Well...that's not too hard but you'll need to do a few things.
1. You need to understand what you local subnet looks like. If you are using a consumer grade router it is likely handing out a class C subnet of IP address. Otherwise know as a /24 subnet. Most of these are similar in configuration out of the box so it is likely handing out some like 192.168.1.0/24 via DHCP. Its local address is almost always .1 unless it has been altered so that would put it at 192.168.1.1. You can ID that by opening a command prompt on your Windows machine and running ipconfig. It will spit back something like the following...
Ethernet adapter vEthernet (Wireless Virtual Switch):
Connection-specific DNS Suffix . :
IPv4 Address. . . . . . . . . . . : 192.168.1.119
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.1.1
The IPv4 address if the address of the machine you are running the command on.
The subnet mask tells the machine which bits are subnet bits and which a host bits. We can get into that later as it is not terribly important to what you are doing.
Default gateway will be the address of your router.
Now letting the router hand out addressing works for most applications. However if you want to share a folder to be accessible to other machines it would be best to set a static IP address on the machine used to host the folder. So in the case of your CAD/CAM workstation you'd want to alter its configuration to be static. There are a couple ways to do that. One is to configure the router to reserve a specific address for that machine. If you want to go that route then you'll need to login to the router and verify that it supports DHCP address reservations. Some of them make it very easy and as simple as clicking a button on a clients configuration page in the router. Due to the need to keep the XP machines off the internet I think statically addressing the individual machines are going to be your best bet. The reason I say this is the best way to keep a machine off the internet is to just take away its default gateway. Easily accomplished via setting a static IP address.
Windows XPStatic IP tutorial. ->
Windows 7/10 static IP tutorial ->
Once that is done and confirmed to be working you can move on the shared folder. The number one thing you'll need to do is make sure that each machine has an account with the same username and credentials. It doesn't have to be the account you use to login to the machine but that would make life easier. Whatever you do please do not use an account that is a local administrator. If you were to get an infection that would allow it to spread easily and infect the other machines.
Windows 10 folder sharing turtorial. -> https://members.iracing.com/jforum/forums/list.page
Please do not use the everyone permission as that leaves the share completely open to any machine that is allowed on your network. This should get you started and YouTube is your friend.