Quickly tapping (threading) holes in plastic

For my day job, I often have to tap (cut threads into) holes in plastic that will receive screws. This can be done by hand with a standard tap wrench, but it's also possible to do with a hand drill. I bought a small 3/8" square drive adapter that has a 1/4" hex shank. This was intended for use in cordless screwdrivers which accept hex shanks, but a drill chuck will grab the hexagon just fine. I also bought a set of adjustable tap chucks that have a 3/8" square drive hole.
The smaller chuck goes from #2 taps up to about #10, and the larger for #10 up to 3/8".

For plastic, I use lots of distilled water as coolant. If I am tapping a blind hole, I usually just fill it up with water. The nice thing about using distilled water is that it leaves no residue after being blown clean with compressed air.

Tapping a hole with the hand drill works best for a certain range of sizes:

In ABS, I would use the drill for any size #2 through 1/4"

In acrylic or Delrin, I would probably only use the drill for #6 through 1/4"

The really small taps sometimes clog up, even with coolant, and the drill doesn't provide enough sensitivity to avoid snapping the tap. ABS is so soft, a clog would just be pushed away, whereas in acrylic or Delrin, it might jam.

For sizes larger than 1/4", I would probably use a tap wrench because they take a lot of torque, and it's difficult to keep the drill straight and have it deliver enough torque to spin the tap.

I like using the chrome-plated taps because the finish will not rust (a concern since I use water as a coolant), and the chrome is supposed to have a lower friction coefficient than bare high-speed steel.

For sizes smaller than 1/4", I would only use a bottoming tap in plastic. There is no reason to use a plug or taper tap because the bottoming tap makes a cleaner cut, and actually produces less friction and heat than plug or taper taps. In harder materials it makes sense to cut gradually, but plastic is so soft, it will just melt if the tap keeps making light passes that rub against the surface.

Google Checkout is making PayPal look good

This post is definitely off-topic for my blog, but I am extremely disappointed with Google Checkout's Merchant services, and I want to publicize the difficulty.

I have a customer that placed an order on my website and paid with Google Checkout. Her credit card was listed as "declined", so she called her credit card company to find out why. The credit card company responded that they had no record of the Google Checkout transaction at all, and there were no declined transactions on the account.

Of course, Google Checkout has no support phone number. Apparently they recently removed their support email address and online support contact form.

UPDATE: I found the "contact us" link at the bottom of the page, which allows merchants to enter an order number and request assistance, but there is no text entry box to actually describe the exact problem.



So now I am stuck with a very frustrated customer and perhaps a lost order. Worse, I have no way of fixing the situation because Google Checkout has no way of being contacted with details about the problem. Thanks, Google.


Here's some links to others who have had problems all stemming from the complete lack of merchant support.

http://www.shaftek.org/blog/2009/03/04/contact-phone-number-for-google-checkout/

http://www.google.com/support/forum/p/checkout-merchants/thread?tid=1c4ddc2e27f4ec72&hl=en

http://news.zdnet.com/2100-9595_22-341375.html

http://www.google.com/support/forum/p/checkout-merchants/thread?tid=67e6e5c02c6f3233&hl=en

Telescope magnification

A friend recently asked me about telescopes, and it made me think about the pictures that I took with my telescope which I haven't looked at in years. This series of photos was a test of the range of magnifications possible with a couple camera lenses and the telescope.

These photos were shot with a Nikon FE2, with some run-of-the-mill 400ASA color film. The photos were developed and printed 4x6 at a 1-hour photo place. I scanned the photos with a flatbed scanner.

50mm Nikon lens. This is essentially the field of view that normal human vision has.

135mm Vivitar lens.

2032mm Celestron Nextar 8" SCT telescope. Prime focus. The ceramic insulator is just visible in the lower right corner.

The same telescope using eyepiece projection -- I forget the focal length of the eyepiece. The white halo in the center of the image is an artifact of my shoddily-mounted eyepiece setup.

A shorter focal length eyepiece increases the magnification even more. This is probably a 6mm eyepiece, giving 2032/6 = 338x magnification. The photo is blurry, but this is mainly due to tiny vibrations in the 'scope and camera. When viewing this directly through the eyepiece, the image is pretty sharp since human eyes and brains have better image processing than cameras.

Stainless steel conical beer fermenter Pt.3

I just finished welding together the stainles racking cane for the beer fermenter project.

Even though this may not look like much, I am quite proud. It is a 5/8" dia stainless tube, but the wall thickness is only .020". I sliced it on my new metal band saw, reoriented the pieces to make an elbow, then tacked it in two places, and welded all the way around. It's air-tight. The trick is to get the tacks done really quickly. I only used filler (.035") to make the tacks, then just fusion welded it in very short sections (a few seconds at a time). I blew through it once, and had to repair it with filler.

Here's the tube welded to a tri-clamp plate, which is welded to one side of a three-piece ball valve.

Here's the other side of the ball valve welded to a hose barb.

The complete assembly is attached to the tri-clamp port on the side of the tank. The purpose of all this hardware is to be able to rotate the racking cane while draining beer from the tank. Thus, the height at which the beer is drawn can be adjusted. This allows the maximum amount of clear beer to be drawn from the tank without getting any cloudy beer that has settled to the bottom.

Inside the tank.

Stainless steel conical beer fermenter Pt.2

The next step in the beer fermenter project is to mount some copper blocks to the outside of the stainless tank. The purpose of these blocks will be to thermally couple two peltier devices to the surface of the tank. The tank is cylindrical (near the top) and the peltiers are flat, so the copper blocks must be curved on one face, and flat on the other. I started by cutting off some chunks of copper and milling the edges square.



Next, I used a long end mill to profile the sides with a radius that matches the outside of the stainless tank. My plan was to silver solder the copper blocks to the stainless tank exterior. This turned out to be a very bad idea. I started by fluxing the copper and stainless, then separately covering them with a thin layer of silver solder. No problem yet. I put the semi-cooled block onto the tank, and planned to heat the tank and block and let the two solder-covered surfaces melt together. This started working, but then the stainless expanded dramatically under the block. The copper was lifted a clear 1/4" off the surface near the edges, while the center was making contact. I removed the heat, and looked inside the tank to find this new disaster:

A monster crack had developed in the wall of the tank! I am still not sure why this happened. It obviously has something to do with the metals' differing rates of expansion, but I had no idea the consequences could be so damaging. Perhaps this has something to do with the stresses in the metal from the spinning (cone-forming) operation?

Luckily it wasn't too difficult to repair the crack. Since the crack went clear through the tank wall, I had to fight the silver solder which was molten and trying to flow into the weld puddle. The copper blocks must be making good thermal contact, since I maxed my TIG machine out at 200A, and it was just enough to comfortably weld.

There are two copper blocks (each made of two pieces of bar). In order to avoid the silver-soldering nightmare again, I used silver epoxy to join the copper to the tank. After both blocks were attached, I mounted the assembly in the milling machine, and flattened the faces.

If I were going to do this again, I would profile the copper backside, flatten the face, then use silver epoxy to attach it to the tank.

UPDATE: This method did NOT work. I was not able to hold the glass without rocking it very slightly. This created a very smooth, but curved surface on the copper. It was unsuitable to mount the peltier.

Here, I am removing the milling marks with sandpaper mounted on a 1/4" thick glass plate. This took a very long time. In fact, I am still "going up through the grits" right now. The next step will be to weld a small threaded boss on either side of the copper blocks. This will serve as an attachment point for the peltier heatsink. The peltier itself will be pinched between the copper and the heatsink.