Straight line cutting test of CNC machines

iusedtoberich

iusedtoberich

AzB Silver Member
Silver Member
Hello,

I would like to ask if any of you who have either commercial or home build CNC machines for turning if you have performed the following test:

1. Program in a straight cut with no taper.
2. Program in a finish cut of about .010 on the diameter.
3. Run the program on a shaft dowel.
4. Repeat the above on a second dowel.

Measurement:
1. Mark the shaft with a pencil every inch.
2. Mike the shaft every inch its entire length. Use micrometers instead of calipers for best results.
3. Graph the results or post them.
4. Repeat on the second dowel.

I'm curious what type of results people are obtaining with this type of test. If you are not getting straight cuts, what type of shape are you getting, ie. angled, curved concave, curved convex, etc. And do you know why you are or are not getting straight cuts.

If you are on a manual lathe with a taper bar you can still check your machine by just using the carriage feed instead of the taper bar and performing the above tests.

Thanks.
 
It'll take a lot of wood until you see a pattern. lol
1. Mine is off about 5 thou at X23.
2 . A little worse but the good part is it's fat where it's off so I just cheat about .004 more going in.
3-4, it's within 2 thou.

It could be better but am happy backlash on the Y-axis and cutter runoff are not factors.
 
Did you ever do a graphical plot to see trends?

For the purpose of this test, you can always cut in the same direction to negate any backlash issues.

I got my machine a couple years ago and was never happy with its performance. Mine is saw based and it cuts a curve when I program a G01 move. I attribute its non-straight cut to flexing in the structure of the machine as the machine moves along its length.

I'm curious how good other people's machines are.
 
iusedtoberich said:
Did you ever do a graphical plot to see trends?

For the purpose of this test, you can always cut in the same direction to negate any backlash issues.

I got my machine a couple years ago and was never happy with its performance. Mine is saw based and it cuts a curve when I program a G01 move. I attribute its non-straight cut to flexing in the structure of the machine as the machine moves along its length.

I'm curious how good other people's machines are.
Click to expand...
I think yours was due to overhang.

I didn't take a graph.
First, I bought a 30-inch precision shaft so I can indicate it for runout .
Got it down to half a thou and with it spining pretty close to 2 thou in the middle.
My rails are not bolted dead-on straight for the 30-inch travel.
I might get it redone if I can't work around it.
Right now, I just know where to cheat the Y feed.
 
Yep, I'm pretty sure mine was due to overhang. I remember spending at least 10 hours tweaking the thing trying to get it to cut straight. I'll have to find my test pieces that I did then but if I recall right my diameters were off by about .010 or more in different sections of the wood.

I since lost interest in making cues, so I haven't run the machine in a couple years...

But its a good test to do, to see where you really are at. Miking at one inch intervals won't lie.
 
almarktool said:
My cnc cuts within .001 over the 29.5 inches
Click to expand...
Mine too. It used to cut the center of a dowel about .011 thinner than the ends but that was due to a slight misalignment of the 2 rails that caused the carrige to travel on an arc rather than straight line.
A good saw and blade will make a diffrence here. I could never get mine to cut perfect untill I upgraded the saw and got a blade reground and finished with a 2000 grit finish. The change in cut quality was amazing. Very smooth finish and I never get any kind of chatter in the center.
 
Mine is actually a 15 year old techno gantry machine,
that i cleaned up and got it to run the way i wanted it to,
any machine should be able to cut it straight as long as you
indicate in your long axis so it is dead nuts with your work piece.
I use a simple taig spindle and tailstock, the key is to mount the
dovetail perfectly straight to the long axis, should only take an hour
after that all else should not matter too much
 
almarktool said:
Mine is actually a 15 year old techno gantry machine,
Click to expand...
I gotta find me one of those!:frown:
Good news is Techno is selling frames next year.
 
Joey,
just don't buy it sight unseen , I personally think that machine could be designed better, I do have a .010 bow in the table from sag over the years, It will get replaced with a 3/4" thick piece of aluminum ground
flat within .001 when i get time to do that, luckly it does not affect me from doing shafts or inlays, but it does stop me from cutting in points,
just be carefully if you buy used the alum plate will cost me 1200.00.
I would give Shop Sabre a good look at
 
When I started building cues, I was a far better programmer than I was a machinist. Honestly, I knew nothing of machining.

I wrote this program that will edit G-Code taper files. It will proportionally change the X values all the way through the file. I ran the same test as described. It was out .01". Told my little program it was out .01" at the tailstock and it outputs a new G-code file with the updated X values.

This is a really goofy solution but all I was capable of at the time. Technically, with this thing, you could move your tailstock over an inch, update the gcode with this program and it would still make a perfect cylinder.

I'd release this to others but my program is really crude and buggy, Honestly, I just know how to use it.

I've since straightened out my machine but I still use this program. Say I have a customer call in wanting some odd ball shaft size. I just run our 13mm shaft taper through this thing and change it down to 12.5mm at the tip end. It works...

Strange solution, ehh?
 
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