Question for the CNC superusers

Pullshocks

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Recently I have worked on learning some basic 2D CAD drawing techniques, and joined a makerspace facility that has a 4x8 AvidCNC router. I bring DXF files to the facility, and use their Vcarve software to do the tool paths. Due to travel time and limited available CNC router time slots, it is not the most convenient setup, but I don’t have room for any kind of meaningful CNC router in my shop. So it’s a lot better than nothing.

I'm still on the learning curve, for sure.

The attached picture shows my 2nd routing job. It consists of various templates to use at home with a table or hand router and template bits. ½” Baltic birch.

On the peghead template in the foreground, the shaft is supposed to be 1.688 (1 11/16).

Using my digital caliper, it measures 1.70 Also, the tuner holes are supposed to be .25” but a 1/4" transfer punch does not fit.

It seems this is due to the end mill I used being under sized. The long kerfs measure about .245 and the bit itself seems to be a tad smaller, as far as I can measure from cutting edge to cutting edge. It is a Freud solid carbide 2 flute spiral.

Do you ever run into under (or over) size bits? Do you test cut width before running a job? It never occurred to me that a new bit would have a cut width different than the nominal size. I know Vcarve has the option to use custom tool sizes, which would be a way to compensate .

Thanks and happy holidays,

Pullshocks / Mark
 

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guitarbuilder

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I'm sure that the bits have some tolerance to them. I think that as long as you are ending up with smaller holes, you can drill them larger. Is it a new endmill or one that may have been sharpened? That would make it undersize.


Is your CAM software putting numbers out to 3 places? Are you using a digital caliper that goes out to 3 places too? Each machine can be tweaked for steps per mm, so it's possible that the calibration on the machine is off.

If you are using Mach, you may be able to do some cutter compensation too.


My X carve is belt driven and I've calibrated it to make the parts just a hair larger than the number calls for.

You may want to try a Whiteside bit too. I think they are the best router bits out there with Freud and CMT coming in under them.


Cutter compensation (milling) (machsupport.com)
 
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RedHills

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I'd just build that bit in the tool library. Or, an offset for the tool path. "Offset allowance" on the toolpath tab....or buy another bit. I'll take a caliper to some bits tomorrow. Got me curious now.
 
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Pullshocks

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Thank you guitarbuilder. I've learned some things as I go but realistically, I "don't know what I don't know."

The bit was virtually new, had only a few minutes previous light cutting. It is by Freud. I can see where machining a spiral cutter, it would be hard to do that without at least a slight loss of width from the shank diameter.

A side issue is I was in a hurry when I bought it. It is a downcut style. I think I should be using upcut for this, right?

When I get more bits I'll probably go with Whiteside as you suggest, or Precisebits.com

You raise some good points about accuracy of measuring tools, and significant figures. I also checked with my trusty mechanical dial caliper and it showed large, by maybe a little less than half a 64th. Which +/- lines up with what the digital caliper. but neither is really calibrated. I'll break out the Starrett micrometer and check further.

Next time I go to the makerspace I'll probably run some pieces with compensation and see how that comes out, as well as with new cutters.
 

GunsOfBrixton

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First off, having access to an avid 4x8 CNC is nice! Now, having said that, your parts are pretty much within tolerance. This isn't rocket science./.military spec. The difference between 1.70 and 1.688 is a quick swipe of some sandpaper. You can play around with cutter compensation but I don't think you need to. Or as someone pointed out, you can create new tool.with the exact diameter of your bit. But again, I don't think it is worth it for guitar building. Just my opinion.
 

Jim_in_PA

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It's a best practice to measure tooling with a caliper for the reason you seem to have discovered...they are not usually exactly the "labeled" diameter, often slightly undersized. By measuring, you can correct the detail in the tool database so that your toolpaths get cut accurately after you create them. Many folks don't bother, but for situations where precision is critical, it's "better than just a good idea" to do so.
 

jvin248

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.

You'll have flex in the machine, resonance of the spindle, and nearly a million small variables. Including measurement error.

Try this: use your calipers to measure the diameter of three different bits you have ten times in random order each time. Record those thirty data points and plot them on a graph. Look at the spread in the data. Likely more spread from measurement errors than what you see between the three cutters.

This is why building guitars on CNCs in not just 'press a button and out pops a guitar' like most of the buyers on the Internet think.

Make cuts and then compensate for the machine, the direction of the cut X or Y may be different, and even wood grain X or Y may be different than the other way.

You will likely build three guitars for every one you make as a final version, unless you are ok with a few things 'off' your intent. You'll make a first cut and realize the travel was too high and the surface finish is poor. You'll cut the next one slower. Then you'll see your Z-axis is flexing and throwing a few dimensions off so you change cutting depth in critical areas, or the cutting pattern. Then you'll get it all dialed in and make a perfect guitar. Now make sure you write down all your notes so you can recreate that guitar from that code file and machine/stock setup.

But you'll want a different guitar for the next one you make and you'll be back to cutting a couple of scrap guitars until you do your final one again.


Then the power interrupts, the bit breaks, or the CNC drops a line of code or something odd and trashes your perfect wood...


Eventually ... 'out pops a guitar'! ;)

.
 

RedHills

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.

You'll have flex in the machine, resonance of the spindle, and nearly a million small variables. Including measurement error.

Try this: use your calipers to measure the diameter of three different bits you have ten times in random order each time. Record those thirty data points and plot them on a graph. Look at the spread in the data. Likely more spread from measurement errors than what you see between the three cutters.

This is why building guitars on CNCs in not just 'press a button and out pops a guitar' like most of the buyers on the Internet think.

Make cuts and then compensate for the machine, the direction of the cut X or Y may be different, and even wood grain X or Y may be different than the other way.

You will likely build three guitars for every one you make as a final version, unless you are ok with a few things 'off' your intent. You'll make a first cut and realize the travel was too high and the surface finish is poor. You'll cut the next one slower. Then you'll see your Z-axis is flexing and throwing a few dimensions off so you change cutting depth in critical areas, or the cutting pattern. Then you'll get it all dialed in and make a perfect guitar. Now make sure you write down all your notes so you can recreate that guitar from that code file and machine/stock setup.

But you'll want a different guitar for the next one you make and you'll be back to cutting a couple of scrap guitars until you do your final one again.


Then the power interrupts, the bit breaks, or the CNC drops a line of code or something odd and trashes your perfect wood...


Eventually ... 'out pops a guitar'! ;)

.

Jackplane salesman ? ;)
 

Pullshocks

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Thanks to all who responded. A lot of good information to be aware of. I am not expecting "out pops a guitar," just trying to make templates that will help me be more accurate with conventional hand and power tool methods

As I examine and measure the pieces and kerfs shown in the photo, I see some of the variability mentioned above. Even the straight kerfs seem to vary slightly in width.

So at my point on the learning curve, compensating based on measured bit diameter would be one approach, another would be to split the difference between the narrowest and widest kerf and use that as my custom bit diameter.

One more question--for cutting plywood templates, is upcut or downcut better.

Thanks again
 

guitarbuilder

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Thanks to all who responded. A lot of good information to be aware of. I am not expecting "out pops a guitar," just trying to make templates that will help me be more accurate with conventional hand and power tool methods

As I examine and measure the pieces and kerfs shown in the photo, I see some of the variability mentioned above. Even the straight kerfs seem to vary slightly in width.

So at my point on the learning curve, compensating based on measured bit diameter would be one approach, another would be to split the difference between the narrowest and widest kerf and use that as my custom bit diameter.

One more question--for cutting plywood templates, is upcut or downcut better.

Thanks again



Downcut, Upcut and Compression, What’s the difference? | Tools Today Buying Guide
 

Jim_in_PA

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Thanks to all who responded. A lot of good information to be aware of. I am not expecting "out pops a guitar," just trying to make templates that will help me be more accurate with conventional hand and power tool methods

As I examine and measure the pieces and kerfs shown in the photo, I see some of the variability mentioned above. Even the straight kerfs seem to vary slightly in width.

So at my point on the learning curve, compensating based on measured bit diameter would be one approach, another would be to split the difference between the narrowest and widest kerf and use that as my custom bit diameter.

One more question--for cutting plywood templates, is upcut or downcut better.

Thanks again

Creating templates is one of the easier tasks for a CNC do do because it's strictly 2D vector cutting. If you are getting variability in your cuts, however, you should very carefully check your machine setup to eliminate as much "backlash" and other things that can cause deviation in movement. Squares should be squares and circles should be circles. If they are not, the machine isn't tight enough. A properly adjusted machine is a must and you need to check it on a regular basis as you do normal machine maintenance. (on my machine, there are periodic lubrication requirements...while I stretch the time intervals because of limited use, they still get done several times a year)

For plywood, the "ideal" tool is a compression bit (combination up/down), but to use these effectively, you really have to be willing to bite into your spoilboard a bit. So my recommendation is make your first pass with a down-cut end-mill and create a second toolpath to use with an upcut end-mill to finish the work for the crispest edges top and bottom. If you are using plywood for your templates, you may want to reconsider, however. A composite material like MDF may be a better choice for this kind of work. It's inexpensive compared to high quality plywood (like BB) and readily available in 1/4", 1/2" and 3/4" thicknesses. Half-inch is very good for template work. I use the 1/4" for layout templates (no actual routing using them) as it's uber inexpensive for that purpose.
 

Pullshocks

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Thank you Jim. I checked some of the items with my engineers square, and they are excellent. Visually, and by feel, the curved cuts are beaitifully fair and free of any ripple, dips or high spots. On the peghead template in the picture, the CAD file has a slight discontinuity where the peghead curves off from the straight shaft. The CNC tracked that discontinuity very uniformly on both sides

.
Peghead template.jpg


So I tend to think they have their machineunder control, but next time I'm up at the makerspace I'll ask about their backlash adjustments and other maintenance.

Good suggestion about MDF. I rarely if ever make anything with MDF in my own shop at home due to the well known dust issues. But the makerspace has a good dust collection system, and using MDF templates in my home shop would not cause any issues. I really like baltic birch plywood but it is getting quite expensive.
 




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