Dado Cut Inaccurate

[Barry K]

I have  an AR8 Pro and am using Vetric aspire software  I  measured apiece of wood at .49 In thick and I want to fit it into anther piece of wood using a dado . I cut the dado using a 1/4 downcut  mill with a Pocket Cut  in a offset  pattern  at .50in  allowing a little room for fit . I didn't fit and after re-cutting the dado several time in ever increasing width it finally fit when I cut a pocket cut at .508 In . Why is that ? Would it be more accurate to my measured with if I used a raster cut 

Can anyone explain how to get a better outcome 

Thank you 

[gerry]

It's better that you post the Vcarve file. If it won't fit, zip it and put it on Google Drive, Drop Box, etc. and post a link here.

[Barry K]

I can post the file ,but I must be not be explaining myself/ the problem accurately . I used a caliper to measure the thickness of a stock which was was .49 In thick. I cut a .20 In. deep dado with a 1/4 downcut mill using a Pocket Cut in a offset pattern at .50in width . The .48 In stock did not fit the dado I cut . It was too narrow .. I just want to know why a Pocket cut cut at .50 In will not fit a .49In piece of stock . Makes no sense to me

[gerry]

The file speaks volumes...

[germanguitars]

Also - if you are plunging a 0.25" bit to 0.20" depth and running fast, it will bend the bit quite a lot depending on speed.

[Barry K]

Test Dado.crv3d (121 KB)In my description of my problem I indicated I cut a .20 In. deep dado ( several passes) with a 1/4 downcut mill using a Pocket Cut in a offset pattern at .50in width . The .48 In stock did not fit the dado I cut . It was too narrow Gerry has asked to see the cut file so please see attached

[joeblow]

Did you measure the dado width with calipers to see if it actually cut .50 wide?

Did you precisely measure the width of your endmill? Just because the manufacturer sells you a .25" endmill does not mean it is precisely .25".

The .49/.48 (not sure which)stock thickness leads me to believe you are using 1/2" plywood? If this is the case, plywood does not hold a precise uniform thickness throughout the sheet. Definitely varies as you measure at different points.

I wouldn't be overly concerned about it myself. I cut a lot of dados and rabbets and typically overcut them by .005 anyways. At the end of the day if they fit good then all is good.

[Barry K]

The Material I am trying to fit into a dado was measured with calipers at .49

[kokopelli2020]

Every point here is valid and I would like to add the issue of humidity levels, type of material being cut and tensional forces when removing material. Mdf is more stable that plywood yet any softwood will tend to curl or curve towards the removed material area. Even fractionally! It’s not a bad practice to add .05 to your removed amount just to accommodate this. Woods are somewhat predictable but it’s not foolproof every time. It’ll fool you one time and treat you like a visionary the next. 🤷‍♂️

[gerry]

The dado slot shown is not the one that will be cut. The toolpath has been changed, but not recalculated. The new path looks but can't tell what the old one is. Old one is displayed when you do a show toolpath.

[Barry K]

I thank you all for your comments . I am still puzzled .. I am a simple man When I measure a piece of wood with a micrometer and get a thickness of .49 In . Then I try to cut a dado using a precision program like Vectric Aspire to cut a pocket cut with a quality Amana 1/4 bit at the thickness measured it doesn't work . I have had several responses suggesting tool bending , humidity and types of wood being cut. If you read my pervious details the explanations make NO sense My question remains . Why is the accuracy so far off . My question is if I need to cut an accurate slot at .49 In why does a pocket cut (cutting outside) not equal .49 In. after process . Why do I need to cut the pocket at .508 In to get a fit This shouldn't be VOODOO

[gerry]

Jan 19, 2021 21:28:17 GMT -5 Barry K said:

My question is if I need to cut an accurate slot at .49 In why does a pocket cut (cutting outside) not equal .49 In. after process . Why do I need to cut the pocket at .508 In to get a fit This shouldn't be VOODOO

That is why I asked you to post a WORKING copy of your project. The last one was not recalculated from the last changes you made. How about the one that actually generated the gcode that you're having a problem with? It sure beats making lots of wild-ass guesses. We need a file that actually produces the problem code.

Have you actually checked the calibration of your machine? Engrave a 5 inch square and a 5 inch circle at your working feeds/speeds with the bit you are using with the profile toolpath. They should be EXACTLY 5 inches. What are your feed/speed for the cut?.....

[Chad]

Bits are very rarely accurate in terms of diameter.

What I would recommend is cutting a single pass in a piece of scrap (with this same bit)...it can be just a few inches long. Then use the calipers to measure the width...You will most likely find that your 0.250 bit is actually 0.247-0.249" wide, which will result in the dado slot being undersized unless a tolerance is used.

This, along with the fact that tools will experience some amount of deflection is often why a tolerance of 0.002-0.006" tolerance if so often used for straight walled inlays.


If cheaper tooling is used...or the tooling has been resharpened, then the amount it is off may be worse. But once measured, its easy to account for.

[mnwoodbee]

Dull tools will make pocket smaller too. Not sure if this is the case, but it’s variable.

[colofan]

I found that if you are using solid carbide cutters they are undersized. I use whiteside and freud spiral bits and they are all undersized. Basically they use stock and machine the bit down to the shank size. They have to further mill some material off to create the profile. My 1/2 inch bits are typically .494 in diameter. I modify my tooling to reflect this in Vectra. cuts a perfect dado everytime. Also my AR6 pro has an accuracy of 0.1mm which translates into being off as much as 0.00394 inches.... Wood moves a lot to humidity mostly so giving yourself a 0.01 inch wider opening would be good.

[bentley]

Hi Guys. I'm gonna give you my 2 cents worth. I had a similar situation come up with my machines. I had a CWI HDX Professor. I thought it was a pretty good machine till I tried cutting a few pockets which are very similar to a dado in that I had to insert a piece of stock into the pocket. Well I found out that machine was so inaccurate that during a trial to find out what was wrong, I did 12 separate pockets and None were the same. They all varied from as little as 3 thousandths to 31 thousandths. Yes that much. I contacted the company I bought it from and he couldn't give me an explanation as to why it varied so much. I also have a AR6 Basic and it to was off. But they were all off the same. Approx 7 thousandths. So long story short. I sold the CWI and now am the proud owner of a ShopSabre 23. I kept the AR6 Basic as my back up machine. The ShopSabre is only off by the amount the cutter is under. My 0.25 down spiral endmill was actually 0.2493. So I hate to say it a lot of it is the machines we use. I need accuracy.
Bentley

[jgarciaa]

Jan 16, 2021 12:36:46 GMT -5 Barry K said:

I have  an AR8 Pro and am using Vetric aspire software  I  measured apiece of wood at .49 In thick and I want to fit it into anther piece of wood using a dado . I cut the dado using a 1/4 downcut  mill with a Pocket Cut  in a offset  pattern  at .50in  allowing a little room for fit . I didn't fit and after re-cutting the dado several time in ever increasing width it finally fit when I cut a pocket cut at .508 In . Why is that ? Would it be more accurate to my measured with if I used a raster cut 

Can anyone explain how to get a better outcome 

Thank you 

Consider exploring the Monport laser for its reputation for being user-friendly, high-quality, and affordable. Unfortunately, I can't access external websites like monportlaser.com/collections/co2-laser-engravers/products/monport-40w-lightburn-ready-12-x-8-co2-laser-engraver-cutter-with-fda-approval?sca_ref=5031521.ka374VZjm3 Regarding your issue with fitting a piece of wood into another using a dado cut, the discrepancy between the measured thickness and the required dado width may be due to factors such as wood compression, tool deflection, or inaccuracies in the cutting process. To achieve a better outcome, consider adjusting your toolpath strategy. Instead of using an offset pattern with a Pocket Cut, try using a raster cut with a smaller width allowance to achieve a tighter fit. Additionally, ensure that your CNC machine is properly calibrated and that the cutting parameters are optimized for accuracy. Testing with scrap material and adjusting the toolpath settings as needed can help you achieve the desired fit.