Question for PRT Alpha owners

[porscheman]

I have a PRT Alpha, which I purchased as a used machine, and I think the original owner may have installed both Z axes (I have dual-Z configuration) in the wrong vertical position - too low, to be more precise...

Here's a photo of the current configuration:


6817

Each Z-axis seems to have four holes through which the mounting bolts and carriage nuts can be installed. As seen in the photo, the top two holes have been used on my machine, which seems to limit the usable machining range.

I checked the PRT assembly manual (still available online), but while it briefly describes the Z installation, it does not specify which set of holes to use. It appears in the B/W photo in the manual that the middle pair of holes is probably used.

So my question for you PRT-Alpha owners is what position you mounted your Z axis in? In carefully measuring my configuration, if I move the Z axis up to the bottom-most set of holes, this will place the bottom of the collet nut on my spindle at the same level as the aluminum rails on the gantry (thus maximizing my machining depth). However, since most of my machining occurs on stock that is less than 2" thick, I'm concerned that I'll get excessive flex in the Z-axis in this mounting position since that axis will be almost fully extended when the bit is touching the top of the table. So maybe the middle pair of mounting holes is the best compromise between available machining height and minimal Z-axis flex?

Any comments or advice would be appreciated. Thanks.

John

[thewoodcrafter]

Looks just like my setup.
I will have to look tomorrow to see which holes mine is using.

You are right. The farther down the Z carriage is the less flex you will get.

[mark_stief]

John
I use the two middle ones with no problems

[beacon14]

Mine is also mounted using the 2 middle holes. The bottom of the square tube that makes up the meat of the Z axis is about an inch above the bottom of the aluminum extrusion. At maximum Z the top of the spindle nut is even with the bottom of the aluminum.

I just checked and in this position I could clear the top of a 6" high workpiece with up to about an inch of bit exposed. I can also reach the table with the shortest bit.

The tallest workpieces I've encountered have been some 4 1/2" thick slabs for outdoor benches. Is there something you want to rout that doesn't fit under the router?

[wardsa]

I never really paid that close of attention. Mine is mounted very high, using the bottom bolt holes. You can't see any holes at the bottom the way mine is set-up. I to bought mine used and never changed it. The really sad thing is I have flex problems and never thought of that as a fix.

[porscheman]

Guys,

Thanks for the responses so far - input/advice from others is still very welcome.

Regarding David's measurements on his setup, here's how mine compares. At maximum Z position, the nose of my spindle sits 3.25" below the aluminum channel on the gantry. So you can see immediately how much potential "headroom" this configuration is sacrificing. I'm sure some of this difference is due to the "nose dimensions" of the spindles that our machines our using. It may be that my Colombo spindle has a longer nose length than David's HSD spindle. I'm not too worried about those variations since I think they are minor.

I'm running into height problems in the following ways. At maximum Z, the nose of the collet is 4.25" above my spoilboard, or maybe closer to 4.75" if I remove my spoilboard and lay pieces directly on the grid of the vacuum platten. So for example, if I want to cut a 2" thick piece of material with a 2" long endmill, I have to pay very close attention to my toolpath creation to make sure that Z safe heights are properly set. I also have to carefully measure how far the bit extends beyond the collet. This cramped situation would be mostly addressed by moving up one hole position (the holes are approximately 2" apart) and would completely disappear if I move the Z axis up 2 holes.

The second situation that's driving this height analysis is the pending installation of a lathe-head and tailstock as a makeshift indexer. If I place this setup running along the Y axis and outboard of my vac table, then I lose a lot of stock length due to the length of the lathe head assembly. 34" would be the max stock length I could squeeze in. So I'm looking at being able to temporarily install this along the X axis, which means that it's placed on the vacuum table and thus milling headroom becomes an instant issue.

I suspect that I'll probably raise the axes by one mounting hole instead of two. It's tempting to move them all the way up for max headroom, but most of my machining is done using the ShopBot Link on 3/4" panels and I'd hate to introduce any flex into the system that would force me to lower my cutting speeds or cause part inaccuracies. The one other option I need to analyze is maybe to raise the A axis all the way to the top set of holes, which would give me max machining depth on that axis while still preserving some degree of rigidity for the spindle on the Z axis.

Again, any thoughts are appreciated.

John

[blackhawk]

John - I asked this same question back in October. http://www.talkshopbot.com/forum/messages/7/47217.html?1256259062
My machine looks just like yours. I still have not tried moving mine yet. I would be reluctant to move it to the highest position because of flex. One more hole is the most that I want to try. I bought mine used and this position was what I inherited also. Mine also came with a 2-1/4" thick spoilboard (3 pcs of 3/4" material), which I didn't understand. I am definitely limited on height. I have been hacking down that thick spoilboard each time that I re-surface, but I still have a ways to go.

[thewoodcrafter]

I looked at mine this morning.
Mine is set to the center 2 holes.
I too mostly cut with the SB Link on 3/4" material so I don't need a lot of space under the carriage.

[loriny]

I have lifted mine as high as it will go. I had to for clearance for 3D work. I haven't noticed any deflection problems cutting cabinets, and I can just reach my plenum if needed for surfacing,
Lorin