[Chuck Bogardus]

I'm going to need a fourth axis. From what I understand, I can mount a stepper to a rotary table, or I can buy something like one of the chicom harmonic drive (maybe?) setups that are on Fleabay... Use will be primarily for small tool cuts in free machining steels, and I'd like to do some simple engraving.

Pros? Cons?

[Jim Dawson]

Either way would work fine. Using a RT might be easier because most of the work is already done in that the chuck/table is already mounted in bearings. If you start with a harmonic drive, then you have to build the table/chuck/arbor system.

Here is what I did http://www.cnczone.com/forums/linear...onversion.html

Attachment 343920

[LongRat]

I mounted a stepper directly to a Vertex HV4 4" rotary table and it has served me well for many years. Unless the zero backlash of the harmonic is an absolute requirement I would just use an RT. Like Jim said, basically all the work is done for you then.

[Jim Dawson]

It is possible to setup the Vertex for zero backlash. You just rotate the worm shaft housing to engage the gears a little deeper. This is not the best as it increases wear on the gears, but with proper lubrication it will last a long time. Since this is an intermittent use tool, I'm not worried about doing this. If it was going to be in continuous use, I would build a direct drive 4th axis. A small, cheap lathe headstock would make an excellent 4th axis starting point. You can pick up a Harbor Freight 7x12 or equivalent lathe pretty cheap from Craigslist or Ebay.

[LongRat]

That's what I have done. I think it would be better if the housing was rotated under spring tension. As standard, you may get zero backlash but only truly at one location in the table's revolution. Elsewhere it can get tight or develop tiny amounts of play.

[Jim Dawson]

That's what I have done. I think it would be better if the housing was rotated under spring tension. As standard, you may get zero backlash but only truly at one location in the table's revolution. Elsewhere it can get tight or develop tiny amounts of play.

I like that idea :idea: Now I'm going to have to figure out the best way to implement the spring loading.

[109jb]

I don't think he is talking about building a 4th axis from harmonic drive, but rather buying one already made for him. You can buy these on e-bay from China for about $400 with a 4" chuck. While not super cheap, it has the stepper motor mounted, and just needs an appropriate stepper driver. If you can get by with a smaller Sherline style lathe chuck, then you can get them for about $200.

If you don't already have a rotary table then the cost is going to be at least as high unless you find a real deal on an RT, and then you probably need a chuck to go with it.

As for eliminating backlash on a RT, I think you will find that this varies with the quality of the RT. I have an import RT that it is not possible to completely eliminate the backlash. It has the eccentric adjustment, but due to mchining quality, it cannot be used to adjust it out everywhere. The worm gear apparently has some runout. If the eccentric is adjusted at the low point, then when cranked to the opposite side, it turns very hard. If adjusted to the high point, then it has some backlash at the opposite side.

[Chuck Bogardus]

This is the sort of harmonic setup I'm thinking of...

CNC Router Rotational 4th Axis A axis 3-Jaw 100mm Chuck Gear Box 1700+Tailstock | eBay

I figure that a 50:1 or 100:1 reduction is better than 20:1, right? Not looking for blazing speed - just moderate accuracy. I don't make 0.00005" parts, but I don't want stuff skittering everywhere.

I already have a rotary table, but it is a 12" but it is staying on the Bridgeport... I'd consider the 5" setup that Grizzly is selling if I was going to convert one.

[Jim Dawson]

I don't see anything wrong with the design of the one in the link you provided as long as the quality is reasonable. The quality of much of the import tooling is a bit hit & miss. The 100:1 would be my choice.

[109jb]

That looks like the one I was thinking about. I however would go with 50:1 rather than 100:1. Reason is because at 50:1. a full step of the stepper motor would get you 0.036 degrees positioning accuracy. Microstepping would be even less, so plenty positioning accuracy can be achieved with a 50:1 ratio, unless you ar making something that needs uber angular positioning then 50:1 should be fine.

[ninefinger]

That chuck is hanging on a 25mm (1") shaft. That is not very beefy, could cause you to have chatter depending on how far out from the center line you are, and how far overhung the part is from the body of the drive.
The 50:1 is likely fine for a small diameter work piece (1 1/2 thou full step resolution at 5" diameter).
Mike

[Chuck Bogardus]

Actually, I thought I saw one with an ER40 collet holder on it - which would work for 90% of what I do... Or maybe I can make one.

[abfa9358]

Hi,
Here's a link to my 4th axis build thread using a harmonic drive and the old head of my minimill. The idea of using a lathe / small mill head stock saves a bunch of time if you go down the harmonic route.


http://www.cnczone.com/forums/benchtop-machines/241988-cnc-cad.html

[rey8801]

Hi! Did you end up buy the harmonic 4th axis. If yes may I know which one and if it working well? The more details you can give the better. I have few drives in the workshop already but not sure if it need something more appropriate. Thx a lot

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