Ramp and lead on last pass?

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Branchy
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Model of CNC Machine: SCM Record 2 and Procam
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Ramp and lead on last pass?

Post by Branchy »

Hi,
I've had a query for a little while now regarding the lead and ramp missing on a final pass. If I programme a toolpath to multipass a profile, I've noticed that the ramp and leads will perform on all passes except the last one.
Can anyone advise if this is correct or is it a fault in the programme?
Cheers
Chris.

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Adrian
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Re: Ramp and lead on last pass?

Post by Adrian »

Is that with the separate last pass option with an offset on it? It's always done it like that as far as I know which is why I don't use that feature. I can't find the post now but I'm sure it was mentioned that is how it is supposed to work. I'm assuming there are some circumstances where the leads might mess things up?

What I do instead is to create two toolpaths, making use of the start depths and offsets, and then use the merge toolpaths with the By Part option. That gives you the same result as using the last pass feature but gives you full control over ramps, leads, feed rates etc.

epicycloid
Posts: 33
Joined: Sat Apr 21, 2018 8:40 pm
Model of CNC Machine: Custom router with Dynomotion KFLOP
Location: southern OR

Re: Ramp and lead on last pass?

Post by epicycloid »

I am searching the posts because I just "discovered" this same issue as well.

I set both circular lead in and lead out, with ramps. I also checked the "Do Separate Last Pass" with an allowance of 0.02".

After making multiple passes, with the lead ins & outs and ramps, the bit plunged straight down the side of the parts to start the last pass at full depth with the allowance cut.

This is wood, so I guess it is ok and can be sanded out, but it wasn't what I expected to happen.

In the machining world, especially with materials like aluminum (aluminium), leads are used to eliminate the witness marks on the finished parts (as opposed to sanding).

I have searched too and can't find the post Adrian mentions. Is this really how the feature was designed to work? Other than making a separate toolpath with a manual offset / allowance, is there some other way to accomplish the desired ramps and leads in one profile operation?

AdamJ
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Re: Ramp and lead on last pass?

Post by AdamJ »

I believe it should place a small lead between the previous pass and the final pass. Ramps won't be included because as you mention, ramping down may produce unpleasant witness marks.
The lead length is restricted because there are lots of circumstances where having a large lead may result a potentially unsafe plunge into a lot of uncut material. So effectively what will happen is that if leads are on, and we can move "safely" across the gap then we will across this distance, but if we can't it will behave as you describe. This shouldn't happen too often though, and typically when there some geometrically significant difference between the allowance vectors and the final vectors. We are always grateful to receive examples where the behaviour is not correct if you email them to our support team.
Small lead across gap
Small lead across gap
lead_across_gap.png (1.45 KiB) Viewed 1918 times
As for alternatives, I think Adrian probably has more to say on this but I know some people use multiple toolpaths with toolpath merging.

Thanks,
Adam

DLWOODWORKS
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Re: Ramp and lead on last pass?

Post by DLWOODWORKS »

I use the method Adrian mentions except I just save the two tool paths into one file. All first passes are done, then the final pass is done. I tend to climb cut all my first passes, then conventional cut my final pass. I get a cleaner and more accurate part when done. The climb cut pushes the bit away from the cut while the conventional cut pulls the bit right to the line. Since you are removing VERY little material on the final pass, bit and machine deflection are negligible.

epicycloid
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Joined: Sat Apr 21, 2018 8:40 pm
Model of CNC Machine: Custom router with Dynomotion KFLOP
Location: southern OR

Re: Ramp and lead on last pass?

Post by epicycloid »

@AdamJ -- Well, I guess I stand corrected. I went back and looked v-e-r-y closely at the toolpath. The leads are there, they are just incredibly minuscule.

Here are three screen shots showing the 0.375" radius leads for the primary passes, then zooming in on the start leads, and then zoomed way in to see the last pass lead, which is less than 0.001". I guess that's why it looked like the bit just fed straight down. It did feed straight down into 0.019" of Allowance left behind, then moved in towards the work the imperceptible remaining 0.001".

I left an allowance of 0.020", so more than 10x the lead that ended up being calculated for the last pass. I can understand caution, but it doesn't make sense to me that the lead would be calculated to be less than the previously left behind allowance amount (at a minimum). The software clearly knows the cutter size and the allowance left on the previous pass. Maybe mine is just the simple case, and there is some fringe case that requires erring on the smaller lead size.

With that said, I can see why some have adopted the "Adrian Technique", and that might be the way to go, just creating two tool paths and either combining them, or as DLWOODWORKS suggests, just writing them to the same file.

At least now I know what's actually happening now, so I can modify my process.

@DLWOODWORKS -- Thanks for the added suggestion of the direction change too.

Thank you for the insights,

--Jon
Overview of full scale leads (0.375" radius on main leads)
Overview of full scale leads (0.375" radius on main leads)
Zoom in on the normal and Last Pass start points
Zoom in on the normal and Last Pass start points
Last Pass Lead zoomed way in, scale visible at top for size reference
Last Pass Lead zoomed way in, scale visible at top for size reference

AdamJ
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Re: Ramp and lead on last pass?

Post by AdamJ »

These are good points Jon, and I will discuss them with the rest of the developers.
I left an allowance of 0.020", so more than 10x the lead that ended up being calculated for the last pass. I can understand caution, but it doesn't make sense to me that the lead would be calculated to be less than the previously left behind allowance amount (at a minimum). The software clearly knows the cutter size and the allowance left on the previous pass. Maybe mine is just the simple case, and there is some fringe case that requires erring on the smaller lead size.
The short lead length is set at the offset allowance, which for linear leads would behave as you describe, but obviously for circular leads then this distance (measured along the curve) amounts to a very small offset from the profile. I understand now why you are seeing these really small leads and I'll see if there is anything that we can do about it whilst still maintaining our safety.

Thanks for your response,
Adam

epicycloid
Posts: 33
Joined: Sat Apr 21, 2018 8:40 pm
Model of CNC Machine: Custom router with Dynomotion KFLOP
Location: southern OR

Re: Ramp and lead on last pass?

Post by epicycloid »

Thanks Adam. I understand safety, and the variety of cases the software has to deal with. This behavior just seemed uncharacteristically strange.

At least with the other feedback there is an easy work around. It is just one of those things that users have to be aware of.

--Jon

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