Over the years, there have been numerous changes in machine tool designs. Most of the changes have resulted in improvements on existing machine designs. The changes from single spindle manual machines, to multispindle cam activated tools, to modern CNC style and Swiss style automatics all have helped shops increase productivity and quality.
However, in many cases the cutting tools most commonly used in the screw machine industry have changed very little. The industry, for the most part, is dominated by hss and brazed carbide parting tools, drills and form tools.
Conversely, these tooling types are seen much less frequently on today's conventional CNC lathes. These machines more commonly use indexable carbide cutting tools. Some reasons for the difference include cutting speeds at which these CNC machines operate and a lack of availability of tools specifically designed to meet the needs of the screw machine market.
Historically, the lower speeds at which screw machines operate often prohibit the efficient use of carbide tools. The geometry, chipformers and cutting edges of the most common indexable inserts are designed for use at higher cutting speeds. Correspondingly, the carbide grades are also made to withstand higher rather than lower cutting speeds and temperatures. Since the major market for these indexable style carbide tools is the traditional CNC machine, until recently very little effort has been made by the carbide tool manufacturers to address the needs of the screw machine industry.
Carbide For Screw Machines
There have been recent advances in carbide technology that allow these types of tools to run at the lower cutting speeds often encountered in screw machines. Also, there has been a greater willingness by the major carbide manufacturers to produce designs for this segment of the market. The benefits to the industry can be looked at from several different points. The most important are as follows.
- Increased cutting speeds and higher production rates
- Decreased down time
- Higher quality
- Lower overall tool costs
- Operator safety
As in any manufacturing environment, the quantifiable advantages have to outweigh the cost and time to justify considering a switch to different tooling. This is probably even more important within the screw machine industry.
The types of cutting tools in use today have been used for years. Operators, setup personnel and designers are familiar with them and feel comfortable using them. In addition, many people in the screw machine industry have a "bad taste in their mouth" when it comes to indexable, throw-away carbide inserts. When these inserts first began to become popular in other aspects of machining in the mid-1960s, people in the screw machine industry unsuccessfully tried them. At that time, the carbide grades and pressing technology did not exist to meet their requirements. A careful examination of the potential benefits will show significant improvements.
Increased Speeds, Productivity, Safety
The use of carbide tooling will usually allow a part to be machined with at least four times the cutting speed when compared to hss. The limiting factor with standard screw machines has been the cutting tool. The cutting speed is limited to the tool that requires the lowest speed, since all of the spindle will have to rotate at the same speeds. The tools that have the greatest limitations are drills and part off tools.
Today, throw-away carbide part off inserts are a standard everywhere except in the screw machine industry. A significant reason for this is a lack of tooling to hold the inserts correctly in these types of machines. There are now available a number of different types of adapters that will allow you to use carbide parting inserts with very little change. There are standard dovetail configurations, block adapters and even replaceable cartridges available.
It is also important to look at the time and safety issues that can be considered. It is obvious that if you can increase the cutting speed that you work with, you will produce parts fast. The faster you can make a part, the more profit.
An overlooked benefit to a change to indexable part off inserts is in the reduction of downtime. An industry standard for changing an hss part off blade, and resetting the machine, is approximately 10 minutes. This is compared to less than a minute for the self-gripping style carbide insert. This may not seem like a lot of time, but over the course of a month and on many machines, it adds up to a large difference in the number of parts produced.
The molded carbide inserts also have very aggressive chip formers in them. This geometry facilitates breaking the metal chips into short manageable pieces.
Operator safety is increased with this chip control. When the chips are small and broken, the operators will not have to stop a machine and clear tangled chips from the tools, part retriever or other areas of the machine. This chip control also saves more downtime because the machine will not have to be stopped because there is a problem with chip wrapping. The sharp cutting edges of these inserts, together with ultra fine or sub-micron grade carbides, have the ability to work at lower cutting speeds than previous grades of carbide did. The sub-micron grades are also stronger than HSS.
Carbide Form Tools
Another change that is a benefit to the screw machine industry is the availability of disposable carbide form inserts. Traditionally, form tools were either made in-house or contracted to a specific supplier.
These tools are built using either an hss blank or brazed carbide that is then ground to the correct form. The HSS tools are limited in the cutting speed at which they can work. The brazed tools are also hindered by the heat in brazing.
Heat creates stress between the bonding material and the carbide and reduces the strength of the tool. Form tools can be difficult to sharpen correctly and are generally expensive. They also require a skilled operator to set them correctly in the machine tool. This setting process is time consuming and results in more lost production time.
Carbide manufacturers have begun to offer alternatives to traditional form tool blanks. There are now carbide insert blanks made to CNC indexable standards that can be used. They still require special grinding but offer a high degree of accuracy. Because they do not require brazing, carbide form tools can be coated with any of the advanced processes available.
The major advantage of indexable carbide inserts is in the reduction of downtime required to change edges. Inserts are repeatable to within ±0.001 inch or closer in length and are held by a screw clamp securely in a steel toolholder.
The least skilled operator can index the insert by simply removing the screw and insert, cleaning the pocket and replacing the insert with a new one. This also allows the insert to have a positive cutting inclination to the workpiece. This reduces the amount of tool pressure and possible distortion of a part.
Carbide For Swiss-Types
Although they are not really all that new, Swiss style screw machines, both standard and especially the CNC models, are increasingly popular in the United States. But they tend to share many of the same problems as more traditional machines.
Most often these machines are applied on small diameter parts. With these small diameters comes the problem of generating enough cutting speed to make carbide work correctly. There is also a very limited amount of space between the tools in the magazine and the tool fixture itself.
The new carbide substrates and sharper cutting edges have reduced most of the earlier problems associated with using indexable tools under these cutting conditions. However, tool holding had to be designed to work effectively and easily.
New screw clamped toolholders with small diameter shanks (down to 0.375 inch square) are also now commonly available. The clamping screws on these holders are put in at a 45-degree angle so an operator can easily index or replace just the insert rather than the whole tool. Once again this reduces the amount of costly down time.
A Drill A Minute
A study done by Iscar Metal with a company that is running 25 New Britain screw machines found that over the course of a 1-month period almost 80 percent of the tool changes that were required came from part off tools and drills. The part off tools were the majority, but drills were a close second.
Whenever a drill is used in a non-rotating application, it has to be almost exactly on center in the X and Z axes. If it is not, the rotation of the part will cause it to deflect. This is not usually a problem with an hss drill unless the difference is too great. In the worst case, short of breakage, the drill will produce an oversized hole or break.
In carbide drills, any such misalignment is a disaster. For this reason, solid carbide drills are seldom used in a non-rotating application. Even a very small (less than 0.0015 inch) mismatch from center will cause the drills to break.
Brazed carbide drills have been the standard for these types of applications. The steel shank that the tip is brazed to allows for some flex in the tool. A minor amount of deflection usually causes no more problems than an hss drill would. Brazed carbide drills allow for a far higher cutting speed and penetration rate than hss and also last longer. These attributes mean more productivity. The drawback is in their initial high cost and loss in performance if they are reground.
On average, shops find that they will only produce between 50 to 75 percent of the holes with a brazed drill that has been reground when compared to new. Another added expense is the regrinding. It requires special equipment and skill to correctly sharpen carbide tipped drills. This has to be factored into the production costs. The cost of regrinding is magnified by the cost of replacement time. The tool must either be preset to the exact length that it was before sharpening or adjusted once in the machine. All of this adds more to the cost of the parts produced. A unique solution is to have a replaceable carbide tip on the steel tool body. Iscar's Chamdrill is an example of such a tool.
The drill body does not have to be removed from the toolholder, so replacement time is almost eliminated. A key is used to rotate a bayonet clamping mechanism in the drill body and matching area on the carbide insert. A quarter-turn of the tip releases or tightens the insert into the body. The total change time is usually less than a minute.
The precision grinding that is done for form tools and other carbide products, together with a "premium grade" carbide substrate, offers a high degree of dependability and accuracy to the screw machine industry. Companies such as Iscar are waking up to the fact that the screw machine industry is not a bastion of "old technology" and traditional process thinking.
Rather, it's an industry on the move to improve and use advancements in cutting tool designs and carbide manufacturing. It's the job of the vendors to help teach "the old dog new tricks."