JE-4887

[jjwharris]

I've been playing around with designs for moving iron cutting heads for awhile - I guess I took Gridlock's advice to heart.

I had a couple of successes trying to build a Presto 1-C/D style cutting head, but without a cutting head in hand it was difficult to get something consistent and reliable, although I'd like to try a take on the Presto again, I've been putting together multiple machines to be able to do small runs, and was needing cutting heads so I based this off the RCA-MI4887 I had on hand.

I'm unsure if this is an avenue most will go, the laser cutting was fairly reasonable, but only because I made enough pole 'laminations' for multiple heads.

The research document on the Universal Cutting head also had some details in it that brought this head into being something that work to something that works really quite well. It's taking 4-6 hours to assemble each one, and part of that is because I'm not great at milling, and I'm stick welding the pole pieces instead of spot welding them which is a major inconvenience. I'm also individually EQing them with a DSP.

I'm embossing to polycarbonate with an eposlab style stylus

opcode/Todd's videos were very helpful in winding tidy coils - I recoiled my original MI-4887 because it had a short on it, but the coils on these are definitely of a better standard.

I still have very little idea on what the armature clamp should look like in the MI-4887s, the head I purchased was essentially dead on arrival, looking back on it now, I think it was brittle/dry dampening which had fallen away, I made something up with aluminium and it started working, then moved to brass and it sounded better.

I was going to try and explain more of what I've done, but I think I'll just post some assembly photos and if anyone has any questions I'm happy to answer them.

I was intending to do a write up and final photos but times got away on me!

[Fela Borbone]

Hey!
Not bad, nice heads!

[Dogtemple]

this is great, I'm liking your work. how does it compare to your stereo head in terms of building it and results?

I'm yet to get my head around mono heads but like the idea of building one. I have a workshop for metal work; got a lathe, mill, tig etc so I have no excuse!

[jjwharris]

I think the stereo heads would be more do-able after doing this.

I'm intending to try and make coil formers that attach to the armature/torque tube from something like brass so there can be a solid brazed/soldered connection. I'm imagining the difficulty may be in making alignment jigs.

The stereo heads I was making had all sorts of issues with alignment, I don't think I ever really got a true stereo signal from them, and then the EQs were wild as there were crazy resonances everywhere - I definitely think there is advantage in not using 3d printed parts, even with these mono heads the 3d printed cases seem to be interacting too much with the sound, it's probably the next upgrade for these heads. I do think when I try and make a stereo head I will be looking to make the prototypes very customisable, in terms of being able to adjust the resonant frequency and magnet positions to determine the 'sweet spot'.

I found the last diamond cut I tried (2-3 years ago) with the last stereo head I tried making, I'll try to remember to post the results tonight.

[Dogtemple]

I always wondered to be honest about the viability of 3D printed parts when making a head. Metal seems to be the only way if rigidity is a key aspect of getting good results. I’ve been working out how to get rigidity with the smallest amount of metal to hold the poles from the speakers and a removable stylus. Made a couple bits so far but way off anything complete.

[jjwharris]

Epoxy Granite is a very interesting material. I'd maybe consider making the case for the stereo head from an epoxy granite.

https://www.adambender.info/post/2017/03/25/epoxy-granite-machine-frame-how-to

[Estrada]

This looks like a cool little head, very similar in concept to my old Audax head. Quick question, how did you attach the stiffening spring to the armature? I'm thinking of either replacing or adding a second spring to the armature on my head as I am getting a lot of distortion which I think is due to lack of stiffness in the armature. Is this brazed on or have you used something like a high strength silver solder?

[jjwharris]

It's a piano wire silver soldered to the armature, but I guess it could be considered brazed, I'm using a map gas torch, and should probably buy a jewellers torch. I load up the armature with cutting fluid where I don't want the silver to go, and then wipe the parts I do want the solder to flow to with acetone.

My first attempt was with an arc welder and that certainly reduced the mass of the entire armature...Not in a good way.

I've got some Sorbothane on the way, I'm hoping that works better than heatshrink as a damper, I'm under the impression the heatshrink is making the response creep around a fair bit, with use and time.

[Estrada]

Thanks for your advice on that. I have a small butane torch which came with my 3D Printer toolkit, I'm thinking that will do the job.

The Sorbathane sounds really interesting, I'll be keen to hear how that goes. I've been using bits of silicon tube but I'm not sure how successful that has been. Presumably you've bought it in some kind of sheet format? Is this available in NZ or have you ordered from overseas?

[Estrada]

Another question, do you know of a good source for piano wire/spring steel rod in NZ? Thanks

[jjwharris]

Acorn Models is my go to for piano wire - fishpond sells sorbothane. I think the shipping time is pretty quick, my payment didnt quite go through till Saturday. I'll let you know how it goes when it arrives.

Where are you based in NZ?

[Estrada]

Thanks for that. I found an online hobby place which stocked various thicknesses so I've placed an order. Also ordered some aluminium rod for the stereo head I will be attempting after I've finished with the mono head. What thickness of Sorbothane did you order? Looks like it is quite pricey, I'll be keen to hear how good it is.

Are you using the Sorbathane for a rear damper attached to the back of the armature or is it for damping around the pivot points of the armature itself? or both? I've had to replace both of these on my head as the head is about 80-90 years old and both are perished. I've used rubber for the rear damper and various bits of silicon tube around the armature. It was working OK for a while, I think because I was using a really thick tube around the armature which put a lot of pressure on the armature and therefore stiffened it up. Sadly the original fixing plate broke due to the pressure and it hasn't been right since. I'm hoping that a stiffer spring will give the armature the stiffness it needs without requiring a lot of pressure on the armature.

I'm based up in Wellington, Lower Hutt actually. You are in Christchurch right?

[jjwharris]

Yup! I'm in Christchurch, might be visiting Wellington soon in the future.Z

I'll test the sorbathane and see if it makes a difference vs the neoprene I'm currently using for the back damper. - I'm imagining it'll make the biggest difference in the armature clamp, I'm having the same issue as you - the dampening material seems to compress over time which lowers the resonant frequency.

As I mentioned earlier, I had a similar issue with the MI-4887 I bought, looking at it now I think the dampening material had perished and fallen out. Leading to a situation like this; https://www.lathetrolls.com/viewtopic.php?t=5208

I had too much pressure on it and broke the clamp. I ended up milling another brass plate and wrapping that with heatshrink and it did end up working, however I'm almost 100% convinced that the pressure is changing over time with the compression of the heatshrink.

[Estrada]

Sounds exactly like what happened with mine. The thick silicone tube I used worked for a while until the clamping plate broke. It is quite thin and snapped at the weak point where it is recessed for the armature.


I made a new one with a piece of 5mm aluminium flat bar. I made it quite thick so that I could apply some pressure without it snapping.

IMG_3408 copy.jpg

Have also been 3D printing drop in coil bobbins, experimenting with depth in order to get the armature at the right height between the magnet poles.


If you are up in Wellington you are welcome to call in and check out what I've been doing, though I suspect it is not quite as impressive as what you have been up to. If you interested send me a PM and we can exchange details. Cheers, Craig

[Estrada]

Not sure what happened with my inline image placement there…. oh well, you get the idea