View Full Version : Need Help! Opinion on table top surface

11-19-2013, 11:48 PM
I have finished the build of my pro machine and I am a stage where I am thinking about the table surface and spoil board. (still have to build electronics)
I have seen machines like the Kronos that use 80/20 extrusion as the table top with no spoil boards, and I have seem other tables purely our of MDF with a spoil board.

My surface is 4'x3'. I plan to cut MDF, starboard and even aluminum. So the challenge of affixing these materials can easily be overcome by inlaying some thing 80/20 (40-8020) extrusions or so I think. I am new to thing, so I am open to advice.

here is what I am thinking of doing:

http://i10.photobucket.com/albums/a146/Profoxcg/HOLDOWN_zps0ebfba58.jpg (http://s10.photobucket.com/user/Profoxcg/media/HOLDOWN_zps0ebfba58.jpg.html)

The idea is to use some woodpecker clamps or others,
T-Track Clamp - Amazon.com (http://www.amazon.com/gp/product/B005BG83WU/ref=ox_sc_act_title_3?ie=UTF8&psc=1&smid=A19VW1BL9ZXZVA)

80/20 25-5013

http://i10.photobucket.com/albums/a146/Profoxcg/imgres_zpsad5ed68c.jpg (http://s10.photobucket.com/user/Profoxcg/media/imgres_zpsad5ed68c.jpg.html)

I have some general questions:

1- what is the advantage if any or doing the top like the guys from kronos (all metal)?

2- It is my understanding that I should plane the MDF. The way I have modeled this, there is a potential for no planing because of the proximity in sizes between the MDF and the extrusion I have chosen. My only option would be to attached a second piece of 1/4" MDF and plane it. That effectively becomes my spoil board.

3- does the placement of the extrusion make sense? should I add more?

11-20-2013, 08:38 AM
If you're going to use extrusions or T track, you'll probably want more than 2.
You'll also definitely need to be able to surface your table, and you'll need to do it periodically.

You may find after you use the machine for awhile that you may want to change to a different table type to better suit the way you work. If you haven't used the machine, it's hard to say what method will best suit you.
I prefer threaded inserts and cam clamps, or vacuum. I'm not a fan of T track and clamps.

11-20-2013, 01:43 PM
If you're going to use extrusions or T track, you'll probably want more than 2.
You'll also definitely need to be able to surface your table, and you'll need to do it periodically.

You may find after you use the machine for awhile that you may want to change to a different table type to better suit the way you work. If you haven't used the machine, it's hard to say what method will best suit you.
I prefer threaded inserts and cam clamps, or vacuum. I'm not a fan of T track and clamps.

when you say "You'll also definitely need to be able to surface your table, and you'll need to do it periodically." is this because of the T track or just overall?

What do you thread into the threaded insert?

11-21-2013, 08:46 AM
Try MFD to start out with. Its cheap compared to the rest of you options and if you don't like it its easy to replace. I used T-nuts in my spoil board.

11-21-2013, 08:47 PM
true, thank you.
so did you make channels in the MDF to use the Tnuts? do you have a pic?

11-21-2013, 09:08 PM
when you say "You'll also definitely need to be able to surface your table, and you'll need to do it periodically." is this because of the T track or just overall?

What do you thread into the threaded insert?You'll be cutting into the spoilboard occasionally while cutting parts, and will eventually want to resurface it to get it flat and smooth again.

Here's how I did my table.

11-22-2013, 01:57 PM
I did the same as Gerry. You can see my spoil board in my build thread. http://www.cnczone.com/forums/open_source_cnc_machine_designs/144173-crp_4x4.html

02-10-2014, 11:32 PM
I am after accuracy for my machine and I need some help understanding the proper implementation of a table top and spoil board.

so the machine currently fitted with 1/2" MDF over a span of 24" The MDF seems to deflect a bit in the middle an I am having a hard time planing it. As a matter of fact, there is more material on the end closer to the picture then on the center.

I am thinking of going with 3/4" (which what I have at hand) unless someone can make a recommendation to go with 1" etc.

Also, I am running the machine one carpet, (I know it was a mass before the dust deputy and shoe) but my concern it with leveling the machine. How important is this? isn't the gantry relative to the table top? why do people level their machines religiously?

http://i10.photobucket.com/albums/a146/Profoxcg/2014-02-10%2023.25.12_zpsrkbo4opr.jpg (http://s10.photobucket.com/user/Profoxcg/media/2014-02-10%2023.25.12_zpsrkbo4opr.jpg.html)

What is the typical of standard construction of these tops in terms of thickness and span?

I also see lots of pictures and videos of people who have tables with T-nuts and attach a spoil surface via clamps, and then attached stock to be cut to the "movable" spoil board (not sure how at this point) but Isn't this redundant?

It seems to me that in order to have the largest cutting area, the top and spoil board should be the same?
am I missing something?

The claps I have are these, are clamps really enough to a fix stock? These are the ones I have (but I wish they were more low profile)
As you can see in my picture above, I am currently attaching the work piece with screws because I do not trust the clamps.
What is the consensus, this is probably why I see so many people building they own camps with the recess. I think getting the top done will bring a a major improvement to my cutting.


02-10-2014, 11:43 PM
Some people like to work differently than others, so you see a lot of different setups.
I'd use at least 3/4" thick, probably even two layers. Possibly add braces in between your extrusions to stiffen it up even more if needed.
If you use two layers, than the top layer should (imo) be as large as it can be so that you can still surface it. It shouldn't be any bigger than that. Remember, you'll periodically be surfacing the spoilboard, so it really should be a second layer above your main top. You can always screw another smaller dedicated spoilboard onto that one for special circumstances.

I really don't like clamps like that, as you have to work too hard to avoid hitting them. But many people do use them, and if tightened securely, they should hold fine. Especially with smaller bits, they should not be a problem. My #1 preference is vacuum, but it doesn't always work for everything. I really like my cam clamps, but a combination of vacuum and threaded inserts is probably my ideal workholding solution.

02-11-2014, 12:03 AM
you have give me some great ideas. I understand what you are describing, I am gong to post a 3d model (image) of what intend to do based on your advice.
regarding table surface, I can true or plane from the front of the frame back 24" but as you can tell from the pictures I stopped at about 22" because I did not want run into the bolts holding the top. But if I do 3/4" that will not be an issue anymore.

I am going to make some clamps, I really don't like to screw into the table to spoil board unless I really have to.

02-12-2014, 07:37 AM
FWIW I used 1/2 plywood on top of the extrusion and then 3/4 MDF on top of that for a spoil board. I've been very pleased with it.

02-12-2014, 09:34 PM
I have 18mm baltic birch plywood as a permanent sub top. On top of that I have mounted 3/4 MDF with aluminum t-slot extrusions set into it. I always mount my projects to their own spoil board if cutting through the workpiece is necessary. It's my first version - I like it so far, but I may change it in future.

02-12-2014, 11:33 PM
How do you attach the work the its own spoil board and how to you attached the removable spoil board to the 3/4" MDF?
Do you surface the works spoilboad?

What I think I am going to do is get 3/4" MDF for the permanent top, surface it, and attached another 3/4" MDF that will be the spoil board. I have room on the ends of the machine that I am going to fill with 8020 extrusion to use as clamp channels.

02-13-2014, 09:25 AM
I attach workpieces using whatever works. Sometimes I'm able to use the t-slot clamps to hold both the work piece and its spoil board at the same time - sometimes I screw the workpiece to its spoil board through some waste area and use tabs on the workpiece to retain it in the waste. I surfaced the MDF before cutting the top to inset the t-slot material.

The 3/4 mdf is bolted through the 18 mm ply into t-nuts underneath battens fastened to the sides of the extrusion extrusions.

02-24-2014, 04:22 PM
I need to help..

Yesterday I tried to make the top and spoil board for my machine, but again, I am not sure why I just can't get the machine to cut the holes in the right place.

I have x2 40-8020 beams 24" apart which means that the centers of the T slot would be at 22.425"
This is because the centers of the t-slot are 20 mm or 0.7874" from the edges.

I have attempted to cut the both set of wholes with the MDF panel in place. However it seems that my machines is unable to same a precise 0.7874 move.

I am thinking of taking the panel, drawing a line on it by hand where the center-line of the wholes need to be and zero the router to that line so that the only move that needs to be made is along that line. Hopefully my gantry is squared and I am able to trace the line.

Once one side if done, I would then rotate the panel 180 deg and make the holes of the opposite site.

This seems like it would work, but I still have the feeling that I should be able to cut it all in one pass and do it repeatedly (because I need the same holes on both the top and spoil board).

I have fitter my machine with home switches, but I am out of ideas...

Do CNC machines have a hard time cutting close to their limits?

Here is a section of the table I want to make:
the red 0,0 is where I am homing and centering the bit, I understand I may be off a little bit.. but I am way off 1/8"+ at times. And then I get to the other end, the wholes are off.

http://i10.photobucket.com/albums/a146/Profoxcg/2014-02-24%2016.01.23_zpswbc1f4i5.jpg (http://s10.photobucket.com/user/Profoxcg/media/2014-02-24%2016.01.23_zpswbc1f4i5.jpg.html)

02-24-2014, 05:14 PM
What I did was to ref all home, then offset x and y by .25 inches and set that spot to 0,0. From there I lined up the X and Y coordinates of the spots where I wanted holes and countersinks cut and created some g-code to do the job. It is reproducible that way. I used the small offset because when I first started out at the local makerspace, the machines there sometimes had a problem working close to the limits. Mine doesn't appear to have that problem but I did it anyway.

02-24-2014, 05:14 PM
Do CNC machines have a hard time cutting close to their limits?

No, they should go exactly where you tell them to go, 100% of the time. If it doesn't, then it's not working right.

Assuming the g-code is correct, you need to do some testing to make sure the machine is not losing steps. Run your program with Z zero set an inch or two above the workpiece, and when finished, return the machine to the 0,0 position. It must return there 100% of the time. If it doesn't, it's losing steps, and you need to tune the machine so it doesn't lose steps.

02-24-2014, 05:25 PM
I looking at both of you responses:

KJK's idea seem good. Only because it gets the machine away from the switches and soft limits.
But regardless it should move as I tell it.

Ger21, what is the "tuning" process - I am "tired" of doing the mach 3 steps calibration.
I have spent a premium in my motors and electronics... And yes, I am loosing faith on the investment every time I am unable to cut something.

02-24-2014, 05:38 PM
The machines I used at the makerspace used mechanical switches which were susceptible to contaminants and wear and tear of the contacts over time. For that reason I used proximity switches - they were pretty cheap on ebay and while they don't have the range of the new ones sold by CNCRP but they work very well on my machine.

Ger is right - if the machine won't return to zero, something is wrong. A thorough check of all the connections/jumpers is in order if the machine will not move reproducibly.

As a suggestion, have a friend compare your wiring to the diagrams - I find that when looking over something like that I tend to see what I want to see - not necessarily what is there. Another set of eyes could help solve the problem. Good Luck.

02-24-2014, 07:37 PM
what is the "tuning" process - I am "tired" of doing the mach 3 steps calibration.

You should only be setting the steps per one time. It's either right, or it's wrong.

What are your velocity and acceleration settings in the motor tuning screens? These are the first things you need to set correctly.

Here's what I'd do.
Set all your acceleration settings to about 4, and all your velocities to 40-50.

Now, run a few programs, starting at 0,0, and make sure it always goes back to 0,0, without losing position. Once you can verify that, then set your steps/unit. Get it as accurate as you can, over a large distance. When it's good, then you should never change it again.

Once your steps per unit is set, then start increasing the velocity settings, a little at a time, until you notice the machine is losing position. If it sdoes, then back off the setting by 15%-25%.
Then start increasing the acceleration, again a little at a time (±5), until you again lose position, then back it off by 15%-25%.

Once you do this, your machine should be 100% reliable.

Before these steps, it's extremely important that all moving parts move freely, with no binding, and are well lubricated where they need it.

If you still find that you're losing position, and reducing acceleration and velocity don't help, then you may need to start looking elsewhere for issues. A lot of things can potentially cause a loss of position.

I'm assuming here that your g-code is correct??

02-24-2014, 09:55 PM
So I looked over the machine, everything is tight.
I used the MPG mode to jog 1" at at time and I was 0.985" in the X and something very similar on the Y. So I ran the Mach 3 Calibration again and I asked for 1" movements.
I dialed it to 1.002" +/- 0.001" Which is pretty good.

I noticed that mach 3 will not adjust the slave motor accordingly to its master, so I made the change manually.
Can you guys confirm this is correct ?

Everything seems to work well close to the origin.

I looked over my g-code in notepad, and I looks good, I even change it to stop and end at the location I tell it to go to.
I also ran two small simple programs (cut a 2"x2" square) and move to a specific location and it is all working well.

I am going to run a larger program in a minute just to make sure that it comes back to 0,0

I don't mind running a little slow if that mean accuracy. My motor settings after the mach 3 calibration os

X = 2043.526 / accel 29.79
y = 2064.314667 / aceel 29.49

02-24-2014, 09:58 PM
What's the velocity?

02-24-2014, 10:03 PM
X = 2043.526 / accel 29.79 / 595.8
y = 2064.314667 / aceel 29.49 / 589.8 (A is the same as Y)

These are the before values.

02-24-2014, 10:15 PM
Just run a "crazy" program that jogged all over the place at 120 IPM.
At the end it returned to 0,0 so the machine is good.

I think I just need to change my workflow a bit for the top and just offset it from the edge, manually set 0,0 and cut each edge at a time.
Once I have all 4 done (2 per MDF sheet) I can bolt the bottom one down, run a program to drill holes for the clamp screws, then attach the spoil boards upside down, machine the recess for the nuts and drill, then remove, flip and install. Hopefully this time it will all line up.

Ger, What is the velocity and should it be set the same on all motors?
I am also gathering that the acceleration value is how many seconds it will take to get to the Velocity?

02-25-2014, 10:07 AM
The velocity setting is right next to the acceleration setting. It may be able to be the same for all motors, but each axis should be tested independently to determine a reliable maximum.

120ipm is slow. If you are losing position, it would usually happen during rapid moves, which is what the velocity setting is.

02-25-2014, 10:26 AM
I need to look at the after settings.

02-25-2014, 10:38 AM
Mach3 doesn't adjust accel and velocity for you. You must do it yourself, through trial and error as I said. I would guess, though, that the 30 and 600 numbers are conservative settings based on testing from CNC ROuter Parts, so you may not need to test them. I'd ask them for their opinion.

Stpes per unit can be calculated, but ultimately depend on the accuracy and consistency of the rack and pinion or screws, which can vary. In some cases the calculate values will be correct. In other cases, they can be much different.

But as I said, jogging at 120ipm doesn't tell you if the machine will lose position, as it moves at 600ipm during rapid moves. I'd make a complex program using all rapid moves. Take the program you had before, and do a find and replace and replace all G1 moves with G0 moves.

02-25-2014, 10:50 AM
Mach3 doesn't adjust accel and velocity for you. You must do it yourself,

What is weird. I will check again, but I don't ever recall changing those values manually.
You are right, the dialog box say, we will adjust step to xxxx.xxxxx. But it also changes acceleration and velocity.

02-25-2014, 09:10 PM
I came home today and ran the calibration again after a short phone call with CNCRP. (Ahren)

He mentioned that I should try to calibrate my machine over a longer distance rather than 1" at a time. So i did this and the steps got very close to the values they had initially provided. He assured me that while they have tested their builds, I should really take their XML as a baseline (which is what I thought).

Now going to back to what is changed by mach 3: Mach 3 will change the steps and will slightly change the acceleration and speed. Ahren explained to me that such a minute change does not affect anything and is essentially the result of Mach 3 liking to work with numbers that are multiples of 2. Anyway, the machine jog 12",24" and 1" "consistently"

I just double checked again and it working good. (moving correctly)

I still think that I am going to cut way from the edge of the table and just do one edge at a time like suggested.