Chris wrote this (UNFINISHED) article on Dec 30, 2013
So I have a small machine shop. Well sort of, the fact that I have a Southbend 13″ by 5′ tool room lathe and a Bridgeport knee mill sort of implies that it is a bit bigger than “small”. When comparing to the people I follow on YouTube such as “Mr Pete/Tubalcain”, “Doubleboost”, Keith Finner(sp), Adam, Oxtools and Tom’s Techniques it feels small.
As I started down this path I had already read many books on how to use the lathe and it seems like a simple enough machine. I’ve read a few more books and watched a boat load of good videos on machining. What I’ve learned is that the machine is cheap, it is all the stuff you have to get to go with it that gets expensive.
In the shop there are a number of different general operations that take place:
- Metal Removal
- Metal Addition
- Metal Forming
- Metal Casting
Metal Removal
Metal removal is the process of removing unwanted metal. There was a joke attributed to some famous sculptor “How do you make such beautiful statues? Well I just remove all the marble that shouldn’t be there.”
Metal removal general starts with a casting or a piece of stock and removes unwanted metal until you get what you want. The goal is to remove the least amount of metal as possible in the shortest period of time safely.
The Milling Machine
Take one very simple thing. Holding something on a milling table. You’ve got this great big table, it has T slots, it has the ability to move a 0.0005 in any direction with the turning of a knob. And you have no way of holding anything to the table.
Enter the Milling Machine Problem. A milling machine is simple compared to a lathe. You attach the work to the table, you set the speed, you insert the tool and then you move the table around until you get the result you want. Nothing real hard about it. That is until you realize that attaching work to the table IS the big issue for milling.
So you get your first holding pieces. This consists of “packing”, “straps”, “bolts and nuts.” The packing should be just a little taller than the work piece. The bolt should be closer to the work piece than the packing. By tightening the nut you squeeze the work against the table. Now you have the work attached to the table.
Except that the work is likely not aligned with the table. So before you can actually attach the work to the table you have to make sure that the work is “perfectly” in line with the axis of motion of the table.
Well, this goes on and on and on with figuring out how to attach and align things. (The quick answer is that you attach something to the table with a small space, such as from an index card, between the item and the table. Then you mill that thing by moving the table in only one axis. This gives you something that is perfectly aligned with the table. From there you attach work to the table and tap it so it is hard against the milled surface, thus aligning the work with the table.)
It turns out that many operations can be done with just a simple vise. If you attach a vise to a milling table then you can attach the work to the vise (close the jaws TIGHT) and be ready to work.
Well, to do this you need to put the vise on the table then to adjust it so that it is exactly aligned with the table movement. This requires that the milling vise have a jaw that is flat. The jaw must be square to the bottom of the jaw mouth. The jaw mouth must be parallel to the bottom of the vise and the bottom of the vise must be flat so it can rest flat on the milling table.
You can pay anywhere from $300 to well over $800 for a six inch milling vise. But things don’t stop there. In order to get the work above the jaws the work has to either be tall enough to begin with or you have to put some sort of packing under the work to lift it up. This is done with parallels. Well parallels come in an amazing array of different types. There are “wavy” parallels that are designed to fit under very narrow items and which can be squeeze a little and still maintain their lift. There are narrow parallels (0.125″) as well as 0.250 and 0.500″ parallels.
You need an indicator holder that can be held in the spindle so that as you move the table you can see if the vise is aligned with the table movement. You need hammers to pound the work down to make sure it is sitting flat on the parallel. You need a stop so that when you put something back in the vise it ends up in exactly the same place again. You need V-Blocks which can come as matched pairs and are precision ground/machined which are used to hold round things. And you still need other hold down devices for when the vise can’t be used.
So at this point you’ve added a few thousand dollars worth of stuff to hold things down and to measure/indicate things. (Measure is to figure out exactly how big something is. This is done via micrometers, calipers, scales and other measurement tools. Indicating is to compare to readings in some way, for example knowing that this end of the vise jaw is 0.001″ closer to the spindle than the other end.)
Now that you have everything held in place you need a way to cut things. This is done via tooling. For a milling machine this is via end mills, fly cutter, face mills, drills, reamers, boring heads and bars, round overs, slotting saws, T slot cutters, woodruff key cutters and other things I don’t know about.
End mills are multi-point cutters, the are relatively small in size, less than 0.750 inches. They can be center cutting so they can plunge straight into the work or they can only cut from the side. They can have square corners or they can have rounded corners to live a fillet. They can be used to cut a grove or widen one. They can be used to completely surface the top of the work but it takes more passes with an end mill than a fly cutter.
A fly cutter is a single point cutter. A cutting tool is inserted into the fly cutter and locked in place. The cutter spins and exactly one point of the cutter touches the work. Fly cutters come in many different sizes from 0.750 to over 2.000″ in cutting diameter. They are designed to cut a small amount off the top of the work to make the top flat (and parallel to the table which should be parallel to the bottom of the vise jaw which should then make the work parallel to the bottom side if that bottom side is already milled flat)
Then there are face mills. These are big honking things that have multiple cutting points and are designed to do the same job as a fly cutter.
So now we know that we need measuring and indicating tools, work holding devices, and cutting tools. But some how we have have to be able to connect those cutting tools to the spindle of the milling machine. This is done via a tool holder. The type of tool holder I use is called an R-8. This gizmo is an arbor that is inserted into the spindle. First it is cleaned of all oil or other contaminants. This is then inserted into the spindle (which should also be wiped down). A grove in the side of the tool engages a pin inside the spindle. This pin is there just to hold the arbor in place as it is pulled into the spindle. At the top of the arbor is a threaded hole. A long bar which has a hex shape on the top passes through the spindle and engages the threads in the arbor. By tightening the draw bar the arbor is pulled up into the spindle. When firmly seated the friction between the spindle and the arbor is high enough to keep the arbor from spinning in the spindle as it cuts the work.
At the other end of the R-8 spindle can be many things. The most common are end mill holders, collets, drill chuck arbors, arbors for boring heads and arbors for saws and shell mills. A end mill holder has a precision hole in it which an end mill can be inserted. A set screw is tightened to hold the end mill in place. A collet has a hole in the very end as well, as the collet is pulled up into the spindle it squeeze the hole closed. This means that it is very easy to change tooling which fits the collet. Collets come in many nominal sizes which means that most standard sized round can be held very securely in them.
So now we have measuring, indicating, work holding, cutting tools, work positioning, and tool holders. To that we need to add some optional but very helpful things.
The first is cooling. As metal is removed by a cutter both the work piece and the cutter heat up. To keep this under control the tool and or work needs to be kept cooled. In the simplest way this is simply a squirt bottle or sprayer that allows you to put cutting fluids where they are needed. A bit more extensive is using a misting system finally there is flood cooling. Flood cooling is where the coolant is poured in a continues stream, often via re-circulating pumps. The biggest problem with flood cooling is that the spinning tools will throw the coolant everywhere. Mist cooling works a bit better but requires compressed air. Using spray bottles works well to begin with.
After this is the power feed. Most milling is performed via a back and forth motion of the table. Each full rotation of the crank moves the table 0.200″ which means that to move the table 6 inches requires 30 full revolutions. Even when the table is perfectly adjusted moving the table back and forth a few dozen times can get tiring. At the same time if the speed at which the crank is turned changes then the surface might not have as nice of a a finish as wanted.
A power feed allows for controlled feed rate which is often very desirable as well as reducing the amount of effort it takes to complete a full operation. (It is often the case that the cut is made at a slow feed rate but the movement of the table back to the start point is done at a much higher speed). My machine has what is known as a “Mechanical feed” which has some parts which are damaged and should be replaced. Unfortunately the part is no longer made. Bridgeport doesn’t even acknowledge that the unit ever really exists. Fortunately, anything on a Bridgeport that is not a casting can be made on a lathe plus mill.
The last piece of tooling for a mill is a DRO or Digital Read Out. This consists of a pickup head attached to the stationary part and a “scale” attached to the moving part. Thus for a 3 axis system there would be a scale attached to the table, the saddle and the knee. The pickups would be attached to the saddle (to read the table location), the knee ( to read the saddle location) and the base/machine (to read the knee location).
The nice thing about a DRO is that it eliminates the problem of dealing with “backlash”.
Backlash is the “space” between gears and threads that exist in order to allow them to move freely. You can see this by putting a nut on a bolt. You should be able to wiggle the nut up and down a little bit as long as it is not snugged up to the end of the treads or tight against something. This wiggle is backlash. It can easily be a 10 thousands or more.
Since the DRO reads exactly where the table is rather than how much the feed screws are rotated you can tell exactly where the spindle is relative to the table. In addition a DRO is easier to read (big numbers on a display rather than small hash marks on a dial) and will often have special features to help with common operations. (Bolt patterns, center of two points, automatic offsets)
Power feed units run a around $700 but might require a new lead screw which is a few hundred more.
Given a day to think about it another “tool” shows up. A rotary table. A rotary table is a table that clamps to the main mill table. It can be turned a controlled amount. Using a rotary table a circular path can be milled or a series of holes can be placed in a circle. Drill a hole, rotate the table exactly 60 degrees, drill another hole. Six holes later you have a “perfect” 6 hole pattern.
In addition a rotary table that can be placed vertically can be used to mill the sides of something or the top of something. This would allow you to make gears or to make a hex at the end of a round bar. All in all a very useful, if expensive, extra.
The lathe
I’ve been saying for more than a few months that the lathe is the most complex machine that I’ve ever used. Just getting ready to use it takes time. There are something on the order of 25-30 different points that should be oiled before starting work, not counting the ways. There are 30+ different controls.
- power switch, fwd, rev, off
- belt tension lever
- step belts
- Bull gear engagement pin
- back gear lever
- power feed screw control: fwd, rev, neutral
- high/low range feed screw control
- high/medium/low feed screw control
- tumbler control
- carriage stop position clamp
- carriage stop adjustment
- carriage motion control
- cross feed motion control
- compound slide motion control
- compound slide rotation
- power feed clutch
- power feed selector, carriage, cross feed, neutral
- split nut control
- threading dial positioning
- Tool post rotation
- Tool holder height adjustment
- Tool holder clamp
- taper attachment engagement clamp
- taper attachment taper selector
- tail stock clamp
- tail stock positioning
- tail stock quill control
- tail stock quill lock
- cross feed graduation dial setting
- cross feed graduation dial lock
- compound slide feed graduation dial setting
- compound slide feed graduation dial lock
- follow rest adjustments * 2
- steady rest attachment and clamp
- steady rest position * 3
- Chuck
- jaw selection (steps pointing in or steps pointing out)
- scroll control to open or close jaws OR 4 worm gears to close 4 jaw checks independently
- chuck selection
- clamping to face plate
- collet controls
- follow rest attachment
In addition to all of the above is tool selection. We select tools for both the tail stock (drill chuck with the right bits, dead center, live center, bull nose center, half center, specialized center) and the tool post (cut off, turning tools (100s of choices), facing tools (100s of choices), boring bars/tools (100s of choices), threading tools (internal, external, left/right handed, forming or not), indicator holders)
Well I think you get the idea… There are lots and lots of controls and choices for doing lathe work. Part of that is because you can do so much on a lathe. Not only can you do turning but with the right attachments you can also do some milling work.
The Bench Grinder
It turns out that many things need tools ground to the right shape. The bench grinder is used to shape High Speed Steel (HSS) to the right shape to cut. These cutters are used in the lathe, in boring bars, in fly cutters. In addition it is sometimes best to use the bench grinder to just grind a little bit more off of something to make it fit.
The Horizontal Bandsaw
It turns out that many many times you need to work with stock that is just about the correct size. While small things can be fed through the lathe spindle larger things can not. In addition a long rod in the spindle tends to bounce around. The benefit is that you waste less because you can extend the rod out to working distance, do your work, cut off the piece, then advance the rod to repeat the operation. This is quick, easy and wastes almost no stock.
If the stock is to big to fit through the spindle or just to long then it will need to be cut. The horizontal bandsaw is the best tool for this operation. A medium capacity saw will take stock up to 6×6 inches. The one I happen to have takes stock up to 9×6.
While it might take a while to make it all the way through such a large piece of metal the bandsaw will eat its way through. The biggest problem is when the piece is wide there is a huge surface area that the band moves across. This makes it very difficult for the blade to cut.
The Vertical Bandsaw(missing)
While the horizontal bandsaw is the right thing for cutting bar stalk for cutting plate it often just can’t and when it can it is very slow and clumsy. The vertical bandsaw can be used for cutting bar stock (bar stock is anything that is nominally small in cross section compared to its length. Nominal because a piece of 8″ round bar stock 6″ long is still considered bar stock, even though it has a larger cross section than length)
A good vertical band saw will cut 1″ thick steel plate at a reasonable rate. It goes much faster with AL and thinner stock. The down side is that it normally will have a limited throat allowing a maximum length of 14-17″. This is not normally an issue.
In perspective the horizontal bandsaw is normally used to cut stock for use in the lathe and the vertical for cutting stock to use on the mill.
The Shaper(missing)
This is a device that has a moving head with a single point tool. The shaper head moves straight in and out. On the back stroke the head is lifted slightly so it doesn’t touch the work. At the end of the back stroke the work is advanced slightly to the side. The head moves forward with the tool down and cut a single sliver off. This is used to make things “flat”.
The Shear(missing)
This device is used to cut a piece of metal. It can be used to cut bar stock or sheet stock. As long as the piece fits in the shear and the shear has the capacity to cut it then the piece can be sheared.
In general the bigger the shear the thicker (and stronger) the material can be. Thus a small bench mounted shear might only be able to cut through 22″ of 20 gauge mild steel while a “ironworker” with a 50 ton hydraulic press might be able to shear mild steel up to an 1/4″ thick and 40 inches long.
Steel mills use shears that can cut huge pieces of metal to size, often these shears operate on red hot metal (metal in the plastic phase)
Cutoff Wheels/Angle Grinder
A cutoff wheel removes metal via a grinding method. A thin disk is spun at high speed and it grinds through metal. Depending on the size and shape this can be fast or slow. It is possible to cut through sheets and plates using this method if a bandsaw is not available. In addition an angle grinder is very portable so can be used in places where a bandsaw can not.
Metal Addition
Unlike wood working in metal working you can add metal. This is done with welding which creates a molten puddle of metal to which more material is added. You can also use a method of spraying molten metal onto a piece of heated stock to build up missing parts. Welding can also be used to connect two or more pieces of metal together.
Metal can also be fitted together so tightly that the become like one item.
Metal Forming
This is the process of deforming or bending metal. It can be applied to bar stock or to sheet metal. It can be done cold or hot. A blacksmith does an extensive amount of metal forming. By heating the metal to a plastic state and then hitting it the blacksmith can make the metal flow to different places.
Metal can be cold worked as well. By hitting sheet metal with hammers with curved faces on top of wooden blocks with cups different shaped curves can be created. Metal can also be rolled to create bends or run through rollers to make long straight curves.
Metal forming can bend, stretch, compress or create complex forms.
Metal can also be formed by pressing between two die.
Metal Casting
Where as in wood working you start with a tree and then remove wood till you get what you want, in metal you can liquefy the metal and then pour it in molten form into a mold. Once the metal is cooled it is removed from the mold and then can be worked via other methods into a final result.
The most common example of this is a cast iron pan. With this a mold is made and molten iron/steel poured in. After the metal is cooled the excess metal is removed and you have a pan ready to use.
More complex objects are often cast in pieces, the pieces are then machined and the machined pieces connected together. A common example of this is the block from a car engine.
Metal casting is a multi-step process. In general it consists of making a pattern. Making a mold of that pattern. Potentially removing the pattern from the mold. Melting the metal. Pouring the metal into the mold. Then extracting the finished product from the mold and finally removing the excess metal.
There are three major types of pattern/molds:
- Reusable pattern
- Lost pattern
- Reusable mold
With a reusable pattern a frame is placed over/around the pattern. A special mixture of clay, sand and water is packed around the pattern. The frame is flipped over and another frame carefully placed on top of the first frame. More sand is packed (rammed) into the frame. Holes are cut to allow molten metal to reach the pattern and vent holes added to make sure that any gases generated during the pour can escape. Finally the two frames are opened up and the pattern removed for use later. The two forms are put back together in preparation for the pour.
With a lost pattern a pattern is constructed and then put into a frame. The sand mixture is packed around the pattern. The holes are cut for the metal to be poured into. When this is done either the pattern is cooked out of the mold or it is left in place to be destroyed by the molten metal.
Lost wax is using a pattern made from wax. The mold is cooked and wax melts and is poured out the fill holes. When the molten metal is added the remaining wax is burned off.
Lost foam patterns are just left in place. When the molten metal hits the foam the foam is destroyed/burned off. You can see this process by applying a flame to Styrofoam. The foam, as it melts, takes up much much less space and the small amount of material left behind is rapidly destroyed/burned off by the molten metal.
Reusable molds are designed so that they can survive molten metal being poured directly into them. The two (or more) parts of the mold are connected together. The molten metal is poured in. When the molten metal is removed the molds can be quickly cleaned and reused. This method could be used for example to cast aluminum items in cast iron molds.
In order to cast an item a mold has to be constructed. The type of mold that I’m interested in is called “green sand” which means it consists of a frame, called a flask, which is packed (rammed) with a sand, clay, oil/water mixture. The sand used is normally very fine and when it is properly mixed you can take a clump of it squeeze it in the palm of your hand and then see your palm print in the clump. The clump will hold this shape and when stressed will break cleanly rather than crumple into pieces.
Most flasks are two pieces but there are flasks that have more than just two pieces. A very simple pattern, one with a flat surface, is placed on the molding board. Parting dust is distributed over the pattern. The bottom flask, called the drag, is placed over the pattern. The green sand is then passed through a sieve into the drag until the pattern is partially covered. This removes any big clumps are left over bits of metal from the last pour.
Once there is sand in the flask a ram is used to tamp down on the sand, compressing it. More sand is added and rammed up. A big flask will have ridges around the inside to help grip the rammed up sand. Filled flasks can be very very heavy so it is important to use the right sized flask. Once the drag is filled to the top a straight edge is run across the top of the flask “striking off” the sand that extends over the top edge. This gives a flat surface.
The drag is then flipped over, a parting dust is sprinkled over the entire exposed drag top. This is the green sand and the pattern. The top of the flask, called a cope, is then placed over the drag. There are normally aligning pins to make this happen in a repeatable way.
If it is a simple pattern then sand is sieved over the pattern, rammed up, then more sand is added until the cope is full. The cope is then struck off. A sprue cutter is then inserted to cut a hole down to the parting line (the line between the cope and drag in the sand). A small well can be formed with a spoon or knife to make a funnel.
If the pattern is big enough a riser will be added as well. This is just like a sprue except it will be filled from the bottom instead of the top. I.e. molten metal is poured into the sprue, fills the space where the pattern exists and then continues filling the cavities including the riser. As the metal cools it will contract sucking metal from the sprue and any risers to keep the pattern full of metal.
After the sprue and optional riser(s) are cut and the sand removed the cope is lifted off the drag and put upside down on the molding board. The parting dust helps keep the sand from the drag from sticking to the sand in the cope. Sort of like flouring a cake pan to help the cake come out of the pan without sticking.
Once the cope is separated from the drag runners are cut by hand from the sprue and riser(s) to the pattern. The runners need to be large enough that the metal flows quickly and freely from the sprue to the pattern cavity and from there to any risers. The depth of the sprue should be slightly deeper than the depth of the runners. This way any sand or junk that gets knocked down the sprue is left at bottom of the sprue and not carried into the casting proper.
At this point the pattern is removed from the drag. Normally there are a couple of small holes in the bottom of the pattern. A screw or eye screw is carefully threaded into these holes. The screws are then gently rapped to loosen the pattern in the mold and then the pattern is carefully lifted out leaving a cavity behind. Any sand is carefully removed, never with compressed air. As one pundit put it: Using compressed air is a good way to get all of the sand out of your flask and all over the work shed.
Anything that is wrong with the cavity that can be repaired by hand is now down. If the cavity isn’t right it is time to knock out the sand and start over again.
Once the cavity is right in the sand the cope is lifted up and flipped over again to be carefully realigned with the drag. This means that the sprue and risers will correctly align with the runners cut in the drag.
At this point more molds can be made up and when all of the molds are ready they are moved to the casting area. The casting area needs to be free from moisture and the top of the sprue has to be lower than the height from which the metal will be poured. I.e. don’t put the flask on top of your table and then expect to lift 10-30 lbs of molten metal in crucible high enough to pour smoothly.
Note: Aluminum weighs in at about 5lbs per qt (5.2359787 lbs/liter at 661C) and Iron is around 15 lbs per qt (15.388266 lbs/liter at 1538C).
Now that the molds are ready it is time to melt some metal. The furnace is started and a crucible placed inside. Ingots or scrap metal is then added to the crucible a little at a time. You don’t want to add the metal so rapidly that the pool of already molten metal cools so much that it solidifies.
At this point some things are added to the molten metal to declassify it and to make any impurities rise to the top. These impurities are scooped off and put to the side to be discarded later. The crucible is then lifted out of the furnace with lifting tongs. It may be transferred to a pouring device or the tongs themselves be used for pouring.
The crucible is then positioned over the sprue and the metal is quickly poured. The metal will immediately start to cool as it comes in contact with the “cold” sand so the pour has to be rapid enough that the molten metal can get everywhere it needs to go with out solidifying. This is why it is hard to cast thin items. The metal solidifies before it reaches all the parts of the pattern cavity.
Once the mold is filled the crucible can be used to fill other molds or returned to the furnace for more metal to be melted. The important thing is that when the pour is done ALL of the metal in the crucible needs to be poured out. Often the metal is poured into ingot molds for use with the next melt.
The metal is allowed to cool to make sure it is completely solid and a bit more. There is no reason to rush this process. Finally the flasks with molds and metal are moved to the knock out area. Here the casting is removed from the sand and as much sand as possible is recovered for reuse. The castings are brushed and poked and prodded until as much sand as possible is cleaned from them.
When the casting is cooled enough the runners are cut off which also removes the sprue and risers. This metal is returned to the raw metal for use in another melt. At this point the raw casting is either sent to finishing or to the machine shop for machining.
Finishing would be using buffers and grinders to remove the last of the runners. This is all that needs to be done for things like cast iron pans.
One of the first castings I hope to attempt is to make a cast aluminum pan. I’ll use one of my current cast iron pans as the pattern and then cast the aluminum equivalent.
Pattern Making
Pattern making is the art of creating something that will create the correct cavity in a mold. The requirements are simple: The pattern needs to be able to be placed into the flask and rammed up then removed without destroying the cavity in the mold. In addition the pattern needs to make correct support cavities to hold any cores that are added.
Consider a simple casting that has an axle that goes through it. The pattern could be made such that item is cast as one solid piece. After that casting is sent to the machine shop and the hole for the axle will need to be drilled and then bored to size. Most of the metal that is removed during this process is likely to be lost. Collecting chips is a pain and they are often contaminated with oils and cutting fluids. They might also be contaminated with different metal. You really don’t want to have steel/iron chips in with your aluminum.
I’m sure that using magnets would work to some extent but it would be a lot of work. Normally when metal recovery is going to be done the mill/lathe is cleaned of all other metals and then the milling takes place.
Regardless, the fact of the matter is that having to machine a complete bore through a casting is time consuming and a pain. In some cases it is impossible without special drills to make the hole deep enough to go from one side to the other.
Thus is born the “core”. A core is merely a casting in sand or other material that can be placed in the cavity created by a pattern to make the hollow. In the example at hand the pattern would have to cylinders attached where the axle would go extending outward. This would create a hole in the mold at both sides. The core, which looks like a rod, would then be placed between these holes filling in an area where the axle will go. Now when the mold is closed over the core and molten metal is poured into the mold it flows around the core.
When the mold is opened and the casting is removed the core can be removed as well. This is sometimes done just by soaking the casting and core in water. There are a couple of ways of making these cores, one is with “liquid glass” another is with a mixture of grains baked in the oven. It doesn’t really matter how the cores are made as long as the are self supporting and can be removed after the fact.
In some cases people will use steel rods as cores in aluminum castings. After the casting cools the steel rod is pressed out of the casting.
Besides cores patterns can also be made in parts. The simplest is the “split pattern.” Again using our example of an axle housing
(The article was unfinished, and ended here.)