[crl1_1]

I have been working religiously on converting a cheap mini mill on ebay, first time doing this some input would be nice. I am doing belt driven and I am wodering what type of backlash to expect.

[cyclestart]

I am doing belt driven and I am wodering what type of backlash to expect

Backlash is usually in the (ball) nut or in the (ball) screw mount bearings. Belt driving the screw won't add backlash.
Hope I understood your question correctly.

I think you need a certain number of posts before being allowed to insert an image.

[crl1_1]

Backlash is usually in the (ball) nut or in the (ball) screw mount bearings. Belt driving the screw won't add backlash.
Hope I understood your question correctly.

I think you need a certain number of posts before being allowed to insert an image.

Yes i was concerned that the belt would introduce backslash. You cant see my pictures? Do you know how many post ?

[cyclestart]

You cant see my pictures?

No, nothing viewable or clickable. If you can see them it might be my browser settings

Do you know how many post ?

I'm not sure, and the rules when I joined 13 years ago may no longer apply.

[hfjbuis]

I have been working religiously on converting a cheap mini mill on eBay, first time doing this some input would be nice. I am doing belt driven and I am wondering what type of backlash to expect.

If the pulleys are not to wide cut, the belt drive shouldn't introduce backlash. But if the belt is not rigid enough, tension on the belt could introduce positioning errors. I agree to the post before,

Backlash is usually in the (ball) nut or in the (ball) screw mount bearings

[crl1_1]

Are these viewable

[crl1_1]

If the pulleys are not to wide cut, the belt drive shouldn't introduce backlash. But if the belt is not rigid enough, tension on the belt could introduce positioning errors. I agree to the post before,

I am actually using 16mm wide belts, I am concerned about tension and step loss. I think i will use closed loop motors with encoders

- - - Updated - - -

Can you verify if you can view the pictures i just uploaded

[iwaskevin]

I can see them

[crl1_1]

I can see them

Thanks

[hfjbuis]

I am actually using 16mm wide belts, I am concerned about tension and step loss. I think i will use closed loop motors with encoders

The encoder on the motor can only adjust for losing steps! It doesn't see how far the belt is stretched. To correct for this, you have to put the encoder on the spindle!
You can't drive your mill any faster than you would do by hand. You can spend a lot of money and time to make the spindle drive 0.001 mm accurate, but in the end, accuracy is determined by the sum off all inaccuracies (end mill, collet, collet holder, spindle run out, Z-axis play, Z-axis deflection, vice accuracy, etc, etc). Your mini mill (and mine) is "accurate" as long as you keep the stress low.
If you lose steps, you are working beyond the limits of the machine. Even if it is corrected at some point in the movement, the part you made is "out of spec". So prevent losing steps by operating in machine limits, not correct them afterwards.

[cyclestart]

If those parts are your handiwork operating machine tools isn't something new to you.
Sites like Misumi have good information on belt profiles and materials. Don't skimp on the pulleys, there's some real junk available.

[dazp1976]

To be fair I'd never touched this kind of stuff until I got my X2 with 350w motor almost 2 years ago.
I didn't do the belt system on my Sieg X2. I made some bearing blocks etc to use the steppers by direct drive. 420oz Nema23.
I cut the base aperture a little further to go from 130mm stock to now 160mm.

I have a belt drive fitted and it just stays on the low (0-1700rpm). If I'm honest I reckon it handles better with the gears (especially with drilling) so I'd prob invest in a metal gear upgrade instead and put up with the noise. (if yours is similar and has the options).
As for backlash the standard acme screws & nuts were awful and there was loads of it. I've been messing with my machine for ages.

So I replaced the acme screws with 1204 chinese ballscrews from ebay. They worked well and was relatively accurate enough, normally say if I was cutting out a part, if I set the cam 'stock to leave' as -0.1 it came out the size of the drawing.
I'm now replacing the screws with 1605 and using thrust bearings. I don't need perfect accuracy but I know the backlash on my X axis is now down to 0.03mm

However, I do have some issues because I've put it all in too tight on the thrust bearings so it's ended up quite stiff and needs taking apart (again). At the moment It rapids at 1300mm/min & cuts at 800mm/min using a 6mm end mill at 1350rpm. The 350w motor lets it down and I can only cut at 1mm DOC without breaking a bit. 8mm bits are at 650mm/min at the same depth.

I'm a total novice but learning a fair bit on the way. Saving up for a new machine that's double the size to play with next.
I also scratch built a 2 nozzle coolant system using a 350w submersible pump and it's fierce. It's the one thing I'm impressed with
Blasts everything away, has a 30L bucket, 2 stage filter boxes. It was that pressured I had to make a diversion valve unit to divert most of the fluid directly back to the bucket and set the nozzles on half. All in cost me around £230gbp to build.

I want to rework the head design next to come down the back instead of down the side. I have a little bit too much backlash in the head but that's due to my design. The 1/2 inch plate I used bends as the screw takes the head up/down. I could change to 1 inch plate but I think I'd prefer to put some plates onto the sides of the head going to the rear and bolt them to a block at the back with the ballscrew.
Also want to put some 15kg gas struts either side so I can get rid of the 4 house bricks & pulley system I'm using now as a counterweight

It's all a learning curve but fun to play with.

[crl1_1]

If those parts are your handiwork operating machine tools isn't something new to you.
Sites like Misumi have good information on belt profiles and materials. Don't skimp on the pulleys, there's some real junk available.

Thanks, I have spent plenty of time dedicated to looking for the most adequate and effective way to retrofit this mini mill. I would appreciate some feedback on what pulleys if you know any that would be more suitable than the chinese 3m pulleys

[crl1_1]

Thanks for sharing, I am curious as to how you fitted the 1605 ball crews I am using 1204 because i could not fit the ballnut in the x or y axis without having to shave the nut to the point where is was going to damage it. I am also concerned that my 1204 maybe too light for the z axis and i have seen the counter weight idea but i am using the spring that is built in the machine to counter it.

[crl1_1]

.....
I want to rework the head design next to come down the back instead of down the side. I have a little bit too much backlash in the head but that's due to my design. The 1/2 inch plate I used bends as the screw takes the head up/down. I could change to 1 inch plate but I think I'd prefer to put some plates onto the sides of the head going to the rear and bolt them to a block at the back with the ballscrew.
Also want to put some 15kg gas struts either side so I can get rid of the 4 house bricks & pulley system I'm using now as a counterweight

It's all a learning curve but fun to play with.

sorry I meant to quote you on the previous message

[hfjbuis]

Direct drive, belt or gears!

I made aluminum gears (on the mill) for my cnc mini lathe. They worked great but made more noise than the plastic (POM) original gears. So i changed them back to the original POM gears. After 2 years I still haven't broken a gear nor wear out one of them, even not at the Nema24 4Nm spindle stepper. The original gears are Modul 1 gears, available as spare parts.
I cut the gears on the mill using a rotary table. I made the gear cutter (fly cutter) on the mill using the "button method". In stead of making that precision button I used an end mill. Using a 4 mm end mill, you can make a gear cutter for 12 teeth, a 5 mm for a 15 tooth gear. It is not that hard to do. If you have a CNC lathe, you can make the gear cutter to, works great.

Backlash on gears!
Gears are cut having a bit of play. International standards describe to cut a Modul 1 gear, 0.16 mm deeper than the theoretical depth. This results in a depth of cut of 2.16 mm. On a 20 ° preasure angle gear, this gives a play of 0.23 mm. Using a 65 Tooth gear would result in a 0.21 ° spindle position error. On a 1605 ball screw the error would be 0.003 mm!

Backlash on belt drives!
An alternative for gears is a belt drive. A belt drive introduces backlash by the flex of belt. On a 40 tooth XL pulley, having a pitch diameter of 64.68 mm, 1 mm flex would result in 0.886 ° spindle positioning error. on a 1605 ball screw the error would be 0.012 mm!

Backlash on direct drive!
Yes on direct drive you could get backlash. These flexible connections are just springs. I don't know how large the back lash would be, that depends on the size of the coupler and the torque it has to deal with. It is just something to keep in mind.

What is the best?
On the lathe I use gears because I had them (original change gears). On the mill i will go for belt drives because i haven't done these before and I want the steppers out of the way because i will use the mill also manual. The belt drives let me put the steppers away from the handles.

[dazp1976]

Backlash on gears!
Gears are cut having a bit of play. International standards describe to cut a Modul 1 gear, 0.16 mm deeper than the theoretical depth. This results in a depth of cut of 2.16 mm. On a 20 ° preasure angle gear, this gives a play of 0.116 mm. Using a 65 Tooth gear would result in a 0.10 ° spindle position error. On a 1605 ball screw the error would be 0.001 mm!

Backlash on belt drives!
An alternative for gears is a belt drive. A belt drive introduces backlash by the flex of belt. On a 40 tooth XL pulley, having a pitch diameter of 64.68 mm, 1 mm flex would result in 0.886 ° spindle positioning error. on a 1605 ball screw the error would be 0.012 mm!

Backlash on direct drive!
Yes on direct drive you could get backlash. These flexible connections are just springs. I don't know how large the back lash would be, that depends on the size of the coupler and the torque it has to deal with. It is just something to keep in mind.

What is the best?
On the lathe I use gears because I had them (original change gears). On the mill i will go for belt drives because i haven't done these before and I want the steppers out of the way because i will use the mill also manual. The belt drives let me put the steppers away from the handles.[/QUOTE]
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For my new one I'm going with dual shaft 420oz nema23 steppers on the X&Y direct driven so I can put manual handles on the back of them.
On the Z a 1600oz nema 34 single shaft. Not as critical needing manual there but may belt this one to give the option.
Building the electronics before I get the machine.
I have some plum couplers to try this time but they seem loose, might have a play with some shim material and try to tighten them up.

[hfjbuis]

I have some plum couplers to try this time but they seem loose, might have a play with some shim material and try to tighten them up

These plum couples can transfer a lot of torque. They need "a bit of play" to compensate for axis misalignment. When they are to tight, they could get hot during use. I would not do anything about it until you are done and can measure the result (less backlash) of the changes made.

I will do X & Y using Nema23 2 Nm (283 oz) steppers. Z will be done using a Nema24 4Nm (566 oz) stepper (these are lying around). I probably keep the original spindles and will do backlash compensation in software. That way the primary conversion can be done cheap without a lot of modifications to the mill. For the controller i will use GRBL and TB6600 for the drivers (same as lathe).

[cyclestart]

I would appreciate some feedback on what pulleys if you know any that would be more suitable than the chinese 3m pulleys

I've purchased pulleys from Chinese Ebay sellers and most suit my needs.
The ones purchased from Stock Drive Products (SPDI) have properly placed and sized bores and I can't say the same for some of the Ebay purchases.

[dazp1976]

These plum couples can transfer a lot of torque. They need "a bit of play" to compensate for axis misalignment. When they are to tight, they could get hot during use. I would not do anything about it until you are done and can measure the result (less backlash) of the changes made.

I will do X & Y using Nema23 2 Nm (283 oz) steppers. Z will be done using a Nema24 4Nm (566 oz) stepper (these are lying around). I probably keep the original spindles and will do backlash compensation in software. That way the primary conversion can be done cheap without a lot of modifications to the mill. For the controller i will use GRBL and TB6600 for the drivers (same as lathe).

True. Prob give as much/little lash as the spring type.
Only thing is (prob just me) I can't get a perfect circle in Mach3. Even with the backlash compensation on it comes out like a slight oval.
Any pointers on that one?
Everything else comes out pretty spot on.