CNC Milling Machine Resolution Question.

[Hackman]

Whoa! That's not right. I think I did the math wrong. That should be around 12 seconds for full table traverse. I've got a 12" traverse @ 60 IPM which should come out to 12 seconds and not 5 seconds. Sorry about that.

To give you more context, I'm running a little X1 mill with 20 TPI leadscrews, so @ 60 IPM that gives me 1200 RPM. Apples to Oranges since you are using different stepper motors, drivers, and your mill is probably about 10x as big as mine, but just for fun, 1200 RPM @ 10TPI would give you 120 IPM, 1200 RPM @ 5TPI would give you 240 IPM.

[Oldmanandhistoy]

Originally Posted by Hackman Whoa! That's not right. I think I did the math wrong. That should be around 12 seconds for full table traverse. I've got a 12" traverse @ 60 IPM which should come out to 12 seconds and not 5 seconds. Sorry about that. To give you more context, I'm running a little X1 mill with 20 TPI leadscrews, so @ 60 IPM that gives me 1200 RPM. Apples to Oranges since you are using different stepper motors, drivers, and your mill is probably about 10x as big as mine, but just for fun, 1200 RPM @ 10TPI would give you 120 IPM, 1200 RPM @ 5TPI would give you 240 IPM.

I’ve been doing a search for “rapids” on this forum (should be in bed really but am on holiday so noprob) and I was seeing 60ipm rapids as a good average. So your 60ipm sounds good for your size of machine. I have been getting up to 82”/min on the stock lead screw so 0.1” pitch sounds like a good size for my X. With my motors and a 0.1” pitch screw I might be able to reach 100”/min rapids (would need to do the maths) which sound good to me. I get good rapids for a hobby machine and higher resolution to boot.

Will go away now and do more research on good cutting speeds and do the maths.

Got any recommendations for max cutting speeds for a hobby machine?

Thanks for your help and no problem with the error it made me get off my ass and find the figure.

John

[S_J_H]

John,
I use the .2 pitch 5/8" diameter rolled ballscrews that many people use. Main reason is simply the price. They are cheap, any other lead or diameter costs more. Add a second nut and preload them and you get pretty good performance at a very good price. Much cheaper to gear them up or down if needed.

I run 1/8 steps. Direct drive is on my X axis and belt driven 2:1 reduction on the yaxis.
So resolution is .000125" for x and .0000625" for Y.

Now I have never tested table movement at such a small increment, and my machine will not come remotely close to holding such tolerances!
But I can get reliable .0002" table movement. I use .0002" movements all the time manually. That's easy to verify. This small movement does come in handy when indicating in bores or edge finding.
If I thought for one second I could somehow get better performance from 1/2 or full steps I would switch. But I have never observed any issues with torque. I can command .0002" movement and get it.

I honestly see no difference in gt2 timing belt drive vs direct drive.
So I would go with whatever is easier to mount. I like belt drives as sometimes you can be much more compact with your motor mounting.

The most important thing I find to keeping backlash low and movements free and accurate, is clean and well lubed ways and well adjusted gibs. I would say I keep the gibs a tad looser than I would for a manual machine. This is necessary for free movement at very small increments.

I like to keep my rapids at 60ipm. I just find it's a good speed for manual jogging and I have never seen it lose a step at that speed. I can go well over 100ipm but see no need to risk lost steps.
I rarely feed at over 30ipm in aluminum. Much lower in steels.
Good luck!
Steve

[Oldmanandhistoy]

Originally Posted by S_J_H John, I use the .2 pitch 5/8" diameter rolled ballscrews that many people use. Main reason is simply the price. They are cheap, any other lead or diameter costs more. Add a second nut and preload them and you get pretty good performance at a very good price. Much cheaper to gear them up or down if needed. I run 1/8 steps. Direct drive is on my X axis and belt driven 2:1 reduction on the yaxis. So resolution is .000125" for x and .0000625" for Y. Now I have never tested table movement at such a small increment, and my machine will not come remotely close to holding such tolerances! But I can get reliable .0002" table movement. I use .0002" movements all the time manually. That's easy to verify. This small movement does come in handy when indicating in bores or edge finding. If I thought for one second I could somehow get better performance from 1/2 or full steps I would switch. But I have never observed any issues with torque. I can command .0002" movement and get it. I honestly see no difference in gt2 timing belt drive vs direct drive. So I would go with whatever is easier to mount. I like belt drives as sometimes you can be much more compact with your motor mounting. The most important thing I find to keeping backlash low and movements free and accurate, is clean and well lubed ways and well adjusted gibs. I would say I keep the gibs a tad looser than I would for a manual machine. This is necessary for free movement at very small increments. I like to keep my rapids at 60ipm. I just find it's a good speed for manual jogging and I have never seen it lose a step at that speed. I can go well over 100ipm but see no need to risk lost steps. I rarely feed at over 30ipm in aluminum. Much lower in steels. Good luck! Steve

As always a wealth of information imo you are an asset to the zone.

Point taken about the cost over there in the US; I only wish we had sources similar (low priced) to what you have over there.

A question or two to you if I may. If you don’t have time or the energy to respond that is fine with me, I am grateful for the info you previously provided.

If you could buy 0.1 pitch screws at the same price as the 0.2” and you had no restrictions of stepper motor and drive ratings what would you have used?

With that question in mind:

Reading your reply you say you are using 2:1 belt reduction on your Y (I will be using a belt drive system similar to yours for the same reason you are) so effectively a 0.1” pitch with 1:1 gear ratio. I think I know the answer but just to be sure can you tell me why you are using that gear ratio?

Would you agree using 0.1” pitch on a Z axis would be a good way to go?

Would you also agree using 0.1” pitch on the X would be a good idea IF your stepper set up would give you over 39”/min cutting speeds and up to 83”/min rapids reliably?

You say you use your machine in manual mode also; do you use hand wheels or do you effectively jog in software?

I have been hoping to still be able to use my mill in manual mode (using hand wheels) so I was thinking the 0.1” pitch screws would better suit this aim. Would you mind giving your opinion on this?


Are you saying that there is no point/advantage using a higher resolution on a hobby machine period?

If you got this far and answered the questions, thanks a million

John

[LongRat]

It sounds to me like you have good rapids and high linear forces with the 0.1" pitch screws. You might as well go with them in that case. However I am in the UK myself and got all my stuff from Homeshopcnc in the US. With the strength of the UK£, it is CHEAP - even with shipping and import taxes. I paid 62p per inch for my screws from them! That is down the back of the sofa prices.

[Oldmanandhistoy]

Originally Posted by LongRat It sounds to me like you have good rapids and high linear forces with the 0.1" pitch screws. You might as well go with them in that case. However I am in the UK myself and got all my stuff from Homeshopcnc in the US. With the strength of the UK£, it is CHEAP - even with shipping and import taxes. I paid 62p per inch for my screws from them! That is down the back of the sofa prices.

Hi again LongRat,

Did you buy 1 preloaded ball nut/axis or two standard nuts/axis?

I think the way things are looking unless some one convinces me other wise (still time yet) I will be going with the 0.1” pitch screws. I have never imported anything but from what I am hearing I might very soon. My dilemma is this, if the 2.5mm screws I can get over here are the best choice and as this is a one off pay out then I maybe better off long term buying them. But if there is not a good enough reason/advantage to them then the Homeshopcnc screws will very likely be the ones I would buy.

What is going on with UK prices when you can import cheaper from the US? Either I am missing something or some one is adding a very large mark up.

The price for 1” aprox of screw over here is £1.28 +P&P and a preloaded ball nut will set me back £61.69 +P&P. Maybe Mr Marchantdice would like to jump in and out of this thread also with a quick sales pitch?

John

[S_J_H]

John,

If you could buy 0.1 pitch screws at the same price as the 0.2” and you had no restrictions of stepper motor and drive ratings what would you have used?

Hmmm,... If my motors could reliably rapid at 80IPM with a .1" lead and price was not a consideration I would say yes, I would have gone with that pitch. I have not seen any negatives from the .2 pitch though.
You may find your cutting limitations will be spindle HP and rpm and overall rigidity and not the ballscrew pitch. That is the case on my x3.

Reading your reply you say you are using 2:1 belt reduction on your Y (I will be using a belt drive system similar to yours for the same reason you are) so effectively a 0.1” pitch with 1:1 gear ratio. I think I know the answer but just to be sure can you tell me why you are using that gear ratio?

Well... when I did my x3 conversion I decided to space out the bearing block and drill the Z column so I could get near 8" of Y travel. But things get sloppy way out at the extreme Y axis travel because of the design of the Y axis dovetails. So the gibs need to be tight. I "thought" a reduction in lead pitch might help provide more torque to the Y axis , and since It was a simple matter of selecting timing pulleys I did just that.
Honestly, I can not say the 2:1 reduction has any real benefit though.

Would you agree using 0.1” pitch on a Z axis would be a good way to go?

I do use .1" pitch on the z-axis plus 2:1 belt driven reduction. I use the factory x3 acme .1" pitch screw on the z-axis. No back drive possible for superior holding power. The only negative is slow rapids because of the inefficient acme thread. But it has good plunging power.

I have been hoping to still be able to use my mill in manual mode (using hand wheels) so I was thinking the 0.1” pitch screws would better suit this aim. Would you mind giving your opinion on this?

Manual mode, I jog and do all manual work with cnc. No hand wheels on my mill.
I probably use the mill in a "cnc manual mode" 90% of the time vs running G-code parts programs.
IMHO forget keeping hand wheels, You'll never need them!! Yes, it does take a little getting used to running the mill with a keyboard, jog dial, MDI, or a gamepad "manually". But you can do things way faster than cranking hand wheels and with power feed on all 3 axis.

are you saying that there is no point/advantage using a higher resolution on a hobby machine period?

No I am not. I think having resolution of at least .0002" is good to have. I am saying not to expect the mill to produce parts with a .0002" tolerance.
All of the above just my $.0002
And, wow, nice job on the scraping of the ways on your mill!
Steve

[Oldmanandhistoy]

Originally Posted by S_J_H All of the above just my $.0002 And, wow, nice job on the scraping of the ways on your mill! Steve

Might be just $0.0002 to you but it means a great deal more to me thanks again. I will go and take another look at your X3 conversion thread and see what motor/drive combo you are using .I do believe it was a Xylotex and 420+ oz in motors. Also from what you have said about your Z axis, I will go see if I can use my X screw for the Z its 1.5mm (0.059”) so a direct drive there might be ideal. The scraping is time consuming but definitely IMHO worth all the effort thanks.

I have enough info now to make an informed decision so thanks to all posters for your help.

John

[LongRat]

I am using double ballnuts preloaded together on my X and Z axes. On my Y, I am using a single 'preloaded' nut filled with oversized balls. I would prefer to use double nuts all round, but there is not enough space to do this on my X2 for the Y axis. On your larger mill, I would go for double nuts on every axis for sure.

[Oldmanandhistoy]

Originally Posted by LongRat I am using double ballnuts preloaded together on my X and Z axes. On my Y, I am using a single 'preloaded' nut filled with oversized balls. I would prefer to use double nuts all round, but there is not enough space to do this on my X2 for the Y axis. On your larger mill, I would go for double nuts on every axis for sure.

Thanks LongRat (strange name ) that is what I thought you would have said.

John