A unipivot made from scrap

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cressy
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A unipivot made from scrap

Post by cressy » Fri Apr 14, 2017 2:10 pm

Ok, i mentioned the lack of posting, so time to pull the head out of ones arse.

Ive made several unipivots before, those were made with the wand having the bearing cup cut into them.

The issue with these was firstly they were dry bearings, and couldnt be damped with anything. Secondly, the bearing contact point was high up in the assembly, which gave stability, with the centre of gravity much lower than the pivot point. But the pivot point was offset from the stylus by quite a bit

The downside is that as the cart traverses warps, the up/Down motion traces 2 curves. One curve is based on the bearing pivot point, and one is the stylus contact point. Those curves means that as the front of the arm goes up, the offset of these 2 curves means the effective length changes by a small amount relative to the rest position of the arm
( i will add a drawing to show what i mean).

The ideal pivot point would be in line with the stylus to minimise this but in practice this is not necessarily viable with this type of bearing.

More to follow
Last edited by cressy on Fri Apr 14, 2017 3:01 pm, edited 1 time in total.

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Re: A unipivot made from scrap

Post by cressy » Fri Apr 14, 2017 2:52 pm

Not the best drawing but will do
Image

So. What we have is pivot point a which is where the arm pivots from, an pivot point b which is the point that we actually align the arm from, as we line the cart up in one dimension, traversal of the record, not vertical tracing.

There are 2 vertical offsets, one is the pivot point above point b, and the second is the stylus which is below point b.

Tracing a curve from point b which we align from gives the green curve. Note that above the stylus there is an offset going to the left of the vertical line, and below there is an offset to the right

Tracing the same curve for point a gives overlapping curves. Note that the effective length will change relative to the physical length of the armtube due to the 2 vertical offsets. The area within the overlap of the curves is the change in alignment as the arm moves up and down.

All very not interesting, essentialy what it means is that the cart alignment perameters change when riding warps.

The ideal pivot point is therefore smack in line with the stylus, if so, there is no vertical offset and warps will not change the alignment very much. It will give a single curve insted of the 2, and the fore/ aft movement of the stylus relative to the record will be minimal, just the slight rearward movement as the arm travels upwards.

If the bearing cup is in the arm pillar instead, moving the pivot point in line with the stylus becomes more viable

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Re: A unipivot made from scrap

Post by cressy » Fri Apr 14, 2017 3:11 pm

Realising this, I have got the lathe out this morning and made an arm pillar and small collar.

Image

The arm pillar is made from a hardened steel bar and has had the cup machined into the end. The pin is a hardened steel scribe for marking timber, so shouldnt deform in use
The collar is a small piece of aluminum, didnt have ant bigger bits, so I might make a wooden one.

The plan is to turn a walnut top section that the pin fits into, and have the pin vertically adjustable

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Re: A unipivot made from scrap

Post by cressy » Fri Apr 14, 2017 3:29 pm

This is the idea
Image

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Re: A unipivot made from scrap

Post by ed » Fri Apr 14, 2017 5:56 pm

Hi Ant

I know squat about vinyl, as most know. Proof is I had an lp12 in the 80s.

however, from a naive standpoint wouldn't it benefit if the surround for the bearing were hanging lower(more mass) there would be more stability, i.e the mass of the bearing surround would be way below the pivot point.
arm bearing.jpg
arm bearing.jpg (12.94 KiB) Viewed 996 times
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Re: A unipivot made from scrap

Post by Paul Barker » Fri Apr 14, 2017 5:58 pm

Agree
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Re: A unipivot made from scrap

Post by IslandPink » Fri Apr 14, 2017 6:04 pm

Doesn't work - more prone to oscillate apparently. I only say this because I read a very good and detailed story about building unipivots from a guy who eventually went commercial with one - quite expensive niche market one aimed at beating the Schroeder. He made this assumption too at first but found a lot of difficulty with record warps if the mass was well below the pivot and eventually found you needed to get the centre of mass just under the pivot point. It's not intuitive but I think James agreed on this too, from his experience.
The guy who wrote the detailed story with pics on his website took the whole thing offline when he went commercial, I believe -worst luck.
Last edited by IslandPink on Fri Apr 14, 2017 6:35 pm, edited 1 time in total.
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Re: A unipivot made from scrap

Post by Paul Barker » Fri Apr 14, 2017 6:08 pm

Although if you look at my Mayware, the skirt below is shallow, which may be due to the asimuth angle at the cup/pivot point to level the headshell. However, Anth could design into his rotation of the asimuth such that the cup of the pivot remains perpendicular. i.e. rotation of headshell.

Image
Image
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Re: A unipivot made from scrap

Post by IslandPink » Fri Apr 14, 2017 6:13 pm

You can get the pivot housing level if you play around with the headshell azimuth ( it rotates on the tube via a small grub screw ) and get the anti-skate fibre pulling perpendicular to the groove via adjustments of the pivot point/crank, again by loosening off a small screw and playing around.

ps. the guy who wrote the above article I was referring to ( about c of g ) makes this arm commercially :
http://www.tonepublications.com/analoga ... rms-talea/
http://www.durand-tonearms.com/About/about.html
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Re: A unipivot made from scrap

Post by Paul Barker » Fri Apr 14, 2017 6:15 pm

There are many ball aches to setting up a Formula IV which result in a bent cantilever. But the biggest ballache is the asimuth adjustment being on the same weight as the tracking force. I suggest stick a wand outrigger with weight on it that moves along its shaft towards or away from the arm. Then another weight that is not eccentric for tracking force. I might adapt this as so. TAnother of the ballaches is that antiscate is over zeallous.
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Re: A unipivot made from scrap

Post by Paul Barker » Fri Apr 14, 2017 6:15 pm

IslandPink wrote:
Fri Apr 14, 2017 6:13 pm
You can get the pivot housing level if you play around with the headshell azimuth ( it rotates on the tube via a small grub screw ) and get the anti-skate fibre pulling perpendicular to the groove via adjustments of the pivot point/crank, again by loosening off a small screw and playing around.

ps. the guy who wrote the above article I was referring to ( about c of g ) makes this arm commercially :
http://www.tonepublications.com/analoga ... rms-talea/
http://www.durand-tonearms.com/About/about.html
I never knew that. Thanks.
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Re: A unipivot made from scrap

Post by Paul Barker » Fri Apr 14, 2017 6:20 pm

You have to find out whta length of arm would be best aswell. I would be more tempted to built a schroeder, as there is an implication this feller decided the task was to beat a schroeder (infurs not an existing unipivot?).
Last edited by Paul Barker on Fri Apr 14, 2017 6:32 pm, edited 1 time in total.
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Re: A unipivot made from scrap

Post by cressy » Fri Apr 14, 2017 6:32 pm

Ive found with the earlier ones I made that having mass hanging too low causes an issue where the arm naturally hangs with its nose down which makes it impossible to get a correct setting for the tracking weight. Once the heavy oak one moved past a certain point in its up and down travel while playing it would pick its nose up and the tracking would go completely to pot.

Marks right in that i found the mass needs to be centred around the pivot point and that the stability of it is derived from putting the mass in the right place.

Its one of the reasons im not fond of rider weights, i think they put a lump of mass where it shouldnt be. A counterweight on its own is sufficient to set tracking weight, the arm osscilates in reaction to the movement of the mass at one end in relation to the other. Adding a third into the mix imo adds an unnecessary complication.

The idea i have will have a small metal ring on the bottom of the bearing housing so that there is some extra weight to aid stability, but it will be just under the pivot point.

Its also why the stub and arm tube are in line on the drawing as I plan to use the lenco dropped counterweight and I dont want the weight hanging too low

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Re: A unipivot made from scrap

Post by cressy » Sat Apr 15, 2017 2:08 pm

Ok so next phase is to make the arm wand now we have a bearing cup. Firstly we need a blank as the intention is to turn a cylindrical mount for the pin.
So one is cut on the bandsaw out of a large block of walnut

Image

This bandsaw is a powerline bk1, an oldie but a goldie. Its a 3 wheel bandsaw, i find these more accurate than the usual 2 wheel ones you see these days.

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Re: A unipivot made from scrap

Post by cressy » Sat Apr 15, 2017 2:24 pm

Next, we drill a hole in the end of the blank and screw it onto the screw chuck with a spacer so that the chisel can get all way to the end and balance the entire blank, even though I wont need to cut the whole thing

Image

Set up with a live centre in in the tailstock as the blank is unbalanced so will waggle all over the place without being secured at this stage

The lathe is an al ko hdm850, i bought this as it is just about man portable. It is very heavy and stable, great bearings and has a 320mm total throw. I could turn a platter on it if I was so inclined. Its based on the hobbymat md65 metal working lathe, and as such the motor is very pokey. Only drawback is that the spindle is an odd size with a 30x1.5 mm thread so a chuck is very expensive for them.....
Other than that, its very very good and extremely high quality compared with the modern equivalents

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