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Wednesday, 30 August 2017

Shortening a chainsaw bar

Shortening an 18” 3/8”pitch Huztl “Holzfforma” brand bar to 14”

Short bar on Huztl 036

Update Dec 2018

The post below was written over a year ago, after my first try at shortening a chainsaw bar. Since then, I have shortened several more and I'm very happy with the results. Some notes on my experience since then:
  • I won't buy any more Holzfforma bars - unless they make a major quality improvement. Poor quality steel results in chips coming off the bar rails, making the bars unusable after a few hours use. Here's my review of Holzfforma bars. Oversize groove gauge also makes for a lot of trouble hammering and dressing bars. 
  • I now use 1.6mm thick cutting discs in my angle grinder, which makes grinding the groove much easier and more accurate on bars with 1.6mm gauge (Stihl saws use 1.6mm gauge on .404", 3/8" and .325" pitch bars and chains).  

Why shorten a chainsaw bar?

I like my chainsaws to have bars as short as practical. This has many advantages: a lighter unit to lift, gives the motor an easy life, less bar and chain to lubricate, more power for the teeth in the cut, quick sharpening, etc. (I go on more about the benefits of short bars in my electric chainsaw entry).
Given all this, I wanted a suitable bar for my newly assembled Huztl 036, (see my post on making this saw here) intended for thinning and limbing small trees. In my experience, the 036 is suitable for no more than a 16” bar in Australian hardwoods. It can comfortably pull 16” of 3/8” chain through lighter hardwood, but at 18” it is bogging down. When the Stihl 036 was relatively new (in the early 1990s), Stihl’s recommended bar lengths were 37cm (15”) and 40cm (16”) and I think they were right. The current Stihl catalogue shows the current version of this saw, the MS362, fitted with a 20" bar as standard. In Australian hardwoods, that's a toddler wearing daddy's boots...
So for my new saw, I wanted to try for 14” – a radically short bar for these days of bar length anxiety. Walking into a Stihl dealer was financially frightening, I couldn’t find a suitable bar online, and the shortest 3/8" pitch bar from Huztl was 18”. So I thought I’d try something I’ve been daydreaming about for some time: cutting a bar to a shorter length. I had a cheap 18" Holzfforma bar from Huztl, and was willing to risk it for science. See my review of Holzfforma chainsaw bars here

How I went about it

First step was to work out what length of chain to use, then to make the bar to fit. I looked at a 60 link chain on a 16” bar and rim sprocket:
Measuring the gap on a 16" bar
I like Stihl’s practice of making bar lengths to fit chains with even numbers of teeth (which means the number of drive links must be a multiple of 4). This means that the alternating pattern of teeth continues around the whole loop. So I started by making a chain the length I wanted (56 drive links) and making the bar to fit. 
The 56 link chain trying the bar for size
I used a genuine Stihl bar to mark out the pattern of slot and holes on the Holzfforma bar.

Using the Stihl bar to mark the slot and holes
Then I punched and drilled the bar.
Drilling the holes
cutting the slot with angle grinder with cutting disc, using a steel bar as guide
I did have an interesting challenge in this process. When centre punching one of the bar tensioner holes, there was a small bang, and I found myself with a splinter of steel surprisingly deep in my pinky finger, a punch with a flattened point and no mark left on the bar. By chance the tensioner holes aligned with spot welds from laminating the bar layers, which had left hardened patches of steel (spot welding brings spots of steel to melting temperature, and when the electrical resistance heat is suddenly stopped, the heated spots are quenched by the cold steel around, which will cause hardening in carbon steels). I first tried to deal with this by moving the hole a little, but ended up having to temper the steel with heat to make it drillable. Thus the tensioner holes are a little misaligned.
The bottom hole is right on the hard spot weld - see the failed punch mark

Cutting the slot between the holes
Filing the slot after grinding
Chamfering the hole edges
Chamfering the hole ends
Cutting the bar to length - there's no turning back now!
cutting off the corners
Grinding a curve onto the bar
Smoothing the bar shape on a linisher
Grinding the bar slot into the bar base with a 1mm cutting disc (I later found 1.6mm discs which work much better)
Chiselling a wafer of middle laminate so it can be ground away
Tempering the bar with LPG-air torch to enable drilling
Drilling the bar after tempering
Drilling the oil holes using the old bar base as template
The new bar fitted to the motor

Outcome

After all this, the bar fit on the chainsaw. I had to make some modifications to the newly ground slot at the bar base, to remove wafers of centre laminate I had missed when grinding the slot - the chain links were catching on a remnant.
The bar looks a little wider than normal at the base, but I can live with that
The job took about an hour and was greatly extended by caution and newness. The most difficult part was grinding the chain link groove into the newly shaped bar base - it was very hard to see what I was doing.
I recall 25 or more years ago when I was doing a lot of chainsaw milling, my chainsaw shop had a machine to regrind the slot in hard nose bars. This had a grinding wheel the right thickness, held the right distance from a table, to reliably grind bar grooves. That would be very useful for this job.
If I shorten a bar again, these would be some tips to help:
  • Temper the steel at the bar base before drilling any holes
  • Use a 1.6mm cutting disc for the groove
With these things in place, shortening bars would be relatively simple - which is good, because there are so many over-length bars around....

Bar hammering video (for worn or badly gauged bars)


Shortening a .325" bar

I've now shortened another few bars, much more quickly and easily than last time - about 30 minutes. 
Most have been .325" pitch bars for a Stihl 024 (my post on repairing 024s and 026s here).
Here's the genuine Stihl 13" bar alongside the Holzfforma 16" bar
I traced directly from the Stihl bar, but the new bar will need to have an open-backed slot because of the existing slot
Here's the freshly repaired 024 with the freshly shortened .325" bar 
Improvements on my method this time:
  • I was able to copy a genuine Stihl 13" bar pretty exactly: tracing around the genuine bar onto the Holzfforma bar with a scriber, giving the same profile (no wide bar base like I have on the first bar). 
  • Once again I found some hardened spot welds when drilling the chain tensioner holes. This time I tempered the steel with an oxy torch, the higher temperature allowing me to quickly heat a smaller area of steel so I don't think I've softened the rails so much. 
  • I bought some 1.6mm angle grinder cutting discs after my first bar-shortening effort. These made grinding the groove at the bar base much easier and quicker. It would still be good to make some sort of jig to hold the disc at the perfect spacing, but I was in a hurry and it worked very well freehand compared to the 1mm cutting discs I used on the first bar. 
  • The bar I used had a narrow slot open to the back (smaller format bar). This made it easy to cut the new slot from the back end with a cutting disc, up to a single hole drilled at the front end of the slot. 
Both chain tensioner holes ran into hard steel from the spot welding: the burnt-looking areas around the holes are where I softened the steel with an oxy flame


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