Monday 9 July 2018
Derek and I were recently debating the merits of machine versus hand planing and the difference in finish given for the amount of time taken and energy expended. Whether you use a separate surface planer and thicknesser or a combined under/over planer/thicknesser, I think the machine planer has to be one of the most time-saving and useful bits of kit in our workshop. When a planer is set up and maintained well it gives a superb clean finish with very accurate results. But throw difficult interlocked grains or rippled timbers into the mix and the finish given can lead to 'planer rage'. Here, I'll look at strategies for keeping your planer as an asset to your timber preparation, even when things get difficult.
So, turning to basics and terminology for a moment, we have various options and names of kit to deal with. Stand-alone surface planers or jointers – as Americans call them – tend to have longer beds than a combined planer/thicknesser. Set up and maintained correctly, longer beds will give you straighter timber and are also less prone to accuracy deviation compared to combined planer/thicknessers, as the surface tables stay set, rather than being moved to accommodate the thicknessing process.
The width of surface beds may not be such a consideration when buying a planer, but with width comes all-important length. The surfacing beds on traditional combined planer/thicknessers tended to be shorter,
as they were fixed and you needed to reach underneath to remove your thicknessed timber; a backbreaking job if done for a long period.
Thicknessers could be relatively narrow at 300mm, or they could be much wider and termed panel planers. Combined planer/thicknessers, also known as under/overs, are more cost-effective to buy; they have one cutterblock where the timber passes either under or over the cutters, just as the name implies. The other advantage they have is all-important floor space, which itself does carry a cost. At the Furniture School we have a combined Hammer under/over and a separate long-bedded Felder jointer.
The reason for having two machines is that flatting and edging is a more time-consuming and critical process than thicknessing, and having the option of two surfacers and one thicknesser keeps the workshop as productive as possible. I feel you can get away with a cheaper planer/thicknesser but cutting corners on a surface planer is false economy. When I set up my first workshop back in the late '80s, I could only afford a fairly cheap pressed steel Startrite under/over; its beds and fence were quite poor but the thicknesser was relatively accurate once I made the decision to partner it with a classic Wadkin jointer.
Choice of cutters used for making planer knives
The steel used for planer blades varies between high speed steel (HSS) cobalt-chrome steel (Co-Cr) and tungsten carbide (TC). HSS cutters can easily be resharpened – either in-house or by your local saw doctor; they're not so expensive to buy and are good for soft woods, but will soon lose their edge on abrasive or tough woods. HSS cutters can be time-consuming to fit accurately compared to more expensive cobalt-chrome thin cutters, which are disposable and are quick to change over. They're harder and more durable and are a good choice of planer blade for the majority of modern workshops. Tungsten carbide blades are the longest lasting and most expensive but can be brittle and tend to be reserved for use in high-production workshops.
Improving the quality of finish from a surface planer
The quality of planed finish from a surface planer is affected by several factors, such as the sharpness of the planer knives and quality of steel used, how many knives are in the block, and most importantly, how many are actually cutting – they must be set correctly for them to all be cutting in the same cutting circle. Other factors are the rpm of the block, your feed speed, and to a lesser degree, the diameter of the cutterblock. The easiest factor to regulate is the amount of timber being removed and the most important one is feeding the timber with or against the grain. The number of cutters, rpm and feed speed determine the wave length or pitch between each cutter, taking a scallop with each rotation of the block. A slower feed speed will give a smaller pitch, which requires less hand planing or sanding to give a clean flat surface, prior to finishing.
Choosing the feed direction for timber is vital to surface finish and reading the grain is required. The fibres must be laid down by the cutters, not picked up and broken off. The cutting action always breaks timber off forward of the cutter, but when working with the grain, the next rotating cutter removes the previous cutter's damage. When planing against the grain the fibres are torn out leaving broken grain below the finished surface. This is normally remedied by turning the timber end-to-end for the next pass, but when we have swirling, rippled or interlock grain, we need other strategies.
Improving the surface finish
When we want to improve surface finish, one of the factors we can easily change when surfacing is the feed speed we use to push the timber over the block. Slower feed speed can make a huge improvement to the surface finish and amount of breakout. However, feeding too slowly reduces the life of the cutters by rubbing and overheating, rather than clean productive cutting; over-rubbing will dull the edge and produce dust rather than shavings.
A smaller amount of timber removed each pass improves the quality of finish, and as ever, cutter sharpness is critical. Blunt cutters will make the timber bounce, which in turn leads to excessive downwards pressure by the machinist; this bends the timber flat only to relax and re-bend when the machinist removes the pressure. Blunt cutters are also more likely to produce dangerous kick back.
When the cutting edge becomes dulled, HSS planer knifes can be sharpened while in-situ with an oil slipstone or, my preference, a dry diamond stone. On an old planer the evidence of this practice can be seen in a flat that has been worn on the cutterblock behind each cutter. To prevent this, I suggest isolating the abrasive stone from the cutterblock by covering the stone with masking tape or similar. The slipstone practice does lower the clearance angle on the planer blades, but gives them a bit more life between regrinding.
If the cutters have a small chip from contacting a hard knot or a metal object in the timber, it can be overcome by sliding the cutters. In my three-knife block I keep one knife in position and move the others 1mm to the left and right offsetting the damaged area. Care must be taken not to take the block out of balance or move the knife so much that it hits the planer's dust extraction hood or planer casing.
Improving the thicknessed finish
When it comes to thickness planing, we can slipstone the cutters as before, and move the cutters left and right if chipped, but most planer's feed speeds are set between 5-12 metres per minute. The more expensive machines can run at one of two feed speeds, fast – 12 metres per minute – and slow – 5 metres per minute – for an improved quality cut.
As we cannot slow the feed speed down any more to improve the planed finish, we have less room for manoeuvre so other tricks have to be deployed on difficult interlocked timbers. On our planer, we cannot reduce our depth of cut too much below 0.4mm as the serrated infeed rollers will leave printing marks on the timber's surface. So we have a couple of options. Note the following should only be done on separate thickness planers rather than combined under over machines. The reason for this is that the trick involves changing the cutter angles, which increase friction, and can be dangerous on hand feed surface planers as it increases resistance when hand feeding.
'Jointing' the cutters
One option is to 'joint' the cutters; this involves resharpening the cutters in situ while the cutterblock is revolving; this allows you to get the cutters in a perfect cutting circle, but can only be done safely on machines with an attached jointing grinder. This was traditionally carried out on larger production panel planers. The new bevel must be kept very small, as it means there will be little clearance, which can cause problems of heat build-up as the small 'jointed land' sitting behind the cutting edge is always rubbing the timber. The advantage of using this technique is that it produces a good quality of cut and a stronger cutting edge, which will last longer on hardwoods and abrasive timbers. It won't work so well on softer woolly timbers – those that will not be improved by cabinet scraping.
Producing a front bevel
In our workshop, the way we overcome tearing of the interlocked grain on our thicknesser is to sharpen the blades with a front bevel at around 10°. This is not unlike having a back bevel on a standard bench plane, or using a high-angle blade in a low-angle plane for cleaning up difficult interlocked grain. You can get your planer blades ground by your local saw doctor when they regrind the main bevel, or you may be able to achieve this on your Tormek with the SVH-320 planer knife jig. We have the Hammer A3-41 planer with disposable cutters, which you would assume cannot be reworked, but we do produce front bevels on these hard chrome cutters using a DMT mesh diamond whetstone.
This front bevel only has to be a maximum of 0.25-0.50mm; this will make all the difference. If we are planing rippled sycamore (Acer pseudoplatanus), or other interlocked timbers, the thicknesser can tear the grain to disastrous effect, but the front bevel produces a less aggressive scraping cut. The downside to this is a significant increase in energy consumed by the thicknesser and lighter-weight thicknessers may not cope with the power required to keep the timber moving. This is an older machining technique, which is now a little overlooked, as so many workshops have moved over to having wide belt sanders. But for those of us less fortunate, front bevels on thicknessers still have their place in the workshop, as hopefully I have demonstrated here.