WTF pictures of amps!

[Captain Kilowatt]

I agree completely. Nothing beats a blind hole done properly with a bottoming tap. but I think you might agree that self tapping machine screws are better than sheet metal screws.

Oh jeesh....no contest on that one. Sheet metal screws may work OK in wood but not when securing devices to a heat sink. If you are not real careful even self tapping screws will cause a ridge around the hole that will prevent the device from sitting flat. Use of a countersink will help greatly with the self tapping but a sheet metal screw is in a league of it's own.

 

[Captain Kilowatt]

An easy way to tap a hole square to the mounting surface is to make up a little guide. Take a piece of material in a drill press or mill and drill a thru hole just large enough for the tap to slide thru. Set your guide on the surface to be tapped and line up your through hole to the one being tapped. When you start spinning the tap into the threaded hole the guide will help you stay square.

The guide can be steel, aluminum etc, but don't hand drill it. Put it in a machine so it's square. The shape or size isn't critical, but rectangular and large enough to hold solidly works well.

Also back your tap off every 1/2 turn to break the chips.

That is something that probably most people do not do.That was instilled in me a loooong time ago. I usually reverse the tap about every full turn or so just enough to break the chips off. It is especially important to do in aluminum as the chips can really mess up the threads. At work we deal with a LOT of machine screws 99% of which are 5/16-18 tpi. stainless. We end up drilling out broken screws and tapping the holes quite often. Most of the tapping is in brass, stainless steel, or cast iron. Sometimes we end up installing helicoils to get the job done and then fill/redrill the holes later. At home I have a tap and die set with both standard and metric threads as well as a complete set of machinists drill bits consisting of fractional sizes, decimal sizes, and even lettered drill sizes. I keep a sheet of info with the tap and die set that has the recommended drill size to use for any given tap. It lists the size hole to drill in aluminum/brass/plastics and stainless steel/iron. you should not use the same size drill for tapping plastic as you would steel. Here is a handy chart similar to the one I have.

http://www.physics.ncsu.edu/pearl/Tap_Drill_Chart.html

 

[GnG8d]

That is something that probably most people do not do.That was instilled in me a loooong time ago. I usually reverse the tap about every full turn or so just enough to break the chips off. It is especially important to do in aluminum as the chips can really mess up the threads. At work we deal with a LOT of machine screws 99% of which are 5/16-18 tpi. stainless. We end up drilling out broken screws and tapping the holes quite often. Most of the tapping is in brass, stainless steel, or cast iron. Sometimes we end up installing helicoils to get the job done and then fill/redrill the holes later. At home I have a tap and die set with both standard and metric threads as well as a complete set of machinists drill bits consisting of fractional sizes, decimal sizes, and even lettered drill sizes. I keep a sheet of info with the tap and die set that has the recommended drill size to use for any given tap. It lists the size hole to drill in aluminum/brass/plastics and stainless steel/iron. you should not use the same size drill for tapping plastic as you would steel. Here is a handy chart similar to the one I have.

http://www.physics.ncsu.edu/pearl/Tap_Drill_Chart.html

Why replace helicoils? In some materials they are stronger than cut threads ...... properly installed of course.

 

[Captain Kilowatt]

Why replace helicoils? In some materials they are stronger than cut threads ...... properly installed of course.

Because of the demands of the application. The screws are installed with battery impact drivers and removed with air impact guns. Occasionally the helicoils will seize the stainless screw and be destroyed when removing the screw. The brass we use is in a constant immersion in chemicals and the holes get eaten away over the years causing old helicoils to loosen as well. When this happens the dies are removed from the machines and placed on the milling machine to have the holes drilled out and plugged with a threaded brass plug and then redrilled and tapped to spec.Helicoils are great for temporary repairs or in low use situations but they just do not stand up to our demanding applications forever. We do have some applications for 10-32 where the screws are repeatedly loosened and tightened when a new setup is being done on a machine. These parts are aluminum and we use 10-32 helicoils in the holes simply to improve the wear resistance as aluminum would strip out in short order.

 

[GnG8d]

Because of the demands of the application. The screws are installed with battery impact drivers and removed with air impact guns. Occasionally the helicoils will seize the stainless screw and be destroyed when removing the screw. The brass we use is in a constant immersion in chemicals and the holes get eaten away over the years causing old helicoils to loosen as well. When this happens the dies are removed from the machines and placed on the milling machine to have the holes drilled out and plugged with a threaded brass plug and then redrilled and tapped to spec.Helicoils are great for temporary repairs or in low use situations but they just do not stand up to our demanding applications forever. We do have some applications for 10-32 where the screws are repeatedly loosened and tightened when a new setup is being done on a machine. These parts are aluminum and we use 10-32 helicoils in the holes simply to improve the wear resistance as aluminum would strip out in short order.

I wonder if "keenserts" or other threaded inserts might work? Oh well, my 20+ years job shop machining/project engineering is coming out. There I said it, lol. Replaceable thread inserts would be awesome here I think.

 

[Captain Kilowatt]

I guess we used to use them YEARS ago as occasionally we will take out a screw and the insert comes out with it leaving a large gaping hole that cannot be filled with a helicoil. I can only assume the switch was made to helicoils because of ease and speed. Even a monkey can install one in a minute however there is never a monkey around when you need one. We actually are trying keenserts in some applications but they have not been working out that great. If/when they come out it is harder to replace them on the fly with a machine down $$$. usually then we just replace the die and take it to the shop for proper repair.

 

[Tallman]

An easy way to tap a hole square to the mounting surface is to make up a little guide. Take a piece of material in a drill press or mill and drill a thru hole just large enough for the tap to slide thru.

You are referring to a bench block. I have done a lot of design machine work on heatsinks for new products using thermoelectric devices. (Solid State Refrigeration)
The tapping had to be square and no rise from tapping could be allowed. So that means you lay out all of the holes to be drilled and tapped then you milled after the tapping the entire mounting surface afterwards. That was scary some times with a 14" fly cutter spinning around at 2900 rpm and metal chips flying all over the place. the aluminum looked like a cd with the prism effect.