Kenwood 700m Supreme knob production (or, the ultimate knob job)

DrumminDaddy

Hit it, Baby !!
I had originally started to post some information on the Kenwood Supreme series knob job in another post:

http://www.audiokarma.org/forums/showthread.php?t=602166

Because this is a separate issue from the electronic repair, and because several AK members have expressed some interest in this matter, I've started a thread specifically documenting the knob production.

The goal of this thread is simply to inform those who may have curiosity about machining, how it may be accomplished. The knob, specifically relating to audio restoration, is a fine little piece of machine work in its own right.

As I complete the operations, I will post more information along with my thought processes. As you might imagine, there can be more than one way to accomplish the same results depending on the machine and tooling available.

Because this knob is not only a functional element, but visually appealing as well, considerations will be made regarding surface finish. In other words, I am going to select my tooling and machining processes to faithfully reproduce the tool marks that comprise the finish as close as I can. I will add examples with photos as I get to this point to better illustrate this concept.

The first step in the process is to reverse engineer the original part. Exacting measurements are taken and these dimensions will be used to create a program for a CNC machine tool (computerized numerical control).
A CNC machine is a machine tool with servo driven axes. There are a multitude of configurations available today. In my shop, however, my choices are a bit more limited.

The axes of these machines are astonishingly accurate. It is commonplace for a machine to be able to maintain repeatability to .0002 inches. To put that into perspective, a human hair is about .002 inches in diameter. So we're talking positional accuracy that is ten times thinner than a human hair. This finesse in accuracy is complemented with some considerable muscle. I can place a piece of carbon steel in a lathe with a 12 inch chuck and easily reduce its diameter by 1/2 inch in a single pass. The manufacturing world has many specialized machines and tooling. We're just going to take a peek at what we can accomplish to make our love for audio just that much more rewarding.

Some basic measuring techniques using calipers, surface plate/height gage and radius gages.

I also measure the shaft of an aftermarket speaker select switch for my
700m. This shaft has a slightly larger diameter than the original. Therefore, the hole in the knob that recieves the shaft shall be made to fit this larger size. I will also make some that will fit the original diameter as well because I have a 700T that is missing a knob.

inspect1.jpg

inspect.jpg

inspect2.jpg

inspect3.jpg
 
I'd like to see this done as well.

Want a bigger project? Duplicate the volume knob on the 700C...
 
This should be a great thread dd! I've got all the knobs for mine but am fascinated by the process and machines.. Will these be anodized?
 
Will these be anodized?

I don't believe that the originals are anodized. It looks like they are clear lacquered. We produce some parts that we have clear anodized and it turns aluminum into a distinctly different shade. It changes from silvery/shiny into a more dull grayish shade. Not even close to the knob finish. I'll try to scrounge up some parts later and show you a before/after. It will be apparent. Anodizing isn't a coating of the metal as one might think. It is actually a chemical process that changes the outer skin of the metal itself forming a thin oxide layer. Hence the the color change. Dye can also be introduced to produce a multitude of colors.

The knobs and faceplate, however, have a bright shiny finish. When you hold them next to virgin aluminum, you can see that the originals have a very slight yellow tinge to them. I think this is even more indicative of aged lacquer.

Regardless, I'll make some extras to play around with. ;)
 
FYI, nearly ALL 70's silver Japanese gear (knobs, faceplates and everything that shows), is anodized, most of it clear.

The Kenwood Supreme Series was supposed to be a champagne-colored anodized finish, but the shading seems to vary from unit to unit (probably more than Kenwood intended).

No Japanese mfgr used lacquer on faceplates and knobs in the 70's (a few did in the 80's). But if we limit our talk to 70's gear, and Kenwood Supreme gear specifically, I've worked on many dozens of Supreme units in every stage of deterioration (from mint to boat anchor), and if the knobs (or faceplate, for that matter) were coated with lacquer I'd have most certainly seen a few chipped knobs by now. But, not an issue, since I know for a fact that they are anodized. Too many things would attack the lacquer, and the mfgr would never know what you might have on your grubby little hands when you went to reach for that volume knob. Thus they used anodizing and don't have to worry about it...
 
...if the knobs (or faceplate, for that matter) were coated with lacquer I'd have most certainly seen a few chipped knobs by now.

Your point about the chipping stands to reason. My mindset is geared toward the parts we make which are hard clear anodized. Geared toward the industrial applications needing wear resistance. It burns into the surface aggressively. Thus changing the hue.

Part on the left is HARD coat clear anodized. Quite a stark change from the freshly machined aluminum on the right.

hard anodizing.JPG

I will be running some parts to our anodizing house this week. I will show them the knob and see what they can do for me.
 
In order to better communicate the processes that I am discussing here, I will define some of the terminology involved in machining.

There are many types of metal removal processes. The ones that we need concern ourselves with in order to produce this knob are 'turning' and 'milling'.

In the broadest sense, one can think of 'turning' as a process where the part is spinning, and 'milling' where the tool is spinning.

Let's take a look at some turning. If you're not quite envisioning what I'm describing, perhaps an analogy is in order. If you held a pairing knife in one hand and spun an apple underneath the knife with the other hand, while slowly traversing the knife across the apple skin, you could peel the skin off the apple in one long coil. That's essentially what I'm doing to the metal.

Pictured here is a typical CNC lathe. A part is clamped into the chuck on the left (in this photo, a flange is being made). The turret to the right is where the tooling is mounted. This particular machine has a 12 station turret. The turret can be indexed to change to a particular tool to perform different operations.
liveToolLathe.jpg


In order to cut the basic profile of our knob, I use what is known as 'single pointing'. Here, an industry standard 3/32" wide grooving tool is used to form the entire profile of the knob. Using a series of straight line moves, I can traverse along each plane until the profile is complete. The tool is essentially a single point of contact (in theory) with the part. Hence the term 'single pointing'.
singlepoint.jpg


In contrast to single pointing would be a form tool. This is a specialized tool bit that would have to be custom manufactured to the profile we wanted. The tool would be plunged into the material and form the profile all in one motion. As you might imagine, the tool bit itself would be costly to manufacture although the time to produce the part would be reduced. This method would only be a consideration where the part was produced in great quantites to justify the tooling cost vs. the reduced cycle time. It would have been commonplace in the days before CNC machinery where the axes of the machine may have been cam driven and the motion limited.
formTool.jpg

The CNC gives us great flexibilty.
From barstock to the profile of our knob using a $12 stock tool bit.
knob_op1.JPG
 
Anodizing

I visited our anodizing house today and spoke with the production manager. She gave me a bit of an edjumicashun.

As I previously showed in the photo of the darkened piece, ours is indeed a hard coat anodizing and cannot be made to a bright finish. They have no other lines in their plant that can do bright. The process is bit different.

The good news is she was able to give me contact information for a couple of local places that can do bright. :D

A very informative four minute video about the anodizing process.
https://www.youtube.com/watch?v=3ZhVOy-ytJY

For those who may be curious about hard coat anodizing, a two minute video.
https://www.youtube.com/watch?v=qcA7oE8yMvc

And I can tell you that it is very hard stuff. I have had to rework parts and trying to cut through the coating dulls tungsten carbide straight away.
 
At this stage in the creation of my prototype, just a little more background as to how I got to this point in terms of the machine control. CNC machines are controlled by specialized logic controllers. Commonplace in the industry is a machine language known as G-code.

A typical method for producing a G-code program would be to use CAD-CAM software. (computer aided design-computer aided manufacturing)

The part dimensions would be used to create a two or three dimentional model in the software. The manufacturing engineer would use that model to apply different tool paths and a simulation can verify that the toolpaths look correct. The modeled tool paths can then be run through a post-processor which is the software that generates the G-code the machine needs to see.

In the case of our knob, I took a different tact. Being a bit of an old schooler myself, most of the time I don't need any fancy simulations or modeling. In the case of this knob, I scribbled my dimensions on a piece of scrap paper, did a few calculations, and opened up my text editor. I typed the G-code into the text editor directly as I have been looking a code for so long that it is work-a-day for me. In terms of geometry, this isnt a complicated part and the lathe only requires programming of two axes. No big deal. If it were some complicated geometry, then I'd turn to the cad-cam.

The lathe that I have pictured earlier is actually a hybrid. It is known as a 'live tool' lathe. That means that I have the capability of not only spinning the part, but the tool as well. It is a lathe that I can do milling with!

After the flange in the picture was turned, the drill came to bear and was used to put the eight hole pattern in. The chuck which normally just spins, now becomes an indexer and can precisesly turn the part to where I want it and then stop. I drill a hole and then index the chuck to the next hole position.

It doesn't take much to turn that scenario into one where I'm chucking the knob and indexing the spindle to each flute position and bringing a cutter to bear to cut each flute. :thmbsp:
 
Workholding

It is often the case in machine shop that we need to machine something before we machine our workpiece. That 'something' is usually associated with workholding. You've got to be able to hang on to your part before you can machine it. Holding it properly is half the battle.

Now in order to machine the back side of our knob, the last thing I want to do is clamp it in any way that would marr the surface that's going to be presented on our showpiece.

The solution: a split bushing made from delrin (plastic).

Delrin is chucked in a conventional lathe (no CNC) which is manually controlled. A drill bit is driven though the center of the spinning part, roughing out the center.
bushing1.jpg

A boring bar takes out the remaining material. I match the size of the hole to the diameter of the knob body.
bushing2.jpg

Moving over to the conventional mill, I machine two flats 180 degrees appart.
bushing3.jpg

I darken the center with a sharpie enabling me to better see the scribe line I made down the centerline. I then cut it in half on the bandsaw.
bushing4.jpg

An internal groove (that was also machined on the conventional lathe) serves as clearance for the raised rib that will become the flutes.
bushing5.jpg
 
I did basic woodworking and metal working back in high school and have really only used nothing more complicated than an electric drill since.

Thanks Daddy for this.
 
Fascinating. I want to see how the flutes work out. Indeed the chuck would have to turn in extremely precise rotational distances in order for the fluting to come out completely even.

Thanks for the demo!
 
Back
Top Bottom