[keyne]

Hello,
I am planning to build a wood router with a cutting capacity of 800x2500mm (32x99in). X and Y axis will be belt driven. I would like to discuss if it is worth trying single side drive. Would save alot of money and avoid some potential problems (sync, ..) and complexity. Gantry racking can be a problem with single side drive and i would like to get around this with a very stiff gantry. I have seen some single side driven commercial machines (Wissner).
The driven side on my machine bears the weight of two stepper motors, and most work would be done close to the driven side. The passive side would be used only for wide workpieces. I will use this machine for furniture making with a 2HP router.
Please see my attached sketches, one uses a torsion box gantry and table and a vertical y-axis, the other is made from steel tube (welded) and has a horizontal y-axis (like shopbot). Linear ways are BWC T4 and wheels W4.

Thanks!
K

[ahren]

The short answer is that it depends on the accuracy you want. Now I'll fill in with a bunch of anecdotes to help confuse the issue .

My current router is about as wide as yours, and is being driven from one side. This is an aluminum machine, built out of old robot parts from my work, and uses IKO linear bearings. It has some flex/racking issues due to the bearing configuration I'm using -- on one side, the bearing rail is mounted vertically, but on the other side it is mounted horizontally (see the attached jpg). When I push hard on the left side, I get almost 1/8" of flex (fear not, re-design in the works!). This is due to a) only having one bearing block on that side, and b) the wimpy 1/2" plate I have for the right gantry side. This plate is definitely twisting, and is the only thing keeping the system from racking. Given that your W4 bearing plate/guide will be mounted vertically, this type of twist could be a problem for you as well. However, having multiple bearings on each side should help a lot.

That being said, the robots that these parts came off of don't have a lot of trouble with this same configuration, and they only drive from one side. However, they are mounted to a very rigid steel tube frame. Even with a high load, they flex only a couple of thousandths (0.002").

To further complicate your decision, I'll say that we have belt drive robots here in addition to the one I described above. These do drive on both sides, but run a shaft across the back of the robot to connect the two sides, which makes the coupling easy. See the second picture. If you are going belt, this is a fairly simple configuration, and can even eliminate having a moving motor on your Y.

[jeffs555]

The carriage can easily be made rigid enough. The major problem I see is your bearings. You appear to show v-groove bearings with a fairly narrow spacing relative to the width of the carriage. V-groove bearings have much more strength in the radial direction than in the axial direction. The axial force on the bearings will be composed of the cutting force in the y direction, plus the cutting force in the x direction multiplied by the ratio of the carriage width to bearing spacing. This may be more axial force than the bearings can take.

[acondit]

Keyne,

Don't to forget that you need to design your bearings to prevent the traveling objects (gantry and z-axis) from lifting up. Weight alone probably won't cut it (unless you are designing a laser).

Alan

[keyne]

ahren:
Thanks for your 2 cents! Valuable practical experience here.
The accuracy i want is about 0.01" at least. What kind of accuracy are you getting in your present setup? It won't flex 1/8" during cutting probably.
I will use a stationary motor belt drive config for the Y axis (on the gantry, motor stays on driven side) but for the long X axis this is not practical, i would need two 7m long belts (280").

jeffs555:
Thanks for pointing to this issue. I am using W4 size bearings and the dynamic axial load rating is 6350N (or 1428lbf). that's about half of the radial rating. Should be fine IMHO. Shopbot and EZ-Router use much smaller W2 size bearings and work fine for years (at least SB). The axial forces are not related to the drive issue i think. Axle to axle spacing is about 14".

Alan:
I will use normal ball-bearings with an eccentric axle as adjustable hold downs. they are in the sketches, just not very visible.

[ahren]

Keyne, the amount of flex I get during cutting really depends on how heavy of a cut I am taking, and how quickly I am trying to cut. My setup right now really isn't adequate, but I think your proposed setup addresses both of the problems I have. First off, your bearing spacing looks good, and having two bearings on the undriven side should help alot. Also, your steel tubes with the V-bearings in them are far more resistant to twisting than my single flimsy aluminum plate.

For the record, my re-design is still only being driven on one side, so if I finish before you do I'll let you know how it turned out. Our gantry widths are about the same, although my bearings are different. Good luck!

[hma]

Hi,

My 2 cents: you can use crossed steel cables and pulleys to eliminate the gantry flexing. Suppose the gantry is powered on the left side. Now place pulleys at the corners of the table, LU, LL, RU, RL corners. Fix the cable at the "powered" site of the gantry, go to the pulley at the LL corner, then to the pulley of the RU corner, fix the cable to the other site of the gantry, the second cable goes from the powered site of the gantry to the pulley at the LU corner, next to the RL corner and again to the other site of the gantry. Of course the pulleys and fixings have to be in a rectangular position.

HUGO

[keyne]

Hugo,
the old shopbot used this cable system. It is too much fuss for me, i want to keep it as simple as possible.
K

[jcc3inc]

Sir,

If oyu are considering single side drive, you should reconsider Bishop-Wisecarver vee wheels since they will slide up the vee ways at some side force which is equal to the down force on them. If you will email me, I can send you pix of the single side drive machine which I have using rectangular cross section rails.

jccinc-at-owc-dot-net

Regards,
Jack C.

[keyne]

Jack,
i sent a mail.
already have the W4 wheels now so no turning back. For the long axis it is the only reasonable option anyway.
How is sliding up the vee ways related to single side drive? I can see how a heavy cut on the Y axis (on gantry) can generate high side loads, and the holddown mechanism must be strong enough to keep the wheels in place.

[jcc3inc]

Sir,

Vee ways do not like side force UNLESS you use hold-downs of some sort.
They are nice to use but do have that limitation!

Regards,
Jack C.