Pre-assemble station
In the last issues of my column, I have talked about different topics all involving cutting, sanding, etc. - in short, processes which produce wood chips or dust. This time I want to touch on a ‘cleaner’ process, one I believe doesn’t get the attention it deserves and needs.
I am talking about product assembly.
The majority of woodworking shops assemble their products before shipping it to the customer or final destination. Most goods created in the secondary wood products industry are not RTA, knockdown or flat-packed furniture. Therefore the assembly process represents an important part in almost every shop or factory. The focus in recent years in most small-to-medium-sized-shops has been to automate the cutting process through investment into CNC machines, beam saws and soft- ware, however little focus has been paid to what happens afterwards. Now that the CNC will cut 40-50 plus sheets a day, can the rest of the shop keep pace? Or will the pile of parts coming into the assembly area grow and grow?
In shops that have recently invested into new production machinery, the assembly stage can become the bottleneck.
The speed and accuracy of the assembly process is dictated in large part by the cabinet construction method used. Let’s have a look at some of the different cabinet construction methods used today.
BUTT & SCREW:
Probably the most common assembly method for small to medium sized non-CNC shops. The cabinet parts usually get cut on a sliding table saw and edge-banded. Staplers are used to fix the parts together and indexed on a stationary assembly table, then screwed together with a cordless drill. Staple and screws provide the benefits most are looking for with very low cost in equipment. Assembling with screws is a simple and effective choice, if other parts of the shop aren’t automated. The butt & screw method becomes a problem with finished ends, a separate end panel is needed to cover screw holes, or the gable end needs to be laminated over, which adds extra time to the process. If the shop gets busy or grows over the years, more workers get hired and assembly tables added as needed. This method needs a lot of floor space and is very difficult or impossible to automate.
BLIND DADO:
Some shops – usually the ones which have just recently switched over to nested-based manufacturing - consider going with a blind dado cabinet construction. The reason behind going with blind dados is that it would help in assembling the box, as parts alignment during assembly is possible, but trade off more machining time. Woodworking shops that have gone the blind dado way and have reached the cutting capacity limit of their nested-based CNC may at some point consider switching over to dowel construction. Shops will look at a horizontal drill & dowel inserter after having adapted to the nested-based system. Casework assembled with the help of blind dados also needs a lot of floor space during assembly and is difficult or impossible to automate.
CONFIRMAT SCREWS & OTHER HARDWARE CONNECTORS:
Confirmat screws or cam locks require special pre-drilling of the cabinet parts. If you are manually pre-drilling confirmats, part alignment can be tricky and blowouts are a real possibility. No parts alignment during assembly is possible. Confirmat screws, cam locks and other hardware are mainly used with RTA furniture (see IKEA).
SUMMARY:
With the above construction methods, screws or staples will show, which means cabinets with finished ends have to go through another process.
As well, all of the above methods re- quire bench assembly, which can be a problem with unskilled assemblers.
DOWEL CONSTRUCTION:
Dowel construction is by far the most used technique for most wood products, whether chairs, cabinets, or frames. Dowel construction is extremely strong, invisible and inexpensive and provides fast, perfect parts alignment. Dowel construction forces the assembler to put the work piece together the correct way. No screws are peeking out and it is virtually impossible to misalign parts.
Shops that use dowel construction for their cabinets, use machinery like horizontal drill & dowel inserters and case clamps to put the frameless case- work together. Dowel construction is the only method that can eliminate stationary bench assembly. While the cabinet is setting up in the case clamp, the back can be installed on the clamp- squared work piece and fixed in place permanently. While the cabinet is in- side the clamp, the next cabinet can be pre-assembled and is ready when the preceding cabinet comes out of the clamp. Depending on temperatures and glue used, about 2-3 minutes of clamp time is required before the next cabinet can be moved into the clamp.
The reason why I stress the difference between bench assembly and case clamp dowel assembly is that bench assembly requires a more skilled worker. If dowel construction is consequently put to use, we can set up different stations, going from hardware fixing, to pre-assembly of the cabinet parts to infeed into the case clamp.
The same can be done on the out- feed side, where drawers can be inserted and so on.
While all of the above mentioned cabinet construction methods have their advantages and disadvantages, it can be said that once a shop reaches a certain size, the only meth- od should be dowel construction, as it is the only one that allows for a non-stationary assembly and a continuous flow of the assembly process (throughfeed assembly).
Apart from the construction method being an important part to optimize the assembly process, I want to briefly look at a few other factors that can have a huge impact on speeding up the assembly.
MANUFACTURING FLOW / SHOP LAYOUT:
Having a properly laid out shop with a clear material flow can be half the battle when it comes to optimizing the assembly process of casework. The material flow should be clear, as short as possible and never cross. As soon as the product is assembled it should leave the assembly area and free the area for the next product. Also, the multiple handling of finished products has to be avoided. Moving completed products from one corner to the other is not only time consuming, it is also another pos- sibility to damage the product.
PARTS IDENTIFICATION:
How much time is wasted by the search for the right parts? The label- ing of parts is one possibility to avoid any searching time. Do not start the assembly, unless all parts are present. This ensures that the assembly area is not stuck with unfinished products, where parts are missing.
QUALITY CONTROL:
Another positive effect of the throughfeed assembly system is that the quality control is done at each station, whereas during bench assembly usually one worker does all steps.
ORGANIZATION OF ASSEMBLY AREA:
One of the most important factors for the efficiency of your assembly is the organization of the assembly process. Instead of doing all necessary steps by one person at one fixed location, it is more efficient to utilize the throughfeed assembly method. By using this method the complete assembly process is broken down into separate process stations. The most common stations are:
• Hardware insertion
• Pre-assembly
• Case clamp
• Drawer / shelf / door assembly
• Packaging
This means the product is moving from workstation to workstation, where specialists perform only part of the complete assembly. This makes the assembly faster, more consistent and allows the use of more efficient tools and machines.
Christian Vollmers is the president of HOMAG Canada Inc. He can be reached at Christian.Vollmers@ homag-canada.ca or visit the website at www.homag-canada.ca
