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Jays Article on Engraving Steel

 

 

Done By Jay Hoffpauir from Xenetech

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1950-2000 – Founder of Xenetech

Rotary Engraving Stainless Steel

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I am often asked by people, in our industry how to engrave stainless steel with computerised machines. Often, these people have tried, quite unsuccessfully, to engrave stainless, and have done little more than ruin a lot of cutters and material.
The causes for this can be traced to, many things, including using the wrong cutters, wrong spindle speeds, wrong feed rates, wrong type of material, and misunderstanding the basic principles of the material involved.
The Engraving Machine:
Stainless is a very strange material with which to work. It is very hard but, depending on the type or grade of material, can be cut freely. The cutter has a hard time penetrating the surface of the stainless, then has an even harder time staying at the proper depth.
The cutter tends to “porpoise” in and out of the material, giving a ragged cut. This process is very hard on the cutter itself, often shattering the tip. This problem can be overcome by applying more down pressure to the spindle, forcing the cutter to remain at the same depth in the material.
How much down pressure is needed? That is hard to say. The diameter of the cutter – the actual cutting size of the tip – has a lot do with the amount of pressure required. The larger the cutter, the more pressure that is needed to keep the cutter in the material.
Many computerised engraving machines have an adjustable spring to the change the pressure on the spindle. These systems work so that the spindle is held up by the actuator, which can be either an air cylinder or a stepper motor. The spindle pressure is regulated by the spring, which holds the spindle down.
This type of arrangement is often called a “Floating” spindle and is great in most applications. The air cylinder or stepper motor has nothing to do with the amount of pressure applied to the work piece.
This helps greatly when the operator is using a nosecone on uneven material by allowing the spindle to ride over the high and low spots in the work piece. This is also good for diamond drag work because it prevents the dreaded diamond dimple syndrome in which the diamond hits the brass or aluminium hard enough to cause a dent or dimple in the material.
The problem I have run into is that many computerised engraving machines do not have the capability of applying enough pressure to the spindle, even at the maximum settings. There is simply not enough spring tension available to keep the cutter down in the stainless.
.Some engraving machines will allow the user to “pin” the spindle arm to the actuator arm. The Dahlgren System 2 is an example of this. Pinning actually allows the air cylinder or stepper motor to push the spindle down onto the work, adding its push to that of the spring. This should be done with care to prevent damage to any of the parts involved.
One of the major problems with stainless engraving is that the engraving machine is not rigid enough. Too much vibration causes the cutters to break. This can be traced to the way the machine is built. Most of the engraving machines we use are designed for plastic and diamond drag engraving, little if any lateral forces are involved.
Materials used in this type of engraving (plastic) are soft and the action of the cutter through the work piece places only a small load on the spindle and carriage. The carriage is the part that mounts the spindle and tracks left to on the machine – the “X” axis.
The problems with stainless – necessary strong down pressure for the spindle and the unique cutting qualities of the material – cause high lateral forces and vibrations in the spindle assembly. This causes the cutter to break even though the user has carefully set the feed rates and spindle motor
There is little you can do about this problem. The dynamics of the machine simply will not allow good engraving in stainless. The way the X, and Z axis are mounted and tracked contribute to this.
‘V’ Wheels:
Some engraving machines use “V” wheels mounted on the carriage and table. These move along a steel rod fixed to the base plate and bridge assemblies on the machine.
“V” wheels are actually roller bearings with a “V” groove cut around the outside surface, in the same position as the tread on a tire. They are mounted on standoffs, usually bolts which give clearance.
The problem with these ‘V’ wheels is that there is no solid surface mounting between the pieces. The inside ring and the outside shell of the wheel are supported by ball bearings and tend to have some slack in them even when new.
The rails on which the bearings ride are fairly small – usually ¼ inch – and must be very straight and parallel or the table will not move freely. These rods also must be perfectly horizontal to the base plate of the machine and the standoffs for the “V’ wheels must all be the same length in order for the table top of the machine to be level.
Another problem is that there is a very small amount of area of the “V wheel in contact with the steel rod at any one time, and this allows play in the moving part.
Some of these problems can be overcome if the machine manufacturer is working to very close tolerances, but most machines designed to do stainless engraving, or any heavy duty application for that matter, shy away from this arrangement. They generally use “Steel Ways” or preloaded recirculating ball bearings called Thompson Bearings. These are mounted in a carrier and ride on a steel shaft of large diameter.
Steel Ways, Thompson Bearings:
Steel Ways are basically two pieces of steel cut to very close tolerances One is mounted on the fixed piece – the base plate for example – and one is mounted on the moving piece – the table.
The ways are well lubricated and slide across each other. The amount of pressure one applies to the other can be regulated and very little, if any, slack is found in such an installation. This is the typical set-up for large milling machines.
These ways must be kept clean, well lubricated, and receive regular maintenance. They are also quite expensive, thus increasing the basic cost of the machine involved.
They are, however, probably the best method of moving a table and maintaining rigidity and flatness because of the large amount of surface area involved between the two pieces or steel. The optimum set-up calls for the steel to be case hardened and to have lubrication grooves cut into the face.
Thompson Bearings are actually many small ball bearings mounted in carrier, cylindrical in shape, several inches long and with a split down one side. These bearings recirculate through the carrier as it moves along the steel shaft.
The steel shaft is very straight (hopefully) and should be mounted parallel to the base plate. These bearings have the advantage of being fairly inexpensive, at least in comparison to the Steel Way described above, and can run dry. No lubrication is required.
The recirculating action of the bearing moves a large percentage or the ball bearings into contact with the steel shaft, increasing the amount of surface area. Increased surface area translates into more rigidity for the moving parts. These bearings can be fitted to a variety of shaft sizes, one-half inch or larger being the norm.
Pre-loading:
The term ‘pre-load” means that the amount of pressure the ball bearings apply to the steel shaft may be regulated by set screws that press the carrier closer to the steel shaft. This method is used on many of the ‘heavy duty” engraving machines on the market.
The mounting of the spindle must be rigid as well. Heavy duty engraving machines often use a steel spindle, very straight and polished, riding in a bronze oil-impregnated bushing.
The bushing is honed to a very close tolerance, usually within several thousandths of an inch, to match the spindle. This method offers a great deal of surface area and thus gives a very rigid but free moving mount.
Many of the engraving machines we use for plastic and diamond drag engraving use a “ball slide”. This slide comprises two blocks of aluminium separated by a series of small ball bearings.
These bearings may be pre-loaded to give a free movement, but the pre-load is set by up to six set screws and great care must be taken to keep from applying to much pressure at any one spot.
The problems with the ball slide is that wear causes the slide to have slack and they must be kept very clean. Also, the amount of surface area available is small so vibrations can build up. If you apply enough pre-load to take out the shack, the ball slide will slick and not move freely.
So, if you get my point, the type of machine used has a lot to do with whether you can engrave stainless steel without ruining your cutters. Your $6,000 computerised engraving machine (how old is this article?) was designed to do plastic and diamond drag engraving not stainless steel.
Spindle:
Most of the engraving machines available use top-loading cutters. These cutters slide though the spindle and are held in place by the threaded knob on the top. The shaft that the cutter slides though must have enough clearance to allow the cutter to slide freely in and out of the spindle,
The only holding point is at the top. This is fine for plastic engraving and burnishing, and is the generally accepted method for holding cutters for light duty machines. The very nature of the spindle allows the cutter to wobble or vibrate at the bottom or the spindle, the most critical point. This causes the cutter to break when used with hard materials such as stainless steel.
The best method of holding a cutter is considered to be a “collet” spindle. A collet fits into the bottom of the spindle and has a taper on the outside. This taper is matched in reverse on the inside of the spindle.
The collet has threads on the non-tapered end and is mated to a draw bar that is inserted through the top of the spindle. The collet usually has three splits or cuts along the taper.
The action of tightening the draw bar pulls the collet up into the tapered part of the spindle, compressing the collet around the cutter. This action causes the cutter to be held lightly at the point where it is needed most, at the bottom of the spindle close to the cutting tip.
Some spindles can use both the normal, top-loaded cutters we see everyday as well as short, one-and-one-half to two inch cutters. The short cutters are generally considered to be better for heavy duty work because there is less chance the cutter will be bent, and there is less weight involved. This should tell you that if you do not have a collet type spindle, your chances of engraving in stainless without breaking a cutter are low.
Cutters:
The cutter you use to engrave stainless steel should be ground to a different angle than the one you use for plastic. In general, different angles should be used for different materials. Plastic engraving stock, phenolic, brass, aluminium, steel, stainless steel, etc., all should have a cutter ground to match the properties or the material.
Because of the hardness of the material involved, cutters ground for stainless need more strength in the tip. This is achieved by utilizing a much larger included angle than normally used for plastic. These cutters should have around an 80 degree included angle.
This can be described by looking a the back of the cutter, with a 40 degree angle going from the tip, up to the shank of the cutter on both sides. A standard plastic cutter uses around a 60 degree included angle.
The 80 degree angle gives the cutter more strength. The clearance angle for the cutter is also different. A plastic cutter uses around a 45 degree clearance. A cutter ground for stainless uses a 25 to 30 degree clearance. I recommend a good grade of carbide for the cutters, although I know many people who swear by high speed steel.
Two good cutters for stainless are quarter round solid carbide and quarter round solid carbide Titanium coated.
Coolant:
A good grade of cutting oil can prolong your cutter’s life. By cutting oil I don’t mean 3-in-1 oil or WD-40. Cutting oil is specially formulated to use cutting applications and won’t break down like the other oils mentioned (use CESCO Green Lube). The idea of the oil is not to act as a lubricant but to help keep the cutter cool. See our article on the cooling tub.
Methods, speeds, and types:
There are many different grades or types of stainless steel. Some harder than others. Some have more nickel than others. The most common grade used in my shop is called “304”. This material is softer than a grade such as “316” but harder than “302”. We do a lot of engraving in this material and find it will satisfy most of our customers’ demands.
Another common problem is created by trying to engrave to the depth needed in three or four passes. The action of the cutter through the material heats the stainless. This actually heat treats the material, making it even harder than it was before!
Every pass hardens the material to a higher Rockwell number and finally makes it nearly impossible to cut. The preferred method is to try to take full cut in one pass, with a clean up pass to remove any burrs done at the same depth.
The engraving machine should be set to as slow a feed rate as possible for this. This type of engraving should be done without a nosecone. Stainless will scratch and mar the finish of a nosecone.
So what have we learned here? To review, all you have to do to engrave stainless steel on your computerised engraving machine successfully is:
• Engrave at a slow feed rate ( the slower the better)
• Engrave without a nosecone
• Engrave using a good cooling lube
• Engrave using a low spindle rotating speed
• Engrave using the right relief and included angle grind on a solid carbide cutter
• Engrave using a rigid engraving machine with a collet loaded spindle set in a solid block that has enough spring pressure to keep the cutter down in the material.

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