Calibrating a new z-zero plate

[mrdovey]

Rather than spend for the stock zzero plate and a piece of wire, I bought a 1/8" x 2" x 6' extruded aluminum bar at my local hardware store and cut two 6" pieces. I bent an end of one up at approximately 45 degrees using a homemade v-block and wedge in my bench vise and drilled a 1/16" pilot hole. I cut a piece of stranded wire and tinned both ends, then attached it to the bent plate with a self-tapping screw. The free end of the wire goes to the terminal block in the ShopBot controller (check your assembly manual for terminal location).

Next, I stacked the two plates under my (unpowered) spindle with a cutting tool in the collet. I lowered the spindle until it contacted the top of the two plates and remembered the z-axis location. Now I removed my new z-zero plate from the stack and held it to the second plate as the spindle was lowered a second time. When the tool contacted the second (not z-zero) plate, I subtracted the z-axis reading from the saved reading and had the thickness of my new z-zero plate to the accuracy of my ShopBot.

To make life easier, I wrote a short SBP file to automate the process (ZCAL.SBP):

' ZCAL.SBP - Copyright (C) 2003 Morris R. Dovey
' Permission is granted to use; but not
' redistribute this original work provided this
' copyright notice is maintained intact - enjoy.

SA

' Hit Enter when ready (2 plates)
PAUSE
ON INPUT(1,1) GOSUB CONTACT
MZ %(3) - 5
&T2 = &T1

' Hit Enter when ready (cal plate)
PAUSE
ON INPUT(1,1) GOSUB CONTACT
MZ &T1 - 0.5

&PT = &T2 - &T1
PRINT "Plate thickness is ",&PT
PAUSE

'Remove plate(s)
PAUSE
END

CONTACT:
ON INPUT(1,1)
&T1 = %(3)
JZ &T1 + 0.25
RETURN

Interestingly, the inexpensive aluminum extrusion measured 0.125 everywhere I measured. I hadn't expected that consistancy in 'cheap' stock.

[rgbrown@itexas.net]

Morris,

Extrusions are very straight and very accurate due to the manufacturing process.

In my "normal" "Z-Zero" routine I placed a pause. The pause is long enough for me to mentally note the number. This "note" allows me to measure a thickness or whatever else I can think of it to do. My "Zero Plate" is a piece of "PC Board".

Ron

[gerald_d]

We put thick double-sided tape under our $0.10 alu. plate so that it cushions a cutter striking the top surface. Our "holster" is the plastic lid of a coffee jar d/sided-taped to the top of the z-car. The wire is a curly cord from an old telephone handset. Why pay good money for something that is very easily produced from normal junk lying around - it is no less accurate?

[mrdovey]

Ron...

After posting I realized that I should have included something like the following code:

OPEN "ZPLATE.SBP" FOR OUTPUT AS #1
WRITE #1,"&ZPLATE =",&PT
CLOSE #1

so that production files need only do

FP "ZPLATE.SBP"

to retrieve the measured value - so that I won't have to remember either the measurement or where I left my pencil 8-)

Gerald...

I installed a drill press center finder (turned to 0.250" on one half and 0.500" on the other, with sharp 60-degree points on both ends) and used that to check out the routine. I expected to see dimples in the aluminum after testing - but couldn't find any even using a magnifying glass.

I like your holster idea; and the coil cord even better.

I'm about to build a dust catcher and have just enough 1/2" Lexan left over to use for a mounting bracket. I'm thinking of drilling that to install a miniature phone plug that I can share between the z-zero plate and the probe. I have a horror of the plate vibrating loose, falling onto the table, and being launched by the cutting tool. I think I will use the curly cord; but locate my holster below the table until I have a bit more confidence in what I'm doing.

[gerald_d]

Morris, our coffee jar lid is about 1" deep, there is no way it will vibrate out of there. The speed of plunge, mass of the router and tension in the balance springs, plus some more factors, will determine how hard the impact is on the plate. Our plate certainly has a couple of scars already. But it really is a very low-tech device and we shouldn't over-analyze it.

[dhunt]

Seems to me that ShopBot makes about one million percent profit marketing a Zero Plate for U.S.$75 + Shipping $10 !!

Never could figure out who would pay that kinda money for what we all make outa scrap stuff, lying around!
...........................................

I recall carefully caliper-ing the Z-plate I used, to discover it was about 2-thou 0.002 less than what the metal-thickness originally was, no doubt due to my shining her up for better contact, over time: I entered the exact thickness in the Zero File and away we go!

[jay_mack1@hotmail.com]

z-zero plate workings?

Looking into buying a shopbot, can you explain in detail the z-zero plate workings. Isn't there a program in the Shopbot software for the z-zeroing. I'm looking into making my own plates. Sounds like to me, its just a circuit switch that completes when the bit touches the plate...

[don_ask]

You are correct. The z-zeroing program is adjustable to the thickness of the plate. It is so easy, I did it.

[Dan B. (Unregistered Guest)]

I made a Z-zero plate out of .250 stock. where do I enter that number for the z-zero plate?
Thank you
Dan B.

[ckurak]

Dan,

Find the file named Zzero.sbp. It should be in the C:\SbParts directory. At approximately line 21 you will find the following:

&ZBOT=0.131 'Put THICKNESS of your plate HERE ... NOW SET FOR 1/8inch Standard Plate
'Our plates seem to mic out at .131 these days ...

Substitute your 0.250 for the 0.131 in the file.

Also, you might want to check your 0.250 stock with a digital caliper just to be sure of its actual thickness. (If you don't have one, you might take your zzero plate with you to a machine shop and ask them to measure it for you.)