DIY CNC Design. criticism wanted.

**joel0407** · 08-04-2017, 10:58 AM

Just a bit of an update guys,

I have my pre-build done. I don't know if I had explained it before but a very good fellow CNCzone member has cut the base parts so they are as accurate as they can be. I have hand cut the parts for the gantry up from MDF. They'll do for the first few cuts but they are far from being as accurate as I'd like. The first cuts of my machine will to be to recut the parts that I have previously hand cut.

Front to back, I'm more than happy with the rigidity of the gantry. Left to Right, not so much. It rapids much faster than I expected and just starting and stopping a rapid is enough for it to wobble.

**joel0407** · 08-04-2017, 01:15 PM

Maybe wobble isn't the right term. More just to jolt and I can see movement.

I'm also not super impressed with the linear bearings. a couple have very slight movement. Nothing I can see or measure but I can feel it when I put force on the bearing in various directions.

**rodw** · 08-04-2017, 05:05 PM

Joel, if you have a dial indicator, set it up against the gantry end and observe the movement on acceleration along the gantry

Here I am set up to measure homing repeatability but if I turned the indicator round so it was on the upright and jog the gantry axis (left to right in the pic), you will see the jerk on the dial when the machine moves.

What you might have to do is reduce the acceleration settings in your LinuxCNC .ini file for that axis. On my machine, I wanted to achieve +- 0.1mm accuracy. This test gave me +- 0.04mm so I was happy. My strategy was that if I did not achieve my desired accuracy, I was going to reduce acceleration and/or add additional bracing on the gantry end (tig welded on).

You can get a cheap dial indicator at Hare and Forbes but from experience, spending about $100 is a much better proposition (says he who upgraded)

https://www.machineryhouse.com.au/M046

**TumbiUmbiTek** · 08-04-2017, 06:07 PM

Originally Posted by joel0407

Maybe wobble isn't the right term. More just to jolt and I can see movement.

I'm also not super impressed with the linear bearings. a couple have very slight movement. Nothing I can see or measure but I can feel it when I put force on the bearing in various directions.

With your linear bearings..sbr1605's? Did you tighten them up?

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**joel0407** · 08-04-2017, 06:10 PM

Thanks mate. Yep. Have a decent dial guage. Have done some variable cam timing on the odd engine rebuild.

Reducing acceleration will stop the wobble but that doesn't mean it won't be moving while driving my router through a cut. I presume it would work for your Plasma as I guess the torch doesn't create drag.

I'm going to have to stiffen up the gantry ends. I've been ginibg it some thought and I have a plan.

**joel0407** · 08-04-2017, 06:11 PM

Originally Posted by TumbiUmbiTek

With your linear bearings..sbr1605's? Did you tighten them up?

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Yes mate. I didn't really crank them but tight.

**TumbiUmbiTek** · 08-04-2017, 06:38 PM

Originally Posted by joel0407

Yes mate. I didn't really crank them but tight.

When you use the word crank... Are we talking the same thing? The grub screw in center at top of bearing block, not meaning the four screws. Cheers

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**joel0407** · 08-04-2017, 06:46 PM

By crank I mean screw them really hard. I just used the long side of the allen key to what I thought was tight. If I replace the loose bearings, I'll be looking for some quality. They seem to be the only thing lacking in with all my chinese gear.

Happy Days

**joel0407** · 08-05-2017, 07:21 AM

I reckon it's still about 3 weeks off being finished yet but it's up and running

**rodw** · 08-05-2017, 07:35 AM

Joel, Good to see it cutting. I wondered when you were talking about jerky movement how many linear guides a side you had.

From what I could see you only had 1. From what I know (and I have never used the guide rods), you should use 2 rails one above the other to control flex. That would prevent the gantry end from rotating around the rail. That is one of the reasons why I chose to use Hiwin linear rails and carriages as you can get away with one rail per side so the cost differential is not that much.

In fact, I sourced my HGR15 rails for the gantry from an Aussie store on eBay. Later on, I bought HGR25 rails from aliExpress for about the same price as the HGR15's. If I build another machine, I'll just go straight to HGR25 rails.