Screw drives and linear rails and wood dust

[Jandyman]

Hi,

I've got an older Zenbot 2448 and it been good to me, but I've recently discovered some of it shortcomings. I'll work around them for now, but it got me thinking about building my own machine at some point. I've tossed a couple ideas around with a friend who is very CNC knowledgeable, and he has warned about about various components not working well in the dusty woodworking environment. I use a dust enclosure rather than trying to capture it as I'm cutting.

In particular, he's warned me about ball screws and linear rails, for the same reason. He says that dust gets into the ball bearings and then they bearings end up sliding vs rolling, and they are are really difficult to clean out. I would be very interested in other's opinions on the best cost effective solutions for rails and for drive systems in the presence of dust. My present machine is a belt drive machine, and there is a lot to like about it, especially speed and resilience in the presence of dust, even though belt stretch is the primary source for lack of rigidity in my current setup. I could see maybe going for another belt drive is the design is right and the belt hefty enough, but I'd really like to gather some opinions, and thought I might find them here.

- Andy

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[Jim Dawson]

It's been my experience that the rails and ball screws are not impacted too much by sawdust as long as the seals and wipers work correctly and they are kept properly lubed. At least this was the case with my 30 year old router, but towards the end of its life the sawdust had taken its toll.

Most modern routers are driven by ball screws and have rails, they seem to work fine. Many times the moving gantry machines are driven by a rack & pinion arrangement, with only the Z axis on a ball screw, these also work fine.

[Jandyman]

Could you link to any rails or screws with appropriate "seals and wipers"? A lot of the parts I'm seeing at the affordable end of the spectrum don't seem to have any seals and wipers at all, from what I can tell.

[Jim Dawson]

McMaster has them, others also. https://www.mcmaster.com/linear-motion-carriages

I think these are sealed but not positive about that. https://www.mcmaster.com/ball-screws

Look for Hiwin, THK, and Thomson products. I think SKF has them also.

It looks like VXB Bearing also sells sealed units

[ger21]

A lot of the parts I'm seeing at the affordable end of the spectrum don't seem to have any seals and wipers at all, from what I can tell.

All profile linear bearings have end and bottom seals that will keep out minor dust. Greasing at regular intervals will help to flush out any dust that does get inside.
If you plan on the rails being constantly covered in dust, then paying extra for double end seals may be a good choice.. Unfortunately, better seals are not cheap, but they probably aren't needed for most hobby machines.

Not sure about seals on cheap ballscrews, but ballscrews are usually located away from the dust chips, so it's less of a concern. Again, greasing them regularly will keep them clean inside.
I've been using big industrial routers for 25 years, where the bearings and screws see a LOT of dust and chips, and have never had any issues.

[teamloks1]

you cant customize your rail? i always buy my rail with double seal+bottom seal+scrapper

[Jandyman]

Thanks for all the help here. I'm digesting all the input.

[peteeng]

Hi Jandy - All good advice. I'm a belt fan too so if you look at belts go 25mm wide minimum. Cheers Peter

[Jandyman]

Hi Peter,

I'm very interested in the case for belts, pluses and minuses. On my current machine, the belts are 3/8" and it is not a "motor in the middle" design. Instead, the motor is on one side and an idler on the other side. This triples the distance between the motor and the carriage anchor point in one direction of force as compared to the motor in the middle design. And an upgrade is not easy with my machine. Hence my budding interest in building a new machine. And along those lines, a belt vs acme vs ball screw discussion is valuable to me. I know the downside of belts pretty well right at the moment! AFAIK, the screw drive issues I've heard of are dust, plus backlash, and from what I hear the screws can whip or "jump rope" when run fast. I'm here to learn.

[peteeng]

Hi Jandy - One starting point is what size machine do you want to build? This drives (sic) many of the decisions. Peter

[Jandyman]

I need a working area 20" by 48"

[peteeng]

Hi Jandy - 20" = 500mm and 48" = 1200mm these are small sizes. Chains, belts, leadscrews, ballscrews and R&P all work over these distances. next Q is accuracy and what do you want to cut? In rough cost order cheapest to dearest:

Chains are stiffer then belts and have lots of accessories. Downside is they need to be lubricated and kept clean. Not often used on routers, seen as cheap but we have been using chains in industrial applications for 100 years
Belts are readily available but idlers need to be made. No lub needed
leadscrews - readily available need to cope with backlash
ball screws - readily available, not too many $$$ for this size machine

R&P not sure where it fits in this size vs cost, but this size machine usually wouldn't use it.

What do you want to cut? Peter

[Jandyman]

I'll be cutting a lot of hard maple, building bass guitar necks and bodies. I would love to experiment with lightweight brass or aluminum cutting, for making parts for bridges or headless tuning assemblies, but that is not a requirement. Your comments roughly correspond with my understanding to date.

Accuracy is pretty important but I don't know how to quantify it - maybe my notions of accuracy is way off. Probably the best way is to visualize the back of a guitar neck. I'd like to minimize the amount of sanding I need to do after all the 3D cuts, and to fix any inaccuracy in registration between cuts on the back and front of the stock. Also, if I were to be lucky enough to be able to build bridge and headless tuner parts out of brass or aluminum, those would need some accuracy.

I tend to lean towards beefy belts of at least 15 mm and with "motor in the middle" design, a la Shapeoko or Ox (but not with their wimpier belts). But a lot of people here seem to like the ball screws with appropriate seals.

Why do you say "idlers need to be made" for belts? Nothing can be bought off the shelf?

https://www.amazon.com/3M30T-Smooth-.../dp/B07SNHWFHT

I don't know enough about the tradeoffs on rails yet.

[peteeng]

Hi Jandy - For timber and a belt I'd use 25mm as they fit on Nema23/24 motors. My router has 16mm belts XY and Z and for timber etc its fine. But you mention aluminium so I say go ball screws. So you should cost the 25mm belt vs the ballscrew.

https://www.aliexpress.com/item/3288...697d7bdelAe3tE

If your earning US$ then the ballscrew is the go. Peter

[Jandyman]

Those ball screws seem pretty affordable, and design of the router is easier too. Do I need to worry about dust seals with such an affordable ball screw? Do I need worry about the ball screw whipping or jump roping on the 48" dimension?

[peteeng]

Hi Jandy - all shafts whip at some point, whirl is the correct technical term if you look for a whirl calculator. Whirl depends on the speed you need. So next step is to figure out some speeds you need or find a similar size machine and use what they use. I don't think dust will be a problem for you... Be careful - saying a router is easy to design is one thing but the devil is in the detail so you need to flesh it out some more and deal with the detail.... Lots to do

There are so many machines running screws across their 1200mm gantry that it must NOT be a problem. Peter

re idlers - there are no off the shelf idlers that I can find for belts. I use a pulley and fit a bearing in it.... Your link is for a 15mm belt and I think you should use a 25mm if you go that way.. A 25mm high stiffness belt is about the same stiffness as a 4mm dia bar. You will be using a 12mm or 16mm screw which the 12mm screw is 9x stiffer & the 16mm is 16x stiffer so go there.

[Jandyman]

Thanks for all your help. I'll try to digest what I've learned here.

[Jandyman]

I'm not making much progress figuring the whirl thing out. It has to do with the resonant frequency, and to know that, I need to know the mass of the screw and probably also the stiffness, neither of which I know. I'll try to look for similar designs with similar screws, but if you know of any ....

[peteeng]

Hi Jandy - Here's a worked example of a whirl calc suitable for this condition. Change the numbers to suit. Typically you neglect the thread and use the minor diameter. But up front you can use the nominal diameter to sort your choice. I expect you will be using 12mm or 16mm or 20mm. Peter

[ger21]

I've never heard the term "whirl" used with regard to ballscrews. Is that an Australian thing?