Fully supported linear bearings - Radial squareness

Hi Guys, Just a very quick question. I am building a new machine and have done my research here and on other forums to build a good sturdy design for milling timber, ali and steel.

I am using SBR20 fully supported rails and they definitely have a very tight fit (zero play) in all directions except for the radial axis with the rod.

I have built the frame and everything is pretty much completely square as I tack welded the whole frame and adjusted accordingly. The only part that is not completely level is the top box steel that the linear rails will be mounted to. Both sides are completely planar but both sides also lean inwards slightly across the top where the rails will mount.

My question is: The bearing are extremely tight except for the radial side and they offer enough play for me to level the bearing mounts flat to the gantry. Is this acceptable as it is considering all other sides are completely square or would I need to level it using epoxy?

I recognize that in a perfect world It should be perfect but I cant see why it wouldn't work the way it is considering the mounts will allow me to twist around the linear bearing.

Photo:Attachment 18488

slixxor
Welcome to the forum.
As i see it your frame still requires some more welding, the base looks sound enough although we prefer some diagonal bracing, but the top rails are just hanging out there only supported @ either end, you really need a couple of stiffners to tie the top rail to the main structure.
I would personally go with epoxy leveling as you will get some more movement when you incorporate the bracing.
Regards
Mike

Thanks for the feedback. Yes I do need to support it more. I will add some gussets and some 45° angle beams to support the top rails and some on the lower section as well. Steel used is 65x65x3mm box.

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I am building a new machine and have done my research here and on other forums to build a good sturdy design for milling timber, ali and steel.

I am using SBR20 fully supported rails

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I wouldn't consider SBR20's to be anywhere near rigid enough for milling steel.

One step at a time. :)

This is my first machine so I will take my time with it. I'm happy for a job to take 5 hours to complete if it has to. The only time I will be milling steel is for small components. I will build it first, try my usual maple ply and then some 5060/6065 ali. If it goes well I will chance mild steel with some carbide bits. but it isn't critical.

I am not a machinist by trade, my profession is custom software and electronic design. I have worked on some GRBL controllers in the past. I decided to do this project to have a worthwhile machine in the lab and learn something on the way. I am writing my software front end (similar to Mach 3 but much nicer) that will work specifically with my machine bed size with a heavily modified version of Arduino GRBL that is running at 200kh on an STM32 ARM cortex board.

From looking around the forum I see a lot of skill gaps. Most users seem extremely proficient on machine design and structrual engineering but lack the electronics skills. I am quite the contrary. Finding myself reading google e-books on structural integrity and CNC/crane gantry designs.

I am more than happy to invest my skills in helping people out and love getting some good advice in return. I will weld up these gussets and some supports tonight.

Hi Slixxor
If you have your design in some kind of software post it, and the guys with more experience than myself will offer advice, really need to see the gantry arrangement.
Regards
Mike

Hi Slixxor,

I am in Melb Aus. If I can assist you in any way.

You might not of seen this thread http://www.mycncuk.com/threads/4356-...upported-rails
The square tube will rhomboid with side load ( assuming gantry uprights are the top of the frame )
Three trucks are better than two trucks ( better load spread )
Regarding tool load what spindle are you using ..kW RPM etc ,, have you seen "CNC Cookbook"

Finally, Hey Ger, Very nice to see you here.

Paul

Hi Paul. Yes I have seen the page. I am planning on using a 2.2kw water cooled spindle. Z axis range of 300mm. My rails for Z are 450mm long. I am adding 45° gusset/supports to both sides of each corner of the frame. I will be running dual ball screws on the X axis to minimize racking each with their own nema34 34HS59-5004S. Still havent decided if I'll be using the same size stepper on Z just yet or slightly smaller.

I haven't done a cad example of my design for reference. May do shortly pending work commitments.

Sure that those Nema 34s are a good fit for this size machine? Nema 23 would be more usual and this is a situation where over-building is definitely a bad thing. I was talking to someone last night who bought someone else's project router and with Nema 34 ("bigger must be better") he was getting no more than 1m/min when with the correct motors he should have been seeing 5m/min or more. Big motors come with much more inertia and high inductance which are both speed killers. Might be worth a bit of investigation?

The bed size is 1300x1000. I am happy to run whatever I need in order to be able to mill aluminium comfortably. I opted for the 34's due their torque at low rpm. Can anyone suggest a nema23 suitable for the application, they certainly are cheaper :)

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Originally Posted by Slixxor

The bed size is 1300x1000. I am happy to run whatever I need in order to be able to mill aluminium comfortably. I opted for the 34's due their torque at low rpm. Can anyone suggest a nema23 suitable for the application, they certainly are cheaper :)

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nema 34 is overkill and won't work as expected try these https://www.cnc4you.co.uk/Stepper-Mo...YGH301B-Nema23 more than capable for that size of machine

Thanks Clive S,

you've saved me $420 AU. Does anyone else agree with Nema23 23HS45-3504S being the best choice for a steel gantry and Z axis?
that will likely weigh over 30kg?

I'm using these in a slightly larger machine, steel gantry probably weighs around 50kg, and I'm not expecting any problems. It's a very common setup. You do need to drive them with a suitable voltage - 60V or more.

Agree with 23's .. ball screws = 10 mm is ok (5mm better axial load deflection needs looking at )
I assume you are in Aust, so these links might be of assistance. Both are in Melb.
http://www.homanndesigns.com/
http://stores.ebay.com.au/CNC-AND-CU...=p4634.c0.m322
CHECK THE SCREW LENGTH This guy normally lists travel length ..

Paul

Thanks @Paul3112, saved in favourites. :)

This is the kit I ordered but had modified for my lengths: http://www.ebay.com.au/itm/331320314...%3AMEBIDX%3AIT

This is the motor kit I wanted to buy: http://www.ebay.com.au/itm/171994192...84.m1436.l2649
This is the motor kit I am considering now: http://www.ebay.com.au/itm/171937212...84.m1436.l2649

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Originally Posted by Slixxor

This is the motor kit I am considering now: http://www.ebay.com.au/itm/171937212...84.m1436.l2649

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Kits are hardly ever a good idea as they are never matched. You need motors with very low inductance and drives that can handle 68V volts to get the best out of them. Go for a toroidal type power supply rather than a switch mode type.

Before you start spending hard earned cash it might be better to do some research as these parts can make all the difference between a good machine and a duff.

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Originally Posted by Clive S

Kits are hardly ever a good idea as they are never matched. You need motors with very low inductance and drives that can handle 68V volts to get the best out of them. Go for a toroidal type power supply rather than a switch mode type.

Before you start spending hard earned cash it might be better to do some research as these parts can make all the difference between a good machine and a duff.

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Thanks Clive, I dont have an issue with switchmode PSU's. As a worst case i'll make a quick back EMF circuit and put some filtering caps across the bridge.

You are dead right about the affect of inductance on supply voltage, especially at higher RPMs. So I have found a reasonably priced torquey Nema23 model

http://www.ebay.com.au/itm/3PCS-3Nm-...EAAOSw37tV85jU

inductance is 3.8mH - The lowest i've seen on ebay in that size.

What is everyones thought on this motor?

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Originally Posted by Slixxor

Thanks Clive, I dont have an issue with switchmode PSU's. As a worst case i'll make a quick back EMF circuit and put some filtering caps across the bridge.

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Not quite sure whether you mean you'll use a switch-mode supply or not. However, the linear supply is often better to drive steppers; the duty cycle of the load is relatively low, even though the peak demand can be high. With a SMPS, you need to rate it for max peak load to avoid it going into current-limiting where a linear supply can handle the peaks even if rated for average load. But then, you can buy cheap SMPS from China...

My 1000x700 build uses Mema 23 425oz 3amp at around £20 each, and they plenty of torque and speed, forgot to change DOC other day set to 3mm 6mm 2 flute for MDF (cuts MDF 6mm 2 flute at 15mm DOC Regularly), so ended up slot cutting 5083 aluminium at 3mm DOC 6mm 2 flute 1000mmpm FR, stepper torque was not a problem and it cut without incident but was working harder than it would normally.