where to start?

[GTJim]

JAZZCNC you have given some excellent advice in this and other threads and I am sure that myself and others will benefit for your experience.
A question, how does rack and pinion compare to ball screws in repeatibilty and accuracy, and does the pinion ever jump teeth under load?
Sorry if this sound dim, but curious.

[trounce]

One last question before I get designing....


As you can see by this image the sheets of 8 x 4 I'll be cutting get used almost right up to the edge of the wood. I will obviously need a frame which is larger so the router will be able to travel past the edge of the wood, I'm thinking 9 x 5 does this sound about right?

[JAZZCNC]

One last question before I get designing....


As you can see by this image the sheets of 8 x 4 I'll be cutting get used almost right up to the edge of the wood. I will obviously need a frame which is larger so the router will be able to travel past the edge of the wood, I'm thinking 9 x 5 does this sound about right?

Not a straight forward yes or no. It will depend on a couple of measurements of the machine to how much extra length and width past the desired cutting area.

First I call the long axis X (length) and Y axis (width) is narrow, Z axis(Up,down) runs along the gantry or Y axis.

Regards Y axis(width) you will want it Just a bit wider than the width of the Z axis, basicly half the width of Z axis at each side plus a bit of stopping room.
The Z axis width will more than likely be determined by the bearings and screw used or the width of the router mounting plate. Wider is stronger because it positions the bearings on the Y axis further apart but wastes travel needing wider Y axis and machine. 200mm is a good width and will fit most routers and spindles plus allow for resonable Y axis bearing placement without wasting travel. 150mm is about the minimum I'd use for a good Zaxis.

Regards the X axis(length) well this depends on the position of the gantry cross beam on Xaxis bearing plates and how far off the front of the gantry the Z axis sticks plus the spindle or router used.
This will ultimatly give you the cutters position relative to where it falls in line with the X axis bearings, this will give you the needed extra for the X axis.

I try to position gantry cross beam back slightly from centre of the X axis bearings so that the cutter position basicly lines up with the front edge of the bearings.
This keeps everything contained within the X axis bearing plate length and nicely balanced weight distribution on the bearings.
No matter how you do it you have to have lose some wasted space at one end or the other.! . . Some sweep the gantry sides back and half the loss at each end.?
I just keep the loss at one end. . . you can always use this space for 4th axis head then it's not taking any table roomand can be left on.?

Doing it the way I do basicly means adding the length of the X axis bearing plates to the required cutting area.

Hope this makes sense.? If not just ask.

[JAZZCNC]

JAZZCNC you have given some excellent advice in this and other threads and I am sure that myself and others will benefit for your experience.
A question, how does rack and pinion compare to ball screws in repeatibilty and accuracy, and does the pinion ever jump teeth under load?
Sorry if this sound dim, but curious.

Well Jim the honest answer would be they don't compare.! Infact if you asked me too place each system in numerical order of effeiciency, repeatabilty and accurecy then R&P would probably come 4th. . . . 1st ballscrews, 2nd lead screws close 3rd belt drive then R&P.!!

That said it's doesn't mean it's rubbish and for large format machines doing lower precision jobs but with high loads and cutting feeds then it's usually the best option.

Ballscrews (and Lead) are too whippy over long length's without taking special measure's IE: rotating nuts or excessive sized screws,(which brings other issue's) plus the accurecy and precision they produce hisn't often required for cutting wood and plastics etc which most large format machines are used for.

Belts are very accurate, effiecent with very good repeatabilty, virtually no backlash. But again they have there issue's over large length's and don't handle high loads as good as R&P. They can be made do-able for large machines but not ideal . . . More suited to large machines with low loads and high speeds.

So IMO R&P is the best all round option for a large format machine with high loads (IE: heavy gantry,deep DOC) needing medium accurecy with resonable repeatabilty and high-ish feed rates. . . Perfect for Cutting wood, plastics, stone.

Regards jumping It's not an issue if designed correctly with correct alignment and pinion tensioning into the rack. The issue's can come from debries getting in-between the rack & pinion but if the rack is positioned with the teeth facing down with the pinion engaged by spring loading into it rack from benneth and positioned out of the firing line as much as possible then it's not an issue normally.
The trick is getting the tensioning just right so it's not so weak it allows teeth too jump or have excessive backlash but at the same time not to high cause's binding which helps to reduce backlash (too a point) and jumping but cause's excessive wear. The required tension will vary from machine to machine depending gantry weight, cutting forces etc and a little setup & adjustment will be required at first but ounce setup it works good for this size an type of machine.

Again at the end of the day it's horse's for course's.! . . . . If you want super precise get a show jumper(screws), if you want light weight and speed get a stallion(belts), if you want a good strong all round work horse then get a shirehorse.(R&P) . . . . . If you want an old knackerd Donkey get some threaded bar and MDF. .:whistling: