XY table

[Fred Bohle]

I tried using mine, put together XY table with stepper, motors, and screw drives, and attempted the Greek key functions on my MDF Rose Engine. There seems to be some slop in my XY arrangement. I wondered what other people use with their MDF Rose Engine.

If I use a light cut, it seem to cut accurately, but with a heavier cut, the path seems to wobble and it looks messy.

 

[Doug Rasmussen]

Assuming there isn't any flex in your cutter..... and the gibs on the XY table are properly adjusted (not to tight or too loose).

There is a slight problem visible in your light cuts too with over run of the cutter at direction change.

A likely problem is too much looseness in your screw/nut combination, looseness in the mounting of the screws, or even the drive coupling between the screw and stepper motors.

Should be easy to test for backlash. Turn the screw in one direction and observe the motion on a dial indicator. Then turn the screw in the reverse direction. If there is backlash the indicator needle won't move until a few degrees of reverse rotation of the screw. That lack of movement would be backlash. Can you put light force on the XY table and see motion on the indicator without turning the screw?

There are various ways to eliminate backlash depending on the cause and construction of your XY table. One is cutting a slot the length of the nut and pinching the nut onto the screw if there's too much clearance between the the nut and screw. There are several possibilities for the problem so pictures of your setup might help along with the above tests.

 

[Mike Harman]

most XY table have a slight movement in them. If you look at how the old masters did ornamental turning then you will see that they did a series of light cuts not one heavy cut. Holspffels vol 2 explains this in turning and manipulation.
It also why on an engineers lathe you make several passes to achieve the end result.
I am in the procees of remaking my attachments for iornamental turning on my lathe and if it helps one thing that I found was to have the X slide on a very fine thread and the Y on a courser thread. this I found helps reduce the free play
What I am also incorporating is a second Y slide that will also have a fine thread this is being mounted on top of the X slide. and will hold the cutters.
When I get a bit further with the build I will put up some photos as instead of a rocking head I am having a sliding base and cross slide that the XY table is mounted onto
Hope that this helps cheers Mike

 

[RichColvin]

The finer X axis threads are also because each movement towards the axis of rotation takes twice that amount off. So, moving in 0.001“ reduces the diameter by 0.002”.

Conversely, a move left or right (the Z axis) is 1:1. So, a 0.001” movement left will only remove 0.001”.

 

[Mike Harman]

Hi Rich
yes that is correct what i based my idea on is from my Myford engineers lathe that i have and also on the other three lathes that I have restored by using the same thread format there is likely no or mininmal movement on the cross slides what there is would be acceptable.
I have been a bit crook lately so havent dne much to the project but hope to get out to the shed this week.
Cheers Mike