David A's New 2x3 Bench Top CNC

[davida1234]

I used a ballbar tester that measures down to the micron to make very tiny adjustments.

0.00004".......

Well, Outlawtaz, yours capabilities are definitely way beyond my capabilities.

As you said, well outside the precision of a homebuilt, let alone some semi-commercial machines.

I do like your different perspectives, though. Keeps me aligned, so to say.

[ger21]

It is true that rack & pinion drives are quite easy to integrate.
One of the reasons I pursued a different path is that I am
not comfortable with their power transfer situation which
creates different load conditions for one direction and the
other. I also wanted to avoid protruding motors.

Yes if you use the spring loaded CNCRP style.

I'm using helical racks with the pinions mounted to a gearbox, that's rigidly mounted in relation to the rack.

This eliminates the all the issues associated with spring loading.

But it does require a machined rack mounting surface parallel to the linear rail mounting surface, and you still have the motor sticking out, potentially even further.

[davida1234]

Welcome to the thread, Gerry.

I had laid out similar sentiments earlier in thread:

Following are my personal thoughts and relates only
to the lower-cost equipment we generally use here:

Option #1:
If you put a servo on a CNCRP drive, you get additional
speed or, with lower gearing, torque. All happening at the
pulley side.

The final gear side, pinion with pressure spring on the rack,
stays the same. Some noise, wear and minute cogging.
Protruding servos. Weight of servos on carriage.

Option #2:
A direct rack drive with a gear box and pinion is expensive
for a good gear box. Can select appropriate speed/torque
combination. Requires perfect gear mesh or pressure. Some
noise, wear and minute cogging. Protruding servos.
Weight of servos on carriage.

Option #3:
With traditional ball nut and screw application, everything is
happening at the nut/screw side. With xx05 nuts, you get
more torque, with xx10 nuts, you get speed. Just two fixed
ratios. Less noise, wear and almost no cogging. Speed limit
due to whipping and/or larger screw inertia. Possible
backlash in coupling. Protruding servos.

Option #4:
With a powered ball nut, everything is happening at the
nut/screw side as well, not the pulley. Less noise, wear and
almost no cogging. With xx05 nuts, you get more torque, with
xx10 nuts, you get speed. Limited speed/torque combinations.
Less inertia than #3. No whipping but possible harmonics noise.
Weight of servos on carriage.

From all four options, #4 seemed the least explored to me. I
prefer ball nuts over rack drive, so I wanted to see what's possible
in that setup.
.

[Outlawtaz]

He already has adjustable feet on the base of the machine, and most do, not over kill it is need for any type of machine build, talking about Bridgeport mills I have done that also to dozens of Bridgeport machines, mainly just for leveling more than anything else

On his machine build he could easy adjust any of the feet if he as a twist problem, on these extrusion builds you have to adjust between twist and level

So if you make it level and it has a twist, then you adjust for the twist then it won't be level, so you have to figure what part is the most important

I acknowledged that he had adjustable feet, that's why I asked about twist. It is possible to level a machine and take the twist out of it. The only one that I ever had problems doing both was on Haas MiniMills as they are a boxed welded base. Its really hard to reduce twist in something that is made out of 1/4" steel plate welded into a roughly 3 foot cube...

[Outlawtaz]

0.00004".......

Well, Outlawtaz, yours capabilities are definitely way beyond my capabilities.

As you said, well outside the precision of a homebuilt, let alone some semi-commercial machines.

I do like your different perspectives, though. Keeps me aligned, so to say.

Precision test equipment for a precision part. I always liked the challenge that was posed in helping the customer make something dead nuts so to speak. Not to mention that customer did all kinds of R&D and experimental stuff that was really cool to see. The above mentioned part was a friction stir welding stylus the had a rounded triangle (like the rotor in a Mazda rotary engine) at the business end. On the faces of the triangle were the vanes that resembled those tiny threads. He actually deburred these "threads" with an exacto knife and a magnifying glass.

Now that I don't do that work anymore, I have entered the world of home built machines. I kinda keep tabs on what other people are doing, collecting ideas and trying to see how precise a machine can be made without spending tons of money. Just a personal mental exercise I am in the middle of that hopefully will turn into something tangible in the near future

[davida1234]

In case somebody is interested.....

The cart under the machine was built out of two sheets
of 3/4" Baltic Birch. Total height of cart is 29 1/2". The
cross braces are at the bottom of the 4x6 or 6x6
(preferable, just didn't have any at hand) corner posts.
Takes about a day or two to built. Total cost about $250
with wheels. If you want to save with smaller wheels,
just make the body taller.

It uses the Harbor Freight large wheels, $25 each.
Very sturdy wheels with turn/swivel lock; easy to roll.
Carries the 350 pound machine with ease but will
probably vibrate when cutting hard.

Here is the concept of it:

[mactec54]

I acknowledged that he had adjustable feet, that's why I asked about twist. It is possible to level a machine and take the twist out of it. The only one that I ever had problems doing both was on Haas MiniMills as they are a boxed welded base. Its really hard to reduce twist in something that is made out of 1/4" steel plate welded into a roughly 3 foot cube...

Even the Haas MiniMill which I have some of them you can take the twist out if you have that problem it's how you approach it that gets the job done, the machine it's self only mounts on 3 points to the base, so it normally does not have a problem with twist

[cyclestart]

Btw, the rotational mass of the power ball nut is just
4.4 ounces compared to several pounds of a ball screw:

Wow, sometimes it's hard to get a sense of scale in a photo.
To put that in perspective the AC bearings I bought for a 'someday' driven nut weigh 7.4 oz each. Most of that weight wouldn't be in rotation but it gives a sense of the overkill in my design.
In your design I see the 'light'. (There's a pun in there somewhere)

In a long span would it make sense to hold the screw in tension ?

[davida1234]

cyclestart, I am amazed at the interest I am getting for the power ball nut system.

The longest screw on my machine is 46" and there is no sagging.

I don't know how it would be with 8' lengths but I think that really should
not make a difference. The screw is guided by the balls in the ball
nut so, even if there is a bit of sagging on the free portion of the screw,
the positioning of the screw inside the ball nut would always be inline.

Besides, I am not sure if the tensioning forces we would be able to apply
here would help much for the little bit of sag. Just look at cable cranes
lifting many tons of loads. Their cables still sag a bit.

[ger21]

I've always read that you can still get "whipping" even when the screw is not spinning. Tension can help there.

If it works fine as is, then there's no need to tension it. But i you see the screws flexing under load, you may need to.

We have a 12' router with a spinning nut on a 40-50mm ballscrew on the long axis. The mount for the screw actually stretches it about .03".

[davida1234]

ger21, valid points.

So far, I did not observe any "whipping" but the screw is just 46" long.
Any possible "whipping" is probably induced by harmonics vibrations.

I guess your machine is one of those $200k + machines. All steel and HD.

There, you might have solid anchoring points. But on a 80/20 machine, how
much could you really tension a steel rod before the extrusion itself gets distorted?

[ger21]

There, you might have solid anchoring points. But on a 80/20 machine, how
much could you really tension a steel rod before the extrusion itself gets distorted?

.
I think even a small amount of tension might make a noticeable difference, if you needed it.

[davida1234]

I will keep that in mind and watch for any "whipping".

Thank you.

[Outlawtaz]

Even the Haas MiniMill which I have some of them you can take the twist out if you have that problem it's how you approach it that gets the job done, the machine it's self only mounts on 3 points to the base, so it normally does not have a problem with twist

I agree, just saying that its the only Haas machine that actually presents a problem with twist. Anything with a cast base can ALMOST be done without thinking. I'm not sure what Haas' reasoning was for a welded base on that model. The only reason I can think of is because the machine was marketed for high speed, small part production where small twists or leveling problems probably wouldn't cause tolerance problems.

Everything else I've ever touched out of Oxnard is cast, even the lower end Toolroom series machines and the original "plastic fantastics"...

[Outlawtaz]

David,

Any new vids from the machine? Curious if you have tried to cut any parts yet (secretly interested in finish quality )

[davida1234]

Sorry to tell you but, I am tied up right now with my real work. Got to pay those bills, right?

As soon as I find a free minute, I will get back reporting.

[Outlawtaz]

Sorry to tell you but, I am tied up right now with my real work. Got to pay those bills, right?

As soon as I find a free minute, I will get back reporting.

I understand... Worked all weekend and then had another 450 mile trip today. Just had time to wonder again...

[davida1234]

Make a list of all your intricate questions.....

[davida1234]

.

I got quite a bit of inquiries about the rotating
ball screw drive I am using on my machine.

If somebody is interested in reviewing,
comparing or trying out the ball screw
drive, I've set up a preliminary website
for you guys to review and evaluate.

Comments and suggestions, constructive
or not, are very welcome.

powerballnuts.com

Thank you for your help.

[Outlawtaz]

Make a list of all your intricate questions.....

No intricate questions... Have another 450 mile trip tomorrow. Gonna look over the website and maybe I will come up with something after that