Mounting 4th axis rotating indexers or lathes under the gantry
 

There is often an interest in mounting rotating indexers or lathes under the gantry of a CNC router. Stephen Hull touched on the issue here and Art Ransom mentioned it here. This thread has been started as a place to throw around ideas on this topic. I personally don't want to get too deeply involved in it yet, but just drop in some observations....

I believe that an indexer/lathe's center height must be adjustable in relation to the height of the gantry. As shown on Stephen's thread, a gantry router ideally cuts things in the range of 125 - 200mm [5" to 8"] below the gantry, therefore:

- a 24" diameter column center should be 17 to 20" below the gantry (9 to 12" below the MechMate table top)

- a 2" diameter table leg center should be 6 to 9" below the gantry (between 2" above table to 1" below table)

Clearly two very different locations and applications.

Edit: This thread started way back in the old forum. These pictures from there are interesting:

Gerald, thanks for the answer and information. This is much of what I have been looking for. I appreciate you venturing into this area.

Since I am new to CNC I often end up it the award position that in latin is called "craius in rectus". My first attempt I had 14" of travel on the Z. There was way too much flex in the gantry and Z. I redesigned using ajustable ways. In the top position I have 8" to the center of the A axis. This should let me turn up to 10". Also the ways can be used to mount a table so I can do routing. There will be stops every 2" increasing the allowable diameter by 4". The A axis asembly bolts to the machine legs in each position. This is a brief overview and I will try and get my site updated this weekend with pictures and details.

thoughts.....

....there is no law which says that the cutting tool shall spin around a vertical axis while doing "indexing" work - the router/spindle can lie on its side and the plunging action can be horizontal....

...bolt a horizontal router/spindle directly to the Y-car...

....indexer/lathe center height can then be fixed (non-adjustable)...

....indexer can be on wheeled trolley parked/bolted against side of MechMate.....

....the inspiration is the last pic in this thread http://www.mechmate.com/Forum/clipart/happy.gif

Posted update to my A axis assembly.
http://turningaround.org/4_axis_mill.htm

Hey Folks,
I have a SB Alpha with Indexer mounted to the side (C-Channels) with a 12" Z-Axis. I am planning to remove it from the side and mount it along the X-Axis upright on the bed angle iron cross-braces. I would like to use a 10' piece of T-Slot to mount the headstock and tailstock. SB Indexer came with short 24"x4"x1" T-Slot machined out of aluminum. Anyone have any suggestions as to what to use for the T-Slot? Like where to buy it at reasonable price? Having it machined at a commercial shop is cost prohibitive. I am not brave enough to attempt to machine it myself. I have thought of using some kind of hardwood, then cut a T-Slot in it, but not sure it would stay stable (stay flat). Also have thought of cutting 2 pieces of aluminum with table saw for T-Slot, then fastening together. But, not sure this would be satisfactory either.

Hoping you all might have some very clever ideas...

Regards,

Wayneo

There is no need to have real precision in the rail. A piece of steel square tube with a slot cut in the one side will work. An angle-grinder is okay for making the slot.

Gerald, I am very much interested in mounting a 4th axis on to the mechmate that I have built. I also do have one spare stepper and a driver. Can you please advice me on this issue.

I do have one spare stepper and a Gecko driver which i bought as spares for the mechmate Can some guy give me some details on how to fix the stepper onto the Head Stock.

Quite frankly, I think a "board-cutter" and an "indexer" are two totally different machines. A combination machine will have to make big compromises.

If I had to build an indexer tomorrow, I would:
- decide on the length and diameter of the workpieces.
- build a MechMate gantry where Y=half diameter. Yes, a very short gantry. (One x-motor may be sufficient on one side only)
- build a very narrow table where X=length and Y=dimension above. The table cross-members will be fewer, lower and height adjustable.

The "headstock" will have interchangeable gearwheels for different diameters of work to be turned. The gearwheel diameter will be approx the same as the workpiece diameter. The standard MechMate spring-loaded motor mount, and motor with standard 20 tooth pinion will ride on the gearwheels.

Your point is well taken. But Gerald, as I would have pointed out to you earlier, in Countries like Sri Lanka where people are new to CNC machines they do not they do not understand and realise as to what could be done with a CNC machine. So due to this we have to initially make samples and show people as to what could be done. Once business starts to grow you could think of having dedicated machines for each type of job.

How about starting from a regular wood lathe and fit the unit on your machine then figure a way to drive the lathe with your stepper? It's an easy way to get started that lots have chosen. You get pretty much all you need; head stock with accessories, tail stock and you can even consider to use the lathe conventionally.

From that point, you got two challenge; connecting your stepper and mounting the lathe on your tool.

Before I (and other) go on with suggestions and ideas... is that what you're looking for to do?

If you only want to make a few samples, then do it like the first couple of pics in this thread that I have linked to before. (across the end of the table).

That thread very much covers the options of mounting locations on a board-cutting table. You can move the Z-axis inside your y-car to reach further over the end of the table if necessary.

Art, What should be the lowest and the highest RPM's that you shoud consider for a indexer (including sanding/finishing work).

Yesterday I finally got all the pieces together and was able to round and flute a 6" by 6" by 96" column. My axis is a 1200 oz. stepper with a 4.5:1 ratio. This ratio needs to be higher probably 50:1 range. Open to suggestions on how to accomplish this. I need a lot more torque to prevent stalling with heavier pieces.
I did the rounding using a straight 2-edge bit and cutting on the side of the piece (Y-axis).
Here is the program
G0 Y0 Z0 X1
M3 F80
G0 x0
M98 P123 L1

M30

O123
M3
G91 x.01
G90 A2200 Y70
G91 x.01
G90 A0 Y0
M99

M3 picks the solid state relay that turns on the router and dust collector and M30 turns them off.
Surface is X0 but G0 Y0 Z0 X-1 places the cutter 1" from the stock so the bit doesn't grab at router start and then it is mover into cutting position at x0
G91 x.01 moves to a cutting depth of .01? deeper cut used for roughing.
G90 A2200 Y70 A rotates 2200 times while Y moves 70?
M98 P123 L1 subroutine runs L number of times.

Values of L, A, Y, and X are changed as needed.
The A axis rotation was 67 RPM and I need to work with motor tuning to see if I can?t get it faster without stalling during positional moves when fluting.
Here is the fluting program
G90 X0 Y0 Z.25 A0 F80
M3
M98 P100 L10

M30

O100
G90 Z-.02
G91 Y4
G90 Z-.02
G90 Y0
G91 Z1
G91 A.1
M99

Cutting is done top center of the piece and this does 10 flutes.
L, Z,Y and A are changed as needed.

These are crude but I am just learning G code so suggestions welcomed

Questions:

How do I home A only?
How do enter comments into program?

The following two statements are mutually exclusive (contradictory)

"This ratio needs to be higher probably 50:1 range."

"The A axis rotation was 67 RPM and I need to work with motor tuning to see if I can?t get it faster without stalling during positional moves when fluting."

Higher gearing will make the moves slower, and vice versa.

An indexer ideally needs gearing optimised for each job diameter - a one-size-fits-all approach can't work.......unless we can get those PC's and control cards that can spit out very high frequency step pulses.....

With a higher ratio I can go into motor tuning and raise the motor RPM and steps without boging the motor.

Mach, when stretched, gives pulses at 45000 per sec, but more reliably at 25000 per sec. To turn a motor a full turn needs 2000 pulses. To turn a 50:1 gearbox a full turn needs 100 000 pulses. At 25kHz, it will take 4 secs to make one rotation at 50:1. If that is fast enough for you, then you are golden. But watch the backlash in a gearbox of that high ratio.

gerald, Art, Initially I intend to use the mechmate for indexing work. I will be mounting the indexer parallel to the Y axis in the machine. Later on if required I would love to have a dedicated machine for turning jobs. The stepper that I initially use with mechmate should be able drive the load of the final dedicated machine as well. Will a 1200 Oz stepper be ok for my purpose.

Fabrica, what are your gearing plans for that 1200 Oz stepper?

Mach seems rock solid with 45000 selected. Getting cycles constant in the 43.5 K range. Running 7568Mg Ram on a 2.2 Mhz $149 new PC from Fry. Came with 1 parallel port, 8 USB ports. CDR, 10/100 NIC, flopppy drive,sound,serial port 40 Mg HD.mouse, speakers, keyboard and 128K of RAM. I added 256Mg of RAM for $49.
The indexer needs more than 1200 steppper at 4.5:1. Not familiar working in oz/in mode but based on my Powermatic 3520A wood lathe which has a 2HPp variable speed motor. The motor has a max RPM of 1750 and on the low pulley setting it's max RPM is 1500. Does that make the gear ratio approx .9:1? My present configuration maxs out at 67 RPM but a 1" bit with a .25" deep cut can move the work. Seem to rember seeing somewhere that 1 HP is 30,000 oz/in. Need recomendation on size of stepper or servo to get for indexer.

Gerald, I am clueless about it. Maybe a stepper with 7.2 gearing ratio connected to the indexer through a tooth belt drive.

Here is a pic of 40 flutes on a 4" column.
http://www.mechmate.com/Forum/messages/19563/3500.jpg

If we have a 600oz/in stepper doing a good job for the linear table, then we don't need a bigger motor for an indexer . . . . . provided that the gearing is correct. The magic is all in the right gearing.

If the recommended Oriental Motor with its 3.6:1 or 7.2:1 gearbox is doing a good job of linear cutting then it will do as well when it turns a small workpiece directly as an indexer. Strictly speaking, a small workpiece with a diameter equal to the pinion diameter (only around 25mm [1"])will give the same surface speeds and loads.

If you want to index a 6" workpiece, then you would theoretically want 6 times more gear ratio. Instead of 3.6 or 7.2 to one, you want to look at 22 or 44 to 1.

Fabrica, your idea of a 7.2:1 box with a further tooth belt drive is good. But I would experiment with cheap bicycle sprockets and chains first......

However, the main point I want to make is that I don't see a need for bigger stepper motors - only for more gearing.

Hi
If you turn even circular object, but if you start carving, the center of gravity change. By how much ? depend on the design. you may need stronger motor?

Yikes, I didn't think of that factor. Valid point.

So finally what do you guys suggest.

Still say that you should get exactly the same motor/gearbox as for the rest of the Mechmate, but then to add further gearing/belting/.... to that motor.

Can we think of a tooth belt driven pulley.

Can some guy recommend to me a good source from which I could buy a good set of head stock and tail stock.

I also need to know weather I could do them in my workshop itself.

On my site at http://turningaround.org/4_axis_mill.htm near the bottom is my head stock and tail stock are shown. All off the shelf iteams except for the #2 Morse tapper in the end of the tail stock quill.

Today I worked on doing motor tuning on the indexer axis and it is running great at 300 RPM. Got too involved and didn’t try faster speeds. At this time I don’t see any need for speeds over 100 RPM except when rounding rough stock. At this speed it takes 11 minutes to make 1 pass of 96”. Also I have order a set of laser cut parts and will replace my crappy gantry with the MechMate one.

Art, What were the prices that you paid for these items and what was the source of that you bought them from.

1.25" allthread Macmaster Carr $40 ?
Nuts $20 ?
4 jaw chuck Harbor Freight $16
Live center $95 Oneway lathe
pillow blocks $10?
Machining #2 moris taper $20
I used the Oneway line center because I have a backup one, the Harbor Freight chucks screw on,and the bearings are replacable.http://www.mechmate.com/Forum/messages/19563/3511.jpg

Gerald, If I am to go by your suggested configuration where the gearing ratio changes depending on the diameter of the stock.

I need to work with stocks having diameters ranging from 1" to 12". The RPM's I would like to work with would range from 1-200. Assume that I will have a 3.2 gearing ratio on the motor. How many sets of pulleys and belts will I need.

Sorry, reference to the earlier post my motor gearing ratio is 3.6:1.

Gerald, I did some quick calculations on a worksheet I got done to do RPM calcualtions for the indexer.

If I use a 3.6 geared motor and set the pulsing speed to 45,000 RPM and also if i set the pulley ratio to 1:1.8 (1" pulley on motor shaft and 1.8" pulley on head stock shaft). I would get a indxer RPM of 200 (This i feel should be the minimum for rounding up work on rough stocks).

AS per your theory which says that the pulley diameter should be equal to to the diameter that you are working on then the max diameter of the stock that I can work with would be 1.8" in diamater.

If I need to work on 12" stock (with 12" pulley mounted on Headstock shaft)I can only operate at a max RPM of 30 RPM.

The other option available to me is to do the rounding of the square stocks on a basic turner with no stepper involvement and then transfer the workpiece on to the machine we propose to build.

Fabrica, you are starting to realise to difference between an "indexer" and a "turner"(lathe)...
- indexer turns the work at low speed while a fast spinning cutter (router) takes deep cuts.
- lathe turns the work at high speed, while a static cutter takes shallow cuts.

They are completely different machines. You seem to want an indexer that has the speed of a lathe? With a router driven cutter, the speed of the workpiece can be much slower.

Rough turning in a lathe is good. Sandpapering in a lathe is also very good. Sandpapering on an an indexer is hard work.

I would pick 2 gear ratios max. Probably 3:1 and 9:1. The idea is that you can have:
- direct drive
- 3x slower than direct drive
- and 3x slower again (9x)

Don't pick the middle ratio in the "middle". If your top ratio is 10:1 the middle appears to be 5:1 but that is actually 5X from the bottom and 2x from the top.

It is easy to do this theoretically, but you need to find practical pulleys or gears....

What you mean is that a diference between a indexer and lathe is that the same job that is done on a lathe at high RPM's could be done on Indexers at a low RPM. But for sand papering and rough cutting lathe is a better option.

OK assuming that If I take the indexer path, I already have a stepper with a 3.6 gearing. If I drive it direct the max RPM would be 375, With 3:1 gearing 125 RPM and with 9:1 gearing 42 RPM.

Your suggestion is to do trials with chains and sprockets.