No longer a 'Wannabe'

[robinsoncr@naxs.net]

Howdy Everyone!

That's right! No more nights laying awake thinking about a Shopbot! No more tireless pacing, debating myself on the merits of a cnc that didn't cost more than my house!

We purchased Rick Adams' PRT60x120. It has one Z-axis, limit switches, z-zero plate, 15hp FPZ vac, 5hp Columbo spindle and an assortment of bits.

We've got the 3ph power supply ready and will set the machine in place this weekend. So, in preparation, I have a few questions. (In preface, I have read the archieves.)

My brother/partner has over 12 years experience as an Industrial Maintenance Engineer. From his experience, a few questions have come up.

1. Is there anyone that has located their PRT, then leveled with metal machine levelers and bolted the legs to the floor? He says it will significantly reduce vibration, reduce the potential for frame movement and reduce the chances that the machine will move over time. All of which contribute to inaccuracy.

2. We're going to add legs in the middle to reduce/eliminate sag. I read in the archives that someone had done this. Any advice?

3. I also read in the archives that the above person used gusseting on each let to further reduce racking. If you have done this, how did you attach from the leg to the frame?

4. Vac Pump noise reduction. I had the thought of locating the vac pump under the table and then building a sound enclosure around it. My thoughts were to take aluminum panels, bond noise reducing foam to the inside and bolting these panels to the outside of the machine, extending from the bottom of the table to the floor. We'd allow for motor cooling air circulation. Has anyone tried this? If so, how did it work for you?

Sorry for the long post. I've been building up for a while! Sorry I couldn't make it to the jamboree but I'm behind in work and couldn't get away.

Thanks,

Chris Robinson

[beacon14]

Chris,

Good luck, I can read the excitement in your voice.
I guess from the lack of response that you and I are the only ones who aren't at the jamboree this weekend.

1. Yes we did bolt the machine to the floor, with no regrets. (The landlord may regret the holes in the floor later, when we move out.)

2. We built a plywood table, doubling up 3/4" plywood for a 1 1/2" frame and rails, and it has center legs. It makes it less convenient for storing materials under the table, but I'm sure it makes the frame sturdier, especially with your 10' table.

3&4. Can't help you there.

Keep us posted,
David B.

[gerald_d]

My reason for lack of response is the fact that your brother/partner is advising you, and I am not going to start a family feud.

[robinsoncr@naxs.net]

Gerald, har, har! It doesn't hurt to ask for other people's opinion! No need to go through the whole ordeal to reinvent the wheel.

I wish I could've made the Jamboree...

Chris

[John S.]

Chris-
All of your brothers suggestions would help quite a bit. I have a PRT 120 and I didn't see any of those other posts and didn't have someone like your brother to advise me. Of course I see a lot of these problems now, but am too lazy to fix them. I guess I'll have to do it now that I see it's not just me that had to do a little tweaking.

The table does sag, so extra legs are a very good idea. Maybe a few more supports horizontally across the x axis that your legs can tie into?

The thing takes so long to square up, that I wish I thought of anchoring to the ground. I bumped the table a couple of times with a forklift and threw it out of square. The upsidedown carraige bolts on industrial slab concrete flooring tends to slide very easily, so even though the table hasn't been bumped, I am sure it has vibrated out of square again.

On the PRT 120, the Y car has a lot of bounce in it. You may consider adding some type of stiffiner to it. I thought of a 1x2 steel tube on the tops of each Y car rail. Drill holes that correspond with the holes on the rail, oversize the top holes (along the 2" wall) for the bolt heads, and a through holes just big enough for the bolts to pass through on the bottom.

I guess gussets can't hurt, but I don't think your machine can move fast enough for the gussets to make a difference. I could be totally wrong, but just my opinion.

[gerald_d]

Chris, if there is a potential for a John's forklift to bump into it, then don't bolt it to the floor. It is better that the machine can slide away, rather than crumple. And that would be the ONLY good reason for not fixing it to a concrete floor.

Gussets/cross-braces are always good, the longer the better. Try to go from a point on the legs near the solid floor. One bolt at each end of an angle iron cross-brace is enough.

Allowing for cooling/air circulation in a closed box under the table could be a problem. Out of sight, out of mind, and the cooling air channels get clogged with sawdust . . . . . . . .

And yes, I agree with John, you should probably look at your gantry first . . . . . .

[John S.]

Gerald-
As always, more very good points! I guess I was smart not to anchor my legs, right! ;-)

[robinsoncr@naxs.net]

John,

Timm is going to bolt the machine to the floor, pin the legs and gusset them. We have a forktruck but only Timm or I drive it. The truck won't be able to get into it because at the front, we'll have a scissor lift for raising sheetgoods up to the spoilboard.

We were looking at the y-carriage today. I was wondering about a frame made from 80-20 stock. Timm was thinking about welding up a thickwall aluminum carriage. (He likes to fabricate things anyway! You should see the Harley he bought in boxes and rebuilt!)

I'll keep you posted. I can't wait to fire this thing up!

Chris

[gerald_d]

For the gantry, the sketches at the bottom of this thread (http://www.talkshopbot.com/forum/messages/29/1352.html?) may give you some ideas.

[logical@gmi.net]

Well, I just made the plunge. I will pick up my new prt 96 the end of this month. I am anxious to get to work. Thanks again to all who gave me information at the jamboree.


Wes
Raleigh, MS

[robinsoncr@naxs.net]

Gerald,

Do you a second servo drive on the y-carriage? How hard is it to add one and what's involved? Any idea about cost?

Chris

[gerald_d]

Chris, we only have one y-motor and are nowhere near to considering a second motor for that. The y-car is very short and our router is mounted nearer to the motor end.

If we had to add a second z-axis (with router twice as far away), then we would look for "racking" deflection of the y-car. But we would investigate gussets before considering a second motor.

No idea of costs, but it looks like a simple plug-in of standard ShopBot components. Maybe one or two holes have to be drilled.

What are you planning to do with your SB that will generate these larger than normal forces?

[noah@noahsart.co.nz]

Chris/Gerald
I am in the process of ordering a prt96 to replace my benchtop. I have decided to include an extra y motor, price i've been quoted is approximately 400 bucks plus an extra 125 for the rack, and it is simple to do. My benchtop does the assymetric wobble due to the off centre thrust on the Y. The drive on the X is in the centre cos the table moves on the benchtop.

[gerald_d]

I cannot recall ever reading here in the Forum that somebody has gone from a single to twin y-motor with any benefit. Granted, the x-drive of the gantry version is in the center, but that proves nothing - surely you didn't have all the router forces in the center of the table? In fact, with the gantry version, you sometimes work 2' away from the rack and if that is not a problem, why worry about the short y-car? If the y-car itself is not stiff enough, then gussets will be more economical than a second motor.

I have resistance to trying to fix things that ain't broken. Simon & Chris, I would suggest getting a standard machine first, and only consider serious mods after you have some experience with it.

[papadaveinwy]

I fully agree with Gerald, I have a (or that is untill the 15th) a prt96 with twin z heads and have never had any problem with the y-car. Infact the more motors the more frequency resonance, therefore more chance of noise interference. I have had one or two times this has caused a problem in the x-car. David in Wyoming

[elcruisr]

OK, here's one guy who solved his problems with a second y motor. We have a 5' x 12' PRT with a 5 hp columbo spindle. When we bought the machine we were told we would be able to achieve certain feed speeds in 3/4 pre laminated plywood. We had problems losing y steps at that speed and also experienced some deflection problems during y moves. The folks at shopbot decided that the way to live up to their word was to add a second y motor. It solved those particular problems, the rest is another story...... If we were to order another one I wouldn't dream of not having the double y motors.

[gerald_d]

Eric, what are those feed speeds, and what diameter bit? It would be useful for the rest of us to know if we are anywhere near to the forces that you are generating.

[elcruisr]

We cut 3/4" Elliotis 9 ply with a 3/8" Onsrud single flute compresion spiral at 2.2" / sec. Sometimes 40 hrs a week. Also cut 3/4" maple or birch 7 ply with HPL prelaminated and a poplar core at the same rate with the same bit. Edge finish is very important and a very good edge is achieved at about 11,500 rpm. I can cut 1/2" ply at 2.5" per second and I cut my vacuum table surface at 3.2" per second with a 1.25" diameter Onsrud fly cutter.

Before the second y motor we started losing steps at 1.9" per second and had to run at 1.7" to 1.5" per second. While the differences may seem small they add up on large runs of thousands of parts and we are in a competitive market. That difference meant getting contracts and our desired price point. We've had contracts where a 1/2 penny a part has meant getting the job from our competition.

What I'm really looking forward to is the new control board design I heard about at the Jamboree. It will reportedly give a 20% cutting speed increase. Rumour has it we might see something in six months and I'm looking forward to it!

[gerald_d]

Thanks for all those details Eric! It is interesting that you opted for the 3/8" bits rather than the 1/2" that most ShopBot/PorterCable users seem to aspire to. We have standardised on 1/4" and I think that you have struck the ideal happy medium between the two. (8mm bits are very popular in Europe).

What is your typical bit life?

But your speed of 11 500 rpm sounds a little slow? What happens to the edge when you go faster?
(The load on the SB-motors will be less if the spindle turns faster). If we extrapolate your 3/8" @ 11 500rpm up to 1/2", it means less than 9000rpm to keep the same tip speed. (and about 18000rpm for a 1/4" cutter).

[elcruisr]

We found the 3/8" diameter bits to be our best comprimise between tool cost and rigidity. We do occasionally still use both 1/2" and 1/4" bits when the job calls for it.

Typical bit life still depends to an extent on the material but take for example the elliotis plywood we are running alot of. We typically get a bit life of 18 to 24 hours of cutting time untill the edge quality drops below acceptable standards. Speed up the RPMs and you get dust instead of chips, a sure sign of too high rpms, and bit life degrades very fast to around a single 8 hour shift. Above that rpm also produces burning when entering a cut, also a great duller of bits and not so acceptable from a parts quality standpoint. Remember also that we use a single edge cutter. If we used a two edge cutter then we would have to decrease feedrate or increase RPMs which would again probably shorten bit life.

We base our approach on the industry standard chiploading formula and the advice of our Onsrud technical rep. He's got over 25 years in the technical end of router bits and cnc cutting so I figure he's seen a thing or two. Remember it's not just about rpm or feed rate, it's about chip load, thats what cools the bit and gives the optimum cutter life. As one veteran of many years of cnc work told me "a cnc machine does two things. It holds a part and a tool. Everything else revolves around that, particularly the tool."

Occasionally we have to cut long runs of 3/4" virola ply. I hate the stuff personally. It's stringy grained and full of abrasive particles. But it's also cheap and one of my customers uses a ton of it. To get acceptable part quality we have to up the rpms to around 14,000 and trash a bit a day. Our tech rep says next contract for that he has a few new bits for me to try and see if we can get better edge quality and cutter life. That should be in about three weeks.

[noah@noahsart.co.nz]

Excuse the delays involved in responding to any suggestions, I am out in New Zealand where it is daytime when the rest of you are up till the small hours on this forum.
Gerald I can appreciate your points about trying stuff only after the standards have failed. I have gone over and over this thing in my mind and it SEEMS completely logical to have two motors on each drive for all sorts of reasons. Pushing power, keeping square, symmetry. Even (from a far away takes time to get here point of view) spare parts! You should appreciate this one in Africa.
PapaDave what is this resonance thing? I would expect 2 motors to override any mechanical resonance.
At least it is working for Eric. Anybody else succesfully using two motors?

[gerald_d]

Simon, after seeing the single y-motor on our SB, I realised that all the parts (spares) are available here in South Africa.

[papadaveinwy]

Simon, the resonance thing is when the noise from electrical products i.e. motors lights etc give off a thing called 60 cycle buzz and when things are just right that can cause havic with the computer. the more electical units the higher the chance for this. also when the bot or any machine is working in a moving mode using electric motors and does a tight radious it can resonate a sound caused buy vibration and will be amplified by the metal parts, this at times will cause problems if you set up your grounding properly this should not be a problem but there are many posts on this subject do a search under missing steps or under electrical noise David in Wyoming