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| MACHINING,JOINTING & FIXING |
Data Sheet 5
Sep 2001 |
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Scope
This sheet describes basic principles of machining and fixing Particleboard and Medium Density Fibreboard (MDF). Any specific requirements related to a particular end use are dealt with in the relevant Installations and Applications sheets.
Particleboard and MDF are simply worked with normal woodworking tools. The uniform and consistent character of the material coupled with a non-directional grain, allows conventional techniques and machines to be readily and economically applied with constant results. Precision techniques are best suited to the machining of these wood panels and when extended to jointing and assembly the user will derive the greatest advantage from the use of the material.
Machining
Particleboard and MDF can be sawn, routed, spindled, planed or bored. The rate of feed should generally be slower than that used for natural timber and cutting edges should be kept thoroughly sharp. This is particularly important in the case of plastic laminate-faced boards as otherwise the edges may bell or spread slightly.
Sawing - Plain Particleboard and MDF
As the grain direction of the wood particles and fibres in Particleboard and MDF are of a random pattern, saw blades with cross-cutting tooth forms should be used.
Handling : Control over the board during machining is important; boards should be properly supported and pressed down firmly against the cutting table and guides to avoid vibration. The most satisfactory results will be obtained with a constant rate of feed which should not exceed 23 m/mm. Generally mechanical feeding, though not essential, is recommended. Because of the difficulty in adequately controlling long boards, a travelling saw will often produce a more satisfactory result.
The projection of the saw above the board has a direct influence on the cleanliness of the cut. Breaking out or chipping of the top surface will occur if it is insufficient and on the bottom surface if it is too great. If either occurs, the projection should be adjusted accordingly until the defect disappears. If the fault persists, the saw speed should be increased or the rate of feed reduced.
Saws: For small quantities of intermittent use, any of the normal hand or power tools can be used with satisfactory results. An aluminium cutting saw blade, 175-250mm diameter, is recommended by one of the major power tool manufacturers. For quantity production, any of the conventional machines used for cutting natural timber are suitable. Tungsten-carbide cutting edges are recommended as they will have at least 20 times the cutting life between sharpenings of a high-speed steel blade and should be capable of about 30 sharpenings before the teeth are worn out.
Circular blades should be 355mm with 60 teeth and have a peripheral saw speed of 3000-3350 metres/min. Teeth angles are important and particularly the need for a positive front angle. The diagram below shows the recommended details:
The saw blade should rotate in the opposite direction to the feed and a riving knife should be fitted to open the cut.
Sawing - Plastic Laminate / Veneered Particleboard & MDF
It is strongly recommended that surfaced boards are reduced to finished component size by sawing, and that spindling or muting cutters are only used on the edges when the shape of the panel precludes sawing.
The techniques of sawing wood panels are well established but cutting boards with a hard brittle skin, such as melamine surfaced and ready to paint boards, can cause unacceptable chipping or break-out of the surface. The need for high quality precision cutting at high output rates has led to the development of sophisticated equipment. Many modem panel cutting saws work on the principle of stationary work piece and a moving saw, cutting a number of boards at a time and with a small scribing saw to eliminate chipping. These machines may be necessary for specialist fabricators and others cutting large quantities of board, but satisfactory results can be obtained from an ordinary saw bench provided the equipment is in good order.
With the variety of equipment likely to be encountered it is not possible to lay down precise details, but the notes below give general recommendations for a normal bench saw and should result in a clean cut with sharp edges on top and bottom surfaces.
Handling: Cut with face side uppermost. Direction of feed should be opposed to the rotation of the saw. Ensure the work piece is supported overall and not balanced precariously by the operative.
The work piece must be firmly bedded on the saw bench. Preferably cut exactly to size on the first pass. If it is necessary to rough cut oversize, note that careless cutting can cause hairline cracks in the surface which will lead to chipping on subsequent operations. This will equally apply if panels are spindled to final size.
Saws: Use tungsten-carbide tipped saws. Saw diameter 350mm or 450mm with 75 or 96 teeth. Tooth shape approximately as illustrated although this may vary according to the saw and the manufacturer's particular ideas. The positive front angle of 4°-5° has been found most satisfactory.
Feed speed should not exceed 15m/min. Mechanical feed is best but if hand feeding, a steady rate is more important than precise speed. Projection of the saw above the work piece should be between about 8mm and 20mm.
If chipping is observed on the top face raise the saw, if on the underside lower the saw.
Spindle and Router
As Particleboard and MDF have a non-directional grain, grooves, recesses and housings can be easily cleanly cut. These processes are normally carried out on a router and tungsten-carbide tipped cutters should be used. The cutter spindle speed should be 18,000 - 24,000 rev/min and the material feed rate 4.5-9.0m)min.
It may be necessary to bring a surfaced panel to size on a spindle or router with the aid of a jig. When minimal quantities are involved, highspeed steel cutters can be used but they rapidly lose their edge and for quantity production tungsten tipped cutters are necessary. However for large volume producers where consistent and accurate profiling is important, diamond tooling is now being used. Tooling might cost ten times the equivalent in tungsten carbide, but it will run 50-100 times longer without needing attention.
Panels will have been cut oversize and the necessity for careful sawing is again stressed. Any excessive chipping at this stage inevitably gives rise to hair-line cracks which "chase through" with the impact of spindle or router cutters. Ideally make a clean saw cut and remove the minimum possible by cutter.
Keep cutters sharp. Dull Cutters will cause the edge to "bell" or spread, which will give trouble when subsequently dressing, edging or lipping flush with the surface. The edges of wood panels can be easily planed and profiled on a spindle machine. Rebates tongues and grooves are the types of moulding best suited to the material and profiles should be kept simple. Because the material can be cut easily and cleanly the use of simple joints in assembly is practical and economical. It is not possible to lay down precise details but in genera] feed speeds should be slower than for solid timber and the maximum number of cutting edges possible provided. The following is suggested:
| Spindle Machine |
|
| Speed |
4000-10000 rev/min |
| Cutter block |
minimum 4 cutters |
| Cutters |
Toe 42 ° Heel 45 ° |
| Material feed |
4 to 5m/min |
| Router |
|
| Speed |
18000-24000 rev/min |
| Cutters |
double edge bit minimum |
|
25mm cutting edge ground 53° angle |
| Material feed |
4 to 5 m/min |
Drilling
As other processes, cutting edges must be sharp if a clean hole is to be cut. Drill speed and the angle of the drill point should be the same as that used for normal woodworking. When drilling through plastic laminates the point angle should be between 80 ° and 90 ° .
Planing
Particleboard and MDF are generally supplied sanded to a uniform thickness so planing should not be necessary. Should it be necessary to reduce the thickness this is best carried out on a belt or drum sander, but it is advisable to avoid reducing the thickness of layered boards on one side only as this may well unbalance the board.
Planing of the edges should similarly be unnecessary as a clean face can be obtained from a saw cut. However, where it is required, there is no difficulty (see notes under 'Spindle Machine' for cutter speeds).
Sanding
As most wood panels are supplied with a sanded finish, sanding is normally necessary only as a finishing process after the machining or fabrication of a component. Sanding is preferable to planing where the edge of Particleboard and MDF is required as a finished face and the saw cut edge considered unsatisfactory. Techniques used are similar to those for natural timber but as these wood panels have no grain direction, boards can be sanded in either direction.
For larger areas or quantity of work, fibre backed papers are recommended. For finishing work, a 120-200 grit grade should be used depending upon the degree of smoothness required. Excessive sanding should be unnecessary and because it could unbalance the board it should be avoided.
Jointing
Two of the outstanding characteristics of wood panels are that they have a non-directional grain and excellent gluing qualities in all planes. Pieces can be cut from a board in the most convenient and economic way irrespective of their orientation in the board. Because the wood fibres and flakes lie in a random pattern there is no end grain and a consistently good gluing surface can be obtained from a clean saw cut irrespective of the direction or angle of the cut.
For the majority of situations, simple glued joints are the most appropriate and economic. They take full advantage of the characteristics of the material and make more complicated mechanical methods both unnecessary and relatively expensive.
Joints between boards
There are many ways of detailing board to board joints. The selection of a particular method will largely depend upon the finished appearance required and the equipment and facilities available. Where boards are to be painted, laminated or veneered, a plain butt joint is normally suitable. If the edges have been cleanly cut, planing will not be necessary. Both edges should be liberally coated with adhesive and pressure applied and maintained until the adhesive is set. A loose thin Particleboard, MDF or hardboard tongue, 4-5mm thick and about 18mm wide can be incorporated in the joint to facilitate surface location but will add little to its strength. Where joints between boards will be exposed, there are various methods that can be used. Some of these are described below.
Joint treatment
The treatment of the joints where two panels meet is very much a matter of taste but it is generally accepted that it is more effective to make a feature of the joint than to try hiding it or losing it.
Small V joint: Suitable for all types of board but particularly plain or painted panels which can be nailed directly to battens.
Board edging
Edging of Particleboard and MDF is carried out to match or harmonise with surface treatment. With its tight, fibrous edge, MDF can be edge profiled and painted to match the colour scheme. Particleboard should have an edging applied.
Boards can be edged or lipped in a variety of ways. Edges can be veneered easily to provide a matching finish to the surface. Provided that a clean saw cut has been made, further treatment of the edge surface is unnecessary. Veneers can be applied by hand or machine and the use of a urea-formaldehyde adhesive is suitable for most cases. An alternative edge details is to use a plastic strip with a toothed tongue on the back face which is pressed into a thin groove cut in the edge of the board.
Solid wood lippings of any suitable width can be satisfactorily glued with a plain butt joint direct to a cleanly cut edge of Particleboard. Edged boards are offered by most manufacturers.
A tongue and groove detail can be used but serves only to facilitate accurate location. Where this detail is used, the groove should be in the wood panel edge.
Although lippings are normally applied after veneering of the surface, they can be satisfactorily applied before if required. In this case, the adhesive should be allowed to dry out thoroughly before veneering to ensure minimal risk of the joint line 'showing through'. Where edges are to be lipped after veneering, it is usual practice to use a slightly oversize lipping and sand down either by hand or drum sander to provide a flush joint .
For most situations, a 6mm thick lipping is adequate but where provided to take hinge fixings, a thickness of at least 12mm should be used.
Carcase Joints
With care in the design and selection of joints, Particleboard and MDF are well suited to carcase construction. Simple glued joints are characteristic of the use of the material for this purpose and one of the main reasons for its widespread application in the mass production of furniture.
The gluing qualities of wood panels are excellent in all planes and full advantage should be taken of this in the design of joints. Deep and/ or wide grooves cut in the edge, or thin and/or wide tongues moulded on the edge of boards will weaken the joint and should be avoided.
Provided that edges have been cleanly cut a plain butt joint provides adequate strength for many situations and is economic. At vertical corner junctions a plain mitred joint can be used successfully.
Some means of ensuring accurate location of the components to be joined is often of practical advantage in assembly; for example, a loose tongue may be incorporated in a mitred joint. There are various other ways in which such provision can be made and some are described and shown below. Generally, they assist only in locating the components and add little, if anything, to the strength of the joint.
Adhesives for Jointing
Although most of the adhesives commonly used for jointing natural timber are equally suitable for jointing wood panels, those made from synthetic resins are normally preferred. Animal glues are not generally suitable as their relatively high water content may cause local swelling of the board.
There are many types and grades of synthetic resin adhesives available for a wide variety of specific applications. The requirements of the user together with the facilities available to him wilt have influence on the choice of adhesive most suitable for his purpose. The following notes therefore give general guidance on the type of adhesive suitable for particular applications.
Synthetic resin adhesives consist of a syrup and a catalyst. The catalyst, which may be in either powder or liquid form, is either mixed in certain proportions or applied separately to one face and the syrup to the other. Adhesives of the mixed application variety, have the catalyst already incorporated. It is essential that the adhesive manufacturer's instructions are closely followed in all cases.
For board to board joints, a urea-formaldehyde (UF) adhesive should be applied to both edges. A mixed application type is recommended. Alternatively, a polyvinyl acetate (PVA) adhesive can be used and should be of high viscosity to avoid starved glue joints.
For edge lipping or veneering with wood, UF adhesives are usually best although a high viscosity PVA is also suitable. Mixed application types are preferred. Where separate application types are used, the syrup should always be applied to the board and the catalyst or liquid hardener to the wood to avoid starved glue joints. Plastic laminates are best bonded with a UF adhesive and it is important that pressure is applied and maintained on the joint until the resin is cured. Alternatively, a rubber-based contact or impact adhesive may be used. In this case, a coat of adhesive should first be applied to board edge and allowed to dry thoroughly. A second coat is then applied prior to bonding. Extruded PVC strip edgings should be bonded with a rubber-based adhesive . The wood panel edge should first be coated with a thinned solution of the adhesive and when dry a second coat applied.
MR Particleboard and MDF have special waxes incorporated to provide water repellent properties. This feature of MR boards may interfere with the curing of adhesives otherwise suitable for standard board. Specially formulated adhesives may be necessary for bonding MR Particleboard and MDF.
For carcase joints, mixed application UF or PV adhesives are recommended.
Fixing
Methods of fixing Particleboard and MDF are basically similar to those normally used for fixing natural timber and other wood-based boards and panels. They can be broadly divided into three groups, nailing, screwing and knock-down methods. The following notes give recommendations on the techniques and types of fastening generally suited to various applications of wood panels.
Nailing
This is the quickest and cheapest method of fixing wood panels to a timber supporting structure. Bullet head nails with either a round or oval shank can be used, particularly for secret nailing through the tongue of t & g boards. They can be punched below the surface easily where a painted finish is required. Helically threaded nails give some increase in the resistance to popping and withdrawal and a significant improvement is obtained by the use of annular ring-shanked nails. These latter types, and particularly annular nails, are preferred for flooring where fixing is through the surface of the board.
For temporary fixings and where easy withdrawal is required, for example in concrete formwork, duplex or double-headed nails are suggested.
Spacing of nails largely depends upon the purpose for which the board is being used and will normally vary between 300 and 600mm. Reference should be made to the relevant Applications & Installations Sheet for recommendations on nails spacing. Nailing should always be through the board into the member supporting or being supported rather than the reverse. Where fixings are required to be made into the board, methods other than nailing are more suitable and should be used.
Some are described in the following notes. Increasing use is being made of portable nailing and stapling machines for site assembly work and they can be used successfully for fixing wood panels. There are many types made by a number of manufacturers and reference should be made to their information in respect of the type of machine and associated fastening best suited for the user's requirements.
Screwing
The screw holding power of Particleboard and MDF is directly related to the type and density of the board and the type of screw used and this data should be obtained directly from the wood panel manufacturer.
Screw holding power is also improved significantly with use of double threaded screws such as the GKN Twinfast. These screws have a greater length of thread than conventional single threaded types and are often threaded up to the head. The plain part or the shank is smaller in diameter than the threads and this virtually eliminates the splitting effect normally associated with single thread screws. They have a self-centering point and driving is claimed to be easier and faster.
Whichever type of screw is used it is recommended that a pilot hole is drilled for the full screw depth to avoid show-through on the opposite face of the board or splitting when screwing into the edge of the board. Screws should not be screwed over tightly and never hammered.
Although screwing directly into Particleboard and MDF for permanent fixings is often suitable, where improved strength or demountability is required, the use of a solid wood insert or special fastening is recommended. Wood dowels should be glued into holes bored of sufficient diameter and length to give a good fit without the need to hammer the dowel home. An alternative to the wooden dowel is a special fastening device comprising a nylon plug or bush which is vertically threaded on the outer face. It is glued into a hole bored to give a good fit and a self-tapping screw is then driven into it. This type of fastener serves the dual purpose of providing a secure but easily demountable knockdown fixing.
Other forms of special fastening devices that provide a high degree of resistance to withdrawal are those that are inserted into a hole pre-drilled through the board and either partly deform or obtain anchorage on the back of the board when tightened. Devices that obtain anchorage on the back of the board are the spring and gravity toggle type. With the gravity type, a portion of the device is held parallel to the screw during insertion and drops perpendicular to the screw when passed through the board. The screw is tightened to obtain a firm anchorage. The spring toggle type normally has two sprung metal wings which are held closed during insertion and then open when passed through the board. Again the screw is tightened to obtain a secure anchorage. The hole size necessary to enable insertion of a toggle device is larger than the screw or shank diameter and thus lateral location will tend to be less rigid than with devices that expand or deform.
Where edge fixings are required to support heavy loads, such as hinge fixings for large doors, the use of a wood lipping is recommended. The lipping should be about 18 mm thick for large doors, although 12 mm would be suitable.
Knock-down Fastening
Only permanent fixing should be made directly into Particleboard and MDF. Where demountability and re-assembly will be required, the use of special inserts or knockdown fastenings is advised.
Reference has been made in previous section to the use of inserts with conventional screw fixing and these notes give a general description of the range of devices specifically made for knockdown applications.
Knockdown or assembly fastenings can be broadly divided into three groups; concealed, surface and flush fitting.
Concealed fastenings are often based on press-stud action. These consist of an insert fixed into a pre-boxed hole in one component and a dowel fixed into a pre-boxed hole in the other. The insert is either ridged or lipped on the inside face and locates into grooves in the dowel when the two parts are brought together. With some types, the dowel is separate and locates into similar inserts fixed into each component and provide a neat and precise joint.
Surface fittings are widely used where visible fastenings are acceptable and economy important. They vary in the type of locating devise used but all are fixed directly onto the surface of each component and do not necessitate any machining. They consist of two parts which locate and are held together either by locking, screwing or wedging.
Locking and wedging devices are usually metal and screw devices either plastic or metal. Some are quite large, but they all tend to be less expensive than other methods.
Precise location of components may be made easier with some of these fasteners if dry dowels are incorporated in the joint.
A number of flush fitting devices are available and they vary widely in their design and method of fastening. Generally, these types of fastener require more machining of the components to be joined and tend to be more expensive but they give a stronger joint.
The range of fasteners available for knockdown applications is wide and varied. The type of applications, visual appearance and cost will influence the user's choice, so it is not possible to make specific recommendations on suitable fasteners.
Health & Safety Information
Normal health and safety precautions should be taken when working with wood panel products. Machine tools should be fitted with dust extraction and work areas should be kept clean. If dust levels exceed The National Occupational Health & Safety Commission's standards, the wearing of a dust mask (AS/NZS 1715 and AS/NZS 1716) and safety glasses is recommended.
Storage and work areas should be adequately ventilated. If large quantities of Particleboard are stored in non-ventilated areas, formaldehyde emissions could accumulate to levels that may irritate some people.
For further information, please contact one of the manufacturers listed below for a Material Safety Data Sheet or download from www.woodpanels.org.au .
Whilst the information contained herein is based on data which to the best of our knowledge is reliable and accurate as of the date hereof, no responsibility can be accepted by us for errors or omissions. Since the information contained herein may be applied under conditions beyond our control, no responsibility can be accepted by us for any loss or damage caused by any person acting or refraining from acting as a result of this information.
Published by the Australian Wood Panels Association Incorporated, PO Box 158, Coolangatta Qld 4225, September 2001
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