Feed Rate | Spindle Design and Condition | Machine Capabilities
Material Handling | Grinding Ability
Grinder Precision | Moulder Condition | Material Used
Finish Quality | Length of Production Run
Operator Training | Cutterhead Selection Chart | Hook Angles Recommended
How to select which type is best for your application.
When making the decision on which cutter style to use, several things must be considered. These include such things as: feed rate, type of spindle design, machine capabilities, material handling, grinding ability, grinder precision, moulder condition, type of material being produced, finish quality of product, length of production run, accuracy of production parts and operator level of training.
The feed rate that you are expecting your machine to run must be known when deciding on the cutter head style. Most small shops have an expected feed rate of less than 50 feet per minute. This feed rate fits in well with the single knife finish. For this type of feed rate, the use of a straight bore tool is most common. This type of tool is slid onto the spindle and locked in place with spacers and a lock nut. The accuracy of this tool type is determined by the slip fit between the bore of the tool and the spindle. Normally this slip tolerance is between .001” and .0015” . This means that the longest knife will produce the finished cut and show the fastest wear. Many small shops have the desire to increase their feed rate by going to the next level of precision in tooling, the hydro locking tool. In the majority of cases, this does not significantly improve the feed rate. This is due to some of the other factors that we will look at later including the spindle design and machine capabilities.
The next group of shops use older machines, many use push type moulders. The feed rate on these machines can vary from single knife finish (35 fpm ) to high speed (250 fpm + ). A large percentage of these machines use self-centering sleeve heads. This type of tool uses a sleeve that locks the head to the spindle. The condition of the tool is very important. Many of the older tools have never been balanced for proper control of the tool. The newer self-centering sleeve tools still will have some inaccuracies due to basic design. The tolerance between the sleeve and the head body, when added to the tolerance between the sleeve and the spindle can cause this type of tool to end up with a tolerance of several thousandths of an inch. The advantage of this tool on many older machines involves the spindle condition. See the next section. The self-centering sleeve can be jointed for high speed production. It is not as accurate as the hydro locking tool. Due to the inaccuracies of the tool design, high speed jointed operations will notice a variation in the amount of flat created from the jointing process. All the knives will not have the same amount of flat. The longest knife will have more flat on it after being jointed. To reduce the increase in flat on the tool, most shops increase the back clearance of the tool. This allows more joints to be done before the longest tool has too much flat to be cutting properly.
The use of hydro-locking tools is normally limited to those shops that run higher feed rates or for those shops that want to attempt to gain accuracy from the tool room. The accuracies gained from the tool, may allow for an increased feed rate if the machine is capable.. What it does do is to equalize better the cut between all the knives in the head body. This in turn may improve the finish quality of the part. To most people , this finish quality gain is not noticeable. To others, the reduction in tear-out and chipping of the part are important.
For those companies that run high feed rates, the use of hydro locking tools is the best choice. This type of tool allows the precision from the grinder to be transferred to the moulder. The tolerance between the bore of the hydro locking tool and the spindle is virtually zero after the tool has been pressurized. The inaccuracies then are found in the machine and spindle design and the grinder precision. For feed rate gain, the use of high speed jointers ( striking the knives ) must be used. With this process all of the knives in the head body are struck with an abrasive stone while the head is rotating and this “grinds” all the knives to the same length. The use of the term “grinds” is used to explain that the knife has steel removed from it. This does not sharpen the knives but actually begins the dulling process.
The spindle design and condition is critical when deciding which tool to use. The spindle design varies greatly between machines. A standard machine may use a spindle design that has 3 or 4 bearings. The use of 2 bearings is becoming common and the use of more than 4 bearings is uncommon. The type of bearings used in different spindles also varies.
On a basic machine, the spindle could have a bearing arrangement with one bearing on the pulley side and one or two or three bearings on the head side. They could also have an arrangement of two bearings on the pulley side and two bearings on the head side. A standard spindle uses mainly a radial bearing, but can use angular contact bearings. The radial bearings do not provide as stable of a rotation as do the angular contact design. Radial bearings are also less expensive. Depending upon the specific design of the spindle, the stability is a key part of how a head design will function. For example, on a standard radial bearing designed spindle, the runout, or allowed inaccuracies of the spindle when turned by hand and checking with a dial indicator, is 30 microns or .00125”. This is not very accurate and will cause a tool that is running at high feed rates to fall out of joint if it can be jointed at all.
On machines with high precision bearings, normally angular contact thrust bearings, the accuracy of the spindle will be 6 microns or .0002”. This accuracy allows the tool to run in a true circle and high speed jointed knives will better maintain their joint.
This simply means that for machines with standard bearings the use of a straight bore or self-centering sleeve type head will provide as much accuracy as needed to produce a good product. This is not implying that all straight bore heads are alike. Different manufacturers make tools to different accuracies. For machines that use the precision bearings and have their spindles designed to handle this type of tool, the use of hydro-locking tools is usually the best choice.
What about the condition of the spindle? The spindle condition is very important to the correct use of the tool. A spindle that has had a cutter head spun on it or that has been sanded or deburred, may be undersized to the point that certain tool types will not work well. On standard moulders. a bent spindle or a spindle that is damaged from abuse can cause the tool to rotate out of round. This will produce an unacceptable finish and increases the noise level of the machine. On a machine where hydro-locking tools are to be used, the spindles must be straight and not undersized by more that .005”. If the spindle is undersized by more that .005”, then many hydro-locking tools will distort and not return to the original bore size. This is due to the fact that steel has a memory, and when stretched too far will not return to its original condition. Some new design hydro-locking tools can be installed on spindles with more wear than the .005” and can be installed and used successfully on spindles with keyways.
Note: Always use a mechanical locking ring or approved device when using hydro-locking tools.
The machine that you are using will have some limitations. These limitations will vary from machine to machine. A standard machine, using standard bearings and spindles, cannot normally be modified to run at a high feed rate. To attempt this is usually a waste of time and money. The spindle design, as described earlier, will not allow it, nor will the guarding of the spindles. The horsepower also will limit what the machine can do.
The cost of adding the required components to run a standard machine as a high speed machine is very hard to justify, if it could be done at all. If you have or are planning to acquire a machine in the future, consider the spindle size. If you buy a machine with 1-1/2” spindles and plan to run high speed production in 3 years, you may need to find out if the tools you are now getting can be used later. Since most high speed moulders are 1-13/16” or 2-1/8” bore, the 1-1/2” tools would not work. On the other hand, if you have a machine at present that uses 1-13/16” tools, you may want to buy hydro-locking tools now and be able to use those tools on a future high speed moulder.
For machines bought with the high speed ability, the use of the hydro-locking tools should be a given. It is wise not to over buy or under buy the tool for the application that you are doing. For instance, use a 60MM (2-3/8') width cutter head body for small knives and use 150MM (6”) width tool for a knife 5 or 6 inches long. By using a small width knife in a long cutter head you are increasing the possibility of damaging the tool or machine.
The diameter of the head body is also very important. Many machines have an overall cutting circle that is smaller than what may be expected. If this is the case with your machine, be sure to get heads that will allow the tool to swing inside the hood or guards. A standard diameter head body may limit the machine to running shallow profiles and not allow for the larger profiles.
The way in which the material is handled to and from the machine will directly affect the feed rate of the machine. Due to its affect on feed rate, the type of tool may change if the material cannot get to or from the moulder.
This is one of the major concerns for many shops. If you have your tools made out of house and the knives must be installed into your head bodies, then the hydro-locking tools provide no advantage at all. The use of straight bore or self-centering sleeve heads provide as much accuracy as the tool requires. If you must have your tools made out of house, use a high quality supplier and tool steel. This will increase the accuracy to the best level short of grinding knives in house.
Without question, it is best to grind and prepare tools in house whenever possible. This puts the control of the tool steel material, balance of the tools and accuracy of the grind into the hands of the people that must use the tools. A properly trained operator is critical in the tool room as well as at the moulder.
If you are doing in house tooling, the machine must be in a condition that produces the highest accuracy possible. If you run a standard moulder, the accuracy of the grinder is slightly less critical than if you run a high speed moulder. It is best to have a high precision grinder and a less quality moulder than the other way around. This is not advising you to purchase a low quality moulder, it simply is stating the importance of the tooling in the operation of a moulder.
Grinders that are used daily, generally require a major cleanup or tensioning every year or 2. This can be done in a few hours with proper training. If the machine is not kept in the best condition, the tooling will suffer. This can cause problems with the moulder and affect which tool is used, as well as greatly effect the finish quality of the product. If a misaligned grinder or low quality grinder is used, then the use of hydro-locking tools may not give you the results that you expect or desire.
In order to run any tooling properly, the machine that the tool is being run on must be in top condition. No tool will work well if the fences, tables, rollers or pressure systems are worn or misaligned to each other. A mistake that is often made is to purchase a high accuracy tool, for example a hydro-locking tool, and expect the new tool to improve quality or speed. If the machine is not aligned, this tool purchase cannot make an improvement. If the tools have been used for several years and problems begin to show up that can not be explained or eliminated, then the wear of the machine parts may be in question. Problem areas that are common will be the fence and table wear and alignment and the feed system condition. Also,the spindle alignment should be looked at. When selecting which tool to use, consider whether or not the machine can perform to the level of the tool. Out of 100 “tool problems”, 80 will be machine problems, 15 will be tools that were missold and only 5 will be true tool problems.
The material being used in today's woodworking goes from one extreme to the other. Solid wood machines differently than MDF does. Hardwoods machine differently than softwoods. The selection of whether to use straight bore, self-centering sleeve or hydro-locking tools can vary due to the material. This also will determine the use of high speed steel or carbide. If you are producing with MDF and want to run at high feed rates then the use of hydro locking tools is a must. If you are producing with hardwoods at a high feed rate then hydro locking tools are by far the most preferred. If you run softwoods at a high feed rate then the hydro locking tool is the best choice but the self centering sleeve heads work well. The difference is the angle at which the knives are ground. With the softwood knives, the angle is very steep and the amount of jointing that must be done to bring all the knives into the same cutting circle does not affect the performance of the knife as much as it does with a blunter angle for the hardwoods. Once again the hydro tool is preferred by most high precision and high speed production shops.
The finish quality of the product will be used to decide which style of tool should be selected. If the decision is not already made by the other factors looked at, then the finish quality may be a deciding factor. By increasing the accuracy of the tool by going to a hydro locking tool, you may be able to reduce the depth of the knife marks. This is done by having all the knives cutting more equally. The accuracy of the tool is transferred from the grinder to the moulder. Do not expect the use of hydro tools to give you enough of a finish difference to be noticeable by most people. If you think this may help you in your operation, then try one or two heads first and do some simple testing before you make a major tool change.
The most common way to predict the surface quality of wood machined with a cutterhead is knife marks per inch (KMPI).
KMPI=RPM x # of knives divided by feed (FPM) x 12.
On non jointed machines, where all the knives are not exactly on one and the same cutting circle, a one must be used for # of knives regardless of how many knives are set in the cutterhead.
Number of KMPI targeted in various industries are:
Ext. wood prdts 8…..12
The length of the production run must be considered. If all of your runs are 5,000 lineal feet or less then the use of hydro locking tools is very hard to justify. If your runs average over 5,000 lineal feet and your machine has the ability to produce at a high feed rate, then the use of hydro locking tools is best.
The level of training and ability of the operator must also be considered. Many companies want to run fast and use hydro locking tools to make this happen. If the operators of the grinder or moulder do not have a good understanding of the process and the machine is not operated correctly, then the system is not likely to work. With proper training and experience, operators can take tools that may not work for others and make them work well.
In general, use the best tool you can that will fit into your budget and needs. It is not recommended to over buy the tool, if you need straight bore head, do not buy hydro locking tools.
Used with permission of The Grinderman’s Association “The Profiler” newsletter. Protected under penalty of law.
HOOK ANGLES RECOMMENDED BY WOOD SPECIE
The following chart is a guideline for recommended hook angles for different types of wood. Check with one of our technicians for more detailed specifics for your application. There are many factors that are used to determine exact hook angles, and our technicians are highly trained in the use and preparation of the cutting tools.
|NUMBER OF KNIVES CUTTING (JOINTED)|
|3,600||10||30 ft||60 ft||120 ft||180ft||240 ft|