Starting MechMate build planning - a broad question on performance specs.

[dneisler]

This thread started by ART RANSOM:

I am Art Ransom located near Dallas, TX, USA. After retirement as a network engineer I started my business TurningARound doing custom and architectural woodturning. Business has grown where the Legacy Ornamental Mill was not a satisfactory solution for my needs so I began looking for an alternative. Commercial machines began at 100K and not feasible for a one-man shop with NO intention of growing. This led to building my own 4-axis mill running Mach3. Unfortunately I didn’t discover MechMate until I was 95% complete with my DIY machine. Progress on my mill can be seen at http://turningaround.org/4_axis_mill.htm (http://turningaround.org/4_axis_mill.htm). Comments welcome. Plans are to finish my mill and get it into production then after a few months start on a MechMate. As a side issue the local high school is talking to me about becoming a part time instructor in building a CNC course, which is another barrel of snakes on which I may be asking your help.
I have been wading through the forum and it is like trying to drink from a fire hose. My mill will have to have modifications, as I need to be able to route 4’ by 8’ sheets and turn/mill 24” by 12’ columns. Here are my questions so far.
1. What is the repeatability/accuracy?
2. Apparently there are no problems with only using one stepper motor to drive the Y carriage and it is direct drive. What size is needed for a 200 lb carriage?
3. It appears that the Z is also direct drive rack and pinion. An idea I like but was told it was a no-no. This is am immediate concern, as I would like to use it in the new gantry I am building.

For the type of work I am doing see www.turningaround.org (http://www.turningaround.org/)


Then Donald replied

Art,
If you want to pick up a set of laser parts, I still have a little time on the next batch to get you a set. I am in Paris, Texas so you could pick up or I could ship.

[Gerald_D]

Welcome Art!

Repeatability/Accuracy is not something I have tried to measure since I inherited that from the ShopBot design. They publish these specs (http://tinyurl.com/26w9mr). Some guys say they are actually way better, while some hint that they don't reach them. I suppose it depends on the application and measuring method...

A single stepper on the Y is completely okay, especially when the cutter is mounted towards the stepper side of the Y-car and the dust hose goes on the other side.

Our gantry weight/mass, with motors, y-car, hose, cables etc (fully loaded) is 85kg [187lbs]. A pair of motors drive that okay. (In fact, we have driven it with a single motor by mistake and there was only a slight fault with the cut parts of that job).

There is an old school that says rack & pinion should be banned and that only screws should be allowed on CNC machines. They grudgingly accept that the X & Y axes are too long for screws but they are still holding out on fighting the Z. Please trust the 1000's of people using Z rack & pinion on woodwork CNC's - they do work. And they are very low maintenance compared to screws. The only perk of a screwed Z is that you don't have to worry about counterbalancing the slide weight so much.

[Arthur Ransom]

It appears to me that the geared stepper motors would be a good solution for all axisis. Presently I am using 640 oz direct drive steppers on X and Y and a 640 with 3:1 on the Z. How about center mounting a 1200 oz dual shaft stepper to drive both sides of the cariage? this would eleminate one Gecko, I/O board and elimanate racking due to lost steps by one of the steppers. Doesen't appear that it would be hard to do.

[Dave]

Did anyone come up with any ideas on how to use one motor and shaft to run both side of the gantry.
I ordered my laser cut part the other day and now working on the steel parts, will stop by the yard tomorrow for prices.