BT32 Alpha not cutting correct dimensions

[DarinB]

I recently purchased a 2007 BT32 Alpha. When I set up the machine I reset it to factory specs as per the latest control software. I re-zeroed the table to the stops. While making my first cuts, I noticed everything was way off. Instead of cutting a circle 12", it measured 8.125. 1" parallel lines measured 0.815. When I drew these and made the toolpaths I adjusted for tool width. The previous owner had some different values for the machine, although I went with the numbers in the software for my machine from Shopbot.

X Axis -2482.8170
Y Axis 2482.8170
Z Axis -2979.3810

Base Coordinate Offset:
X Axis 54.267
Y Axis 4.182
Z Axis 0.000

I don't know if this is a control software problem or if it's somewhere else. I already emailed Shopbot support, just wondering if anyone else had this issue before.

[Gary Campbell]

Darin...
Previous owner may have changed a couple things to increase resolution. Either 10:1 motors or 25T pinions. Both are easy to figure. What is the model number on the end of the motors? Should be something like ASM98AA-T7.2? If that's correct, remove a motor and count the teeth on the pinion gear. 25 or 30? You may have to use the unit value number from the previous owner.

[dana_swift]

Darin,

I noticed the following quote in your post "When I drew these and made the toolpaths I adjusted for tool width". What? Drew the toolpaths with what software?

If you enter MX 0, followed by MX 2, does the X axis move exactly 2 inches? That is the key question. If that works, the shopbot is operating the X axis correctly. Check the other two the same way.

Now if its not cutting what you expect, the phrase you entered "When I drew these and made the toolpaths I adjusted for tool width" bothers me. If you are doing your design in Vectric products (VCarve Pro, or Aspire) you should NOT compensate for the size of the bit. The design software does that for you. So my question is what software did you "draw" the toolpaths in?

I do most of my designs in the Vectric software (great stuff, I can highly recommend. Worth the price!). In that software you draw what the result should be: a hole of a given diameter, or an island with the material removed from the perimeter. Draw these at the size you wish the result to be. Then when you run the toolpathing operations of the software and select a bit to use at that point, the geometry of the bit will be taken into account. Then you can use any bit and get almost identical results.

There I used the word "almost". And that is where experience with the machine and materials comes in. As a beginner you should be getting accuracy of about 0.02 inches. Later when you figure out what you are cutting and how to cut it efficiently, you will learn other techniques to improve that accuracy. I have been stunned at the accuracy I can get out of the BT-32 Alpha. Mine is an older machine with a lot of years of use. Its still amazing

Hope that helps-

D

[DarinB]

Just got done re-calibrating the values. It was a bit off, which was my problem. I got everything as close as I could get it. I do have another question, how precise do you calibrate the values? By this I mean, what do you use to measure the movements? Tape measure, machinists gauge, v-bit, 1/32 tapered ball nose?

[Gary Campbell]

Darin...
Historically, due to the use of high quality gearboxes, ShopBot machines have not required "recalibrating". This is mostly due to known reduction ratios and pinion sizes that they provide. That said, the previous owner could have changed something. If you would provide the info I listed in post #2, you will be given an EXACT unit value that will allow you to maintain the advertised .002" positional accuracy. I wouldn't recommend anything less.

[DarinB]

Darin...
Previous owner may have changed a couple things to increase resolution. Either 10:1 motors or 25T pinions. Both are easy to figure. What is the model number on the end of the motors? Should be something like ASM98AA-T7.2? If that's correct, remove a motor and count the teeth on the pinion gear. 25 or 30? You may have to use the unit value number from the previous owner.

Gary, it does have the ASM98AA-T7.2 motors. If my count was correct it has 25 teeth on the pinion.
I did the adjustment as per Shopbot. I moved the machine 10" on the controls then measured the actual movement, which came out to 8.315. I took the current settings, multiplied by 10 then divided by 8.315 and used the result as my new value(2986.8475). I checked the actual movement again and it was pretty spot on. I was doing this with a 60 degree bit and a tape measure, but I did get it to the 16th of an inch.

[Gary Campbell]

Are you just a little long? With the 7.2 gearmotors and 25 tooth pinions the unit value should be 2979.3810. If they return less than accurate movement at that unit value, they may be worn.

[DarinB]

Are you just a little long? With the 7.2 gearmotors and 25 tooth pinions the unit value should be 2979.3810. If they return less than accurate movement at that unit value, they may be worn.

Yes, you're right. I did it again and it came out to 2980.7586. I think I need a better tape measure, lol. I have a friend coming over later who has a dial indicator and has done this a few more times than I have. I'll find out whats up with my measurements tonight.
Thanks for the help. I'm sure this won't be the last problem I run into.

[DarinB]

I know this thread is a few months old, but I just wanted to give an update on the issues I had earlier. From suggestions, I replaced all the pinions. After replacing, the old pinions looked like they might have been originals. The difference was night and day, very clean and no bumpy cuts. I cut this the other day for a friend at work, I just have to paint it now!
Thanks for the help guys.

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