[Mr.ChronoM]

Thx for this very helpful information ! After another successful test I have the same marks and they are indeed at the place where the tool is going down, I have this in all corners (not as bad as on the pictures as my material was only 9mm MDF instead of 22mm).

Looking at the machine very closely there is some flex in the gantry (no surprise to me) and in the bed holding the pieces (I didn't see this coming).

So I will have to make both of those stronger, but overall I'm *very* happy with my first piece (top part of a dust shoe héhé)

[aarggh]

Ideally if you can ramp your cuts instead of plunging in your software you'll get a much smoother and better finish, as well as longer tool life and reduced load.

cheers,
Ian

[Mr.ChronoM]

Thanks for the tip Ian, but unfortunately my gcode generator has no support for these kind of things. I'm using only opensource software at this point, but will have to upgrade to something with more features I guess .

I attached some pictures showing the first test pieces that are 'pretty good'. The above problems are still in these ones. I have been testing different speeds, but with my current bit (and no experience at all) I really don't know how far I can push machines like this in MDF.

Some info:

• The 'dust shoe plate' has been cut from 8mm MDF in 4mm steps at 400mm/minute (=15IPM) with a Bosch 8mm bit from a handtool (don't have any real CNC bit yet)
  
• The 'Lamp' has been cut from 18mm MDF in steps of 6.5mm at 2500mm/minute (= 100IPM) with the same Bosch 8mm bit.

Issues to solve:

• Make gantry stronger (adding a backplate should do the trick)
• Make the bed (a lot) stronger (Adding 3-4 steel bars under it)
• Write myself a gcode sender that can be used to home the machine, edit speeds on the fly, pause the machine, ...

Ordered:

• Kress 6mm collet
• 4mm/6mm/8mm 2 flute hss end mill (Cheap eBay ones for testing)
• Super strong magnets for dust shoe (20 N48 ones from eBay)
• Brushes from McMaster-Carr for dust shoe

[br1]

Interesting build. Nice job.

[Mr.ChronoM]

Thanks

Another small update. After the initial tests and cutting some pieces from 18mm MDF I started looking into more detailed settings of the GRBL controller.

I found the acceleration and feed rate settings and started to play with it (After looking into the IPM rates of other projects). I quickly learned that I wasn't getting above 20IPM and that this was *very* slow even for a plywood machine. Higher numbers made the machine lock up in the middle of the X and Y axis.

So I started taking everything apart to find the alignment issues in the machine. After about 6-8 hours work I got to the 2000mm/minute (80IPM) I wanted, put it back to together and did some new test without problems. (Running faster still locks up the machine on the X axis)


I also started writing a small .NET program to jog the axis, home the machine, speed control, ... and the important thing for me will be the ability to control the axis from my Android phone.

[legout]

Hi,

very nice machine. I´m planing (since couple of years ), to build a CNC machine too. After intensive searching the web, I stumbled over the machine of michealghammer (DIY CNC Router for modelers - RC Groups. And I like this one a lot. It seems to be the basis of your machine too.

But I don´t like the base made of cement. To heavy (although this is is a important aspect), to less experiences with...
Your solution looks easier to implement while still being stable enough. However, I would insert some additional braces in the middle of your base.

There is one thing, you should change on your machine. The base plate of your z-axis is placed disadvantageous. The lower end should be identical with the lower linear rail of the y-axis. You can see this on the machine of michealghammer.

This would increase the stiffness of your z-axis considerably. To get the same height for the z-axis, you can decrease the height of your gantry side boards, which would additionally increase stiffness.

About the speed of your machine. Use a power supply with 36 or even 48 V and your machine will run much faster.

legout

[Mr.ChronoM]

I'm pretty sure that no one would use the same design when building a second machine, but only by building one you can find the problems in the designs you made. So I do understand the remarks you make and they are correct, but still the machine performs way better then I ever hope (I planned for 4 weeks and build is the following 2 weeks, without any CNC knowledge).

If I can give you some tips about the design of michael hammer, they would be the following :

• If you want to build it as big as mine, use the different design to start from (Joe's 2006 is just a better design)
• The concrete base is not needed, but just don't start without a torsion box as base. It so much stiffer and easier to build once you have a reference surface
• Use at least one (I wouldn't rebuild without 2) torsion box for the gantry. This design simply sucks and is almost un-usable. Even adding braces is not good enough on a big machine like mine.
• And just for your info, the only thing that is really stiff and that I haven't considered changing when using the finished machine is the Z axis. I'm sure I can't make is move even if I use a big hammer, the amount of rails + blocks on that small amount of space will keep it as stiff as can be.
  The only consideration is that I can only rout materials that are less then 4-5cm, but I will never think about that.
• The speed at this point isn't limited by the power on the motors, but by the alignment in the rails/ball screw. The system simply locks itself up from 3000mm/min and above. I just can't do better with the tools I have. Rebuilding it using my CNC machine would easily fix this (I'm not planning on doing that )

I'm sure I will rebuild my machine with a torsion box as base, the ballscrew UNDER that torsion box and not above it. A gantry with a torsion box UNDER the torsion box base and one as the backplate of the gantry.
The rails will be mounted on top and under the gantry torsion box and on the sides of the base.

[legout]

Hi,

it´s always the same, learning by doing is in must cases the best. However, planing and discussing with other cnc builders could be helpful to avoid a number of mistakes. Therefore, thanks for your tips.

I do know, that torsion boxes are way better than a pure ply-wood, even when it is thick. However, building a straight and stiff torsion box isn´t easy with the handtools available to me. Or do you know an "easy" way to build a torsion box without a cnc machine?

That´s why I wanna start with a less stable, but easier to build design and rebuild the wood construction afterwards with parts made by the first cnc router. Another advantage would be, that I´ve already have expericences in building a CNC machine which would be helpful when designing the new machine.

In contrast to your machine, I will add some more braces in the first machine.

Do you have a plan available of your machine?

legout

[Mr.ChronoM]

That's almost the same plan as I started with, build a machine with my handtools and then maybe rebuilding it with the machine itself .

For the torsions boxes, this link is the best that I have seen. It was mentioned in the 400$ topic on the same forum as this topic. That guy also builds his torsion box by hand.
Torsion Box Assembly Table | The Wood Whisperer

I do have a pretty good design, but not a 100% detailed one in sketchup (no screws, bolt and nuts on there). I can post it when I'm behind my laptop.

[legout]

Wow, thanks for that link. This should be possible, even for me . However, making this box "dead" flat and even with absolute square corners is still a challenge.

So, the question remains if its better to build the first machine without such torsion boxes and afterwards replace some parts with torsion boxes build from pieces cut on that cnc machine.

[lerman]

One thing I noticed is that the space between the carriages on each rail is smaller than the distance between rails. That can cause skewing of the table and may be the source of your lockup.

A general rule of thumb is that the space between the carriages should be greater than or equal to the distance between the rails. (I believe I read that in a paper by one of the MIT machine design people.)

Ken

[Mr.ChronoM]

Wow, thanks for that link. This should be possible, even for me . However, making this box "dead" flat and even with absolute square corners is still a challenge.

So, the question remains if its better to build the first machine without such torsion boxes and afterwards replace some parts with torsion boxes build from pieces cut on that cnc machine.

The 2 questions that I see (if I should start over) are:

• Do you have a dead flat surface to build your machine on, I found it very difficult without
• Can you make a design that allows for a 'quick build' and that lets you replace some parts with torsion boxes ( without starting all over). In my design there isn't any space left :/

I pretty sure that the easy way is to build a machine without torsion boxes and then rebuild it when the machine is ready. In the near future I will be doing the same I think.

One of the first thing I will do is build a torsion box to put my current machine on. After that I can re-use that surface as the bed of the new machine.

But first, is the dust shoe, bought myself a big vacuum cleaner last week and the design is almost ready to be cut

[Mr.ChronoM]

One thing I noticed is that the space between the carriages on each rail is smaller than the distance between rails. That can cause skewing of the table and may be the source of your lockup.

A general rule of thumb is that the space between the carriages should be greater than or equal to the distance between the rails. (I believe I read that in a paper by one of the MIT machine design people.)

Ken

My English is failing me here I think ... what part of the machine is the carriage ?

[legout]

The 2 questions that I see (if I should start over) are:

• Do you have a dead flat surface to build your machine on, I found it very difficult without
• Can you make a design that allows for a 'quick build' and that lets you replace some parts with torsion boxes ( without starting all over). In my design there isn't any space left :/

I pretty sure that the easy way is to build a machine without torsion boxes and then rebuild it when the machine is ready. In the near future I will be doing the same I think.

One of the first thing I will do is build a torsion box to put my current machine on. After that I can re-use that surface as the bed of the new machine.

But first, is the dust shoe, bought myself a big vacuum cleaner last week and the design is almost ready to be cut

I think I will choose the same way, as it is easier. I´m still not sure about the specs of the machine and wheter I will use the ball screws (the ones from china) or a chain drive. I don´t know why, but I like the chain drive, although ballscrews are way more accurate.

WIll you build your torsion boxes with MDF or plywood?

[Mr.ChronoM]

I think I will choose the same way, as it is easier. I´m still not sure about the specs of the machine and wheter I will use the ball screws (the ones from china) or a chain drive. I don´t know why, but I like the chain drive, although ballscrews are way more accurate.

WIll you build your torsion boxes with MDF or plywood?

I'm very happy with the rails and ballscrews from China. Great quality for that price and easy to install (if you order the correct size couplers héhé :/)

I will be building the torsion boxes and the updates to my machine in MDF. I used plywood, but would not do it again (or at least with a better quality plywood). The good think with MDF is that it is 'engineer', so if you get a flat piece from the shop, cut it in piece, the pieces will be flat also. When cutting plywood, you can easily end up with bend pieces ...

[Mr.ChronoM]

I'm still waiting to start making the dust shoe because my ebay magnets haven't arrived. But in the mean time I did manage to make the machine a lot stronger and run some test pieces.

I'm also pretty close at completing my own 'gcode sending program' for GRBL, so that's gone ask for some more testing later on.

Some pics of the reinforced bed and the test pieces I made for my girlfriend (who is a teacher for 5 year old kids).

The biggest problem (next to the dust) I'm still facing is clamping the material to the bed. I'm screwing it in at the moment, but the last few millimeters are still a problem (especially on the test pieces here ...)

[harryn]

Perhaps consider to double side tape your wood down, and then cut. That will help hold things still better.

Another way is to cut in a way that leaves behind some material to hold it still, and then do this last cutting by hand. For example, if your material is 5mm thick, then cut 4.5 mm deep with the cnc, and use a knife to cut the last 0.5 mm.

[Mr.ChronoM]

Forgot mention that I did try double sided tape, but I partially ripped of the top layer of my plywood trying get it of . Maybe I'm using the wrong stuff, but I will try to leave some material in place on the following pieces, that sounds like a good plan, thanks !

Perhaps consider to double side tape your wood down, and then cut. That will help hold things still better.

Another way is to cut in a way that leaves behind some material to hold it still, and then do this last cutting by hand. For example, if your material is 5mm thick, then cut 4.5 mm deep with the cnc, and use a knife to cut the last 0.5 mm.

[aarggh]

I don't known what tape you used, but I use the 50m rolls of double sided sisalation tape from the roofing/building suppliers at around $25AU roll. I get mine from robottrading here in Vic, OZ, but I couldn't find a brand name on their site sorry

It adds about 0.5mm thickness, holds like a wife's death grip on her purse, but pulling it from the edge removes extremely easily with no damage to materials. If the materials are clean you can re-use a few times without hassle. I actually machined a 38mm thick alu billet using only this to hold it down, and it only moved because of the tremendous heat buildup!

cheers,
Ian

[Mr.ChronoM]

It's been a while since the last update, so here it goes. I have been testing all kinds of stuff to see what the machine (and myself) manage to make. My 'End Mills' from ebay for testing have been 4/6/8mm 2flute ones. And managed to burn the 6mm one 2 weeks ago. All end mills are HSS for price reasons, so I will need some good ones soon.

Now after my tests I discovered multiple problems and fixed most of them. I reinforced the Y-axis in multiple places and it seems to be better, but there is one thing I can't figure out at all. I don't even know where to looks, so I hope someone here has an idea (I'm still a complete newbie, so it could be obvious to you guys).

The attached pictures show my last 'test' in black MDF. I did cut the same thing twice in one job, next to each other. Pic 3 shows the one that has been completed 'almost OK', pic 1 and 4 show the 'not OK version'. It looks like the first pass is completely off on both pieces and it gets restored from the second pass on. The strange thing is that I haven't seen this on other materials like in pic 2.

Is there someone here who has an idea where to start looking ? Gcode is made with vCarve and the controller is GRBL, I don't think that is a problem. I guess there still is some design/build error, but if I was missing steps I think the problem would get worse (which isn't the case at all) ?

Second guess was to much flew on the Y axis, but in pic 4, the 'bump in the long side' is away from where it should be, so that sound strange also.

I'm also sure my HSS bit at 12000rpm (Kress milling engine) isn't the correct one, but to produce things that look that bad :/ ?

Hope someone can point me in some direction ... I made multiple thing that look perfect, but with these big jobs (far away from the starting point) it going really wrong