of confusion. I didn't actually post a spreadsheet, I just recommend using one. Every type of arm will be made of different parts so it is difficult to produce a universal spreadsheet without making it over complex and confusing. What you need to do is work out the effective mass that each part of the arm contributes then add all of these together to get the overall effective mass. The more "parts" you divide the arm into the greater the accuracy. The spreadsheet helps to keep track of all the separate parts. If you know the density and dimensions of the parts and how far they are from the pivot (which you should know if you are building the arm yourself) the spreadsheet can calculate the effective mass for you. Any changes of material or dimensions can be easily made without having to manually recalculate everything.
When I say "part" I don't just mean a complete component but a small piece of the component. For instance you might divide the armtube into 5mm long segments each being classed as a part. If you can't find the original post where I detailed how to do this I will try to explain it again for you. The maths is quite simple and shouldn't be a problem as long as you are familiar with how to use a spreadsheet. It might take a while for me to get this done as I am in the middle of moving house.
Niffy
Hi Stefano,
No. The arm mis-tracked at 5 Hz. I used What Hifi Test LP.
You may use following site to estimate resonance frequency. For vertical resonance frequency, you need to calculate vertical effective mass first. In order to calculate vertical effective mass, you need to draw all the parts and the construction of arm first.
Resonance Frequency
Jim
No. The arm mis-tracked at 5 Hz. I used What Hifi Test LP.
You may use following site to estimate resonance frequency. For vertical resonance frequency, you need to calculate vertical effective mass first. In order to calculate vertical effective mass, you need to draw all the parts and the construction of arm first.
Resonance Frequency
Jim
I use the same Hi-Fi News (If I understood correctly) and my cartridge on the Terminator never mistracks or skips, but on the scope, I can see the at one point, the signal disappearing which I consider that to be the resonant frequency. Maybe at one point I should post a video as well.
Regarding the equivalent vertical mass, these calculations certainly go over my head. I will do some more research to see how to calculate the vertical mass, but I in the end, I am trying to get the TOTAL mass of the arm to be as small as possible and below 90gr and use the same layout that Kuzma has i.e. arm tube on top of the bushing and counterweight below the bushing. I assume this should ensure the effective vertical mass to be around 80gr. Parts will be all magnesium to minimize weight.
However, I can definitely see the benefit of being able to calculate the vertical mass in order to optimize length of wand and length of counterweight shaft to minimize vertical mass.
Is there an article that describes calculation of masses of a pivot system that I could look at?
Thanks!
Hi Stefano,
I've had a search and found the post where I described how to calculate effective mass. It's page 207 of the linear arm thread #2061.
DIY linear tonearm
I also explained a bit about the effects of effective mass in the audio band in #2063. Ignore the graph in #2063 as I got the vertical scale wrong. I rewrote this description in #2923 with the correct graph and a bit more detail .
DIY linear tonearm
Hope this helps. If you have any questions then please feel free to ask for clarification.
Niffy
I've had a search and found the post where I described how to calculate effective mass. It's page 207 of the linear arm thread #2061.
DIY linear tonearm
I also explained a bit about the effects of effective mass in the audio band in #2063. Ignore the graph in #2063 as I got the vertical scale wrong. I rewrote this description in #2923 with the correct graph and a bit more detail .
DIY linear tonearm
Hope this helps. If you have any questions then please feel free to ask for clarification.
Niffy
Sorry, it was Hifi News Test LP.
If the Terminator never mis-tracked on the same test LP, it indicates its reasonable frequency is above the test frequency range. If its reasonable frequency is above 20 Hz, it is not good.
For a linear arm, its lateral effective mass is same as its total lateral mass. Since the stylus will carry the total mass, or lateral effective mass, the total mass is a very critical factor for linear arms. For a medium compliance cartridge, I think total mass should be about 70-100 g based upon my own experience. For a low compliance cartridge, its total mass should be around 90-140 g. I personally prefer low compliance with heavy arm.
Vertical effective mass for a linear arm is same as pivot arm. Niffy just showed you how to calculate vertical effective mass. You need at least to get a rough estimate.
Jim
If the Terminator never mis-tracked on the same test LP, it indicates its reasonable frequency is above the test frequency range. If its reasonable frequency is above 20 Hz, it is not good.
For a linear arm, its lateral effective mass is same as its total lateral mass. Since the stylus will carry the total mass, or lateral effective mass, the total mass is a very critical factor for linear arms. For a medium compliance cartridge, I think total mass should be about 70-100 g based upon my own experience. For a low compliance cartridge, its total mass should be around 90-140 g. I personally prefer low compliance with heavy arm.
Vertical effective mass for a linear arm is same as pivot arm. Niffy just showed you how to calculate vertical effective mass. You need at least to get a rough estimate.
Jim