LumberJocks

CNC router build #5: It's Alive (almost)

  • Advertise with us
Blog entry by Mainiac Matt posted 12-08-2017 09:21 PM 2805 reads 0 times favorited 6 comments Add to Favorites Watch
« Part 4: Using a CNC to make a CNC... the bots are taking over! Part 5 of CNC router build series Part 6: more progress »

Got a lot of assembly work done today. Tested the X & Y axis using a cordless drill.

’’‘

Here's a YouTube video of the motion

https://www.youtube.com/watch?v=SnWdjIwWqxA

-- I yam what I yam and that's all what I yam



6 comments so far

View Mike_D_S's profile

Mike_D_S

491 posts in 2386 days


#1 posted 12-09-2017 04:01 PM

Matt,

First off, nice clean CNC build and the motion looks plenty smooth.

I’ve got a Probotix Nebula CNC that’s a similar design with a 37×50” working area. There are a couple of things that are a little bit of a weak spot with the gantry design in general which is fairly similar to yours. I know you’re basically finished with this one, but I figured I’d share what I see using my machine that might be helpful to you in the next version. You said the big one will be for glue application, so these issues probably won’t matter for that application.

On the gantry, when the spindle is in the middle of the X span and cutting in the Y axis, there is a little bit of deflection in the bit. I see it mostly when doing fairly rapid dado cuts in cabinet sides when the cut is long enough for the machine to accelerate close to the max allowed feed rate. For hobby use, being off 4 or 5 thousandths is not a big deal, but if I was going to rebuild my gantry I’d double up the gantry beam cross section, taking mine from a 30×90 section to a 60×90 to improve torsion resistance.

With the gantry in the middle of the Y span, I get a fairly small amount of deflection in the Y axis frame. I’d recommend increasing the depth of the section (30×60 to 30×90) to help with this, but my machine uses dual Y steppers instead of an underslung single stepper. So I took the poor boy approach and just shimmed the bottom of the frame to provide some additional support.

The last thing I see is around the MDF spoil board I use. I’m not sure what you are planning on for a spoil board, but with the size of my machine, after a few months I get sag in the middle of the spoil board (humidity, etc). While the gantry deflection and sag in the Y frame are all in the thousandths, the spoil board sag can easily be more than 0.01”. For my machine I added some additional 30×60 cross members to support the spoil board, but one again I don’t have the underslung gantry design you have. But it’s worth thinking about.

I took the easy route and just bought a machine, but I have a lot of respect for guys who take the DIY route. It’s not easy at all.

Mike

-- No honey, that's not new, I've had that forever......

View Mainiac Matt 's profile

Mainiac Matt

8497 posts in 2500 days


#2 posted 12-10-2017 01:44 AM

Thanks for the detailed comments Mike,

The next build will have twin ball screws and a slaved stepper instead of the under carriage, as I’m seeing some racking in the gantry when I push one side and pull the other… and the gantry is only ~30” long

Lot’s to learn…

The “plan” for the full size machine will be something in the neighborhood of 45” x 55”, with 7” clearance under the gantry and ~15” Z-axis travel and will dispense hot melt adhesive via. an industrial hot melt system with a 25# glue pt and variable speed gear pump. I’m hoping to have M-code control of the pump speed so I can tweak the size of the glue bead and still run fast. With no cutting forces pushing back on the travel, it’s just weight and inertial forces that I have to consider.

-- I yam what I yam and that's all what I yam

View Mike_D_S's profile

Mike_D_S

491 posts in 2386 days


#3 posted 12-10-2017 08:55 AM

In a general sense, you could probably adapt the spindle speed signal to your glue pump.

You’ll probably have to do some looking at the glue feed along with the accel/decel at the start and stop of moves and with tighter turns. Playing with the path following tolerance might work for you there. It’ll round off the corners (sometimes quite a bit), but keep the speed up through the direction changes which should keep a more even bead without having to worry about trying to adjust the glue feed on the fly.

Mike

-- No honey, that's not new, I've had that forever......

View Mainiac Matt 's profile

Mainiac Matt

8497 posts in 2500 days


#4 posted 12-10-2017 01:45 PM

Our oscillating knife CNC has a “look ahead” feature and will slow down in anticipation of tight corners … but it will only stop at a right angle turn.

-- I yam what I yam and that's all what I yam

View Mike_D_S's profile

Mike_D_S

491 posts in 2386 days


#5 posted 12-10-2017 04:45 PM

I’m only familiar with the Probotix setup with linuxcnc and the Probotix controller. You use a G64 Px Qy code to set the path blending. This helps to improve speed in corners and tight arcs.

With a normal spindle or a knife, the corner behavior is really only an issue of machine time to make the cut. But I’m thinking with glue you’ll have some new things to consider.

If we assume the glue pump rate is set to optimize glue volume deposited on straight runs where the machine reaches the specified feed, then anytime the machine slows down for a significant direction change you’ll get more glue deposited in those spots. If that’s acceptable, then it doesn’t really matter.

If the amount of glue being laid down in the slower parts of the path is not acceptable, then your choices are to either control the glue flow rate on the fly during the cut to compensate for machine velocity or to program the path to maintain a minimum acceptable velocity through the whole path.

For on the fly, a plugin which computes machine velocity as a percentage of optimal velocity and binds that as a signal out might be an option. For velocity optimization, you could look at path tolerance settings and try and keep the gantry mass down in order to increase the accelerations you achieve.

Apologies if this is all stuff you’ve already gone through. Between my interest in CNC and being a Mech E, this kind of discussion is super interesting to me. But I don’t want to keep rattling on about it if it’s not helpful.

Mike

-- No honey, that's not new, I've had that forever......

View Mainiac Matt 's profile

Mainiac Matt

8497 posts in 2500 days


#6 posted 12-11-2017 01:25 PM

All comments are appreciated Mike,

I knew going into this project that setting up an X-Y-Z gantry was going to be the easy part and that glue bead control and a suitably easy operator interface were going to be the challenges. I’ll have to read up on the G64 command, as I haven’t used that before. The sales rep for our hot melt system (Hot Melt Technologies) is both knowledgeable and willing to help and has his equipment in use on commercial built CNC applicators, so I may need his help.

We don’t need a perfect glue bead, but we can’t have a puddle at each end of the path and a pin size bead in the middle. It will help that we don’t need to break the sound barrier for this machine to be considered a success. 300-400 ipm will likely get the job done.

-- I yam what I yam and that's all what I yam

Have your say...

You must be signed in to post the comments.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics

HomeRefurbers.com