Hello Guest it is July 05, 2020, 03:27:41 AM

Author Topic: Guideway compensation/mapping?  (Read 3567 times)

0 Members and 1 Guest are viewing this topic.

Guideway compensation/mapping?
« on: May 03, 2007, 04:00:43 AM »
Hi,
does Mach software have any provision for compensation for mounting irregularities or wear/sag/flex in machine guideways?

Cheers
Splint

Re: Guideway compensation/mapping?
« Reply #1 on: May 03, 2007, 09:50:43 AM »
There is screw mapping and there is a section on the Wiki about it.
Thanks
Brian
Fixing problems one post at a time ;)

www.newfangledsolutions.com
www.machsupport.com
Re: Guideway compensation/mapping?
« Reply #2 on: May 04, 2007, 12:41:06 AM »
Thanks Brian,
yes, I was aware there is screw mapping, but I'm curious to know if there is anything for compensating inaccurate guideways. When you look at the dimensions of Chads 5 axis router any error in the mounting of the linear rails (and unless you can precision grind the full area of the  mounting faces there will be some error) will be multiplied over the length of the beams in question. Also, as the z axis moves closer to the middle of the y axis, the y axis will tend to sag and twist due to gravity, this could in theory have a compensation model which adjusts the axies accordingly to minimise the effect. Is there currently any way of doing this or is there plans in the pipeline?

Splint

Offline chad

*
  • *
  •  361 361
  • When the going gets weird, the weird turn pro.
    • View Profile
Re: Guideway compensation/mapping?
« Reply #3 on: May 04, 2007, 02:29:18 AM »
There is another way. Now I haven't had time to play with this yet but would like to look into it soon.

Look for Formulas under Function Con-figs.  It is my understanding that this could be used to correct for this error.

My machine does exactly what you were describing. I have about 1/8 of an inch difference between the middle and the edges of travel on the y (the bridge). Essentially the 1/8 error is on the tangent of around a 500 foot circle (guessing).  Unfortunately I am maths handicapped so i am going to enlist one of my friends who is much smarter than i am to help me figure it out.

After we finish some stuff at the shop i am going to spend some time on the big machine. I have a new 5 axis head in the works and I will be building a new y bridge that will be beefed up to hopefully get rid of the twist problem. 

As for keeping things totally straight and and square i have struggled greatly with this. I have found at this scale it is very much a chicken and egg problem. It is really difficult to build really flat and paralell things without something that is big and flat and parallel to use as a reference. Or a machine big enough to machine said surfaces.

All in all i am not too worried about it. The machine has already paid for itself in the molds we have built with it. and nothing is off more than .1 on the final molds. For our purposes that is entirely acceptable. Aerospace tolerances it ain't, but for home brew for us it is fine and will be getting better soon.

Chad
 
« Last Edit: May 04, 2007, 02:30:58 AM by chad »

Offline fdos

*
  •  326 326
    • View Profile
Re: Guideway compensation/mapping?
« Reply #4 on: May 04, 2007, 05:56:36 AM »

As for keeping things totally straight and and square i have struggled greatly with this. I have found at this scale it is very much a chicken and egg problem. It is really difficult to build really flat and paralell things without something that is big and flat and parallel to use as a reference. Or a machine big enough to machine said surfaces.



Yes it it difficult ;)   Many years ago(about 20)  I worked for a company that built some special x-y- tables for cartographic use.   They were called EMMA tables for some reason.  There was a speacial camera mounted on it's z axis..  I think the customer was Marconi or Quest Automation. 

Anyway these tables were a cast alloy and bolt up construction and used linear rails and ballscrews.   I guess they were about 5ft x 3ft/ Quite a bit smaller than your machine, BUT they had to be built more accurately than your average mill.

I can't remember the exact tolerance but it was certainly in "seconds of arc" not minutes.   They used a Taylor Hobson TalyVel to determine error, and shimmed up the rails until it was spot on.   Took a fair amount of time.

Wayne....
Re: Guideway compensation/mapping?
« Reply #5 on: May 04, 2007, 11:52:56 AM »
Thanks guys,
yeah, I know about how painstaking truing up the axes can be. I've not long gone through and shimmed the x rails (round shaft) on my router, 16 points on each rail, ended up with each rail having variation of 2 thousandths of an inch overall which I'm fairly happy with. I think if I were to take on building a router of the proportions of Chads machine I would consider running a twin beam y axis with the z axis located between the beams such that the gravitational loading would have equal exposure to both beams and hang centerally, this way only the tool loading and accell/decel would cause any significant deflection.
Do you have any pictures to show us of your new head Chad?

Cheers
Splint