[Momentz]

HI
Basic question.

Why dont i see many people using these supported linear shafts on there CNC machines. is it just pure and simply because they can be expensive or are they no good?

Thanks

[HorridHenry]

Why dont i see many people using these supported linear shafts

Better products came on the market,I suppose its about what you can afford,on saying that I find all types far too expensive for what they are.

[Momentz]

Better products like what?

[HorridHenry]

HWIN guide rails

[louieatienza]

There are a lot of builds in the DIY sections that use supported linear shafts! Plain shaft is cheaper and easier to implement, though it might not be the best choice if cutting forces are high.

Also, the higher priced linear rail systems actually need more care and precison in assembly, as any misalignment can cause binding and premature wear, and you wouldn't get the benefit that the higher precision rails would otherwise have.

[ger21]

Pros:
Better than unsupported round shafts, and cheaper than profile rails (THK, Hiwin)

Cons:
Load capacity is much lower than profile rails.
Work best with shaft on top. Other orientations aren't as strong.

[Momentz]

Thanks for all the replies, so what i gather thou is that any true linear guide or shaft is better than a few skate bearings on steel. so if i got this right.


Skate bearings on steel = OK/Average (like cncrouterparts.com torchmate for example)
Supported linear shaft = Good
Linear Guide Rail = Best

So surely the supported shafts and bearing support more weight than the skate bearings and provide better accuracy
Thanks

[HorridHenry]

Skate bearings

I wouldn't even give these the time of day when building my CNC and anyone seriously thinking of trying to get some precision out of CNC would not neither.

I've opted for supported linear shaft but not the commercial ones ie my shaft will be 25mm hardened ground with two drilled and tapped holes for two bolts and I will use 25mm open bearings.

[ger21]

I wouldn't even give these the time of day when building my CNC and anyone seriously thinking of trying to get some precision out of CNC would not neither.

There are thousands of skate bearing machines doing very precise work. Mine uses skate bearings on EMT conduit, and it's very accurate, although not the most rigid.

And the CNC RouterParts skate bearing carriages have proven to be a viable option. I wouldn't be surprised if they can handle higher loads than open bearings.

In the end, it all comes down to budget. If you look at Glacern's pricing, it would appear that their supported rails are not much more expensive than CNCRouterParts carriages and the CRS rails.

And if you look at the surplus Hiwins from automation4less, you'll see that you can get Hiwins for about $150-$250 per axis more than the Glacerns. I paid about $425 for a set of 1700mm rails. Glacern is $240 for 1800mm. But the Hiwins are much stronger.
If you're going to be spending $4000 or more on a machine, it might be worth spending an additional $500-$700 on better componenents.

[LeeWay]

I wouldn't even give these the time of day when building my CNC and anyone seriously thinking of trying to get some precision out of CNC would not neither.

I've opted for supported linear shaft but not the commercial ones ie my shaft will be 25mm hardened ground with two drilled and tapped holes for two bolts and I will use 25mm open bearings.

I have been running my skate bearing router since about 04 and it is pretty accurate. It also has about 150 pound gantry. I took it apart about a year ago with intention of replacing the bearings. They were still in good shape though. This is a semi production machine that runs about 1.5 days a week. Strong a heavy duty for it's size.

I would not consider using skate bearings on a mill, but they are well suited for a router if it's designed well enough.

[louieatienza]

I wouldn't even give these the time of day when building my CNC and anyone seriously thinking of trying to get some precision out of CNC would not neither.

I've opted for supported linear shaft but not the commercial ones ie my shaft will be 25mm hardened ground with two drilled and tapped holes for two bolts and I will use 25mm open bearings.

Running ABEC-7 bearings preloaded on ground tool steel plate can be just as accurate as linear rail/bearing. I probably wouldn't use them on a metal cutting mill, but for most woodworking, laser, plasma, and other applications, they're more than accurate enough.

There are many DIY machines that use skate bearings and get good accuracy, well within the needs of hobbyists or even small-scale production work.

Heck if you want precision than even hardened ground rod and open blocks might not give that to you, since the open blocks do not have the same load capacities in every direction. So you have to figure your max load to actually be in the weakest direction (pulling away from the rail) and I read that's about 30% of it's strongest state (mounted flat)

[HorridHenry]

So are they any good for when it comes to 'Glove Fit' cutting dovetail joints? in wood.?

[LeeWay]

I would say absolutely. That is with a good frame design. I probably get .003" accuracy and that is mainly because I use a cheaper double nut rolled ball screw.

[ger21]

So are they any good for when it comes to 'Glove Fit' cutting dovetail joints? in wood.?

There are a lot of factors that can come into play here, and you can't really make a blanket statement that "skate bearings are bad".

There are two important factors in getting good performance out of them.
1) Whatever they are riding on must be straight.
2) They need to be preloaded enough to prevent any play in the system.

If the bearings are tight, and they're rolling on a straight guide, then, yes, they can be very accurate, as Lee said.

I would say that the precision of your screws and the rigidity of the entire machine would have more effect on the fit of your dovetails than skate bearings.