Crash course in CNC

1459 Views 10 Replies 7 Participants Last post by difalkner, Aug 20, 2018

Raistlin

Discussion Starter · Aug 9, 2018

Hi,

I have recently purchased a Multicam II CNC table for an exceptionally good price. I now have the challenge of getting it to do what I want. It came complete with a Windows 95 computer, CRT monitor and running Toolpath software. I have actually managed to to get it all working using 6 different computers to transfer files from the office to the workshop.:laugh2:

I've been trying to make an acrylic shower caddy that can be assembled without glue. I design 2D drawings in Corel Draw, export them in DXF format, import them into Toolpath which creates the G-Code and sends it to the CNC machine. I have successfully cut one of these caddies out of acrylic and now I am trying to fine tune the joints. The joints are like a tenon that slides into a slot, moves sideways and locks. The challenge is to cut the slots as close to the thickness of the acrylic so the joints fit snugly. I have cut numerous test pieces out of the material to determine the optimal measurement in Corel Draw to translate to the toolpath. However, I'm encountering inconsistencies that I cannot pin down. The slots are marginally narrower on one end compared to the other. Having chosen the best slot thickness I ran the job again only to discover that all the slots varied in thickness even though they were identical in the drawing file.

I thought it may be related to feed rate so I ran more test pieces using different feed rates (from 7200 down to 5000) and multipass versions using 2 and 3 passes on 3mm acrylic. Interestingly the multipass versions cut narrower slots than their single pass counterparts. However, on all test cases I still keep getting varying thickness from one end of the slot to the other.

I am using a 2mm upcut router bit in a Bosch GGS 27 die grinder. Can anyone suggest where these discrepancies may originate?

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difalkner

· #2 · Aug 9, 2018

Welcome to the forum! When you get a minute complete your profile with first name and location. That will get rid of the N/a in the left panel and your location helps us to help you.

First off, congrats on getting your new (to you) CNC. How rigid and accurate is your machine? Is it mechanically just not holding tolerances or is there a problem with your toolpath? Are you taking a roughing cut and then a follow up final cut?

I'm not familiar with that Bosch die grinder; what's the runout on that? I would think it isn't near as good as even a router for shaft runout, and certainly not as good as a spindle. You may just be seeing the best a die grinder can do in that situation.

David

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old55

· #3 · Aug 10, 2018

Welcome to the forum.

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MEBCWD

· #4 · Aug 10, 2018

You don't say what model CNC you got and that will make a difference.

Like David I question the use of the die grinder. That die grinder is rated at 300w so it might be underpowered for this use. I would think that there would be some runout in the die grinder that could contribute somewhat to the poor quality of your cuts, just don't know without testing.

How are you mounting the die grinder in the Z-axis? Is it loose enough that it is moving slightly during your cut? You might not notice the movement because it would be small. Is there movement in the die grinder between the case and spindle? The difference in thickness of your slots sounds like looseness somewhere in the mount, spindle or both.

I'm not sure what you are talking about when you say your feed rates are (from 7200 down to 5000), are these feed rates or are they spindle speeds? If they are feed rates then they are probably in mm/min so that would be from about 285 in/min to 200 in/min so the feeds are probably way too fast for you die grinder. I think most of their CNCs actually us 4hp spindles and up.

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gjackson52

· #5 · Aug 10, 2018

Welcome to the forum !

Gary

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marecat3

· #6 · Aug 15, 2018 (Edited)

Welcome

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DesertRatTom

· #7 · Aug 15, 2018

Good question and thanks for the sequence of file creation and conversion. That is one of the questions that has bugged me, but didn't know to ask. We almost need a primer on CNC as a standing pdf and post. Videos are OK, but you can't study them the way you can with print.

What would you think about putting together such a document, Mike? My 17 things post is something like that, full of basics. I wrote, edited and revised is many times over a couple of years, and still fiddle with it now and again.

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MEBCWD

· #8 · Aug 15, 2018

DesertRatTom said:
Good question and thanks for the sequence of file creation and conversion. That is one of the questions that has bugged me, but didn't know to ask. We almost need a primer on CNC as a standing pdf and post. Videos are OK, but you can't study them the way you can with print.

What would you think about putting together such a document, Mike? My 17 things post is something like that, full of basics. I wrote, edited and revised is many times over a couple of years, and still fiddle with it now and again.

Right now just can't find the time to put something together that would be pinnable.

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Raistlin

Discussion Starter · #9 · Aug 20, 2018

difalkner said:
Welcome to the forum! When you get a minute complete your profile with first name and location. That will get rid of the N/a in the left panel and your location helps us to help you.

First off, congrats on getting your new (to you) CNC. How rigid and accurate is your machine? Is it mechanically just not holding tolerances or is there a problem with your toolpath? Are you taking a roughing cut and then a follow up final cut?

I'm not familiar with that Bosch die grinder; what's the runout on that? I would think it isn't near as good as even a router for shaft runout, and certainly not as good as a spindle. You may just be seeing the best a die grinder can do in that situation.

David

Thanks David, I actually did fill out the profile but it doesn't seem to have saved. My first name is Kerry and my location is NSW Australia. From my understanding the Bosch Die Grinder did not come with the table. The company I purchased the table from replaced the original router with the die grinder. As for the "runout" I am not familiar with that term so I cannot tell you. Can you please explain what that it is. :| As for the specs on the die grinder, I seem to recall that it stacked up better than the equivalent router. It is designed for machining metal dies with very small tolerances.

I am not doing a rough cut. I am cutting through the 3mm acrylic with one pass using a 2mm bit. I have run some further tests and will post the results once analysis is complete. I'll also try and upload some images.

Kerry

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Raistlin

Discussion Starter · #10 · Aug 20, 2018 (Edited)

MEBCWD said:
You don't say what model CNC you got and that will make a difference.

Like David I question the use of the die grinder. That die grinder is rated at 300w so it might be underpowered for this use. I would think that there would be some runout in the die grinder that could contribute somewhat to the poor quality of your cuts, just don't know without testing.

How are you mounting the die grinder in the Z-axis? Is it loose enough that it is moving slightly during your cut? You might not notice the movement because it would be small. Is there movement in the die grinder between the case and spindle? The difference in thickness of your slots sounds like looseness somewhere in the mount, spindle or both.

I'm not sure what you are talking about when you say your feed rates are (from 7200 down to 5000), are these feed rates or are they spindle speeds? If they are feed rates then they are probably in mm/min so that would be from about 285 in/min to 200 in/min so the feeds are probably way too fast for you die grinder. I think most of their CNCs actually us 4hp spindles and up.

Thanks Mike.

In my reply to David I said the die grinder is not the original tool that came with the table. However, it was purchased because it fitted into the mounting bracket. So the mounting bracket is original. The tool is not fixed rigid in the Z-Axis. There is a built-in buffer, so you can manually lift the mounting bracket when the grinder is locked into place. It does not force the tool down into the material. It relies on gravity, so the grinder will cut its way to the lowest mount position and stay there for X Y movement.

You are correct. The feed rates are in mm/min. They are based on guide published by Multicam for estimating feed rates based on bit size and material.

A recent update. I did find a broken grub screw in the mounting bracket. However, the problem still persists even after replacing it. I have also noticed some difference between slots cut with a single pass to those cut using multipass. I will post the results later, hopefully with some pictures.

Oh I almost forgot. I did say what the model of the machine was. It's a Multicam II. Difficult to find information on the web. I am lucky the previous owners kept all the documentation.

Kerry

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difalkner

· #11 · Aug 20, 2018 (Edited)

Raistlin said:
As for the "runout" I am not familiar with that term so I cannot tell you. Can you please explain what that it is. :|
Kerry

Shaft runout is the amount of wobble in the shaft relative to running perfectly true. Die grinders are on the low end for maintaining high tolerances, meaning shaft runout is higher than routers, routers higher than spindles, etc.

Here's a graphic that may give you a better idea on the shaft runout -

David

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