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Saturday, December 25, 2010

Silent air compressor

For a long time I have wanted to have pressurised air available, especially for glass bead blasting of machined parts. The main reason I haven't just bought an air compressor is that they are loud as a jet engine, usually in the order of 85-95 dB and that just doesn't cut it in an apartment complex.

About a year ago I stumbled upon articles in the internet of how to use a freezer or fridge compressor to produce compressed air silently. The only drawback is low air output volume, but my need is occasional and very little, so this fits for me.

A week ago I was able to obtain a working compressor out of an old freezer. This was an old ZEM compressor with 110 W of power and it runs from 230 VAC 50 Hz supply. The best thing is that it was from a freezer, as these are usually more powerful than compressors from fridges. The compressor had its original oil still in it, so I poured it in to a junk oil container and replaced it with a regular oil meant for automatic transmissions. The only requirements are low viscosity and that the oil is of mineral variety.

These freezer compressors are able to produce enormous pressures and also vacuums, so care must be taken that the system is able to withstand the pressure. This usually means a pressure switch that cuts of the compressor when the pressure reaches a set upper limit and starts the compressor when the pressure drops below a set lower limit. Also an overpressure valve is needed, as it vents if the pressure switch doesn't switch off the compressor.

To have a big supply of air for some time the compressor needs a pressure tank after it. This can be an old carbon dioxide extinguisher bottle or gas bottle that is rated for the intended pressure. There is also a need for the pressure switch, pressure regulator, connectors, piping, oil/water catcher and air drier. If you have to buy everything as new, the total will cost probably around 100-200 EUR plus all the work needed to assemble everything.

This is the reason I thought of buying a brand new compressor unit and just replacing the compressor unit with the freezer compressor. So I spent 89 EUR to buy a Herkules brand compressor from Bauhaus, as it was the cheapest I found that had a 24 liter pressure tank. The original compressor unit fills the tank in about 2 minutes from 0 to 10 bar, but produces an ear breaking noise of 97 dB at the same time. Not good.

So I started disassembling the unit to get the original compressor out and to take measurements of the pipe size used so I can make or buy a proper sized fitting between the pressure container and my freezer compressor. It was quite easy to just unscrew the plastic cover to reveal the compressor unit, but the Abiko connectors to the power switch didn't want to come off so they got a quick treatment from pliers.

After removing everything not needed, I was left with a tank, a one way valve, a pressure switch, overpressure vent and pressure gauge with an adjustment valve. The output is a 1/4" female quick connector and the input to the tank is from the end of the one way valve that has G 3/8" male thread.

The freezer compressor had a 1/4" OD copper tube as its output, so I bought a hose connector that had a 1/4" hole through it. From this I hacksawed the hose connection part off and after cleaning everything, I soldered this to the compressor output tube. To the one way valve I rolled few turns of PTFE tape and screwed on a G 3/8" -> 10 mm hose connector. This way I can attach the compressor to the tank with a small length of hose and avoid induced vibrations to the tank and also avoid messing with rigid tubing and bending.

The pressure switch was factory set to have 10 bar upper limit and 8 bar lower limit. I plugged the modded compressor to the wall for a quick test and started a timer to see how long it takes to fill the 24 liter tank from 0 to 10 bar. The pressure rise was slow but steady and finally after 24 minutes the pressure switch turned the compressor off. This means that the compressor can provide about 10 litres per minute. The 0-5 bar rise time was exactly 10 minutes.

I was a little bit doubtful if the freezer compressor could start against pressure, so I blead the tank slowly until I heard the pressure switch click on at the 8 bar marker. The compressor immediately started to purr and building more pressure to the tank. I timed how long it took to see how much the air output volume would change from the first run as this was against pressure. It took 4 minutes 50 seconds until the compressor shut off, so against pressure the compressor can provide about 9.93 litres per minute, which doesn't differ from the 0-10 bar rise time.

The compressor temperature climbed to quite warm during the half hour test, meaning I could not keep my hand on it for long. I'm quite sure my air need is so small that this will not be a problem as I can switch the compressor off when I have pressure in the tank, but I could add a small 230 VAC fan to blow cooling air over it.

Of course mounting the freezer compressor meant that I had to fabricate some sort of plate that attaches to the tank mounting holes and that has mounting holes for the freezer compressor. I used a proper sized piece of aluminum plate that I countoured with a CNC mill and drilled and threaded for M6 screws. I used the freezer compressors own rubber feets to isolate vibrations from the unit to the rest of the system. In the rear end I also installed a small water separator unit that is screwed to the plate and inside the plate is an 8 mm passage for the air. On the back side there is one electrical connection to the pressure switch, which is wired in series with the fridge compressor and shuts it off when there is 10 bar in the tank.

So how silent is my silent compressor? Well, I can just hear it running, sounds like a fridge humming its things. Yep, beats the so called "silent" compressors they advertise in hardware stores with about 68-70 dB sound output.

Added 06.01.2011: I bought a small Simota brand air filter that had a connection for a 9 mm diameter tube. This was probably meant for some RC car or something, but works wonders in this application and totally eliminates the small intake sound of the compressor.

I also had an idea for a decal to replace the original "Herkules" tape and so I used 5 minutes with KolourPaint and printed off a nice new decal and attached it (poorly) with a piece of tape meant for covering books. The word "Perkules" is a kind of twist from a Finnish curse word "perkele" and the original brand "Herkules". The word below it is my own drunk misspelling of the word "kompressori", which is Finnish for a compressor.

Friday, December 17, 2010

Cleaning of SIEG C4 lathe

As the lathe was covered heavily with protective red grease, my first task was to clean it all off and lubricate the machine properly. As I was cleaning the lathe, I notice that the bed number was 00425, meaning that this is a very new model and I got one of the "first ones". Just for comparison, the previous SIEG C2 lathe I had was with a number of 22xxx.

For cleaning I used paper towels and lots of turpentine that I spread with a small paint brush after getting most of the smears of grease away with the towel and finished with a turpentine soaked towel. The bed way was quite easy as well as the slides and took me about 30-45 minutes.

As I removed the chuck to clean the spindle end, I rotated the spindle to get to the bolts and heard small grinding sounds. Thought first that the spindle bearings are already broken, but when I disengaged the spindle from the auxiliary axel the sound stopped. This required a peek in to the headstock, so I unscrewed the cover on the casting and noticed grease on the gears.

Nope, it was not grease. Metal gears covered in thick greasy like substance that felt more like a coarse lapping compound. I don't know if they have lapped the gears in place for quieter operation and in a rush didn't bother to clean it up or is their grease gun full of small grinding particles and chips.

As I didn't want to disassemble the headstock at this time, I tried soaking the gears in engine oil and rotating them. The oil was mainly picking up individual partcles, so I tried with turpentine and found out that it flushed the grease quite nicely, but didn't get everything. I ended up using both, first soaking with turpentine and then with oil and couple of times wiping with a paper towel between the teeth of the top most gear. About five minutes later I had a nicely working and soundless headstock gear pair.

All the accessories went through the same turpentine treatment. The change gears had thick red grease on them, mixed with dust or something similar. The paint brush and a pail of turpentine got rid of everything easily. A nice feature of these all metal gears is that the hole in the center is a free siliding fit to the respective axel as opposed to the C2 lathes plastic gears, that had a hard friction fit which was a PITA.

I removed the lead screw cover to examine the lead screw closer. The lead screw was covered with the red grease and a closer look revealed that there is only grease on it, so for now I left it alone. However, I was intrigued to take a peek of the pillow blocks that hold the screw to see if there is anything better than the C2 lathes cast iron blocks. The tailstock end seemed to have a bronze or brass bushing where the screw rotates and the block itself was pinned and bolted. These hardened pins are an indication for me of precision, as they lock the block in certain place and position.

In the headstock end of the leadscrew I had to remove the plastic electrical box to see the screw. The electric wires were protected from the screw with a small floded steel sheet and under that was a very positive surprise: A flexible connector and a thrust bearing! Seems that SIEG really has thought about these things and this really is showing up as a quality product so far.

The carriage also had some interesting features. In the photograph to the right I circled some items from the top view. Yellow ones are threaded holes that have a grub screw to protect it from swarf and these are for some sort of attachments/accessories. The blue ones are for adjusting the front side gibs that hold the carriage down. The green ones are yet again hardened pins to align the apron to the lead screw. The red one is for locking down the carriage to the ways when for example parting off or taking a truing cut with the cross feed.

I removed the carriage front side gibs or "paws" to clean them up. They had the same red grease all over and some paint chips on them. I'm just a little bit confused as to how I should adjust these, as the lathe manual is of no help at all and these are different than in the C2. There is no obvious adjustment screws that would counteract each others, there is only those two small socket head cap screws that hold it in place. I just snugged them up a little bit so the carriage won't lift, but in the end the cutting forces are pushing the carriage down to the prism way, so I think these paws are just to keep it from lifting up easily. If someone knows better, write about it in a comment to this article.

Behind the right hand side paw there is a locking element that locks the carriage to the ways. I did not have a magnet at hand so I didn't remove it, but once I find one, I'll check it also. I'm suspecting that it is just a little bit of L-shaped and the only screw just lifts it up to lock the carriage from movement.

To get best access to the electronics compartment located in the back of the lathe, I had to get the gear cover off. This means removing every gear and the drive belt. While removing parts I cleaned them all and noticed a nice desing feature on the gear axles: They have an oiling port in the visible end and a small hole goes halfway through the axle and then there is a cross drilling. This provides oil to between the axle and gears to lubricate them properly. Very clever idea!

The gear covers frame however had one error: It will not come off if you don't first remove the four small screws on the backside of the electrical box that is located inside this gear compartment. I think I'll just take a Dremel or a round file and make some sort of small cutout to the frame where the screws are located so it slides in and out without unscrewing those small screws. Otherwise the end shows the usual SIEG style of finishing with a paint on parts half assembled, as indicated by the paint spots.

The controller board, as seen in the above photos, is located in the large box behind the lathe. There is a hefty finned heat sink that provides cooling together with a small fan to full wave bridge rectifier and six huge (probably) MOSFETs. The capacitors are so big that there is a cutout in the heat sink for them. The board itself has nothing special except very good connectors. There is a small additional daughterboard connected with a couple of rows of pins and this contains an ATMEL microprocessor. Didn't look that closely but I suspect that it is responsible for operating the display and keypad.

As I put everything back, I ran into a problem: How to get those small wires through the white plastic tube that goes through the headstock and is already full of wires? Well, found a length of copper wire, pushed it through and made a small loop and to this loop I attached the wires and pulled them through.

Wednesday, December 15, 2010

SIEG C4 lathe arrived

My new lathe arrived on Monday, 13.12.2010, brought by a TNT courier. Incredibly fast service and delivery from Axminster Tool Centre in UK, as I ordered the lathe 03.12.2010 and they did not have it in their warehouse at the moment. This was my first purchase from Axminster and I was very happy with their customer service, so I can easily recommend them as a good source of tools for quite cheap.

The freight from UK to Finland cost me about 140 EUR, so not bad for a 130 kg shipment that measures over 1000 x 700 x 500 mm in size. I already blogged about this lathes specs previously, so I won't go in to details in this post. As I knew the package dimensions I checked my doors and elevator that the package fill fit through them all and with the weight in mind I bought a hand truck capable of lifting 200 kg for a price of 39.90 EUR (from Biltema).

I think it was about 27.4 seconds after I got the package inside and the doors closed when this happened. As usual, the safety inspector already did some jumping tests on the crate and tried to tip it over.

Using pliers I ripped the steel bands off of the crate and with a regular hammer I pried the top open. The lathe looked as expected and nothing obvious was missing or damaged. The usual aroma of fresh new machinery and a lingering smell of grease filled my nose as I pulled the plastic cover away. Most of the space in the crate is occupied by the chip shield that is located behind the lathe. It weighs quite much and makes moving the lathe more difficult, so I unscrewed the four socket head cap screws that were holding it.

All the accessories and tool were inside a small wooden box. This includes the change gears for threading and/or feed, the chuck key and a small wrench for tightening the screws in the tool post. The gears are all metal which is nice, compared to the plastic gears in the SIEG C2 lathe. And these gears include a 127 tooth gear that enables the lathe to thread imperial threads without approximation.

Removing the sides of the crate was a small task, as someone had went haywire with a stapler. But there's nothing a big hammer can't fix ;) Opening the crate revealed the lathe in all its might and it really looks like a slick machine. I even like the color, don't know why. In this photo I was looking for the bolts that secure the lathe to the crates bottom and also was looking for some good lifting points. While searching I also discovered myself looking at my cat :D She sneaked up to sniff and snoop around and was very curious...I think too curious, might have to keep on eye on her...

This is the lathe in the setup I used to lift it off from the pallet to the floor. I removed the tailstock and wound the carriage all the way to the headstock end of the bed. Then I grabbed under the headstock, left hand under the motor and right hand under the small ledge and lifted it with a straight back so that the tailstock end of the bed was resting on the pallet. My helping hand removed the pallet and I could then lower the headstock on the floor and lift the tailstock end down also.

Next I removed the plastic chuck guard and the small metal lid that covers the spindle opening in the gear trains cover. I parked my hand truck behind the headstock and lifted the lathe to stand on the hand truck. The gear train cover is metal and it will easily handle this load if not shock loaded.

While the lathe was like this, I could take measurements from the mounting holes easily and accurately. I discovered that the holes are spaced 110 mm apart and there is 590 mm from the tailstock holes to the middle holes and then another 90 mm to the headstock holes. And curious as I was, I HAD to unscrew the bottom plate of the apron off to see what is inside. This lathe has power feed for carriage and cross slide and these small gears provide that. The drive comes from a worm screw that is rotated by the lead screw. This turns the bronze gear which in turn turns the small, movable gear. This small gear can be moved to engage with the apron front-most gear for cross feed and the gear near the bed for carriage feed. If you engage the power feed, the threading half nut is locked from moving to prevent an accident.

The cross slide and compound both have a thread pitch of 1 mm and adjustable bronze nuts. The only bad thing is that the carriage dial is measuring the radius. Would be nice from SIEG to see diameter reading cross slide dial, makes working easier. Oh well, have to make a modifications for this :) The best thing of these is that they slide so smooth compared to the C2 and this is because there is a bearing in those handles.

And WOW! 1000 W brushless DC motor, delivering awesome torque even in low speed (100 RPM). This thing is just huge and looks very good. This is connected to the spindle driving mechanism via a toothed belt. And yes, that is my dirty hand (try polishing molds for 8 hours and it will get that way).

After I had drooled enough, I grabbed the hand truck and rolled the lathe to my "shop". Really it is just a 13 m² room that doubles as a storage as you can see. The bench I made for C2 lathe is just behind me and is to be tested with me and heavy weights that it will hold without squeaks under the C4 mass.

Now I need a few hours of time, a small container, brush, rags and kerosene or some other solvent to remove the packing grease. And maybe a beer or two along with this operation ;)

Wednesday, December 8, 2010

Materials and their properties

A machinist have to know the basics of metals and plastics and their properties in order to machine them correctly and to know their pros and cons when building something. I will give a short introduction to material science in this article and describe the most basic materials as best as I can.

Most of the metal material you use in your projects are actually alloys, meaning they compose of different metals melted together. This is to provide better properties than the base material itself has. For example, aluminum as such is very soft and gummy and doesn't have any strength, but if you add a small amount of magnesium in to the mix, the material will have added strength and it isn't gummy any more. This means more easily machinable material, as the material holds to cutting forces applied (think of machining rubber vs. steel) and doesn't stick to tools so easily. Different metals can be used as additives and they all function differently and provide different advantages.

Alloying elements are chosen for the properties wanted. Examples of such properties could be strength, corrosion resistance, machinability, weldability, ease of anodisation, melting point, hardness and hardenability. Most of the materials properties arise from the base material and the rest is provided by the alloying elements. Usually the metal alloy consist of 70-100 % of base material and the rest is for the added alloying elements. Usually the percentages for alloying elements are 0.05-2 % per element, but this again depends on the properties wanted. A rule of thumb is that if there is 5 % or over of something in a steel alloy, the resulting material can be hardened quite easily, but this is not exact.

Probably the most known alloy is steel. Steel is a mixture with carbon content being between 0.05 to 1.7 % and the rest being iron. If carbon content is between 1.7 and 6.67 %, the result is cast iron. However, if you add other elements like nickel and chromium, you get stainless steel which has a better corrosion resistance than plain steel. This is due to chromium and nickel forming oxide layer on the metal which prevents further oxidation of the metal, meaning rust. For ease of machining the alloy may contain sulfur or lead. These provide "soft spots" so that when the cutting tool creates a chip, the softness allows the chip to break up very easily and the resulting chips are like flakes. These are called free machining alloys and are highly used in NC-machines where long and curly chips could block the machine operation as they will not exit through the chip conveyer.

Most probably the three most often used metals in your homeshop will be aluminum, brass and steel. Aluminum is easy to machine, light weight, quite cheap and provides good starting point for learning to use your machine. Brass is quite heavy, quite expensive but it is very easy to machine, it gives very nice flakey chips but also very pointy sticks and provides an attractive look and finish to your projects. Steel is a little harder to work with unless it is free machining steel, as it is very tough to machine with mini machines capabilities and it tends to form long curly chips. However, when you need that extra strength, go for steel.

Aluminum is probably the most machinist friendly material to start with, as it is easy to machine and doesn't cost that much. It has a density of 2700 kg/m³ so it is about a third of the density of steel and a melting point of about 660 degrees Celsius. Cost is about 2-20 EUR per kg, depending of the exact alloy wanted and if it is from a scrap pile or not. Aluminum is perfect for anodisation, although some alloys do not anodise well at all. Welding aluminum is quite difficult, but can be done with the proper machines and skill.

Aluminum coldwelds to tools easily, so use plenty of coolant like isopropanol or some other alcohol, but due consider that the vapors are extremely flammable. The coldwelding can be seen on cutting edges as a silvery color or as a piece of aluminum sticking to it. This can sometimes provide uneven surface quality, as a piece of aluminum blocks the cutting edge and literally pushes (shears) the material ahead instead of the tool cutting it.

Brass is another easy metal to work with as it forms short chips and cuts with just about any sort of tool. It is an alloy of copper and zinc, but the most often used machinable brass includes also lead about 3 %. The downside is the huge price. Brass will not rust, but it will tarnish. Rubbing with a cloth shines it up nicely and freshly machined surfaces have a nice gold like appearance. Brass is quite soft, so it can be used to make a brass hammer that is nice when in need of assembling a tight fitting steel part or when a plastic hammer won't do it.

Cast iron has a high carbon content and flakey structure which makes it very suitable for machine parts, as the structure dampens vibrations and provides a good sliding surface. Very often model engine makers use this for their pistons and/or cylinders due to these properties. Cast iron is very easy to machine and leaves a nice surface finish.

Steel is a little bit tricky to work with these small machines due to low powers available, but still doable. Steel provides a harder material for your projects than aluminum or brass, but it is also quite heavy, 7860 kg/m³. Steels melting temperature depends mostly on the carbon content, but it is minimum 1130 degrees Celsius. The cost is usually under 1 EUR per kilo when bought as scrap metal and even as new it doesn't cost that much if it isn't stainless or tool steel. If possible, I would advice you to find free machining steel, meaning for example leaded steel as this type of steel machines easily and leaves a nice surface finish.

Stainless steel
is literally hard to work with small machines due to power issues, but leaves a nice surface finish. Stainless has such a high alloy percentages (10-20%) that it work hardens very easily. In machining, the cutting tool removes a layer but the heat generated may harden the just cut surface, making it very hard to penetrate. High speed steel bits will not go through that layer easily, so the best option is to have lots of power and use plenty of coolant. A good sign of hardening is when a drill bit starts to squeek very hard and the drilling action stops, the drill bit gets dull fast and might also snap. Stainless steel can be identified from 'normal' steel with a small magnet, as stainless is non-magnetic.

Tool steels are steels that have quite high carbon content and added alloying elements such as vanadium, molybdenum, chromium, tungsten and lots of others. Tool steel is what it says on the tin, it is usually used to make tools as it can be hardened easily and is usually sold as ground to certain tolerance. Silver steel is one example of low cost tool steel, although the price for tool steel is not that low. The most 'known' tool steel is high speed steel (abbreviated HSS) which is used extensively in drill bits and lathe tools. Usually tool steels are sold in annealed form, meaning they are heat treated to be soft so they are easier to machine and thus they have to be hardened and tempered to achieve the hardness wanted. Although tool steels are tougher to machine, they almost always produce a very nice surface finish no matter what you do.

Plastics, like PMMA (a.k.a. acrylic), ABS, PTFE (a.k.a. Teflon), nylon and lots of other are a nice alternative for metals when making a prototype, training to use the machines or when the project requires lightweight parts or electrically non-conductive material. Plastic is nice to work with as it is soft and machines easily, but the downside is that you need sharp tools and low speeds to cut plastic or otherwise the heat generated just melts the plastic, leaving rough and ugly surface. Cutting tools heat up very quickly when machining plastic, as plastics are more like insulation than heat absorbers, so almost all of the cutting action induced heat will end up in the tool bit.

Some plastics like acrylic are prone to crack easily when for example drilling a hole and the drill bit is just going through the workpiece. This means that you have to be quite careful with your feed pressure. Plastics have the nice property of being very light and some plastics like PTFE and POM (a.k.a. Delrin) have self-lubricating properties which makes them very useful as for example light duty bearings. POM also is very engineer friendly material, as it holds tolerance well, is quite tough and withstands lots of chemicals.

Copper is nasty stuff to machine as it is very gummy and soft like pure aluminum, but with a sharp tool and lots of lubricant (coolant) it produces a very nice and shiny surface finish. Copper is work hardening metal, but can be softened again by annealing. Usually the copper comes in annealed form. If your project needs a good electrical conductor or heat capacity or the like, use copper. Or if you are doing EDM work, copper is the most basic electrode material and holds very well in that process.

Graphite is really easy to machine but it dulls the tools quite fast and of course it is brittle. There will be nothing but dust generated from machining it, so have a vacuum cleaner near the cutter on full suction to keep the level of free dust to a minimum and clean the machine very well afterwards. Graphite conducts electricity very well and is used in EDM as electrode material. Price depends on the particle size and is quantified usually per litre and is expensive, unless you get it from a tool & die makers scrap pile.

Sunday, December 5, 2010

Mod: Reversible spindle for X2 mini mill

Almost from the day I got my X2 mini mill I have been thinking of adding a reverse switch to it. Countless times I have tapped with power but have had to wind the tap out by hand as the spindle has no reverse.

All I know of the mills power and voltage demand for the motor that it is about 300 W, so about 1-1.5 A of current from mains at most. The voltage is probably just full bridge rectified mains voltage, so about 325 V at maximum. I'm thinking of a 250 VAC and at least 2 A rated DPDT (double pole double throw) ON-ON switch for this application. Now you may think that the switch can't handle the requirements and it is true. But it is true only if used to switch under a load. The closed switch easily handles the voltage and the current and there is no problems.

Well, finally I decided that enough is enough, as I had to tap over 50 M4 holes and my hand said no go. I went to the electronics store, bought a DPDT ON-ON type switch, rated for 250 VAC / 3 A. Price was 1.85 EUR, so nickels to save my hand.

Quickly drilled a small hole to the controller box and installed the switch. Then I clipped off the quick connector clips from the motors leads and soldered the leads from the controller to the switch's centre poles and the leads to the motor to the other end of the switch. After that I soldered jumper connections from the other side of the switch connections to the motor cables but crossed them. This way the switch flips the polarity going to the motor and thus the motor runs in reverse.

Tested after assembling the controllor box and it worked! I also got the switch mounted so that regular clockwise rotation is achieved when the switch is down and counter clockwise when it is up. Easy to remember from tapping: Down goes down and up comes up :)