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bit getting "sucked" down

Posted: Fri Dec 05, 2014 4:09 am
by jdunne
Hello shapers,

So I have been putting a lot of time in on my S2... Upgraded to acme screw, nema23, and the 400W quite spindle. Adjusted and re-adjusted for square and removed as much play as possible without binding, using the eccentric nuts. I am using 1/4 single flute upper cut bit....

So my first goal is to cut some new motor mounts, dust shoe, and and camera mount for visually zeroing the machine....

on Foam, everything is working great. I can cut the hell out of foam. But now I am trying to cut acrylic. I have a bunch of .75in thick casted acrylic. So far I have not been able to get any decent results. I have done a bunch of test cutting .25 in slots and different feeds and depths... then used those settings to try and cut out some shapes. And no matter what I try, at some point it will bind, or slip an axis, or start cutting way too deep.. .I have already ruined several bits.... Typically after a few hours of running smoothly, something goes haywire....

Short of setting the cut depth at .5mm and feedrate of less than 50mm/min.... (it would take days to finish simple jobs.) I am trying to come up some process to get reliable results in .75 in thick acrylic.. reading all the information online about cutting acrylic they recommend cutting at 3x the dia of the bit.. and pretty fast feedrates...

I am going to list 2 problems I am stumped on.

1. After boring a .75in, 52mm dia circular hole. I find that the sides of the bore are actually tapered. it should have been a perfect cylinder, Instead I get a cone???? I am going to guess that the spindle is not stiff enough? and the bit is tilting against the side as the bit gets deeper? I mean a S2 should be capable of creating a true 2in bore hole in .75 acrylic shouldnt it?

2. for whatever reason when a cut starts to "struggle" maybe it is feeding a little too fast or too deep, then it seems to suck down into the material even deeper, until the machine comes to screeching halt. So literally on a 3mm deep cut, moving in a straight line, it will work ok on a couple passes, and then randomly it seems to start to dig into the material and within a distance of 80 mm the depth goes from the set depth of 3mm, all the way to 20mmm and then game over...... I have no way to predict when this will happen. I have a run where pass after pass is working great, and I am down to about the last 10 mm of material, then something catches and it just starts to dig in..

I have have tried large roughing strategies before the contour cuts. I completely gave up trying to use using a contour cutting strategy alone . .75in acrylic seems to be way too deep. I set the each cut depth to be .5mm, but once the bit gets to .5in or more... it starts to bind.... so I went to rough out the profile area before using the finish contour...

Anyway I am at a loss as to how to successfully machine thick acrylic... I am pretty sure if I went to to .25in or maybe .5in sheets, I would have more success. But I want to cut real, 3D machined shapes in .75 acrylic... then maybe alum.. baby steps....

Is the uppercut bit the problem? I can see that it naturally want to suck the spindle down if it starts to grip the material.... should I be using a down cut bit?

So any suggestions? I have cranked up the pots on the glb shield and the nema23s have a LOT more ummfff. I guess I could sit there and spray lubrication on the part for 3 hours... But that does not seem like it should be required...

Is the 400W spindle a problem? Maybe I should look at trying a bigger motor like the DW? since the Dewalt router have 1.5 hp.. the 400w spindles are what, .5 hp... is the machine just not stiff enough? If I push on it a a little at the spindle there is a bit of springiness. But I think that is just the reality of the Shapeoko?

I need some advice.... and I bet there is some old time machinist out there just laughing at me....

Re: bit getting "sucked" down

Posted: Fri Dec 05, 2014 5:53 am
by Dhpenner1
I built an electronics enclosure out acrylic for my machine before it was all together. I have a full wood shop and figured "how hard can it be"... I did successfully build the enclosure, but can't see myself ever doing it again. It's such a tricky material to work with, I think aluminum would be easier.

I doubt you need more HP, it's going to be a bit or a speed issue. Did you try making fast, shallow, passes?

If you want to stiffen up your X axis, try putting a few bolts in between the makerslide. It helped a lot on mine. Check it out

Re: bit getting "sucked" down

Posted: Fri Dec 05, 2014 9:50 am
by RobCee
I found acrylic to be a very tricky material to work with too. It tends to melt and stick to the cutter, causing lots of issues. Also, your cutter will get very hot, causing lots of the issues you are talking about.

I found two things helped with it:

1. Using a straight cutting bit was much more successful, as it didn't want to dig in as much as the spiral shaped ones.
2. Slower spindle speeds will generate much less heat, so I went for a low speed and gradually ramped up the feed rate until I found one that worked well for me.

The only other thing I can think of that may be causing issues is the amount of tool deflection you will be getting. You will have to stick a lot of your tool out in order to cut through .75" of material, so you may find that it deflects a lot.
You may also have to adopt some machining strategies that minimise the amount of 'rubbing' that your tool may do on already cut surfaces, which would generate a lot of heat in the tool.

Luckily (for me) I only had 3mm acrylic to cut, but I did manage to get some nice clean components.

Re: bit getting "sucked" down

Posted: Fri Dec 05, 2014 2:28 pm
by Schruminator
A few of the symptoms also sound like they could be a result of missed steps (in addition to acrylic being a pain to machine). I know that even with 2 fans blasting my grbl shield with air, I had to back off the trim pots from 100% max to about 85-90% otherwise the drivers still got uncomfortably hot and I had issues.

Do you have sufficient cooling on your grbl shield? 30 minutes or an hour into the job have you checked to see if the driver chips are scorching hot or not?

Re: bit getting "sucked" down

Posted: Fri Dec 05, 2014 4:04 pm
by Damin69
I found cutting acrylic to be very hard. Love HDPE for cutting. slow spindle/router speed fast shallow cuts seem to work the best. Polypropolyne was worse than acrylic..