Thread: 4th Axis questions

Hi Fred,
The formula for calculating resolution for a rotary axis is
( Motor Steps X Driver Microsteps X Reduction Ratio ) divide by 360.
So a typical Nema Stepper Motor has 200 steps per revolution and say a Gecko Driver has 10 microsteps and you have a reduction ratio of 6:1 through pulleys then it calculates as follows.
(200 X 10 X 6) / 360 = 33.33333 steps per degree.
This should be adequate for engraving on pens.

I would be looking at a Nema 23 and 387oz/in motor with a Gecko Driver. The motor needs to resist cutting pressures so needs to have a bit of grunt.

I use this seller to get motors/drivers and pulleys/belts.

Thanks Rod, so if I understand correctly I need a 10 tooth pulley on the motor and a 60 tooth pulley on the spindle?

Ausxmods doesn't list a Nema 23 387oz motor, only a 269oz, but does list a 387 Nema 24, what's the difference? Physical size wise the Nema23 would be preferable.

Fred,
The Nema 23 and Nema 24 are the same motor mount and the difference is the body of a Nema 24 is 60mm square compared to 57mm of the Nema 23. I don't see a problem using the Nema 24. You can start with a Nema23 269 oz/in and swap it for the 387 without modification to your mounts if you have a problem. Homann has the 387 in stock also.

You are correct on the 10 tooth and 60 tooth pulleys and no coincidence I chose those as it is his largest and smallest sizes. The prices are very good and should get you past concept with only a small outlay. A higher ratio will give more resolution but speed is compromised and I figure you may have to skim the pen surface to get it parallel to the cutting path. If you don't do this your engraving will be deeper in some areas. There is a floating cutter you can get to compensate for this type of thing and this might be a preferable solution for you but they are expensive. The cutters are spring loaded and a nose cone rides on the surface. The other option is to cut a flat on the pen and engrave on that area.

Hi Rod

I don't understand why the engraving would be deeper in some places than others.

If the pen is rotating under the cutter then surely this is the equivalent of engraving on a flat surface?

Hi Bob,
You are correct if the pen in turned by the same CNC machine then engraved. Not knowing how Fred is planning this and looking at the ER collet chuck he is using I am allowing for manual turning of the pen then mounting and engraving on the CNC machine after. Obviously a manual turned pen would not have a parallel surface to a cutting path and at least be very difficult to draw. I have given various options depending on the method to be used.

I may be over complicating things but old retired farts tend to let the mind wander a bit.

Here is a link to the spring loaded engraving tool. Note there are nose cones to control depth of cut on the same site.
While I don't expect it to travel across the radius of the pen as the rotary takes care of that it will compensate for variation along the length of the pen if the nose cone is used.

Originally Posted by rodm

Hi Bob,
You are correct if the pen in turned by the same CNC machine then engraved. Not knowing how Fred is planning this and looking at the ER collet chuck he is using I am allowing for manual turning of the pen then mounting and engraving on the CNC machine after. Obviously a manual turned pen would not have a parallel surface to a cutting path and at least be very difficult to draw. I have given various options depending on the method to be used.

I may be over complicating things but old retired farts tend to let the mind wander a bit.

Yes, that is correct Rod, manual turning followed by CNC machine engraving/embellishing.

Originally Posted by rodm

Here is a link to the spring loaded engraving tool. Note there are nose cones to control depth of cut on the same site.
While I don't expect it to travel across the radius of the pen as the rotary takes care of that it will compensate for variation along the length of the pen if the nose cone is used.

Interesting tool there Rod, food for thought.

Finally had some time in the shed after all the hot days we've had, only 26C today.

I had to shorten the part that holds the ER25 spindle as there wasn't enough sticking out of the back to mount the 60 tooth pulley, only 15mm should be 21 mm.

So I shortened the "headstock" to give me about 20mm of the ER25 spindle sticking out the back.

Next step was to bore out the pulley, it only had an 8mm bore, the ER25 spindle is nominal 20mm.

On the lathe it went, drill out 10mm then 13mm.



Then use the boring bar to get to the nominal 20mm to give a nice snug fit on the spindle.



Re-assemble everything and this is what it looks like with the 60 tooth pulley mounted.



Next step will be to mount the Nema23 motor, have already designed the mount.

I bought this one
Diamond Drag Engraving Tool Spring Loaded 1 2" Shank | eBay
They are sold out at the moment but he keeps relisting them as he makes more.

There are also these ones
Spring Loaded Engraving Tool Holder for 125 Engraving Tool | eBay
they are a helluva lot cheaper than the other ones.
Last time I asked him, he would ship to Australia

Thanks SAISAY

Another cool day in the shed.

Machined up a mounting plate for the Nema23 motor, manually - haven't got a CNC router yet!









I will start a WIP thread for getting the engraver operational.

I think that I would have been inclined to use proper bearings on the shaft rather than the bronze bushes. They would be less inclined to wear in the one place.

How are you controlling end float?

Originally Posted by Bob Willson

I think that I would have been inclined to use proper bearings on the shaft rather than the bronze bushes. They would be less inclined to wear in the one place.

How are you controlling end float?

Fair comment Bob, but I am not building a heavy duty properly engineered 4th axis to handle a 1.5Kw spindle etc here.

I am just building a "proof of concept" 4th axis here to fit on a small CNC engraver. I thought that the bushes I turned up were a step up from the Lignum Vitae I was going to use.

I am using the materials I have to hand where possible which gives me a lot of satisfaction. If in time the bushes wear (highly unlikely given it's likely duty cycle) then I will probably look at building a "properly" engineered 4th axis or buy one like rodm has just bought.

Hi Fred

I agree. I too would rather make things myself from existing parts and I think that what you made looks very strong and workmanlike.

The comment was more to do with the end float because as you tighten up the spindle you are going to put more pressure on the end of the chuck and so that could lead to binding. Proper bearing would obviate this and also remove end float

OTOH, if all you are going to be using it for is pen turning, then I don't imagine that you will be applying too much pressure anyway.