Hello everyone, I had an ancient scm cnc, running bobcad for a while. It never really worked out. After 15 yrs without one, I am looking to get into it again. My question is about accuracy. I really want to get into 3d carving with some of my art work, but right away the router would be replacing some of my boring and cutting tasks with standard casework. I am concerned that the rack and pinon set up with the 4x8 won’t hold tolerance for really detailed carving, and on the other hand, I would like to machine full sheets 4x8 for the cabinet shop duties. Does anyone have any advise on this? At double the cost it is a leap to go for the larger machine, but it would me more directly applicable to the already profitable casework I build. The original idea was to use the machine for my art, which would be a massive bummer if it can’t. Thanks for any imput.
Hi Evan,
Right off the bat, I have no altmill of any kind so am a wee reluctant to be too confident with advice on the difference between the versions, but I believe the accuracy of any altmill is equal. I am however not familiar with rack and pinup setup and what it would do for accuracy.
If you are affraid a bigger machine would be less accurate on smaller projects, don’t. I have cut and engraved key chains with my longmill 30x48. It feels rediculous to work on 60x20mm projects when you have that huge machine bed, but the machine does not know nor care. It’s as accurate on the dime as it is on max range.
The machines sienci produces are all capable of some pretty fine detailed work, if you use the endmills that can produce it. I don’t think a 4x4 differs from a 4x8 in that regard.
I am curious why you think a 4x8 wouldn’t be. Oh and I am curious on what kind of artwork you are planning to do. Do you have an example?
I worked for a large CNC manufacturer for a couple of decades. They made both ballscrew driven and rack and pinion machines, sometimes both on the same machine.
Early on (with rack and pinion), competitors would try to convince their customers that R&P machines could not possibly hold the tolerances needed for CNC work. Early on, they probably had a point. But before long we proved them wrong.
The inherent problem with ballscrews (aside from cost) is mainly when you exceed a certain length - you get whipping in the screw. That’s why Sienci went with R&P on the 4 x 8 AltMill. Rotating ball nuts can mitigate it, but are very expensive. Improvements in machining tolerances of the R&P components pretty much did away with the argument, along with software compensation, etc. Higher end machines now use helical R&P for even tighter tolerances.
Long story short, the overwhelming majority of machines larger than 4’ in any axis use R&P now. It takes up much less space than the equivalent coverage of a ballscrew driven machine due to the moving table.
In the late '90s, they came out with a 4 x 8 table that had ballscrew on X and R&P on Y. It wasn’t long before they realized that the backlash on X was higher than on Y, so they switched to R&P on both axes.
Knowing what I know, I wouldn’t hesitate to go with an R&P driven router for wood applications.
Hope this helps.
@evanscottcabinet I doubt that there’s much slop in the rack and pinion system itself. So the accuracy of the system would be dependent on the stepper motors. And as @Spamming_Eddie pointed out, you’ll get the same accuracy on a small portion of the mill as you would on the entire bed. I haven’t looked at the differences between the stepper motors of the 4x4 and the 4x8 but they should be published on the Sienci site somewhere.
Thanks for the reply. I am looking to do some 3d carving into the furniture I make, as well as inlay work in furniture doors etc. Right now I do the inlay stuff with the old router and template sets. It is cool because all the designs are freehand off the bandsaw, but it takes a lot of time and the chance to screw it up is high. I also have some cad and cam experience that is just going to waste and I am a bit burnt out so I’m looking to branch out. I have a profitable cabinet shop booked until next May, which is usually the case, but I really don’t want to buy a machine with the current business in mind, because most of those systems are pretty dialed between my current equipment and the shops I outsource to.
With all the back-lash and related topics understood in this interesting discussion, surely a remaining question is beam rigidity. For the same beam, it is reasonable to expect that 8’ would flex more under cutting stresses more than 4’, but…
- Beefier beams could have been used to offset that
- The rigidity of the AltMill could be it’s main strength, so 4’ or 8’ isn’t the problem in reality (eg: Z-axis nod, bounce)
- For wood cutting maybe this is mostly irrelevant, but what of people wanting to cut Alu or Brass for example, where cut quality can quickly become affected by rigidity/resonances etc
I have 4 cnc machines. 1 4x8 altmill with atc. i love it and have no issues with alignment. i make a lot of cabinets and other interlocking items on it. again, it is pretty fast and accurate, and repeatable.
hope this helps.
btw, i have take a 3d carve off mid carve(i use fence to locate) and moved across brands to altmill, used spoil as surface to zero, and used fence as known xy. stopped about 40 percent done, needed other machine to complete job, swapped to altmill and finished with no issues. can’t tell. started on close to same line of code. that says something.
First off, I haven’t any engineering training so these are just my thoughts or gut instinct if you will. The 8’ mill has 8 legs instead of 4. Without measuring I’m pretty sure the unsupported spans of the Y beams are shorter than the 4’ mill. There is also bracing on the legs that the smaller mill doesn’t come with. I would expect those two points to increase stiffness.
I’m also thinking that with the gantry side plates always being at the same Y, the longer Y beams don’t give any extra leverage to the cutting forces.
Educate me if I’m missing something because I’m thinking the 8’ is probably more rigid not less.