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​Metal Spinning How to, Part 4 -- Trim Tool

Posted by Earl Powell, edited by Lydia Harry on

Metal Spinning How to, Part 4 -- Trim Tool

In part 3, I described the metal spinning tool. Now I will describe and explain the importance of the trim tool. There are three basic types of tools used in metal spinning. The first and most important is the spinning tool, however it would be difficult to execute a metal spinning without the second type, the trim tool. The third type of tool would be the beading tool. The beading tool is not required for spinning but is used to perform a post spinning operation, namely putting a bead on the spinning. Post 5 in this series will cover the beading tool.

The trim tool has two major functions. The first is to trim off any runout or eccentricity that develops at the perimeter of the metal spinning. When trimming off this runout or eccentricity the part is often cut down to size at the final trimming. The second important function of the trim tool is to cut out the end of the metal spinning as required to secure the design intent. Sometime this needs to be done just to get the metal spinning off the tool, (mandrel).

Looking first at trimming off the runout or eccentricity one might ask why is there eccentricity? The raw material for metal spinning is sheet metal, more precisely metal circles. However, these metals circles may be precisely stamped out yielding a nearly perfect circle with a pristine edge 

or they could be cut out by scribing a circle of the appropriate size and then cutting out the circle with aviation snips. While I have cut circles for many years using aviation snips and I am pretty good at doing it, the edges are far from perfect. 

So if you cut the circle by hand you most likely need to trim it. Further, these circles are made of sheet metal. Sheet metal is a rolled product and therefore has a grain. Because of the rolling operations used to bring the sheet down to the appropriate thickness and grain, the circle will not stretch symmetrically. While in “conventional” metal spinning the goal is to achieve a consistent cross section thickness throughout the part, even the most experienced metal spinner will have some stretching. As the spinning process proceeds you will generally see more and more runout or eccentricity. Beginning metal spinners are likely to be stretching the part and see rather severe runout. This runout needs to be trimmed off before the runout gets too bad. You also want to watch for cracks that could be starting at the perimeter of the circle. This is more likely to happen on hand cut circles. You must also trim off these cracks because they will simply propagate and get much worse requiring you to abandon the spinning because it would be far to dangerous to proceed. Obviously you would want a nice clean concentric edge on your part if for no other reason than aesthetic; it should look nice. If you are going to perform a post spinning operation such as beading of the assembly, these eccentricities must be trimmed out.

The trim tool is also used to remove the end from the spinning, as in the example of spinning a jelly jar funnel as shown in the image.

In general, you remove the end or a portion of the end, as done with the jelly jar funnel, or just so that the part can be removed from the spinning mandrel as shown in the example of the venturi.

Now that I have convinced that you need a trim tool, I will describe the trim tool. The trim tool is similar to the spinning tool in that it has the steel rod, handle and ferrule.

Rather than having a specially prepared end used for spinning, the trim tool steel rod has a hole and set screw used to secure tool bits. 

The tool bits may be high speed steel (HSS) or Carbide. In general high speed steel is preferred for nonferrous metal such as aluminium, pewter, copper, brass, bronze, and silver. However, this requires that sharpening equipment and/or the expertise for sharpening needs to be available. A dull and/or incorrectly sharpened HSS tool bit is more dangerous than a carbide tool. I use carbide tool bits almost exclusively. These carbide tool bits come in several configurations, last a long time, and are inexpensive. I use the 5/16” BL-5 C2 carbide tool, and these are included when you purchase the trim tool.

The trim tool is similar enough to the metal spinning tool that I am not going to go over how the tool is made in the detail presented for the spinning tool. The handle is shorter and the tool bit is inserted into the tip of the tool and secured with a set screw.

Once again, while you could fabricate the trim tool yourself, may I suggest that purchasing one for $122.99 would be your best option.

In the next post I will describe the metal spinning beading tool.

This is part 4 in our series of Metal Spinning How To posts. If you missed posts 1, 2, or 3, I have provided a link below:

Metal Spinning How To Part 1-- Lever and Fulcrum

Metal Spinning How To Part 2 -- Tool Rest

Metal Spinning How To Part 3 -- Spinning Tool

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