Scale Factor Question

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Rik
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Joined: Tue Sep 27, 2005 10:50 pm
Location: New Hampshire

Scale Factor Question

Post by Rik »

Is scale factor 1 a true 1:1 (inches) size. I mean at scale factor of 1 will it machine out as the true size I want it to be?

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BrianM
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Post by BrianM »

Hi Rik,

I'm not sure I've understood your question correctly, so if I've got it wrong just ask again.

The only place I'm aware that the term 'Scale Factor' is used in VCarve Wizard is in the "Estimated Machining Time" form. This form shows the estimated machining time of all the toolpaths for the job. At the bottom of the form are two fields ...
Rapid Rate and Scale Factor.

Both these fields are used to help make the estimate of machining time more accurate, they have NO effect on the size of the job or the output toolpath.

There are two problems we face when estimating the machining time for a toolpath...

1) How fast are rapid moves? Most machines have a code such as G0 which tell the machine that a move is a 'rapid positioning move' which is not cutting material and so should be performed as quickly as possible. To estimate the machining time, we need to know how fast a machine really moves during these rapid positioning moves and this field is where you, the user, can tell us how fast your machine goes when doing a rapid move.

2) All machines have to accelerate and decelerate as they machine a toolpath. It is impossible to go from stationary to say 8 inches per second instantly. When VCarve Wizard calculates the estimated machining time, if a toolpath segment is 100 inches long, and the programmed feedrate is 10 ips, VCW will estimate the time to machine this segment to be 10 seconds (100 inches / 10 inches per second). However, this takes no account of the machine having to accelerate at the start of the segment and decelerate at the end. In theory, we could ask users to enter the acceleration / deceleration parameters for their machine, but very few people would know this information, and from experience in the real world, it still does not lead to particularly accurate results as there seems to be many other factors involved.

What we have found instead, is that is for a given machine, working on similar types of jobs, the user quickly finds that there is a 'Scale Factor' (usually between 1.2 and 4.0) which gives acceptable results for their machine. For instance, with the default scale factor of 1.0, If the estimate for the toolpath is 5 minutes, but it actually takes 10 minutes to cut, setting a scale factor of 2.0 (10 / 5) will mean that subsequent estimates are likely to be more accurate. Although this system can sound quite crude, we have found over many years, that users quickly get a feel for the appropriate scale factor for their machine on particular kinds of jobs and hence can extract very useful time estimates from the software.

Hope the above makes sense

Happy Christmas (it is now 12:07 on Christmas morning here in th U.K - I'm waiting up for Santa :D )


Brian
Last edited by BrianM on Mon Dec 26, 2005 6:53 pm, edited 2 times in total.

hbhbbb

Post by hbhbbb »

Thank you. Yes this expanation makes sense. I noticed a change in the amount of time but mistook that to mean it would be twice as large. OK now that you've explained it. I hope Santa was as good to as he was to me, Happy Christmas.

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BrianM
Vectric Staff
Posts: 1961
Joined: Mon May 16, 2005 10:15 am
Model of CNC Machine: A few ...
Location: Alcester U.K
Contact:

Post by BrianM »

Hi Rik,

Glad it makes sense (must be a first for me :D )
Glad you had a good Christmas, I'm happy to say mine was excellent as well, so ...

Happy New Year

Regards

Brian

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