Charcoal bags

Charcoal bags

Search This Blog

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

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. 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.

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. This means that the alternating pattern of teeth continues around the whole loop. Given this, the next shorter length of chain was 4 links shorter, so I made a chain this length and tried it in place (56 drive links). 
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 saw cuts fine with the new shortened bar. It's currently only had a few minutes to test, but bar problems would normally show quickly: pinching in the groove, failing oil flow, tensioning problems etc..
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
  • It would be really good to have a proper bar groove grinding setup
With these things in place, shortening bars would be relatively simple - which is good, because there are so many over-length bars around....

Update on the bar in action

After about 15 hours run time, the bar and saw work very well. I really like the shortness and lightness of this bar.
After 3 hours of metered running hours (I've fitted an hour meter to this saw to see how long it goes), the bar was working perfectly, mostly thinning very small cypress trees in a thicket. I removed and inspected the bar, and found some burring of the bar rail edges at the base, on the top side, in the section where the chain meets the bar groove after going around the sprocket. 
see the burred bar edge above the tensioning hole, in the heated section
It looks like the steel I softened here with the gas torch is deforming under chain impact. 
Chainsaw bar rails are heat treated to make them harder, you can see in this photo below: 
This bar lost a patch of paint, showing the heat treatment colours along the rails, bottom side
The blueish strip along the bottom side of the bar shows that there has been some heat treatment - probably induction hardening - to make the groove rails harder and more durable. Heating the bar with a torch would have softened this area and made it prone to working up a burr. 
I ground off the burr on the linisher, and over the next few hours work the steel seems to have work hardened and settled down, without rapid burring again.
The Holzfforma bar has had some other damage, but this time I don't think it's caused by my modifications. After the chain jumped off (not a sign of good practice I know. Excuse: new chain was stretching rapidly, while cutting 100s of small vertical stems, and a twisting tree knocked the chain off) I later noticed some chipping in the bar rails:

See the chips in the rails, mostly on the inward side 
I think this chipping is caused by the opposite problem of the burring I caused by softening the steel at the bar base: brittle steel, left too hard by poorly controlled heat treatment. There's not much I can do about it, re-tempering it would be a very tricky task. For a bar that cost under AU$12, I'm happy to leave this side of the bar on top for now. Perhaps later I can grind off 1 or 2mm of the rails, and see if it's better - I suspect induction hardening and tempering could easily have left a thin layer  of over-hardened steel, with better steel close below.

Holzfforma Bars

I have a few of these bars from Huztl on the go currently. I think they are worth using, but there are things to watch out for. 
Firstly, cheap bars often need their bar rails dressed before first use. Sometimes there's significant out of square when new, suggesting the bar layers are spot welded together but then never dressed around the edges when manufactured. With cheap or old bars, it's always important to dress the bar rails with a file or linisher, to make them square, before use. 
Secondly, Holzfforma bars, like many Chinese chainsaw bars, seem to be made with a wide gauge of chain groove, allowing a lot of side-to-side wobble in the chain, even when new. This may be because the bars are generously painted, including inside the groove, so they avoid the chain being jammed by the paint by making the groove wide. Of course the paint rubs off very quickly, leaving a over-gauged groove and making it difficult to cut straight.
After working this out with a couple of bars, I now scrape out as much paint as I can before installing the bar on a saw. After a few minutes cutting, I then remove bar and chain, and hammer the bar to close the groove. This video shows how I do it:

It can take a few hammerings before the groove settles down. 

Shortening a .325" bar

I've now shortened another few bars, much more quickly and easily than last time. 
This time it was a .325" pitch bar for a Stihl 024 (I found the motor in the scrap metal bin at the local tip, fixed it with Huztl parts inc. piston, cylinder, carby, cowlings, etc). Huztl's shortest .325" bar is 16", and we like to use 13" bars on our 026 and 024. A genuine Stihl 13" bar cost about AU$75 delivered. A Holzfforma 16" bar cost about $10, and took about 1/2hour to cut down - definitely worth a try. 
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 Holzfforma 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


No comments:

Post a Comment