Best R+P combo for precision aluminum cutting
 

I'm sorry if the following has been addressed before, but I have I have a friend who is soon to be returning from an extended vacation in the US, and has offered to bring back at no charge what ever, any parts that I might need for my MM build.

That being said I am in desperate need of advice on choosing the best combination of rack and pinions for my coming MM build. Here are a few details on what I am planning to do, and the items already on order.

I have placed an order for four PK299-F4.5A Orientals # , and will be setting them up with belt reduction drives of 4:1 ratio. They will be a combination of the models shown from Chopper, and J.R.

My plan, or should I say the principal use of my MM will be cutting aluminum sheets for aircraft parts, and occasionally thin stainless, and steel metal. I have no need to do any work that involves cutting, or shaping any wood products.

I am going to try and do my best during the build, and in my choice of parts, to do what ever is necessary to get the closest tolerances on my MM. My question is the following, what would be the best combination ( size and number of teeth ) should I be purchasing for the rack and pinion. ?

I am looking for very smooth, and precise cutting, and I am in no way interested in saving time, or increasing the speed at which the machine will be cutting.

Thank's to all of you, who will take the time to share with me, your knowledge on the above issue.

Robert

Bob,
Sean had good luck with the rack from standard steel and I believe he used 20 tooth pinions. They are a little slower but give you the most resolution. I believe these are the smallest that will still leave enough meat left on the pinions for grub screws. :) Bigger pinions will get you faster speeds but lower resolution.

Stepinwolf,

I'm using the 7.2 geared Oriental Motors with 20 tooth pinion. We've been cutting a fair bit of 5083 marine grade alum with it with great success. No lube, no bit cooling, 72ipm, .125 spiral carbide bit (not a bit designed specifically for alum, just some I happen to have), we cut 1/8" in 2 passes.

Going for the reduction drives and 20T pinions will bias your machine toward the greatest resolution and you'll still have plenty of speed for fast jogging. You might want to put some thought into a vacuum system for holding thin sheet down. Having done some aircraft work myself I can say that the Mech will easily meet your accuracy needs for sheet parts, even in standard configuration.

Chuck

Chuck,
Exactly my setup on machine #5 and resolution is great. I can 2nd that mechanical set up!

Photo of nice cuts in clear acrylic here.

Sean

Bob,
I am using the 20dpi, 20degree rack and pinion as stated it the plans,
I, like you, cut mostly aluminum, and that is what made me build the 4to1
belt reductions and they are the single biggest improvement I have made to my machine, I also built a set for a friend and he says the same thing, he is also running the same rack and pinion I am, getting the same quality of cut as I am also, you need to keep the pinions small to keep the resolution tight, and the belts help eliminate back lash, I am also running kelling 450 ozin motors that have a low inertia that allow fast ramping and slowing of motor speeds, I know you are not interested in speed but I also believe that this makes them run smoother, ( I to do not need the speed it was a side effect of the setup I run )
so with this setup you will end up with the best all around performance,
and with the reductions you will get smoother cuts in aluminum, well everything as far as that goes..
//chopper

I'm going to second Chopper's recommendation for the belt drives. I'm finding more backlash in the PK296 7.2 gear reduced motors than I like to see though in fairness I've not seen any error in final cuts that is attributable to lash. But if I was starting from scratch I'd go with the belt drives, lower cost to boot.

Chuck

Chuck,
when I had my 7.2's there wasn't much I could attribute to the gear reductions either, except not as smooth of cut as I wanted not bad but it had a little chatter in them and that is what lead me to the belt drives,
I do not think the 7.2 are bad, it just depends on what you want for cut quality and repeatability, on my friends machine there was so much back lash that when he cut a sign you could see the letters get smaller as it went down the sign, it was like it was missing steps, and when it was done cutting and returned to zero it was off by more than .5 of an inch, (in the defense of the geared motors they were really large files) I believe they were hundreds of thousands of lines of codes long, but it caused errors in the machining from the back lash, the belt drives that I made were modeled after JR's drives, but in doing some calculations on ratios, I decided that the optimal ratio would be either 4 to 1 or 5 to 1 since it would be tough to get a 5 to 1 ratio with out stacking pulleys (pulleys that are available go up to 4 to 1 but are hard to find any larger than that ) and that is also what made me decide to go with steal with the stand offs that JR used it makes it difficult to get a 72 toothed pulley to fit,( so I designed the drives so I could reuse the stock motor plates and welding them together seemed like a good idea) since going to the belt drives when the machine returns to home it is exactly where it started, so I know I am not loosing or gaining from the backlash, and my friends machine is the same, I also want to add that the geared reductions my friend was using were made by a different manufacture than oriental, the belt drives were an experiment to see if they would work, and so far they have been a great success...
//chopper

That is NOT backlash - that is lost steps from overloading the stepper motor. (or an under-powered stepper motor)

The rest of the backlash discussion moved here