Using two motors is mechanically simpler and has lower rotational
inertia but I am not a fan of this setup. If you use a tube rather than
a solid shaft, you won't add a lot of inertia. I'm thinking of building
another plasma cutter and it will probably use a shaft rather than 2 motors.
But the thing is, I'm planning to use round guides
with bronze adjustable bearings.
Do you mean guides that are only supported at the ends? This is a very
bad idea. They'll flex and bounce all over the place. You are also
likely to get a lot of wear unless you pressure feed lubricant. If you
do that oil will go everywhere. My router uses box ways on the Y and Z
axes with oil feed. It gets pretty messy at times.
Most modern commercial routers and many machining centres use
rectangular linear ways, such as this
<https://www.qualitybearingsonline.com/lwl25r240bhs2-iko-maintenance-free-linear-guide-rail/>.
They are very rigid and lasts a long time with very little wear. The
only disadvantage is that you need to be careful to make sure everything
is perfectly aligned. These have very little give in them. Another
option is supported round rail such as this
<https://www.amazon.co.uk/TEN-HIGH-Supported-SBR40UU-BlockBearing-Bearing/dp/B01N10JF5N>.
For the sort of size machine you are talking about you'll need at least
40mm round rail. Round rails wear faster than rectangular but are a lot
less fussy about alignment.
So, to sum up, with these kind of bearings I expect more resistance on the
joints, and also the router is 2 meters x 3,8 meters long so to have enough
rigidity I'm planning to use steel and cast iron, so that's why I'm
oversizing the motors.
To give you an idea about motor sizing the motors on my router (1 per
axis) are about 1.8kw and it's scary. My tool changer is mounted on a
bracket made from 50mmx50mm box section. I messed up the tool change
sequencing a while back and it pushed the tool changer out of the way
without breaking a sweat. It tool a lot of effort with big levers to
twist it straight again. Here is a link to a similar machine to mine but
without a tool changer
<https://www.bidspotter.co.uk/en-gb/auction-catalogues/cjm-asset/catalogue-id-cjm10389/lot-47df49af-bf1a-4676-ab88-a75a00f5f92b>.
Lots of heavy steel and cast iron. Mine originally had 4 drill heads and
2 spindles. If it was easy to dial back the power I would. If something
goes wrong the machine will keep pushing until something breaks.
I do maintenance work on a router with 750W motors. A while back the
spindle stalled while it was cutting. It bent the 1/2" cutter nearly 90
degrees and carried on.
Les
On 04/10/2018 13:47, Leonardo Marsaglia wrote:
First of all, thank you guys for your advices as always!
I'm gonna try an asnwer this on one message because sadly gmail doesn't
have the quote selected text feature anymore.
About the oversized motors. Yes, I also think that for a normal router 1 kw
per side is too much. But the thing is, I'm planning to use round guides
with bronze adjustable bearings. I decided this because I want more
rigidity for an eventual need of machining aluminum, and also because I
think this kind of guides with whipers are much more reliable than the
recirculating ball ones. Also, I don't think I can have the adjustable
feature with the slotted ball bearings. I'm attaching a picture of the
bearing I plan to make, there are no lube channels on the model but they
will be on the final part.
So, to sum up, with these kind of bearings I expect more resistance on the
joints, and also the router is 2 meters x 3,8 meters long so to have enough
rigidity I'm planning to use steel and cast iron, so that's why I'm
oversizing the motors. Besides, there's no much difference between a 400W
and a 1Kw chinese servo motor and drive on ebay.
About how to drive and home the gantry. From what we've been talking and
thinking it through a little more, I'm thinking that the best solution is
the one Gregg suggested. To have a transversal shaft on the gantry driven
by the servo motor by a worm and gear reduction with the timing pulleys on
each end of the shaft driving the pinions. This way I can adjust and square
the two columns and it should stay squared at any time. This is really
important because this is going to be used by a regular operator, so this
has to be as reliable and fail proof as possible.
About the last question. Is there any disadvantage other than may be a
little more mechanical complexity with the one motor and shaft approach?
Because I've seen lots of routers driven with two motors that I almost
think it's mandatory for some reason.
Thanks again!
Leonardo
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