First plaque, sloppy lettering problem

[DarinB]

I finished my first project for a friend. It went ok, but the Vcarving was a little sloppy looking. The letters have little waves in them and the circles have flat spots, as do the C, O, P, etc... Is this a problem with the lettering I chose or my machine? I used a brand new CMT 60 degree laser point bit. I meant to inlay the coins, but time did not permit. The molding was my first attempt with VCarve.
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[Chuck Keysor]

Hello Darin. At first glance, I thought your letters were OK when viewing the over-all plaque. But then when I looked at the close-up, it did look like things were a bit uneven. Then I realized that the paint that you used to fill the letters looked like it was almost over-flowing the edges of the V-carving. That obscures what your machine may have done.

Do you have a picture of the lettering from before you put in any paint? If not, can you do a fresh cut of the same text and skip the painting? I think that such a photo would help people evaluate how your machine is carving.

Your up close photo is pretty good, so you should be able to get something that will give more clues if there is no paint applied. Please try taking pictures from various angles, as I can have a letter look better or worse, depending upon how it is lighted.

And as to your bit, the CMT Laser bit is about the best I have found.

Thanks, Chuck

[steve_g]

Darin…
It would also help diagnosis if we knew which machine you have and its age. Some of the issues you describe can be caused by a loose or worn pinion gear.
SG

[myxpykalix]

go back to your original file and zoom in as close as you can get in your 2d view and check to see if you don't have broken vectors or vectors with too many points that are jagged and misaligned. Where did the text come from? Did you copy and paste it, or create it originally? Try remaking the file (not recutting) with a different font and see if that makes a difference.

[DarinB]

Here is a pic without the paint and right after carving. I re-ran the toolpath right after the first run to clean it up. It seemed that there were thin lines of wood inside the lettering, on the straight lines of the text. The straight edges of the letters also do not look straight, they sort of bump in and out. I used Franklin Gothic Medium font. The preview looked fine, even at very high resolution. There wasn't an excessive amount of nodes and no movement in the spindle. I checked the pinion gears and they looked fine. I'll try and re-calibrate it today. I have a Shopbot BT32 Alpha, 2008.

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[jerry_stanek]

what program did you use to design it can you show us the preview.

[RossMosh]

How fast were you moving? My opinion is you needed to slow down. Looks like the machine was constantly trying to get up to speed and thus caused a jerky motion.

[DarinB]

Well, upon the recommendation of a few, I checked the calibration. It was off on the y by +.007 & +.018 on the X. I put in an engraving bit with a .002 tip and set up a ruler on the bed. The Y is dead on now, from home to 10". But here's my problem, if I move the X axis 10" it's dead on. If I move it 1" it's off by +0.015. This is by adjusting the values in the shopbot software. Where am I going wrong? I know it controls the resolution, which might have something to do with the sloppy lines, but why the difference in 1" and not at 10?

Jerry, I am using VCarve Pro 8.5.

[Chuck Keysor]

Thanks Darin for posting an unpainted view. Though it isn't as zoomed in as your earlier painted detail photo, it does look like your letters are sloppier than I get from of my 2004 PRT alpha. I hope some of the experts weigh in. I am no expert, but as I recall, the model resolution only applies to 3D models, so I don't believe you can gain any insight by changing your resolution.

While I run my V-carving no faster 1.5IPS, people who know often comment that there are software limits on the speed/acceleration characteristics of your machine on all the curves, which in such work, is virtually everything. But you can go and change those limits. Maybe your limits on ramping speeds, etc as defined within your machine have been incorrectly set by a previous owner, allowing excessive cornering speeds.

There is an extensive write-up on this subject, as I recall, by Brady Watson. (But as my machine worked OK, I have not had to wade into those weeds, though I know that I should, just because this topic is key for obtaining maximum performance.) It may well be that you have to investigate that subject in order to improve the performance of your machine.

Again, let's hope some of the experts weigh in. Chuck

[Gary Campbell]

Darin...
Your issues are most likely due to mechanical backlash or slop in the drive system. The pinions either have loose grub screws or they are not adjusted fully into the rack. The X axis under the table is the hardest to see, but is the most likely to require adjustment on a regular basis. This condition would return the results you mention above. Debris or lack of lube can cause the pinion to be not fully engaged in the rack.

The only condition that would require adjusting the unit values (calibration) would be a worn pinion. I suggest you revert yours back to the default for your model and leave them alone. You are far better off replacing the pinions than adjusting the unit values to compensate.

You can use a dial indicator to find which axes have backlash and how much. A firm push/pull with the indicator set will quantify the amount.

[DarinB]

Thanks Gary. I think I'm going to order some replacement pinions. Being that it's an 8 yr old machine, and I have no idea how many hours are on it, it might need it by now. I'll check out the pinions and seating today. I've been keeping everything lubed pretty often, but I have not taken the motor off to check the pinion on the gantry yet. This is definitely a learning process, and some frustrating times.

[Gary Campbell]

Darin...
You may have to add a block to the Buddy able to set the indicator to, but here is a doc that shows how to use a dial indicator to measure actual backlash.

[Chuck Keysor]

Gary, thanks for the article on pinion back-lash measurements. However, the article contained no specifications for acceptable amounts of backlash. So, I figured Darin would want to know that next, so I did a Google search of the Shopbot forum, looking for limits which I could post here for Darin. But oddly, whenever I found information that looked like backlash specifications, I would see slightly jumbled text! Such as: "PRT .003" to .004". It's gotta be in the gears."

Where should Darin look to get his pinion backlash limits? Thanks, Chuck

[Gary Campbell]

Chuck...
There is no hard and fast number as the acceptable backlash can vary with the gearbox type that is on the motor. Intended use of the machine, pinion diameter and age of machine will also be determining factors of what is acceptable.

In most cases .004 to .006 could be considered acceptable, maybe even good. But if you had a thousandth more would you wish to replace all your gearmotors to make birdhouses as a hobby? I think not. You may want to consider it if you have a 4-5 year old machine and cut close tolerance parts commercially.

I think Darin will find that he has at least one axis that has substantially more play than I mention above. That will allow him to concentrate his efforts and learn how to detect and correct pinion wear and maladjustment problems. Don't get hung up in a decimal point number!

[Chuck Keysor]

Thank you Gary for your reply. I hope it will be useful for Darin to not only know how to measure the backlash of his machine, but what range of numbers he should be looking for on his dial indicator. Thanks, Chuck

[Joe Porter]

Darin, I have the same age and size Buddy as you, although mine is a standard. Some while ago, I noticed some slop in my X axis. I finally found that the slop was in the gearbox of the motor itself. Supposedly you can separate the gearbox from the motor and adjust the looseness, but I really haven't had a whole lot of luck with this. On top of the advice already given, I would consider the size of my plaque material and mount it to your table at the center of the machine directly over the gear and rack. Also, release the pinion and check that your table is still in good contact with the v-rollers and guide rails. Hope this is of some help...joe