Maslow Home Maslow Community Garden Newsletter

Cutting accuracy problems with long arcs etc

Ok, so I’ve managed to get everything up and running after a few trials and tribulations (including having to put in a UPS because of big voltage drops we’re getting where we’re work - another story!) and have a linkage kit set-up. However, I’ve got problems with getting consistent profile cuts. I am cutting quite extreme profiles, i.e. almost full width (2400mm), slow arc (2060mm radius, about 100mm depth) roof spars out of 18mm hardwood ply for a project, with half-height centre slots and right-angle corners at each end. I have extended the board to support the sled at the bottom and sides, but lose the profile shape at the ends (and the centre with the top couple of spars), plus have difficulty in maintaining good slot/right angle cuts. I’m using a 6mm straight 2-flute cutter, with 4.5mm depth cuts at 800mm/min. I notice at the far left/right cuts (surprise) the longer chain goes a bit slack and the linkage drops, only to be pulled taight again when it starts cutting. I’ve messed around with using climb/ conventional cuts to try to optimise the cutting at the end, but this seems only to help so far. Should I:

  • try to get a single flute cutter to get into the right chip removal rate?
  • reduce the plunge depth?
  • add more weight to the sled?
  • replace the (smooth) MDF sled I’ve made with something else?
  • go for (yet) another beer?
    Ok, the last one’s a given, but any useful suggestions would be very welcome.

Cheers,
Jules

1 Like

Ok, so I’ve managed to get everything up and running after a few trials and
tribulations (including having to put in a UPS because of big voltage drops
we’re getting where we’re work - another story!) and have a linkage kit
set-up. However, I’ve got problems with getting consistent profile cuts. I am
cutting quite extreme profiles, i.e. almost full width (2400mm), slow arc
(2060mm radius, about 100mm depth) roof spars out of 18mm hardwood ply for a
project, with half-height centre slots and right-angle corners at each end. I
have extended the board to support the sled at the bottom and sides, but lose
the profile shape at the ends (and the centre with the top couple of spars),
plus have difficulty in maintaining good slot/right angle cuts. I’m using a
6mm straight 2-flute cutter, with 4.5mm depth cuts at 800mm/min. I notice at
the far left/right cuts (surprise) the longer chain goes a bit slack and the
linkage drops, only to be pulled taight again when it starts cutting. I’ve
messed around with using climb/ conventional cuts to try to optimise the
cutting at the end, but this seems only to help so far. Should I:

  • try to get a single flute cutter to get into the right chip removal rate?

a good idea, but won’t help this problem

  • reduce the plunge depth?

this is the first thing to do. 4.5mm depth of cut is fairly aggressive, and on
hardwood you need to go easier, and since you are correctly identifying the
problem (the sled gets stuck, chain goes slack, then sled moves) a less
agressive cut may help.

  • add more weight to the sled?

possibly, but you may have problems in the top center. you will need to
re-calibrate if you change this. Along similar lines, putting blocks under the
back legs to tilt it closer to vertical may help (again re-calibration needed)

  • replace the (smooth) MDF sled I’ve made with something else?

reducing friction would help. how smooth is it? try waxing it, how rough is the
workpiece you are cutting? can you make it smoother in any way?

David Lang

2 Likes

Hi David,
Thanks for the reply, this has been a fairly steep learning curve. I’ll take up your suggestions, give it a go and let you know how it went.
Cheers,
Jules

The other thing you may need to consider is increasing the distance between your motors. Of course that might be a non-trivial modification as you would need to:

  • Design a way to extend your top beam
  • Extend your top beam and mount the motors
  • Increase you chain lengths
  • Recalibrate

In this case, he’s describing a chain going slack and the sled then jerking. If
it’s only the long chain going slack, then it’s possible a wider top beam will
help, if it’s the short chain going slack, then a slicker sled or shallower cut
are needed.

David Lang

2 Likes

Also, depending on your software you may need to add more points or nodes to the line if you begin having problems with the smoothness of your cut. Curves under populated with nodes can look fine on the screen and end up cutting with angular sections.

And echoing what others have said, the maximum step down I use is 1/2 the diameter of the tool in plywood. With harder woods I would go less than 1/2 the diameter of the tool.

I would recommend a similar mentality with speed 31 in/min (800 mm/min) is fast for me. I normally run 20-25 in plywood, slower in harder woods.