building a tonearm

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Vinyl-ant
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building a tonearm

Unread post by Vinyl-ant » Wed Apr 10, 2019 1:31 pm

This follows on from an arm build I did about 18 months ago, not the first arm I've built, but a collection of ideas I've been kicking about for a while....
Jump forward to now, and having had something of a lean period in terms of work, I decided to revisit the unipivot. I had some other ideas for it over the last couple of months, and having had the md65 lathe for a good while I decided to use it for this project. Or series of ideas rather than project.

So having got hold of some 25mm round steel bar, and an electric sharpener that I got for drill bits and wood chisels and modified to allow me to sharpen the metal lathe tools, I thought I'd build the ideas I had into something. To see if it would work.

It's taken about a week on and off to make all the parts, probably about 8 hours work in total. I didn't want to buy any bits in, I wanted to make them myself because I'm stubborn. Only thing left to make is the bias weight. All turned from steel apart from the headshell which is made from 3mm aluminium to keep weight down.

A collection of parts
ImageCx unipivot mk iv by anthony cresswell, on Flickr

Built up into the arm
ImageCx unipivot mk iv by anthony cresswell, on Flickr

The part not fitted is a heavier counterweight. The fitted on is 75g, the other is 100g

a top view
ImageCx unipivot mk iv by anthony cresswell, on Flickr

The cylinder stuck out at an angle is the setscrew for locking the arm height. I borrowed this idea from the audiomods arm as it is alot easier to get hold of rather than fiddling with an allen key or screwdriver to lock the arm in place.

and a side view
ImageCx unipivot mk iv by anthony cresswell, on Flickr

This is an inverted bearing, the spike is fitted into the arm wand pointing down, an sits in a cup in the top of the arm pillar. this can be filled with silicone fluid to damp the pivot, same as the original. I also kept the wire arm rest and bias weight 'thing' of the original as it was nice and simple

original
ImageLenco with u/p mods by anthony cresswell, on Flickr

This one however does not have the bearing pivot point at the same height as the stylus, as this led to unpredictable azimuth on stuff that was quite warped, so the pivot is set just a little higher than the top of the arm tube to get all the weight lower than the pivot. There were ways around this characteristic, by moving the weight around and reducing weight above the pivot, thats for the next prototype.

The problematic bias location on the original has been moved so that it follows the Hadcock pattern of being on the left rather than out of the back of the housing as on the original. this caused issued with setting the cart tracking weight and was too fiddly.

the counterweight is not dropped and is on axis with the main arm tube instead, as I wanted to separate the azimuth and the tracking weight adjustment. The azimuth is adjusted by the two weights on the left and right of the bearing housing and is much less fiddly than skewing the main counterweight off to the left or right, and gives plenty of clearance at the back which was a problem with the original, plus, the majority of the weight of the assembly is lower than the pivot on this version so its not necessary from a stability point of view.

The arm tube is a composite of 8mm stainless tube at the back and Makore hardwood at the front. The idea was gotten from an origin live arm that uses aluminium and carbon fibre sections to alter the resonance of the arm, and I liked the wood and steel idea. the wooden section was turned on the wood lathe into a larger blank, which was turned down on the metal lathe so that a 30mm section could be glued into the steel, then it was all turned and trued on the metal lathe into a complete wand.

To get the headshell on, I bodged myself a rudimentary milling machine out of the pillar drill and an old lathe cross slide. Stuck a twin flute router bit in the drill chuck and used the cross slide to mill the 3mm deep section out of the end of the wand. Worked perfectly. No good whatsoever for metal, but fine for this particular job. It was spot on for drilling out the holes to make the bolt slots in the headshell though, it meant that the chain of holes were positioned perfectly.

One thing I did forget to do was to drill out the inside of the wooden section of the arm tube to take the wiring, so I'll have to tape the wiring on the outside to test it. There is always something.......

It took abit longer than strictly necessary to make the parts, but in my defence, I needed to get a feel for the lathe and how the steel turned having not really made anything in steel on it other than some really small things where dimension weren't important. Also used a shit-ton of wd40 as cutting fluid especially when boring and tapping as the proper stuff I ordered still hasn't turned up... Martin at the local engineering place suggested it in the interim as he didn't have any I could buy, he just had a huge great drum of the stuff that they use in the workshop. Worked nicely though.

I also got an aerosol air duster to blow the chips away at his suggestion, they use an air line, but also have some of these knocking about for if the air line is in use on something else. A quid from the local pound shop, it was really useful. Nothing worse than the chips building up on the tool and sticking to the cutting fluid so you cant really see the cutting edge of the tool, the steel cut fine dry on the exterior cuts but abit of fluid was needed to get a nice fine finishing cut. The interior boring needed loads of fluid though, you can feel it when it needs abit more.

Haven't got the hang of thread cutting with it yet though so I just used taps this time. Need more practice....
BTE designs type 2 lenco, Audio Technica AT1100 AT150mlx (his), JVC QL-Y5F Audio Technica AT150sa (hers)
Benedict audio hothead mm phono stage
BTE designs type 1 custom khozmo shunt build passive pre,
PASS F5 dual mono Power amp
modified Philips cd371
Squeezebox classic
fane 15" sb speakers
BTE designs balanced power supply

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Latteman
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Re: building a tonearm

Unread post by Latteman » Wed Apr 10, 2019 4:30 pm

Most impressive
I’ll be round for some that wd40
I have 2 hand made arms- by someone else- will do one myself one day😉
RIP Doc
Analogue Source-
ARB Turntable & 10” dual pivot arm, Grado Reference Prestige 1 MI
Funk Firm Vector V, Acos modified tonearm, Grado Gold Prestige MI
Nva Phono 1 (external psu)
Digital Sources- Denon CEOL Piccolo, iPhone 6s
Amplification-NVA P50, A20 (x2), A60
Speakers- Diy BBCubes on Nva Stands;
Ls6, Ls5; SSP
Mini BMU

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Dr Bunsen Honeydew
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Re: building a tonearm

Unread post by Dr Bunsen Honeydew » Wed Apr 10, 2019 4:42 pm

Luv it, but I am not a fan of single point bearings but it is the only way a home build can be made unless you use an existing bearing system like the Rega. I also like switching carts especially as I have to test both mm and mc phono stages, so detachable headshells for me is a must. But for *normal users* this looks a fantastic project, promote it, get users, make some dosh.

keepitsimplestupid
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Re: building a tonearm

Unread post by keepitsimplestupid » Wed Apr 10, 2019 6:58 pm

Ant, if you fancy it I have a ruby jewel bearing and a micron polished shaft lying round from a unipivot build that I never completed once I bought my 12" Kuzma S. They're pretty small parts so mounting them would require some precision, but you'd struggle to find a lower friction bearing. Yours if you want em, no charge.

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guydarryl
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Re: building a tonearm

Unread post by guydarryl » Wed Apr 10, 2019 10:13 pm

Looks beautiful. Always feel inadequate when I see what others can achieve in terms of craft/engineering.

Hope you don't mind a question though Vinyl-ant: you say that you used aluminium for the head shell to keep mass low, why is this necessary? As the arm is balanced, is it a matter of reducing momentum? but then if the head shell is heavy and so has high inertia isn't that a good thing as the stylus/cantilever are only responding to to changes in the groove rather than "fighting two things at the same time?

Sorry for questions, (after a few pints of Adnams Old and a couple of glasses of red wine) but reading your post has really got my mind going overtime :guiness;

(By the way, very nice offer SQ)
LP12,Ittok,DV10X5,Phono2(twin supply),P50/A40's,LS5,SSC,Rega Ela mk1/Cube 3's
Sony cdp xb930,Alessandro ms1

Vinyl-ant
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Re: building a tonearm

Unread post by Vinyl-ant » Wed Apr 10, 2019 10:25 pm

It's to keep the effective mass down as the tube is a split mass design; I. E the heavier stainless section of the arm wand is closer to the pivot so effects the total movement of inertia of the assembly less. the lighter wood end has less mass so affects the total moment of inertia less too. The further from the pivot the mass is, the more inertia it has so keeping the mass centred as close to the pivot as possible aids stability as the arm can react to changes quicker as it doesn't take as much effort to move it. Also keeping the effective mass lower makes cart matching in terms of compliance.
I prefer higher compliance carts, my audio technicas are all med/high compliance, I'm not a 103 person.
@ keepitsimplestupid, that would be great thanks, I've thought about getting hold of a jewelled bearing for one of these before but never got round to it
Cheers ant
BTE designs type 2 lenco, Audio Technica AT1100 AT150mlx (his), JVC QL-Y5F Audio Technica AT150sa (hers)
Benedict audio hothead mm phono stage
BTE designs type 1 custom khozmo shunt build passive pre,
PASS F5 dual mono Power amp
modified Philips cd371
Squeezebox classic
fane 15" sb speakers
BTE designs balanced power supply

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guydarryl
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Re: building a tonearm

Unread post by guydarryl » Wed Apr 10, 2019 10:44 pm

:) Cheers Ant, I think that makes sense - will read again tomorrow
LP12,Ittok,DV10X5,Phono2(twin supply),P50/A40's,LS5,SSC,Rega Ela mk1/Cube 3's
Sony cdp xb930,Alessandro ms1

setting son
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Re: building a tonearm

Unread post by setting son » Thu Apr 11, 2019 10:42 am

Nice. Very nice indeed.

Vinyl-ant
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Re: building a tonearm

Unread post by Vinyl-ant » Thu Apr 11, 2019 9:25 pm

made the last of the arm parts today and permanently assembled it. The stub and wand are epoxied into the bearing housing to make sure everything is rigid rather than just using setscrews

Made the mounting plate (wrong wood, I couldn't find the bit of Makore big enough for this bit...) which is glued to the steel height adjuster base, and the bias weight. I ended up with a 3.5g bias weight after checking the figures with the James Kogen bias research paper, this gives a bias range of about 0.75 to 2.4g bias which is plenty of range.

also made a new bias guide as I wasn't happy with the loop one. the new one presents a smoother travel for the string as it's a smooth curve rather than the tight bend in the first one, and it allows the string to run at the same height as the bias arm which is in line with the pivot point so the pull doesn't affect stability. Now it doesn't affect the azimuth by pulling it upward either.

ImageCx unipivot mk iv by anthony cresswell, on Flickr

I also sorted some wiring out, this needs a terminal box making for it before it can be connected. Next job.

ImageCx unipivot mk iv by anthony cresswell, on Flickr

onward!
BTE designs type 2 lenco, Audio Technica AT1100 AT150mlx (his), JVC QL-Y5F Audio Technica AT150sa (hers)
Benedict audio hothead mm phono stage
BTE designs type 1 custom khozmo shunt build passive pre,
PASS F5 dual mono Power amp
modified Philips cd371
Squeezebox classic
fane 15" sb speakers
BTE designs balanced power supply

Vinyl-ant
Posts: 233
Joined: Sat Jul 01, 2017 12:51 pm
Location: South yorkshire
Contact:
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Re: building a tonearm

Unread post by Vinyl-ant » Fri Apr 12, 2019 10:15 pm

Ok, got the wiring loom bodged on. It is a big fat bodge, I should've drilled out the wooden part of the arm tube, but it suffices for testing the arm..... It is sat in the mdf block as I was fed up of it falling over every time i looked at it funny...

ImageCx unipivot mk iv by anthony cresswell, on Flickr

The loom is also a little compromised as it's abit shorter than I would like, but it was pulled from one of the old wands, the mk 3 flat one that was not great. Thats on an old audio talk thread, was this one

ImageDSC_0622 by anthony cresswell, on Flickr

No need to buy in some more wiring if its already there and all terminated already

ImageCx unipivot mk iv by anthony cresswell, on Flickr

Ive reused the small locking connector from that loom aswell as it was pretty good and really solid.

ImageCx unipivot mk iv by anthony cresswell, on Flickr

The loom would have followed the same pattern though, exiting the arm tube through the top at the same point the bodged loom bends up, through a grommet to insulate it from the tube. Probably with a silk sheath around it so it remains flexible.

The connector box is what I had in mind, turned from a block of walnut with a cover on the bottom turned to fit in it as a resistance fit like a box lid I will put an earth post on it if the arm needs one. Dunno if it will yet as the cart will be isolated from the rest of the assembly by the wooden bit of the wand. It might hum, it might not, will see. I didn't have any issue with hum with the flat wooden wand

ImageCx unipivot mk iv by anthony cresswell, on Flickr

So other than the wiring being taped to the arm rather than being inside it, this is pretty much how it will be.
Too late tonight to start dismantling a deck to put it on, a job for another day
BTE designs type 2 lenco, Audio Technica AT1100 AT150mlx (his), JVC QL-Y5F Audio Technica AT150sa (hers)
Benedict audio hothead mm phono stage
BTE designs type 1 custom khozmo shunt build passive pre,
PASS F5 dual mono Power amp
modified Philips cd371
Squeezebox classic
fane 15" sb speakers
BTE designs balanced power supply

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