Deflection + Backlash Testing Video


Hey @everyone,

I just finished putting together a “quick” video showing how the LongMill 30x30 performs under load. It’s 10pm on the Friday of a long weekend but hopefully it all makes sense. For regular use (just under 3kgs load) this ends up at around ±0.13mm (±0.005") in the x-direction and ±0.2mm (±0.008") in the y-direction.

In the x-axis, the movement comes from a small amount of looseness in the linear guides, the delrin v-wheels, and a small amount of backlash. In the y-axis, the majority of this movement arises from twisting in the long x-axis rail as well as a tiny contribution from the delrin wheels and backlash.

It’s hard to compare these results to other similar CNC routers since documentation like this is hard to find from other companies: if you do know of any then please contribute in the comments. Otherwise, I’d speculate that this testing is easily comparable to any other in-class machinery out there.

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LOL Comparable? I think your being way too kind my friend. This machine is tougher than 90 percent of them out there. Thanks for the work and it did not have to be done today you nut. Go get some rest my man.

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So good to see the updates here and everyone else joining the forums to see them. Excellent!
And very nice to see these details shared. Thank you!

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Chris is your 3kg “regular load” reference to the weight of a work piece of the weight of the completed machine with a 3/4" MDF spoil board?



The regular load I’ve indicated is the maximum cutting force the tool should experience when the machine is running a high demand job i.e. really high chip load. This includes surfacing at high feed rates, cutting deep slots, etc.

I suppose I could clarify that this isn’t the deflection you should normally expect, but instead closer to a maximum. I don’t currently have the means to verify this load estimate so instead I read through some papers, performed some rough calculations, and tested based off my high-end estimate so that everyone can draw their own conclusions. Keep in mind that normal loading during a v-carve, detail work, or otherwise wouldn’t get close to 3 kilograms of force. I hope that clears everything up for you.


I would doubt you would get close to a 1/4 kg force during a v carve project. Plus during a hard surfacing maybe a 2 kg but it would not deflect at all during that process.


Sounds like a reasonable approach. To make it easier to relate to day to day cutting, what would the load be of a 1/4" compression or downcut bit at 3/8" depth of cut in to baltic birch plywood or similar? Is there a formula somewhere I could use to work backwards if I had the chip load and feed rate to convert that in to force as you have done here?



I don’t think there is one. But I did a ton of baltic birch cutting in their shop while beta testing it. I don’t think it would have had more than 1 kg on a 1/4" down cut 2 flute bit. It would depend on the speed the bit is traveling through the plywood of course. I had it running about 900 mm a min travel speed and it had very little effect in the bit. Now I was only taking 1/8th inch cuts because we didn’t want to over heat the bit. It was a fair bit of money to buy for me at the time. So the deeper its cutting the higher heat build up you have in it. So safety concerns have to be taken into account. A lot of people have talked about taking large cuts out of aluminum or plywood but fact is that its just not safe at times. Or you have to slow down the travel speed quite a bit. You are limited by the size of the bit and what it will take to overheat the bit or snap it off. So safe practice has to be taken into account when working on a machine like this. Always no excuses.

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