Mr.Chips wrote:And yes my Z screw is a one start, my thinking was the stepper could transmit more torque than a five start. Using a 2 start acme on the X axis and a 5 pitch ball screw on the y axis.
You all have got me thinking about rack and pinion, looking around at different builds and how they handle backlash.
TR do you have pictures posted on your machine?
Put my Z stepper back on made some hold downs out of maple and V cut a couple of samples for the women’s lunch, no steps lost, funding fine, no hard to cut Chinese characters, just old fashioned English. But ran at my super conservative settings IE 40 IPM. And your recommended V&A settings.
Yes there off center this is just showing what the font and material look like.
You are correct that your one start screw will transmit more torque. However that torque comes at the price of increased rpm required to achieve a given lineal feed rate. That motor has to really fly to achieve higher feed rates. When you are plunging you really do not need a whole lot of torque because gravity is on your side.
OK, looks like you have it basically running. Now that you understand the gcode controls the feed rate you can start tuning the machine. That is why Mach3 calls it "motor tuning". You do not need to cut stuff while tuning until you feel you are close to the optimum settings. The place to start is with X/Y axis. First set the velocity much higher, on a screw machine like yours I would set maybe 250. Create a program that does stuff like profiling with the feed rate set to increasingly higher values until you either fail and start to lose steps or you reach the maximum velocity. If you reach maximum velocity then it is time to increase the accel and rerun. I would increase the accel by about 5 each time and when it fails back it off by about 2 until it runs correctly.
This was all done with air cutting so there was no side load on the spindle. Run your test cuts in wood. If the machine fails back off the accel until it does not. Eventually you will reach a point where the feed rate vs the accel is acceptable to you. There is no way for anyone to tell you what those settings will be because we do not know your machine.
Important: There is a feed rate that no matter what you have your accel set to the machine will fail because the motor does not have the power to run it at that feed rate. You would not want to run at that level anyway because the accel would cause your parts to look weird.
My machine is essentially an old style CNC Router Parts machine. I use the trucks from them and the motor side plate that spring loads the pinion into the rack. That is how backlash is handled, essentially there is none.
I am not crazy about linear bearings on these type of machines mostly because of the dust issues. On the industrial machines I designed we used expensive way covers to protect them but that is not practical for most of us home hobby type guys. I like the trucks that CNC Router Parts provides. Simple, effective and adjustable. They run on flat cold rolled steel.
Here is a link to a YouTube video I posted a few years back that shows the machine cutting at 800 IPM:https://www.youtube.com/watch?v=Pt8n43_YRjI
It is a little blurry but you can pretty much see what is going one and how the machine is constructed.
Ok, I am going to say something a bit snarky. I take what I read over on the CNC Zone with a grain of salt. There are some folks over there who know what they are talking about, but there are also a whole bunch of wannabe engineers who enjoy postulating about theoretical issues and others who rely on what is "common knowledge". A really good way to see how to build a machine is to look at a manufacturer or supplier who has been successfully building machines and does not have all sorts of complaints about the machine they provide. In other words, I did not engineer this machine. I took what looked like a good design that worked and then figured out how to build it and save some money at the same time. But I used the key pieces they had already designed and proven to work.
Is my machine perfect? Far from it. But the basic design and construction are very good. As time goes by I make incremental improvements to it. My first machine was a pretty typical wood construction el cheapo machine. I was not sure how much I would actually use the machine in my day to day projects so did not want to put a bunch of money into something that would sit in the corner most of the time doing nothing. Well, it proved so useful building lots of other things like jigs, fixtures, tools, mortising, tenoning, etc that I decided to build a much better one.